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Boron Determination—A Review of Analytical Methods

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Abstract

This paper reviews published methods of sample preparation, determinand purification, and the determination of boron concentration and isotopic composition in a sample. The most common methods for the determination of B concentration are spectrophotometric and plasma-source spectrometric methods. Although most spectrophotometric methods are based on colorimetric reactions of B with azomethine-H, curcumin, or carmine, other colorimetric and fluorometric methods have also been used to some extent. These methods, in general, suffer from numerous interferences and have low sensitivity and precision. Application of nuclear reaction and atomic emission/absorption spectrometric (AES/AAS) methods has remained limited because these methods have poor sensitivity and suffer from serious memory effects and interferences. Among a large number of published nuclear reaction methods only prompt-γ spectrometry has been of practical use. The prompt-γ method can determine B concentration in intact samples, which makes this method especially useful for some medical applications, including boron neutron capture therapy. However, this is a time-consuming method and not suitable for detection of low levels of B. Inductively coupled plasma optical emission spectrometry (ICP-OES) created a new dimension in B determination because of its simplicity, sensitivity, and multielement capability. However, it suffers interferences and is not adequately sensitive for some nutritional and medical applications involving animal tissues that are naturally low in B. All methods involving the measurement of B isotopic composition require a mass spectrometer. Thermal ionization mass spectrometry (TIMS) and secondary ion mass spectrometry (SIMS) have been used to measure isotopic composition of B; however, these methods are time consuming and require extensive sample preparation and purification. Development of inductively coupled plasma mass spectrometry (ICP-MS) not only overcame most of the drawbacks of earlier methods, but also its capabiltiy of measuring B isotopes made possible (1) B concentration determination by isotope dilution, (2) verification of B concentration by isotope fingerprinting in routine analysis, and (3) determination of total B concentration and B isotope ratio for biological tracer studies in the same run. Therefore, plasma source MS appears to be the method of choice among present-day technologies.
MICROCHEMICAL JOURNAL
56, 285304 (1997)
ARTICLE NO.
MJ971428
Boron DeterminationA Review of Analytical Methods
R. N. Sah and P. H. Brown
Department of Pomology, University of California, Davis, California 95616
Received April 12, 1996; accepted September 1, 1996
This paper reviews published methods of sample preparation, determinand purification, and
the determination of boron concentration and isotopic composition in a sample. The most common
methods for the determination of B concentration are spectrophotometric and plasma-source
spectrometric methods. Although most spectrophotometric methods are based on colorimetric
reactions of B with azomethine-H, curcumin, or carmine, other colorimetric and fluorometric
methods have also been used to some extent. These methods, in general, suffer from numerous
interferences and have low sensitivity and precision. Application of nuclear reaction and atomic
emission/absorption spectrometric (AES/AAS) methods has remained limited because these meth-
ods have poor sensitivity and suffer from serious memory effects and interferences. Among a
large number of published nuclear reaction methods only prompt-
g
spectrometry has been of
practical use. The prompt-
g
method can determine B concentration in intact samples, which
makes this method especially useful for some medical applications, including boron neutron
capture therapy. However, this is a time-consuming method and not suitable for detection of low
levels of B. Inductively coupled plasma optical emission spectrometry (ICP-OES) created a new
dimension in B determination because of its simplicity, sensitivity, and multielement capability.
However, it suffers interferences and is not adequately sensitive for some nutritional and medical
applications involving animal tissues that are naturally low in B. All methods involving the
measurement of B isotopic composition require a mass spectrometer. Thermal ionization mass
spectrometry (TIMS) and secondary ion mass spectrometry (SIMS) have been used to measure
isotopic composition of B; however, these methods are time consuming and require extensive
sample preparation and purification. Development of inductively coupled plasma mass spectrome-
try (ICP-MS) not only overcame most of the drawbacks of earlier methods, but also its capabiltiy
of measuring B isotopes made possible (1) B concentration determination by isotope dilution,
(2) verification of B concentration by isotope fingerprinting in routine analysis, and (3) determina-
tion of total B concentration and B isotope ratio for biological tracer studies in the same run.
Therefore, plasma source MS appears to be the method of choice among present-day technologies.
q
1997 Academic Press
INTRODUCTION
Boron is an essential element for plants. Boron is present in animal tissue in low
concentrations (about 1 mg B/L) and is probably an essential micronutrient for humans;
however, no essential biochemical function has yet been positively identified to estab-
lish its essentiality to animals and humans (1). Boron deficiency in plants may result
in reduced growth, yield loss, and even death, depending on the severity of deficiency.
Excess B is toxic to plants and animals. Boron toxicity symptoms may range from
necrosis of some plant organs to death of the whole plant depending on the extent
and severity of the toxicity. The tendency of B to accumulate in animal and vegetable
tissues constitutes a potential hazard to the health of those consuming food and water
with a high B content (2).
Boron occurs as a significant component of steel, glass, and the dielectric borophos-
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q
1997 by Academic Press
All rights of reproduction in any form reserved.
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SAH AND BROWN
phosilicate glass films. Boron is widely used as a thermalizing agent in nuclear reaction
materials and as a dopant in silicon wafers in the semiconductor industries. Boron
carbide, used as control rods, is an important nuclear material (3). The flow and other
properties such as etch rates of borophosphosilicate glass films are directly dependent
on their B and P concentrations (4). A small change in B concentration/content can
influence the properties of semiconductor-grade silicon (5) and physical properties
such as hot workability, hardenability, and creep resistance of steel and alloys (6–8).
Boron is also used as a source of short range
a
particles in cancer treatment using
boron neutron capture therapy (BNCT) (9–10).
BNCT is a novel technique for the treatment of cancer that uses
10
B-labeled com-
pounds and neutron radiation to kill cancerous cells. The significance of B compounds
in BNCT stems from high neutron cross section or capture probability (3838 barns)
of the
10
B atom compared to other biologically ubiquitous atoms such as carbon (0.003
barns), hydrogen (0.33 barns), nitrogen (1.8 barns) and oxygen (0.0002 barns) atoms.
The short range (10
m
m) cytotoxic
a
radiation released in the neutron capture reaction
kills the targeted cancerous cells without affecting the neighboring healthy cells (11).
Precise determination of B and its isotopes is necessary for the evaluation of the tumor
specificity and pharmacokinetics of B compounds for BNCT. Often it is necessary to
be able to measure B in very small samples (e.g., biopsy-needle samples) to make
sure that the drugs are actually localized in target tissue before exposing the patient
to neutron sources (12). Therefore, accurate determination of the B concentration is
very critical for these applications.
Naturally occurring materials may vary enormously in B isotope proportions (13).
B isotope ratios (
11
B:
10
B) in naturally occurring rocks and minerals varied from 3.8
to 4.2 depending on the source and the nature of the materials (14). The
11
B:
10
B ratios
of weathered rocks may show negative shifts while those of the marine sediments
show
11
B enrichment relative to their natural abundance ratio (15). Aggrawal and
Palmer (16) have recently reviewed the methods of B isotope analysis. The National
Institute of Standard and Technology (NIST) certified Standard Reference Materials
(SRMs) such as NIST-boric acid standard for B isotope ratio and NIST-botanical
standards for total B concentrations are widely used for verifying the accuracy of a
determination. A small amount of siliceous or a calcareous mineral matter present in
these SRMs may contribute to the errors that may not be resolved by analytical
methods. Therefore, one must use caution to ensure that the heterogeneity found in
SRMs is appropriately considered and dealt with (17).
The purpose of this article is to review the significance, strengths, and weaknesses
of various techniques for B determination. The role of sample preparation methods
for B determination is also addressed.
EXTRACTION OF B AND SAMPLE DISSOLUTION
Extraction from Soil and from Geological and Miscellaneous Materials
The hot water extraction method (18) has been widely used for establishing the
index of plant-available B in soil. However, the amount of B extracted by this
method is affected by the extraction time and temperature (19) and the potential
resorption of B during the cooling period (20, 21). The hot water extract of some
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BORON DETERMINATION METHODS
soils may be colored which may affect spectrophotometric B determination. Treat-
ment with activated charcoal to remove the color of the extract was suggested (22);
however, this treatment may lower B concentration in the extract due to B sorption
on charcoal (21). On the whole, the hot-water extraction procedure is difficult to
standardize (23), time consuming, and tedious for routine usage if reproducible
results are to be obtained (24).
A number of reports suggested the extraction of soils with dilute CaCl
2
solutions
to alleviate the problems associated with hot-water extraction (25, 26). However, cold-
dilute CaCl
2
extracts less B than hot water or hot-dilute CaCl
2
(27, 28). Studies
involving 31 US soils (29) and 100 Dutch soils (23) confirmed these findings but also
showed that the values of cold extraction (water or 0.01 MCaCl
2
) were highly corre-
lated with those of hot extraction (water or 0.01 MCaCl
2
). A more convenient cold
0.05 MHCl extraction method was recommended for predicting plant-available B
status of acid soils (30, 31). However, Fe extracted with the acid extractants often
interferes in B determination by commonly available methods such as azomethine-H
spectrophotometry (32) and inductively coupled plasma optical emission spectrometry
(ICP-OES) (32–34). Alternative methods such as extraction with sorbitol as B-sorbitol
chelate (35) and with BaCl
2
or water using microwave heating (24) were found to
give results in agreement with those obtained by the standard hot-water extraction
method for ICP-OES and spectrophotometric determinations. In light of clay dispersion
and filtration problems commonly encountered in the extraction with either cold or
hot water, alternative extraction methods using BaCl
2
recently suggested by Vaughan
and Howe (35) and Deabreu et al. (24) appear appropriate as they would also allow
the determination of Ca in the same extract.
Decomposition of Biological Materials
Biological samples are commonly decomposed by dry ashing, wet ashing, and
microwave dissolution. However, several other methods have also been reported for
specific applications. Alkali fusion of biological materials caused negligible volatiliza-
tion loss of B and resulted in 8095% recovery of spiked
10
B(36) but the high salt
environment of the fused materials may cause matrix interferences (14). Low tempera-
ture radio frequency (RF) current ashing followed by ion exchange separation of B
reduced matrix interferences in the hollow cathode emission method (37). Alwarthan
et al. (38) decomposed the seeds and pulp of date palms by treatment with saturated
Ca(OH)
2
, drying at 105
7
C, volatilization over a burner and then dry ashing in a muffle
furnace for colorimetric determination of B. For an interlaboratory B determination,
timber samples were extracted with HCl or H
2
SO
4
(39).
Dry ashing of a sample is commonly performed in a muffle furnace using a suitable
container (e.g., quartz or platinum crucibles) by a method similar to those described
by Gaines and Mitchell (40) for the spectroscopic azomethine-H method and by Brown
et al. (41) for inductively coupled plasma mass spectrometry (ICP-MS). Dry-ashed
samples are usually dissolved in dilute HNO
3
for determination.
Wet ashing is performed by placing samples and strong mineral acids such as
HNO
3
,H
2
SO
4
, HClO
4
, or their mixtures in digestion containers (ideally B-free) and
heating (usually with a reflux system) to decompose the sample. The use of B-
containing digestion containers such as borosilicate glass could contaminate the sample
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SAH AND BROWN
and lead to higher blank values. HNO
3
is preferred for wet ashing for plasma source
methods because it provides a simpler matrix than other mineral acids. If HF or HClO
4
is used for wet ashing for ICP-MS determination, the digest is usually evaporated to
dryness and redissolved in HNO
3
(42).
Several analytical methods for B determination have utilized high-pressure micro-
wave dissolution of biological materials in closed Teflon (PTFE) vessels. Examples
of these methods include spectrophotometric (32, 43), ICP-OES (44, 45), and ICP-
MS methods (4648). The microwave dissolution is also used for the decomposition
of nonbiological materials such as steel samples (6) using diluted aqua regia
(3HCl:1HNO
3
v:v) and for coal ash samples (49) using HCl
/
HNO
3
/
HF. High-
pressure microwave digestion is faster than other methods, requires less acid, avoids
sample volatilization and cross contamination, and results in generally low blank
values. However, the digests of biological materials may contain a large amount
of dissolved carbon and organic compounds, which tend to interfere in some meth-
ods of B determination. For the microwave digestion of plant and animal materials,
the ICP-OES and MS methodswere the best, the carminicacid method was unrelia-
ble, and the azomethine-H method was satisfactory only for materials with high B
content (32).
Published findings on sample decomposition methods for B determination are incon-
sistent and conflicting. Sample-B concentrations found by the wet ashing method were
reported equal to (50, 51), lower than (52), or higher than (45, 53) those found by
dry ashing and the microwave dissolution methods. Some examples of these contradic-
tions are as follows: (a) decomposition of human hair samples by dry ashing gave
higher B values than by wet ashing with acid or base when B was determined by the
azomethine-H method (52); (b) wet-acid digestion of plant tissues gave substantially
higher B concentrations than dry ashing (53) and microwave digestion without predi-
gestion (45); and (c) several reports have stated that there is no significant difference
among these digestion methods. Boron values of NIST SRM citrus leaves and other
materials for closed vessel microwave dissolution agreed well with those for dry-
ashing (50) and open vessel wet ashing (43) determined by ICP-OES. Spiers et al.
(51) also found no difference between the decomposition of plant materials by wet
ashing (with HNO
3
) and dry ashing. Most recently, Bratter et al. (54) found good
agreement between microwave sample dissolution and high-pressure wet ashing of
diet samples for a number of elements determined by ICP-OES. We found quantitative
recovery of B and no difference in B values in the NIST SRM biological tissue
samples decomposed/dissolved by either (1) microwave acid dissolution with HNO
3
and H
2
O
2
, (2) dry ashing, (3) wet ashing with HNO
3
and H
2
O
2
or (4) wet extraction
with hot 1 MHNO
3
when digests/extracts were analyzed by ICP-MS(55).
Decomposition of Miscellaneous Materials
Complete decomposition of soil and of geological and silica-rich materials is gener-
ally accomplished either by alkali fusion (36, 56) or by wet digestion using HF or a
mixture of HF with other acids (42, 57, 58). Although Na
2
CO
3
is the most extensively
used flux for alkali fusion, use of other fluxes such as NaOH, KOH, and Cs
2
CO
3
has
also been reported (14, 36, 56). Beary and Xiao (3) noted significant advantages of
Cs
2
CO
3
over Na
2
CO
3
for TIMS determination of B. When the sample is decomposed
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BORON DETERMINATION METHODS
with HF, excess HF is usually destroyed by evaporating the digest to dryness. Signifi-
cant amounts of B may be lost during the decomposition and the evaporation processes
(33, 34, 59), owing to the high volatility of BF
3
, if proper precautions are not taken.
The volatilization loss of B during the evaporation process occurs more readily from
acidic solutions than from neutral or alkaline solutions (60). Addition of mannitol
during the HF digestion and the control of evaporation temperature below 80
7
C(61)
or at 70
7
C(62) were effective in preventing B volatilization loss (61) and isotopic
fractionation (62), probably due to the formation of Bmannitol complex. Hu (63)
used mannitol to control B volatilization during the evaporation of HF
/
HNO
3
from
digests of high purity silica to near dryness (0.5 mL volume). The use of orthophos-
phoric acid instead of mannitol in the HNO
3
/HClO
4
/HF digest also gave 100% recovery
of B following the evaporation of the digests to dryness at 70
7
C(64).
SEPARATION AND PRECONCENTRATION
Some sample matrices may interfere with B determination. Matrix interference
reduction methods such as matrix matching, standard addition, and isotope dilution
methods are often adequate to overcome matrix effects. However, the presence of
substances such as high salt concentrations (e.g., in soil extracts, ocean waters, and
alkali fusion digests), organic substances, and the determinand species that directly
interfere with B signal make some sample matrices too difficult to analyze by the
above-mentioned methods. The isotope dilution technique, one of the most reliable
methods for many difficult matrices, cannot be used for the determination of isotopi-
cally altered samples commonly encountered in biological tracer studies. Similarly,
the determination of isotope ratios would be in error when species in the sample
matrix interfere with one of the two isotopes differentially. Under these conditions,
the determinand of interest needs to be separated from the sample matrix. If the
concentration of the determinand in the original sample is too low to be accurately
measured, then separation is combined with determinand preconcentration in the new
matrix. Several methods of separation and preconcentration of B from aqueous solu-
tions have been reported in the literature. These include solvent extraction (65), ion
exchange separation (14, 66), chelation (14), B-specific resins (36), chromatographic
separation, and separation of B as gaseous methyl borate (67, 68) or boron fluoride.
Solvent Extraction
Boron may be extracted as one of its organic complexes such as (a) the boron-2-
ethyl-1,3-hexanediol complex of B into chloroform or benzene (69, 70), (b) the 2,4-
dinitro-1,8-naphthalenediol complex of B into toluene (71), or (c) the complex of B
with a 24% solution of 2,4-dimethy-2,4-octanediol into isopentanol (72). Novozamsky
et al. (73) suggested a solvent extraction system based on the formation of BF
0
4
ions
that were extracted with liquid ion exchanger (Aliquat 336, tricaprylmethylammonium
chloride) in xylene for ICP-OES determination. The separation may be performed on
line and the separated phase is introduced directly into an ICP using pulse nebulization
(72). However, the use of solvents affects plasmas and often limits the applicability
of these methods for B determination (74).
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SAH AND BROWN
Exchange Separation
Boron may be converted to tetrafluoroborate (BF
0
4
) ions using 10% HF and separated
from the sample on an anion exchange resin (66). The HF digest of rocks often
contains high concentrations of several major cations. The digest was evaporated to
dryness, redissolved in an aqueous solution, cations were separated using a cation-
exchange resin and then B was separated from the digest matrix using an anion-
exchange resin (62). Hemming and Hanson (75) described a separation and preconcen-
tration procedure using Amberlite IRA-743 B-specific anion-exchange resin that gave
100
{
1% recovery of B. Many authors have also used B-specific resins for sample
purification from complex matrices or for sample preconcentration (36).
Chromatographic Separation
High performance liquid chromatography (HPLC) may be used as a separation
device with several analytical methods. Sample B is converted into an ionic compound
such as BF
0
4
ion using 10% HF (66) or a complex with organic compounds (76–78)
and separated from the sample matrix on an ion-exchange column prior to detection.
The ionic complexes of B with chromotropic acid (76) and H-resorcinol (78) were
separated by HPLC and detected spectrophotometrically by measuring absorbance at
350 and 510 nm respectively. When HPLC separation of inorganic ions is used in
conjunction with a conductivity detector, it is commonly called an ion chromatograph,
as several ions can be analyzed sequentially. Ion chromatography has been used for
B determination (58, 66); however, the sensitivity of this method is lower than that
of ICP-OES (79).
Gas Phase Separation
Conversion of sample B to volatile species such as boron fluoride or methyl
borate provides the basis of gas-phase separationof B from the sample matrix. This
technique is used to separate B from a number of matrices, such as wine (80),
waters (67), metal and metal alloys (70, 81), plants (68), and biological tissues
(12), for determination by atomic absorption spectrometry (AAS) as well as by
plasma-source spectrometric methods (80–82). Musashi et al. (83) described a
manual methyl borate distillation method for the separation of B from alkali fusion
digests of rocks. Novozamsky (84) and Johnson et al. (12) described continuous-
flow techniques for on-line generation and separation of methyl borate from the
sample matrix for determination by ICP-OES. The accuracy of the method, however,
depends on the quantitative conversion of the sample B to methyl borate. Some
workers have generated methyl borate in a concentrated sulfuric acid medium (67,
68) to utilize the heat generated by the hydration of sulfuric acid in the reaction
vessel for a rapid volatilization of methyl borate. This technique was used for the
sulfuric acid
/
hydrogen peroxide digest of wine for flame emission spectrometric
determination of B with 0.03 to 0.04
m
g detection limits (82).
Pyrohydrolytic Technique
There are some reports of the separation of B from steel samples by the pyrohydro-
lytic technique (7). The pyrohydrolytic separation of B does not require sample decom-
position. It resulted in 1.0 mg kg
01
detection limits by ICP-OES (7).
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BORON DETERMINATION METHODS
MEMORY EFFECTS
Boron tends to raise the base line in spectrometric and other procedures by adhering
to instrumental components, which affects subsequent readings of many determination
methods. This phenomenon is called ‘‘memory effect’’ and presents a major problem
in B determination (13, 85). Measures to minimize memory effects include running
only dilute concentrations of B, typically less than 0.2 mg/L, and increasing wash
time (86, 87); the use of a direct injection nebulizer (DIN) (36), and running an
acidified (0.02 mol/L HNO
3
) 2 mg/g sodium fluoride (NaF) wash solution for 60 s
between samples (48). Luguera et al. (85) suggested the use of 4% m/v solution of
NaF between the atomization cycles to transform residual B into a volatile species,
BF
3
, which decreased the memory effect in AAS. In a comparative evaluation of
nebulizers for the memory effects of a 1.6 mg B/L solution during the washing cycle
with 2% HNO
3
, a DIN significantly outperformed the concentric Meinhard nebulizer.
With the concentric Meinhard nebulizer, the B signal fell to within 2% above the
original baseline value after 8 min wash time while with the DIN it fell to about
0.03% above of the original value in one minute (36).
BORON DETERMINATION METHODS
Spectrophotometric Methods
Colorimetric Methods
A number of spectrophotometric methods based on the use of specific reagents for
the color development are employed for B determination. Examples of these methods
are curcumin (88, 89), carmine (88, 89), methylene blue (89), azomethine-H (90),
and others such as quinalizarine, arsenazo, and crystal violet (65). Two B-curcumin
complexes, rubocurcumin in the presence of oxalic acid and rosocyanin in the presence
of strong sulfuric acid, are of practical significance for B determination (91). The
commonly used curcumin method employs the reddish-brown rosocyanin complex,
which has a 545 nm absorption maximum at about pH 1.0. The detection limit in
high purity silicon was 3.0 ppb B (92).
The Azomethine-H method is perhaps the most commonly used spectrophotometric
method of B determination. This method is fast, simple, and sensitive and does not
require concentrated acids, which make it desirable for automation (93). In a compara-
tive evaluation of azomethine-H, carminic acid, and curcumin methods for B determi-
nation in water, the azomethine-H method suffered the least interferences and was
the most sensitive (94). Zenki et al. (95) reported that the use of azomethine-HR (1-
(2,4-dihydroxy-benzylidene-amino)-8-hydroxynaphthalene-3,6-disulfonic acid, a de-
rivative of azomethine-H) increased the sensitivity 3.5-fold relative to the azomethine-
H method, but the azomethine-HR method suffered interferences from A1, Cu, Fe,
Ti, and Zr.
A number of flow injection (FI) spectrophotometric methods have been presented
in the recent years. Some of these methods have employed FI systems for sample
introduction in the existing spectrophotometric methods of B determination. Examples
under this category are the use of FI in (1) the chromotropic acid method (96), (2)
the azomethine-H method following an on-line partial dissolution of directly intro-
duced solid soil samples (97), (3) the azomethine-H method following an on-line
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SAH AND BROWN
TABLE 1
A Partial List of Published Spectrophotometric Methods for B Determination
Author Material Method(s) Interferences
Kaplan et al. (29) Soil Azomethine-H Organoborates
McGeehan et al. (21) Soil Azomethine-H Charcoal
Banuelos et al. (53) Plant Azomethine-H
Zarcinas (103) Soil, plant, fertilizer Azomethine-H Fe
Ciba and Chrusciel (52) Hair Azomethine-H
Lopez et al. (94) Water Azomethine-H, curcumin
Chen et al. (97) Soil Azomethine-FI and
carminic acid
Sekerka and Lechner (99) Water Azomethine-FI
/
precon
Zenki et al. (95) Water Azomethine-HR Al, Cu, Fe, Ti, and Zr
Hulthe et al. (171) Sea water Curcumin
Ostling (172) Sea water Curcumin
Ishchenko et al. (173) Steel
/
alloys Curcumin
Higgs (174) Carminic acid
Hofstetter et al. (56) Geological material Carminic acid
Nose and Zenki (101) Water, eye solution Sorbitol/methyl orange Mo, Fl
Garcia et al. (65) Plant and natural Crystal violet
waters
Campana et al. (107) Soils, waters, plants Fluorimetric
Alwarthan et al. (38) Dates Quinalizarin Al, Cu, Na, Mg, and
analine
Lussier et al. (96) Light water and Flow injection analysis
heavy-water
samples
Nogueira et al. (100) Plants Monosegmented FI with
relocating detectors
sequential injection and zone trapping for soil and plant tissue samples (98), and (4)
the azomethine-H method following an on-line ion-exchange preconcentration (99).
Other FI methods have used different approaches. Nogueira et al. (100) described a
monosegmented FI method with detector relocation. The method of Nose and Zenki
(101) determines B by measuring the change in the methyl orange indicator as a result
of the change in acidity of a sorbitol solution in presence of boric acid. A partial list
of published spectrophotometric methods of B determination is given in Table 1.
The spectrophotometric methods, in general, suffer interferences from several spe-
cies including Al, Cu, Fe, Zn, and Mo (102). The sample pH, especially in the range
of 6.4 to 7.0, affects the color of the B-azomethine-H complex (98). Color of the
sample (especially in soil extracts) and high Fe levels may cause severe interference
and a wide variability in spectrophotometric B values in the azomethine-H and car-
minic acid methods (21, 32). The presence of Fe enhances azomethine-H values of
B. Fe interference may be suppressed by thioglycolic acid treatment (103); however,
thioglycolic acid reduced the sensitivity of the azomethine-H method. These interfer-
ences and lack of sensitivity limit the application of spectrophotometric methods for
the samples with low B concentrations and complex matrices.
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BORON DETERMINATION METHODS
Fluorimetric Method
Boron forms fluorescent compounds with a number of reagents in concentrated acid
as well as under milder conditions. A list of these reagents is given by Chimpalee et
al. (104). The differences in the reported fluorimetric methods are mainly due to the
choice of reagents used for fluorimetric reaction. The FI fluorimetric method of Mo-
tomizu et al. (105) and the spectrofluorimetric method for plant tissues, fertilizers,
and natural waters employing B determination at the constant wavelength difference
of 24 nm (106) used chromotropic acid as a fluorimetric reagent. The fluorimetric
methods using Alizarin Red S (the sodium salt of 1,2-dihydroxyanthraquinone-3-
sulfonic acid) were described by Chimpalee et al. (104) with a 0.34
m
g/mL detection
limit, by Campana et al. (107) with a 7.2 ng/mL detection limit, and by Blanco et al.
(108) for simultaneous determination of B and Mo using first and second derivatives
of the synchronous spectra. However, this method is sensitive to pH and temperature
and suffers from inferences from a number of chemical species. The fluorimetric
method using carminic acid also suffered from several problematic interferences (32).
Ionometric Methods
For potentiometric determination, B is generally separated from the sample matrix,
treated with HF and the resulting tetrafluoroborate ion (BF
0
4
) is measured potentiomet-
rically with a suitable BF
0
4
selective electrode. The BF
0
4
conversion is complete within
a few minutes (109, 110). Ionometric methods not requiring B separation from the
sample have also been reported (2, 111, 112). These methods, however, are severely
affected by the sample matrix which may shift the potential. To achieve reliable
results, it is essential to either remove the matrix or match the calibration matrix with
that of the sample (2). A polarographic method based on the adsorptive characteristics
of the B complex with beryllium (III) (4-((4-diethylamino-hydroxyphenyl)-azo)-5-
hydroxy-2,7-naphthalenedisulfonic acid) at the dropping mercury electrode in a solu-
tion of potassium hydrogen phthalate (pH 3.74.6) was reported for the determination
of trace amounts of B (1
1
10
09
g/mL detection limit) in foods (113). The amounts
of B obtained by this method reportedly agreed well with those by the ICP-OES
method. The ionometric method has not been very popular; however, some interest
in this technology remains for borophosphosilicate glass (114), rocks and ores (115),
and environmental samples (116).
Atomic Spectrometric Methods
Atomic Emission/Absorption Spectrometry Methods
Atomic emission spectrometry (AES) and atomic absorption spectrometry (AAS)
generally involve introduction of samples into a flame (usually of acetylene–air or
acetyleneN
2
Oair), where elements of the sample are atomized. The AAS measure-
ment is based on the principle that free atoms of an element (e.g., B) in their ground
state absorb photons of discrete energy values (a characteristic wavelength) generate
by a hollow cathode lamp containing that element (e.g., B). The AES methods measure
emission from the atomized and excited species when they fall to ground state. The
AES/AAS determination of B often requires separation and preconcentration of B
from the sample matrix for acceptable results (117). After the separation of B from
ah0b$$1428 06-12-97 21:28:22 mica AP: MCH
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SAH AND BROWN
the sample matrix as volatile methyl borate, atomic emission measurement of BO
0
2
radical at 548 nm (67) or absorbance measurement at 149.7 nm (68) improved the
detection limit and sensitivity. Separation and preconcentration methods for B are
discussed in the earlier sections. Boron determination by the AAS/AES was recently
described by Usenko and Prorok (118) and Zakhariya et al. (119). This method
generally has poor sensitivity (120), serious memory effects of previous samples, and
numerous interferences.
The ET-AAS method can analyze solid or liquid samples without requiring sam-
ple digestion and atomization at high temperatures (117, 121, 122). The ET-AAS
method without the use of chemical modifiers has poor detection limit and sensitivity
due to (a) inefficient thermal dissociation of B-containing species (probably oxides
and carbides) produced by dissociative desorption of B
2
O
3
and (b) severe memory
effects resulting from B atoms apparently undergoing a series of condensation
vaporization steps, which causes a persistent plateau in the tail of AAS signals (85,
122). Use of chemical modifiers (a list has been compiled by Botelho et al., 117),
and the treatments of the pyrolytic graphite tube are often necessary for acceptable
results by ET-AAS. Coating of the graphite tubewith tungsten carbide or lanthanum
carbide increased the optimum pyrolysis temperature of B from 850
7
Cto
ú
2200
7
C
and the addition of a Ca–Mg modifier to the B solutions increased the pyrolysis
temperature to 1200
7
C(122). Higher pyrolysis temperature would be expected to
increase the efficiency of thermal dissociation of B-containing species. A chemical
modifier composed of nickel and zirconium salts and the treatment of the graphite
tube with zirconium solution mitigated the interference of iron for the determination
of B in iron and nickel-based alloys by ET-AAS (8). Barnett et al. (123) described
a method of matrix modification using totally pyrolytic graphite tubes. The use of
two chemical modifiers, (1) diammonium hydrogen phosphate to suppress matrix
interferences and (2) NaOH to retain determinand on the surface of the tungsten
boat furnace vaporizer, suppressed the loss of B during the ETvaporization process,
enhanced detection, and overcame matrix interferences (124). Ideal conditions for
ET-AAS determination in cell-suspension B were 4 s hold time at 2500
7
Cfor
atomization and 249.7 nm wavelength for the detection (120).
Plasma-Source Methods
Introduction of plasmas as ionization sources and the development of plasma-source
analytical instruments (plasma-source-OES and MS) provided higher sensitivity and
lower detection capability for B determination than was possible by spectrophotomet-
ric, flame AES/AAS, and time consuming nuclear methods. There are several types
of plasma (125), namely, the direct current (DCP) (126, 127), the inductively coupled
plasma (ICP) (4, 6, 128), the microwave induced plasma (MIP) (129) and the glow
discharge plasma (GDP) (130). Plasmas have been generated from a number of gases
or their mixtures (125, 131); however, most commercial plasma-source instruments
use an argon ICP (i.e., ICP generated from argon) for ionization. The plasma source
instruments are of two kinds, based on the detection method they employ —(a) plasma-
source optical emission spectrometry (OES) such as ICP-OES and DCP-OES and (b)
plasma-source mass spectrometry (MS) such as ICP-MS, DCP-MS, and microwave
induced plasma (MIP)-MS. OES has also been called atomic emission spectrometry
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BORON DETERMINATION METHODS
TABLE 2
Examples of the Application of ICP-OES Methods for B Determination
Author(s) Material analyzed Method used Remarks
Novozamsky et al. (73) Plant ICP-AES N/A
Spouncer et al. (19) Soil ICP-AES
Evans and Krahenbuhl Plant, animal ICP-OES, ICP-MS Fe for ICP-OES
(32, 48) Azomethine-H
/
other
Hu et al. (133) Plant leaves ICP-OES Fluorinating ETV method
Ferrando et al. (43) Plant, animal ICP-OES N/A
Fucsko et al. (4) Borophosphosilicate glass ICP-OES, ICP-MS
Hu (63) High-purity silica DCP-ES
Hunt and Shuler (175) Biological materials ICP-AES Low-temperature wet
ashing
Coedo et al. (6) Steel ICP-OES
(AES), mainly in the older literature. In order to maintain uniformity, we will use
OES consistently, even when AES was used in the original articles cited in this work.
Generally, samples are converted to liquid and introduced to the plasma of instru-
ments, though, several alternate modes of sample introduction (e.g., slurry, powder,
gases, laser ablation, and electrothermal vaporization (ETV)) are used for specific
purposes, mainly to avoid sample preparation and to reduce interferences (125, 131).
When HF is used for the sample dissolution, its presence can be expected in the
digest. Presence of HF in the digest generally causes problems in B determination
and has damaging effects on the sample introduction and interface regions of the ICP-
MS instrument. For plasma source determinations, HF should be removed from the
sample by evaporation to dryness.
Plasma-source OES. Development of ICP-OES revolutionized the determination of
several so-called ‘‘problem elements’’ such as B, S, Mo, and all hard to detect trace
elements by virtue of its low detection limits, large linear range, and multielement
(several elements in the same run) detection capability. Reported detection limits for
B are 10 to 15
m
g B/L in soil solutions and plant digests by ICP-OES based on a
linear self-scanning photodiode array (51), 0.1 ppm (100
m
g B/L) in as little as 50
mg animal tissues (tumor, blood, liver, skin), or as few as 5
1
10
7
blood cells in cell
suspensions by DCP-OES (132), and 25
m
g B/L in the microwave digest of mice
tissues using Ar-ICP-OES with a Babington nebulizer (44). Some examples of the
applications of the ICP-OES method for B determination in various sample types are
presented in Table 2. Boron determination in plant leaves by slurry introduction in a
fluorinating ETV ICP-OES system resulted in good sensitivity, avoided carbide forma-
tion, and decreased memory effects and interferences (133). The ICP-OES determina-
tion of small biological samples following an in situ conversion of sample-B to gaseous
methoxyborate improved the detection limit by more than tenfold over the conventional
ICP-OES procedures using liquid sample introduction (12). The FI-ICP-OES resulted
in a near unity dispersion ratio, high linearity of the concentration-peak height relation-
ship and increased the sample throughput up to 320 per hour (128).
Interferences in ICP-OES: If the wavelength of the elements of interest is near the
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SAH AND BROWN
wavelength of another element (in the sample) within the search window, then the
peak search routine becomes less reliable, even erroneous. Iron interferes with the
two most sensitive B lines at 249.773 (B1) and 249.678 (B2) in the ICP-OES method
(33, 34, 134). If the sample has high iron concentrations (as encountered in the digests
of soil, metals, geological and some biological materials) then the B 249.773 and B
249.678 lines cannot be used because of the overlap of Fe at 249.782 on at B 249.773
nm and Fe at 249.653 on B at 249.678 nm (64). Boron determination by ICP-OES is
also affected by other interfering species; for example, Si interferences may render
low levels of B determination unreliable (135, 136). The presence of Fe, Ni, Cr, Al,
and V depressed, while Mn, Ti, Mo, and high concentrations of Na enhanced B signals
(33, 34, 137).
Novozamsky et al. (84) found it essential to separate B (as methyl borate) from
the iron alloy digests for reliable B determination by ICP-OES. Din (136) suggested
successive fusion of geological materials with potassium dihydrogen phosphate and
potassium hydroxide to obtain iron-free solution. Alternatively, Kavipurapu et al.
(137) suggested a multiparametric linear regression model to mathematically correct
for Fe interference without separating B from digests of steel. Kato and Takashima
(138) suggested the use of Cu as internal standard to manage interferences for B
determination in sea water.
The presence of HF in a sample increases B transport into the plasma and B emission
signal (4). The presence of HF favors the formation of boron trifluoride, which has
higher transport efficiency as aerosol particles than boric acid. In fluoride-free metha-
nolic solutions, most of B volatilizes in the spray chamber and reaches the flame
mainly as vapor (139, 140). The presence of HF in methanolic solutions deceased the
ICP emission signals by the replacing BOCH
3
bond with BF bonds and the forma-
tion of a less volatile B-fluoro complex (139141).
ICP-MS. The ICP-MS is often the method of choice over ICP-OES and spectropho-
tometric methods for B determination (32). The advantages of ICP-MS over other
methods are higher sensitivity, lower detection limits, and simultaneous measurement
of
10
Bto
11
B isotopic ratio and total B concentration in a sample. The ability of ICP-MS
to measure B-isotope ratios renders this instrument especially suitable for biological B
tracer studies (142). The reported detection limits are at the ppb level, e.g., one ppb
(36) to 3 ppb (48) in biological materials, 0.15 ppb in saline waters (143), and 0.5
ppb in human serum (47). The uniqueness of ICP-MS is also due to its capability to
carry out B determination by the isotope dilution method which is considered the
most precise for quantitative determination.
The external calibration with an internal standard is most widely used method for
plasma-source OES and MS because of its simplicity and labor efficiency. Other
methods, such as standard addition, addition calibration, and isotope dilution methods
are less common and are generally employed to deal with difficult sample matrices
or to improve precision. For the external calibration method, an internal standard as
close as possible to the mass number of the determinand elements should be selected
(57). Beryllium is the closest in mass number to B and therefore it is the most
commonly used internal standard for B determination. Use of Be as an internal standard
was effective in mitigating matrix interferences (32, 48) and corrected the matrix-
induced suppression of
10
B signals (85.6
{
5.2% recovery relative to 0.14% HNO
3
),
ah0b$$1428 06-12-97 21:28:22 mica AP: MCH
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BORON DETERMINATION METHODS
yielding 99.6
{
2.5% relative signal intensities (47). For the microwave digests of
biological materials, the external calibration method using Be as internal standard was
as good as isotope dilution or standard addition methods for B determination (32, 48).
Boron determination is not affected by isobaric interferences or by spectroscopic
interferences from the elements originating from water, acid or plasma gas (144).
However, if samples vary in acidity, use of Be as an internal standard may result in
error in the determination of B/Be ratios by ICP-MS (32).
Mass discrimination in ICP-MS is the resultant effect of unequal transmission of
ions of differing masses through the interface, ion lenses, quadrupole mass filter, and
detector. Measurement of B isotope ratios by ICP-MS may be influenced by mass
discrimination effects arising from instrumental parameters as well as the sample
matrix (145). The apparent B isotope ratio (
11
B/
10
B) for NIST SRM 95 boric acid
(certified value
Å
4.0436) varied from 3.2 to 4.7, depending on the ion lens voltage
setting alone (14). As a light element (atomic mass 10.8) with relatively low ionization
in the argon plasma (approximately 58% at 7500K) B is expected to have serious
nonspectroscopic interferences from species present in the sample solution (14). The
matrix related mass discrimination, in general, results in more severe suppression of
10
B relative to
11
B, thus increasing the
11
B/
10
B ratio (14, 143, 146).
Spectral overlap of the
12
C peak on the
11
B peak may interfere in
11
B/
10
B ratio
determination if the sample solutions or the digest contains high levels of organic
carbon as commonly found in microwave digests of biological materials. Evans and
Krahenbuhl (48) noted a significant interference of
12
C on B in the microwave digests
of biological materials when the measurement was made in the normal resolution
mode (0.8
m
). In the high resolution mode (0.6
m
) the effects depended on the sample
types: the
11
B peak of the hay sample was resolved completely, but those for the
kidney and the flour sample were resolved only 75 and 55%, respectively. An analyst
should always watch for the possibility of error in B isotope ratio measurement due
to anticipated enhancement in
11
B signals by
12
C. Where the isotope ratio measurement
is not required, total B can be computed on the basis of
10
B to avoid the error due to
the overlap of
12
C-peak (47).
A number of approaches have been suggested to deal with the interferences in ICP-
MS determination. The readers are referred to two recent reviews in this area (125,
144). Nebulization of HF-containing solutions using a borosilicate glass sample intro-
duction system results in the high B blank values due to the dissolution of glass by
HF. Boron determination in an HF digest of titanium using, a Galan-type nebulizer
made of an organic polymer and a PTFE sample introduction system to eliminate the
sample-glass contact, resulted in low background and detection limits (57). Sample
purification techniques discussed in the earlier section are also employed to overcome
interferences.
Other Mass Spectrometry
Thermal ionization mass spectrometry (TIMS): TIMS provides the high degree of
accuracy and precision for the determination isotopic composition of an element in a
sample. The TIMS method for B isotopes determination is described in detail by Xiao
et al. (147) and Xiao (148). Bassett (15) reviewed published sample preparation
procedures and analytical methods for B
11
/B
10
ratio measurements in minerals using
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SAH AND BROWN
TIMS. In general, TIMS procedure involves the following steps: (1) sample decompo-
sition, (2) separation of the determinand of interest from the sample or the digest
matrix, (3) ionization of the determinand, and (4) the determination of m/zratios by
a mass spectrometer. The biological samples are ashed to destroy organic matter while
the nonbiological samples are wet digested. Vengosh et al. (149) found no difference
in isotopic ratios whether the sample were analyzed with or without chemical separa-
tion. The detection limit was 0.06 ppm with approximately
{
2% precision. Isotope
ratio (
11
B/
10
B) determination by TIMS may utilize thermal ionization of BO
/
2
or
BO
0
2
salts of Na, Cs, or Rb (62, 150). The TIMS procedure for
10
B/
11
B ratio measure-
ment involving Cs
2
BO
/
2
(mass peaks Cs
210
B
16
O
16
O
/
at m/z
Å
308 and Cs
211
B
16
O
16
O
/
at m/z
Å
309) resulted in better precision than that involving Na
2
BO
/
2
when measured
ratios were corrected for the interference from Cs
210
B
16
O
17
O
/
at m/z
Å
309 and its
oxygen isotope composition (3). Negative TIMS may have a significant advantage
over the positive TIMS methods for B isotope determination; however, this procedure
may be affected by organic materials which (1) interfere with the ionization of
BO
0
2
and (2) increase the occurrence of isobaric interference at mass 42 (75). Draw-
backs of TIMS are the long sample determination time, usually 0.5 to 3.0 h (151),
and laborious sample preparation steps (148).
Spark source mass spectrometry (SSMS) is mainly used to determine concentrations
of trace elements. Lukaszew et al. (152) noted a satisfactory performance of SSMS for
the determination of isotopic composition. Secondary ion mass spectrometry (SIMS)
employs sputtering of the surface atoms from small areas (typically, 250
1
25
m
m)
of the sample by an energetic primary ion beam (153). The secondary ions generated
from the sample are detected in a mass analyzer. The applications of SIMS to BNCT
were reviewed by Moore (11). While SIMS can be used for quantitative B determina-
tions in small samples also, it is particularly suitable for the determination of intracellu-
lar B concentrations in BNCT.
Nuclear Reaction Analytical Methods
A number of nuclear reaction analytical (NRA) methods have been reported for B
measurement. Some of these methods may be of mere academic significance, having
little practical value for routine B determination. All these methods involve the bom-
bardment of B nuclei and the measurement of the reaction product(s). For convenience,
the reported NRA methods are divided into two classes(a) neutron activation analy-
sis (NAA) and (b) other NRA methods.
Neutron Activation Analysis
In general, the sample is bombarded with neutrons, the elements of interest are made
radioactive and quantity of the element is determined by measuring the radioactivity or
radioactive decay products. NAA is a nondestructive method capable of handling solid
samples with multi element detection capability and generally low detection limits.
However, it is not suitable for sample mass or liquid volumes that pose a threat of
radioactive leaks after activation (86). Ward et al. (86) found good agreement between
the value of 18 elements determined by NAA and ICP-MS. The nuclear methods for
B determination were recently reviewed by Pillay and Peisach (154).
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BORON DETERMINATION METHODS
The measurements by the NAA methods require access to a nuclear reactor for the
production of thermal neutrons for bombardment to convert isotope(s) of interest in
a sample to radioisotopes (151). Boron is an exception to this strategy. The activation
of
10
B by an incident beam of thermal neutrons does not make it radioactive but
causes the following neutron-capture reaction (11):
10
B
/
neutron
Å
7
Li
/
a
particles (2.31 MeV)
/
gamma-ray (478 KeV).
This reaction involves only the
10
B isotope, which has approximately 20% abundance
in naturally occurring B. All NAA methods for B determination are based on the
measurement of one or more products (
a
-particles and
g
-photons) of this reaction.
Perhaps the most important method based on the measurement of
a
particles is neutron
activation (NA) MS while that based on the measurement of gama rays is prompt
g
-
ray spectrometry. Reported methods of B determination based on the above nuclear
reaction are listed below.
Methods based on
a
particles. Neutron activation mass spectrometry (NA-MS):
Iyengar et al. (155) and Clarke et al. (156) described an NA-MS method for simultane-
ous determination of lithium and B in biological materials. The sample was placed
in an ultrapure polyethylene ‘‘liner’’ and freeze-dried. The liners containing the freeze-
dried samples were placed in lead containers and evacuated to about 10
05
Pa. The
lead containers were pinched-sealed following evacuation for neutron irradiation. A
static mass spectrometer was used to measure
4
He (from
10
B) and
3
He (from
6
Li)
generated by the NA reaction. The error rate at
¢
1 ppm B concentration was 15%
but at 6 ppb B the error rate increased to 75%. This sensitivity is not adequate for
BNCT and some nutritional and environmental applications.
a
-track etching. This technique, also called neutron capture radiography, is generally
used to determine microscopic distribution of the
10
B isotope in tissues. This technique
was reviewed by Moore (11). The sample containing
10
B is placed in contact with a
detector film and is irradiated with neutrons. Following irradiation, the film is stained,
reversed, and etched with KOH or NaOH. This technique has been used for mapping
the distribution of natural B in histological sections of mouse tissue (157) and in
parenchyma cells of clover leaves (158). The quantification of B is possible using an
image analyzer (159).
Neutron depth profiling (NDP). This method has been used for near surface analysis
of isotopes that undergo neutron-induced positive Q-value charged particle reactions
such as
10
B(n,
a
)
7
Li for B determination where
10
B is the target isotope, n is the
neutron as an irradiation source, and
a
particles and
7
Li are the products of the
reaction. Lamaze et al. (160) used the NDP method to measure B in CVD diamond
surfaces. The samples were irradiated with cold neutrons and resulting particles escap-
ing from the surface of the sample were detected with a silicon surface barrier detector.
Methods based on the measurement of
g
-rays. Prompt
g
spectroscopy is an exten-
sively used method for the measurement of
10
B(154, 161, 162). The
g
-ray emitted
from the disintegrating
10
B nuclei due to the action of the neutron is detected. This
is also a nondestructive method; however, it is not sensitive for the detection of low
B levels (generally,
õ
5
m
gg
01
) in the sample (163). As B concentration in the sample
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SAH AND BROWN
TABLE 3
Other Nuclear Methods
Author Material Method/source Reaction Particle detected
Strijckmans et al. (167) B in titanium Deuteron activation
10
B(
d
,n)
11
C
11
C, half life
Å
20.4 min.
Moncoffre et al. (165) B-implanted Prompt nuclear
11
B(
a
,
a
)
a
with a surface barrier
steel methods (thin layers) silicon
detector
11
B(p,
g
)
g
-ray with a NaI(T1)
detector
McIntyre et al. (166) B in thin
a
activation
10
B(
a
,p)
13
Cpby surface barrier
films detector
11
B(
a
,p)
14
C Same as above
Szegedi et al. (168) Glass Neutron transmission Thermal neutrons
Pillay and Peisach (169) B tablets
3
He activation
g
-ray
Bayulken et al. (170) Boron Neutron radiography
Moore (11) Cells Electron energy loss
spectroscopy
decreased, counting time to achieve desired precision increases logarithmically (164).
For example, counting time necessary to achieve 1% precision was 10 h for a sample
containing one ppm B and 50 h for a sample containing 0.5 ppm B.
Other Nuclear Methods
A number other methods involving non-neutron irradiation sources such as
a
parti-
cles, protons, and deuterons (165167) are reported in the literature and are listed in
Table 3. These methods have not been commercially adopted.
CONCLUSIONS
This paper compiles methods of determining total B concentration and its isotopic
composition. The evolution of B determination methods has generally progressed
with developments in analytical instrumentation. The application of nuclear reaction
methods (mainly prompt-
g
spectrometry) has remained limited to some specialized
fields. Atomic spectrometric methods such as AES and AAS revolutionized the deter-
mination of a large number of elements, but these methods were not very sensitive
for the elements such as B, P, Mo that occur mainly as their oxy-ions. As a result,
spectrophotometric methods remained the methods of choice for most routine applica-
tions until the development of ICP-OES. ICP-OES was also not adequately sensitive
for nutritional and medical research involving animal tissues that are naturally low in
B. Development of plasma-source MS (e.g., ICP-MS) not only has overcome most
of these drawbacks, but also its capability of measuring B isotopes has made possible
(1) B concentration determination by isotope dilution, (2) verification of B concentra-
tion by isotope fingerprinting in routine analysis, and (3) determination of total B
concentration and B isotope ratio in the same run for biological tracer studies. There-
fore, plasma source MS appears to be the method of choice among present-day
technologies.
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BORON DETERMINATION METHODS
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... Organic matter was estimated following the loss on ignition method (Benbi, 2018). Initial available B concentration in soils was determined after hot water extraction with the Carmine method (Sah and Brown, 1997;Pena-Pereira et al., 2020). The main differences observed in these soils were in texture, electrical conductivity, total equivalent calcium carbonate and organic matter. ...
... and TBNa (pH=7.3). The B in the solution, after shaking time, was measured by the colorimetric method Azomethine-H (Sah and Brown, 1997;Pena-Pereira et al., 2020). The spectrophotometer used was T80 (a high-performance double beam spectrophotometer). ...
Article
Full-text available
BACKGROUND AND OBJECTIVES: Boron is a micronutrient of high importance, both for plant development and normal growth. The range between boron deficiency and toxicity is very narrow, which makes boron unique among the essential micronutrients. Boron adsorption is one of the most important factors determining the release and fixation of this micronutrient, though its adsorption has not been widely studied in semiarid Tunisian soils. This study aims to improve knowledge of B adsorption process in calcareous salt-affected soils in semiarid areas. It equally focuses on the type of cation (monovalent and divalent) in function of the soil texture and time of shaking. These three latter factors influence boron adsorption, which also influence the availability for plants. METHODS: A study was carried out on boron adsorption at different shaking time intervals (1, 3, 6 and 9 hours) in two soils of different textures in the absence and presence of different background electrolytes solutions (0.02 N CaCl2, 0.02 N MgCl2 , 0.02 N sodium chloride and 0.02 N potassium chloride. FINDINGS: The soil-A (clay loam) adsorbed more boron than soil-B (sandy loam). Boron adsorption was the highest in Soil-A under the presence of potassium chloride, close to the mean values given when using calcium chloride. In Soil-B, it was found with calcium chloride background electrolyte. Minor boron adsorption was observed in both soils when boric acid solution was used without background electrolytes. Adsorbed boron showed significant differences with the shaking time in all treatments used with background electrolytes solutions, except for boron solution treatment without background electrolyte in both soils. As a comparison of divalent and monovalent cations, boron adsorbed content was higher with the solution containing calcium than in sodium chloride solution, due to the fact that calcium carbonate is an important boron adsorbing surface. CONCLUSION: This study reveals that the best conditions for maximum boron adsorption are defined by calcium chloride background electrolyte in this type of soil in a determined shaking time interval of 3 hours. This causes a low rate of boron assimilated by plants, which leads to the decrease of the crop yield and the agricultural production, and subsequently hurt the Tunisian national economy.
... Water samples were taken from 41 stations in the Northern Basrah Governorate and were collected in April 2021. The measurement of Boron concentration in water samples by the (ICP/OES) method [13] was performed: The (ICP-OES) is an effective instrument to determine how various samples contain metals. Samples will then be injected in radiofrequency induced argon plasma using several nebulation devices or methods for injecting samples. ...
Article
Full-text available
For both vegetation and human beings, boron is not a uniformly scattered, all-embracing essential micronutrient. The reason for this study is to determine Boron concentrations, 10B5, in Iraq in northern Basrah. The measurements were carried out using ICP-OES methods by analysis of the water samples collected in 41 different locations. This study showed that the concentration of 0.26 mg/L (Al –Alwa Al Qurnah) to 1.7 mg/L (Al Huwair Al Sagher). The findings of the study will be given, and they will be compared to other papers. These results might be used to introduce a novel concept additional contribution to the preservation of radioactive contaminantfree water samples required by people if an event of pollution occurs and to implement requirements for water quality for associated organizations. In addition, the survey found that 41 samples of water had more boron than levels detected. This is because of increased surface-water fluidity outside the root level via monsoon rain. There will therefore be a chance of acute boron contamination soon.
... Boron concentrations were determined by a modified spectrophotometric technique using the reagent azomethine-H (Kiss, 1988). The Azomethine-H method is a common technique for boron determination (Sah and Brown, 1997;Gross et al., 2008), as briefly described: 1 ml of sample was set into a polypropylene flask, followed by addition of 2 ml of acetate buffer solution (250 g of sodium acetate, 15 g EDTA, 125 ml acetic acid glacial until a pH of 5.1 was measured in 1 L of deionized water) and 2 ml of azomethine-H solution (0.45 g azomethine-H with 1 g ascorbic acid in 100 ml of deionized water). The samples were then left for 30 min at room temperature for color development. ...
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The La Escalera geothermal system, a southern region of the Sierra Mil Cumbres, Mexico, is considered a potential prospect for geothermal resources. Geochemical exploration techniques such as thermal water chemical analysis, soil gas flux measurements, mineralogy analysis, and geochemical modeling were applied to develop a conceptual model of the geothermal area. Thermal water samples were analyzed to examine their chemical features. The thermal waters are mainly near-neutral bicarbonate type with sodium as the major cation. A high variation of gas fluxes was observed, and the correlation of CO2 diffuse degassing with fault system in the La Escalera geothermal area is related to local failure stresses. The integrated multicomponent geothermometry method was performed to rebuild deep fluid chemical composition and estimate the optimal bottom temperature. Results show a bottom temperature of 120 °C where concentrations of Al, Mg, CO2, and selected minerals for geothermometry modeling are critical variables for successfully constraining the mixing processes. Geochemical modeling was performed to evaluate the water-rock equilibria. The chemical characteristics of the thermal waters indicate interaction with andesitic rocks, and the chemical processes governing the formed hydrothermal mineralogy include both cation exchange and hydrolysis reactions. The methodology of this study can be helpful for other potential geothermal areas with similar conditions.
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This research aims to introduce the easy, inexpensive and fast analytical method for spectrophotometric measurement of boron in water samples. Here, the BF4—Methylene blue (MB) formation was selected and accomplished faster using ultrasonic and, then the dispersive liquid microextraction (DLLME) condition was optimized for the flow-based determination of the boron. Some influential factors such as reaction time (10 min), the concentration of NaF (0.3 M), H2SO4 solution (0.2 M) and Methylene blue (0.5 M), type and volume of organic solvent (300 µL of chloroform), type, and volume of disperser solvent (1 mL Acetone), were studied. Under the optimum conditions, this work showed a limit of detection of 0.01 µg/L and a linearity range of 0.1–1.2 mg/L. Also, the relative standard deviation and the enrichment factor were 4% (N = 7) and 25, respectively. Finally, the boron contents in the samples of reverse osmosis desalination process were measured. The results showed that the optimal BF4—MB formation time could be two times shorter using an ultrasonic bath and flow base analysis could make the determination of the collected sediments of DLLME, easier and faster.
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The separation of boron in nuclear fuels by cloud point extraction (CPE) has been a challenge due to high acidity of digested sample solutions. High acidity hampers the coacervation of micelles. As a result, the cloud point temperature increases and thus could cause the inevitable loss of boron as volatile species. Herein we have proposed a novel CPE-assisted colorimetric method for the quantification of traces of boron (B) in uranium-based fuels. A 1:1 mixture of 2-ethyl hexane-1,3-diol (EHD) and curcumin dispersed in Triton X-114 surfactant was used in the proposed CPE process. We had investigated several compounds to act as micelle surface modifiers. Among them, only bromine water (Br2) was found not only to lower the CPT (from 80 °C to 42 ± 2 °C) but also resulted in the quantitative recovery of boron (≥95%). The CPE of boron from uranium matrix in a 2.0 mol L⁻¹ HCl medium was suitable for direct chemical quality assurance of routine uranium-based fuels. The molar extinction coefficient of the boron-EHD-curcumin complex was found to be 4.75 × 10⁵ L mol⁻¹ cm⁻¹ (λmax at 458 nm) in N,N-dimethyl formamide medium. The linear dynamic range and detection limit of the proposed analytical procedure were calculated to be 10–150 ng mL⁻¹ and 0.8 ng mL⁻¹ respectively. The proposed analytical methodology was validated by analysis of three in-house working reference materials of uranium. Determination of traces of boron in two uranium dioxide and two metallic uranium samples were found to demonstrate the applicability of the method. The relative standard deviation of the proposed method was found to be of 3–5%.
Article
Boron is an important element in nuclear reactor technology due to its high neutron absorption cross section of ¹⁰B isotope. Isotopic composition of B (IC, ¹⁰B/¹¹B atom ratio) determination in finished neutron absorbers is a necessity under chemical quality control (CQC). We report an innovative greener method for rapid and non-destructive approach of isotopic composition determination of B in “as received” boron based ceramic neutron absorbers including boron carbides and hexa-borides by external (in air) Particle Induced Gamma-ray Emission (PIGE) using 3.5 MeV proton beam. It involves irradiation of “as received” powder samples wrapped in a thin Mylar film and measurement of prompt gamma rays at 429, 718 and 2125 keV from ¹⁰B(p,αγ)⁷Be, ¹⁰B(p,p’γ)¹⁰B and ¹¹B(p,p’γ)¹¹B, respectively, using a HPGe detector system. The method was standardized with natural and enriched B4C powders. For validation, the results of isotopic composition obtained from “as received” samples were compared with that obtained from pellet samples using both external and vacuum chamber PIGE methods. IC values obtained for natural to ¹⁰B enriched samples (19.8–67 atom % of ¹⁰B) are very encouraging with 1–2% and 0.3–0.7% uncertainties from single and replicate sample experiments. The method is truly non-destructive as the samples can be returned back as such after the experiment as they are not radioactive. Compared to existing PIGE method for IC of B, the developed method keeps promise for wide applications as it is simple, sensitive and rapid and it does not require vacuum, pellet preparation with a binder, exact mass of the sample and beam current measurement.
Article
The exact and precise determination of the boron concentration in silicon is still a challenge. A systematic investigation dealing with the digestions of 60 silicon samples with HF-HNO3 and subsequent boron determination by ICP-OES revealed that the concentration found could be up to 60% lower than the actual boron concentration depending on the composition of the sample solution. As the original boron–silicon compound that was identified was colloidally precipitated in the presence of an excess of hydrofluoric acid and then partially retained by filtration or by the sample introduction system, systematic lower boron concentrations were determined. In acidic, HF-free digestion solutions, this compound existed in a soluble form parallel to the borate in B(OH)4-. In an excess of hydrofluoric acid, the compound was converted into the colloidal form and, in parallel, B(OH)4- was converted to tetrafluoroborate, BF4-. For the composition of the colloidal compound, a molar ratio of boron to silicon of 1 : 4 could be determined. 11B-ss-NMR analysis revealed a tetrahedral geometry compound with a central boron atom surrounded by four silicon atoms. It is assumed that a soluble form with four –Si(OH)3 groups was present in the hydrofluoric acid-free solutions, while an insoluble form with four –SiF3 groups was present in HF-containing solution.
Article
The use of the boron content and isotopic composition of secondary silicate minerals and siliceous organisms to trace weathering reactions and past ocean pH requires characterizing the fundamental reactions that govern the incorporation and subsequent isotope fractionation of this element in these materials. Toward this goal we have investigated boron adsorption on the surface of amorphous silica (SiO2·0.32H2O) and quantified its isotopic fractionation. Boron adsorption envelopes and corresponding isotope fractionation factors were measured in dilute aqueous solutions (0.01 M) of NaCl and CaCl2. B maximum adsorbed fraction was found to be about 2 times higher in CaCl2 solutions than in NaCl. The modelling of chemical and isotopic data in NaCl solutions allowed to identify the formation of two main B surface species at the SiO2(am)/water interface: a neutral trigonal (B3) inner-sphere complex, >SiOB(OH)2⁰, characterized by a fractionation factor of ∼ -16 ‰ relative to aqueous boric acid, and a negatively charged tetrahedral (B4) inner-sphere complex, >SiOB(OH)3–, with fractionation factors of -5 ‰ with respect to aqueous borate. In CaCl2 solutions the data modelling indicates the presence of B4 inner-sphere complexes with a fractionation factor of -6.5 ‰ relative to aqueous borate, but excludes the presence of trigonal surface species and suggests instead the formation of a Ca-B(OH)4– complex that could partly account for the observed increase of boron adsorption in these solutions. These observations indicate that changes in the surface charge density and interfacial water structure due to different background electrolytes can induce changes in concentration and both chemical and isotopic composition of B adsorbed on silica surfaces. Although this study suggests that the B adsorption reaction on SiO2(am) plays a minor role in the B incorporation and isotopic signature of clay minerals and siliceous cements forming during weathering reactions, the acquired data should allow for an improved knowledge of the biomineralization reactions. The observed B isotopic fractionations between adsorbed and aqueous B species should help determining the relative contribution of various processes, such as cellular transport, biological mediation and silicification reactions, on the amount of B incorporated in siliceous microorganisms and its isotopic signatures. In addition, the presence in NaCl solutions of both trigonal and tetrahedral boron complexes at the silica surface could explain the observed weak pH dependence of the boron isotopic composition of marine diatoms.
Article
In this study, a glassy carbon electrode (GCE) modified with gold nanoparticles (AuNPs) and coated with electropolymerized p-aminothiophenol (p-ATP) was used as a modified electrode (GC/AuNPs/PATP electrode) for the indirect electrochemical determination of boron (B) using boric acid-mannitol complexation. The developed electrode showed good electroactivity for B determination. Firstly the current intensity of mannitol was measured, then B (as H3BO3) was added to the medium and the unreacted mannitol was measured. The decrement in current intensity was recorded against B concentration to build linear calibration curves within the range of 5-50 mg L⁻¹ of B using square wave voltammetry (SWV), and the oxidation peak potential of mannitol was observed at 0.97 V. The limit of detection (LOD) and limit of quantification (LOQ) were 1.43 mg L⁻¹ and 4.77 mg L⁻¹, respectively. Boron (5 mg L⁻¹) was determined with quantitative recovery in the presence of 100-fold concentration (except 5-fold for Fe³⁺) levels of the potentially interferent ions (Cl⁻, SO4²⁻, HCO3⁻, CO3²⁻, NO3²⁻, PO4³⁻, NH4⁺, Cu²⁺, Mg²⁺, Ca²⁺, Na⁺, K⁺, Fe³⁺) in water using the proposed SWV method. Fe³⁺ and Cu²⁺ interferences could be overcome with cation exchange resin removal. Additionally, boron was determined in synthetic and real seawater samples. The proposed SWV method was statistically validated against the spectrophotometric carmine method using seawater, antibacterial hand sanitizer gel and boron/potassium nitrate (BPN) pyrotechnic formulation.
Chapter
A prompt gamma neutron activation analysis (PGNAA) facility has been built at the 5 MW MITR-II Research Reactor to support our ongoing boron neutron capture therapy (NCT) program. This facility is used to determine the concentration of B-10 in NCT relevant samples such as blood and urine. The B-10 concentration is needed to determine the radiation doses that tumor and healthy brain receive during neutron irradiation of a patient (1). Assaying for B-10 by PGNAA has several advantages over conventional chemical methods. It is rapid, accurate, nondestructive (allowing for re-analysis), inexpensive, sensitive (ppm level), generally independent of the chemical or physical matrix of the B-10, and does not require chemical manipulations of the sample.
Article
This work is concerned with a study of the effect of various factors on the determination of tetrafluoroborate ions with the help of an ion selective electrode. The membrane electrode was prepared from a mixture of polyvinylchloride, dibutylphthalate, and the electrode active substance, tetradecylammonium tetrafluoroborate. A AgCl reference electrode and a salt bridge filled with 1 M Na//2SO//4 were used in the execution of the work. The results of the determination of boron in standard samples of rock compositions, mixtures of these samples, and in a control sample are shown.
Article
The quantitative determination of boron in ores is a long process with chemical analysis techniques. As nuclear techniques such as X-ray fluorescence and activation analysis are not applicable for boron, only the neutron radiography technique, using the high neutron absorption cross section of this element, can be applied for quantitative determinations. This paper describes preliminary tests and calibration experiments carried out at a 250 kW TRIGA reactor.
Article
The effectiveness of different acid mixtures for the de-mineralization of total diet samples of different origin and composition by means of a closed microwave-based technique and a pressurized-ashing technique were investigated. The aim was to obtain complete acid solubilization for the subsequent accurate determination of Al, Ca, Cu, Fe, K, Mg, Na, P and Zn by means of inductively coupled plasma atomic emission spectrometry (ICP-AES). The sampling and sample preparation steps for the trace element analysis are described. The special care taken in the sampling design to ensure the representativeness of the sample for the population studied is also mentioned. The regions and population groups were selected from the high mountain valleys, where a high incidence of stomach cancer exists, and a low altitude valley at the foot of the Andes, which is a low stomach cancer incidence region, both located in the state of Tachira, Venezuela. Total diets including breakfast, lunch, supper, snacks and drinks from the whole-day intake of 140 adults were collected. The collection of the total diet was made according to the double-portion technique. The most reliable digestion procedure for the determination of Al, Ca, Cu, Fe, K, Mg, Na, P and Zn in total diet by means of ICP-AES was a microwave-based technique using HNO 3 alone or with the mixture HNO 3+H 2O 2 (2+1). In comparison, a pressurized-ashing technique is faster although lower values for K (13 and 11%) were obtained. Despite low sensitivity for the ICP-AES measurements, the microwave-based digestion using HNO 3+H 2SO 4 (4+1) also gave good agreement for the determination of Al, Ca, Cu, Fe, K, Mg, Na, P and Zn by means of ICP-AES. Because of the simple and rapid procedure, the pressurized-ashing technique using the mixture HNO 3+H 2O 2 (2+1) was selected to perform the digestion of the total diet samples collected. The results of the mean daily intake of the population investigated showed for the high mountain valleys (high stomach cancer incidence area, n=77) as compared with the low altitude valleys (low stomach cancer incidence area, n=33) significantly higher daily intake with respect to Na (2082 versus 1471 mg), K (1190 versus 731 mg), P (640 versus 381 mg) and a significantly lower daily intake with respect to Ca (925 versus 1379 mg) and Cu (2.78 versus 4.66 mg). No differences were obtained with respect to Al, Fe, Mg and Zn.
Article
Extraction of soils by shaking for 1 h at 20" with 0.01 M CaC1, + 0.05 M mannitol was found to be a more convenient soil test for plant-available boron than the standard hot water soluble (HWS) method, and to be as good in predicting the response in boron uptake by plants. Wheat plants were grown in a pot experiment involving 18 different surface soils having a range of HWS-boron concentrations of 0.22-2.52 wg g-I. Two treatments applied were with and without the addition of 1 pg g-' H,BO, to the soils. The relative boron uptake by the plants in response to the treatment was correlated with extractable boron in the untreated soils, as measured by the HWS method and six alternative extractants. Boron determined in the low availability range using a CaC1,-mannitol extractant was significantly related to the HWS boron, although less boron was extracted. At potentially toxic concentrations the mannitol method extracted more boron than the HWS method. Optimization of the mannitol procedure is described, and the advantages of the extractant are discussed. The clear, colourless extract obtained was suitable for analysis by inductively coupled plasma spectrometry and by spectrophotometry.
Article
The hot water extractable boron (HWB) soil‐test procedure does not lend itself to rapid routine analysis. This study was conducted to evaluate boron (B) chelates in extracting soil B in comparison to the HWB soil‐test procedure. The following B chelates were examined as soil B extractants: mannitol, salicylic acid, 2‐hydroxyisobutyric acid, and sorbitol. Soil B chelate extractants were prepared in a buffered solution that contained IN ammonium acetate and 0.1M triethanolamine. For each respective chelate, chelate concentrations were evaluated at 0.05, 0.2, and 0.5M at a soil:extract ratio of 1:4, extract pH of 7.3, and a shake time of 15 minutes for extracting soil B. For only the sorbitol B extractant (0.2M), various soihextract ratios (1:2, 1:4, 1:6, and 1:10), extract pH (5.0, 7.3, and 8.8), and shake times (15, 30, 60, and 120 min) were examined. All soil B determinations were by ICP. Maximum amounts of B were extracted at a chelate concentration of 0.2M for all four chelates evaluated. Maximum amounts of B were extracted at pH 5.0, 1:6 soiL:extract ratio and a shake time of 120 min. Based on laboratory efficiency and instrument detection limits, a chelate concentration of 0.2M, 1:4 soil extract ratio, pH 7.3, with a shake time of 15 minutes was selected for standard operating procedures. Soil extractants were ranked in terms of amount of soil B extracted as follows: HWB>mannitol=sorbitol>salicylic acid>2‐hydroxyisobutyric acid. Quadratic regression equations explained 84 to 88% of the HWB variability when mannitol, salicylic acid, or sorbitol were used as the independent variable. Stepwise regression equations were developed that explained 90 to 92% of the HWB variability when soil pH in addition to chelate extractable B was entered into the equation. Based on chelate expense and ability to extract soil B, sorbitol would be suggested as a replacement for the HWB soil test procedure in determining available soil B.