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Binding Medium, Pigments and Metal Soaps Characterised and Localised in paint Cross-sections

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Katrien Keune at Rijksmuseum Amsterdam
Katrien Keune
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Description Chapter 3 SIMS studies on oil binding media in paint cross-sections
File ID 88547
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SOURCE, OR PART OF THE FOLLOWING SOURCE:
Type Dissertation
Title Binding medium, pigments and metal soaps characterised and localised in paint cross-sections
Author K. Keune
Faculty Faculty of Science
Year 2005
Pages 182
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SIMSS studies on oil binding media in
paintt cross-sections
SecondarySecondary ion mass spectrometry (SIMS), a relatively novel
techniquetechnique in painting research, was applied to identify and
localiselocalise of oil binding medium constituents in paint cross-sections.
ThisThis chapter is divided into three sections: J) the interpretation of
thethe SIMS spectra. 2) the localisation of organic constituents of oil
paint,paint, and 3) the enhancement of the organic ion yields.
InIn the first section, three mass spectromelric techniques.
SIMS,SIMS, direct temperature resolved mass spectrometry (DTMS) and
gasgas chromatography/mass spectrometry (GC/MS), are applied for
thethe analysis a natural and accelerated aged linseed oil paint
reconstruction.reconstruction. The comparative study facilitated the significance
ofof the ions produced by SIMS and demonstrates the advantages
andand limitations of oil medium identification with SIMS.
InIn the second section, the ratio between palmitic and
stearicstearic acid (P/S ratio) indicative for the type of oil used in the
paints,paints, is determined directly from the paint cross-section by
analysisanalysis of the negative ions of palmitic and stearic acids. The P/S
ratiosratios are determined in test samples and in various layers present
inin paint cross-sections from 15
th
- to 19'
h
-century paintings. The
positivepositive ion SIMS mass spectrum gives information on the specia-
tiontion of the fatty acids in free, ester-bound or metal carboxylate
form,form,
which is indicative of the drying stage of the oil.
InIn the third section, the surface of a cross-section is coated
withwith an ultra-thin gold layer, which improves the yields of
secondarysecondary ions from the fatty acids and diacids. A comparative
studystudy of a native and gold-coated surface from an aged linseed oil
paintpaint reconstruction demonstrates the enhancement of the yields
ofof organic ions produced by SIMS in a paint cross-section rele-
vantvant for painting studies.
Of)Of) oil hi
3.11 Secondary ion mass spectrometry characterisation of tradi-
tionall oil paint: comparative studies with DTMS and GC/MS
3.1.11 Introduction
Bindingg medium analysis, on a paint scraping of 10-I00 ug, is traditionally
carriedd out with well established mass spectrometric techniques, such as GC/MS and
i-j j
DTMS..
The disadvantage of analysing a scraping is the loss of the information
concerningg the spatial distribution of the organic constituents in the multi-layered
paintt systems. Secondary ion mass spectrometry (SIMS) offers the opportunity to
overcomee this problem as it is a highly sensitive surface analytical mass spectrometry
technique.. With this technique the oil binding medium can be identified and localised
inn a multi-layered paint system by analysing paint cross-sections. SIMS is a rela-
tivelyy new technique in painting research and it is therefore important to compare its
analyticall information with that from other mass spectrometric techniques such as
GC/MSS and DTMS. Exact attribution of the mass peaks in the SIMS spectra is
complexx as no separation method is coupled to this mass spectrometric technique.
Inn this chapter we focus on the oleaginous-containing binding media. Oil is a
historicallyy important binding medium and much work has been reported on the
compositionn and drying properties.
~
The main components of oil are triacyglyc-
erols,,
which react during drying and aging of the paint film. The polyunsaturated
moietiess in the triacylglycerols of the fresh oil form radicals, which react with oxygen
too form a cross-linked network. As the three dimensional network ages hydrolysis of
thee ester bonds occurs leading to free fatty acids groups, diacids and acid-rich network
oligomers.. These acid groups are immobilised and stabilised by coordination to metals
(suchh as lead), originating from pigment or drier, to form metal carboxylate bonds.
Thee obtained mature aged oil paint therefore consists of polyanionic ionomer
networks. .
GC/MSS or DTMS studies on oil paint result in detailed identification of inl-
andd saturated, oxidised and unsaturated, mono- and di-carboxylic acids in free form,
esterr or metal bound as well as, various oxidation products and metal present. Some
informationn of the inorganic components of the paint film can also be obtained by
DTMS..
A systematic SIMS study on the characterisation of mature oil paint systems
iss not found in the literature. The few oil studies described reported focus mainly on
thee characterisation of the fatty acid constituents of oils. Fatty acids are have been
extensivelyy studied with SIMS in self-assembled monolayers, such as Langmuir-
44 4
Blodgettt films. '" Furthermore, analyses on lipids e.g. phospholipids in biological
sampless provide relevant information on the ionisation processes of fatty acids.
Fattyy acids under SIMS conditions form quasi-molecular ions [M+H]' in the positive
ionn mode and
[M-H]
-
in the negative ion mode. The main fragmentation of di- and
monocarboxylicc acids in the positive ion mode is lost of water [M + H - H
2
0]~ and
20..
-J. 25
losss of formic acid [M + H - HCOOH]
+
in case of a dicarboxylic acid. Recently.
Spooll (2004) has postulates proposed a model or the ionisation and fragmentation
pathwayss of carboxylic acids and corresponding esters."
Inn this chapter, we compare the analytical data obtained from well defined
paintt samples, by three distinct mass spectrometric techniques: SIMS, py-TMAH-
GC/MSS and DTMS. The selected samples are naturally and artificially aged tradi-
tionall lead white oil paint reconstructions. The naturally aged oil paint sample is a
relativee young paint whereas the artificially aged sample approaches a mature oil
paint.. These two aging stage will be characterised with SIMS, which will be evaluated
inn a qualitative manner by comparing with pyrolysis-tetramethylammonium hydroxide
melhylationn gas chromatography/mass spectrometry (py-TMAH-GCVMS)and DTMS
equivalentt spectral data. SIMS spectra of reference materials are used to assist the
detailedd characterisation of the different oil constituents analysed.
3.1.22 Methods and materials
3.1.2.13.1.2.1
Analytical techniques
3.1.2.LI3.1.2.LI SIMS
Thee static SIMS experiments were performed on a Physical Electronics (Eden
Prairie,, MN) TR1FT-1I time-of-flight SIMS (TOF-SIMS). The surface of the sample
wass scanned with a 15 keV primary ion beam from an
I,5
In
+
liquid metal ion gun.
Thee pulsed beam was compressed (bunched) to ~1 ns to obtain a better mass resolu-
tion,, a current of 60 pA and the spot size of- 120 nm. The primary beam was raslered
overr a 150 x 150 uin sample area, divided into 256 x 256 pixels. The measurements in
bothh positive and negative mode were made, each with a total primary ion dose of 1.0
xx I0
12
ions/cm-, well within the static SIMS regime. The surface of the sample was
chargee compensated with electrons pulsed in between the primary ion beam pulses. To
preventt large variations in the extraction field over the large insulation surface area of
thee sample a non-magnetic stainless steel plate with slits (1 mm) was placed in top of
thee sample.
Tinyy paint samples of the paint film ZD and ZDC taken from the Melinex
SIMSSIMS studies
45 5
sheetss (see section 3.1.2.2 for further details on these samples) were attached to a
glasss substrate and analysed. Stearic acid (Sigma-Aldrich Chemie GmbH) was pressed
intoo a tablet using a high power KBr pellet press. Tristearin (Sigma-Aldrich Chemie
GmbH)) was dissolved in a mixture of ethanol (Biosolve) and dichloromethane (Fluka)
(33 : 7) and dropped onto a silicon substrate. After evaporation of the solvents the tris-
tearinn was measured with SIMS. A well homogenised mixture of chalk (Merck) with
1%%
lead stearate (kindly supplied by M. Verhoeve, University Leiden, The
Netherlands)) was pressed into a tablet using a KBr pellet press.
3.3.
1.2.1.2
DTMS
Althoughh the sensitivity of the DTMS method allows analysis of smaller
samples,, approximately 50-100 jig was scraped off the Melinex support and
homogenisedd into ethanol {- 100 jil), in order to ensure statistically more representa-
tivee sampling. Aliquots of 2.5 ul of the sample suspension were deposited on a 0.1
mmm diameter, platinum/rhodium (90 : 10) filament (Drijfhout, The Netherlands) of the
DTMSS probe. DTMS analyses were performed on a JEOL JMS-SX/SX 102A four-
sectorr instrument of B/E-B/E geometry. In the ion source of this instrument, the wire
wass resistively heated by ramping the current at a rate of 0.5 A/min. Using this ramp
thee temperature was linearly increased from ambient to approximately 800 °C in two
minutes.. Desorbed and pyrolysed material was ionised by 16 eV electron ionisation.
Thee mass spectrometer was scanned over a m/z range of 20-1000 using a I s cycle
time. .
DTMSS analysis of the reference materials was performed under the same
conditions,, using \\M ethanol solutions. Similarly to previously described, aliquots of
1-22
JAL
were applied on the platinum/rhodium (90 : 10) filament.
3.1.2.1.33.1.2.1.3 On-line TMAH (tetramethyktmmonium hydroxide) methylaiion Py-TMAH-
GC/MS GC/MS
Thee samples were analysed by Curie point Py-TMAH-GC/MS equipped with a
reagent-ventingg module. A small sample (50-200 |ig) was placed in a GC vial and 50
p:LL of TMAH (2.5% w/v in H
2
0) was added. The vial was capped and placed in the
ultrasonicc bath for 5 minutes until a
Cmc
suspension was formed. About 2-5 faL of the
paintt film suspension was applied to the rotating 610 °C Curie point wire and the
samplee dried in vacuo. The ferromagnetic wire was inserted in a glass liner and placed
inn the pyrolysis unit (temperature of the base of the pyrolysis unit 185 °C). Curie-
pointt pyrolysis was performed with a FOM 5-LX pyrolysis unit. The ferromagnetic
wiree was inductively heated for 9 s in a 1 MHz RF field to its Curie-point temperature
(6100 X). Methylated compounds were Hushed into the pre-column/column set-up
46 6
mountedd in a Carlo-Erba gas chromatograph {series 8565 HRGC MEGA 2) coupled
directlyy to the source of a JEOL SX 102A/1U2, a double sector instrument via an
inhousee build interface, kept at 300 °C. Pre-column: Chrompack VF-lms, length 3 m,
idd 0.32 mm, film thickness 0.10 jum. Analytical column: Chrompack VF-5 ms, length
300 m, internal diameter 0.32 mm, film thickness 0.50 urn.
TMAHH is a basic reagent, known to attack the bonded phase of the column. In
orderr to eliminate most of the unreacted TMAH, a venting module was installed
betweenn the pre-column and the analytical column. This module is open for 10
secondss during pyrolysis to remove the very volatile reagent while concentrating the
samplee on the retaining pre-column. The slightly less polar bonded phase of the
retainingg pre-column was chosen due to its higher resistance to reagent attack. Helium
wass used as carrier gas at a How rate of approximately 2 ml/min. The initial oven
temperaturee was 50 °C, maintained for 2 minutes then increased to 320 °C at a rale of
88 °C/min. Ions were generated by 70 eV electron impact ionisation. The mass spec-
trometerr was scanned from m/z 40-800 with a cycle time of Is. A JEOL MS-
MP9020DD data system was used for data acquisition. The 70 eV electron spectra of
thee eluted compounds were used for structural identification, normally insufficient in
lake/pigmentt identification.
3.1.2.23.1.2.2
Samples
Thee lead white-containing linseed oil paint reconstruction (ZD) was prepared by
Carlylee in 1999 in the course of the MOLART project at FOM-AMOLF using freshly
pressedd linseed oil (linseeds provided by MACOS bv., Swifterbant, The Netherlands)
mixedd with Dutch stack process lead white {loodwit Schoonhoven de Kat, in stock at
MOLART).. The paint was applied on polyester film (Melinex) and kept under
ambientt conditions. A three-year-old sample was taken from these paint films in 2002
andd analysed with SIMS. Part of the ZD paint film was artificially aged for 30 days at
500 °C and 80% RH marked as ZDC.
3.1.33 Results
3.1.3.13.1.3.1 SIMS of linseed oil-containing paint reconstructions
3.1.3.1.13.1.3.1.1 SIMS of reference compounds - positive ions
Thee spectra of reference materials relevant for the oil paint composition, such
ass triacylglycerols (a source of ester-bound fatty acids), free and metal-bound mono-
andd dicarboxylic fatty acids are the basis for the interpretation of the SIMS spectra of
sampless ZD and ZDC. The characteristic peaks in positive and negative ion mode will
bee attributed; small hydrocarbon fragments (below m/z 100) are not specific enough
andd will not be discussed here.
Thee positive SIMS spectrum of tristearin, a triacylglycerol of stearic acid,
showss main fragments at m/z 607 (diacylglycerols of palmitic stearic acids), m/z 341
(monoacylglycerolss of palmitic stearic acid) and m/z 267 (acylium ion of palmitic
stearicc acid) (Fig. 3.1.1a). Uncharacterised fragments of triacylglycerol are observed
att m/z 325 and 395. The quasi-molecular ion of tristearin detected at m/z 891 is minor
(nott shown). The main peaks in the SIMS spectrum of the free monocarboxylic fatty
acid,, stearic acid, are the protonated molecular ion (m/z 285) and its corresponding
acyliumm ion (m/z 267) in similar intensities (Fig. 3.1.1b). The positive ion spectrum of
aa metal-bound monocarboxylic fatty acid, stearic acid lead soap, shows ions at m/z
489-491..
The peaks corresponding to the protonated stearic acid and its acylium ion
aree not detected (Fig. 3.1.1c). Other ions delected in the analysis of lead soaps are of
leadd (m/z 206-208. 412-416), lead oxides (m/z 428-432; Pb
+
2
0) and lead hydroxides
(m/z(m/z 223-225. 445-449; PbOH", Pb
2
0
2
H~). The peak at m/z 323 is calcium stearic
acidd soap; the calcium is derived from the chalk matrix, particular to this reference
sample.. The spectrum of free nonanodioic acid (azelaic acid), shows a peak at m/z
171..
the acylium ion, which is which is dominant over the peak more intense than the
quasi-molecularr ion at m/z 189, the protonated molecular ion (not shown). The posi-
tivee ion spectrum of a metal-bound dicarboxylic fatty acid, lead azelate. shows ions at
m/zm/z 393-395, and ions attributed to the lead-bound acylium ion at m/z 375-377 in
equall intensities (not shown). Neither the protonated azelaic acid (m/z 189) nor its
acyliumm ion (m/z 171) is detected. Dominant peaks in the spectrum are lead (m/z 206-
208..
412-416). lead oxides (m/z 428-432; Pb'
:
0) and lead hydroxides (m/z 223-225.
445-449;; pbOH . Pb
2
0
2
H
+
).
J..
1.3.1.2
SIMS description of sample ZD - positive ions
Thee positive ion SIMS spectra oi' the top surface side of the lead white-
containingg linseed oil paint film sample ZD shows characteristic fragment peaks of
48 8
607 7
2500 300 :150 400 450 500 550 600
m/z m/z
FigFig 3.1.1 The SIMS spectra (mass range m/z 235 - 615) in positive mode oj tristearin (A), stearic acid
(B).(B).
stearic acid lead soap fC). lead white-containing oil paint sample ZD natural aged (top side of
thethe film) (D) and lead while-containing oil paint sample ZDC artificially aged (top side of the film)
(E). (E).
49 9
off the film, as the Melinex is an impermeable support. Light, oxygen and humidity
cann only be introduced into the paint film via the top side.
3.1.3.1.43.1.3.1.4 SIMS of reference compounds - negative ions
Thee negative SIMS spectrum of tristearin shows a main peak at m/z 283 repre-
sentativee attributed to deprotonated stearic acid (Fig. 3.1.2a). Aliphatic chain fragment
ionss of fatty acids with a mass increment of 14 amu (m/z 71, 85, 99, 113, 127, 141,
155,,
169, 183, 197, 211, 225, 239 and 253) are detected in lower yields. These nega-
tivelyy charged fragments are of the type CoH^CF^^COO. The negative SIMS spec-
trumm of free and metal-bound stearic acid contain a significant peak of deprotonated
stearicc acid at m/z 283 (not shown). In both spectra, aliphatic chain fragment ions of
stearicc acids are detected in lower yields (not shown). From these results it can be
concludedd that in the negative SIMS mode free, ester- or metal-bound fatty acids all
givee origin to deprotonated fatty acid peaks. However, the deprotonated ion yields for
thee free fatly acid is higher than those for the ester- (in the ratio 1:2) and metal-bound
fattyy acids (in the ratio 1 : 1.5)/
Thee intensity of the protonated fatty acids, their acylium ions and the ions of
thee metal carboxylate fatty acid salt in the positive ion mode provides an indication of
thee relative amount of free, ester- and metal-bound fatty acid in an oil paint." The
SIMSS spectra of free and lead-bound azelaic acid reveal a deprotonated molecular ion
peakk at m/z 187 as main peak. Aliphatic chain fragment ions of fatty acids are
observedd at m/z 71, 85, 99, 113, 127, 141 and 169 (not shown).
3.1.3.1.53.1.3.1.5 SIMS description of sample ZD - negative ions
Thee negative SIMS spectrum of the top side of the paint film of sample ZD
showss two dominant negative ions peaks at m/z 255 and 283 from deprotonated
palmiticc and stearic acid, respectively {Fig. 3.1.2.b). The reference materials indicated
thatt these ions are not informative about the origin of the fatty acid moiety (free,
ester-- or metal-bound). The detected aliphatic chain fragment ions of fatty acids (m/z
71,85,99.. 113, 127, 141, 155, 169, 183, 197, 211, 225, 239 and 253) are marked with
ann asterisk in Fig 3.1.2b. The peaks detected at m/z 143, 157, 171 are attributed to
deprotonatedd short chain monocarboxylic fatty acids, octanoic, nonanoic and decanoic
acidd and the peak at m/z 187 for the dicarboxylic acid, a/elaic acid (Fig. 3.1.2b).
Thesee short chain fatty acids and dicarboxylic acids are known oxidation and degrada-
tionn products of aged oil paint. Other negative ions observed, characteristic of oil
paint,, are m/z 311 and 339 corresponding corresponding to monoacylglycerol esters of
palmiticc and stearic acids.
Thee negative SIMS spectrum of the bottom side of the ZD sample is identical
51 1
25000 -
1999 113
600000
;
** *
20000--
1400000 -
283 3
10x x
127 7
265 5
155 5
169 9
1833 197 211
WVA^WAJU»—>JAI I
L.LuL'x* *
283 3
ULmiLl) )
143*,157 7
171 1
187 7
255 5
255 5
5x x
208 8
^ML^JVAJUIJL...^^
:i
283 3
A A
B B
C C
D D
" "
150 0
250 0
m/z m/z
FigFig 3.1.2 The SIMS spectra (muss range m/z 98 - 289) in negative mo Je oftristearin (A), lead white-
containingcontaining oil paint sample ZD natural aged (lop side of the film) (B) and lead white-containing oil
paintpaint sample IDC artificially age: top side (C) and bottom side of'the film (D). The asterisks in B-C
areare indicative for the aliphatic chain fragment ions of fatty acids. The counts of mass peaks before
thethe dotted line indicates that the counts of the mass peaks before the line have to be multiplied by 4. 5
oror 10 times (value indicated in the figure I.
52 2
too the spectrum of the top side (not shown).
3.1.3.3.1.3. J.6 SIMS description of sample ZDC - negative ions
Thee deprotonated palmitic and stearic acid at m/z 255 and 283, respectively, are
dominantt in the spectrum of the top side of the paint film of sample ZDC (Fig.
3.1.2c),, similarly to what was observed for the sample ZD. The characteristic frag-
mentss of compounds for the first stage of drying are either present in low yields (in
thee case of the short chain fatty acids m/z 143, 157 and 171) or not detected (diacids
m/zm/z 187) in the top surface of the artificially aged sample ZDC (Fig. 3.1.2c). As
observedd in the positive ion mode, the composition of the top side and bottom side of
thee paint film ZDC is different. In the spectrum of the bottom (Fig. 3.1.2d) the small
chainn fatty acids, as well as the diacids are present.
Summarising,, on the top side of ZDC paint film, which is exposed to light,
oxygenn and moisture, the more volatile degradation and oxidation products are not
detected.. However, these products are still abundant in the environmentally shielded
bottomm side (Fig. 3.1.2d).
3.1.3.23.1.3.2
DTMS of linseed oil-containing paint reconstructions
3.1.3.2.13.1.3.2.1 DTMS of reference compounds
Detailedd interpretation of the different components found in oil paint can be
foundd elsewhere however it is relevant to describe here the most significant features
inn the DTMS spectra of linseed oil-containing paint reconstructions, by comparison to
thee analysis of reference materials.
Duringg the DTMS analytical process, the non-bonded or free components of
thee oil (resulting from the hydrolysis process and prior to being metal coordinated) are
volatilee and desorb early from the paint particles coated on the pyrolysis wire at lower
temperature.. These free fatty acids ionise readily and the main peak detected is the
molecularr ion. As an example the case of stearic acid is described here where the base
peakk is m/z 284. Minor peaks include the chain fragments m/z 241, 185, 129, 73 and a
Y-HH rearrangement ion at m/z 60. Free diacids also desorb at low temperatures the
earlyy scans and the main electron ionisation generated peak is [M -
2^0]*"
(e.g. in
thee case of azelaic acid m/z 152). Other peaks include m/z 84, 98, 111 and 124. The
mainn peak of the metal-coordinated monocarboxylic fatty acids is the acyiium ion,
however,, the molecular ions can also be detected. For example in the case of lead
stearatee m/z 267 and 489-491 are the main peaks. Metal-coordinated diacids show a
differentt behaviour. The example of lead azelate is studied here: lead ion isotopes (m/z
206-208)) are the main peaks. Other important peaks detected are m/z 348-350
resultingg from decarboxvlation, as well as the molecular ions at m/z 392-394. Minor
53 3
fragmentt peaks include fatly acid chain fragments such as m/z 73, 87, 101 and a y-H
rearrangementt ion at m/z 60.
Withh respect to the ester-bound fraction, the analysis of pure triacylgly-cerols
showss that the main ions resulting from their desorption/ionisation of these contain
eitherr one or two fatty acid moieties. It is expected that in the case of paint samples
wheree some of the original fatty acid moieties of the triacylglycerols have reacted and
aree incorporated into an oil network, the main peaks would be the acylium ions and to
aa lesser extent the mono- and diacylglycerol ions. In practice, elimination reactions
releasingg fatty acids with M*~ are more prominent than formation of acylium ions.
Thee inorganic components of the network or pigment particles can also be
detectedd in some cases. Not all metals will go through the reduction, pyrolysis and
ionisationn steps required for their detection. For this reason not all inorganic elements
aree detectable by DTMS. In the cases studied here lead is the only metal present and
iss detected in the higher scan number with its isotope pattern {m/z 206, 207, 208).
Theree is another important fraction in the paint system for which it is difficult
too find adequate reference materials and therefore the interpretation of the different
peakss detected is complex. This is the highly cross-linked network fraction of the
paintt sample. Due to the pyrolysis of this fraction it is detected as an unresolved peak
patternn with mainly aromatic components {e.g. m/z 105, 91).
Onn the basis of this information Table 3.1.1 was assembled and will be used for
interpretationn of changes in the chemistry of the lead white-containing oil paint recon-
structionss studied.
3.1.3.2.23.1.3.2.2 DTMS description of sample ZD.
Thee TIC of the DTMS analysis of ZD lead white sample (Fig. 3.1.3) shows
thatt there are four separate events in the analysis.
Eventt A (Fig. 3.1.3a), Scans 1 - 54. The spectrum is dominated by the molec-
ularr ion of palmitic and stearic acids. The monocarboxylic acid molecular ion corre-
spondss to the desorption of the free form {m/z 256 and 284 for palmitic and stearic
acidss respectively). The desorption profile of palmitic acid {m/z 256 in Fig. 3.1.4)
howeverr suggests that the monocarboxylic fatty acids are not present in the free form,
butt also in a bound form. Although m/z 256 is not present in the spectrum of pure
tripalmitinn or lead palmitate, this ion can be formed by an elimination reaction of the
boundd fatty acid when in a complex organic matrix. The acylium ions of palmitic and
stearicc moieties {m/z 239, 267 respectively), the corresponding monoacylglycerols
{m/z{m/z 313, 341) and diacylglycerols (m/z 550, 578, 606) suggest that the hydrolysis of
thee ester bonds is not yet complete in the ZD sample. This is in agreement with the
SIMSS data.
54 4
Eventt B (Fig. 3.1.3b), Scan 55 - 67. A small amount of lead can be detected in
thiss event suggesting the presence of organic lead (lead carboxylates for example).
Fattyy acid fragments present include m/z 60. 73, 129. 171. 185. These fragments {e.g.
m/zm/z 129 in Fig. 3.1.4) are detected in different events suggesting that the fatty acids
aree present in different forms. In addition, the acylium ions of the saturated monocar-
boxylicc fatty acids (e.g. m/z 239 in Fig. 3.1.4) desorb in two distinct maxima (scan 49.
eventt A and scan 62 event B) corresponding to the ester- and metal-bound fractions
FigFig 3.1.3 TIC and partial mass spectra of the ZD sample. (A) Scan
1
' I - 5-/. tBl scans 55-67. (C) scans
6H-H0,6H-H0, (D) Scans
<S7-
/ 30.
55 5