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A rapid method for determination of ethanol in alcoholic beverages using capillary gas chromatography

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A simple and rapid method was developed to determine ethanol content in alcoholic beverages using megapore polar column (CP-Wax 58 CB, 30 m ¥ 0.53 mm) with direct injection gas chromatography. Contrary to packed GLC method, distillation and/or stepwise dilution of samples were not necessary by the method developed in this study. Ethanol in sample was injected directly into GC for analysis, after adding suitable amount of internal standard, acetonitrile solution. Using this method, less than 8 min was required to obtain result since sample preparation, and the limit of quantitation (LOQ) was about 0.5 µg/mL. Recovery studies were performed using 0.5 mL of red wine and whisky. Each was spiked with ethanol at 50 and 100 mg, respectively. The recoveries were found in the range of 99~104% and 99~101%, respectively. The coefficients of variation were less than 3.4%. Comparisons of the AOAC method (AOAC 969.12 and 920.57) with current method showed no significant difference. These results suggested that precision of direct injection GC method was higher than that of AOAC methods. Several commercial alcohol beverages, including distilled and non-distilled spirits, were analyzed by the current method. The ethanol content of distilled and non-distilled spirit were found as: 165.2 ± 4.9~415.7 ± 17.6 and 28.2 ± 0.8~141.2 ± 4.9 mg/mL, respectively. Using this GC method, we could change the concentration in gravi-metric percentage (%, w/v) to volumetric percentage (%, v/v) by the equation (%, w/v) = 0.814 (%, v/v) with linear coefficient R 2 , higher than 0.999.
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Journal of Food and Drug Analysis, Vol. 11, No. 2, 2003, Pages
133
Journal of Food and Drug Analysis, Vol. 11, No. 2, 2003, Pages 133-140
A Rapid Method for Determination of Ethanol in Alcoholic
Beverages Using Capillary Gas Chromatography
MEI-LING WANG1, YOUK-MENG CHOONG2*, NAN-WEI SU3AND MIN-HSIUNG LEE3
1. Department of Food Health, Chia-Nan University of Pharmacy and Science. 60, Al-jen Road. Sect. 1,
Pau-an, Jen-teh Hsiang, Tainan Hsien 717, Taiwan, R.O.C.
2. Department of Food Science and Technology, Tajen Institute of Technology, 20, Wei-shinn Rd., Yan-puu hsiang,
Pintung Hsien 907, Taiwan, R.O.C.
3. Department of Agricultural Chemistry, National Taiwan University, 1,sec. 4, Roosevelt Road., Taipei 106, Taiwan, R.O.C .
(Received: May 16, 2002; Accepted: September 30, 2002)
ABSTRACT
A simple and rapid method was developed to determine ethanol content in alcoholic beverages using megapore polar column (CP-
Wax 58 CB, 30 m ¥0.53 mm) with direct injection gas chromatography. Contrary to packed GLC method, distillation and/or stepwise
dilution of samples were not necessary by the method developed in this study. Ethanol in sample was injected directly into GC for
analysis, after adding suitable amount of internal standard, acetonitrile solution. Using this method, less than 8 min was required to
obtain result since sample preparation, and the limit of quantitation (LOQ) was about 0.5 µg/mL. Recovery studies were performed
using 0.5 mL of red wine and whisky. Each was spiked with ethanol at 50 and 100 mg, respectively. The recoveries were found in the
range of 99~104% and 99~101%, respectively. The coefficients of variation were less than 3.4%. Comparisons of the AOAC method
(AOAC 969.12 and 920.57) with current method showed no significant difference. These results suggested that precision of direct
injection GC method was higher than that of AOAC methods. Several commercial alcohol beverages, including distilled and non-
distilled spirits, were analyzed by the current method. The ethanol content of distilled and non-distilled spirit were found as: 165.2 ±
4.9~415.7 ± 17.6 and 28.2 ± 0.8~141.2 ± 4.9 mg/mL, respectively. Using this GC method, we could change the concentration in gravi-
metric percentage (%, w/v) to volumetric percentage (%, v/v) by the equation (%, w/v) = 0.814 (%, v/v) with linear coefficient R2,
higher than 0.999.
Key words: alcoholic beverages, ethanol, direct injection gas chromatography, quantitative analyses.
INTRODUCTION
Ethanol content is very important for the mouth-feel
and flavor of alcoholic beverages. Ethanol contents of
wine, liqueur, and beer range from 7~21%(v/v),
20~50%(v/v), and 3~6%(v/v), respectively(1). In general,
ethanol contents serve as the quality index and taxation
factor for alcoholic beverages(2). After entering WTO,
alcoholic beverages in Taiwan are taxed according to the
ethanol contents, like the taxation system in United
States(3). The higher the ethanol content in an alcoholic
beverage, the higher the tax. A simple, accurate, and quan-
titative analysis method for the determination of different
levels (0~50%, v/v) of ethanol content is needed as the
standard method of quality control for beverage manufac-
turers and as a guide for government-related sanitary
agencies.
Currently, ethanol determination in alcoholic
beverages can be performed in several ways: (1) boiling
point depression of the ethanol solution relative to water,(1)
(2) densimetric analysis(4), (3) refractive index method(1,4),
(4) oxidation of the distillate(1,4-5), (5) dichromate oxidation
spectrophotometry(1,4,6), (6) enzymatic method(7-9), (7)
biosensor(10), (8) potentiometry(11), (9) gas chromatography
(GC)(1,4,12-21), (10) capillary electrophoresis(2), (11) high
performance liquid chromatography (HPLC)(22-23), (12)
modular Raman spectrometry(24), (13) near-infrared (NIR)
spectroscopy(25), (14) beer analyzer(4), and (15) flow
injection analysis(26-27). Due to complicated pretreatment
procedures (e.g. sample distillation and accurate weighing
process) and large sample volume required, the two most
popular methods, picnometry and densimetric analysis, are
not applicable for samples with small amount. As for
oxidation of the distillate and dichromate oxidation spec-
trophotometry, more than 5 mL of sample volume is
required for analysis. Besides, the reagents used are highly
toxic and classified. Low stability, low reproducibility and
low accuracy are the disadvantages for enzymatic method,
biosensor, and potentiometry(9). Raman spectrometry and
capillary electrophoresis, are not popular due to the
expensive instruments required. Since sample distillation
and accurate weighing process are still required as the pre-
treatment procedures, and only spectrophotometry is used
for analysis, HPLC method obtains a comparatively low
sensitivity(22). Recently developed NIR spectroscopy and
beer analyzer are time consuming in establishing calibration
curves and have low accuracy as they can be interfered by
* Author for correspondence. Tel:886-8-7624002 ext 352;
Fax:886-8-7621972; E-mail: ymchoong@ccsun.tajen.edu.tw
other alcohols in alcoholic beverages. In conclusion, GC
method is the most appropriate and rapid method for deter-
mination of ethanol contents in alcoholic beverages with
complicated alcohol contents and small sample amount.
Bouthilet et al. had developed a packed GC method to
analyze ethanol contents in alcoholic beverages(14).
However, the method required at least 100 mL of sample
and distillation as the pretreatment process. The packed
GLC(4,12-13,15-16) and capillary GC(17-21), which required
only small sample amount, were then developed. The GC
methods currently available for determining ethanol
contents in alcoholic beverages are US official methods,
AOAC 968.09, 984.14, and 986.12(4), and Taiwan official
method, CNS N6181(15). These official GC methods adapt
different sample preparation procedures when dealing with
different sort of samples, and these samples require distilla-
tion or dilution as the pretreatment process. Since packed
column is used, lower resolution and interference by other
alcohols in alcoholic beverages still exist(12,21). Besides,
durability of packed column and reproducibility of the
retention time are relatively low(12). When capillary GC is
used in determining ethanol contents in alcoholic
beverages, time-consuming pretreatment procedures, e.g.
solid-phase extraction(17), head space balancing(18), solvent
extraction(19) and distillation(20-21), are still required. The
capillary GC method is one of the most important modern
analytical techniques because of its advantage of high reso-
lution and sensitivity. Owing to years of research using
GC, we found that insertion of a ball of glass wool into a
liner of GC injector could effectively prevent the non-
volatile compounds form getting into the analytical column
and thus, moderate the interference from the contaminants.
We find that commercially available megapore capillary GC
column is superior in water resistance. Even when an
aqueous solution sample is injected directly into the
column, the resolution and reproducibility of retention time
are almost the same as a new column(28-30). In this study,
we will prepare and inject the alcoholic beverages with
adequate amount of internal standard solution directly into
a GC analyzer without any pretreatment process,. Together
with the application of appropriate column and GC condi-
tions, we plan to develop a simple, rapid and accurate
method in determining ethanol contents in alcoholic
beverages. Meanwhile, the AOAC methods will be
compared and assessed in recovery and standard deviations
of intraday/ interday analysis in order to evaluate the
accuracy and precision of the study method.
MATERIALS AND METHODS
I. Materials
Twenty-six alcoholic beverages, including 12 distilled
spirits (whisky, brandy, kaoliang wine, rice wine, Mizhiu
Tou, and medicine wine) and 14 non-distilled spirits (millet
wine, grape wine, fruit wine, Shaohsing wine, and beer),
were purchased from supermarkets in Tainan, Taiwan. LC
grade (purity > 99.5%) ethanol, acetonitrile, 1-propanol,
isopropanol, acetone, 1-butanol and t-butanol were obtained
from ALPS (Taiwan).
II. Methods
(I) Preparation of standard solution and internal standard
solution
Ten grams of ethanol and acetonitrile were dispensed
into 1000 mL volumetric bottle and then distilled water was
added to the 1000-mL mark. This was the 1% (w/v)
ethanol standard solution or internal (acetonitrile) standard
solution.
(II) Relative response factor (RRF) of ethanol to acetoni-
trile
Ethanol, 1% (w/v), was mixed with 1% (w/v) acetoni-
trile in various ratios (ethanol:acetonitrile = 15:1, 10:1, 5:1,
2:1, 1:1, 1:2, 1:5, 1:10, and 1:15). A linear regression line
was generated with the GC peak area-under-curve (AUC)
ratio of ethanol to acetonitrile (Y-axis) against the concen-
tration ratio of ethanol to acetonitrile (X-axis). Relative
response factor (RRF) is the slope of the regression line, as
in the Equation 1: RRF = (AS/WS) ÷ (AIS/WIS); in which,
AS= ethanol AUC, AIS = acetonitrile AUC, WS= ethanol
weight (mg), WIS = acetonitrile weight (mg).
(III) Quantitative ethanol determination
(1) Direct injected GC method
Beverage sample solution (0.5 mL) was dispensed into
an l-mL caped sample vial, and then 5 mL of 1% internal
standard solution (equivalent to 50 mg) was added. After
mixing, 0.1 µL of the sample solution was injected directly
into a GC with syringe. Ethanol content was calculated
according to the Equation 2: Ethanol (mg/mL) = (AS/AIS) ¥
(WIS/RRF) ¥1/V; in which, V = sample volume (mL).
(2) Dichromate oxidation method(1, 4)
Beverage sample solution (1~5 mL) was steam distil-
lated to obtain alcoholic eluate (> 50 mL), and then
oxidized with acidified dichromate. The excessive
potassium dichromate was then titrated with ferric oxide.
The ethanol content in beverage sample could be obtained
by calculating the volume difference of potassium dichro-
mate consumption between sample solution and control
solution.
(3) Distillation-hydrometric method(1, 4)
Alcoholic volatile compounds in beverage samples
were separated by distillation, and the gravity of the distil-
late was measured by hydrometer. The ethanol content was
Journal of Food and Drug Analysis, Vol. 11, No. 2, 2003
134
then converted.
(IV) The limit of quantitation (LOQ) of ethanol by GC-FID
Ethanol standard solution (10 mg/mL) was initially
diluted with distilled water to make concentrations of 50,
25, 10, 5, 2, ,1, 0.5, 0.1, and 0.05 µg/mL, and then the
internal standard solution was added to each solution.
Solution was injected directly into a GC, which was
equipped with an FID detector. The FID signal was set at
range = 1, and attenuation = 1. The coefficient of variation
(CV, %) of ethanol recovery was set at 15%. This was the
LOQ of ethanol(28, 31). Each analysis was carried out in
triplicate.
(V) Recovery
5 and 10 mL of 1% (w/v) ethanol standard solution
(equivalent to 50 mg, and 100 mg, respectively) was added
into 0.5 mL of red wine and whisky (in 20-mL vial),
respectively; and then 5 mL of 1% (w/v) internal standard
solution was added. After gentle mixing, 0.1 µL of sample
solution was injected into a GC with syringe for the deter-
mination of the ethanol content. Each analysis was carried
out in triplicate. Blank sample was analyzed at the same
time.
(VI) Validation of the analysis method
Ethanol standard solution (500 mg/mL) was diluted
with distilled water to make concentrations of in 500, 250,
100, 50, 20, and 10 mg/mL. Each solution (0.5 mL) was
dispensed into a 20-mL caped vial. After gentle mixing
with 5 mL of 1% (w/v) internal standard solution (equiva-
lent to 50 mg), 0.1 µL of the mixture solution was injected
into a GC with syringe. Each concentration of ethanol
standard solution was measured in triplicate in 1 day
(intraday) or in 3 successive days (interday). Standard
deviation (SD) and coefficient of variation (CV, %) were
measured to evaluate precision of the method. The relative
error of the mean (REM) was measured to evaluate
accuracy of the method. REM was calculated according to
the equation: REM (%) = [(measured value - true value) ÷
(true value) ¥100%].
(VII) GC conditions
This study used Trace GC 2000 (TermoQuest, Milan,
Italy), which was equipped with computer integrator
software (Chrom-Card version 1.06 for Trace GC,
TermoQuest, Milan, Italy) and an FID detector. The flow
rates of H2and air were set at 30 and 300 mL/min, respec-
tively. The temperature of the FID detector and the
injection port was set at 285ûC, and 225ûC, respectively.
Nitrogen (N2) in the flow rate of 2 mL/min was used as the
carrier gas. The CP-Wax 58 CB separation column (30 m ¥
0.53 mm, Chrompack, Netherlands) was used. Oven tem-
Journal of Food and Drug Analysis, Vol. 11, No. 2, 2003
135
perature was set initially at 45ûC for 2 min, and then
increased to the final temperature of 245ûC in 1 min at the
rate of 45ûC/min. Injection volume was limited to 0.1 µL.
Splittless injection mode was selected.
RESULTS AND DISCUSSION
I. Analysis of GC Conditions
During the selection of GC column, we had compared
the polar and mid-polar megapore capillary columns in the
determination of ethanol contents in alcoholic beverages.
Because there was no sample pretreatment procedures
required, and the sample could be injected directly into a
GC for analysis, polar CP-Wax 58 CB column was the most
appropriate column for the determination of ethanol
contents in alcoholic beverages. When applying the GC
conditions described in Materials and Methods, the
retention time of ethanol standard solution was 2.73 min
(Figure 1A, 1B, and 1C). The resolution of megapore
capillary column (Rs = 5.8) was better than the packed
GLC method (Rs = 1.4~1.9), as described in AOAC(4) and
CNS(15). The GC peaks of the other minute components in
alcoholic beverages could be separated effectively with
megapore capillary GC (data not shown).
In the selection of internal standard, distilled spirits
(e.g. whisky) and non-distilled spirits (e.g. red wine) were
47.4
37.6
27.81
(mVolt)
18.01
8.22
-1.580.0 1.613
(A) 2.75
1
3.35
2
3.226 4.839 6.452 8.065(min)
408.2
322.2
236.2
(mVolt)
150.2
64.1
-21.90.0 1.613
(B)
2.76
13.35
3.87 4.56 5.6825 5.69 7.77.7239
2
3.226 4.839 6.452 8.065(min)
575.2
455.1
335.0
(mVolt)
214.9
94.8
-25.30.0 1.613
(C)
2.73
13.31
3.463.83 4.54 5.56 5.886 6.26 6.74 7.7
6.94
2
3.226 4.839 6.452 8.065(min)
Figure 1. Gas chromatograms of (A) ethanol and acetonitrile
authentic compound; (B) ethanol in non-distilled spirit (red wine) and
(C) ethanol in distilled spirit (whisky) by splittless injection method.
Peaks: 1 = ethanol, 2 = acetonitrile (IS).
added with minute quantity of 1-propanol, 2-propanol, ace-
tonitrile, acetone, 1-butanol, and t-butanol. The ethanol
contents were then analyzed according to the above-
mentioned method. The results had shown that retention
times of the 6 standard solutions were 4.43, 4.37, 3.32,
4.06, 5.96, and 5.72 min, respectively (data not showed).
When GC chromatograms of distilled spirits were
compared with non-distilled spirits, no overlapping of ace-
tonitrile GC peaks was observed. Meanwhile, the GC
peaks of acetonitrile and ethanol were closer to each other
than the other compounds. Acetonitrile was then selected
as the internal standard. Due to the possibility of its
existence in some sort of alcoholic beverages(1,12), 1-
propanol, acetone, t-butanol, or 1-butanol, which acted as
the internal standards GLC methods (AOAC(4), CNS(15),
and other researches(12, 16)), are not appropriate as the
internal standard for the determination of ethanol contents
in alcoholic beverages. Acetonitrile is the most appropriate
internal standard for the determination of ethanol contents
in alcoholic beverages, because no acetonitrile could
possibly exist in alcoholic beverages.
For selection of the GC conditions, we initially
selected low temperature, 45ûC for 2 min, and then
increased rapidly (at the rate of 45ûC/min) to 245ûC in 6
min. Ethanol and acetonitrile were eluted at 80ûC and
100ûC, respectively. Through this process, sample compo-
nents will be eluted very rapidly and it takes 7-8 min to
complete a sample analysis.
II. Relative Response Factor of Ethanol to Iinternal
Standard
In this study, we selected acetonitrile as the internal
standard for quantitative determination of ethanol contents
in alcoholic beverages. In order to obtain an accurate quan-
tification, the RRF value of ethanol against acetonitrile
needed to be specified, then the ethanol contents could be
calculated according to Equation 2. When the AUC ratio of
ethanol to acetonitrile (Y-axis) was plotted against the con-
centration ratio of ethanol to acetonitrile (X-axis), a linear
regression equation (Y = 0.952 X) was generated (R2
0.999). The slope of the linear regression line, 0.952, is the
RRF of ethanol to the internal standard. The linearity of
the adjusted regression line is in the range of 0-500 mg/mL.
III. The Effect of Internal Standard Volume to the Accuracy
of Ethanol Content Quantification for Various Alcoholic
Beverages
Ethanol contents in commercial alcoholic beverages
vary a lot, even for the same sort of beverages, e.g. 3~6%
(w/v) for beers, 7~21% (w/v) for fermented wines, and
20~50% (w/v) for distilled spirits.(1) Therefore, different
pretreatment procedures are required when using the
current official packed GLC methods for the determination
of ethanol contents in various alcoholic beverages. In this
study, a rapid capillary GC method, which requires smaller
Journal of Food and Drug Analysis, Vol. 11, No. 2, 2003
136
sample amount and simple pretreatment procedures, is
developed and can be adapted in different sort of alcoholic
beverages with ethanol contents between 0~500 mg/mL.
Ethanol standard solutions in the concentrations of 5,
10, 20, 30, 40, and 50% (w/v) were prepared, and then 0.5
mL of each solution was dispensed into a 20-mL caped
vial. After adding and gentle mixing with 0.5, 1.0, 2.0, 2.5,
3.0, 4.0, 5.0, 6.0, 7.0, and 8.0 mL of 1% (w/v) internal
standard solutions, respectively 0.1 µL of each solution was
then injected directly into a GC for analysis. In order to
achieve better accuracy, samples with high contents of
ethanol needed larger amount of 1% (w/v) internal standard
solution (Figure 2). For different beverage samples with
ethanol contents of 5, 10, 20, 30, 40, and 50% (w/v), the
ratios of 1% (w/v) internal standard solution required were
2, 4, 6, 8, 10, and 12, respectively. In order to obtain
results with high consistency, 10x amount of the internal
standard solution was added when ethanol contents in
alcoholic beverages were determined in this study.
In conclusion, we have developed a method which
requires only 0.5 mL, even 0.1 mL (data not shown), of
beverage sample amount mixed with adequate amount
(10x) of 1% internal standard solution. This sample
solution is suitable for injecting directly into a GC for
determination of ethanol content. No pretreatment
procedure is required. This study method, is more simpli-
fied than official methods AOAC(4) and CNS(15), which
require dilution or distillation as the pretreatment proce-
dures. This study method is also easier in operation
compared with methods developed by Collins et al.(2) (CE
method) and by Antonelli(12) (packed GLC method), which
require ultra-filtration (0.45 µm filter) and dilution as pre-
0
20
40
60
80
100
0246810121416
Volume ratio of IS to ethanol
Recovery (%)
5%(w/v)
30%(w/v) 10%(w/v)
40%(w/v) 20%(w/v)
50%(w/v)
Figure 2. Effect of the volume ratio of 1% internal standard (IS)
solution to various concentration of standard ethanol solution(1) on
the quantitative determination(2) of ethanol content.
(1) The volume ratio of IS solution to standard ethanol solution, v/v,
0.5 mL of standard ethanol solution was used in each analysis.
(2) Recovery(%) = (level of ethanol detected)/( ethanol level of the
standard solution) 100%.
Journal of Food and Drug Analysis, Vol. 11, No. 2, 2003
137
treatment procedures.
IV. The Limit of Quantitation (LOQ) of Ethanol by GC-FID
In this study, 1% (w/v) ethanol standard solution was
successively diluted, and then mixed with adequate amount
of internal standard. The mixture solutions were then
injected directly into a GC, equipped with an FID detector.
The FID signal was set at range = 1, and attenuation = 1.
The coefficient of variation (CV, %) of ethanol recovery
was set at 15%. This was the LOQ of ethanol.(28, 31) The
results showed that the LOQ of ethanol was around 0.5
µg/mL (Table 1).
V. Validation of the Analysis Method
The recoveries of ethanol standard solutions, which
spiked into a non-distilled spirit (red wine) and a distilled
spirit (whisky), were found in Table 2. The results show
that the recoveries of ethanol with 50 and 100 mg fortifica-
tion level into 0.5 mL of red wine and whisky were in the
range of 99~104%, and 99~101%, respectively, with coeffi-
cient of variable (CV) less than 3.4%. When 6 ethanol
samples with known concentrations (10~500 mg/mL) were
analyzed using the current method, the coefficient of
variant (CV, %) for intraday (1 day) and interday (succes-
sive 3-day) analyses was 1.0~4.7%, and 1.5~3.6%, respec-
tively. These results indicate that the precision of the study
method is very high. In addition, the REM of intraday and
interday analyses was -3.5~4.2% and -1.0~3.0%, respec-
tively. It indicates that the study method possesses very
good accuracy (Table 3). Using the study method, after
beverage samples are mixed with adequate amount of
internal standard solution, they are ready for injecting
directly into a GC for the determination of ethanol contents.
It takes 7~8 min to complete an analysis. The GC chro-
matograms were illustrated in Figure 1A, 1B, and 1C. This
study method is one of the most simple, rapid, and accurate
methods published for quantitative determination of ethanol
contents in alcoholic beverages. It also can be proposed as
the reference method for routine analysis.
VI. Comparison of Study Method with Dichromate
Oxidation Method and Distillation-Hydrometric Method
The study method was compared with dichromate
oxidation method (AOAC 969.12) and distillation-hydro-
metric method (AOAC 920.57) for the determination of
ethanol contents in red wine, rice wine, and whisky (Table
4). No statistically significant difference between these 3
methods (p < 0.05, n = 3) was observed, which indicates
Table 1. Limit of quantitation, LOQ, of ethanol using gas chromato-
graphy with FID detector by direct injection method
Ethanol content (mg/mL)aRecovery (%)bCV (%)c
50.0 105.4 ±0 2.4 2.3
25.0 97.5 ±0 4.4 4.5
10.0 98.4 ±0 6.6 6.7
05.0 104.7 ±0 9.2 8.8
02.0 106.7 ±0 5.4 5.1
01.0 095.7 ±0 8.9 9.3
00.5 105.3 ±13.3 12.6
00.1 107.2 ±22.3 20.8
00.05 138.4 ±41.1 29.7
a: FID range = 1, Attenuation = 1.
b: Average of triplicate analyses.
c: Coefficient of variation (cv %).
Table 2. Recoveries of the ethanol in spiked commercial alcoholic beverage by direct injection gas chromatographic method
Sample Blanka(mg) (A) Amount added (mg)(B) Amount foundb(mg) (C) Recovery(%)cCV(%)d
Red wine 90.5 ±2.7 50 142.7 ±4.0 104.0 2.8
100 189.1 ±3.9 98.6 2.1
Whisky 317.9 ±11.6 50 367.3 ±10.3 99.0 2.8
100 418.5 ±14.4 100.6 3.4
a: Ethanol in 0.5 mL alcoholic beverage.
b: Average of triplicate analyses.
c: Recovery(%) = (C–A) / B ¥100%.
d: Coefficient of variation (cv %).
Table 3. Precision and accuracy of intraday and interday validation for concentration range from 10 to 500 mg/mL
Concentration Intraday (n = 3)aInterday (n = 9)b
(mg/mL Precision Accuracy Precision Accuracy
Mean ±SD (CV %) REM(%)cMean ±SD (CV %) REM(%)
10 9.9 ±0.1 (1.0) -1.0 10.2 ±0.2 (2.0) 2.0
20 19.7 ±0.4 (2.0) -1.5 19.8 ±0.3 (1.5) -1.0
50 52.1 ±1.9 (3.6) 4.2 50.7 ±0.8 (1.6) 1.4
100 96.5 ±3.1 (3.2) -3.5 102.4 ±2.7 (2.6) 2.4
250 259.3 ±8.5 (3.3) 3.7 255.6 ±7.1 (2.8) 2.2
500 487.8 ±23.1 (4.7) -2.4 515.2 ±18.7 (3.6) 3.0
a: n = 3, Repeated injection three times on the same day.
b: n = 9, Repeated injection three times each day and a successive three-day.
c: REM = relative error of the mean.
Journal of Food and Drug Analysis, Vol. 11, No. 2, 2003
138
the study method we proposed obtains very high accuracy.
The ethanol contents determined by the study method were:
107.4 ± 2.9 mg/mL in red wine, 158.2 ± 6.0 mg/mL in rice
wine, and 326.3 ± 7.8 mg/mL in whisky (CV = 2.4~3.8%).
The ethanol contents determined by dichromate oxidation
method were: 110.6 ± 14.2 mg/mL in red wine, 151.4 ±
16.8 mg/mL in rice wine, and 311.3 ± 37.7 mg/mL in
whisky (CV = 11.1~14.2%). The ethanol contents deter-
mined by distillation-hydrometric method were: 114.1 ±
13.8 mg/mL in red wine, 153.2 ± 14.7 mg/mL in rice wine,
and 307.6 ± 40.5 mg/mL in whisky (CV = 9.6~13.2%).
These results also indicate that the study method has better
precision than the AOAC methods. Both AOAC methods
require dilution and distillation as the pretreatment proce-
dures. High deviation may be found if the connection tubes
are not tightly secured, condensation efficiency is low, or
the sample contains high contents of volatile compounds
(e.g. acetic acid or sulfur dioxide) in distillation process(1).
Also, highly toxic reagents used in dichromate oxidation
method make the operation and waste disposition trouble-
some and dangerous.
In the application point of view, the simple, rapid, and
direct-injected capillary megapore column GC method
developed in this study has shown more advantages and
values compared with the AOAC(4) and CNS official
methods(15). Currently, different beverage samples require
unique sample preparation procedures when the official
AOAC and CNS packed GLC methods are applied, and dis-
tillation or dilution is required as the pretreatment
procedure. Since the resolution is low for packed GLC
methods(12), the accuracy for quantitative analysis and
reproducibility of ethanol contents will be interfered for
those beverages with high and complicated contents of
volatile compounds (e.g. fruit and medicine wines)(21). In
addition, packed GLC column has shorter shelf life and
lower durability. Besides, currently adapted capillary GC
methods require time-consuming sample pretreatment pro-
cedures, such as solid-phase extraction(17), headspace
balancing(18), solvent extraction(19), and distillation(20-21).
The study method we developed is a simple and rapid
method, without any sample pretreatment procedure
required. After beverage samples mixed with adequate
amount of the internal standard solution, the samples are
ready to be injected into a GC for determination of ethanol
contents.
VII. The Ethanol Contents of Commercial Alcoholic
Beverages
The ethanol contents of commercial alcoholic
beverages are usually labeled in volume percent (%, v/v).
Sometimes, it is convenient to be labeled in weight-volume
percent (%, w/v). In this study, the ethanol standard
solutions, 10%, 20%, 30%, and 40%, were prepared both in
weight-volume percent (%, w/v), and in volume percent (%,
v/v). After beverage sample were mixed with adequate
amount of internal standard and injected into a GC for the
determination of ethanol content, the relative response
factor (RRF) was then calculated according to the above
mentioned equation. The linear regression line of ethanol
weight-volume percent (%, w/v, Y-axis) against ethanol
volume percent (%, v/v, X-axis) was: Y = 0.814 X, with R2
= 0.999 (Figure 5).
Table 5. Ethanol levels determined in alcoholic beverages.
Sample Ethanol Content
Distilled spirit mg/mL %, v/v
Whisky 1 327.6 ±11.8 40.3 ±1.4
Whisky 2 317.8 ±12.4 39.0 ±1.5
Whisky 3 331.3 ±13.2 40.7 ±1.6
White liquor 1 328.9 ±09.5 40.4 ±1.2
White liquor 2 413.6 ±19.7 50.8 ±1.8
Kaoliang1 327.5 ±12.4 40.2 ±1.5
Kaoliang2 415.7 ±17.6 51.1 ±1.7
XO 313.0 ±13.6 38.5 ±1.7
VSOP 289.7 ±08.2 35.6 ±1.0
Rice wine 165.2 ±04.9 20.3 ±0.6
Mizhiu Tou 324.3 ±10.3 39.8 ±1.3
Medecine wine 197.7 ±05.3 24.3 ±0.7
Non-distilled spirit
Millet wine1 77.2 ±02.9 9.5 ±0.4
Millet wine2 81.3 ±01.4 9.9 ±0.5
Red wine 1 90.1 ±03.7 11.1 ±0.5
Red wine 2 106.5 ±04.5 13.1 ±0.5
Red rose 1 128.4 ±05.5 15.8 ±0.7
Red rose 2 117.1 ±03.7 14.4 ±0.5
Fruit wine1 91.2 ±02.6 11.2 ±0.3
Fruit wine2 112.5 ±03.4 13.8 ±0.5
White wine 1 100.4 ±03.1 12.3 ±0.4
White wine 2 103.9 ±03.7 12.8 ±0.5
Shao-Hsing wine 1 141.2 ±04.9 17.3 ±0.6
Shao-Hsing wine 2 138.6 ±05.5 17.0 ±0.7
Beer 1 28.2 ±00.8 3.5 ±0.1
Beer 2 29.8 ±00.6 3.7 ±0.1
a: Average of triplicate analyses.
Table 4. Comparison of AOAC methods and proposed method for the determining of ethanol content in alcoholic beverages.
Sample Proposed methodaDichromate oxidation methodbDistisllation hydrometric methodc
Ethanol (mg/mL) CV (%)dEthanol (mg/mL) CV (%)dEthanol (mg/mL) CV (%)d
Red wine 107.4 ±2.9 2.7 110.6 ±15.7 14.2 114.1 ±13.8 12.1
Rice wine 158.2 ±6.0 3.8 151.4 ±16.8 11.1 153.2 ±14.7 9.6
Whisky 326.3 ±7.8 2.4 311.3 ±37.7 12.1 307.6 ±40.5 13.2
a: Direct injection GC method in this study.
b: AOAC 969.12 method.
c: AOAC 920.57 method.
d: Average of triplicate analyses, coefficient of variation (cv%).
The ethanol contents of the 26 commercial alcoholic
beverages (12 distilled spirits and 14 non-distilled spirits)
were analyzed with the study method. As shown in Table
5, the ethanol contents of 12 distilled spirits were:
317.8~331.3 mg/mL (39.0~40.7%, v/v) in whisky,
328.9~413.6 mg/mL (40.4~50.8%, v/v) in white liquor,
327.5~415.7 mg/mL (40.2~51.1%, v/v) in kaoliang wine,
313.0 mg/mL (38.5%, v/v) in XO, 289.7 mg/mL (35.6%,
v/v) in VSOP, 165.2 mg/mL (20.3%, v/v) in rice wine,
324.3 mg/mL (39.8%, v/v) in Mizhiu Tou, and 197.7
mg/mL (24.3%, v/v) in medicine wine. The ethanol
contents of the 14 non-distilled spirits were: 77.2~81.3
mg/mL (9.5~9.9%, v/v) in millet wine, 90.1~128.4 mg/mL
(11.7~15.8%, v/v) in grape wine, 91.2~112.5 mg/mL
(11.2~13.8%, v/v) in fruit wine, 100.4~103.9 mg/mL
(12.3~12.8%, v/v) in white wine, 138.6~141.2 mg/mL
(17.0~17.3%, v/v) in Shaohsing wine, and 28.2~29.8
mg/mL (3.5~3.7%, v/v) in beer. The ethanol contents of
these alcoholic beverages were consistent with their labels,
without any fraud found.
CONCLUSIONS
In this study, it takes only 7~8 min to complete a
sample analysis for the determination of ethanol content in
a beverage sample. A sample solution (0.5 mL) is mixed
with adequate amount (5 mL) of 1% (w/v) internal standard
solution (acetonitrile, equivalent to 50 mg), and injected
into a capillary GC. The study method we developed can
be applied to alcoholic beverages with different alcoholic
contents, and with the advantages of simple sample pre-
treatment procedures, rapidity and accuracy, and maybe a
routine analysis method in substitution of current AOAC
and CNS methods.
ACKNOWLEDGEMENT
This research was supported by Project 90-FH-10 from
Chia-Nan University of Pharmacy and Science. The authors
would like to thank Dr. Hui-Cheng Chen for his translation
work.
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