Determination of Impurities in Whiskey Using Internal Standard Techniques A GC Experiment for Quantitative Analysis Gary W. Rice College of William and Mary, Williamsburg, VA 23185 Quantitative determinations in gas chromatography (GC) are routinelv incornorated into auantitative or instrumental analysis labbrator; curricula t o dstahlish experimentally the concents of detector resoonse factors and internal standard techniques. These determinations are generally carried out on solutions fabricated in the laboratow.'-" We recently berame aware of a column parking esperially designed tor the direct separation and determination of congeners in whiskey. T h e method is fairly rapid and adequately separares a variety of rompounds commonly found a s impurities in distilled alcoholic spirit^.^." Over the past two years we have developed a n experiment in our instrimental inalvsis course t o determine five imnurities in American and scotch whiskey quantitatively using internal standard techniaues. T h e students are in turn familiarized with response factors for GC detectors and evaluation of a "real world" sample.
Equipment A Hewlett-Packard Model 5710 GC equippedwith a flame ionization detector (FID), temperature programming,and a Model 33808 Integration unit was used; however, any GC equipped withtemperature programming, on-column injection port, and integration capsbilities would be suitable. A glass column (2 m X 2 mm i.d.) was packed with 5% Carbowax 20m on 801120 Carbopak B (Supelco, Ine., Bellefonte, PA). The injector and detector ports were maintained at 200 T , and the oven was programmed from 60 'C to 160 "Cat a rate of 8 Wmin. A helium carrier gas was used at aflow rate of 30 mllmin. Reagents Students were supplied with ethyl acetate, 1-propanol, 2-metbyll-propanol, 1-butanol (internal standard), 2-methyl-l-hutanol, and 3-methyl-l-butanol. A 40% ethanol-in-water mix was used as the solvent to approximate the whiskey matrix. American and Scotch whiskey samples, known to contain measurable congeners, were supplied, or students were at liberty to bring their own samples. A minimum of 60 mL is required for the analysis. Students bringing their own samples were requested to present the samples to the instructor prior to the laboratory exercise in order to screen the samule for the desired imourities and to insure that an insignificant amount of 1-hutanol was present, which would interfere with the internal standardization and quantitations. Procedure Preparation of the whiskey sample with on internal standard: Pipet 25 @Lof I-butanol, using a 50-pL glass syringe, into a clean 50'Miller, J. M. J. Chem. Educ. 1970, 47, 306 (and references contained therein). Karasek, F. w.; DeDecker. E. H.; Tiernay, J. M. J. Chem. Educ. 1974, 51, 816. Pacer, R. A. J. Chem. Educ. 1976, 53.592. DiCorcia. A,; Samperi, R.; Severini,C. J. Chromatogr. 1980, 198. 347. Martin, G. E.; Burgraff. J. M.; Dyer, R. H.; Buscemi. P. C. J. Assoc. Off. Anal. Chem. 1981. 64. 186.
Time (min) Chromatograms obtained from 500gpm standard solution (top1and whiskey samole s ~ i k s dwith 500-.om . l-butanol as an internal standard (bottom).FID ATT 10 X 32 Pea* tdent t#cat#an (I)acetaidehyoe.(21 methanol. (3)etnanol. (4) emyl acetate. (51 l-propano. (6) 2-methyl-l-propanol,(7)I-o~tanol.( 8 ) aceuc acid. 191 Pmcmyl-l-o~tanol, an0 ( I 0 3-memyl-l-bJtanoi
ml. volumetric flask that has been rinsed with the whirkey sample. Dilute I,, the 50-ml. mark with [he whiskey sample, and mix thoroughly. Preparation of the standardsolution: Use the 50-@Lglass syringe to pipet 25 @Lof each compound listed above into a clean 50-mL volumetric flask approximately three-fourths full of the 40% ethanol-in-water mix. The compounds are injected into the partially filled flask to reduce evaporative losses of the more volatile compounds during the preparation time. Dilute to the 50-mL mark, and mix thoroughly. Make duplicate, ~ - , Linjections L of each solution on the GC, and obtain the chromatograms. From the peak areas tabulated from the standard solution eh;omatograms, calculate the response factors for each component, using l-butanol as the internal standard. Determine the averme factors from the two ehromato~rams. " resoonse . IJse the averaee r ~ ,resoonse .. ~ factors to calculate the Dart . ~. e million Irl\*tJamounts of each compound in the whiskey .iample.Calrulate the averrgr value of each rurnponent from the two chromatograms. ~~~~~
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Volume 64
Number 12
December 1987
1055
Response Factors and ppm Levels ol Flve Compounds In Whlskey Samples
Average Response Factors
Brand # I b
Ethyl Acetate
1Propanal
2-MethylI-propanol
2-MelhylI-butanol
0.671(* 0.032)
0.903(10.028)
1.0591~0.015)
1.039110.0231
215, 217 216(11.0)
107. 105 106(11.0)
529,484 507(123)
491.485 488(+3.0)
3MethylI-butan01
1.0761*0.045l
Brand #2
'Re8panse relafiveto l-bufanol based an 13 determinations. ' F a each brand and compaund, tap numb= are IMividual determinations, bouom numben are me mean wlues with average deviations
Dlscuoslon The concentration of each component in the standard solution, and for l-butanol in the whiskey sample, is 500 ppm (v/vO/o). This concentration is an approximate midrange value for typical congener concentrations found in whiskey samples. Typical chromatograms, and conditions under which they were obtained, for a standard solution and whiskey sample spiked with l-butanol are shown in the top and bottom of the figure, respectively. Methanol, acetaldehyde, and acetic acid were not quantitatively determined in the experiment. The attributes of using such a special column for the separations isexemplified by the partial resolution obtained for the pentanol isomers, which differ in boiling points by only 4 O C . The FID correction or response factor (F) for each compound with respect to l-butanol is given by:
where (AIM'), is the area and weight for a given component, and (AIM'),, is the area and weight for the internal standard (1-butanol). Since 500 ppm is taken as the "weight" for all components, the response factor for each compound simplifies to the ratio of the compound area to the area for 1hutanol. Average response factors obtained over a threeweek period are given in the table. Under the same detector conditions, these factors can be
1056
Journal of Chemical Education
used to calculate the ppm concentrations of each component from the whiskey chromatograms using eq 1in the form:
where A, and A,, are the areas of the compound and internal standard in the whiskey chromatogram, W,, is the weight of the internal standard in the whiskey sample, and F, is the response factor for the compound. A list of results obtained by individual groups for the five compounds, in four brands of American and Scotch whiskeys, is given in the table. The overall precision of the measurements for each brand was excellent, with relative average deviations never exceeding 5%for any given component in any brand of whiskey. The entire experiment can be performed in a two-hour period. Our total expense to incorporate the lab into the curriculum was ap~roximatelv5140. which covered the elass column and packing material; and some whiskey sampl;?~. Although we have always screened samples hrouebt in hv students for the contaminants being det&mined,ke as df yet have found no whiskey that does not contain all of the five compounds determined. The use of a variety of brands allows students to assimilate how quantitation by GC can be a very useful tool for fingerprinting the quality and brands of products from the levels of congeners present. Needless to say, student interest and comments concerning the experiment have been highly favorable.
