Quantitative Study of Gas Chromatographic Analysis of Head Space Gas of Dilute Aqueous Solutions SIR: Weurman (2) in studying enzymatic formation of volatile materials in raspberries reported linear relationships when chromrttographic peak heights from head space vapor analyses were plotted against concentrations for isopropanol, ethyl acetate, propionaldehyde, and acetone a t 0.001 to 0.02570 (10 to 250 p.p.m.) in water. By modifying conventional hertd space sampling methods, Bassette, Ozl3ris, and Whitnah ( I ) were able to analyze some volatile organic compounds rtt concentrations in the range of 0.01 to 1.0 p.p.m. The research reported here was undertaken to establish the quantitative aspects of this modified techniqul:. EXPERIMENTAL
Apparatus. The apparatus, operating conditions, and head-space gassampling procedure were identical to those reported ( I ) except that a 9.5-ft. commercially prepared, l/s-inch stainlesq steel column packed Kith 20% Carbowax 20 M on 60- to 80-mesh, acidwashed Chromosorb IT was employed. Chemicals. Aqueous sample solu1.0, tions containing by volume: 0.1, and 0.01 p.p.ni.-propanal, nbutanal, n-pentanal, 2-butanone, 2pentanone, 2-hexanone, methyl formate, ethyl formate, ethyl acetate, ethyl propionate, allyl sulfide, and diethyl sulfide. 10.0, 5.0, and 1.0 p.3.m.-methanol. 10.0, 1.0, and 0.1 p.p.m.-ethanol, 12propanol, and n-butanol. 1.0, 0.8, 0.6, 0.4, 0.2, 0.1, and 0.01 p.p.m.-acetone. Procedure. Each of the dilute aqueous solutions was analyzed in duplicate as head-spExe vapor by the proposed method. RESULTS AND DISCUSSION
Standard curves showing chromatographic peak heights plotted against p.p.m. concentration of the chemical compounds studied are shown in Figure 1. -411 of the lines in the figure are defined by three different concentration. except the one for acetone; seven concentrations of acetone were analyzed. Each point in the figure represents the average peak height in duplicate analyses. Duplicates varied no more than 595. As can be seer: from Figure 1, linear relationships existed between chromatographic peak heights and concentration. Within each homologous series, slopes of the standard curves varied considerably. Differences in slopes of curves in Figure 1 draw attention to some in-
01
I
I
CONCENTRATIOE(ppm)
(yo
Figure 1 . Relationship between p e a k heights full scale deflection X attenuation factor) and concentration of organic compounds analyzed
teresting relationships between the analytical procedures and the type of organic compounds studied. Alcohols could not be detected at below 0.1 p.p.m. In fact, the lower limits for the determination of methanol was 1.0 p.p.m. All of the other compounds could be determined a t 0.01 p.p.m. In contrast with alcohols, diethyl sulfide produced a higher peak, for a concentration, than any of the other compounds. Within each class, lower molecular weight homologs generally yielded shorter peaks. Two exceptions were the five-carbon aldehyde (n-pentanal) and the six-carbon ketone (2-hexanone), each of which produced less response than the respective shorter chain homologs. Some differences in peak heights observed in this study undoubtedly can be attributed to the chromatographic phase of the analysis per se. Other differences probably result from the sampling procedure. The concentration of material drawn into the head space and injected into the instrument
varies in relation to the characteristics of the material being analyzed. Interrelations between factors i;fluencing the relative peak heights would>makeinterpretation of these differences highly speculative and beyond the scope of this correspondence. Development of standard curves utilizing head-space gas analysis in the analyst's laboratory should provide a basis for quantitative analysis of trace organic chemical compounds a t and below the p.p.m. level. LITERATURE CITED
(1) Bassette, R., bzeris, S., U'hitnah, C. H., ANAL,CHEM.3 4 , 1540 (1962). ( 2 ) Weurman, C., Food Technol. 15, 531 (1961). SCHEYLA
OZERIS
RICHARD BASSETTE Dept. of Dairy Science Kansas State University Manhattan, Kan. Contribution No. 313, Department of Dairy Science, Kansas Agriculturd Experiment Station, Manhattan, Kan. VOL. 35, NO. 8, JULY 1963
1091
