An Introductory QuantitativeGC Experiment for the Organic Chemistry Laboratory Phyllls A. Leber Franklin and Marshall College, Lancaster. PA 17604 Experiments related to gas chromatography (GC) can he found in ever-increasing numbers in organic laboratory manuals. However, most of these experiments are devoted to product analysis involving semiquantitative measurements of relative composition based on area % calculations. While GC should be an integral part of the core chromatography oroeram in the oreanic chemistrv lahoratorv. we have found ;ha; the instrumentation is complicated enough that students need an introductory exposure to the technique before embarking on GC applications. To accomplish this, we have develo~edan introductorv GC exueriment that allows students to explore the scope and limits of the technique. In subsequent experiments students are then required to set up the instrument independently for a variety of product analyses. In addition to orovidine students with a thoroueh introduction to the technique, the experiment involves a nontrivial ouantitative aoolication of CC tu the identification of an .. unknown mixture. A survey of some quantitative GC experiments recentlv cited in THIS JOURNAL^ reveals that thev have been deiigned for use in an advanced analytical or instrumental analvsis laboratorv rather than in a lower level laboratory.2 The advantage of the auantitative ex~erimentis that students gain a greater appreciation for thk scope of GC usage, including the extent of altering GC variables such as temperature, flow rate, and the nature of the stationary phase. Finally, students are asked to rationalize elution order trends as an exercise in correlating structural features with chromatographic phenomena. The choice of the six-component alcohol system (t-amyl, 2-pentyl, l-pentyl, cyclopentyl, l-hexyl, and l-heptyl) is predicated on satisfying the following criteria: (1) a short analysis time to reduce problems with peak broadening and to allow students time for d u ~ l i c a t einiections, (2) relatively low volatility to eliminate sample evaporation as a serious source of error, (3) near baseline resolution of the sample components on two GC instruments in common use in organic teaching labs (GOW-MAC 69-150 and Carle 6500),(4) ieady availability of the components in most chemistry department stockrooms, and (5) a series to illustrate elution order trends involving the effect of homologation, chain length and branching, and relative conformational flexibility.
-
rate injection. Questionable assignments are verified by coinjection. Sample chromatograms of the mixture acquired on the Carle and the Gow-Mac instruments are shown in Figures 1 and 2, respectively. Typical GC parameters are
-
Figure 1. A GC Chromatagram of the 1:1:1:1:1:1 (by volume)alcohol midure obtained on the Carle 6500: t-amvl . Ill. . . 2-oentvi . . 121. . . l-oentv . . 131, . . CvclODentyi . . . (4). I-hexyi(5). I-heptyi (6)with retemion tlmes (relativetoair)of 0.2.0.3.0.5. 0.7. 0.9. and 1.5 mi", respectively.
Experimental
Students calibrate the instrument using a 1:1:1:1:1:1(by volume) standard mixture of the six known alcohols mepared with volumetric glassware. Each component alcohbl is then characterized by its unique retention time upon sepa-
'
(a)Van Ana, R. E.; Van Atta, R. L. J. Chem. Educ. 1980,57,230. fbl Benson. G. A. J. Chem. Educ. 1982.59.344. icl Rudzinski. W. E.:
--~ - . . .-
- - ~ -
~~.
A referee has suggested that the experiment is designed for a laboratory sectfonwith a ow student:teacher ratio. in fact, in a large laboratory section (approximately 28 students)we achieve this effect by staggering the GC experiment over a four-week interval to accommodate this number of students using four GC instruments, on which
students work in pairs. 550
Journal of Chemical Education
Figure 2. A GC ChromatDgram of the 1:1:1:1:1:1 (by volume)alcohol mixture obtained on lhe GOW-MAC 69-150: t-amyl (I). Bpemyi (2). I-pew1 (3% cyciopentyl(4).1-hexyi(5). l-heplyi (6)wilh retention times (relativelo air)of 0.6. 1.0. 1.7. 2.3. 2.6, and 3.9 min, respectively.
Table 2. GC CornDonents
Table 1. GC Instrument Parameters Cede
GOW-MAC
Injection sire (pi)
1
+ 5 (air)
1
TCD
+ 5 (air)
Column
Phase Dimensions Temperature (OC) Injection port
Carbowax
...
D (148)
Column Gas flow (mllmin)
150 40-50
C(112)
Liquid
Recorder speed
(emlmin)
Y4"
Carbowax %" X 5'
X 4'
singie-component Total analysis time (mi")
Response Factor
1-amyl alcohol 2-pentyl alcohol 1-pentylalcohol cyclopentyl alcohol lhexyl alcohol lheptyl alcohol
0.91 0.88 0.91 0.67 0.96 1.0
bp ('C)
density (glml)
102
0.805
119
0.812
136-8 139-140
0.811 0.949 0.814
156-7 176
0.822
13-15
2.5
5
16
5 25 2
Attenuation Mixture
Component
64 5
given in Tahle 1. After standardization is complete, the student is eiven an unknown mixture consisting of three or four of the-original six alcohols. Retention time comparisons enable the student to identify the components in the unknown mixture. Quantitative data analysis relies on peak area integration and detector response factors. Integration of peak areas is performed using the peak height times peak width a t halfheight formula. This is the method of choice for inteeratine when baseline resolution is not achieved. Since thermal conductivity detector (TCD) response factors are reported in Tahle 2 based on masslarea ratios, the volume composition of the standard is converted to a weight composition using the density of each component (see Tahle 2). In order to obtain the relative detector response factor for each component, the mass of each component is divided by its corresponding peak area and then the masslarea ratio of each is divided hv the lareest masslarea ratio. After reneating the calculat
