F. W. Karasek, E. H. De Decker, and J. M. Tiernay University of Waterloo Waterloo, Ontario, Conado
Qualitative and Quantitative Gas Chromatography for the Undergraduate
The piezoelectric crystal detector (P/Z) is a recent development in gas chromatography (gc) (1-6), which can readily be employed in an undergraduate instrumental analysis course. The P/Z detector uses a quartz crystal which is coated with the stationary phase of interest. When a solute passing through the detector dissolves in the stationary phase, the resonant frequency of the crystal is changed. This frequency change is converted to a dc voltage and recorded; a block diagram of the device is shown in Figure 1. The sensitivity of the detector depends on the quantity of solute which dissolves in the liquid phase (the partition coefficient), and can he adjusted merely by changing the type of liquid phase on the crystal. A simple and completely portable analytical gas chromatograph which uses a P/Z oscillator detector is current-
Figure 1 . Block Diagram of P/Z Chromatograph.
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ly available (Model P/Z-7250, Laboratory Data Control, P.O. Box 10235, Riviera Beach, Florida 33404). The P/Z gc has several advantages for undergraduate laboratory instruction: the instrument uses the more economical carrier gases of nitrogen or air, operates at room temperature, and is sensitive to concentrations in the ppm range. The warm-up and stabilization times are very fast, requiring only a few minutes before use. While the usual cylinder supply of helium and nitrogen can he used for the carrier gas, an ability to use air as a carrier obviates the need for gas cylinders, regulators, and associated apparatus. A simple aquarium aerator pump is generally a satisfactory and inexpensive supply for carrier gas. The simple yet rugged construction of the instrument and the absence of heating units also allow extensive trouble-free undergraduate use. Any university-level chemistry student today should he familiar with gc because of the extensive analytical capabilities and widespread use of the technique. The P/Z gc offers not only one of the most recent advances in detector design, hut the potential of the instrument suggests that today's student may well find one a t his d i s ~ o s a luoon pradu~lionand en[& into industrial or academic'!irldi. ' In an? insfrumentation rourse i r w ~ ~ u lhe d iml~ossibleto provide extensive training in all areas of gc; it ib therefore desirable that the course introduction provides a solid foundation in both theory and practice. To this end, we have designed two experiments which bring within reach of the beginning student a broad general knowledge of the field. In the first experiment, the student obtains data which illustrate the use of gc retention times for qualitative analysis. The second experiment requires the student to perform quantitative analysis using weight factors for each of the gc peaks. He is carefully led through the theo-
Figure 3. Kavats Retention Index Graph for an n-hydrocarbon mixture. Figure 2. Chromatogram of known n-hydrocarbon mixture: n-hexane, nheptane and n-octane. Measurement of retention time. In, is illustrated lor n-octane. Conditions: column. 20 X 118 in. a d . . S.S.. OV-17 on 801 100 mesh Chmmosorb P: carrier gas. dry grade air; flaw rate. 30 ml/ min: temperature. 23-C.
ry of gc analysis, and although the results are only a first approximation, the surprising accuracy should be impressive to him. An important component of each of the experiments are the questions which the student is required to answer in his report. These arise naturally from the experimental work. Through the use of referenced literature to answer them, the student learns how to refine gc measurements, and can also discover the multitude of capabilities of the chromatographic technique. Experiment 1
Using the gc conditions indicated in Figure 2, the student is given or asked to prepare a preselected n-hydrocarbon mixture with equal parts by volume, such as a hexane, heptane, and octane mixture. Injections of this mixture are made into the gas chromatograph, with the point of injection accurately marked on the chromatogram
so that the retention time of each gc peak can be accurately measured (Fig. 2). At least three separate runs should he made to obtain a reasonable average of retention values (Table 1). Since the P / Z instrument uses nitrogen or air as a carrier gas, no gc peaks for air (tn) will he observed. An adjusted retention time ( t ~ = ' t x - tn) may be calculated by subtracting the retention time of methane (normal laboratory gas) from each of the individual component retention times. The experimenter is then instructed to make a plot of log retention time (or log adjusted retention time which gives a more linear function) versus 100 times the carbon number for each component in the chromatogram, and draw the best straight line through the points as shown in Figure 3. This gives a graphical representation of the Kovats Retention Index, a widely used concept for qualitative work in gas chromatography (7). The Kovats RI value is unique for each compound, and is relatively insensitive to changes in instrumental or gc column parameters (such as carrier gas flow rate, amount of liquid load on the packing, and sample size). The student is now given an unknown mixture of n-hydrocarbons to 'chromatograph and identify. Since n-hydrocarbons gives Kovats RI values
Table 1. Typlcal Experimental Data for Experiment 1
RI
Absolute retention time'' 1 2 3 Ave.
retwtirm time
Log adjusted retention time
600 7W 800 1W
2.0 2.0 2.0 2.0 4.3 4.3 4.5 4.4 11.0 11.3 10.6 11.0 0.5 0.5 0.5 0.5
1.5 3.9 10.5
0.176 0.591 1.021
?
28.5 28.3 28.3 28.4
27.9
1.446
?
76.3 76.6 75.6 76.2
75.7
1.879
Adjusted corn~~~~
POY"~
Herane Heptane Odsne
Methane
Unko0am 1 unknom 2
Kovsta
...
Table 2.
Knm
srt%
m8asud area
50 .- 50 1W
40 60 1W
".CVHrn
z P
Determination of Relative Walght Factors
% ealcd from
Toluene
'.,=.-: f
'Retention timea measured in arbitrary units.
Benzene
%*
?
Cald
wt factor ca1ed
1.25 0.83
srt% (area % X sst factor) 50 50 100
A.
1
J
L
~ i g u r e4. Chromatogram of unknown n-hydrocarbon mixture (expanded for illustrative purposes), n-nonane and n-decane. Column and conditions identical to Figure 1.
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Figure 6. Chromatogram of a mixture at aromatic hydrocarbons. Column and c?nditions identical to Figure 5. Figure 5. Chromatogram of a benzene-toluene mixture of unknown wt % illustrating the parameters used la calculate peak areas. Column. 20 X 118 in. a d . , $3, Carbowax 400 on 80/100 mesh Chromosorb W carrier gas, dry grade air: flow rate, 30 ml/min: temperature, 23% of 100 times their carbon number, a value of 900 obtained for an unknown identifies it as n-nonane (Table 1and Fig. 4). Other compounds may be identified using the same method and giving the student a predetermined table of Kovats RI values for compounds among which will be his unknown. Experiment 2 The student is given approximately 1 ml of a sample containing a mixture of benzene and toluene on which he will perform a quantitative wt-% analysis. The student first identifies the sample constituents by chromatographing the mixture and comparing component retention times to pure benzene and toluene. From his chromatogram (Fig. 5) he calculates the relative peak area percentages using an approximate method A = 1.06 H W U ~
where A is the peak area, H is peak height, and Wuz is the half-height peak width. The factor 1.06 is included since the area calculated by multiplying H by W m is approximately 6% less than the actual peak area. The student is reminded next that each component peak area is dependent on the detector response to that compound. Therefore, in order to obtain the actual wt-% composition of the sample, detector weight factors must first be determined. To accomplish this, a mixture of the sample components must be prepared in which the wt % of each constituent is accurately known. A chromatogram of this mixture is run and the relative weight factors determined according to the method illustrated in Tahle 2. Essentially, the weight factor is a constant required to correct the measured oeak area to the true wt % in the mixture. Using this method the student can readily calculate the percentage composition of his unknown mixture. This same pro-
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cedure can be used for samples containing many components. Each weight factor can further be referenced to the weight factor of one component, arbitrarily set at 1.00. The calculation involving area percent times weight factor must then be normalized to a total of 100 wt %. The P/Z instrument, because of its room temperature o~erationand high sensitivity can emplov columns of only 18 in. and yet maintain adiquate component separation with short retention times. For example, Figure 6 illustrates the chromatogram of an aromatic hydrocarbon mixture at room temperature (this feature also allows temperature-sensitive comoounds to he chromatoeraohed). It is therefore possible fkr students to prepare more than one standard and repeat chromatoma~hic - . runs to reduce random errors. The following are the follow-up questions to which the referenced literature provides adequate answers (8-11). 1) How would you more accurately determine weight factor
values? 2) How do weight factors for the P/Zdetector differ from those of a thermal conductivity detector? 3) List the possible pitfalls in the analytical procedure you have fnllnu..rl
4) Describe as concisely as possible the theory of the piezo-elec-
tric crystal detector in gc and its gravimetric capabilities.
5) Discuss the concept of the Kovats Retention Index and its use
in qualitative gc analysis. Literature Cited 11) King. W.H..Aml.Chem.. 36,1735(19&11. ipl King. W. H..Re8./Daudop. 20. (41.28 119691. 13) Guilbault,G.G..andLopez-bmen.Ano1. Left. 5.225i19721.
(4 Janghorbani. M.. and Freund. H..Aml. Chem., 45,3250973). 151 Ksraeek. F. W.. and Gibbons, K. R..J Chromatogr. S l i . 9.535 119711. 161 Karaaok. F. W.. and Tiemay, J. M., J Chmmnlo~r.,I974 linpreeal. 171 Mc Nair. H. M..and Bonelli, E. J.. "Basic Gas Chmmatoersphv." Varisn Asrograph. Walnut Creek, Calif.. 5ihEd.. 1 9 M pp. 137-169. 181 Dai Nogare, S.. and Juvet. R. S.. "Gas Chromatography." Wiley-lnfar~iencs, New York, 1962. pp.46-48,241-266.
(91 Keulemena, A. I. M.. "Gas Chromatagraphy." 2nd Ed.. &inhold Publishing Cow., NewYork, 19.59. pp.26-57. I101 Etfre. L. S.. and Zlafkis. A,. "The Practice of Gsa Chmmatography," Wilay-Interscience, New York. 1967. pp. 373-405. 1111 Peenok, R. L., and Shields. D. L., "Modem Methods of Chemical Analysis." John Wiley & Sona.Inc.. New York, pp.79-82.
