In the Laboratory
A Unique Qualitative GC Experiment for an Undergraduate Instrumental Methods Course Using Selective Photoionization Detectors Justin Notestein, Nadège Hélias, Wayne E. Wentworth,* and Julie G. Dojahn Department of Chemistry, University of Houston, Houston, TX 77024-5641 Edward C. M. Chen University of Houston–Clear Lake, Houston, TX 77058 Stanley D. Stearns Valco Instruments Co., Inc., P. O. Box 55603, Houston, TX 77255
X + hν → X+ + e¯
2
Adiabatic Ionization Potential
1
U (ev)
This experiment is an instrumental analysis method for gas chromatography that is quite suitable for students at the undergraduate level. It is unique in that the responses from three detectors can be used in conjunction with retention times to qualitatively identify a GC eluent. The pulsed discharge photoionization detector (PDPID) used in this experiment gives a universal response, is virtually nondestructive to the sample, and is highly sensitive. It gives a positive response to fixed gases (shows an increase in standing current) with an MDQ in the low ppb range (1– 3). This detector is commercially available and uses helium, argon, and krypton, which are nonflammable gases. The experimental method is simple, safe, and reliable. This experiment can be performed in a 3-hour laboratory period. It provides students with an understanding of basic gas chromatography and the use of an internal standard, as well as photoionization and vertical ionization potentials. In addition, the emission from the discharge provides an understanding of the continuum that can be obtained from He2 and resonance lines obtained from Ar and Kr. Photoionization is the vertical process that occurs when a species is irradiated with ultraviolet or X-ray photons to produce a positively charged ion and an electron. This process for a gas, X, can be represented by:
AB+
0
AB
- 1 Vertical Ionization Potential (Photoionization) - 2 1
v=1 v=0
2 3 Internuclear Distance (Angstroms)
4
Figure 1. General Morse potential energy curves for species AB and AB+.
(1)
The source of ultraviolet photons in this experiment is one of the gases used in the discharge of the detector: He, Ar, or Kr. The vertical ionization potential represents the energy difference that produces the strongest band in the spectrum and occurs between the zero-point vibrational level in the neutral species and a higher vibrational level in the positively charged species. The nature of the cation determines to what level that transition will occur (4, 5). The adiabatic ionization potential is defined as the energy difference between the zero-point vibrational energies of the neutral and ionic species. Figure 1 shows the Morse potential energy curves for a neutral species, AB, and the positively charged species, AB+. The vertical and adiabatic ionization potentials are indicated on the plot. The internal standard used in this analysis will account for variations in sample size and detector response. The use of capillary columns is essential, a column length of about *Corresponding author.
360
Figure 2. Schematic diagram of the pulsed-discharge photoionization detector.
Journal of Chemical Education • Vol. 75 No. 3 March 1998 • JChemEd.chem.wisc.edu
In the Laboratory
25 m being required for good separation. Another incidental benefit of this method is the use of air in the analysis. The helium discharge will show an air peak of significant proportion followed by a peak for water vapor. The argon- and krypton-doped helium discharge will show very little or no response to air or water peaks. Experimental Procedure The gas chromatograph used in this experiment was a Varian Aerograph equipped with a PDHID. A 25-m fused silica glass capillary column with an inside diameter of 0.18 mm was maintained at an oven temperature of 40 °C. The bias voltage was held at 300 V and the electrometer range set at 1 × 10¯10 amps on a full scale of 1 mV. The purity of the He, Ar, and Kr was five-nines or 99.999% and each of these was further purified by VICI (Valco Instruments Co., Inc.) gas purifiers operated in the “Bake-out” mode. The carrier-gas flow rate was measured at 27 mL/min. A block diagram of the experimental apparatus is shown in Figure 2. A discharge gas, helium or helium doped with argon or krypton, enters the detector and is ionized by the high-voltage arc from the discharge electrode. The photoemission from this process is then used to ionize the sample to be analyzed. The collector electrode records the current produced by the ionization of the sample. Figure 3 shows a schematic diagram of the experimental setup, showing the flow path configuration for the stream selection valve. For this experiment, a Valco four-port multiposition SF flow path valve was used. Helium is backflushed through the detector to maintain a constant flow rate for the makeup gas.
Ar
Gas Purifier
He
To Column Gas Purifier
Gas Chromatograph Detector
Blank
Argon Inlet/Outlet
Common Outlet
Helium Outlet
Gas Purifier
He
Krypton Inlet/Outlet
Four Port Multiposition Flowpath Valve
Gas Purifier
Kr
Figure 3. Diagram of the flowpath configuration for the stream selection valve.
In this experiTable 1. Composition of Sample ment, a sample mixture Mixture was prepared according Volume Compound (mL) to Table 1. The listed volume of each volatile Carbon disulfide 0.05 liquid and 1 mL of benMTBE 0.1 zene were placed into a n -Hexane 0.1 10-mL screw-capped Ethyl acetate 0.3 vial fitted with a gas1,1,1-Trichloroethane 0.2 lock valve. Benzene Benzene 1 served as an internal 1 , 4 D i f l u o r o b e n z e n e 0.8 standard for this analy1 , 1 D i b r o m o m e t h a n e 1 sis. The other compoc i s 2 H e p t e n e 2 nents of the mixture were chosen because of 1-Bromobutane 2 their volatility and difToluene 4 fering elution times. The sampling technique used was that of headspace analysis. The sample mixture was allowed to equilibrate in the sealed vial for at least 15 min before injection. This is to ensure adequate vaporization of each of the components. After equilibration, a syringe was inserted through the gas-lock valve in the top of the cap and approximately 3 mL of vapor was drawn from the center of the head space. These vapors were then injected into the gas chromatograph through a 10µL sample loop. Initially, the vapors of each component were chromatographed separately in helium to determine the relative retention times for each. Once the retention times were established, the mixture was chromatographed in the same manner. The analysis was repeated using pure helium, helium doped with 3.36% argon, and helium doped with 1.68% krypton as the discharge gases. Results and Discussion Figure 4 shows the resulting chromatograms from this analysis. The chromatogram shown in Figure 4A was obtained using pure helium. The air and water peaks are very pronounced at retention times of 1.20 and 1.25 min, respectively. The chromatograms for helium doped with argon and krypton are shown in Figures 4B and 4C, respectively. The air and water peaks are almost nonexistent, as would be predicted from the ionization potentials. Table 2 presents the results for the mixture described previously. For the helium detector, the relative retention time and peak heights were measured for each compound and a ratio calculated relative to the benzene internal standard. Because the relative retention times of the compounds do not vary significantly for the different detectors, only the peak heights were determined for argon and krypton, along with the ratios relative to the benzene internal standard. Equations 2 and 3 represent the calculation of the relative retention time and peak height ratios. RX = RTCOMPOUND /RTBENZENE
(2)
HX = HCOMPOUND /HBENZENE
(3)
The relative values of the detector responses eliminate concentration dependence. The detector response for benzene is measured in He, He/Ar, and He/Kr. The differences in response are determined by the ionization potential (IP). The
JChemEd.chem.wisc.edu • Vol. 75 No. 3 March 1998 • Journal of Chemical Education
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In the Laboratory
mean value for the ratios obtained from eq 3 is determined and a percent deviation is calculated according to eq 4.
% deviation =
Σ
H x – H MEAN
2
m m – 1 H MEAN
1/2
2
× 100
(4)
In this equation, Hx are the peak heights of the individual samples relative to benzene, HMEAN is the mean value of these ratios, and m is the number of samples taken. For the purposes of this analysis, five sets of data were obtained and the mean values calculated, from which the percent relative standard deviation was then determined for each compound. For each compound in the mixture, a relative photoionization cross section or RePIX is calculated. The RePIX value is characteristic of a compound and is independent of other factors. Three experimental variables must be considered in obtaining the RePIX value. These are the concentration distribution of the gas chromatographic peak, the variation in the split of the column effluent to the detectors, and the power applied to the PDPIDs. The responses can be determined from the following equations.
Figure 4. Chromatograms showing retention time in minutes for the sample mixture using (A) pure helium, (B) helium doped with argon, and (C) helium doped with krypton. Peak assignments are 1: air/ water; 2: carbon disulfide; 3: MTBE; 4: hexane; 5: ethyl acetate; 6: 1,1,1-trichloroethane; 7: benzene; 8: 1,4-difluorobenzene; 9: dibromomethene; 10: cis-2-heptene; 11: 1-bromobutane; 12: toluene.
RHe,X = kHe,X fHe PHeCX
(5)
R He,Std = k He,Std f He PHeCStd
(6)
R Ar, X = kAr,X fAr PArC X
(7)
R Ar,Std = kAr,Std fAr PArCStd
(8)
R Kr,X = k Kr, X f Kr PKrC X
(9)
R Kr, Std = k Kr,Std f Kr PKrCStd
(10)
where R is the response, k represents the linear response fac-
Table 2. Retention Times and RePIX Values for the Sample Mixture Helium Detector
Compound Carbon disulfide MTBE n -Hexane Ethyl acetate 1,1,1-Trichloroethane Benzene 1,4-Difluorobenzene 1,1-Dibromomethane cis -2-Heptene 1-Bromobutane Toluene
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Mean % Dev. Mean % Dev. Mean % Dev. Mean % Dev. Mean % Dev. Mean % Dev. Mean % Dev. Mean % Dev. Mean % Dev. Mean % Dev. Mean % Dev.
Argon Detector
Krypton Detector
RT/ RT(BZ) H/ H(BZ)
H/ H(BZ)
RePIX
H/ H(BZ)
RePIX
0.582 0.833 0.636 0.691 0.72 0.55 0.777 0.781 0.924 0.798 1 0 1.111 0.646 1.261 0.972 1.424 0.64 1.547 0.739 2.033 1.245
1.779 1.065 0.382 0.95 0.546 0.74 0.283 0.672 0.37 0.797 1 0 0.55 0.651 0.131 2.289 0.525 1.485 0.583 2.474 0.481 3.325
2.481 2.426 0.692 1.609 0.843 1.533 0.643 0.858 0.604 1.007 1 0 1.16 1.193 0.244 4.753 0.974 3.167 1.121 3.83 1.394 5.123
0.287 2.246 0.237 1.715 0.035 1.708 0.035 1.708 0.008 2.588 1 0 0.382 0.974 0.08 3.921 0.227 2.97 0.094 4.106 0.376 3.813
0.4 3.13 0.429 2.151 0.054 2.173 0.08 1.789 0.013 2.66 1 0 0.806 1.396 0.149 5.72 0.421 4.08 0.181 5.041 1.09 5.452
0.717 2.18 0.552 1.299 0.648 1.343 0.44 0.533 0.613 0.616 1 0 0.474 1 0.536 4.165 0.539 2.797 0.52 2.924 0.345 3.897
Journal of Chemical Education • Vol. 75 No. 3 March 1998 • JChemEd.chem.wisc.edu
In the Laboratory
tors and is proportional to the photoionization cross sections, f is the split ratio, P is the power supplied to the respective detectors, C X is the concentration of the analyte, and CStd is the concentration of the internal standard, benzene. Because the responses are taken relative to the standard, the variation of the split ratio between the detectors and the differences in power supplied to the detector are cancelled. Thus,
R He, X R He, Std
=
R Ar, X R Ar, Std R Kr, X R Kr, Std
k He, X k He, Std
=
=
k Ar, X k Ar, Std k Kr, X k Kr, Std
C = X C Std
(11)
=
CX C Std
(12)
=
CX C Std
(13)
The relative photoionization cross section or RePIX is then calculated by dividing eq 12 by eq 11, and eq 13 by eq 11, giving eqs 14 and 15, respectively:
RePIX Ar =
RePIXKr =
k Ar, X k He, Std k He, X k Ar, Std
=
k Kr, X k He, Std k He, X k Kr, Std
=
R MEAN, Ar
(14)
R MEAN, He R MEAN, Kr
(15)
R MEAN, He
Thus, the quantity RePIX accounts for the concentration dependence and variation in chromatographic properties such as the split ratio and the instrumental properties such as change in the sensitivity of the detectors due to variation in the applied power to the discharge (1). In this experiment, benzene is used as the internal standard and all relative responses are referenced to it. Benzene was selected as the internal standard because it has a low ionization potential and is easily detected
by all three detectors. The values of the RePIX are found to be reproducible and quite accurate over long periods of time. The percent deviation for the RePIX values is calculated to determine the precision of the data. This is accomplished using eqs 16 and 17: 2
2
2
2
% deviationRePIX =
% deviationAr + % deviationHe
% deviationRePIX =
% deviationKr + % deviationHe
(16) (17)
As can be seen from the calculations of the errors for the data presented in Table 3, the percent deviation from the mean for the retention time ratios ranges from 0.55 to 1.245, thus showing excellent reproducibility. For the response ratios, the percent deviation for pure helium ranges from 0.53 to 4.17, that for argon-doped helium is 0.65 to 3.33, and krypton-doped helium shows a range of 1.40 to 5.72. The percent deviations of the RePIX values range from 0.86 to 5.12 for argon-doped helium and from 1.40 to 5.45 for krypton-doped helium, indicating fairly good precision for the experimental method. Because the values of the peak heights for a given lamp are taken relative to the internal standard benzene, variations in detector sensitivity and sample size are not significant factors in this analysis. This method assumes constant detector sensitivity for the chromatogram, as the retention time of benzene is the same for each lamp and the ratio of the discharge constant. In addition, spectral emission characteristics from each lamp are based on the concentration of the dopant. The retention times are determined relative to benzene to account for changes in flow rate. This does not, however, precisely account for changes in column temperature. Use of Alternate Detectors One might ask, “Are there alternate detectors presently on the market that can be used for this experiment?” The answer is simply “No.” Photoionization detectors are available but they are made with sealed photoionization lamps, which obviously must use a window to transmit the radia-
Table 3. RePIX Values: Mean and Standard Error of the Mean Helium RT/RT(BZ)
Compound
IP (eV)
Mean
SE
Argon RePIX Mean
SE
Krypton RePIX Mean
SE
Air/O2
12.1
0.375
–
N/A
Water
12.6
0.391
–
N/A
Carbon disulfide
10.07
0.582
0.00485
2.481
0.0602
0.4
0.0125
9.24
0.636
0.00439
0.692
0.0111
0.429
0.00923
MTBE
N/A N/A
n -Hexane
10.13
0.72
0.00396
0.843
0.0129
0.054
0.00117
Ethyl acetate
10.01
0.777
0.00607
0.643
0.00552
0.08
0.00143
1,1,1-Trichloroethane 11.0
0.924
0.00737
0.604
0.00608
0.013
0.00035
Benzene
9.24
1
0
1
0
1
0
9.14
1.111
0.00718
1.16
0.0138
0.806
0.0113
1.261
0.0123
0.244
0.0116
0.149
0.00852
1,4-Difluorobenzene 1,1-Dibromomethane
10.5
cis -2-Heptene
8.84
1.424
0.00911
0.974
0.0308
0.421
0.0172
1-Bromobutane
10.13
1.547
0.0114
1.121
0.0429
0.181
0.00912
8.82
2.033
0.0253
1.394
0.0714
1.09
0.0594
Toluene
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tion. Either LiF or MgF2 is used for the window and these allow the Kr resonance line at 10.6 eV to be transmitted. This PID is similar to our Kr-doped helium PDPID and should give a chromatogram similar to that shown in Figure 4C. There are no windows available to transmit the highenergy emission from the helium discharge and the chromatogram shown in Figure 4A could not be obtained. The argon resonance emission at 11.8 eV can be transmitted with some attenuation through LiF. However, the transmission is further attenuated at high temperature and in general finds very limited use in GC. It is possible that one could obtain a chromatogram such as in Figure 4B with an existing PID, but this is very questionable for an instrumental analysis experiment. The commercially available PIDs using sealed lamps also have the serious restriction that the windows become contaminated and need frequent cleaning. This cleanup procedure must be carried out with extreme precaution, since the windows can be damaged rather easily and this will further restrict the transmission. I would certainly not leave the lamp cleanup procedure to an inexperienced undergraduate. Furthermore, the lamps are somewhat expensive and have a limited lifetime. In conclusion, this experiment would not be practical as an undergraduate experiment with previously developed PIDs. The windowless operation is essential for this instrumental analysis experiment. Furthermore, the operation of the detector is very safe and can even be used in process analyses where initiation of combustion is of great concern. In regard to the qualitative identification of a compound based upon multiple detector responses, other detector combinations can be used. However, without the PDPID the selection is quite limited and the application very restrictive. For example, the PID and the flame ionization detector (FID) can be used (6 ). However, the PID has the restriction of detecting compounds with IP < 10.6 eV and this limits its range of application. Everyone is familiar with the FID and its dependence on organic carbon containing compounds, but with some exceptions to this. For example, the FID does not respond to formaldehyde—nor would the PID, since its IP is > 11.6 eV. So again, this aspect of the experiment would be impractical without the PDPID. To further emphasize the simplicity of operation of this detector and the simplicity of this experiment, I should point out that one of the authors, Justin Notestein, carried out the experimental operation when he was a junior in high school. He was attending a summer institute at the University of Houston for advanced science students. The institute was
364
sponsored by the Robert A. Welch Foundation and ran for six weeks. Summary This experiment will give students experience in capillary gas chromatography and an understanding of the pulseddischarge helium photoionization detector in terms of photon energies and vertical ionization potentials. Additional benefits include a unique qualitative identification of some organic compounds and the error analysis of the data obtained. The concept of an internal standard is also demonstrated experimentally. The experimental procedure should minimize students’ exposure to the organic compounds, since only the headspace vapors are used. Furthermore, different groups of students can use the same mixture, thus eliminating waste and disposal of chemicals. The amounts of compounds injected are quite small; but if there is fear of laboratory contamination, a trap can be placed on the outlet of the sample injection loop. Even lower amounts of compounds will be injected into the GC, but if one is concerned about these vapors entering the laboratory, a trap can be placed on the outlet tube from the PDPID. If there is a desire to replace some of the compounds in this mixture, this can be done provided the compounds substituted are satisfactorily separated from the others in the mixture. Acknowledgments We would like to express our gratitude to the following organizations for financial support: Robert A. Welch Foundation (grant no. E095), Applied Technology Project (grant from the State of Texas), and Valco Instruments Co., Inc. Literature Cited 1. Gremaud, G.; Wentworth, W. E.; Zlatkis, A.; Swatloski, R.; Chen, E. C. M.; Stearns, S. D. J. Chromatogr. A 1996, 724, 235. 2. Wentworth, W. E.; Li, Y.; Stearns, S. D. J. High Resol. Chromatogr. 1996, 19, 85. 3. Cai, H.; Wentworth, W. E.; Stearns, S. D. J. Anal. Chem. 1996, 68, 1233. 4. Alberty, R. A.; Silbey, R. J. Physical Chemistry, 1st ed.; Wiley and Sons: New York, 1991; page 507. 5. Turner, D. W.; Baker, C.; Baker, A. D.; Brundle, C. R. Molecular Photoelectron Spectroscopy; Wiley: New York, 1970; pp 1–14. 6. Wentworth, W. E.; Hélías, N.; Zlatkis, A.; Chen, E. C. M.; Stearns, S. D. J. Chromatogr. A, in press.
Journal of Chemical Education • Vol. 75 No. 3 March 1998 • JChemEd.chem.wisc.edu
