ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
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14
I 12
c
1
I
I
I B E N 2 0 :a) PYRENE I n presence o f l p g BENZO(k) FLUORANTHENE
I
delay
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-
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STEADY -STL\TC c a r r e l o t i o r coef‘ 3 998
I 200
430
600
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time resolution arises largely from the fact that the analyte, BaP, exhibits a greater fluorescence decay time (78 ns) than the internal standard, BbF (q= 37 ns); the data plotted in Figure 5 were obtained at a long delay time (90 ns). The limit of detection for BaP (defined as the quantity of BaP required to produce a difference in fluorescence intensity between a sample and a “blank”, containing 1pg BkF and no BaP, equal to 3 times the standard deviation of the “blank” fluorescence intensity a t 4006 A) in the presence of 1 pg BkF was 40 ng by steady-state MI fluorometry but was reduced to 2 ng by use of time resolution. Additional improvement in detection limits in cases of this type should be possible by using the argon ion laser to synchronously pump a dye laser (7). A major shortcoming of the fluorometer shown in Figure 1 is its restriction to a single excitation wavelength (2573 A); this is an especially important limitation in MI fluorometry because excitation as well as emission spectra are sharpened by use of cryogenic matrices. While time resolution is unlikely to achieve the status of a routine technique in low-temperature fluorescence spectrometry, it should prove very useful in special cases (as, for example, when a carcinogenic PAH such as benzo[a]pyrene is a minor constituent of a complex sample). Future developments in time-resolved MI fluorometry in this laboratory will include use of a synchronously-pumped dye laser (7) as a tunable source, and the complementary use of time-resolved and steady-state MI fluorescence spectra to obtain detailed profiles of PAH content of complex real samples, such as coal liquids and shale oils.
10~0
nq 33P
Flgure 5. Analytical calibration curves (A = 4006 A) for benzo[a]pyrene in the presence of 1 p g benzo[ k ] fluoranthene, by time-resolved (A) and steady-state (0)MI fluorometry. In all cases, benzo[ blfluorene (BbF) was used as internal standard
1 pg BkF and no BaP) were normalized to yield the same intensity for the internal standard, BbF, a t 3377 A. The normalized blank intensity at 4006 A was then subtracted from each of the normalized “sample” intensities a t 4006 A; the resulting BaP fluorescence signals were expressed as the ratio of BaP (4006 A) to BkF (3377 A) fluorescence intensities. The greater slope in the analytical calibration curve observed for
LITERATURE CITED (1) Jones. P. W.; Freudenthal, R. K. “Polynuclear Aromatic Hydrocarbons: Second International Symposium on Analysis, Chemistry, and Biology”; Raven Press: New York, 1978. (2) Stroupe, R. C.; Tokousbalides, P.; Dickinson, R. B., Jr.; Wehry, E. L.; Mamantov, G. Anal. Chem. 1977, 49, 701. (3) Tokousbalides, P.; Hinton, E. R., Jr.; Dickinson, R. B., Jr.; Bilotta, P. V.; Wehry, E. L.; Mamantov, G. Anal. Chem. 1970, 50. 1189. (4) Mamantov, G.; Wehry, E. L.; Kemmerer, R. R.; Stroupe. R. C.; Hinton, E. R.; Goldstein, G. Adv. Chem. Ser. 1978, 170, 99. (5) . , Mamantov. G.; Wehrv, E. L.; Kemmerer. R. R.; Hinton. E. R. Anal. Chem. 1977, 49, 86. (6) Harris, J. M.; Chrisman, R. W.; Lytle, F. E.; Tobias, R . S. Anal. Chem. 1976. 4 ., 8 . 1937. .. (7) Harris, J M.; Gray, L M.; Pelletier, M. J : Lytle, F E. Mol. Photochern. 1977, 8, 161.
Richard B. Dickinson, Jr. E. L. Wehry* Department of Chemistry University of Tennessee Knoxville, Tennessee 37916
RECEIVED for review December 1, 1978. Accepted February 2, 1979. This research was supported by the National Science Foundation (Grants MPS75-05364 and CHE77-12542).
Electron Mobility in a Plasma Chromatograph Sir: A plasma chromatograph is one of the analytical instruments for investigating both positive and negative ions resulting from a series of ion-molecule reactions, taking place at atmospheric pressure, between molecules of interest and the reactant ionic species ( I ) . The generation of positive reactant ions has recently been studied in detail ( 2 ) and that of negative reactant ions has also been discussed in many reports ( 3 ) . However, when nitrogen gas is used as both drift and carrier gases, only thermal electrons are expected to be 0003-2700/79/0351-0780$01 .OO/O
generated as the negative species, which were found to be much less reactive than the positive or negative reactant ions ( 4 ) . This mode of the plasma chromatograph operation has been utilized by Karasek, Tatone, and Kane ( 5 ) in studying the electron capture mechanism, which is of interest to many gas chromatographers. However, with use of a Franklin GNO Beta/VI model plasma chromatograph, the authors reported that these thermal electrons appeared as a continuum across the plasma chromatographic scan because the gate grids did 0 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 6,MAY 1979
781
S y n c Pulse
S y n c pulse
S i g n a l (lV/div.)
S i g n a l (lV/div.)
A
S y n c pulse
S y n c Pulse
Signal (lV/div.)
S i g n a l (0.5V/div. ) B
S y n c pulse Signal (lV/div.)
ulse
Siqnal ( l V / d i v .
)
Figure 2. Electron drift velocities at a gate width of 50 ps and at field strengths of (A) 214 V/cm, (B) 143 V/cm, and (C) 107 V/cm. The sync pulse width is 25 p s , which is one-half of the gate pulse width
S y n c pulse S i g n a l (lx:/div.)
< - -time Flgure 1. Thermal electron peak shapes at a field strength of 214 V/cm and at gate pulse widths of (A) 50 ps, (B) 100 ps, (C) 200 ps, (D) 500 ps, (E) 1 ms. The sync pulse widths are one-half of the gate pulse widths
not close perfectly to exclude such high speed particles. If the grid could exclude these electrons, they predicted that the ion drift spectrum would show a single peak with a drift time of 10 ps, which would be too fast for the response of the recording system. However, Spangler and Collins (6) did detect a very sharp peak near the start of the spectrum and observed a base-line current in some of the ion drift spectra obtained on a Franklin GNO Beta/VII plasma chromatograph when nitrogen was used as drift gas and a considerable lower flow of air as carrier gas. Because their interest was in the generation of negative reactant ions, no discussion was made about their observation. Because of the above described ambiguity, the electron mobility, i.e., the electron velocity per unit field strength, was re-investigated with use of a plasma chromatography and the results obtained are discussed in this paper. EXPERIMENTAL A model Beta/VII-S plasma chromatograph, manufactured by the Franklin GNO Company, now PCP, Inc., of Palm Beach, Fla., was employed in this study. The detailed description of the instrumentation and the experimental conditions have been reported elsewhere (7). The plasma chromatographic oven was kept at 200 ‘C and prepurified nitrogen, after passing through a Linde Molecular Sieve 13X trap, was used as both drift and carrier gases with flow rates of 500 cm3/min. and 150 cm3/min, respectively. The data obtained were recorded by the following ways: a Polaroid camera attached to a dual beam Tektronix oscilloscope for measuring electron drift times, a Varian CAT, a digital signal averager, for obtaining the ion drift spectra, and a PAR Boxcar integrator, an analog signal averager, for monitoring the reaction between electrons and CC1,. R E S U L T S AND DISCUSSION When nitrogen was used as both drift and carrier gases, a very sharp peak was observed on the oscilloscope near the origin of the negative ion mobility spectrum. The oscilloscope was triggered by the positive slope of the plasma chromatographic sync pulse, which is one half of the gate pulse. As
I n
1
I 6.6
-+rift tire
(nsec)--->
24
Figure 3. Ion mobility spectrum after the introduction of 2 pL of CCI, vapor into the drift gas stream
the pulse width increased, the peak width of the observed peak also increased. This flatness of the peak indicates the time required for the negative species to arrive a t the collector is shorter than the gate pulse width. This is shown in Figure 1, where the gate pulse width varied from 50 ps to 1 ms and the plasma chromatographic drift field strength was 214 V/cm, or in other words, 3 kV was applied to the entire 14-cm plasma chromatographic tube. The tube consists of two regions the ion-molecule reaction region, 6 cm in length, and the ion drift region, 8 cm in length. With the shortest gate pulse available which is 50 ,us, the drift times of these negative peaks were measured as referenced to the time of the one-half of the gate pulse, as shown in Figure 2, to be 100, 110, and 120 ps a t the drift field strengths of 214, 143, and 107 V/cm, respectively. Assuming that these negative particles are thermal electrons, the drift times are much longer than 10 ps, predicted by Karasek et al. ( 5 ) . The drift velocities a t the above drift field strengths were calculated to be 0.80, 0.73 and 0.67 X lo5 cm/s. The reduced mobilities,
where d is the drift distance, t the drift time, E the drift field strength, P the pressure in mm Hg and T the temperature in K at the experimental conditions, were calculated to be 0.22, 0.30, and 0.36 X lo3 cm2/V-s a t the above respective field strengths. Under the present ambient pressure experimental conditions, the E / N values, where N is the number density of the gaseous molecules, were calculated to be 0.85,0.57, and 0.43 T d a t 293 K (1 T d = lo-’’ V cm2 molecule-’). The electron drift velocities a t the corresponding E,” value at 293 K were found to be 4.2, 3.7, and 3.3 X lo5 cm/s (8). The reduced mobilities are therefore calculated to be 1.8, 2.4, and 2.9 X lo3 cm2/V-s which are much higher than our experi-
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
Figure 4. Reaction of CCI, f e-
-,GI-
-t CCI,
after the
introduction of 2 p L of CCI, vapor into
mental values, even though the effect of the gate width time (t,) is taken into consideration (9). In order to understand more about these observed peaks, 2 pL of CCll vapor was injected into the drift gas stream by adding a T connection with a septum inlet in the drift gas line. The ion mobility spectrum was obtained and is shown in Figure 3. The first peak (peak A) is the aforementioned negative peak and the second peak (peak B) is at the same drift time as C1-, which is the expected dissociative electron capture product from CCl,. The distorted portion between these two peaks is interpreted to result from the reaction between the negative reactant of peak A and CC14 in the plasma chromatographic drift region. This experiment indicates that not only did the detected negative reactant of peak A transfer charge to CCl, but also that the drift gas got into the ion-molecule reaction region. Spangler and Collins (6) also observed similar distorted reactant ion peaks toward the shorter drift time when the air carrier gas flow was increased. This was explained to be due to the presence of oxygen and water in the drift region, which trap detected electrons to recycle them through ion-molecule cluster reactions before arrival at the collector. Both experiments point out that the drift gas can get into the carrier gas region and vice versa, which then would affect the plasma chromatographic performance as well as the relative densities of thermal electrons and negative reactant ions in the ion source. When the plasma chromatograph was tuned to the peaks shown in Figure 3, the intensities of the two peaks were recorded as function of time after 2 pL CC14 vapor was introduced into the drift gas stream. This is shown in Figure 4. That an increase of the C1- peak intensity correlated with a decrease of the negative reactant peak intensity is direct evidence for the reaction between CC14 and the negative reactant of peak A. Because of their extremely high mobility, peak A cannot be due to negative species with significant mass. The smallest negative reactant species other than electrons would be H-. The generation of H- under the present experimental conditions is chemically unlikely. Furthermore, the collision cross section was estimated to be 43 A2 when the radii of H- and the N2molecule were assumed to be 2.08 8, (10) and 1.6 A ( l l ) , respectively. The reduced mobility of H- was then estimated, according to the Langevin equation (11,12),to be 12 cm2/V-s.
the
drift
gas
stream
This value is much smaller than the reduced mobilities obtained herein for peak A. Therefore, the evidence presented above indicates that peak A is due to thermal electrons. With use of a plasma chromatograph of the same model as that of this study, M. Cohen and R. Wernlund (13) confirmed the magnitude of our measured value of the electron mobility. The discrepancy between the electron mobilities reported in literature and measured by a plasma chromatograph is attributed to the undesired capacitance coupling in the plasma chromatographic detection system. This will result in a longer integration time of a signal from the fast arriving electrons than the actual drift time of the electrons. Therefore, the present plasma chromatograph is not suitable for an accurate measurement of the electron drift mobility. ACKNOWLEDGMENT The authors express their gratitude to M. Cohen and R. Wernlund of PCP, Inc., for their efforts in confirming our measurement and to D. Ruhl for his assistance in characterizing the electrometer. LITERATURE CITED (1) Karasek, F. W. Anal. Chem. 1974, 4 6 , 710A-20A. (2) Carrol, D. I.; Dzidic, I.; Stillwell, R. N.; Homing, E. C. Anal. Chem. 1975, 47, 1956-9. (3) Carr, T. W. Anal. Chem. 1977, 49, 828-31. (4) Karasek. F. W.; Kane, D. M. J . Chromatogr. 1974, 9 3 , 129-39. (5) Karasek, F. W.; Tatone, 0. S.;Kane, E. M. Anal. Chem. 1973, 45, 1210- 14. (6) Spangler, G. E.;Collins, C. I. Anal. Chem. 1975, 4 7 , 393-402. (7) Tou, J. C.; Boggs, G. U. Anal. Chem. 1976, 48, 1351-6. (8) Huxley, L. G.; Crompton. R. W. "The Diffusion and DriR of Electrons in
Gases", Wiley: New York, 1974; Chap. 14. (9) Spangler, G. E.; Collins, C. I. Anal. Chem. 1975, 4 7 , 403-7.
(IO) Coulson, C. A. "Valence", Oxford University Ress: London, 1961; p 316. (11) Griffin, G. W.; Homing, E. C.; Wentworth, W. E. J . Chem. Phys. 1974, 60,4994-9. (12) Hasse, H. R. Phil. Mag. 1926, 7 . 139-60. (13) Cohen, M. J.; Wernlund, R. F., PCP Inc. Palm Beach, Fia.. private communication, 1976.
J. C. TOU* T. Ramstad T. J. Nestrick Analytical Laboratories The Dow Chemical Co. Midland, Mich. 48640
RECEIVED for review May 5,1978. Accepted January 15,1979.
