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ANALYTICAL CHEMISTRY, VOL. 51, NO. 6 , MAY 1979
Table 111. Quantitation Data for 1,4-Butanediol and Glycerol response factor for concn, 1,4-butanediol concn, vs. n-octanol ppm ppm 80 0.521 i: 0.008 200 40 0.258 i: 0.004 100 0.104 i 0.002 50 16 8 0.051 * 0.001 30 4 0.026 * 0,001
response factor for glycerol vs. n-dodecanol 0.446 * 0.008 0.221 * 0.004 0,109f 0.004 0.056 i 0.005
noteworthy that using hydrogen as carrier gas lowers the analysis time to a matter of minutes. Figure 2 is the reproduction of a chromatogram of glycerol and triethylene glycol contained in water at the level of about 50 ppm. The column packing used is the same as that cited above, but the column temperature was raised to 125 OC in order to contain the analysis time within about 30 min. As far as column stability is concerned, we noted that an uninterrupted use of 0.8% THEED-coated Carbopack C a t 115 OC for ten days caused a final decrease of the retention
time of 1,4-butanediol not larger than 10%. A more evident column deterioration was observed keeping the temperature a t 125 “C. ACKNOWLEDGMENT We thank E. Sebastiani for his experimental assistance. LITERATURE CITED (1) G. Manius, F. P. Mahn, V. S. Ventureila, and B. 2. Senkowski, J . Chromatoor. Sci.. 0, 367 11971). (2) I. L. Weatiierail, J . Chromatogr.’,28, 251 (1967). (3) K. Assmann, 0. Serfas, and G. Geppert, J. Chromatcgr.,26, 495 (1967). (4) Applied Science Labs., State College, Pa., Technical Bulletin, no. 24. (5) S. B. Dave, J . Chromatogr. Sci., 7 , 390 (1969). ( 6 ) L. H. Phlfer and H. K. Plummer, Anal. Chem., 38, 1652 (1966). (7) B. A . Swinehart, Anal. Chem., 40, 427 (1968). (8) A. Di Corcia, A. Liberti, and R. Samperi, J . Chromatcgr., 122, 459 (1976). (9) A. Di Corcia and A. Liberti, Adv. Chromatogr., 14, 305 (1976).
Antonio Di Corcia* Roberto Samperi Istituto di Chimica Analitica Universiti di Roma 00185 Rome, Italy RECEIVED for review July 19, 1978. Accepted December 5, 1978.
Time-Resolved Matrix-Isolation Fluorescence Spectrometry of Mixtures of Polycyclic Aromatic Hydrocarbons Sir: The environmental presence of complex mixtures of polycyclic aromatic hydrocarbons (PAH) containing species which are mutagenic or carcinogenic has long been recognized as a potential health hazard. Identification and quantitative determination of specific constituents in such mixtures remains a difficult task because of the large number of species usually occurring in a given sample and the similarities of their chemical and spectroscopic properties (1). Steady-state fluorometric analysis of PAH mixtures using matrix isolation (MI) as a sampling technique has been developed in this laboratory as a procedure for qualitative and quantitative analyses of PAH in mixtures; the method has been demonstrated useful in very challenging cases, including the resolution of PAH isomers (2-4). We now report the application and potential utility of laser-excited time-resolved fluorescence procedures for characterization of matrix-isolated PAH mixtures. EXPERIMENTAL A block diagram of the time-resolution spectrofluorometer is shown in Figure 1. When operated in the mode locked/cavity dumped regime, the argon ion laser produced light pulses of 800 ps FWHM at repetition rates ranging from single shot to 1 MHz. The 5145-A line was frequency doubled with an Interactive Radiation 5-1 temperature phase-matched SHG system (ADP crystal). The doubled beam, which was separated from the fundamental by a prism, was directed onto an MI sample deposited on a sapphire window in a closed-cycle cryostat ( 2 , 5). Fluorescence from the sample was analyzed with a Spex 1702 0.75-m grating spectrometer equipped with a 600 groove/mm grating and was detected by an Amperex XP2020 photomultiplier. Temporal resolution was achieved by use of a sampling oscilloscope, which limited the sampling rate (and hence the repetition rate of the laser) to a maximum of 32 kHz. Control of the sampling sweep by an external ramp generator was used to plot fluorescence decay curves with an X-Y recorder. Use of a “waveform hold” control of the ramp generator permitted plotting of time-resolved fluorescence spectra in which the “time window width” was defined by the rise time of the sampling oscilloscope (e.g., 75 ps for the Tektronix S-2 head), and the delay (with respect to the
laser pulse) was defined by the voltage at which the ramp was “held”. Performance of a time-resolved spectrometer of similar design for temporal separation of fluorescence and Raman scattering has been characterized by Harris et al. (6).
RESULTS AND DISCUSSION In Figure 2 are shown steady-state MI fluorescence spectra for a six-component PAH mixture in Nz a t 16 K. Excitation at a minimum of two wavelengths (248 and 280 nm) is required to fully illustrate the presence of all six components. While the major spectral peaks are sufficiently well resolved to be identified easily, quantitation is complicated by the presence of partially-overlapping band systems and the resulting difficulty in base-line definition. Note, for example, the partial overlap of the anthracene and pyrene bands at ca. 3700 A, that of pyrene and benzo[e]pyrene at 3900 A, and the severe overlap of the benzo[a]pyrene and benzo[k]fluoranthene features a t ca. 4000 A in Figure 2. In this mixture (which was deliberately chosen to highlight partial overlap of spectral features), quantitation of anthracene, benzo[e]pyrene, benzo[a]pyrene, and benzo[k]fluoranthene would be extremely difficult. Time-resolved MI fluorescence spectra of this same mixture, a t three different delay times, are shown in Figure 3; use of a somewhat larger emission monochromator bandpass than that employed in steady-state M I fluorometry was required to observe an adequate signal. Despite the loss in spectral resolution thereby introduced, comparison of Figures 2 and 3 shows that time resolution succeeds in alleviating the difficulties of band overlap and base-line definition evident in Figure 2. For example, though anthracene absorbs only weakly a t 2573 A, its temporal resolution from pyrene is apparent. I n addition, while benzo[a]pyrene (BaP) and benzo[k]fluoranthene (BkF) cannot readily be distinguished from each other by steady-state M I fluorometry, their fluorescence decay times in N2 matrices are sufficiently different (78 ns for B a P and 13 ns for BkF) to produce excellent temporal “separation” of the emission spectra for these
0003-2700/79/0351-0778$01.00/0Q 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
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Figure 2. Steady-state matrix-isolation fluorescence spectrum of six-component PAH mixture. Sample contained pyrene (P; 3 pg), anthracene (A; 1.5 pg), benz[a]anthracene (BaA; 400 ng), benzo[elpyrene (BeP; 3.5 Kg), benzo[a]pyrene (BaP; 27 ng), and benzo[k]fluoranthene (BkF; 2 5 8 ng). Matrix: N,; temperature: 16 K
Figure 3. Time-resolved MI fluorescence spectra, obtained at three different delay times, for the same mixture used to obtain the steady-state spectra in Figure 2. Fluorescence decay times in N2 matrix (not corrected for finite laser pulse width): BaP. 78 ns; BkF, 13 ns; A, 8 ns; BeP, 100 ns; BaA, 115 ns; P, 430 n s
compounds. The presence of BaF’ fluorescence in the virtual absence of interference from BkF is especially noticeable a t long delay times (e.g., 60 ns in Figure 3), though it is obvious that use of very long delay times in time-resolved fluorometry involves a trade-off of sensitivity for resolution. Time resolution can be especially useful for identification of minor sample constituents. For example, Figure 4 compares steady-state and time-resolved MI fluorescence spectra of BaP in the presence of a large excess of BkF. While the steady-state spectrum provides only a vague indication of the presence of BaP, the time-resolution data provide strong
confirmatory evidence for the presence of (carcinogenic) BaP in this mixture. In cases of this type, the fluorescence decay time serves as an additional parameter useful for identification of minor sample constituents in complex samples and thus serves to enhance the utility of MI fluorescence as a “fingerprinting” technique. Quantitation of BaP in the presence of 1 pg BkF is compared for steady-state and time-resolved MI fluorometry in Figure 5. In both cases, measurements were made a t 4006 A, using benzo[b]fluorene (BbF) as internal standard ( 2 ) . Fluorescence spectra for all samples and a “blank” (containing
ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
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LITERATURE CITED
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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.
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Flgure 5. Analytical calibration curves (A = 4006 A) for benzo[a]pyrene in the presence o f 1 p g benzo[ k ] fluoranthene, by time-resolved (A) and steady-state (0)MI fluorometry. In all cases, benzo[ b l f l u o r e n e (BbF) was used a s 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
(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 ) . T h e 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
