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Anal. Chem. 1988,60,1362-1365
Supersonic Jet/Synchronous Scan Luminescence Spectrometry Totaro Imasaka, Atsushi Tsukamoto, and Nobuhiko Ishibashi* Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka 812, J a p a n
Supersonic jet spectrometry is combined with synchronous scan lumlnescenoe spectrometry; the fluorescence wavelength Is synchronously scanned wlth the excltatlon wavelength. When the wavelength difference Is adjusted to 0 nm, a single component gives a slngle peak in most cases, like chromatography. The resoivlng power, the ratlo of the dynamic range and the line width, Is 2.5 X lo4, whlch corresponds to chromatography having a theoretical plate of 10". Sample Introduction from a solution is useful for quantltatlve analysls. The detectlon limit for anthracene is 7 X lo-' M. I t takes 10 min for recording a spectrum and 5 mln for sampie replacement. Thls spectrornetrlc technique is advantageous for construction of the data base In supersonic jet spectrometry. The seiectlvlty and sens#lvny can be improved by dlfferentiatlng the Wavelengths for excltatlon and fluorescence detection. The detection iimlt achieved is 3 X lo-' M in this method.
Supersonic jet expansion cools a sample molecule to several K, so that it gives a very sharp line structure in the spectrum. It greatly improves selectivity in chemical analysis and allows reliable identification of the molecule (1-3). Supersonic jet (SSJ) spectrometry is a flowing analytical technique, and therefore it can be directly coupled with chromatography (4-1 1). This hyphenated technique provides ultrahigh selectivity, which is sometimes required for analysis of the sample containing many isomers. However, SSJ spectrometry has inevitable disadvantages, some of which are ascribed to the characteristics introduced as advantages described above. There are vibrational levels in the excited and ground states, so that many lines are observed in the excitation and fluorescence spectra. The spectrum consisting of many sharp lines allows reliable identification of the sample molecule. But, it makes the spectrum more complicated, and the assignment of the chemical species becomes rather difficult for a mixture sample containing many components. In fact, we sometimes suffer from even counting the number of components observed. On the other hand chromatography gives a single peak for a single component, so that the assignment is more straightforward though the retention time changes depending on the experimental conditions used. For quantitative analysis, SSJ spectrometry should be combined with chromatography. I t is mainly due to the difficulty in controlling the partial vapor pressure to be proportional to the sample concentration inside the nozzle. In the application to the chromatograph detector, the knowledge for the spectral parameter is necessary before measurements. It is originating from the fact that the wavelengths for the dye laser and the fluorescence monochromator cannot be scanned in a short period as eluting the sample. Therefore, a data base is strongly required in SSJ spectrometry. However, the number of aromatic molecules reported is only 100-200, which is much smaller than a value of 10000 for infrared absorption or mass spectrometry. Then, many standard samples should be measured for construction
of the data base. Unfortunately, SSJ spectrometry has several difficulties. The spectral lines are so sharp that it takes time to determine the spectral parameters for excitation and fluorescence detection. Furthermore, it is also time-consuming to replace the sample in SSJ spectrometry. Therefore, development of a new spectrometric technique is necessary to overcome these problems. In this study we report SSJ spectrometry based on synchronous scan luminescence (SSL) spectrometry. This is simply a combination of two spectrometric methods, but it provides us a quite versatile analytical tool in chemical analysis, which has never been achieved by conventional spectrometric and chromatographic techniques.
EXPERIMENTAL SECTION Principle. The approach of SSJ/SSL spectrometry is schematically shown in Figure 1. In the SSL spectrum measured at room temperature, a broad single peak or several peaks are observed, depending on the spectral shape in the excitation and fluorescence spectra. In this cme the wavelength difference should be optimized experimentally. SSL spectrometry gives slightly narrower and simpler spectra in comparison with conventional fluorometry. By rotational cooling the SSJ spectrum gives several sharp lines in the excitation and fluorescence spectra. On the other hand, SSJ/SSL spectrometry gives only a single peak for a single component, when the wavelength difference is adjusted to 0 nm. In this scheme resonance fluorescence from high vibrational levels in the excited state is assumed to be weak. This assumption is correct in most cases especially for a large molecule though there are some exceptions (12, 13). The peak position corresponds to pure electronic transition (0-0 transition), which probably is the most important parameter in SSJ spectrometry. It gives a very simple spectrum, and optimization of the experimental condition is unnecessary. Then, it may be very useful for analysis of a mixture sample containing many chemical species. The spectral line width is determined by a spectral resolution of the dye laser or the rotational envelope determined by the jet temperature. The fluorescence monochromator is used for isolation of the peak for the 0-0 transition, so that it improves selectivity in spectrometry. It is noted that the observed line width in the SSJ/SSL spectrum is not affected by a resolution of the fluorescence monochromator. When further spectral selectivity is necessary, it may readily be achieved by differentiating the wavelengths for excitation and fluorescence detection, though the wavelength difference should be optimized experimentally for a specified molecule. Apparatus. A block diagram of the experimental apparatus is shown in Figure 2. The solid sample is placed in the reservoir, which is evaporated by raising the temperature to 150 "C. For a solution sample a pump for high-performance liquid chromatography (HPLC) (Shimadzu, LC-5A) is used, the flow rate being adjusted to 5-25 wL/min. The sample solution is introduced through a stainless steel tube (0.026 mm i.d., 0.51 mm 0.d.) into the reservoir heated to 200 "C. The top of the tube is restricted to maintain a high pressure (40 atm) to prevent boiling of the solution, which is necessary to reduce sample deposition in the tube. The concentration of the sample is typically adjusted to M. The vaporized sample is introduced into a pulsed supersonic jet nozzle (5), which is operated at 20 Hz. The sample is expanded into a 6-in. vacuum chamber, which is evacuated by a 4-in. oil ejector pump (Ulvac, PBL-04, 14 m3/min) equipped with a water baffle (Ulvac,BW-04B). It is further evacuated by
0003-2700/88/0380-1362$01.50/0 @ 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 14, JULY 15, 1988
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I
Anthracene
2-Ethylanthrecene
0-0 Trknsilion (A)
Conventional Spectrum
(c) SSJ Spectrum
i. I '
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SSL Spectrum
(0)SSL-SSJ
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Figure 1. Principle of supersonic jet spectrometry combined with
synchronous scan luminescence spectrometry.
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U
Reservoir
/I\ sample
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Wavelength /nm Figure 3. Supersonic jethynchronous scan luminescence spectrum for mixture of polycyclic aromatic hydrocarbons volatilized from solid sample.
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Mirror
n
305 Excitation Wavelength /nm
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Figure 4. Improvement of spectral selectivity by differentiating the Wavelengths for excitation and fluorescence detection.
Figure 2. Experimental apparatus for supersonic jet spectrometry.
a mechanical booster pump (Ulvac,PMB-OOlB, 1800 L/min) and a rotary pump (Ulvac, D-330,300 L/min). The pressure in the chamber is measured by a pirani gauge (Ulvac, GP-02) and it is maintained below 0.003 Torr when the nozzle is operated. This chamber is separated by a skimmer from another chamber evacuated to a lower pressure for multiphoton ionization spectrometry, though it is not used in this study. An excimer-laser-pumped dye laser (Lambda Physik, EMG102MSC, FL2002) excites the sample molecule in a supersonic jet at 10 mm away from the nozzle. The two pairs of three baffles are placed below and above the excitation region to reduce light scattering from the windows. Fluorescence is focused onto the slit of a monochromator (Jasco, CT-4OD) equipped with a photomultiplier (Hamamatsu, R931A). The signal was measured by a boxcar integrator (NF Circuit Design Block, BX530A). The scanning speed of the laser wavelength is synchronized by a homemade pulse generator to that of the fluorescence monochromator. The resolution of the monochromator was adjusted to 1-1.5 nm. Reagents. Polycyclic aromatic hydrocarbons were obtained from Tokyo Kasei (anthracene), Aldrich (2-methylanthracene, 2-ethylanthracene, 1-chloroanthracene, 9-chloroanthracene), Nakarai (9-methylanthracene), and Wako (pyrene). The-laser dyes of phenylbiphenylyl-l,3,4-oxadiazole(PBD) and 4,4'-bis(butylocty1oxy)-p-quaterphenyl (BBQ) from Nakarai were dissolved in 1,4dioxanefrom Kishida The carrier gas used was argon from Iwatani. The carrier, methanol, was purchased from Kishida and used without further purification.
RESULTS AND DISCUSSION SSJ/SSL Spectrum. The SSJ/SSL spectrum for a mixture of seven polycyclic aromatic hydrocarbons (PAHs) is shown in Figure 3. Very sharp lines indicate sufficient rotational cooling by supersonic jet expansion. One may notice that a single component gives a single peak, other unassigned small peaks in the spectra are reproducible and may probably
be due to resonance fluorescence from higher vibrational levels in the excited state. The signal intensity of the peak is proportional to (1) absorptivity, (2) fluorescence quantum yield, (3) partial vapor pressure at the specified temperature, and (4)the Franck-Condon factor for the 0-0 transition. The last factor is strongly affected by configuration of the molecule in the excited state. When the molecule is twisted in the excited state, overlap of wave functions for the excited and ground states may be small. It decreases a Franck-Condon factor and gives a very small signal. SSJ/SSL spectrometry is unsuitable for sensitive detection of such molecules. The SSJ/SSL technique requires no spectral data before the sample measurement, since the wavelength difference between the excitation and fluorescence wavelengths is always adjusted to 0 nm. On the other hand, the wavelength difference should be optimized in conventional SSL spectrometry. In SSJ spectrometry optimization of the fluorescence and excitation wavelengths is a more serious problem. It is emphasized that the SSJ/SSL technique not only simplifies the spectrum but also simplifies the analytical procedure. Therefore, it may allow a direct measurement of a mixture sample containing many unknown chemical species. Improvement of Selectivity. By differentiation of the wavelengths for excitation and fluorescence detection, it is possible to improve spectral selectivity. Figure 4 shows a SSJ/SSL spectrum for a mixture of two PAHs. These compounds have intense fluorescence bands at 24.3 nm away from the 0-0 bands, then both components are observed in the spectrum. On the other hand, 9,lO-dimethylanthracene has a strong fluorescence band at 5.7 nm away from the 0-0 band, but no band for 9,lO-dichloroanthracene.Then, the latter component has not been observed in the lower spectrum. This spectral selectivity may give an additional advantage. Even when two or three compounds have almost identical wave-
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ANALYTICAL CHEMISTRY, VOL. 60,
NO. 14, JULY 15, 1988
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2
Anthracene
$
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P
2-Melhylanlhracene l', ~
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2-Ethylanlhracene
365
1
9-Melhylanthracens I
",
9-Chloroanlhraceoe
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370
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Wavelength lnm
Figure 6. Supersonic jet/synchronous scan luminescence spectrum for mixture of polycyclic aromatic hydrocarbons volatilized from the solution sample.
I 380
385
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Excitation Wavelength lnm
Figure 5. Effect of temperature on the spectrum.
lengths for 0-0 transitions, they are distinguished by differentiating the excitation and fluorescence wavelengths. It is noted that this selectivity can be improved with a spectral resolution of the fluorescence monochromator. This approach is also useful for determination of a specific trace component in the presence of a large quantity of diverse molecules. However, the optimum wavelength difference should be determined experimentally before the measurement, as in the case of conventional SSL spectrometry. It makes it difficult to apply this method to unknown chemical species. Temperature Dependence. When the sample is directly volatilized from a solid phase, the signal intensity changes depending on the partial vapor pressure of the sample in the stream. Figure 5 shows the spectrum measured at 125 "C and 170 "C. The peak for 9,lO-dimethylanthracene is appreciable in the spectrum measured a t 170 O C , but it is not observed in the spectrum at 125 O C . Then, the temperature should be carefully controlled for quantitative analysis, but it is practically difficult. An alternative approach for quantitative analysis might be the use of a deuteriated compound as a standard (14). But, it is also impractical, since it takes a long time for synthesis of such compounds. Therefore, development of some analytical technique is necessary for quantitation, as described below. Solution Sample. For quantitative analysis, a solution sample may be continuously volatilized in the reservoir. The sample and solvent molecules can be immediately and completely vaporized in the reservoir by sufficient heating, so that the concentration of the sample in the stream might be maintained to a constant value, as long as the flow rate of the pump is kept constant. The spectrum for a mixture of seven PAHs dissolved in methanol is shown in Figure 6. Sharp spectral peaks are observed, which is due to sufficient mixing with argon as an expansion gas. It is noted that more broader lines are sometimes observed when the flow rates of methanol and argon are not carefully optimized. The broad background signal is originating from light scattering from the laser; the spectral distribution was exactly identical with that of the laser intensity. This background is difficult to be removed at present, since the wavelength for the fluorescence monochromator is adjusted to the same wavelength for the exciting laser. Time-resolved fluorometry may be useful for reduction of this scattered emission, though it requires fast-response instruments for signal measurements. Detection Limit. An analytical curve was constructed for M range, anthracene. I t was straight in the 0 to 1.5 X the detection limit being 7 x 10" M. The detection limit was determined by light scattering of the exciting laser. Further
improvement in the detection limit might be achieved by differentiating the wavelengths for excitation and fluorescence detection. The analytical curve was also constructed in the 0 to 2.5 X lo4 M range by adjusting the wavelength difference to 19.3 nm. The detection limit could be improved to 3 X lo4 M. It is apparently due to rejection of background light scattering from the exciting laser. In this case the detection limit is determined by the intensity of the laser and a shot noise of the photomultiplier. Further improvement may then be achieved by using the laser with a larger output power, since no saturation effect is observed in the fluorescence signal. Construction of the Data Base. SSJ spectrometry has a great performance in sample identification, but it gives no direct information such as a molecular weight as in the case of mass spectrometry. Therefore, a data base is essential for assignment of the chemical species in SSJ spectrometry. However, the experimental work for construction of the data base may have several difficulties. For example, it is not straightforward to optimize the experimental conditions for measurements of the SSJ spectrum because of sharp spectral features. For recording the excitation spectrum, a knowledge of the fluorescence spectrum is required to adjust the monochromator wavelength to one of the specified peaks. But, the excitation spectrum should be known in advance for recording the fluorescence spectrum. The line widths are so sharp that it is time-consuming for a trial-and-error approach to determine both parameters simultaneously. The rapid sample replacement is another practical but serious problem for construction of the data base. A powder sample is usually placed in the supersonic jet nozzle, before it is attached to the vacuum chamber. Then it takes more than 1 h for cleaning the nozzle and for sample replacement. I t prevents the measurement of many samples in a short period. In SSJ/SSL spectrometry demonstrated in the present study, the time required for the measurement of a sample is 15 min: 10 min for recording the spectrum and 5 min for cleaning the stream line and the nozzle and for replacing the sample. It may then be very suitable for construction of the data base. Comparison with Chromatography. The SSJ/SSL technique gives a single peak for a single component in most cases, like chromatography. Both techniques give a narrow signal peak under a wide dynamic range, so that many sample components can be determined simultaneously. This is one of the reasons that chromatography is so useful for analysis of the sample containing many chemical species. This advantage is apparently coming from a large ratio of the dynamic range and the line width, namely, a high resolving power. The SSJ/SSL technique provides us a resolving power of 2.5 X lo4,by assuming a spectral line width of 0.01 nm. The number of theoretical plates in high-resolution capillary gas chromatography is IO6 in the best case, corresponding a resolving power of 250. This fact means that the SSJ/SSL technique has a 100 times better resolving power and corresponds to a chromatograph technique having a theoretical plate of lo1'. Such an ultrahigh resolution has never been achieved in chromatography and will not probably be achieved even in
Anal. Chem. lS88, 60, 1365-1369
the future. Therefore, the SSJ/SSL technique has definitely better selectivity in comparison with chromatography. This SSJ/SSL technique has several additional advantages. The observed wavelength is a spectral parameter, so that it is independent of the experimental conditions used. While, the retention time in chromatography is frequently changed by the conditions, for example, by a separation column, the temperature of an oven, the flow rate of a carrier, etc., the experimental condition should strictly be kept constant in chromatography. The time for the measurement is similar or slightly shorter than chromatography, since the time for high-resolution chromatography sometimes requires more than 1 h. It is worth mentioning that the time for recording the SSJ/SSL spectrum can be shortened to a second by increasing the scanning speed of the dye laser wavelength in the future. The detection sensitivity of SSJ/SSL spectrometry in the concentration unit is comparable to chromatography. SSJ/ SSL spectrometry has, of course, some disadvantages. The amount of the sample required for the measurement is 200 pL, so that the detection limit in the amount unit (100 ng) may be poorer than a picogram detection limit for capillary gas chromatography. The chromatograph technique can be combined with the other spectrometric method such as mass spectrometry, which is quite useful for assignment of the sample molecule. A similar hyphenated technique is also possible in SSJ/SSL spectrometry. The use of a spectrometric multichannel analyzer allows the repetitive measurements of the fluorescence spectrum (11). It is useful for sample identification from the spectral feature. A combination with multiphoton ionization/mass spectrometry gives us additional information con-
1365
cerning molecular weight and chemical structure of the molecule (3,6). If a rapid wavelength scan becomes possible in the future, this SSJ/SSL spectrometry might be combined even with chromatography. This approach may provide us much greater selectivity in chemical analysis. Registry No. Anthracene, 120-12-7; 2-methylanthracene, 613-12-7; 2-ethylanthracene, 52251-71-5; 1-chloroanthracene, 4985-70-0; pyrene, 129-00-0; 9-methylanthracene, 779-02-2; 9chloroanthracene, 716-53-0; 9,10-dimethylanthracene,781-43-1; 9,10-dichloroanthracene, 605-48-1.
LITERATURE CITED Hayes, J. M.; Small, G. J. Anal. Chem. 1983, 55,565A. Johnston, M. V. TrAC Trends Anal. Chem. 1984, 3,58. Lubman, D. M. Anal. Chem. 1987, 59,31A. Imasaka, T.; Shigezumi, T.; Ishibashi, N. Ana/yst (London) 1984, 109, 277.
Imasaka, T.; Okamura, T.; Ishibashi, N. Anal. Chem. 1986, 58, 2152.
Imasaka, T.; Tashiro, K.; Ishibashi, N. Anal. Chem. 1986, 58, 3242. Hayes, J. M.; Small, G. J. Anal. Chem. 1982, 54, 1202. Pepich, B. V.; Callis, J. B.; Danielson, J. D. S.; Gouterman, M. Rev. Sci. Instrum. 1988, 57,878. Pepich, B. V.; Callis, J. B.; Burnes, D. H.; Gouterman, M.; Kalman, D. A. Anal. Chem. 1986, 58, 2825. Stiller, S. W.; Johnston, M. V. Anal. Chem. 1987, 59,567. Imasaka, T.; Tanaka, K.; Ishibashl, N. Anal. Sci. 1988, 4 , 31. Beck, S. M.; Powers, D. E.; Hopkins, J. B.; Smalley, R. E. J. Chem. Phys. 1981, 73,2019. Beck, S. M.; Hopkins, J. B.; Powers, D. E.; Smalley, R..E. J. Chem. Phys. 1981, 74,43. Yang, Y.; D’Silva, A. P.; Fassei, V. A. Anal. Cham. 1981, 53,2107.
RECEIVED for review November 7,1987. Accepted March 3, 1988. This research is supported by Grant-in-Aid for Scientific Research from the Ministry of Education of Japan and from the Nissan Foundation.
Evaluation of a Supercritical Fluid Chromatograph Coupled to a Surface-Wave-Sustained Microwave-Induced Plasma Detector Debra R. Luffer, Leonard J. Galante,’ Paul A. David, Milos Novotny,* and Gary M. Hieftje
Department of Chemistry, Indiana University, Bloomington, Indiana 47405
A capillary supercritical fluid chromatograph (SFC) was coupled to a surface-wave-sustained microwave-induced plasma (MIP) sustained wlth a surfatron. The chromatographic system, interface, and plasma source are described. The plasma was optlmlzed for sulfur emisdon at 921.3 nm and used to detect a mixture of sulfur-containing polycyciics that had been separated by SFC. The linear dynamlc range for these compounds is 3 orders of magnitude with detection iimlts of 25 pg/s sulfur for thlophene. The relative standard deviations for repetltlve injections are typlcaiiy I-5% at concentrations well above the detection limit.
Capillary supercritical fluid chromatography (SFC) has been recognized as the separation method of choice for compounds that are not easily amenable to either gas or liquid chromatographic analysis (2-3). The former is ultimately limited by ICurrent address: Glaxo, Inc., Crown I1 Bldg., 1035 Swabiz Ct., Morrisville, NC 27560. 0003-2700/88/0360-1385$01 S O / O
the involatility of large compounds or thermal lability a t the high temperatures required in gas chromatography (GC). The latter suffers from a lack of element-selective detection methods (4)due to solvent interferences. A supercritical fluid mobile phase has been able to overcome some of these limitations because. it can operate optimally at significantly lower temperatures than GC, its viscosity and solute diffusivities are between liquid and gaseous phases, and its solvating power approaches that of a liquid ( 5 ) . In addition, the column effluent at atmospheric pressure is compatible with many GC detectors that offer sensitive and selective detection. It is for this reason that many detection schemes are being borrowed from GC and adapted to SFC. Some examples of this include SFC/MS (6,7), SFC/FTIR (8),and SFC/FID (9-12), as well as thermionic (12), dual flame photometric (13), and ion mobility (14). Of the many varied detection methods for chromatography that have been investigated, those based on atomic spectroscopy have become increasingly attractive due to their inherent selectivity, freedom from interferences, and multielement detection capability (15). In particular, plasma 0 1988 American Chemical Society
