Anal. Chem. 1989, 61. 797-800
797
and visible radiation a t -2 ps is a bulk pyroelectric effect resulting from eventual heating of the entire element. In the case of UV excitation a different mechanism is operating. The transient pyroelectric signal is completely absent and the bulk pyroelectric effect is delayed substantially. Crystalline PLZT is known to absorb strongly below 500 nm (9). The absorbed energy is possibly stored in a long-lived electronic state, the relaxation of which produces the delayed pyroelectric signal (10). The identity of this electronic state, which may be due to an impurity in the ceramic (9), requires further investigation.
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ACKNOWLEDGMENT We wish to thank Sivaram Arepalli and Daniel Robie for their assistance with some of the measurements.
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LITERATURE CITED
a W
m
(1) Herbert, J. M. Ferroelectric Transducers and Sensors; Gordon and UY
0
R(mm)
Flgure 1. Profile of COP laser beam measured by scanning with a masked PZT detector. The curve is a Gaussian function.
approximately to that of the laser pulse, whereas for dye laser radiation (25 ns) it is several times longer than the pulse rise time and is independent of laser spot size. The PZE oscillations that follow the transient pyroelectric signal are due to strain developing from the heating of the element. This combination of pyroelectric followed by piezoelectric response has also been seen by Glass and Abrams (6) and by Simhony and Bass (7, 8 ) in the case of COz irradiation of very thin ferroelectric crystals. The broad maximum observed for IR
Breach: London, 1982. (2) Hodni, A. Pyroeiectrocity and Pyroelectric Detectors. I n Infrared and Milimeter Waves; Academic Press: New York. 1980; Vol. 3. (3) Optical and Infrared Detectors. Topics in Applied Physics; Keyes, R. J., Ed.; Springer-Veriag: Berlin, 1980; Voi. 19. (4) Jaffe, B.; Cook, W. R., Jr.; Jaffer. H. Piezoelectric Ceramics; Academ ic Press: London, 1971. (5) Nowak, R.; Miniewicz, A.; Samoc, M.; Sworakowski, J. Ferroelectrics 1983, 4 8 , 225. (6) Glass, A. M.; Abrams, R. L. J. Appl. Phys. 1970, 4 1 , 4455. (7) Simhony, M.; Bass, M. Appl. Phys. Lett. 1979, 3 4 , 426. ( 8 ) Simhony, M. Ferroelectrics 1980, 2 8 , 373. (9) Haertiing, G. H. J. Am. Ceram. SOC. 1971, 5 4 , 303. (10) Merkie, L. D.; Poweii, R. C.; Wilson, T. M. J. Phys. C 1978, 7 1 , 3103.
RECEIVED for review October 17, 1988. Accepted December 27, 1988. This research was supported by the Department of Energy under Grant No. FG02-88CH13827. The lasers used in this study were purchased with instrumentation grants from the National Science Foundation (CHE84-08342) and the Department of Energy (DE-FG05-84ER75155).
Chemiluminescence Sulfur Detection in Capillary Supercritical Fluid Chromatography Darryl J. Bornhop* and Brett J. Murphy
Lee Scientific, Znc., 4426 South Century Drive, Salt Lake City, Utah 84123-2513 Lisa Krieger-Jones
Hauser Laboratories, 5680 Central Avenue, Boulder, Colorado 80301 The need for selective detectors in supercritical fluid chromatography (SFC) is particularly evident for the sensitive analysis of sulfur-containing species. These sulfur species are of interest to the environmental community, for the analysis of pesticides ( I ) and atmospheric pollutants (2),as well as to the petroleum industry (3). Various detectors have been employed for the analysis of sulfur. Among these are the Hall electrolytic conductivity detector (HECD) (4-6), a microwave-induced plasma detector (7),a helium after glow plasma detector (B), a nonflame source induced S z fluorescence detector (9),and a dc plasma detector (10). The most popular sulfur-selective detector for gas chromatography (GC) is the flame photometric detector (FPD) (11)which has also been applied to capillary SFC (12). When used as an SFC detector, the FPD suffers from several limitations. Among these limitations is a signal that has a squared dependence on sulfur concentration (as in GC). While this nonlinear response can be electronically compensated, the ideal analytical instrument provides linear calibration. Another major drawback in ap-
plication of the FPD to SFC is a limited dynamic range. Poor sensitivity (detection limits of about 25 ng on-column), a severe response dependence on pressure, and a significant base-line shift during pressure programming are also among the SFCFPD limitations that had been initially reported (12). It should be pointed out that the FPD-SFC results were found during a preliminary study and that further optimization has produced detection limits (13) approaching those for an FPD-GC system. While optimization of the FPD has produced improved detection limits, the detector still suffers from the disadvantage of a squared dependent response and a base-line perturbation with pressure programming. Multidetector compatibility is a major advantage of SFC over either GC or high-pressure liquid chromatography (HPLC). Fluid phase optical detectors such as the ultraviolet-visible (UV-vis) spectrophotometric detector and the fluorescence detector have been employed successfully in capillary SFC (14,15). While gas-phase detectors such as the flame ionization detector (FID) (16) and the dual flame
0003-2700/89/0361-0797$01.50/00 1989 American Chemical Society
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inside the oven with a zero dead volume union (Scientific Glass Engineering, Austin, TX) to maintain the necessary pressure in the separation system. The restrictor was positioned in the transfer line to the SCD such that the outlet end was approximately 1 mm inside of the chemiluminescencereaction chamber. The fluorine-induced sulfur selective chemiluminescencedetector (SieversResearch, Boulder, CO) is comprised of a stainless steel reaction chamber equipped with four ports, a photomultiplier tube and associated electronics, a high-voltage discharge power supply, and a Edwards vacuum pump. The reaction chamber was operated at a base pressure of 0.2 Torr. F2reagent gas results from a high-voltage electrical discharge of the reagent grade SF6 gas. A 6 in. long heated transfer line was used to maintain a constant temperature between the oven and the detector reaction chamber. This transfer line also provides the necessary heat preventing the frit restrictor from damage due to the coolinginduced thermal stresses during supercritical CO, decompression. Control of the transfer line heat tape is accomplished through the use of three thermocouples and the necessary feedback circuitry which has been incorporated into the SCD. Reagents. All chemicals are reagent grade or of high quality. Hexane was used as the solvent for the thiol calibration solutions. Stock solutions of isopropylthiol and dodecanethiol (Sigma Chemical, St. Louis, MO) at 1.63 and 1.21 mg/mL in hexane were used. A number of dilutions were prepared to construct calibration curves. A mixture of ethanethiol, isopropylthiol, allyl sulfide, benzenethiol, and dodecanethiol was used as a test mixture for chromatographic and detector performance. Toluene and hexane were used for the determination of selectivity for sulfur. Chromatographic Conditions. SFC grade COz (Scott Speciality Gases, Plumsteadville,PA) was the chromatographicmobile phase. All analyses were performed under split flow conditions with split ratios of about 15:l. Linear pressure or density programming was employed for the determination of dodecanethiol detection limits and for the separation of all multicomponent mixtures. Isopropylthiol was chromatographed at a constant pressure of 80 atm. In all cases, the oven temperature was held constant throughout the run. A splitter device (22) (Lee Scientific) was used for simultaneous detection with the FID and the SCD. This simultaneous detection approach permits the direct comparison between the universal and selective detectors.
u-----1 I I
I
r
I
Supercritical fluid chromatographic system with sulfur chemiluminescence detector interfaced. Electronic connections are designated with a dashed line, while fiuid connections are solid lines. SCD is the Severs chemiluminescencedetector. SFC pump and oven are controlled through the pc computer based controller. The analog output from the SCD is unconditioned and sent directly to a strip chart recorder or integrator. The column is 50 pm i.d. fused silica capillary column. Figure 1.
photometric (FPD) (11)are also amenable to SFC, Later et al. (17) have illustrated that various detectors, including mass spectrometry (MS) and Fourier transform infrared spectrometry (FTIR), can be used effectively with SFC. Among the attributes of SFC are the ability to efficiently separate and quantify thermally labile (18) and relatively high molecular weight molecules. The addition of a sensitive and selective sulfur detector to the repertoire of currently available SFC detectors further advances the usefulness of SFC. The chemiluminescence sulfur detector (SCD) (19,20) has been successfullly interfaced with a gas chromatograph and with a high-performance liquid chromatograph, but until this report, it has not been used in capillary SFC. The SCD is based on the detection of the chemiluminescent radiation resulting from the reaction of fluorine (F,) gas with a sulfur atom contained in the solute. The specific reaction and its dynamics have been reported by Birks and co-workers (21) who demonstrated that the H F vibrational overtone emission dominates the chemiluminescence signal produced and that the signal is dependent on the specific reaction conditions. This rich chemiluminescence produced by the F, reaction is well suited to the sensitive and selective detection of chromatographic solute species. Due to the nature of the chemiluminescence phenomenon, the detection sensitivity can be quite high. We describe the use of this SCD for the analysis of sulfur-containing organic analytes as separated by supercritical fluid chromatography. Detection limits of 35 and 115 pg of isopropylthiol and dodecanethiol, respectively, have been obtained for the SFC-SCD system with the linear dynamic range of about lo3. Sulfur detectability appears to be independent of chromatographic pressure and the utility of the system is illustrated by the presentation of several real world analyses.
EXPERIMENTAL SECTION Instrumentation. A schematic diagram of the apparatus is provided in Figure 1. The supercritical fluid chromatograph (Model 602, Lees Scientific, Inc., Salt Lake City, UT) was equipped with a 500 nL volume injector valve (Rhecdyne, Cotati, CA) and a commercial flame ionization detector (Lee Scientific). The SFC oven module was elevated approximately 4 in. to allow the transfer line from the SCD to enter the side of the oven. Supercritical fluid chromatography was performed on either a 10- or 3-m SB-Methyl-50or SB-Methyl-100capillary column (Lee Scientific). A ceramic frit restrictor (Lee Scientific)was connected
RESULTS AND DISCUSSION The design of the detector interface is critical in order to achieve optimum system performance. The simple interface between the SFC and the SCD is accomplished by using a conventional length (25 cm) frit restrictor which is connected to the capillary SFC separation column via a zero dead volume butt connector. The frit is threaded through the temperature-controlled transfer line such that the entire separation column is still inside the SFC oven. Control of the transfer line temperature is provided through the SCD. A transfer line temperature greater than 200 "C was necessary to avoid loss of the ceramic frit material responsible for the required pressure restriction for SFC. In the described experiments, the chromatographic performance appears to be unaffected by high transfer line temperatures. Thermally labile species for which SFC is so ideally suited were not investigated; however, a simple modification of the transfer line, which involves heating only a small portion of the frit (ca.2 cm long) at the end of the transfer line, is currently under investigation. This approach to a heated transfer line would minimize the thermal stress problems associated with the frit during decompression to ensure the integrity of the ceramic frit material, while allowing the chromatographic performance to be optimized. It would also minimize any possible degradation of labile components in the transfer line. A comment is in order on the simplicity of the connection for the SFC column to the SCD. The two major criteria for an effective transfer line are that (1) the frit restrictor be positioned in the reaction chamber such that only 1 mm of fused silica protrudes into the chamber and (2) a pressure-tight connection is made at the entrance point of the transfer inlet
ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989
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Table I"
solute
inj mass DL, pg
S mass DL,b pg
amt of S per 9,
PPI s
isopropylthiol 35 15 0.71 dodecanethiol 115 18 0.75 'Detection limits calculated at 3u. All injections were split flow injections and results have included loss due to sample split. *Calculatedat 43 % S and 15.8% S in solute for isopropylthiol and dodecanethiol, respectively. (easily made with a graphite-Vespel ferrule). Calibration curves were constructed from standard solutions of isopropylthiol and dodecanethiol. Isopropylthiol was chromatographed a t constant pressure and temperature, but it was necessary to employ pressure programming at constant temperature to elute dodecanethiol. Calibration curves for each solute were linear with correlation coefficients of 0.998 for isopropylthiol and 0.997 for dodecanethiol. Linear calibration extends over 3 orders of magnitude in concentration from the detection limit (ca. 5 X mg/mL) to greater that 1.5 mg/mL. Detection limits in SFC were determined a t 3a values for two solutes. The solute mass and sulfur mass detection limits are summarized in Table I. Mass detection limits in the range of 15 to 18 pg of sulfur are about 3 orders of magnitude better than those reported with use of an FPD as a sulfur-selective detection scheme in capillary SFC (12) and compare well with those obtained in GC-SCD (20). Superior mass detection limits are a result of both the performance of the SCD and the inherently narrower peaks seen when a capillary-based separation system is employed. The linear response of the instrument is particularly attractive since it provides assurance in quantitative analysis, unlike the second-order dependence of the FPD (12). A further advantage of the SCD for SFC is that the response appears to be independent of pressure. Inspection of Table I illustrates this pressure independence. Upon normalization of the response of the SCD by the percentage of elemental sulfur in the solute, and to allow for differences in peak width (solute dilution), the detection limits for isopropylthiol and dodecanethiol are 0.71 and 0.75 pg of S/s, respectively. These two numbers are identical within the experimental error of the method. The normalized detection limit of 0.71 pg/s of sulfur for the isopropylthiol analysis was performed at a constant pressure of 60 atm, while the normalized dodecanethiol detection limit of 0.75 pg of S/s was obtained during pressure programming with the solute elution pressure of about 163 atm. Selectivity is a key issue for selective detection methods. Various definitions can be used to represent the selectivity of a method. We utilize the selectivity ratio (SR) (20) as defined in eq 1. Each of the two solutes, isopropylthiol and SR = (signal/mole of S cmpd)/(signal/mole of interfering cmpd) (1) dodecanethiol, were compared to toluene and hexane to determine the selectivity ratio in SCD-SFC. Values of 5.5 X lo4 for hexane and 8327 for toluene (each relative to isopropylthiol) were obtained. The selectivity of dodecanethiol versus toluene and hexane was determined to be 7950 and 3.8 X lo4, respectively. Comparison of the values obtained in SFC-SCD with selectivity ratios found in GC-SCD (20) and HPLC-SCD (19) indicates that SR values for SFC should fall between the two related chromatographic techniques. SCD with SFC is more selective than SCD with HPLC, yet is less selective than SCD detection in GC. Currently, we are determining SR values of sulfur-containing species compared to nitrogen-containing and phosphorus-containing compounds as interferences.
/I/I
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15
Figure 2. SFC-SCD chromatogram in order of elution of ethanethiol, isoproytthiol, allyl sulfide, benzenethiol, dodecanethiol. Chromatographic conditions were pressure program from 70 to 415 atm at 10 atm/min. SCD integration was set at 0.25 s. An oven temperature of 45 OC was held constant and CO, was the mobile phase employed. The injection split was set at a flow ratio of 151. The column used was a 3 m long, 50 pm i.d. SB-Methyl-100 fused silica capillary column.
FID
0
15 TIME(rnin)
3b
Flgure 3. SFC-SCD chromatogram of a polysulfide performed at a constant temperature of 80 OC. Density programming from 0.2 to 0.74 g/mL ramped linearly at 0.005 (g/mL)/min separated the mixture. The SCD integration time was set at 1.0 s. A 3 m X 50 pm i.d. MethyClOO capillary column was employed. Sample concentration was 15 mg/mL. The inset chromatogram was run under the same conditions, but detection was provided by an FID.
In the application of any detector to a separation technique chromatographic performance is a primary criteria. Additionally, an applications base proving the utility of the method is vital. Figure 2 presents a capillary SFC chromatogram of a mixture of ethanethiol, isopropylthiol, allyl sulfide, benzenethiol, and dodecanethiol prepared in hexane. The rapid density program from 70 to 415 atm contributes to the narrow peaks. Detector selectivity is evident from the lack of a detected solvent peak. The absence of a significant base-line shift due to pressure programming is also seen and is further illustrated in Figures 3 and 4. A polysulfide sample was chromatographed on the capillary SFC-SCD system and is shown in Figure 3. An inset found in the figure presents an SFC-FID trace obtained on a system configured in a similar manner. Again the absence of a solvent peak for the SCD trace while the FID trace exhibits one illustrates the type of solvent response expected with a universal detection method. Excellent chromatographic performance is achieved in the SFC-SCD system. Further
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989
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0
SCD
I
is
30 TIME!minI
45
0
15
30
TlME!m nl
Figure 4. Simultaneous SCD-FID supercritical fluid chromatogram of TMI crude oil. Oven temperature was set at 120 ‘C for the pressure program of 70 to 415 atm at 5 atrnlmin. Injection split was set at 1O:l while the detector split was set at approximately 1:l. The SCD integration time was 0.5 s and the transfer line was operated at 303 ‘C. The FID was operated at 415 ‘C and a range of 1 and attenuation of 6. Supercritical C02 was the mobile phase.
seen in Figure 3 is the polymeric distribution for sulfur-containing species found in the complex mixture which is different from the compounds which respond in the universal mode (FID). The complexity of the chromatogram is greatly simplified due to the selective detection of the SCD, allowing for a greater understanding of the sample matrix. Base-line stability is maintained during the programmed run. Bornhop and co-workers (22) have shown that simultaneous selective and universal detection can be employed in capillary SFC. They have demonstrated that utilizing simultaneous detection gives added solute information. This splitter assembly was applied to simultaneous detection with FID and SCD in capillary SFC for the analysis of crude oil. The simultaneous FID and SCD chromatograms of this crude oil are shown in Figure 4. The information gained from the early part of the separation through use of the SCD is dramatic. Various low boiling, sulfur-containing compounds are present in the sample which would not be detected if only the FID were employed. An expected oligomeric hydrocarbon distribution is detected by the FID yet is ignored in the sulfur-selective SCD. There are two peaks which are detected in the SCD and FID at an elution time of about 25 min that are thought to be sulfur-containing aromatic compounds. A reduced precolumn split would allow more sample to enter each detector for an even greater information gain. One advantage of the use of a sulfur-selective detector with a universal detector is seen in this example-one chromatographic analysis yields two types of information.
CONCLUSION The chemiluminescence detector, based on the gas-phase reaction of molecular fluorine with organosulfur compounds, has proven to be a valuable tool for selective detection in capillary SFC. Much work remains with respect to understanding the reaction mechanism, sensitivity for sulfur heterocycles, and selectivity for sulfur versus various interfering species. However, it appears that the guidelines set in GC and HPLC provide a basis upon which to make reasonable predictions about the SFC-SCD performance. The detector is safe since the production of F2 gas via electrical discharge is not possible at pressures above 5 Torr. In fact, the only gas leak possible is nontoxic SF, gas. Op-
eration of the SCD is quite simple, and the interface for capillary SFC is easily done. The high efficiency of capillary chromatographic separations is maintained even when a split for simultaneous detection is employed as previously demonstrated (22). Supercritical fluid chromatographic systems are ideally suited to the use of the SCD since the most commonly used mobile phase (CO,) does not react with the Fz reagent gas. Density or pressure programming can be performed without perturbation to the base line and with reasonable confidence that the detector response function is constant. Since the SCD has been employed in HPLC using organic mobile phases and the selectivity for sulfur-containing compounds is reasonably good, the authors feel that the potential to use SCD with certain modifiers in COz (in the region of 1-15%) is excellent. Although the use of organic modifiers in COP is most often applied to packed column SFC in order to “condition the stationary phase”, Schmidt and co-workers (23)have shown that capillary SFC can also benefit from the use of modified mobile phases. We are currently investigating the potential of the SCD-SFC system for the analysis of the sulfur-containing drugs analyzed by modified COz with capillary SFC (23). ACKNOWLEDGMENT The authors thank Sievers Research for their gracious loan of the SCD and for the continued support of the interface project. Special thanks are extended to all of the scientists at Lee Scientific for helpful discussions. D.J.B., B.J.M., and L.K.J. would like to thank Dr. Douglas Later for supporting the project and Dr. Bruce Richter for help with this paper.
LITERATURE CITED Nelson, J. K. Ph.D. Dissertatlon, University of Colorado, Boulder, CO.
1984. Sulfur in the Environment Part I ; Nriagu, J. O., Ed.; Wiley: New York,
1978. Wenzel, 8. E. J. Chrometogr. Sci. 1974, 2 0 , 409-41 1. Halt, R. C. J . Chromatogr. Scl. 1982, 12, 152-160. Ehrlich, B. J.; Hall, R. C.; Cox, H. G. J. Chromatogr. Sci. 1981, 19,
245-249. Gluck, S.J. Chmatogr. Sci. 1982, 2 0 , 103-108. Genna, J. L.; McAnnich. W. D.; Reich, R. A. J. Chromatogr. 1982, 238, 103-112. Skelton, R. J., Jr.; Markldes. K. E.; Farnsworth, P. B.; Lee, M. L. HRC CC.J . High Resolut. Chromatogr. Chromatogr. Common. 1986, 1 1 ,
75-8 1. Gage, D. R.; Farwell, S. 0. Anal. Chem. 1980, 5 2 , 2422-2425. Treybg, D. R.; Ellebracht, S. R. Anal. Chem. 1080, 5 2 , 1633-1636. Burnett, C . H.;Adarns, D. F.; Farweli, S. 0. J. Chromafogr. Sc;. 1978, 16, 68-73. Markides, K. E.; Lee, E. D.; Bodlick, R.; Lee, M. L. Anal. Chem. 1986, 54 - . , 740-743 . .- . .-. Olesik, S. V.; Pekay, Lars A.; Paiiwcda, Elizabeth A. Anal. Chem. 1989, 6 1 , 58-65. Fields, S. M.; Markides, K. E.; Lee, M. L. Anal. Chem. 1988, 6 0 ,
802-806. Fjeldsted, J. C.; Richter, B. E.; Jackson, W. P.; Lee, M. L. J. Chromatogr. 1983, 279. 423-430. Richter. B. E. HRC CC, J . High Resolut. Chromatogr. Chromatogr. Commun. 1985, 8 , 297-300. Later, D. W.; Bornhop, D. J.; Lee, E. D.; Henion, J. D.; Weiboit, R. C. LCJGC 1987,5 , 804-816. White, C. M.; Houck, R. K. HRC CC J . High Resolut. Chromatogr. Chromatogr. Commun. 1985, 8 , 293. Nelson, J. K.; Getty, R. H.; Blrks. J. W. Anal. Chem. 1983, 55,
1767-1770. Mishaianie, E. A.; Birks, J. W. Anal. Chem. 1988, 54, 918-923. Glinski, R . J.; Mishaianie, E. A.; Birks, J. W. J. Photochem. 1987, 3 7 , 21 7-23 1. Bornhop, D. J.; Schmidt, S. S.; Porter, N. L. J. Chromafogr. 1089, 459, 193-200. Schmidt, S.;Blomberg, L. G.; Campbell, E. R. Chromatographis 1986, 2 5 , 75-780.
RECEIVED for review September 28,1988. Accepted December 29, 1988.
