Anal. Chem. 1989, 61,872-876
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Laser- Induced Phosphorescence Spectrometry of Halogenated Naphthalene Derivatives Prepared in Vapor-Deposited Parent Molecule Matrices Charles F. Pace' and Jon R. Maple*s2 Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131
The analytlcal merits of laser-Induced phosphorescence spectrometry for the mufflcomponent analysis of halogenated aromatk hydrocarbon derlvatlves have been assessed for a prototype system conslstlng of chloronaphthalenesand bromonaphthalenes. By preparatlon of samples In low-temperature, vapor-deposlted parent molecule matrlces and use of laser excltatbn, the high resolutlon, high selectivity, and low detection llmb that characterlre laser-Induced fluorescence spectrometry have been extended to analytical phosphorlmetry. A detection llmlt of ca. 0.8 pg for 1,Cdlchloronaphthalene Is demonstrated, and hlgh-resolution phosphorescence spectra wlth monochromator-hlted bandwldths of ca. 10 cm-' are presented, along wlth examples of analytlcal selectlvlty In the spectral and time domains.
Conventional analytical phosphorimetry offers the advantages of wide linear dynamic ranges and low detection limits ( I ) . Room-temperature phosphorescence (RTP) spectrometry, when employed with cyclodextrin-stabilized solutions (to minimize the effects of quenching), has resulted in picogram detection limits for polycyclic aromatic hydrocarbons (PAHs) such as acenaphthene, which has a detection limit of ca. 15 pg (2-6). It has also been demonstrated that solid substrates are useful for the analysis of PAHs with both room-temperature and low-temperature phosphorescence spectrometry ( I , 7-10). For example, RTP has been observed at the 200-ng level for certain phenols, hydroxynaphthalenes, and hydroxybiphenyls ( 7 ) . However, a limitation of analytical RTP spectrometry is that the phosphorescence peaks are often broad and structureless, and sample separation procedures are often required for multicomponent analysis. Fluorescence spectrometry is also recognized as an extremely sensitive tool for trace analysis. However, until the introduction of various procedures for improving spectral resolution, the applications of fluorometry in multicomponent analysis have been limited. The usage of laser excitation coupled with improved sampling methodologies (e.g., laserexcited Shpol'skii (11 , 12), fluorescence line narrowing (13, 141, or site-selection (15,16)spectrometry) has resulted in high spectral resolution, low detection limits, and capabilities for the direct analysis of complex samples, such as oil shales and coal tars. In view of the spectacular results that have been achieved with laser-induced fluorescence (LIF) spectrometry, it would be highly desirable to extend the methodology to compounds that phosphoresce, rather than fluoresce. Although the fluorescence line narrowing methodology has resulted in high-resolution fluorescence spectra, it is not well-suited for compounds that phosphoresce. With this method a monochromatic excitation source (Le., a laser) is 'Present address: Sandia National Laboratories, Organization 7241,P.O. Box 5800, Albuquerque, NM 87185. 2Present address: Biosyrn Technologies, Inc., 10065 Barnes Canyon Rd., San Diego, CA 92121.
tuned to an inhomogeneously broadened absorption band and only those molecules in resonance with the excitation wavelength will absorb radiation. The resulting fluorescence is characterized by narrow emission line widths (13-16). In vitreous solids the singlet states of the energy-selected molecules are energetically equivalent, even though they are positioned in different sites in the low-temperature solid, and the triplet states will often be energetically nonequivalent. The resulting phosphorescence spectrum will consequently consist of broad bandwidths. Thus, in order to observe spectral line narrowing from phosphorescent species, it is often necessary to utilize direct So TI excitation ( 1 7-21). Because of the extremely small absorption coefficients for these transitions, high-resolution phosphorescence spectra are acquired by sacrificing sensitivity and low detection limits. Recently, the applicability of laser-induced fluorescence spectrometry for the multicomponent analysis of aromatic hydrocarbon derivatives has been extended by the usage of aromatic crystalline matrices (22-26). High-resolution fluorescence and excitation spectra of methyl, fluoro, and chloro derivatives of naphthalene result when these compounds are prepared in a low-temperature parent molecule (i.e., unsubstituted naphthalene) crystal (24-26). The advantage of a parent molecule crystalline matrix is that the derivatives of the parent molecule can often be incorporated in well-defined sites in the crystal, which are approximately the same size as the derivative or guest molecules. Because each guest or analyte molecule is contained in the same environment, inhomogeneous broadening of fluorescence spectra is reduced to insignificant levels, and each molecule absorbs and emits the same frequencies of light. By use of vapordeposition sample preparation procedures, solubility problems are circumvented and subpicogram fluorescence detection limits result for some naphthalene derivatives (24-26). Since this method directly addresses the problem of inhomogeneous broadening, it should be possible to extend the analytical advantages of low-temperature laser-induced fluorescence spectrometry to the multicomponent analysis of samples containing components which phosphoresce. Consequently, this paper will assess the analytical merits of laser-induced phosphorescence spectrometry for the multicomponent analysis of aromatic hydrocarbon derivatives prepared in a low-temperature, vapor-deposited parent molecule matrix. Results for a prototype system consisting of a mixture of chloronaphthalenes and bromonaphthalenes will be presented. Since the methodology presented herein is dependent on the usage of a parent molecule matrix, the method is designed for analytical problems in which derivatives of a particular aromatic hydrocarbon derivative are of interest. Different matrices must be utilized for different families of aromatic hydrocarbon derivatives, and the methodology is not suited for the characterization of the parent molecule composition of samples. The importance of developing highly selective and sensitive procedures for the measurement of phosphorescent species is due in part to the fact that many environmentally hazardous
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compounds, such as polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and biphenyls (PCBs), phosphoresce rather than fluoresce (27-28). Chloronaphthalenes and bromonaphthalenes are also wide-spread pollutants in our environment and have chemical and physical properties similar to PCBs (29).
EXPERIMENTAL SECTION Naphthalene, 1-chloronaphthalene (1-CN), l-bromonaphthalene (1-BN),2-BN, 1,4-dibromonaphthalene (1,4-DBN), and 2,3-DBN were obtained from Aldrich Chemical Co., while 2-CN and the remaining dichloronaphthalenes (DCNs) and dibromonaphthalenes (DBNs) were acquired from Ultra Scientific. Naphthalene was purified by sodium and potassium fusions and by zone refining. All of the naphthalene derivatives were used as received. Samples were vapor deposited into a naphthalene matrix prior to spectroscopic examination at 10 K. Purification and vapor deposition procedures have been described elsewhere (24-26). The excitation source was an excimer laser (Lumonics TE861T) pumped dye laser (Quanta Ray PDL-1E). UV excitation was achieved by frequency doubling the dye laser output with an angle-tuned KDP crystal. Phosphorescence was dispersed by a 0.64-m grating monochromator (Instruments SA HR-640),which was set at a band-pass of about 0.1 nm. Phosphorescence was detected by an RCA 8850 photomultiplier tube, and a photon counting apparatus processed the signal. Data collection was controlled by a Digital Equipment Corp. PDP-11/23 laboratory computer. Further information concerning the instrumentation and data collection is given elsewhere (22-26, 30). Time-resolution procedures were utilized for some of the spectra. The phosphorescence signal was measured in 1.0-ms increments following each excitation pulse. Thus, a multidimensional spectrum of phosphorescence intensity versus time and wavelength was acquired. For situations in which a significant difference existed in the phosphorescence decay times of the components that contributed to the spectrum, the individual contribution of each component to the spectrum was computed and displayed on a recorder. The procedure for computing the time-resolved spectra is similar to the least-squares algorithm developed by Margerum and Pardue et al. (10, 31, 32) and is described in detail elsewhere (26).
RESULTS AND DISCUSSION The potential usefulness of a conventional Shpol'skii matrix for analytical laser-induced phosphorescence (LIP) spectrometry of halogenated naphthalenes was assessed by vapor depositing 2-CN in n-pentane and observing the LIP spectrum that is depicted in Figure la. The LIP spectrum, as well as the two-photon excitation spectrum (24),reveals a considerably greater amount of inhomogeneous spectral broadening than the previously reported (energy-selected) LIF spectrum of 2-CN in a pentane matrix (24). Further evidence of inhomogeneous broadening is revealed by exciting a t different energies along the inhomogeneously broadened excitation peak. The observed LIF spectra are identical except for a shift in energy (corresponding to the shift in excitation energy). Al'shits et al. have explained the observed spectral broadening of the LIP spectrum in terms of a different effect of the local field on the energies of S1 and Tl states (18). Hence, molecules with equal So S1 transition energies can possess different So T1transition energies, and phosphorescence emission can occur from energetically different triplet levels (18). Thus, substantial spectral broadening is observed in the phosphorescence spectrum of Figure l a but not in the previously reported energy-selected fluorescence spectrum of 2-CN. Therefore, when inhomogeneous spectral broadening is significant, Shpol'skii matrices are not necessarily well-suited for high-resolution, low-temperature phosphorimetry. Nevertheless, in some cases analytically useful examples of lowtemperature Shpol'skii phosphorimetry have been reported (33, 34).
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WAVELENGTH (nm) Flgure 1. Laser-induced phosphorescence spectrum of (a) 2-chloro-
naphthalene vapor-deposited into a pentane matrix (320.0 nm excitation) and (b) 2chloronaphthalenevapordeposited into a naphthalene matrix (324.0 nm excitation). Previous studies by Wolf et al. have demonstrated phosphorescence from @-halogenatednaphthalene derivatives in naphthalene single crystals originated from X-traps, rather than the guest molecules (35).X-traps are host molecules that have been displaced by neighboring guest molecules and trap the energy acquired by the guest molecules. Thus, spectra of X-trap emission and host emission are identical except for differences in spectral broadening and an energy displacement (i.e., wavelength shift), which is dependent on the nature of the specific interaction between the guest and X-trap molecules. The same X-trap emission phenomenon is observed when 2-CN and 2-BN are vapor-deposited into a naphthalene matrix. Figure l b illustrates the sharp spectral features of X-trap emission from a 2-CN sample. The spectrum of 2-BN is identical with that of 2-CN, except for a red-shift of ca. 129 cm-' and the absence of the two peaks at ca. 478 and 480 nm in Figure Ib. Wolf also reported that the two nonenergetically equivalent sites for each @-halogenatednaphthalene derivative in the naphthalene single crystal give rise to only one X-trap when the concentration of the analyte was low (35). We have observed the same effect for vapor-deposited 2-fluoronaphthalene, 2-CN, and 2-BN. X-trap emission was observed from 2-CN and 2-BN, but not for any of the other CNs or BNs vapor deposited into a naphthalene matrix. The LIP spectrum for each of those compounds is unique and no longer resembles the vibronic series of naphthalene. Apparently, the triplet states of these molecules lie lower in energy than the naphthalene X-trap. Presumably, the laser excitation energy is transferred directly from the S1 state of the analyte to the Tl state of the analyte, rather than to the naphthalene X-trap. The Sl and Tl origins of several halogenated naphthalene derivatives have been determined (in our lab) and are listed in Table I. The S1 origins of most of the CNs were determined by observing the total fluorescence emission during a twophoton excitation of fluorescence scan and have been reported elsewhere (24). For the centrosymmetric 1,B-DCN molecule
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Table I. SIand T,Origins for Bromo- and Chloronaphthalenes
1-bromonaphthalene 2-bromonaphthalene 1,4-dibromonaphthalene 2,3-dibromonaphthalene 1-chloronaphthalene 1,2-dichloronaphthalene 1,4-dichloronaphthalene 1,5-dichloronaphthalene 2,3-dichloronaphthalene 2,7-dichloronaphthalene
324.1 nm 325.2, 325.8 nm 329.1 nm 329.2 nm 322.2 nm 328.8 nm 328.1 nm 332.8 nm 327.6 nm 328.0 nm
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487.3 nm 477.7 nma 494.3 nm 481.5 nm 488.7 nm 489.6 nm
495.3 nm 508.6 nm 480.3 nm 479.4 nm
X-trap emission origin.
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the two-photon 0-0 transition is forbidden by selection rules, and the 0-0 transition was identified from one-photon LIF emission and excitation spectra. The T1origins of the CNs were determined from LIP spectra obtained with S1 excitation. The T1 origins of the bromonaphthalenes (BNs) were originally determined from LIP spectra obtained by exciting the S1 origin of the naphthalene matrix. A rapid and efficient energy transfer from the delocalized host exciton band to the guest BN molecules resulted in BN phosphorescence which was sufficiently intense that it could be readily distinguished from matrix (and other impurities) emission. Since the BNs do not fluoresce, the SIorigins of the BNs were subsequently determined from LIP excitation scans. In all cases, excitation of the SI origin resulted in the greatest intensity of CN or BN emission. Emission from the T1 0-0 transition was always relatively intense, but longer wavelength transitions were sometimes more intense. The spectrum observed by using 0-0 excitation was identical with that observed when the matrix was excited, except for occasional impurity peaks and minor differences in relative peak intensity. Except for 2-CN and 2-BN, which induced X-trap emission, all of the halogenated naphthalenes exhibited a double exponential phosphorescence decay. A similar observation has been reported for phosphorescence of aromatic hydrocarbons on solid substrate surfaces (9). The decay times of each component have been measured and reported elsewhere (30). In general, the decay times of the short-lived component ranged from 0.6 to 3.6 ms for the BNs and from 0.3 to 40.4 ms for the CNs. Decay times for the long-lived component ranged from 6.7 to 14.6 ms for the BNs and from 110.6 to 261.4 ms for the CNs. In order to gain an understanding of the origin of the double exponential phosphorescence decay, the phosphorescence spectra of each compound was time-resolved. As an example the time-resolved spectra of the short and long-lived emission components of 1,4-DBN are illustrated in parts a and b of Figure 2, respectively. The phosphorescence decay times of these components are 1.3 and 6.7 ms (30) and are sufficiently different to ensure that the time-resolution methodology (10, 26, 31, 32) resulted in the complete resolution of the two components. In both spectra the peaks are quasi-linear and the spectra are identical, except for small variations in the relative peak intensities. Consequently, both phosphorescence components probably originate from molecules residing in identical sites in the crystalline matrix, and the two components apparently originate from two degenerate triplet states. Although emission wavelengths from each of the three degenerate states in a triplet are by definition equal, the emission lifetimes of these three states may be substantially different, as in the cases reported herein. This possibility has been discussed by El-Sayed (36). In order to establish the analytical utility of LIP for multicomponent analysis, a 10-component mixture of CNs and
Flgure 2. Time-resolved laser-induced phosphorescence spectra of (a)the short-lived component of 1,4dibromonaphthalene and (b) the long-lived component of 1,4dibromonaphthalene.
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Figure 3. Laser-induced phosphorescence spectra of 10-component mixture consisting of 50 ng of each substance listed in Table I. Selective excltatlon wavelengths for (a) 1,5dIchloronaphthalene(332.8 nm excitation), (b) 2-bromonaphthalene (325.8 nm excitation), (c) 2,3dichloronaphthalene (327.6 nm excitation), and (d) 2,7dichloronaphthalene (328.0 nm excitation) were employed. BNs (each at the 50-ng level) was vapor-deposited into a naphthalene matrix. The mixture consisted of the 10 components listed in Table I, and 0-0 S1 excitation was employed for selectively exciting each component of the mixture. Selective excitation of six of the ten components yielded spectra virtually identical with the pure compound. LIP spectra acquired by selectively exciting 1,5-DCN, 2-BN, 2,3-DCN, and 2,7-DCN are illustrated in parts a 4 of Figure 3, respectively. The spectra of 1,4-DCN, 1,4-DBN, 2,3-DCN, and 1,2-DCN, however, exhibited extra peaks not observed in the pure compound spectra. In the case of 1,4DCN, the impurity peaks occurred at ca. 492,494, and 498 nm and were smaller than the two major analyte peaks at 495 and 503 nm. For 1,4-DBN, 2,3-DBN, and 1,2-DBN an intense impurity peak was observed at 498.7 nm in the LIP spectra. The intensity of the impurity peak was much lower in each of the pure compound spectra and was not present in the naphthalene blank, so it evidently was present as an impurity in several of the commercially obtained samples. The impurity peak was also highly resolved and did not overlap analyte peaks in any of the spectra. Thus, the presence of this impurity does not impair either the qualitative or quantitative analysis of the mixture. The high selectivity and spectral discrimination that is afforded by laser
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NANOGRAMS 1,4-DCN Flgure 5. Analytical calibration curve for 1,4dichioronaphthaiene
vapordeposited into a naphthalene matrix. The internal standard is 1-chioronaphthalene.
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WAVELENGTH (nm) Figure 4. (a) Laser-induced phosphorescence spectrum of a 10component mixture, utilizing the optimal 329.1-nm selective excitation wavelength for 1,edibromonaphthalene. (b) Time-resoived version of the spectrum in (a). The latter spectrum was obtained by summing the spectra from the short- and long-lived components of 1,4di-
bromonaphthalene. excitation and by the quasi-linear absorption and phosphorescence line widths is more than adequate for this mixture. For more difficult analytical problems substantially increased component discrimination could be readily provided by utilizing time-resolved phosphorimetry (2,10,32,37). As an example of the potential usefulness of time-resolution, the selectively excited LIP spectrum of 1,4-DBN in the 10-component mixture is illustrated by Figure 4a, while the timeresolved spectrum is depicted in Figure 4b. Since the phosphorescence decay time of the large impurity peak at 498.7 nm was 160 ms, time-resolved phosphorimetry readily discriminated between phosphorescence from this impurity and the 1,4-DBN, which has decay times of 1.3 and 6.7 ms. As demonstrated by Figure 4b, the impurity peak at 498.7 nm was completely removed. The possibilities for successfully utilizing time-resolution procedures for discriminating between phosphorescence from impurities and analytes is significantly improved when the analyte has two distinctively different lifetimes, as in halogenated naphthalenes. This is because of the enhanced probability that at least one of the emission components of the analyte will have a significantly different (i.e., factor of 2) lifetime than the impurities. In order to assess quantitative LIP, vapor-deposited naphthalene crystals were prepared with the following amounts of 1,4-DCN 10 pg, 100 pg, 1ng, 10 ng, and 100 ng. For an internal standard 1ng of 1-CN was codeposited. The calibration curve is shown in Figure 5 and is linear from 10 pg to 10 ng with a correlation coefficient of 0.999. Measurements of five deposits of l ng of 1,4-DCN yielded a standard deviation of 9%. The 9% reproducibility that was obtained is characteristic of analytic measurements of vaA 0.8-pg detection limit por-deposited samples (11,15,24). for 1,4-DCN was estimated by observing the emission of the (M phosphorescence peak for the 10-pg sample. The detection
limit is defined as the amount of substance needed to obtain a signal that is 3 times the standard deviation of the noise. These results demonstrate that the linear dynamic range, reproducibility, and sensitivity of the LIP spectrometry of halogenated naphthalene derivatives are comparable to the previously determined LIF results for the CNs (24). A new method for characterizing complex mixtures of halogenated aromatic hydrocarbon derivatives has been tested. It has been demonstrated that the wide linear dynamic range of analytical calibration curves and the high sensitivity, selectivity, and discrimination of LIF spectrometry can be extended to LIP spectrometry. Vapor-deposited parent molecule matrices allow the characterization of complex mixtures of a wide variety of aromatic hydrocarbon derivatives, including isomers which differ only in the number or position of nonpolar, polar, and halogenated substitutents (24-26). The LIP methodology developed herein is complementary to LIF methods (11-16,22-25) and utilizes virtually the same equipment (30).
LITERATURE CITED (1) Barnes, C. G.; Winefordner, J. D. Appl. Spectrosc. 1885, 3 8 , 214-227. (2) Weinberger, R.; Cline Love, L. J. Appl. Spectrosc. 1885, 39, 5 16-5 19. (3) Scyplnski, S.; Cline Love, L. J. Anal. Chem. 1984, 56, 322-327. (4) Femla. R. A.; Cline Love, L. J. Anal. Chem. 1884. 56. 327-331. (5) Scypinskl, S.; Cline Love, L. J. Anal. Chem. 1884, 5 6 , 331-336. (6) Cline Love, L. J.; Habarta, J. G.; Dorsey, J. G. Anal. Chem. 1884, 56. 1132A- 1148A. (7) Daiterio, R. A.; Hurtublse, R. J. Anal. Chem. 1884, 56, 336-341. (8) Bower, E. L. Y.; Wlnefordner. J. D. Anal. Chlm. Acta 1878, 102, 1-13. (9) Sharf, G.; Smith, B. W.; Wlnefordner, J. D. Anal. Chem. 1885, 57, 1230-1237. (10) Goerlnger, D. E.;Pardue, H. L. Anal. Chem. 1978, 51, 1054-1060. (11) Maple, J. R.; Wehry, E. L.; Mamentov, G. Anal. Chem. 1880, 52. 920-924. (12) Yang, Y.; D'Silva, A. P.; Fassel, V. A. Anal. Chem. 1881, 5 3 , 894-899. (13) Brown. J. C.; Duncanson, J. C., Jr.; Small, G. J. Anal. Chem. 1880, 5 2 , 1711-1715. (14) Heislg, V.; Jeffrey, A. M.;McGlade. M. J.; Small, G. J. Sclence 1984, 223, 289-291. (15) Maple, J. R.; Wehry, E. L. Anal. Chem. 1881, 53. 266-271. (16) MacDonald, B. F.; Hammons, J. L.; Gore, R. R.; Maple, J. R.; Wehry, E. L. Appl. Spectrosc. 1988, 42, 1079-1083. (17) Cunningham, K.; Morris, J. M.;Funschilling, J.; Williams, D. F. Chem. Phys. Lett. 1875, 32,581-585. (18) Al'shits, E. I.; Personov, R. I.; Kharlamov, B. M. Chem. Phys. Lett. 1878. 40, 116-120. (19) Al'shlts, E. I.; Personov, R. I.; Kharlamov, B. M. Opt. Spectrosc. (USSR) 1878, 41, 474-479. (20) Wlillamson, R. L.; Kwiram, A. L. J . Phys. Chem. 1879, 63, 3393-3397. (21) Suter, G. W.; Wild, U. P. J . Lumln. 1881, 2 4 / 2 5 , 497-498. (22) Thornberg, S. M.; Maple, J. R. Anal. Chem. 1884. 56, 1542-1544. (23) Thornberg, S. M.;Maple, J. R. Anal. Chem. 1985, 5 7 , 436-439.
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(24) Pace, C. F.; Maple, J. R. Anal. Chem. 1985, 57. 940-945. (25) Pace, C. F.; Maple, J. R. J . Opf. SOC.Am. 8 1985, 2 , 1582-1588. (26) Pace, C. F. Ph.D. Dissertation, The University of New Mexico, Albuquerque, NM, 1986. (27) O’Donnell, C . M.; Harbaugh, K. F.; Fisher, R. P.; Winefordner, J. D. Anal. Chem. 1973, 45, 609-611. (28) Pohland, A. E.; Yang, G. C. J . Agric. Food Chem. 1972, 2 0 , 1093- 1099. (29) Brinkman. U. A. T.; Reymer, H. G. M. J . Chromatogr. 1978. 127, 203-243. (30) Pace, C. F.; Thornberg, S. M.; Maple, J. R. Appl. Spechosc. 1988, 42, 891-896. (31) Wiilis, B. G.; Woodruff, W. H.; Frysinger, J. R.; Margerum, D. w.; Pardue, H. L. Anal. Chem. 1970, 42, 1350-1355.
(32) Meiling, G. E.; Pardue, H. L. Anal. Chem. 1978. 50, 1611-1616. (33) Colmsjo, A. L.; Zebuhr, Y. U.; Ostman, C. E. Anal. Chem. 1982, 54, 1673-1677. (34) Qrrigues, P.; Ewald, M. Anal. Chem. 1983. 5 5 , 2155-2159. (35) Wolf, H. C.; Port, H. J . Lumin. 1978. 12/13, 33-46. (36) El-Sayed, M. A. In Molecular Luminescence; Lim, E. C., Ed.; W. A. Benjamin: New York, 1969; pp 715-736. (37) Harbaugh, K. F.; O’Donnell, C. M.; Winefordner, J. D. Anal. Chem. 1974, 4 6 , 1206-1209.
RECEIVED for review November 10,1988. Accepted January 23, 1989.
On-Column Capillary Flow Cell Utilizing Optical Waveguides for Chromatographic Applications Alfred0 E. Bruno,* E r n s t Gassmann, Nico PericlBs, and Klaus Anton Central Analytical Department, CIBA GEIGY Ltd., CH-4002 Basel, Switzerland
The high separatlon efficiencies provided by various chromatographic technlques employlng microbore and capillary columns, countered by current detection llmltatlons, present new challenges In detection cell design. I n thls contrlbution we descrlbe a device sultable for on-column simultaneous absorption and fluorescence detection that utlllzes optical waveguides. The deslgn of this cell is based upon the aMlty of a capmary to functlon as a strongly focuskrg lens along one of Its axes. I n order to use thls attribute to the fullest advantage, a three-dimensional ray tracing algorithm, which sbnuiates the optlcal phenomena at the Interfaces and In the travellng media, was developed. Thus, from a trial set composed of commercially available components, we were able to choose optkal flber/capillary tube geometric configurations that yielded theorellcai transmittances of ca. 90%. Subsequent lmphmtatbn of the cell In capillary supercrltical flukl chromatography led to verlflcatlon of these flndlngs. Statlc and dynamlc refractive index effects, which are known to dlstort recording signals, were carefully Investigated and mlnimlzed. Two guldeHnes based upon experimental and theoretical observations are formulated-the first statlng the maxhnum source mer dlameter for a given choke of cap#lary tube and the second relating the minlmum dlameter of the co#ectlng fiber to an already chosen source Hber and caplllary tube-enabilng rapld selection of the cell optical hardware.
INTRODUCTION The development of small optical detectors for microbore and capillary chromatographic techniques is gaining considerable momentum (1-3). The demand for improvements in presently available models or, these improvements being inadequate, for the design of new devices is a direct consequence of today’s advanced state of the art of separation methods such as microbore liquid chromatography (4),capillary supercritical fluid chromatography (CSFC) (4, 5 ) , and capillary zone electrophoresis (CZE) (6). The high resolution delivered by these techniques is often lost at the detection stage as a result of coupling with insufficiently sensitive detection devices. 0003-2700/89/036 1-0876$01.50/0
Furthermore, the performance of these techniques increases with decreasing capillary tube inner diameters, thus compromising concentration sensitivity. The fiist problem one encounters when designing an optical on-column flow cell is the focusing of a considerable amount of light into the small light/solute interaction volume defined by the capillary tube inner diameter. Lasers undoubtedly provide the best light source for this purpose. Virtually the entire laser beam (exclusing Fresnel losses, which are unavoidable) can be readily focused into a tiny spot inside the capillary orifice. Excellent sample selectivity and sensitivity fluorescence (8,9),light have been reported for absorption scattering ( l o ) ,refractive index (2,1 1 ) , and thermal lens (12) methods employing coherent laser light. Unfortunately, due to their relatively high degree of sophistication, laser-based detection methods are primarily suitable for research-oriented laboratories. For industrial analyses, where chromatography is widely used, we designed a flow cell which, while retaining many of the virtues of laser detection methods, utilizes conventional lamps as the light source. The efficient focusing of spatially incoherent light from conventional lamps into capillary tubes with internal diameter i.d. ca. 5100 pm is not trivial. On-column absorption measurements are commonly made simply by intercepting the optical path of an absorption spectrophotometer with the capillary tube (3,13,14). A pair of masking slits ensure that only light that has passed through the capillary tube reaches the photosensitive detector. A major drawback of this experimental arrangement is the associated alignment procedure, which must be carried out exceedingly carefully to ensure good reproducibility and a high signal-to-noise (S/N) ratio. Optical fibers, originaUy developed primarily for the purpose of communication technology, considerably simplify the construction of small on-column detection cells. In fact, a few cells based upon these fibers are now emerging (15-17). The most recent and promising detector of this type, developed by Foret et al. (17),consists of a pair of optical fibers that bypass the optical path of a UV-visible spectrophotometer. One fiber, the source fiber, transfers light from the source to the capillary tube while a pair of fibers, the collecting and emission fibers, carry the absorption ( 17) and fluorescence
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0 1989 American Chemical Society
