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Anal. Chem. 1982, 5 4 , 1174-1178
monomer. Figure 8 shows separation of two inhibitors, TBC and hydroquinone, in a monomer system and demonstrates the excellent reproducibility of the procedure. Thus, monomer and inhibitors do not passivate the PECB electrode.
100 ppm /Hydroquinone
ACKNOWLEDGMENT
---
0
lo
20
0
20
lo
0
IO
20
Time, Minuter
Flgure 8. Determination of inhibltors in monomers with a PECB electrochemicaldetector with replicate injections of an alkyl-substituted styrene monomer.
An improved method was desired for determination of polymerization inhibitors in an alkylated styrene monomer. p-tert-Butylcatechol (TBC) is almost universally used as an inhibitor and antioxidant in manufacture and storage of styrene (11). An ASTM procedure is available for determination of TBC in styrene (IZ),but an extraction is involved. Mixed inhibitor systems are also employed and such phenolic compounds can be separated by LC but the monomer itself interferes with UV detection. Because use levels of inhibitors are in the 10-100 ppm range ( I I ) , monomer can be diluted 1000-fold in the 29% acetonitrile eluent system and injected directly onto the column used for phenol separations. Sufficient sensitivity is available with the PECB electrochemical detector to determine inhibitors with no interference from the
The help of M. W. Long in selecting and preparing polymer electrodes, H. D. Woodcock for machining electrodes, and R. E. Reim for evaluating the electrodes is gratefully acknowledged. Thanks are also due to V. A. Stenger and R. M. Van Effen for helpful discussion and Harry Baker for photomicrography.
LITERATURE CITED (1) Gaylor, V. F.; Conrad, A. L.; Landerl, J. H. Anal. Chem. 1957, 2 9 , 224-228. (2) Adams, R. N. "Electrochemistry at Solid Electrodes"; Marcel Dekker: New York, 1969; pp 280-283. (3) Kissinger, P. T.; Refshauge, C.; Drelllng, R.; Adams, R. N. Anal. Lett. 1973, 5 , 465-477. (4) Armentrout. D. N.; McLean. J. D.: Long, M. W. Anal. Chem. 1979, 5 1 , 1039-1045. (5) Pungor, E.; Feher, 2 ; Nagy, G. Magyar Kemiai Fokoirat 1971, 77, -288-293 - - - - -. (6) Clem, R. G.; Sciamanna, A. F. Anal. Chem. 1975, 4 7 , 276-280. (7) Panzer, R. E.; Elving, P. J. J . Nectrochem. SOC. 1972, 119, 864-874. ( 8 ) Klatt, L. N.; Connell, D. R.; Adams, R. E. Anal. Chem. 1975, 4 7 , 2470-2472. (9) Mascini, M.; Pallozzl, F.; Llbertl, A. Anal. Chim. Acfa 1973, 6 4 , 126-13 1. (IO) Anderson, J. E.; Tallman, D. E.; Chesney. D. J. Anal. Chem. 1978, 50, 1051-1058. (11) Hodges, K.; McLean, J. D. "Encyclopedia of Industrial Chemical Analysis"; Whey: New York, 1974; Vol. 18, Styrene, pp 285-313. (12) ASTM D 2120-68 "Test for Inhibitor, 4-Tertiary-Butylcatechol in Styrene Monomer"; American Society for testing and Materials: Philadelphia, PA, 1970.
RECEIVED for review December 14, 1981. Accepted March 25, 1982.
Refractive Index and Absorption Detector for Liquid Chromatography Based on Fabry-Perot Interferometry Steven D. Woodruff and Edward S. Yeung" Ames Laboratory and Department of Chemistry, Iowa State University, Ames, Iowa 500 11
The combination of Fabry-Perot interferometry and a single frequency laser is probably the most sensitive way to detect refractive index changes. The added finesse and the increased monochromaticity provide an order-of-magnitude improvement in detectability over commercial high-performance llquid chromatography refractive index detectors. I f a second laser Is used to interact with the anaiyte, absorption can be monitored as a change in refractive Index. For a 60-mJ laser pulse, we have achieved detectabiifty 2 orders of magnitude better than standard absorption detectors In high-performance liquid chromatography.
With the increasing interest in environmental, clinical, and other biological problems, there is a growing need for trace analytical methods that are suitable for complex organic mixtures. While gas chromatography (GC), particularly in combination with mass spectrometers, has been successfully 0003-2700/82/0354-1174$01.25/0
applied to the volatile species, high-performance liquid chromatography (HPLC) is often the choice for the nonvolatile components. Although HPLC technology has made big gains recently ( I ) , the overall separatory power is still not competitive with GC. This is why new concepts for HPLC detectors can become beneficial. Further, since small sample sizes are required for these highly efficient HPLC separations, the detectors must be improved with respect to their detectabilities. Of the three most commonly used HPLC detectors, the fluorometric detector (2, 3) has already been developed sufficiently to be suitable for most situations. For nonfluorescing samples, the absorption detector must be used, but the detection of small differences in two large signals limits range in absorbance. conventional detectors to the low3to When the species of concern does not show convenient absorption bands, e.g., saturated organic compounds, the refractive index (RI) detector is commonly used, despite its poor sensitivity. Since the scope of application of HPLC is inversely related to the detectability of the detectors, it will be useful 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
to improve the R1 and the absorption detectors along these lines. It is apparent that the absorption detector can be improved only if some associiated effect rather than the decrease in light intensity is monitored. The most convenient associated effect is the generation of heat through relaxation of the excited molecules. The nonuniform heating resulting from a laser beam gives rise to thermal lens calorimetry (4).If instead the temperature gradient, and thus a RI gradient, that is developed is used to deflect a probe laser beam, the technique of photothermal deflection is created (5). One can also use the heat waves that are generated by a pulsed or choppecl excitation source as the basis for photoacoustic detection (6). This concept has already been demonstrated as a detection scheme for HPLC (6, 7). Since the magnitudes of all of these associated effects incirease with the power of the excitation (absorbed) light source, one can in principle achieve lower detectabilities compared to conventional measurements. Perhaps the most sensitive way to monitor small changes in the refractive inidex is interferometry. The same technology that allows one to achieve high frequency stability in lasers and to measure tlhese frequencies to great precision and accuracy can be applied to the detection of RI changes. By using a Mach-Zender interferometer and a single frequency laser, one can monitor plhase delays in the sample due to absorption and the subsequent heating to detect trace gases (8).This type of phase-fluctuation optical heterodyne spectroscopy has been shown to be a decent GC detector (9). I t is clear that detectability depends on the quality of the interference that can be achieved. The Mach-Zender interferometer has relatively low finesse because a low reflectivity mirror (50%)is used for splitting the beam into the two paths. It also suffers from having an “idle” arm that can be perturbed by acoustic waves unless the system is evacuated. Further, arrangement of the optical components is such that system rigidity is difficult to maintain. The Fabry-Perot interferometer, on the other hand, typically has very high fiiesse, has no “idle” optical paths, and is commercially available with excellent rigidity using materials with low coefficients of expansion, such as super invar. A Fabry-Perot interferometer using plane parallel mirrors shows maximum constructive interference whenever h/n = 2d/m
(1)
where h is the wavelength of light, n is the refractive index of the medium, rn is any integer, and d is the spacing of the mirrors. Typically, the interferometer is used in air for accurate wavelength determinations. If however a single-frequency laser is used, the RI of whatever medium that is inside the interferometer can be monitored. From eq 1, it can be seen that AX/h = An/n. Since Fabry-Perot interferometry has been used to determine laser frequencies to 3 parts in lo1’ (IO),one can expect a similar detectability in the change in RI in the ideal case. T o interface this with HPLC, a flow cell must be put in the cavity. Fortunately, it is not necessary to have the liquid filling the entire cavity. One can readily see that the sensitiviity simply decreases in proportion to the fraction of the optical cavity length (RI weighted) not covered by the liquid. Thle reflectivity finesse of the system is given by
F = 7rfi/(l-
R)
where R is the reflectivity of each of the mirrors. Since this determines the ultimate resolution of the interferometer, one wants as high a value as possible. This is why a Fabry-Perot scheme is better than a Mach-Zender scheme in detectability. The cell windows contribute to loss via natural reflection at the air interface and effectively reduce the reflectivity R. This
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can be overcome however by using Brewster’s angle as the angle of incidence, so that the p-polarized component suffers essentially no reflective losses. Additional losses at the liquid to cell window interface can be neglected. We can estimate the detectability allowed by a combination of commercially available components. Single-frequency helium neon lasers have a stability of about 1MHz (1s), or about 2 parts in lo9. Interferometers readily provide d = 15 cm, so that the free spectral range is about 1 GHz. The half-width of the interference fringe is thus 10 MHz for a system finesse of 100. The peak of the interference fringe, however, can be located to better than one-tenth the width, so that the ultimate resolution is also 1 MHz. One can therefore expect improved detectability over standard R I detectors for HPLC. To function as an indirect absorption detector, a collimated, secondary light source matching the absorption band can be introduced along the optical path of the interferometer to induce a RI change. One can explicitly relate the RI change, An, and the absorbance of the sample, A . For small absorptions, which is typically true in HPLC, the amount of light absorbed is 2.303AI, where I is the excitation source intensity in joules. This eventually all becomes heat if thermal relaxation is slow compared to the excitation process. Then, the temperature increase, AT, in the interaction region of cross sectional area a cm2 and unit length is
2.303AI A T = - (K) (3) CPa where C, is the specific heat in J 8-l K-l and D is the density of the medium in g cm-3. We then have An = AT(dn/dT)
(4)
where dn/dT is the temperature dependence of the RI. When thermal relaxation rates are comparable to the excitation process, as in this work, eq 3 must be modified by a proportionality constant, which decreases the sensitivity but maintains the linear dependence. Using typical values I = 0.1 J, C, = 3 J g-l K-l, D = 0.8 g ~ m -a~=, 0.01 cm2,and dn/dT = -5 X one finds that the minimum detectable absorvide supra. bance is 4 X lo-’ for a RI detectability of 2 X The system is thus superior to standard absorption detectors in HPLC. A subtle point is that traditional absorption measurements gain linearly with increasing interaction length. In interferometry, increased length increases the absorbed amount, but at the same time more volume must be heated up. There is no net gain in An. However, the resolution of the interferometer generally increases with the distance d, so that longer light paths are still desirable. It is also interesting to note that for gases, D is smaller and favors AT for the same absorbed amount. But, d n / d T is also smaller by a similar factor, and no enhancement is possible. In the following, we report the first successful adaptation of Fabry-Perot interferometry to HPLC detection in a system that allows the measurement of both RI and absorption. Detectabilities superior to the corresponding HPLC detectors are shown, and the potential for further refinements is discussed.
EXPERIMENTAL SECTION RI Detector. The experimental arrangement to detect RI changes is shown in Figure 1. A single-frequency HeNe laser (Tropel, Fairport, NY, Model 100) is used to provide a frequency stability of =k1 MHz/s, or &3 MHz/min or & l o MHz/h, respectively. To avoid feedback into the stabilization mechanism of the laser, we placed a Glan-Thompson polarizer (Karl Lambrecht, Chicago, IL, Model MGT-25E8-45) in the polarization orientation of the laser, followed by a quarter-wave plate (Oriel, Stamford, CT, Model 2562). The interferometer (Burleigh, Fishers, NY, Model RC-110 with RC-670-C2.3mirrors) is chosen
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ANALYTICAL CHEMISTRY, VOL. 54,NO. 7, JUNE 1982 ?
xr14
FP
;REc
L Figure 1. Refractive index detector: HENE, single frequency laser;
P, polarizer; X/4, quarter wave plate; FP, interferometer: PMT, phototube; REC, chart recorder.
Figure 2. Absorbance detector: P i , P2, polarizers; AO, Bragg cell; L,
lens; other symbols as In Figure 1.
for its thermal stability and mechanical stability. A chromatographic flow cell is machined locally to fit inside the interferometer and is made out of a 10 cm long, 5 cm diameter aluminum cylinder with a central bore having a volume of about 200 pL. The cell i s mounted on the same base plate used for the interferometer through a rough positioning mount. The windows are oriented at Brewster's angle with respect to the entering laser beam, and the cell is thus oriented accordingly. The windows are in. fused silica with a surface finish of X/20 (Oriel, Stamford, CT, Model 4492) and are held by a hard epoxy. The exciting laser beam is directed to a photomultiplier tube (RCA, Lancaster, PA, Model 6342A) operated at 700 V after passing through a small monochromator (PTR Optics, Waltham, MA, Model Minichrom 1) to reject room light. The entire optical system is mounted on a rigid optical table (Newport, Fountain Valley, CA, Model LS-48) without further vibrational isolation. In operation, a linear ramp is generated by a minicomputer (Digital Equipment, Maynard, MA, Model PDP 11/10 with LPS-11 laboratory interface) and is amplified by a high voltage operational amplifier (Burleigh, Fishers, NY, Model PZ-70) to scan the interferometer. For each step in the ramp, the output of the photomultiplier tube is digitized and stored in the computer. After each scan, which takes less than 0.3 s, the computer decides on where along the ramp the maximum phototube output occurs, i.e., the value of d for maximum constructive interference. This value is then converted to an analog signal to be displayed on a chart recorder (Houston Instruments, Austin, TX, Model 5000). In practice, the gain of the op amp is adjusted so that a complete ramp corresponds to one free spectral range for the interferometer. This way, only one interference peak is present along each scan and no confusion arises. When a peak drifts off the scan range, the next order comes into the range to resume monitoring. There is a corresponding reset on the chart display, which degrades the esthetic value of the display but does not affect the usefulness of the measurement. When such a reset occurs, one simply adds one free spectral range to the peak position and continues monitoring. In fact, allowing resets and accounting for these properly in principle provides an unlimited dynamic range for the measurement of An without adjusting the interferometer. Absorption Detector. The experimental arrangement for detecting absorption is similar to the above and is shown in Figure 2. An argon ion laser (Control Laser, Orlando, FL, Model 554A) operating a t 514.5 nm is used for excitation. Light pulses of well-defined total energies are selected from the CW laser by a Bragg cell (Coherent Associates, Danbury, CT, Model 304 and 305 D). The chromatographic flow cell is modified to include a second optical path for the excitation laser. The ion laser is
TIME (MINI Figure 3. Refractive index chromatogram (unsmoothed)of benzene. Ordinate is proportional to refractive index as discussed in text. The main peak corresponds to An = 3 X IO-'.
collimated to match the size of the HeNe laser by a 50-cm focal length lens. Introduction into the optical path of the interferometer is achieved by using the excitation laser at a polarization direction perpendicular to that of the probe laser to take advantage of the natural reflection of the cell window. About 15% of the incident intensity is thus introduced. The ion laser beam is not maintained after two or three passes in the interferometer cavity because of the unfavorable losses at the windows for this polarization direction, because of the collimating lens, and because of the wavelength-dependent refraction at the cell windows. The excitation light in the direction of the phototube is conveniently rejected by a second polarizer, P2. To minimize the volume of the detector, we used a cell length of 1.0 cm instead. The reduced size allows the use of a smaller interferometer (Trope], Fairport, NY, Model CL-100) and allows one to actually build the cell as an integral part of the mirror mount of the interferometer. To operate, the computer generates a ramp and stores the interferometric scan as described above. The Bragg cell is then turned on under computer control for a period of time, typically 1 s. While waiting, the computer determines the location of the peak for the scan before irradiation, and provides an output at a chart recorder as before. Immediately after irradiation, a second scan is taken to determine the shift of the peak due to absorption. The difference in peak locations is then stored and plotted as the absorption chromatogram. To assure that thermal relaxation of the system is complete between data points, the data gathering cycle is repeated once every 15 s. It is interesting to note that this scheme simultaneously provides a RI chromatogram and an absorption chromatogram. Chromatography. All reagents and eluents used are reagent grade materials without further purification. The chromatographic system is conventional and consists of a metering pump (Micrometrics, Norcross, GA, Model 750), a pulse dampener (Handy & Harman, Norristown, PA, Model Li-Chroma-Damp11), a 20-pL sample loop at a conventional injection valve (Rheodyne, Berkeley, CA, Model 7010),and a 25 cm X 4.6 mm, 10-FmCISreversed-phase column (Alltech, Deerfield, IL). Acetonitrile (100%) is used throughout as the eluent. For comparison, a conventional UV absorption detector (Rainin, Woburn, MA, Model 153-00)operated a t 254 nm is placed in series with the interferometric detector.
RESULTS AND DISCUSSION RI Detector. Benzene diluted in acetonitrile was chosen as the test mixture. The solvent front commonly associated with injection can thus be eliminated, and simultaneous monitoring with the UV absorption detector provides a convenient check on the chromatography. Solutions were prepared by successive dilutions with no greater than 1OO:l for each step to minimize dilution errors. The procedure is chosen so that if any error occurs, one obtains a sample that is definitely lower in concentration than expected. This then allows us to establish an upper limit on the concentration. Figure 3 shows a chromatogram of a sample prepared in this manner, with the ordinate being the position of a particular interferometric peak as determined by the computer. This is linear with concentration, since Ad/d = - A n / n from eq 1, and is confirmed experimentally by using five standard solutions over the range An = 3 X to An = 3 X lo4. The chart recorder
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
output was refreshied by the computer at 0.3-s intervals, and no time constant was introduced at the output. The main peak (benzene) corresponds to a RI difference of 3 X lo-', as determined from the differences in RI for benzene (nD = 1.5011) and for acetonitrile (nD = 1.3442), and the dilution factor from the sample loop size and the volume of the whole benzene peak. The "bump" after the benzene peak corresponds to an unidentified contaminant which also shows up in our other trials. We have also injected a sample of one-tenth this concentration. Using a 5-9 time constant generated digitally by the computer, we estimate the signal-to-noise ratio to be 6. This implies a deof the peak of An = 3 X tectability of RI changes of 1.5 X lo-@( S I N = 3), which is roughly an order-of-magnitude better than those of commercial RI detectors. At the low concentration levels of these trials, many sources of noise become important. Since the entire system is directly coupled to the floor of the laboratory, vibrations lead to jitter in the peak positions. Acoustic noise in the room affects primarily the part of the interferometer cavity not covered by the cell (about 2 cm). Some improvements can be expected through vibrationail isolation and acoustic isolation, although the use of a time constant in the recorder output also helps. Temperature chan,gescertainly affect the peak locations. The super-invar (50.36 X lo4 K-l) construction of the main interferometer is unlikely to contribute significantly. The thermal expansion of aluminum (2.5 X K-l) cannot compensate for the change in RI of the eluent (-5 X K-l, estimated) and the latter becomes the major contributor. However, temperature drifts with such a bulky cell are long term effects on the base line, which in principle can be correded for by the colmputer. Naturally, a thermostat-controlled enclosure (e.g., Bwleigh Model RC-75) is a desirable addition. In our simple arrangement, we found that opening and closing the doors to the laboratory changes the base line. Flow gradients and pulsations generated by the chromatographic system can affect the interference peak. At our level oi sensitivity, we did not find these to be problems, since the noise level seems to be independent of flow from 0 to 1 mL/min. It was however necessary to wait until the flow has stabilized from a cold start before performing the final adjustments on the interferometer. Apparently the epoxy is still flexible enough to distort differently with and without flow. From one day to the next, no adjustments were necessary on the interferometer if tbe same flow rates were used. Most of the above-mentioned contributions from noise can be compensated for in a dual-beam arrangement. Two parallel optical paths can be introduced into the interferometer by splitting the output of the single-frequency laser, one for the HPLC flow cell and one for a reference cell containing the eluent only. The difference in positions of the peaks in the two paths, as oppolsed to the absolute positions, can then be registered using separate photoelectric detectors. If the two cells are machined into the same metallic block, thermal effects are minimized. In fact, drifts in the interferometer and in the laser, vibrations, and acoustic noise are automatically compensated for. A dual-beam arrangement also allows the adaptation of modulation and lock-in detection. For example, the interferometer can be set a t the half-maximum point of an interference peak, and is then modulated around this point piezoelectrically. The difference signal between the two optical paths can then be extracted by using a lock-in amplifier to provide an extremely sensitive measure of RI changes. In comparison to similar modulation techniques (8), one gains from the much sharper slope in the Fabry-Perot arrangement. It should lbe noted that the Brewster angle arrangement is not an absolute requirement. If a high-efficiency antireflection coating is used on the windows, losses are still negligible at
3
..................... .....................
1177'
... . ........ ....
Figure 4. Absorbance chromatogram of NBD and decomposition
products. Ordinate is proportional to absorbance as discussed in text. The main peak corresponds to A I 2.6 X normal incidence. Rotation of the cell to fit eluents of different refractive indexes is then avoided. An extra benefit is that normal incidence allows the interferometer mirrors to be placed much closer to the windows, thus increasing the fraction of the optical path covered by the eluent and decreasing the contribution of acoustic noise and air turbulences. Also, the photoelectric detector can be a simple photodiode rather than a photomultiplier tube, since the transmission characteristics of available Fabry-Perot mirrors are very good. Finally, confocal Fabry-Perot interferometers have twice the free spectral range as plane ones for the same physical length. The optical beam is also naturally focused at the center. These two properties can lead to smaller detector volumes at the same level of detectability. Absorption Detector. To test the ultimate performance of the detector in absorption, one needs standard solutions with very small absorbances. A convenient system is a solution in acetoof NBD (7-chloro-4-nitrobenzo-2-oxa-1,3-diazole) nitrile. While NBD is stable in methanol solution, it degrades in acetonitrile solutions over a period of days (11). Both NBD and its decomposition product(s) show very weak absorption a t 514.5 nm. Using standard spectrophotometers, we have determined that a (by weight) solution of NBD in methanol has an absorbance of 0.013 (*0.0005) at 514.5 nm in a 1-cm path. Taking into account dilution in the eluent, a 20-pL injection of this stock solution diluted 1 : l O in acetonitrile gives an absorbance at the chromatographic peak of 2.6 X So, dilution steps are minimized in preparing the injected samples. Also, the separation can be conveniently monitored using standard UV detectors at the much stronger absorption bands. The result of such a trial is shown in Figure 4. The ordinate represents the differences in the interferometric peak positions before and after each irradiation with the ion laser pulse. This quantity is found to be proportional to 2.6 x to the absorbance over the range of 2.6 X using five standard solutions. The ion laser was maintained at a constant CW power, and, judging from the various reflectivities in the optical path, we estimate that a 1-s pulse contained 60 mJ. Since data points are gathered at 15-s intervals, the chromatogram, without any smoothing, shows up as individual points. NBD is eluted at 17 min, as confirmed by the UV absorption detector. Its decomposition products show up between 12 and 14 min and are not separately identified. The main chromatographic peak then represents an absorbance of no greater than 2.6 X 10". Except for three stray points, the average noise is about 1/30 of the peak height, implying a detectability of 2.6 X lo4 absorbance units ( S I N = 3), or about 2 orders of magnitude better than commercial units. Further enhancements can be expected using higher laser powers. To show that Figure 4 is not an artifact of changing RI in the absence of absorption, we have simultaneously obtained the RI chromatogram by recording the interferometric peak positions before each exciting laser pulse but after the effects of the previous pulse has decayed. This is shown in Figure
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
Figure 5. Refractive index chromatogram of NBD and decomposition products taken simultaneously with that in Figure 4. Ordinate is proportional to refractive Index as dlscussed in text. Dashed lines indicate resets as discussed in the text.
5. The “resetting” of the interferometric scan is shown as the dashed lines, as discussed above. We note that even in the presence of a rapidly drifting RI base line, which is typical of the system before complete warm-up, particularly since a lower quality interferometer is used for this part of the work, the absorption chromatogram in Figure 4 shows little effect because only 1 s was needed to measure the difference. At the location of the main chromatographic peak, no significant R I changes occur. We thus demonstrate that the present system is suitable for simultaneous monitoring of RI and absorption. The 15-s interval between measurements is dictated by the optical arrangement in Figure 2. Since the excitation beam never quite exits the Fabry-Perot cavity, the cell body is heated up to a certain extent even with no absorbing species present in the eluent. We have studied the time dependence of the peak shift after the excitation pulse. Immediately (submilliseconds), the peak shifts to a new value, presumably due to heating of the liquid, and decays in the 1-s time scale (thermal diffusion and replenishment of the liquid by the flow). Also in the 1-9 time scale, a second, additive shift occurs, which is consistent with heating of the aluminum block by stray radiation and eventual equilibration with the liquid. This is confirmed when a Nd:YAG-pumped dye laser is used to excite the vibrational overtone absorption of acetonitrile around 620 nm in the same flow cell. Then, after about 10 s, the original peak position is reestablished, indicating complete thermal relaxation of the system. To shorten the time interval required, one does not really need to wait uiitil complete relaxation. If a pulsed laser is used, the same amount of energy can be deposited in a shorter time. The instantaneous shift can then be registered with minimal effect from
the slower thermal relaxation processes. More frequent data collection allows some signal averaging and thus increased SIN. A second approach to minimize contributions from thermal relaxation is to modify the optical arrangement. If instead the flow cell is a glass (or quartz) capillary tube, the excitation laser can be introduced perpendicular to the probe laser. Radiation not absorbed in the liquid can exit and will not contribute. Thermal relaxation will then be determined by the rate of replenishment of the liquid in the interaction region and is expected to be in the subsecond time scale for typical eluent flow rates. An additional benefit is the more efficient utilization of the laser power compared to relying on the natural reflection. Finally, the Brewster angle entrance can be avoided to provide a more compact interferometer cavity. As with the RI detector, a dual-beam arrangement should be advantageous. The exciting laser can be directed to both a sample and a reference flow cell, and the difference in the two paths is recorded either by direct methods or by a lock-in amplifier synchronized with the modulated exciting laser. Improvements similar to a dual-beam RI detector can be realized. In addition, residue absorption in the solvent, which ultimately limits any absorption technique, can be electronically substracted.
ACKNOWLEDGMENT The authors thank Steven A. Wilson for the data on absorptivities.
LITERATURE CITED (1) Scott, R. P. W.; Kucera, P. J . Chromatogr. 1979, 169, 51-67. (2) Yeung, E. S.; Sepaniak, M. J. Anal. Chem. 1980, 52, 1465 A-1481 A.
(3) Dieboid, G. J.; Zare, R. N. Science 1977, 196, 1439-1441. (4) Harris, J. M.; Dovichi, N. J. Anal. Chem. 1980, 52, 695 A-706 A. (5) Boccara, A. C.: Fournier, D.; Jackson, W.; Amer, N. M. O p t . Left. 1980, 5,377-379. (6) Oda, S.;Sawada, T. Anal. Chem. 1981, 5 3 , 471-474. E.; Juraensen, A.; Winefordner, J. D. Anal. Chem. 1981, (7) Voiatman, . . 53 ,- 1921- 1923. (8) Davis, C. C. Appl. Phys. Lett. 1980, 3 6 , 515-518. (9) Lin, H. 8.; Gaffney, J. S.;Campilio, A. J. J . Chromatogr. 1981, 206, 205-214. -.. (IO) Woods, P. T.; Shotton, K. S.;Rowiey, W. R. C. Appl. Opt. 1978, 17, 1048- 1054. (11) Sepaniak, M. J., unpublished results.
RECEIVED for review November 3,1981. Accepted March 19, 1982. The Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. W-7405-eng-82. This work was supported by the Office of Basic Energy Sciences and the National Science Foundation Scientific Equipment Program.
