Anal. Chem. 1984, 56, 2975-2977
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Laser Intracavity Photothermal Beam Deflection Spectroscopy Sir: Photothermal beam deflection (PBD) spectroscopy is a spinoff of photoacoustic spectroscopy (PAS) that is gaining increased attention. It is based on the “mirage effect” and was first reported by Boccara et al. (1) in 1980. In PBD spectroscopy a beam of electromagnetic radiation approximately normal to a sample surface is intensity modulated at an audio frequency and illuminates a small area of a sample. If the sample absorbs the incident radiation the radiationless transitions in the sample produce a modulated thermal gradient in the sample and a consequent modulated refractive index gradient in a layer of fluid (gas or liquid) just above the heated sample surface. This refractive index change is detected by the deflection of a low-power CW laser beam (propagating parallel to and almost grazing the sample surface) that strikes a position sensor aproximately 1 m beyond the sample. Photothermal beam deflection experiments have been subjected to quantitative theoretical analysis (2-4). The key advantage of the PBD technique over microphonic PAS is that the detector is outside the sample cell. Thus, contamination of the sample by the outgassing of a microphone (5) is precluded, and concern for possible damage to a microphone by high sample cell temperatures or pressures is eliminated. As is true for photothermal techniques generally, a poor signal-to-noise ratio is characteristic of PBD experiments of the sort described above. Perturbation of the sample surface by the energy of the illuminating beam (6, 7), environmental vibration (8), and weak temperature dependence of the refractive index of gases (9) pose obstacles to the application of PBD spectroscopy. A straightforward improvement in sensitivity should accrue by the introduction of the sample into the optical cavity of the grazing incidence detecting laser. In this case the signal, represented by a decrease in the net gain of the detecting laser, would arise from diffraction losses in the cavity due to misalignment of the resonator by the thermal deflection process. The decrease in laser output can be up to 100% for lasers operating near the threshold. Additional advantages are that no special position sensing photodiodes are required for intracavity PBD and that the angular stability of the laser resonator can be exploited, thus avoiding the need for a stable detector platform outside the laser cavity. Johnson et al. (10) have demonstrated that the signal dependences on spatial and excitation laser modulation frequency parameters are similar in both internal and external cavity PBD methods but no quantitative comparison with the external cavity beam deflection method was shown. The present study reports a quantitative improvement in the signal-to-noise ratio available with the intracavity PBD technique. This improvement is further illustrated by a quantitative study of several dyes adsorbed on HPTLC plates. EXPERIMENTAL SECTION For the intracavity PBD experiments,the samples were placed at grazing incidence under the laser beam in the intracavity space provided in a Spectra Physics Model 165 argon-ion laser operated at 70-mW power on the 488-nm line as shown in Figure 1. Power levels nearer the threshold incresed the sensitivity to ambient noise sources. A portion ( - 5 % ) of the argon-ion laser output was monitored with a Silicon Detector Corp. 5D-076-12-12-211 photodiode external to the cavity while the chopped (2.25 Hz) output of a Coherent Innova 90 krypton-ion laser operated at 100-mW power on the combined 350.7- and 356.4-nm lines was focused onto that part of the sample lying directly under the grazing incidence argon-ion laser beam. No change in the argon-ion laser transverse mode (TEM,,,,) was observed in the far field. The output argon-ion laser beam noise was reduced by inserting a current-to-voltage converter summed with offset trim 0003-2700/84/0356-2975$01.50/0
into a sixth-order low-pass active filter, 3db point at 30 Hz, between the photodiode and output electronics. The signal was presented to a PARC Model 124A lock-in amplifier through a PARC 116 differential preamplifier and then to a Tohshin Electron Model TCO 2001 chart recorder. Signals were simultaneously monitored in every case by using a Tektronix 549 oscilloscope with a Type W plug-in. The planar rear mirror of the argon-ion laser was slightly tilted (using the vertical adjustment) to a position which provides maximum change in output intensity for a given intracavity off-axisdeflection. The optimum tilt of the rear mirror was found by plotting the intensity of the laser beam vs. the rear mirror tilt as measured outside the laser cavity with a United Detector Technology (UDT) Model LSD/5O position-sensingphotodiode placed at a distance of -1.5 m and connected to a UDT Model 431 position monitor (Figure 2). The sensitivity and detectivity measurements described below were made at an optimum laser mirror setting where the change in laser intensity with mirror tilt was maximum. This occurs 5 s of an arc away from the alignment for maximum laser intensity. The 488-nm line was chosen for stable output and a greater dispersion through the intracavity prism relative to the 514.5-nm line. A small intracavity aperture setting was employed to increase the diffraction losses associated with beam deflection. The setup for the external cavity PBD experiments was identical with that described above except that the sample was located external to the argon-ion laser cavity, and the external deflection of the argon-ion laser beam was monitored with the UDT position-sensingphotodiode connected to the UDT position monitor described above. It should be noted that both the krypton-ion and argon-ion lasers used here have adequate intracavity space for these experiments. The choice of the krypton-ion laser as the excitation laser was based on the availability of ample UV power for the TLC experiments. The TLC samples were Merck silica gel 60 HPTLC plates without fluorescent indicator, and the dyes used were Eastman Chemical trans-azobenzene and Congo Red. Mixtures of the dyes were prepared in US1 absolute ethanol diluted to 90% by volume with distilled water. Stock solutions of the dyes (2.4 X and 2.87 X lo4 M for trans-azobenzene and Congo Red, repsectively)were prepared by weight, and dilute solutions were prepared from the stock solutions by volume. Of each solution 1 pL was applied to the HPTLC plates with Drummond Scientific 1-pL disposable pipets. Dispersion of the individual sample spots was minimized by first developing the sample-impregnatedHPTLC plate with a 9 1 by volume mixture of methanol and ammonia. This had the effect of compressing the spot containing the sample mixture to a narrow line (3 X 0.5 mm) parallel to the solvent front. The HPTLC plate was then developed with a 51:4 solution of butanol, acetic acid, and water which separated the sample mixture line into several narrow, parallel lines. The output of the exciting laser was cylindrically focused so the exciting laser spot dimensions approximated the dimensions of the sample spots. The TLC plate was moved under the laser beam by a Burleigh Inchworm positioner/translator that provides precise, vibration-freemotion. The vibration of a stepping motor proved troublesome, thus necessitating the use of the Inchworm. In addition, Norite A carbon black samples applied to HPTLC substrates were used for signal-to-noise comparison studies.
RESULTS AND DISCUSSION Signal-to-noise ratios for the intracavity and external PBD methods described above were compared by using the carbon black samples. Measuring average signal voltage and peakto-peak noise voltage gave approximately a 16-1 advantage in signal-to-noise ratio for the intracavity method over the external PBD. Significant noise contributions in this experiment were ambient mechanical vibrations and air currents. These sources were diminished through vibration isolation and enclosure of the experiment. The residual vibrational noise @ 1954 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984
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Mirror
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Figure 3. Chromatograms of Congo Red and trans-azobenzene on two HPTLC plates detected by the intracavity laser PBD technique shown in Figure 1. (a) The amounts of Congo Red, 1, and fransazobenzene, 2, in the spots are 30.9 and 230 ng, respectively. (b) The corresponding amounts are 1.4 and 10.5 ng. trans-azobenzene, ng (-C-) 50 IO0
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Figure 2. Dependence of detecting laser intensity on beam deflection determined by trimming the mirror angle at one end of the argon-ion laser cavity. degrades the intracavity results to a lesser extent due to the superior mechanical stability of the laser resonator and the higher sensitivity to thermal gradients from the sample than in the extracavity arrangement. Figure 3 shows typical chromatograms produced by the intracavity PBD experiment. The chromatogram of greater amplitude, a, is of stock solutions of Congo Red and transazobenzene. The chromatogram b shows the corresponding peaks at greatly reduced concentrations of the dyes. The small peak to the left of the Congo Red peak is apparently an impurity in the Congo Red. The detecting argon-ion laser was operated a t X = 488 nm with 70 mW of power. The krypton-ion laser illuminated the samples with two wavelengths simulataneously, X = 350 and 356 nm, with a total power of 100 mW. The chart speed was 30 cm/h. The sensitivity, S, and detectability, D , calculated (11)for the range of linear response of the analytical curves shown in Figure 4 give for Congo Red and trans-azobenzene S = 8.6 mVJng and D = 0.5 ng and S = 4.4 mV/ng and D = 0.9 ng, respectively. Chen and Morris (12) reported similar sensitivities and detectabilities for an external PBD experiment
0
I I IO 20 Congo Red, ng (+)
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Figure 4. Calibration curves for frans-azobenzepe (0)and Congo Red (0)on HPTLC plates determined with the setup shown in Figure 1. The
diameter of the experimental polnts Indicates the uncertainty in the measurement. with other TLC adsorbed samples. However, the strong dependence of S and D on experimental parameters (including sample optical and thermal properties) in either PBD method precludes a direct comparison of their results with the results noted above. The positive deviation from linearity of the calibration curves (Figure 4) at large sample concentrations probably results from thermal lens defocusing in addition to deflection of the intracavity beam. This effect was observed in an external cavity PBD experiment as a distortion of the deflected beam at higher sample concentrations. Sensitivity in the intracavity method can be further enhanced by destabilizing the argon-ion laser cavity by insertion of an output mirror of greater radius of curvature. The stock output mirror of the argon-ion laser, with a radius of curvature of 200 mm, was replaced by one with a radius of curvature of 500 mm. This modification, however, made the intracavity method more sensitive to the vibration and air current noise
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Anal. Chem. 1984, 56, 2977-2979
sources mentioned above and required greater effort in isolating the experiment from them. In spite of this difficulty, there was a 25% improvement in signal to noise (measured in the same manner as above) using the intracavity method with the 500-mm radius of curvature mirror vs. the same method with the 200-mm radius mirror. This result represents a 2O:l detection advantage for the intracavity method over the external PBD method. Registry No. Carbon, 7440-44-0.
(8) Low, M. J. D.; Lacrolx, M.; Morterra, C. Spectrosc. Lett. i982, 75, 57. (9) Fournier, D.;Boccara, A. C.; Badoz, J. Appl. Opt. i982, 21, 74. (10) Johnson, C.; Brundage, R. T.; Giynn, T. J.; Yen, W. M. J . fhys., Colloq. (Orsay, Fr.) 1983, 44, 248. (11) Coddens, M. E.; Poole, C. F. Anal. Chem. i983, 55, 2429. (12) Chen, T. I.; Morris, M. D. Anal. Chem. 1984, 56, 19.
Tsutomu Masujima Asutosh N. Sharda Lindsay B. Lloyd Joel M. Harris Edward M. Eyring*
LITERATURE CITED (1) Boccara, A. C.; Fournier, D.; Badoz, J. Appl. Phys. Lett. i980, 36, 130. (2) Jackson, W. B.; Amer, N. M.; Boccara, A. C.; Fournier, D. Appl. Opt. 1981, 20, 1333. (3) Aamodt, L. C.; Murphy, J. C. J . Appl. fhys. 1983, 54, 581. (4) Mandeiis, A. J . Appl. Phys. i983, 54, 3404. (5) Low, M. J. D.; Morterra, C.; Severdia, A. G. J . Mol. Catal. 1983. 2 0 , 311. (6) Pichon, C.; Leiiboux, M.; Fournier, D.; Boccara, A. C. Appl. fhys. Lett. 1979, 35, 435. (7) Low, M. J. D.; Lacrolx, M.; Morterra, C. Appl. Spectrosc. 1982, 36, 582.
Department of Chemistry University of Utah Salt Lake City, Utah 84112
RECEIVED for review June 11, 1984.
Accepted July 31, 1984. Financial support of this work by a grant from the Department of Energy (Office of Basic Energy Sciences) is gratefully acknowledged.
Solution introduction Mass Spectrometry: A Simple Alternative to Desorption Techniques Sir: A considerable number of techniques have recently been developed for obtaining mass spectra from nonvolatile and thermally labile samples. These new techniques generally require specialized source facilities such as field desorption ( I ) , secondary ion mass spectrometry (2), fast atom bombardment (3), liquid ionization (4),electrohydrodynamic ionization (5), and laser desorption (6). Plasma desorption mass spectrometry (7),which is one of the more successful of the new techniques, requires unique mass spectrometry facilities. Direct introduction of liquid chromatographic effluents into mass spectrometer ion sources requires the presence of both a liquid chromatograph and an interface system in the mass spectrometry laboratory. The thermospray (8)interface and other methods of direct liquid introduction (9) have allowed determination of mass spectra of compounds that would not normally vaporize under conditions of direct probe introduction. It was the objective of this research to develop a procedure for obtaining mass spectral data on nonvolatile compounds with minimal modifications to a commercial mass spectrometer. The experiments with direct interfaces between liquid chromatography and a mass spectrometer have clearly shown that overcoming a compound’s lattice energy by introducing the compound in solution makes i t possible to obtain mass spectral information from compounds that would be destroyed during attempts to evaporate them directly from a solid matrix. Many LC/MS interfaces that have been described have typically used nebulizers with holes a few micrometers in diameter for solution introduction. This report presents a method for using the same principles for sample introduction without the need for modifying the instrument or utilizing a liquid chromatograph td present the sample to the mass spectrometer. EXPERIMENTAL SECTION The mass spectrometer used in these experiments was a Finnigan 4510 GC/mass spectrometer equipped with a temperature controlled direct insertion probe. The instrument was operated in ita normal positive or negative chemical ionization mode with 0.4 torr of isobutane, as the reagent gas. Both source temperature 0003-2700/84/0356-2977$01.50/0
and direct insertion probe temperature were treated as experimental variables and are reported with the spectra. Primary ionization was accomplished with an 89-eV beam of electrons. Solution Injection System. Figure 1 illustrates the tubes that we have used for direct injection of solutions of nonvolatile compounds. A 2 mm 0.d. Pyrex capillary was fashioned to replace the quartz or ceramic probe tip that we usually use with this mass spectrometer, The conventional probe tip has a diameter of 2 mm and a length of 11 mm. After an approximately 12 mm segment of capillary tubing sealed at one end was cut, the open end of the tube was fire polished in a micro oxygen-natural gas flame. The fire polishing was continued until the outlet diameter was between 250 and 220 pm. This was checked periodically by cooling the capillary and inserting a 220-pm fused silica needle from a conventional on-column capillary GC injection syringe. With a small amount of practice, probe tips like that in Figure 1 can be made in any quantity desired. Solytions were prepared by dissolving substrates in HPLC water from a Milli-Q system to obtain a concentration of approximately 1 mM/L. Approximately 5 p L portions of these solutions were used to fill the capillaries. The capillaries were introduced into the mass spectrometer as probe tips in the conventional way except that the pump-down time in the direct probe interlock was limited to 2 s.
RESULTS AND DISCUSSION Figure 2 illustrates a solution introduction mass spectrum obtained by inserting a capillary containing 5 KL of a 2 mM solution of erythromycin A in water into an isobutane CI plasma. The evaporation profile for the solution is illustrated in Figure 3. The background positive and negative CI spectra of water are given in Figure 4. The protonated molecule of erythromycin is prominent in Figure 2 at m / z 734. Fragment ions at m / z 658,876, and 716 correspond to loss of C2H403, C2H202,and water, respectively, from the protonated molecule. The fact that the protonated molecule is not the base peak in this chemical ionization mass spectrum suggests that the ionizing conditions in the water-isobutane plasma generated during the injection process are more energetic than in a simple isobutane chemical ionization plasma (10). The smooth evaporation profile which is illustrated in Figure 3 was characteristic of only approximately half of our 0 1984 American Chemical Society
