is now being carried out for the surface characterization of industrially used materials such as carbon fibers, plastic films, and metals by applying a thin Ag evapo- rated film onto these surfaces. ~9

ACKNOWLEDGMENTS

We would like to express our grateful acknowledgment to Mr. Umemura, Musashino Electrical Communication Laboratory, N.T.T., for supplying us with the samples of Ag contacts and also for the AES measurements. Our thanks are also expressed to Professor Yamada, Kwansai Gakuin University, and Dr. Ka- tagiri of our laboratory for their helpful discussions. We also thank Mr. Soeda and Dr. Nagasawa of our laboratory for the preparation of the carbon layers and the measurement of their thicknesses.

1. R. K. Chang and T. Furtak, Surface Enhanced Raman Scattering (Plenum, New York, 1982).

2. C. A. Haque and A. K. Spiegler, Appl. Surface Sci. 4, 214 (1980).

3. P. Dhamelincourt, F. Wallart, M. Leclercq, A. T. N'Guyen, and D. O. Landon, Anal. Chem. 51,414A (1979).

4. M. Delhaye and P. Dhamelincourt, J. Raman Spectrosc. 3, 33 {1975}. 5. M. E. Anderson and R. Z. Muggii, Anal. Chem. 53, 1772 (1981). 6. F. Tuinstra and J. L. Koenig, J. Chem. Phys. 53, 1126 (1970). 7. M. Nakamoto, R. Kammereck, and P. L. Walker, Jr., Carbon 12,259 (1974). 8. R. P. Cooney and M. R. Mahoney, in Proceedings of the VIIth International

Conference on Raman Spectroscopy (1980), p. 404. 9. M. W. Howard and R. P. Cooney, Chem. Phys. Lett. 87, 299 {1982).

10. I. Pockrand and A. Otto, Appl. Surface Sci. 6,362 {1980). 11. A. Otto, Surface Sci. 75, L392 (1978). 12. J. C. Tsang, J. E. Demuth, P. N. Sanda, and J. R. Kirtley, Chem. Phys. Lett.

76, 54 (1980). 13. R. Iwamoto, M. Miya, K. Ota, and S. Mima, J. Chem. Phys. 74, 4780 (1981). 14. M. Delhaye, M. Dupeyrat, R. Dupeyrat, and Y. Levy, J. Raman Spectrosc.

S, 351 (1979). 15. E. Kretsehmann, Z. Phys. 241,313 (1971). 16. J. Trilhe and A. T. N'Guyen, Lfic. Chim. 45 (1980). 17. L. V. Delpriore, C. Doyle, and J. D. Andrade, Appl. Spcctrosc. 36, 69 (1982). 18. H. Yamada, Y. Yamamoto, and N. Tani, Chem. Phys. Lett. 86,397 (1982). 19. H. Ishida and A. Ishitani, to be published.

Convective Effects in Thermal Lens Spectroscopy

C L I F F O R D E. B U F F E T T and MICHAEL D. MORRIS* Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109

Thermally induced solution stirring can cause periodic signal fluctuations which can be the sensit ivity l imit ing factor in t h e r m a l lens spectroscopy w i t h a chopped pump l a s e r . P u m p and probe laser fluctuations are s h o w n to be random, w h i l e thermal lens signals are s h o w n to h a v e a s m a l l p e r i o d i c c o m - p o n e n t caused by stirring. T h e p r o b l e m is serious in dilute solutions because of the high solvent background absorbance. Index Headings: T h e r m a l lens spectroscopy; Laser, argon ion ; Techniques, spectroscopic.

I N T R O D U C T I O N

The thermal lens effect has been known since the early 1960's and has been used by physical chemists for meas- urement of small absorbances in both the gas and liquid phase. 1-3 Several experimental configurations have been used and both pulsed and cw lasers have been used. With pulsed lasers an independent probe laser, generally a small He-Ne, is used to monitor the lens formation as the expansion of the probe beam over an aperture. A similar configuration can be used with cw lasers and is necessary if a dye laser is scanned to generate a spectrum. In this case the laser generating the lens is chopped at a low frequency and the modulation of the probe laser is detected with a lock-in amplifier. 4 For a fixed frequency laser, however, one can simply monitor the time-depend- ent defocusing of the beam through a limiting aperture. 5' 6 Generally, in two laser experiments the lasers are colli- near, although crossed beam geometries could allow spa- tial profiling and may prove advantageous in background subtraction. 7

Received 22 Sep tember 1982. * Au tho r to whom correspondence should be addressed.

Volume 37, Number 5, 1983

Recently, analytical chemists have begun to examine thermal lens spectroscopy as an alternative to photoa- coustic spectroscopy for measurement of small absor- bances. Measurements in flowing systems have been reported, s Thermal lens detectors for liquid chromatog- raphy have been described. 9' 10 Application to enzymatic analysis has been demonstrated. 11

It is now realized that mixing effects can degrade thermal lens measurements in moving systems s and that probe laser intensity fluctuations may be the limiting noise source in two-beam configurations. 7 In this com- munication we demonstrate that convective mixing may cause small, regular signal undulations. These small sig- nals can become a major problem in solution measure- ments, where the desired signal must be subtracted from a large solvent absorbance.

I. E X P E R I M E N T A L

The experimental configuration has been previously described. 1° The 514.5 or the 488.0 nm line from the argon ion laser was used as required. Power was con- trolled at 200 mW at the laser head and was about 190 mW at the sample. Glass spectrophotometer cells were used as sample holders. For most experiments 2-cm path length, 1.8-cm diameter cylindrical cells were used. For certain experiments 1-cm square cross section cells were employed. The lenses used to focus the beams into the cell and to relay the signal to the optical fiber were 100 mm focal length achromats. Standard glass filters (Corn- ing 3-66 or 3-67) were used to prevent the argon laser beam from entering the optical fiber. The filters were placed just before the relay lens, where the argon beam

APPLIED SPECTROSCOPY 455

1500 was expanded to a diameter of about 1 cm. The thermal lens signal was demodulated with a lock-

in amplifier (PARC, model 5101). The lock-in output time constant was always maintained at 1 s. The lock-in signal was sampled, digitized with 12-bit resolution, and stored.

Samples were prepared in distilled water or ACS re- agent grade carbon tetrachloride and filtered through 0.22 ~ membrane filters to remove any suspended solids. Both cobalt(II) ion and the peroxidase-catalyzed oxida- tion product of 4-aminoantipyrine were used to generate aqueous solutions of known absorbance. The latter com- pound was used because the monitoring of enzymatic reactions by thermal lens spectroscopy is one of the ultimate aims of this work. 11 Iodine was used as the solute in carbon tetrachloride. The system was calibrated against a spectrophotometer by preparing solutions of absorbance of approximately 0.1 and diluting these to the useful range of 1 × 10 -3 to 1 × 10 -6 absorbance.

II. R E S U L T S A N D D I S C U S S I O N

We have found the thermal lens signal in aqueous solution to be linear over the absorbance range 3 × 10 -5 to 9 X 10 -4, the upper limit of our measurements. In each case the signal is superimposed on a water absorbance of 2 × 10 -4, as measured from the intercept of a (nominal) absorbance vs signal line. The measured water absor- bance is within the range, 1 × 10 -4 to 4 × 10 -4 c m -1, observed around 500 nm by other workers. 12 The stand- ard deviation is about 7 × 10 -6 absorbance.

The chopper period dependence of the lensing signal is summarized in Fig. 1. The signal increases with in- creasing chopper period, as expected. The signal is linear in chopper periods below about 0.05 s (20 Hz) and approaches a limiting value at long chopper periods, as predicted from theory. 4 Although the magnitude of the thermal lens signal decreases with increasing chopper frequency, the signal/noise ratio is approximately con- stant over the range of frequencies examined. Thus, there is no penalty for the use of modulation frequencies higher than the conventional 5 to 20 Hz. This finding suggests that modulation frequencies can be chosen sufficiently high to "stop" mobile phase motion in liquid chromatog- raphy, for example, 1° without loss of sensitivity.

Our earliest measurements were made with 1-cm square cross section cells. We observed that the meas- urements often appeared to have a clearly periodic com- ponent. Moreover, the magnitude and regularity of these fluctuations increased or decreased in puzzling ways with small changes in cell position or with changes in sample volume. Because these fluctuations are a serious experi- mental problem, their source and possible cure were investigated.

The thermal lens apparatus is mounted on an ordinary laboratory table which provides no isolation from room or chopper vibrations. Attempts were made to cure pos- sible vibration problems by shock-mounting the chopper and the argon laser power supply. These measures pro- vided only small improvement. The apparatus was then moved to a vibration isolation system, but thermal lens signals showed the same large and regular variations.

456 Volume 37, Number 5, 1983

1200

c

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._1

300

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0 I I I 0 30 60 90 120

Chopper Period (ms) FIG. 1. Plot of thermal lens signal vs chopper period for chopper frequencies from 10 to 150 Hz.

Thus, vibration was eliminated as the cause of these variations.

Intensity fluctuations of both the argon ion and he- lium-neon lasers were monitored at several chopping frequencies by observing the signal collected after the beam was spatially integrated by scattering off a ground glass plate. In every case the standard deviation was approximately 0.1%.

The pointing stability of each laser beam was measured by attenuating that beam to a suitable intensity, chop- ping, and measuring the attenuated signal with the de- tector system used for thermal lens measurement. Stand- ard deviations for each beam were approximately 1% and power spectrum plots showed the fluctuations to be ran- dom. The source of this noise is probably table vibration, since the known pointing stability of the lasers, about 0.01 arc s, is much better than this value.

Taken together, these measurements indicate that noise in the thermal lens system is generated by the measurement process itself within the sample cell. Nei- ther room vibrations nor laser intensity fluctuations are limiting factors in the precision of these measurements.

The nonrandomness of the variations in signal inten- sity is demonstrated by a typical power spectrum, as shown in Fig. 2. For comparison we have included the power spectrum for the He-Ne probe fluctuations at the same chopper frequency. The He-Ne fluctuations are random, while the periodicity in the lens signal is very evident. Power spectra were measured at several chop- ping frequencies between 10 and 150 Hz. In every case strong periodicity was observed.

C

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)/ b

[ l I 0.00 o.25 0.50 0.75 1.00

Frequency (Hz) FIG. 2. a. Power spectrum of a typical thermal lens signal at a fixed chopper frequency, b. Power spectrum of the probe laser, chopped at a fixed frequency with no sample cell present.

solution. It is this effect, schematically illustrated in Fig. 4, which generates the periodicity in the observed ther- mal lens behavior.

The thermal properties of carbon tetrachloride are substantially different from those of water. Both the heat capacity and density change with temperature of carbon tetrachloride are greater. For these reasons, circulation patterns can be set up over the entire cell. In water, circulation remains more localized. Thus, undulation frequencies are higher in water, about 0.1 Hz, than in carbon tetrachloride (less than 0.001 Hz).

It is possible to break up these convective resonances by stirring the solution. However, a stirrer also breaks up the thermal lens, particularly at low modulation fre- quencies, and causes a decrease of I order of magnitude in the signal, as well as a serious degradation of the signal/noise ratio.

Because the interferences are small, they are not a serious problem in many applications. Indeed, they are important only if the measured signal is superimposed on a large background, such as a solvent absorbance. Because a solvent is relatively opaque, thermal lens measurements in solutions will often be superimposed on a background. This situation will occur in applications such as detection of liquid chromatography bands or enzymatic analyses. It is likely to be exacerbated by the use of very small sample cells, as encountered in chro- matography detectors. Indeed, we have observed that stopping the pump in our liquid chromatographic system causes formation of very obvious periodicities, due to circulation around the l-ram diameter cell. This behavior

To verify that the periodicity was real, further exper- iments were performed in solutions of iodine in carbon tetrachloride, using the 488 nm line. Carbon tetrachlo- ride was chosen because it generates the strongest ther- mal lens signal of any common solvent. 3 Solutions were made up to 4 × 10 -4 absorbance/cm, and the thermal lens behavior in both square and cylindrical cells was examined. Fig. 3 shows typical time dependences. With the beams centered in the cell, the thermal lens signal oscillated with a period of about 22 rain, with oscillations of about 10% peak-peak. When the beams were asym- metrically placed, parallel to the cell axis, but close to one wall, the undulations showed a regular sawtooth behavior, with the same 22-min period.

These experiments are steady-state experiments in which data are taken after the system has been in the laser beams for some time. If the lasers are abruptly switched into the cell and data taken from the start of the illumination, the thermal lens starts at its maximum value and begins its regular oscillation after a few min- utes.

Simple experiments demonstrate that these undula- tions are due to currents set up in the solutions by the laser-induced temperature gradients. A few drops of a solution of Oil Blue N dye placed on the top of the solution make strong circulation patterns very visible. Dye tracing experiments show that there is a strong upward circulation from the point of formation of the lens. The flowing solution hits the cell walls and is deflected, ultimately generating regular stirring of the

o c o

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I I I I I I I I

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Time (minufes) FIG. 3. Thermal lens signal of iodine in carbon tetrachloride, 2 cm cylindrical cell. a. Lasers positioned at center of cell. b. Lasers posi- tioned near edge of cell.

APPLIED SPECTROSCOPY 457

FlG. 4. Convective flow pa t t e rn in cylindrical sample cell.

is masked by other effects in high flow rate/large diam- eter liquid chromatographic systems, but may become much more important in low flow rate/low volume cells used in microbore column chromatography.

Until now most efforts in thermal lens instrumention have been devoted to design of detector systems. The present work shows that careful study of cell design is also needed. Efforts to optimize cell design are currently under way in our laboratories. A more rigorous theoret- ical description of convective effects under realistic boundary conditions is also in progress.

ACKNOWLEDGMENT

This work was supported in part by National Institutes of Health Grant RM- GM28484.

1. J. R. Whinnery, Acc. Chem. Res. 7, 225 (1974). 2. D. S. Kliger, Acc. Chem. Res. 13, 129 (1980). 3. J. M. Harris and N. J. Dovichi, Anal. Chem. 52,695A (1980). 4. R. L. Swofford and J. A. Morell, J. Appl. Phys. 49, 3667 (1978). 5. N. J. Dovichi and J. M. Harris, Anal. Chem. 51,728 (1979). 6. N. J. Dovichi and J. M. Harris, Anal. Chem. 53, 106 (1981). 7. W. B. Jackson, N. M. Amer, A. C. Boccara, and D. Fournier, Appl. Opt. 20,

1333 (1981). 8. N. J. Dovichi and J. M. Harris, Anal. Chem. 53, 689 (1981). 9. R. A. Leach and J. M. Harris, J. Chromatogr. 218, 15 {1981).

10. C. E. Buffett and M. D. Morris, Anal. Chem. 54, 1824 (1982). 11. J. P. Haushalter and M. D. Morris, Appl. Spectrosc. 34, 445 (1980). 12. C. K. N. Patel and A. C. Tam, Rev. Mod. Phys. 53,517 (1981).

Specif ications for Infrared Reference Spectra of Molecules in the Vapor Phase

P r e p a r e d by the VAPOR P H A S E S U B C O M M I T T E E OF THE C O B L E N T Z S O C I E T Y S P E C T R A L E V A L U A T I O N C O M M I T T E E

Chairman:

Members:

Chairman, Spectral Evaluation Committee:

Peter R. Griffiths University of California-Riverside A n d r e w R. H. Cole University of Western Australia Philip L. Hanst U.S. Environmental Protection Agency Wal te r J . Lafferty National Bureau of Standards

R. Norman Jones NRC Ottawa (retired)

Robert J. Obremski Beckman Instruments, Inc. John H. Shaw Ohio State University

I. PREAMBLE

A. In t roduc t ion . Infrared spectrometry is being used to an increasing extent for the qualitative and quantita- tive analysis of trace components of the atmosphere. Several collections of reference spectra of samples in the vapor phase at ambient temperature are now available. 1-6 The spectra in these collections are measured at medium resolution (Av = 1 to 4 cm-1); in some the samples are

Received 22 November 1983.

458 Volume 37, Number 5, 1983

neat at low pressure while in others an atmosphere of air or nitrogen has been added. Although these reference data are certainly useful to the many users of medium resolution spectrometers (both grating and Fourier transform), the spectra have been measured at too high resolution to provide even semiquantitative calibration factors for users of selective wavelength analyzers and at too low resolution for users of tunable diode laser (TDL) spectrometers. There is now a very definite need for collections of spectra of samples in the vapor phase, measured at a variety of resolutions, with and without air broadening, in as uniform a format as possible.

APPLIED SPECTROSCOPY