RESULTS AND DISCUSSION

The anticipated pulse broadening was calculated by means of procedures outlined in an earlier publication from this group, 4 and the results for an f /30 system are shown in Fig. 2. Pulse broadening is expected to be min- imal for optical fibers that are less than 200 m in length, but for longer fibers the effect of broadening is antici- pated to be more severe.

The pulse widths (full width at ha l f -maximum, FWHM) measured after the laser-beam passage through fibers of different lengths and core diameters are not statistically different from the 310-ps pulse width mea- sured before the fiber. Table I lists the instrumental variations and the average pulse widths that were mea- sured. The choice of fiber length was dictated largely by the availability of optical fibers in our laboratory. No appreciable pulse broadening is observed for fibers up to 60 m in length. Figure 3 shows several of the oscillo- scope tracings obtained in this experiment.

Because there is considerably more light reaching the active area of the first photodiode than that of the pho- todiode located after the fiber (see Fig. 1), the signal-to- noise ratio is worse for after-fiber measurements. Simi- larly, the signal-to-noise ratio decreases with increasing fiber length, because of loss of light from the fibers. (In an ideal fiber, such loss would not occur, but the fibers and wavelengths used here do not exhibit total internal reflection.) It is important to note that each photograph shows several oscilloscope tracings; pulse widths report- ed in Table I are average values from many tracings.

A 600-m length of 100-/zm:core fiber was aligned with the optical system, but no pulses were observed on the oscilloscope screen. Although it is possible that the signal is broadened so much as to be indistinguishable from the noise (theory predicts a 430-ps pulse width), it is more likely that the signal is too weak to be detected, because of losses in the fiber. Increasing the laser tube current (and, thus, average radiant power) does not help the situation because it leads to degradation of the pulse profile into two- and then three-pulse trains. The signal from a Hamamatsu R928 photomultiplier tube, placed at locations before and after the optical fiber, was mon- itored on the oscilloscope. The pulse profile is consid- erably broader for light exiting the fiber than for scat- tered light from the beam translation optics (light that has not entered the fiber), but the P M T response time is insufficient for accurate pulse resolution.

In short, no appreciable pulse broadening is observed for optical fibers of length 60 m or less. These lengths are comparable to those used in most laboratory or in- plant monitoring experiments. The experimental results agree well with the theory. In fact, loss of beam intensity in the optical fiber appears to be a more serious instru- mental consideration than pulse broadening. It is im- portant to note that pulse broadening will be much more severe for a broad-band signal (such as fluorescence) than for a nearly monochromatic laser pulse, because the re- fractive index of the fiber varies with wavelength. Hence, a broad-band signal will experience additional broad- ening. The importance of this additional broadening will have to be assessed in individual applications because of their differing spectral-bandwidth requirements.

ACKNOWLEDGMENTS This work was supported by the National Science Foundation, Grant

CHE 90-20631. M.K.C. acknowledges support from an ACS Analytical Division Full-Year Fellowship, sponsored by Perkin-Elmer Corpora- tion. The authors extend their thanks to Dr. Dean Geraci at Galileo Electro-Optics for the donation of the 100-~m-core-diameter optical fiber used in this research.

1. O. S. Wolfbeis, Trends Anal. Chem. 4, 184 (1985). 2. W. R. Seitz, CRC Crit. Rev. Anal. Chem. 19, 135 (1988). 3. O. S. Wolfbeis, Chem. Anal. (N.Y.) 77 (Mol. Lumin. Spectrosc., Pt.

2), 129 (1988). 4. G. H. Vickers, R. M. Miller, and G. M. Hieftje, Anal. Chim. Acta

192, 145 (1987). 5. R. D. Petrea, M. J. Sepaniak, and T. Vo-Dinh, Talanta 35, 139

(1988). 6. M. E. Lippitsch, J. Pusterhofer, M. J. P. Leiner, O. S. Wolfbeis,

Anal. Chim. Acta 205, 1 (1988). 7. M. K. Carroll, F. V. Bright, and G. M. Hieftje, Anal. Chem. 61, 1768

(1989). 8. K. Gustafsson, B. Hoek, L. Jonsson, and C. Ovren, Sens. Actuators

19, 327 (1989). 9. S. H. Lieberman, S. M. Inman, G. A. Theriault, Proc. SPIE - Int.

Soc. Opt. Eng. 1172, 94 (1990).

A New Sample Cell for Diffuse Reflectance IR Spectroscopy of Air-Sensitive Solid Samples*

S H E N G DAI, J . P. Y O U N G , t and G L E B M A M A N T O V Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6142 (S.D., J.P.Y.) ; and Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996 (G.M.)

Index Headings: Infrared; Air-sensitive samples; Reflectance; Spectros- copy techniques.

INTRODUCTION

The advent of diffuse reflectance (DR) IR spectroscopy 1 as a routine technique has greatly facilitated the study of otherwise intractable solid samples. In particular, re- search involving coals, 2 catalytic processes, 3 and solid- state photochemical reactions 4 is being carried out by this technique. However, infrared spectra of air-sensitive solid powders are often difficult to measure with con- ventional DR accessories. For such studies, several ven- dors of IR accessories (Spectra-Tech, Inc. and BIO-RAD, Inc.) recommend techniques involving the use of an en- vironment chamber. This is a relatively expensive com- ponent (103 $) and is more elaborate than instrumen-

Received 18 February 1991. * Research sponsored by U.S. Department of Energy, Office of Indus-

trial Processes, Conservation and Renewable Energy, under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.

t Author to whom correspondence should be sent.

1056 Volume 45, Number 6, 1 9 9 1 0003-7028/91/4506-105652.00/0 APPLIED SPECTROSCOPY © 1991 Society for Applied Spectroscopy

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FIG. 1. A side view of the sample cell where A is the sample cap; B, a ½-in. o.d., ¼6-in. O-ring; C, a ½-in.-dia. window; D, the sample cover; and E, the copper beam blocker. The top view of the assembled sample cell is shown above.

tation required for many routine studies. In an effort to provide a DR cell for routine measurement of hydroxide species in various anhydrous but extremely water-sen- sitive inorganic salts, we have developed a cell of simple design which fits in sample-cap holders of Spectra-Tech DR accessories.

DISCUSSION

This new sample cell is composed of three parts: a window, a stainless steel sample cap which is a simple modification of one made by Spectra-Tech, and a stain- less steel cell cover. The cell is shown in Fig. 1; the window is compression-sealed to the sample cap with a rubber

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FIG. 2. Diffuse reflectance IR spectra of magnesium chloride powder in the sealed sample cell: (upper spectrum) anhydrous sample loaded in an inert atmosphere box; (lower spectrum) the same sample after momentary exposure to air.

O-ring and a close fitting sample cover. A slot is cut across the periphery of the sample cover (Fig. 1, top) so that a copper beam blocker s can touch the window surface to prevent specular reflectance from the window. The win- dow can be made from any appropriate material; we have used yttrium oxide which is nonhygroscopic and trans- parent in the 4000-1400 cm -1 region of the spectrum. Although this apparatus was specifically developed for a particular type of reflectance measuring attachment, the design idea is generic, and similar devices could be fabricated that fit other accessories. The key for obtain- ing useful spectra is to prevent specular reflection of the cell window from interfering with the desired diffuse reflectance.

Air-sensitive samples are loaded into the sample cap in an inert atmosphere drybox. The sample cell is as- sembled as shown in Fig. 1 (bottom) and transferred outside the drybox for DR IR measurement. Figure 2 shows DR IR spectra of laboratory-prepared anhydrous magnesium chloride before and after a short exposure to the open air. As originally loaded, no evidence of any absorption is seen in the region of 3400 cm-1; the spectral trace (upper) is essentially flat except for the water vapor absorption originating within the instrument in the re- gion blue of 3500 cm -1. After momentarily exposing the sample to laboratory air (i.e., removing and replacing the window and cell cover), one obtains the lower spectral trace. This latter spectra shows strong absorbance from water that was absorbed by the sample.

No evidence of hydroxide species or water has yet been found in anhydrous magnesium chloride samples pre- pared by the decomposition of ammonium carnallite, e,7 A calibration curve of the absorbance of hydroxide as a function of concentration in simulated samples was de- termined, however, and is shown in Fig. 3. The samples were prepared by mixing various amounts of calcium hydroxide and potassium carbonate for 4 min in a Wig-

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FZG. 3. Absorbance vs. hydroxide concentration for mixtures of calcium hydroxide and potassium carbonate; mixtures were ground together prior to measurement.

L-Bug grinder (Crescent Dental MFG. Co.). Quantitative results obtained with this cell in that study seem quite satisfactory.

ACKNOWLEDGMENTS

This research was sponsored by the Office of Industrial Processes, Conservation and Renewable Energy, U.S. Department of Energy, un- der Contract AC05-840R21400 with Martin Marietta Energy System, Inc. One of us (Sheng Dai) is a participant in the U.S. Department of Energy Postgraduate Research Program at the Oak Ridge National Laboratory administered by Oak Ridge Associated Universities.

1. P.R. Griffiths and M. P. Fuller, in Advances in Infrared and Raman Spectroscopy, R. J. H. Clark and R. E. Hester, Eds. (Heyden and Son, London, 1982), Vol. 9, Chap. 2, pp. 63-129.

2. E. L. Fuller, Jr., and N. R. Smyrl, Appl. Spectrosc. 44, 451 (1990). 3. I. M. Hamadeh, D. King, and P. G. Griffiths, J. Catalysis 88, 264

(1984). 4. P. W. Yang and H. L. Casal, J. Phys. Chem. 90, 2422 (1986). 5. R. G. Messerschmidt, Appl. Spectrosc. 39, 737 (1985). 6. W. K. Behl and H. C. Gaur, J. Sci. Industr. Res. 20B, 231 (1961). 7. G. P. Smith and C. R. Boston, J. Chem. Phys. 46, 4051 (1965).

1058 V o l u m e 45, N u m b e r 6, 1991