Quantitative Analysis of a Binary Mixture by Fourier Transform Infrared Photoacoust ic Spectroscopy

M A R K G. R O C K L E Y , D E N N I S M. D A V I S , a n d H U G H H. R I C H A R D S O N

Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078

The isotopic shift of the r2 N O 3 -1 absorption of mixtures of K15NO3 and K14NO3 at 800 and 825 cm -~ has been used to dem- onstrate that Fourier transform infrared photoacoustic spec- troscopy is a technique which is useful for quantitative analysis of solid mixtures. Index Headings: Fourier transform infrared photoacoustic spec- troscopy.

centage of each salt, the p2 absorpt ion feature at 825 c m - ' was analyzed. The intensi ty of this absorpt ion feature (generally consisting of the two bands due to the two different salts) at 800 cm -1 was rat ioed to the sum of the intensities at 825 and 800 cm -1. In this manner , a relative signal s t rength for the 'SN salt absorpt ion was obtained for each mixture.

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

Fourier t ransform infrared photoacoust ic spectroscopy (FT-IR-PAS) of solids is a relat ively recent exper imenta l development 1-4 which holds great promise as a technique, complemen ta ry to diffuse reflectance, ~ for infrared anal- ysis of solid surfaces. I t is essential, therefore, tha t the question of whether or not F T - I R - P A S can be used for quant i ta t ive analysis of mul t i component surfaces and bulk solids be answered. This work describes an experi- men t designed to find an answer to this question.

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

The exper imental setup used to measure the FT- IR- PAS spectra has been described elsewhere. 2' 3 In an at- t e m p t to see if the F T - I R - P A S signal s t rength was cor- related with the percent by weight concentrat ion of the sample, the spectra of a set of mixtures of K15N03 and K14NO3 were measured. The K'~NO3 was obtained f rom Stohler Isotope Chemicals Co. The K'4N03 was obtained f rom Baker Chemical Co. Both were >99% isotopically pure. A set of mixtures of the two salts was carefully weighed out and ground with an agate mor t a r and pestle to increase the sample surface area and ensure sample homogeneity. In practice, the F T - I R - P A S signal s t rength is dependent on the part iculate size. However, because these exper iments relied on the measu remen t of relative absorpt ion peak heights of the two salts, the absolute signal magni tude was not an impor tan t consideration. Consequently, the salt mixtures were not sized.

Fig. 1 shows the unsmoothed, unre touched F T - I R - P A S spectra of pure KN03 and a mixture of the two KNO3 salts. The inset, which shows an expansion of the spec- t r um in the vicinity of the v2 absorpt ion at 825 cm- ' , clearly shows the isotopic shift expected and the presence of the two components in the 50:50 mixture. The bo t tom spec t rum is of the pure K'4NO3 while the top spec t rum is of a 50:50 mixture of K'SNO3 and K'4NO3.

To obtain an es t imate of the quant i ta t ive dependence of the F T - I R - P A S signal s t rength on the relative per-

Received 3 September 1980; revision received 4 November 1980.

Volume 35, Number 2, 1981

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

Fig. 2 is a graphical representa t ion of the relative absorbance by K15N03 as a function of the weight per- centage of this salt in the mixture. A least squares fit of the data gives a s t ra ight line with a slope of 0.66 and a correlation coefficient of 0.98. This line intercepts the ordinate at 0.24. This is not to say tha t KI4N03 was 24% K~NO3. Rather , this in tercept represents the relative height of the shoulder on the K'4NO3 absorbance. For similar reasons, 100% K15NO3 registers as 90% purity. T h a t is, there is a smaller but finite high frequency tailing of the K'SN03 absorption.

~pas

4 0 0 0 2 4 0 0 8 0 0

C M -1

Fro. 1. FT-IR-PAS spectra of K~4N03 (lower spectrum) and a 50:50 mixture of K'4NO3 and K'aNO.~ (upper spectrum). The inset shows an enlargement of the two spectra in the vicinity of the 825 cm -~ absorp- tion. For each spectrum 1000 scans at 8 cm -~ resolution were coadded. Analysis time on the evacuated FTS-20 C Digflab spectrometer was ~16 min per sample. An internal 16× amplifier on the FT-IR spectrom- eter was used for additional amplification.

1.0

0.5

O

0.¢

/ 2~ ' 7's ,oo

% 15N

FIG. 2. Relative peak intensity at 800 cm-' vs 825 cm -~ plotted against percent KlaN03. The least squares line fits the data with a correlation coefficient of 0.98.

APPLIED SPECTROSCOPY 185

Thus, FT-IR-PAS has potential for use as a quantita- tive tool for analysis of the relative composition of mul- ticomponent solid substrates. The use of this method for depth profiling of a surface should be viewed with cau- tion, however, since the FT-IR-PAS spectrum arises from sampling depths which vary with the infrared modulation frequency and consequently, the infrared spectral fre- quency.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the support of this work by the National Institutes of Health (Grant 5RO1GM25353-03). The authors especially appreciate the generous donation of the nitrate salts by Professor J. Paul Devlin.

1. M. G. Rockley, Chem. Phys. Lett. 68, 455 (1979). 2. M. G. Rockley and J. Devlin, Appl. Spectrosc. 34, 407 (1980). 3. M. G. Rockley, Appl. Spectrosc. 34, 405 (1980). 4. D. W. Vidrine, Appl. Spectrosc. 34, 314 (1980). 5. M. P. Fuller and P. Griffiths, Anal. Chem. 50, 1906 (1978).

Analysis of a Polarizing Michelson Interferometer for Dual Beam Fourier Transform Infrared, Circular Dichroism Infrared, and Reflectance Ellipsometric Infrared Spectroscopies

M. J. DIGNAM and M. D. BAKER Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 1A1

The p rope r t i e s of a Four ie r t r a n s f o r m in f ra red (FT-IR) spec- t r o m e t e r des igned a round a polar iz ing Michelson in te r fe rome- ter (PMI) are analyzed wi th r e spec t to appl ica t ions in bo th convent iona l abso rp t ion as well as polar iza t ion spect roscopies . The PMI des ign is t ha t of Mart in and Pup le t t and cons is t s of a meta l gr id polar iz ing b e a m spl i t ter combined wi th roof top re- t r o m i r r o r s set to ro ta t e the polarizat ion direction in each beam by 90 ° . I f used in conjunct ion wi th addi t ional gr id polar izers , pos i t ioned according to applicat ion, the resu l t is an FT-IR in- s t r umen t t ha t can funct ion bo th as a dual beam absorp t ion spec t rome te r and as a polar iza t ion spec t romete r . Opera t ing in the fo rmer mode, the i n s t r u m e n t is predicted to achieve speeds comparab le to those achievable us ing a nonpola r iz ing dual beam Michelson i n t e r f e rome te r but to cover a much wider spec t ra l r ange due to the achromat ic na tu r e of meta l gr id po- lar izing beam spl i t ters . In the la t te r mode, it is predicted to give at least double the effective throughput achievable w i th con- vent ional i n s t r u m e n t s equipped wi th ex te rna l polar iza t ion mod- u la t ing optics, and again to cover a much wider spec t ra l range . It would ex tend the capabi l i ty of de t e rmin ing circular dichroism spec t ra into the far IR (down to - 3 cm -1) for the f i rs t t ime. With the deve lopment of a p rocess for forming a meta l gr id polar izer of fine spac ing (~0.1 pm) on a KBr subs t ra te , the i n s t r u m e n t would be capable of cover ing the spect ra l r ange 25 000 to 3 cm -1 or lower us ing only two b e a m spl i t ters , i.e., in two ranges : 25 000 to 450 cm -1 us ing a KBr - suppor t ed meta l grid, and 600 to 3 cm -1 or less us ing an u n s u p p o r t e d wire grid.

Index Headings: Inf rared; Four ie r t r ans fo rm; Polar izat ion; In- s t rumenta t ion .

INTRODUCTION

In the near infrared, conventional beam splitters for

Received 30 June 1980; revision received 10 October 1980.

186 Volume 35, Number 2, 1981 .

use in Fourier transform infrared (FT-IR) spectrometers 1 consist of dielectric flats, coated with a dielectric layer of thickness chosen to result in a reflectivity of the order of 50% over the desired spectral range (e.g., Ge-coated KBr). In the far infrared, a thin Mylar film (polythene tere- phthalate) is commonly used in beam splitters, with the thickness chosen to give rise to an optical path length close to ~/2 in order to achieve adequate reflectivities. In both cases, the optimum performance of the beam split- ter (-50% reflectance, 0% absorption) is achieved over a narrow wavelength range only. This is most serious in the far IR, where four or five beam splitters are required to cover efficiently the entire spectral range of 500 to 10 cm -1, a range otherwise requiring only a single detector (liquid helium cooled bolometer or triglycine sulfate) and a single light source (mercury arc lamp). Recently Burton and Akimoto 2 described an FT-IR instrument designed around a polarizing beam splitter which covered the entire range 3 to 400 cm -~ at high efficiency.

In this paper we investigate the properties of an FT- IR spectrometer based on a polarizing Michelson inter- ferometer (PMI). In addition to both single and dual beam operation, it is capable of operating in a polari- metric mode, determining both circular and linear di- chroism spectra along with the corresponding differential dispersion spectra. In a specular reflection mode it is therefore capable, of determining the relative absorption and dispersion spectra of surfaces, leading, for example, to high sensitivity vibrational spectra of species adsorbed onto single crystal metal surfaces. 3' 4

The design of the polarizing interferometer is that given by Martin and Puplett. '~' 6 A metal grid polarizer is used as the beam splitter with its transmission axis set at 45 ° to that of an input grid polarizer. Combined with fixed and moving rooftop retromirrors in the arms of the

APPLIED SPECTROSCOPY