Spectrochimica Acta, 1965, Vol. 21, pp. 2135 to 2139. Pergamon Press Ltd. Printed in Northern Ireland

Infrared spectra of powders by means of internal reflection spectroscopy

N. J. HARRICR and N. H. RIEDERMAN*

Philips Laboratories, Briarcliff Manor, New York

(Received 5 April 1965)

Abstract-Studies have shown that Internal Reflection Spectroscopy can be used to record the optical infrared spectra of powders and that there is no scattering of light. Possible applications of this technique include lunar soil analysis.

RECENT unpublished studies at Philips Laboratories have demonstrated that Internal Reflection Spectroscopy can be used to obtain the infrared optical spectra of powdered rocks without the extensive light scattering accompanying standard spectroscopic techniques. These results have been confirmed in the course of a NASA? supported program designed to determine the feasibility of using previously developed tech- niques of Internal Reflection Spectroscopy [l] for lunar soil analysis. This report presents results+ + obtained during this program on test powders ranging in particle diameter from 0 to 43 p.

Briefly, the principles of Internal Reflection Spectroscopy are the following. Light (electromagnetic energy) conducted into materials of refractive index higher than that of its surroundings will be totally reflected internally when the internal angle of incidence exceeds the critical angle. The electromagnetic energy actually penetrates beyond the reflecting interface to depths of a fraction of a wavelength of the light being used. This electromagnetic field can interact with an absorbing medium brought to within a penetration depth, and by proper choice of angle of incidence and refractive index spectra can be obtained which closely resemble those obtained via transmission spectroscopy.

The properties of this interaction and technique are such that, in many instances, significant advantages are obtained over conventional absorption techniques. Some of the advantages previously reported are as follows:

Samples preparation is minimal, i.e. the sample to be studied need only be brought to within the penetration depth of the light beam, and can often be analyzed in its natural state.

* Present address-Ford Instrument Co., Division of Sperry Rand Corp., Long Island City, New York

t This work was supported by NASA Lunar and Planetary Programs, Washington, D.C. under contract number NASw-964.

$ Some of the results reported here were discussed in talks with R. J. P. LYON of NASA Ames Research Center at the Eastern Analytical Symposium in New York, November 12, 1964 and at the Third Annual Meeting of the Working Group on Extraterrestrial Resources at Cape Kennedy, Florida, 17 November, 1964 (to be published in the Journal of Geophysical Research).

[I] N. J. HARRICP, Phy8. Rev. Lettera, 4, 224 (1960); J. FAHRENFORT, Spectrochim. Acta 17, 698 (1961); J. FAHRENFORT and W. M. VISSER, Spectrochim. Acta 18, 1103 (1962); N. J. HARRICK, Ann. N.Y. Acad. Sci. 101, 928 (1963).

2135

2136 N. J. HARRICK and N. H. RIEDERMAN

The bothersome interference phenomenon associated with thin films (liquid or solids) in conventional transmission or reflection spectroscopy is absent.

Trace amounts, less than lo-’ g, of infrared absorbers can be qualitatively and quantitatively analyzed, because the sample can be efficiently placed on the multiple reflection plate [2].

Another important advantage, not previously reported, is that the spectra of the powders can be recorded by simply placing the powder in contact with the internal reflection plate (e.g., by dipping the plate into the powder) and that there is no scattering of light due to the powder.* (It may be recalled that mulls or KBr pellets are required in conventional spectroscopy because of the high degree of scattering, with resultant loss in sensitivity, which occurs when making measurements of powders. Mull techniques can be used successfully where the refractive indices of the mull and particles can be matched and when the particle size is not too large.)

Two double-pass multiple reflection plates (Ref. [3]), with about 100 reflections in each plate, were employed in an optically balanced double-beam arrangement at an angle of incidence of 45--one plate was for the sample beam and the other the reference beam. The plates were constructed of intrinsic germanium single crystals. Germanium has a region of high transparency from 2 to 12 ,LJ, and there are no losses due to molecular resonance absorption in this region. Beyond 12 ,u, however, the transparency of germanium falls off due to lattice bands and/or surface oxidation; two broad absorption bands were noted between 12 and 14.5 ,LJ.

The following materials were obtained and analyzed:

Lo-Micron Silica powder of six different size groupings ranging in diameter from 0 to 43 /J. Quartz powders of diameters 0 to 300 ,u (40 mesh) and 0 to 150 ,u (100 mesh). Frosted soda glass plate and frosted fused quartz plate.

The Lo-Micron Silica powder consisted of ground naturally occurring quartz which is not pure SiO,, since it contains mineral impurities of low metal content. An X-ray

diffraction analysis performed at Philips Laboratories indicated that the powder contained approximately 50 o/o kaolinite clay. The powder was separated into particle size groupings of O-3.5, 3.5-5, 5-10, 10-20, 20-30 and 30-43 p diameters by the process of air elutriation.?

Each powder sample was placed against the sample plate, and an internal re- flection spectrum of each sample grouping was obtained: the 2 to 14.5 p wavelength range was investigated.

As an example of these measurements, the spectrum of the 5-10 ,u diameter sample is presented in Fig. 1, and a plot of percent absorption vs. wavelength is shown in

* Although the reasons for the absence of scattering in the interaction of the penetrating electromagnetic field with the powdered samples are uncertain, we suspect that the explanation involves the non-propagating nature of the standing waves near the reflecting surface.

t Although previous experience with this air elutriator gave reliable results, a check was made to verify the particle separation. A sample count of the 3S43 p grouping showed 81 y0 of the particles to be above 30 ,u, 17.5% in the 20-30 ,U range, and only 15% in the O-20 ,u range.

[2] Work done at Philips Laboratories under Melpar contract No. SU-60397/63. [3] N. J. HARRICK, Anal. Chern. 36, 188, (1964).

Infrared spectra of powders by means of internal reflection spectroscopy 2137

,Zero level ,Bolance curve

14.2 2.8 4.3

:: F Lz

I II 9 9.79.9

Wovelength, p -

Fig. 1. Internal Reflection Spectrum of Lo-Micron Silica Powder (5-10 y diam.) in contact with a germanium internal reflection plate, 0 = 45”.

Wavelength, p

Fig. 2. Plot of percent absorption ~8. wavelength for Lo-Micron Silica Powder (5-10 p diam.).

Fig. 2. The data in Fig. 1 are raw data, i.e. not corrected for change in infrared power level with wavelength. No loss of infrared power was observed in the non-absorbing regions of the spectrum. In the absorbing region, however, all losses can be attributed to the molecular resonance absorption bands of quartz and kaolinite. Similar results were obtained for the other powder particle size groups, as well as for unelutriated

2138 N. J. HARRICK and N. H. RIEDERMAN

Lo-Micron Silica, and for the 0 to 150 ,u and 0 to 300 ,u quartz powder samples. The positions of the quartz absorption band peaks obtained by Internal Reflection Spectroscopy agree with those obtained by Lyon (Ref. [4]) who utilized conventional absorption spectroscopy techniques. The general assignments for the absorption band peaks of quartz are 9.1 to 12.5 ,u (Xi-stretch) and 12.5 to 16.7 ,u (Si-Si stretch).

The measured half-width of the composite absorption band (8 to 10.5 1~) of Lo- Micron Silica powder exhibited no dependence on the particle size. The effect of particle diameter on infrared absorption is shown in Fig. 3 which shows increased absorption for the smaller particle diameters at all absorption band wavelengths.

45,

30-

%

- &

25-

E 8 20-

P 2 $ 15-

IO -

5-

I I I I I I I 0 I I

10 20 30 40 50 60 70 60 60 I I I 100 110 120

Absorption, arbitrary units

Fig. 3. Absorption ‘us. particle diameter for Lo-Micron Silica Powder.

A reason for this dependency of absorption on particle size is that the effective volume of powder being sampled by the penetrating electromagnetic field decreases as the packing fraction decreases.

Transmission spectra via KBr pellets were recorded for a number of powders. Excellent spectra were obtained for the O-3.5 ,u and 3.5-5 ,u powders with only a small degree of scattering. Poorer spectra with large loss of power were obtained for the larger powders, and no useable spectra were obtained for the 30-43 ,U powder. The usual Christiansen and mosaic effects accompanied the spectra obtained via KBr pellets for the larger diameter powders.

A spectrum of frosted fused quartz plate is presented in Fig. 4, and similar data were obtained for frosted soda glass plate. The plates were roughened by grinding with a 240 mesh alumina/water mixture until they became diffuse reflectors. The plate samples were analyzed in the same manner as the powder samples, and their

[4] R. J. P. LYON, Final Report on the Evaluation of Infrared Spectrophotometry for Compositional Analysis of Lunar and Planetary Soils, Stanford Research Institute, Pert I (p. 73), Sept. 1962, Part II (p. 83), Feb. 1964.

Infrared spectra of powders by means of internal reflection spectroscopy 2139

0

I ii 8 1

I 1

Absorption cuwe

bi

8.2

s!s J6

00%

1 f = d

Wovelength. p -

Fig 4. Internal Reflection Spectrum of frosted fused quartz plate in contact with a Germanium Internal Reflection Plate.

spectra, as those of the powders, showed no power loss which could be attributed to light scattering.

Based on the experimental results obtained, it was concluded that the technique of Internal Reflection Spectroscopy is a feasible method for obtaining high contzast infrared spectra of powders and rough solids.

Acknowledgements-The authors are indebted to Dr. W. PARRISH of Philips Laboratories, for

obtaining and interpreting the X-ray data. They are also indebted to Dr. R. C. GORE and Dr. R. W. HANNAH both of Perkin-Elmer for recording the transmission spectra via KBr pellet technique for a number of the powdered samples. Mr. F. ROCK and Mr. J. TEEKLE of Philips

Laboratories provided able technical assistance. The authors profited from discussions with

Dr. R. J. P. LYON of NASA/Ames during the course of this work.