ANALYTICAL
1478
and Refining Company and the Atomic Energy Commission for permission to publish these data. LITERATURE
CITED
(1) Delfosse, J., and Hippie, J. A., Phys. Rev., 54, 1060 (1938). (2) Evans, M., Bauer, N., and Beach, J. Y., J. Chem. Phys., 14, 701 (1946). (3) Fischer, R. B., Potter, R. A., and Voskuyl, R. J., Anal, Chem., 20, 571 (1948). (4) Flexner, L. B., Gellhorn, A., and Merrell, M., J. Biol. Chem.. 144, 35 (1942). (5) Hevesy, G., and Jacobsen, C. F., Acta Physiol. Scand., 1, 11 (1940). (6) Mohler, F. L., and Dibler, V., Phys. Rev., 72, 158A (1947). (7) Orchin, M., Wender. I., and Friedel, R. A., Anal. Chem., 21, 1072 (1949).
CHEMISTRY
(8) Smyth, H. D., “Atomic Energy for Military Purposes,” Princeton, Princeton University Press, 1945. (9) Swann, H. G., U. S. Air Force Air Matériel Command, A.F. Tech. Rept. 5972, Wright-Patterson Air Force Base. Dayton, Ohio, August 1949. (10) Swann, H. G., Brucer, M., Moore, CM and Vezien, B. L.. Texas Repts. Biol. Med., 5, 423 (1947).
(11) Taylor, R. C., and Young. W. S. Ind. Eng. Chem., Anal. Ed., 17,811 1945). (12) Thomas, B. W., and Seyfried, W. D., Anal. Chem., 21, 1022 (1949). (13) Turkevich, J., Friedman, L.. Solomon, E., and Wrightson, F. M., J. Am. Chem. .Sac., 70, 2638 (1948). Received February 27, 1950. Presented at the Consolidated Engineering Corporation Mass Spectrometer Annual Conference, New York, N. V.. May 1949.
Infrared Absorption Spectra of Minerals and Other Inorganic Compounds JOHN M. HUNT, MARY P. WISHERD, AND LAWRENCE C. BONHAM1 The Carter Oil Company, Tulsa, Okla. is described for utilizing the infrared spectrophotometer in the analyses of minerals and other inorganic compounds. Mineral samples are ground to a powder having an average particle diameter smaller than 5 microns. The The powder is deposited as a film on a conventional rock salt window. spectra of 64 minerals, rocks, and inorganic chemicals are shown for the wavelength region from 2 to 16 microns. Spectral positions of the principal absorption bands of these compounds are tabulated. The technique is used in the study of fine-grained rocks. Analytical chemists in other fields such as catalysis, soils, and ceramics may find the same methods applicable to their problems. A method
spectroscopy' is used extensively for the identificaapplication to the field of inorganic analysis has been rather limited. Inorganic solids cannot be analyzed in polar solvents such as water because such solvents generally' have strong absorption bands of their own in the infrared range. Spectra have been obtained from thin sheets, but many' substances cannot be sliced thin enough to prevent them from being opaque to infrared radiation. Conventional methods of mulling the sample in Nujol (S) produce spectra which are poorly defined because of reflection and refraction of
tion of organic molecules (2, 4), but its INFRARED
the incident radiation by' crystal particles. In addition, the absorption bands of Nujol must be distinguished from those of the substance being analysed. The infrared technique described herein was developed as a means of analyzing rocks, but the same methods are applicable to analytical problems in other fields such as catalysis and ceramics. The technique involves grinding the sample to a powder having an average particle diameter of less than 5 microns. This powder is spread as a thin film over the sodium chloride window through which the infrared radiation passes. Qualitative determinations of the major constituents of rocks can be obtained rapidly' by this technique. Semiquantitative data may' be obtained if all of a sample is ground to the desired particle size. The infrared absorption of very small particles was first studied extensively by Pfund and associates (9, 10). They' found that when a crystalline material is ground to a size finer than the wave length of the infrared radiation, its spectrum is sharpened considerably. 1
Present address, Washington University, St. Louis, Mo.
Pfund also found that films made from particles larger than the wave length tended to scatter the infrared radiation at all but At these few positions most of the radiation a few wave lengths. was transmitted through the film, giving rise to transmission bands. Barnes and Bonner (1) demonstrated that the scattering of radiation by large particles was due mainly to refraction. They' established that the transmission peaks observed by' Pfund were due to a lack of refraction at wave lengths where the refractive index of the particles is 1.0. Such abnormally low refractive indexes occur on the short wave-length side of absorption bands as a result of the phenomenon of anomalous dispersion. Henry (6) studied the effect of particle size and layer thickness True on the infrared transmission of dry films of quartz powder. absorption spectra were obtained from thin films of quartz particles 1 micron in average diameter. Scattering by' refraction occurred only' at the very' short (less than 3-micron) wave lengths. With large particles none of the light was transmitted in the 2- to 16-micron range except at the few wave lengths where refraction was at a minimum. The authors separated fine particles into uniform sizes by sedimentation. Transmission spectra were obtained from powder films made up of only large particles (>20 microns), whereas absorption spectra were obtained from powder films of only small particles (66/a
1479
ments, and inorganic chemicals were obtained and are shown for tlie wave-length region from 2 to 16 microns. Spectral positions of the principal absorption bands of these compounds are tabulated. EXPERIMENTAL
The absorption spectra were obtained from powders having particle diameters smaller than 5 microns in all cases except for those minerals which were difficult to disperse by sedimentation,
ANALYTICAL
1480
such as pyrite and galena. About 0.2 gram of powder smaller than ,5 microns is sufficient for analysis. This can be obtained by grinding about 5 grams of the sample to a fineness that will pass a 150mesh screen. This pow'der is added to 250 ml. of distilled water with a small amount of a dispersing agent. For clays the dispersing agent is 15 ml. of 0.2 N sodium oxalate, and for carbonates, silicates, and the inorganic chemicals it is 5 drops of sodium metasilicate. The mixture is violently agitated in a Waring Blendor for about 10 minutes. The suspension which forms is poured from the blender into a 250-ml. graduated cylinder and
allowed to stand 2 hours. At the end of this time the upper 5 inches of the suspension will contain particles less than 5 microns
in diameter. The exact particle-size distribution can be calculated from Stokes’ law. It -will vary somewhat, depending on the specific gravity of the sample and the temperature. The upper 5 inches (12.5 cm.) of suspension are drawn off and centrifuged to separate the sediment from the solution of the dispersing agent. The sediment is then dried in an oven at 105° to 110° C. for 24 hours, and placed in a desiccator prior to use.
WAVE
COMPOUND
TREMOLITE AND
CALCITE AND
SOURCE
PURITY
CONNECTICUT
SOLIO
STATE
TEMPERATURE
25-50
C
THICKNESS
I.Smgm/em8 I-REF SHUTTER OPEN 2-REF SHUTTER
CURVE CURVE
LOWERED
16
TURNS
MAGNESITE SOURCE
AND
PURITY
WASHINGTON
S0..0 TEMPERATURE 25-30*C THICKNESS 0.2 mgm/em2
STATE
DOLOMITE Ca
SOURCE
r/g:C02 )2 AND
PURITY
NEW YORK
SOLID TEMPERATURE 25- 30’C THlCKN'ESS c.2 rngm/em8
STATE
COMPOUND
CALCITE SOURCE
AND PURITY
MEXICO
STATE
TEMPERATURE
THICKNESS 0
SO.ID 25-30
C
3 mgm/em2
CHEMISTRY
NUMBERS
IN
CM’1
VOLUME
2 2,
NO. 12, DECEMBER
1950
A number of samples can be sedimented concurrent!}·, so that the time required for any one sample is short. Smaller particle sizes may be obtained by drawing off less suspension or by allowing it to stand longer. Some difficulty is experienced in preventing coagulation of fine suspensions, particularly clays. If a sharp line of demarcation forms in the graduate while standing, it indicates that coagulation has taken place and the sample must be dispersed again with
1481
another agent. Some samples have a tendency to gel. This be broken by diluting the suspension about 4 to 1 with distilled water and adding 0.1 gram of sodium hexametaphosphate. Clays such as montmorillonite have a tendency to coagulate on drying. Sometimes this may be avoided by drying at a lower temperature. If not, the sample is sedimented in isopropyl, alcohol and kept as a paste until analyzed. If equipment for very fine grinding is available, it is preferable can
COMPOUND
ARAGONITE SOURCE AND PURITY
CALIFORNIA CALCITE
IMPURITY
SOLIO
STATE
TEMPERATURE 25-30*0 THICKNESS 0.4mgm/cm*
COMPOUND
RH0D0CHR0SITE MnC0$
SOURCE
PURITY
AND
MONTANA
SOLIO TEMPERATURE 25-30*0 THICKNESS 04 mgm/cm*
STATE
VE
NUMBERS IN
CM'1
COMPOUND
S1DERITE SOURCE
PURITY
AND
CONNECTICUT
SOLID
STATE
TEMPERATURE THICKNESS 0
25-30*0 5
mgm/cm*
WAVE NUMBERS
COMPOUND
SMITHSONITE SOURCE
AND
PURITY
MISSOURI
SOLID TEMPERATURE 25-30*0 THICKNESS 0.5 mgm/cm*
STATE
IN
CM'1
ANALYTICAL
1482
to grind a small sample completely to particles less than 5 microns in diameter. Such samples can be run directly by infrared without going through the steps of sedimenting, centrifuging, and drying to isolate the fine particles. This saves considerable time, and, in addition, the substance is handled in the dry state.
The spectra were recorded on a Baird Model B double-beam infrared recording spectrophotometer. The oven-dried sample is crushed in a small mortar to separate particles which have ad-
hered together, and some of the powder is placed on a standard sodium chloride window. A few drops of isopropyl alcohol are added to form a paste. The paste is smoothed out on the window with a microscope slide whose edges have been beveled and polished to prevent scratching. When the slide is removed, the alcohol evaporates, leaving a thin film of sample on the window. The window- with the powder film on it is inserted in the sample beam and a blank sodium chloride window is placed in the reference beam. The shutters in front of both source mirrors are left wide open. When the recording is finished, the window with
NUMBERS
IN
CM-1
WAVE NUMBERS
IN
CM'1
WAVE
COMPOUND
BARITE SOURCE
AND
PURITY
MISSOURI
SOLID
STATE
25-30 C THICKNESS 0.7 mgnvemZ TEMPERATURE
WAVE
COMPOUND
PYRITE SOURCE
AND
PURITY
UTAH
SOLID TEMPERATURE 25-30*C THICKNESS 0.3 mgm/em*
STATE
CHEMISTRY
LENGTH
IN
MICRONS
VOLUME
2 2,
NO. 12, DECEMBER
1950
the powder film is weighed before and after removing the film with alcohol. The difference in weight divided by the area is recorded on the spectrogram as milligrams per square centimeter. Care must be used to handle the window with rubber finger tips, or tongs, to prevent it from changing weight.
For purposes of correlation it is sometimes desirable to expand the scale of the spectrum in order to bring out weak absorption bands. By mounting a very thick sample the intensity of the
1483
absorption bands can be increased, but the over-all transmitted radiation is seriously decreased. With a single-beam spectrophotometer the slits can be opened to compensate for the over-all loss in transmitted radiation. With a double-beam instrument the same result can be achieved by lowering the shutter in front of the reference beam. This has the effect of increasing the difference between the two beams. The expansion of the spectrum of a mixture of tremolite and
WAVE
NUMBERS
IN
CM'1
WAVE
NUMBERS IN
CM*1
WAVE
NUMBERS
CM*1
COMPOUND
GALENA SOURCE
AND PURITY
ILLINOIS
SOLID
STATE
25-30*0
TEMPERATURE
THICKNESS
O.S mgm/cm8
COMPOUND
d-OUARTZ s;o8 SOURCE
PURITY
ANO
OTTAWA, CANADA
SOLID TEMPERATURE 26-30* C THICKNESS 0.2 mgih/e/n*
STATE
COMPOUND
CHERT
_»o,_ SOURCE
AND PURITY
MISSOURI
SOLID TEMPERATURE 25-10*0 TH'CKNESS 0 3 mgm/cm8
STATE
COMPOUND
OPAL SOURCE
AND
PURITY
WYOMING
SOUD TEMPERATURE 26-30*0 0.2 myn/cm8
STATE
ThiCKNESS
IN
ANALYTICAL
1484
calcite by this technique is illustrated by the first set of curves in Figure 2. Curve 1 was obtained with the reference shutter open, and No. 2 with it partly closed. The peaks in the 13- to 15micron range are more prominent in curve 2 than in No. 1. For quantitative work, the measurement of optical densities from a background line on the expanded scale is inaccurate, because the line is actually off the scale. However, “base line” optical density measurements ( ), which are independent of the background
COMPOUND
HEMATITE SOURCE
AND PURITY
MINNESOTA
SCL-0 TEMPERATURE 25-10 C THICKNESS 0.5 mgm/cm2
STATE
ILMENITE SOURCE
PURITY
AND
NORWAY
TEMPERATURE
THICKNESS
ß
25- 30* C mgm/em*
OLIVINE (Mg, F$)2 Si 0«
SOURCE
AND
PURITY
NORTH CAROLINA
TEMPERATURE
THiCKNESS
25-10*0
0.2 rngm/em^
COMPOUND
GARNET
SOURCE
AND. PURITY
NEW YORK
1
TEMPERATURE 2S-30°C THICKNESS Clmgrr./cm2
CHEMISTRY
line, may be used. The spectra of minerals and inorganic compounds are shown at two powder film thicknesses. The thin films were run with the normal transmission scale, but in a few cases the thicker films were run with the expanded scale. MINERALS
AND ROCKS
Absorption spectra of minerals and sediments are shown in Figures 2 to 13. The clay minerals were obtained through the
VOLUME
2 2,
NO. 12, DECEMBER
1950
courtesy of R. E. Grim of the Illinois Geological Survey. Most of the other minerals were purchased from Wards Natural Science Establishment. Chemical analyses of the minerals were not available, but from optical examination it was estimated that the impurities in most cases did not exceed 5%. Wave-length positions of the absorption maxima (or shoulders) for each mineral The spectrometer was calibrated from the are listed in Table I. known wave lengths of absorption bands of benzene and pyridine.
1485
The wave-length measurements are accurate to about ±0.04 micron in the 2- to 5-micron region, and ±0.02 micron in the Sito 15-micron region. The amount of powdered mineral traversed by the beam is expressed in terms of milligrams per square centimeter. The relative intensities of the absorption bands are indicated by the letters s, m, and w, signifying strong, less than 25% transmission; medium, 25 to 75% transmission; and weak, moré than 75% transmission. These values are for the transmittance
COMPOUND
AUGITE CaMg!S.0$)2
SOURCE
PURITY
AND
ONTARIO, CANADA
S
SOLO
THICKNESS
,
’AT E TEMPERATURA
25-30° C
·-2
COMPOUND
TREMOLITE ;OH)2 Co2Mg5(S
