Attenuated Total Reflectance vs. Transmission Infrared Spectrometry in the Quantitative Evalwation of Paint Vehicles RAYMOND J. McGOWAN Physics and Mathematics Division, U. S. Naval Civil Engineering Laboratory, Port Hueneme, Calif.
b Certain problems are encountered in using infrared transmission spectrometry for the quantitative evaluation of paint vehicles; therefore, an investigation of attenuated total reflectance spectrometry was conducted. In thinfilm analysis, it i s very difficult, if not impossible, to know the sample thickness; this knowledge is essential in quantitative infrared transmission spectrometry. The attenuated total reflectance technique eiiminates this problem. Therefore, it was concluded that this method is superior to transmission measurements for quantitative evaluation of paint vehicles.
A
SERIOUS PROBLEM encounted in infrared transmission studies of paint vehicles is that of controlling sample size and thickness in order to produce infrared spect,ra of quantitative value. The intensity of the measured spectrum is related to the thickness of the sample and the concentration of the functional groups. Quantitative analysis of the spectrum is dependent on the intensity
Table 1.
location,
microna 3.4 3.5 5.8 7.9 8.9 9.4 13.5
Infrared Transmission and ATR Studies. I n the field study to deter-
mine the quantitative value of infrared transmission, samples of a paint vehicle (alkyd resin) were prepared and spectrograms were made at NCEL and two other laboratories also using IR-5's. The data were evaluated a t NCEL.
Summary of IR-5 Infrared Transmission Results
Average peak values Bremerton Cal-Colonial NCEL 3.25 1.90 67.80 23.00 4.66 3.66 1.91
Table It.
Peak location, microns
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EXPERIMENTAL
To compare ATR with infrared transmission, the field study was repeated by the three laboratories. Sample Preparation. The samples evaluated by infrared transmission were prepared as follows: the pigment and vehicle were separated, using a Federal test method ( 3 ) with extraction mixture A , and diluted to a volume of 25 ml. with ether. Aliquots of 0.2 ml. were delivered on a salt window (NaC1) and dried for 15 minutes a t 50" C. For the evaluation of the ATR technique the samples were prepared as follows: the pigment and vehicle were separated, using the Federal test method (3) with extraction mixture A and diluted to a volume of 25 ml. with ether. Aliquots of 0.4 ml. were placed on the prism of the ATR unit (KRS-5) and dried for 15 minutes a t 80" C. RESULTS
The two techniques are compared in the tabulations shown in Table I ; four random spectrograms were used to obtain the average peak values for each laboratory. Ranges were calculated as follows: Xi = average peak value arranged in ascending order (4 = 1, 2, 3,). DISCUSSION
Peak
3.4 3 5 5.8 7.9 8.9 9.4 13.5
of the variations in absorption peaks caused by the concentration of the functional groups. A very new technique, attenuated total reflectance (ATR), developed by Fahrenfort (a), offers a solution to this problem. It differs from infrared transmission spectrometry in that the infrared beam is reflected from the sample into the slit of the spectrophotometer instead of passing straight through the sample as in the case of infrared transmission. Consequently, the sample thickness will not affect the spectrum if the sample is a t least 5 microns thick, because the reflection is from the first 5 microns.
3.23 2.18 6.71 5.26 2.92 2.63 1.73
1.91 1.46 3.13 2.76 1.81 1.71 1.23
ANALYTICAL CHEMISTRY
Range
2.80 1.85 25.88 10.34 3.13 2.67 1.69
1.34 0.72 64.67 20.24 2.86 1.95 0.69
Summary of IR-5 ATR Results
Average peak valuee Bremerton Cal-Colonial KCEL 1.28 1.25 2.12 2.03 1.82 1.59 1.43
Overall average
1.26 1.20 2.34 1.09 1.94 1.69 1.38
1.45 1.25
2.25 1.87 1.78 1.60 1.42
Overall average
Range
1.33 1.23 2.24 1.96 1.85 1.62 1.41
0.19 0.05 0.21 0.16 0.17 0.10 0.05
Because of the sensitivity of infrared transmission to sample size and thickness, its application to quantitative evaluation in thin-film studies is limited (4). In quantitative evaluation, sample size must remain constant, because the intensity of the functional groups is directly proportional to the concentration and sample size. If the thickness is known, it can be compensated for. In the case of thin films, such as paint vehicles, it becomes a problem t o determine the exact thickness. In some cases, variations in thickness can be compensated for mathematically by the absorbance-ratio method (1). A practicable technique for the infrared evaluation of paint vehicles should be free of problems related to sample thickness. These results show that infrared transmission has a high variability (3 to
62%) and range (0.7 to 64.6), as shown in Table 1.
The variability technique 'llo'ls a range loner (4 to 16%) mid (0.05 to 0.21), as shown in Table 11. Thus, ATR offers a solution to the problem of sample size and make< quantitative evaluation of paint vehicles practicable.
ACKNOWLEDGMENT
The author expresses appreciation to George E. Hayo, NCEL statistician, for his assiqtallcc in analyzing the data. LITERATURE CITED
(1) Chicago Society for Paint Technology, "Infrared Spectroscopy-Its Uee as an Analytical Tool in the Field of Paints
and Coatings," Infrared Spectroacopy Committee, Chicago, Ill., October 31, 1960. (2) Fahrenfort, J., S P e c t ~ o c h Acto ~ ~ * 179 698-709 (1961). (3) General Services hdminietration, No. 141 Federal Test Method Standard, Method 4021, May 15, 1958 (4) Harris, R. L., Svoboda, G. R., ANAL. CHEM.34,1655-7 (1962). RECEIVEDfor review April 8, 1963. Accepted July 22, 1963.
Attenuate($ Total Reflectance Infrared Analysis of Aqueous Solutions BERNARD KATLAFSKY and ROBERT E. KELLER Research Department, Organic Chemicals Division, Monsanto Chemical Co., Sf. Louis 77, Mo.
b The attenuated iota1 reflectance (ATR) technique i s shown to be a practical infrared methocl for the qualitative and quantitative analysis of aqueous solutions. Characteristic spectra are obtained throughout the rock salt region except iri the immediate vicinity of the very intense 3300 ern.-' water band. Water soluble components that are difficult or impossible to determine by conventional infrared transmittance techniques are easily identified and measured using ATR. Spectral data for carboxylic acids and salts, sulfonic acids, amino acids, phenols, amides, carbohydrates, and inorganic components are presented. Data are included to show that two-phase systems containing suspended matter in water, such as dispersed solids or emulsions, can be analyzed directly by this method. Advantages and litmitations of the method are discussed.
T
HE LOW SOLUBILITYof
many watersoluble inorganic, rrolar organic, and biological materials in the common organic infrared solvents limits the infrared measurement of these materials. Spectral data for materials of this type must be obtained by the analysis of water solutions or of the compound in the solid state. Solid state spectra are often unsatisfactory because additional absorption bands due to the crystalline state can be confused with fundamental vibrations. The use of water as an infrared solvent, for transmittance studies was demonstraked as early as 1905 by Coblentz (6). Gore, Barnes and Petersen (12) and Blout and Lenormant (4) have shown that deuterium oxide can be used, in conjunction with water, to obtain infrared spectra in the rock salt region. Plyler and Acquista ($5) have shown that the tmnsniittanw spertrum of
water contains very intense absorption bands a t 3300 ern.-' for the OH stretching vibration and a t 1640 cm.-I for the HOH deformation vibration. They attribute a third, very intense band a t 660 cm.-' not observed in the water vapor spectrum to an intermolecular vibration arising in groups of water molecules that exist in the liquid state. The latter vibration interacts with the fundamental water vibrations to produce combination bands a t 3920 and 2110 cm.-' They showed also that a window is present from -1540 cm.-' to -1000 cm.-1 where there is still enough infrared transmittance in reasonable path lengths of water to permit its use as a solvent for quantitative purposes. Potts and Wright (16) proposed the use of a transmittance screen in the reference beam of a double-beam infrared spectrophotometer to obtain a more useful 10in the 1540 to 1000 cm.-l region. Kaye (16) and Sternglantz (87) have investigated optical window materials suitable for use with water. Reviews by Blout (8) and Goulden (IS), covering applications of infrared transmittance spectra of aqueous solutions of inorganic, organic, and biological materials, and recent papers dealing with studies of lactates ( 1 4 , amino acids (SS), vitamins (893, biogenetic amines (17), metal chelate compounds of a-amino acids (f9),and iminoacetic acids (10) attest to the growing interest in the use of water aa a solvent for infrared spectrometry. Spectral data are usually observed only in the 915 to 1540 cm.-l region using water and in the 1500 to 1850 cm.-' region with deuterium oxide as the solvent. Cells with barium fluoride or silver chloride windows with path lengths of 10 to 50 microns are the usual experimental conditions. The use of deuterium oxide introduces the complications of hydrogen-deuterium exchange reactions
which may result in complex spectra. The attenuated total reflectance (ATR) technique developed by Fahrenfort (11) offers an attractive alternative to conventional transmittance methods to overcome or rrinimize the intense water absorption band. which limit the range in which useful infrared data can be observed. A simple explanation of the principles of the ATR effect has been given by Wilks (28). The absorption-like infrared spectra obtained result from an extremely shallow (less than 5 microns) penetration of energy into the sample. ATR has been applied mostly to solid samples (7-f0,16, 94). The work reported in this paper shows that ATR can be applied equally as well to aqueous solutions. Except in the region of the 3300 cm.-' water band, infrared data can be obtained throughout the entire rock salt region. Since ATR is independent of sample thickness, cell designs which accommodate large volumes can be used to simplify filling, emptying, and cleaning of the cells. The results of the application of the ATR technique to the qualitative and quantitative infrared study of organic and inorganic compounds in water are presented in this paper. EXPERIMENTAL
The ATR spectra were obtained with a Connecticut Instrument Co. Model ATR-1 attachment and an Irtran-2 (Eastman Kodak Corp.) prism a t a 40' angle of incidence. The spectra were recorded with a Perkin-Elmer Model 221 double beam infrared spectrophotometer with rock salt optics. Energy losses were compensated with a Model BA-1 (Connecticut Instrument Co.) variable beam attenuator in the reference beam. Materials for this study were reagent grade or C.P. quality and were used without further purification. VOL. 35, NO. 11, OCTOBER 1963
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