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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RESULTS AND DISCUSSION
The infrared transmittance spectrum of water in a 28-micron path length cell with Irtran-2 windom is shown in Figure 1.1. Superimposed on this spectrum is shown the effect of placing a transmittance screen in the reference beam of the spectrophotometer to produce a more useful lo in the 950 to 1540 cm.-' region. The decrease in transmittance a t lower frequencies is due principally to the absorption of the 660 cm.-' water band. Figure 1B shows the ATR spectrum of water. The intensity of the 1640 em.-' water fundamental is greatly decreased, the combination band at 2100 ern.-' produces a very weak absorption, and only the region of the OH stretching band a t 3300 cm.-l is opaque. The 2335, 2070, and 890 cm.-l bands in the ATR water spectrum are due to the Irtran-2 prism. The fine structure on either side of the 1640 cm.-' water band is due to atFREQUENCY 3000
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WAVELENGTH ( M I C R O N S
ATR spectra of carboxylic acids in water
Figure 2.
Water spectrum
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spectra reported by Coulden (IS) and Sternglantz (27) for the 950 to 1510 cm.-1 region. The acid carbonyl band a t 1705 cm.-' is well defined above the water background. This region is opaque in transmittance spectra of water solutions. The citric acid spectrum shows the carbonyl band a t 1705 cm.-1 and the positions of the C-0 stretching band a t 1210 cm.-' and the minor bands in the 950 to 1510 ern.-' window region agree with Parker's (11) transmittance data. The ATR spectra of sodium acetate and sodium citrate in water are shown in Figure 3. The positions of the symmetrical carboxylate bnnd a t 1400 cm.-I
mospheric water vapor resulting from an approximately 4-inch increase in the optical path of the sample beam introduced by the optics of the ATR attachment. The Irtran-2 prism absorbs strongly in the 5000 em.-' region and becomes opaque to infrared radiation a t about 710 cm.-l Spectra were obtained, therefore, only in the 4000 to 740 cm.-l region. Carboxylic Acids and Salts. Figure 2 shows the ATR spectra of acetic acid and citric acid in water. The position of the C-0 stretching band a t 1270 cm.-' and the minor absorption bands in the acetic acid spectrum agree with the aqueous transmittance
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3. ATR spectra of carboxylate salts in water Irtran-2 prism, 6 = 40' 4. Sodium acetate, 4 M E. Sodium citrate, 2 M
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Figure
4. ATR spectra of sulfonic acids in water Irtran-2 prism, 0 = 40' A. Benzenerulfanic acid, 2 M B. 2-Naphthalenerulfonic acid, 2 M C. 1,3,5-BenzenetrisuIfonic acid, 1.5M
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Figure 6.
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and the minor bands in the spectrum of sodium acetate complement the aqueous transmittance data reported by Parker (21). The position of the asymmetrical carboxylate band a t 1540 cm.-' agrees with the data obtaincd by Gore, Barnes, and Petersen (12) for heavy water transmittance spectra. The ATR spectrum of sodium citrate shows the symmetrical and :tsymmetrical carboxylate bands a t 1280 and 1560 cm.-' and agrees with the aqueous transmittance data reported by Sternglantz
\
ATR spectra of amino acids in water
Irtran-2 prism, 6 = 40' (- - ATR woter blank) A. DL-Alanine, 2 M E. P-Alanine, 3M (- - - - - sodium salt 3 M )
obtaining infrared data even in the immediate area of the 1640 cm.-' water band, which is opaque in transmittance spectra. Figure 5A shows the ATR spectrum of glycine in water. The absorption pattern matches exactly the aqueous transmittance spectrum obtained by Parker and Kirschenbaum (23) for the window region of 950 to 1510 cm.-' The dipolar ion (zwitterion) appears as a broad band a t 1615 em.-' and is clearly distinguishable over the background of the 1640 em.-' water band. Figure 5B shows the spectrum (27). Sulfonic Acids. The XTR spectra obtained when an equivalent of hydroof benzenesulfonic acid, 2-naphthachloric acid was added to an aqueous lcnesulfonic acid, and 1,3,5-benzen~- glycine solution. The free acid appears trisulfonic acid in water shown in a t 1740 cm.-l and additional absorption Figure 4 exhibit some large diff wences is found superimposed on the 1640 over the solid statt: spectra of these cm.-' water band that is probably asmaterials. -4doubht band system in sociated with the NH3+ ion. The addithe 1200 em.-' region degenerates into tion of an equivalent of sodium a single broad band and a band in the hydroxide to an aqueous glycine solution produced the spectrum shown in 1020 to 1040 cm.-l is enhanced. By analogy with carbox:rlic acid ions, ionic Figure 5C. The carboxylate ion absulfonic acids should produce a pair of sorption band shifts to 1500 em.-' bands in the 1000 and 1200 cm.-l Gore, Barnes, and Petersen ( l a ) , reregions and Bellamy ( I ) has assigned sorted to the use of DzO, DC1, and the ranges of 1260 LO 1150 em.-' and XaOD to obtain the spectra of the three 1080 to 1010 cm.-' to the ionized sulionic species of glycine. fonic acid group. In water, benzeneThe ATR spectra of DL-alanine and sulfonic acid absorb: a t 1175 and 1020 @-alanine in water are sh0n.n in Figure em.-', 2-naphthalenesulfonic acid a t 6. The dipolar ion absorption for DL1170 and 1032 crn.-l, and 1,3,5-benalanine appears as an unresolved doublet zenetrisulfonic acid a t 1190 and 1035 with bands a t 1600 and 1625 em.-' The em.-' band for 0-alanine, however, is shifted to Amino Acids. The ATR spectra 1560 cm. -I, the region where simple of glycine, glycine hydrochloride, and carboxylate ions usually absorb. The sodium glycinate i 1 water illustrate fact that the 1560 em.-' band is due to the advantages of this technique for the dipolar ion is demonstrated in
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Figure 6B where the carboxylate ion band for the sodium salt of p-alanine in water is superimposed on the spectrum of the neutral solution. The two absorption bands are very close, but the sodium salt band a t 1540 em.-' is displaced by 20 cm.-l from that of the dipolar ion. The ATR spectrum of L-lysine monohydrochloride in water shown in Figure 7A is interesting in that only a single broad band is observed a t 1615 crn.-l and no absorption is found in the 1700 crn.-' region for the free acid. The presence of only one half an equivalent of hydrochloric acid apparently results in salt formation of only the e-NHZgroup and has little effect on the a-KH2. Therefore, only the normal dipolar ion absorption is observed. Figure 7B shows the ATR spectrum of L-cysteine hydrochloride and, like its homolog glycine hydrochloride, shows the free acid band a t 1740 cm.-' and some additional absorption a t 1629 cm.-l which is probably associated with the SH3+ ion. Phenolics. Figure 8 showb the ATR spectra of resorcinol, pyrocatechol, and pyrogallol in water. These spectra match exactly the solid state transmission spectra of these materials. In each case the C=C skeletal vibration in the 1600 cm.-' region can be readily detected over the 1640 cm? water band. Amides. Figure 9 shows the ATR spectra of acetamide, S-methylacetamide, and S,.\;-dimethylacetamide in water and presents an interesting series for studying the amide I and amide I1 bands. The over-all spectral patterns match those of the solid state and capillary liquid film transmittance spectra of these materials. The amide I and amide I1 bands for acetamide are found a t 1655 and 1600 cm.-' in water. For N VOL. 35, NO. 11, OCTOBER 1963
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