chloride. I n this case, cell resistance was 0.2 megohm. ACKNOWLEDGMENT
The authors gratefully acknowledge T. the helpful suggestions of -vyron Kelley and D* J. Fisher Of Oak Ridge Sational Laboratories.
LITERATURE CITED
(1) Arthur, P., Vanderkam, R. H., ANAL. CHEM.33, 765 (1961). ( 2 ) Jackson, W., Jr., Elving, P. J., Ibid., 28, 378 (1956). ( 3 ) Kelley, hl. T., Jones, H. C., Fisher, D. J., Ibid., 31, 1475 (1959). (4) Xcholson, M. M., Ibid., 27, 1364 (1955).
(5) Oka, S.,Ibid., 30, 1635 (1955) Presented at the 14th Pittsburgh Conference on Snalytical Chemistry and Applied Spectroscopy, March, 196%. Financial assktance of the National Science Foundation by an “Undergraduate Seience Research Participation Grant” to one of the authors (KJH) is gratefully acknowledged.
Infrared Differential Technique Employing Membrane Filters Howard J. Sloane, Beckman Instruments, Inc., Fullerton, Calif.
c o m m of routine operation Iis occasionally of a spectral laboratory, the analyst confronted with the need N THE
to obtain the infrared spectra of small amounts of materials which have been trapped on a filtering medium. Sometimes it is possible to scrape off or dissolve the sample with a suitable solvent. More frequently, especially for microgram quantities, this is impossible to do without introducing considerable and indeterminable amounts of contaminant from the filter or from subsequent sample handling. At best, such a procedure generally is inconvenient. The purpose of the study described in this note was to find a filtering material with an infrared spectrum sufficiently weak at a useful thickness to permit direct examination of the sample by means of the infrared differential technique. K i t h such a filter, one could expect to obtain the sample’s spectrum, differentially compensating out the filter’s absorption spectrum by placing an equivalent thickness of filter in the reference beam of the double beam instrument. The initial portion of this work, then, involved examination of spectra for a wide variety of cellulosic and noncellulosic filters. Among the commercial materials examined were A, a standard cellulose filter paper (0.15 mm thick-
ness); B , a glass fiber filter (0.2 nim.); C, a Teflon-impregnated glass fiber filter (0.05 mm.) ; D, a cellulose acetate foil (0.15mm.); E, an acrylic fiber sheet (0.11 mm.); F , a poly(viny1 chloride) filter (0.225 mm.); and G, a cellulose nitrate-acetate membrane filter (0.025 mm.). Of these materials, all except G gave spectra far too intense for the differential work. However, all these filterj, especially D, E, and F, showed considerably improved transmission of infrared radiation when they were “wetted” with mineral oil to decrease scattering losses. These losses are, therefore, deemed to be a major contributor to the high degree of opacity observed. The spectrum of cellulose nitrateacetate membrane filter is shorn in Figure 1. This filter is available conimerically from the Millipore Filter Corp., Bedford, Mass., in a xariety of pore sizes, diameters, and thicknesses. It has been used extensively in bacteriological work and air pollution studies and to remove contaminants in fuels and hydraulic fluids. The spectrum shown in Figure 1 is that of the thinnest membrane filter available, Millipore type TH, 0.025 mm. and is the type used in all of the Lvork described below. With this thickness. it is
unnecessary to add oil to reduce beattering losses. The infrared spectra shown here ivere recorded on a Beckman IR-9 spectrophotometer equipped with Automatic Slit Control. The function and utility of this latter device have been demibed previously (3, 5 ) ; however, its importance in the present work n arrants home additional remarks. Briefly, the purpose of Automatic Slit Control is to provide a constant energy background for differential Jvork by automatically opening and closing the instrument slits to the degree necessary to compensate for energy being absorbed in the reference beam. The result 13 a uniformly high energy background (except in regions of total absorption) tliroughout the entire spectral r s n q being scanned. -2lthough ,iutomatic Slit Control is highly beneficial for such work. nieaningful results can still be obtained on the smaller, less versatile instruments not so equipped. I n this case, honever, i t should be remembered that bpurious bands or “dead” regions will occur in those portions of the spectrum nhere the filter is heavily absorbing. T o obtain a reasonably flat base line n hen filters are scanned differentially, it is imperative to match the thicknesses of sample and reference materials as
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Figure 1.
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Spectrum of cellulose nitraie-acetate membrane filter, Millipore, type TH, 0.025 mm. nominal thickness
ANALYTICAL CHEMISTRY
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Figure 2.
Spectra obtained through filtering material
Upper curve: Baseline obtained differentially with well-matched filters in sample and reference beams. Lower curve: spectrum of polystyrene film obtained with matched filters in the beams. Noteihe 8 4 0 cm.-' and 1070 cm.-' regions where filters are heavily absorbing
closely as pas-ible. Tf the filter> are not well matched, a spectrum of the membrane polymer is obtained iyhich i. indicative of the di'ference in thickIlPSS.
I n our experience, the best match in thicknesses was consistently obtained Iiy cutting adjacent piwcq from the
same sheet of filter material, which is available in large sections from the manufacturer. When a good match is obtained, a remarkably flat base line may be achieved (upper curve, Figure 2 ) . Under nearly ideal matching conditions. then, only the apparently opaque regions of the filter near 1285
cm.-1 and 1660 em.-' (nitrate absorptions) need be regarded with suspicion when making interpretations of differential spectra. That the highly absorbing regions near 840 cm.-' and 1070 cm.-' are still quite useful is shown by the lower curve of Figure 2 This spectrum of a polyatyrene
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Figure 3 A . Spectrum of di-n-octyl phthalate obtained differentially with matched thickness of Millipore, type TH, in the reference beam. B. Spectrum of di-n-octyl phthalate in 0.02 1 mm. sealed potassium bromide cell VOL. 35, NO. 10, SEPTEMBER 1963
1557
calibrator film, inserted additionally in the sample beam, was obtained while matched thicknesses of filter were present in the trvo heani. I t is quite obvious that the instrument rrniains “live” and faithfully traces the 840 em.-’ and 1072 ci11.-~ bands of re 3A is an illustration of the qiectrum obtained from apl)roximately 3 pl. of ail actual chc>mical sample flcyosited on a meinbrarir filt,er, and vompensated with a matching thicklies ( i f filter in the reference lieam. The +ample ih di-n-octyll)ht~halatr. Except ror the anomalous region near 1660 (.mi.-1; the spectrum compares farorabljivith that, obtained by scanning the +xmple undiluted in a thin, sealed i)ot:rsi.ium bromide cell (Figure 3 B ) . The membrane filter has several liinit,ations, ho\vever. Care mu-t be rserciieti in the choice of solvents to be used with this polymeric material. While most hydrocarbons. chlorinated hydrocarbons mineral acids, bases, id higher alcohols do not affect thr
filter, the lower alcohols, hetone>, esters, and ether alcohols dissolve the polymer
(4)’
Second, these filters, especially the thin TH type, normally pick up a considerable electrostatic charge and are difficult to handle and n-eigh. This charge may be partially dissipated with an alpha ionizing Source +uch as a polonium d r i p (available from Suclear Products Co., El Monte, Calif.). ,\pplication of the infrared differential technique employing membrane filters ha. not been thoroughly investigated. \Ye used the method to examine contaminants filtered from hydraulic fluid, and condenyate. from tobacco -moke. Potential applications -et’m mort’ di1 e r v than this. The ?\Illlipore membrane filter has already becn . h o w to be a rather eficient traj) for ga3 cliromatographic fraction\ ( 2 ) and it may be po-ible to examine the trapped GC effluent, h] infrared in the manner d e w 4 x d . The twhnique ma to air ~iollutioninve
amiliation of materials coliected from “clean” room atmospheres. There is also some indication that the Xllipore filter i i a useful medium for “paper” chromatography and electrophoresis ( I ) , in which case infrared identification of the spots or s t r e a h obtained seems promising. This probably will require xi-ork with microapparatus. LITERATURE CITED
J . L., JIillipore Filter Corp., Bedford, Mass.! private commiiniratiori, 1!16?. ( 2 ) Hajra, -4.K.) Radin, S.S..J . Lipid Kes. 2, 131 (1961). ( 3 ) M a t t h e m , I
