Powder-Polyethylene Film Technique for Spectral Measurements Karl J. Schwing, Department of Chemistry, Upsala College, East Orange, N. J., and Leopold May, Department of Chemistry, The Catholic University of America, Washington, D. C. 20017
the sodium chloride region by impregnating plastic samples with polyethylene (8). Because some solids react with sodium chloride plates ( I I ) , coatings of polyethylene have been prepared on the surfaces of the salt plates (10, I I , IS). It is possible to spread a mineral oil mull on a sheet of polyethylene (3) or between two polyethylene sheets held between cardboards (2, 12). A simpler method that has been developed for solids using polyethylene sheets as the support is described in this paper.
have been a M popular method for sampling of solids for infrared spectral measureINERAL OIL hIULLS
ments. Its disadvantage, the difficulty of compensating for the intense absorptions a t 3.4, 6.8, and 7.2 microns, has led to the search for other methods. The potassium halide disk method was developed to eliminate the bothersome interference from the mineral oil absorptions. However, in preparing the disk, the sample is esposed to high pressure that sometimes causes alterations in the sample. Polyethylene can be used to prepare a disk without the use of pressure (6) or with high pressure (9). I t has been particularly useful as a suspending medium in the far-infrared (1, 6, 6, 14, 15) and the ultraviolet regions (4, 7 ) where it has no absorption bands. Polyethylene has been used in
EXPERIMENTAL
Apparatus. The following spectrophotometers were used in this study: ultraviolet region (250 to 400 mp), Bausch and Lomb Model 505; nearinfrared region (0.6 to 3.0 microns), Perkin-Elmer Model 13 equipped with
0.9
LiF prism; S a c 1 region (2.5 to 15 microns) , Perkin-Elmer Rlodels 21 and 337; CsBr region (15 to 38 microns), Perkin Elmer Model 21 equipped with CsBr prism; far-infrared region (38 to 100 microns), Perkin-Elmer Model 301. Procedure. The procedure consists of two operations: preparation of a finely ground sample (particle size less than 1 micron) and suitable means to fix the powder on the polyethylene sheet. The sample can be prepared by grinding the sample in a mullite mortar until it cakes t o the walls of the mortar. The cake is loosened with a spatula and reground. This is repeated five or six times to assure that the sample is small enough. Alternately, the sample can be ground using a Wig-L-Bug mixer. To fix the powder on the polyethylene sheet, a dry or wet method can be used. I n the dry method, a few milligrams of the ground sample are placed on a flat, clean glass plate and covered Nith a polyethylene sheet ll/z X 2 inches, 0.004 or 0.0015 inch thick (Bel Art, Nannet, N. Y., Cat. S o . F-23974 or F-23975, respectively). The sheet is then pressed down on the sample and
0.8
0.7
0.6
0.5
z
U
m
a
W
u)
z
a
a 0.4
00.4
i
0.5
/
a m
m
0.3 -
a
m
a
0.3
0.2
-
0.2 /
0.I
-
I
1
380
I
I
,
360
Figure 1.
,
340
,
,
320
,
A,miIIimicrons
300
,
300
2$0
260
240
I..
Spectra of polyethylene sheets in ultraviolet
C. Single, 0.001 5 inch; d. Compensated, 0.004 inch single sheet; e. Compensated, 0.001 5 inch, double and single sheets
a. Double, 0.004 inch; b. Double, 0.001 5 inch;
280
260
A, millimicrons
240
220
220
Figure 2. Ultraviolet benzoic acid
spectrum
of
Top and bottom, on polyethylene sheet (less acid on sheet in bottom curve); middle, in methanol
VOL. 38, NO. 3, MARCH 1966
523
I
I
2.8
3.0
2.6
24
2.2
A,
,
20
I
,
,
I
1.8 1.6 1.4 1.2
Figure 4. Spectra of polyethylene sheet and cr-isoaminobutyric acid in NaCl infrared region
I
1.0 0.8
0.6
MICRONS
Top, a-isoaminobutyric acid, double 0.001 5-inch sheet, Compensated; bottom, polyethylene sheet, double, 0.001 5 inch
Figure 3. Spectrum of polyethylene sheet nea r-infra red region
(0.004inch) in
Top, single sheet compensated; Bottom, single sheet UncomDensated
moved with a circular motion until a fine layer adheres to the sheet. Polyethylene gloves or finger cots are worn to avoid contamination of the film. With samples that have little affinity to polyethylene, it is best to wet the powder with a few drops of a low-boiling organic solvent, such as methanol, acetone, or ether, before rubbing it with the polyethylene sheet. Once the mixture has adhered, the solvent will completely evaporate leaving a suitable film of the sample. I n the case of a sample that has a low melting point or is hygroscopic, a second polyethylene sheet is placed on top of the sample film, This is advisable for samples stored for future use. The sheet is then stapled directly between two cardboards containing holes for the spectral beam and cut to fit the holder on the spectrophotometer. The holes are best cut using a suitable 1-inch circular die, with a soft plastic
30
25
35
plate as the supporting base. A cork borer size No. 9 is also useful for this purpose. It is necessary to prepare a blank sheet (or blank sheets when two polyethylene sheets are used for the sample) for each sample. This blank is used whenever the sample is examined. This is necessary to match thickness and the number of exposures of the polyethylene sheets to the analyzing rays. RESULTS
Ultraviolet and Visible Region (200
to 800 mp). Figure 1 shows t h a t polyethylene begins t o absorb strongly at about 250 mp. The absorption is related to the thickness of the film and the number of sheets. For example, the double 0.OOPinch sheets completely absorb above 250 mp (curve a, Figure 1).
4 0 MlcRoNS 5 0
80
60
7.5
9.0
8.0
Compensation with a similar set does not completely eliminate the absorption (curve d). Double sheets of 0.0015inch polyethylene also absorb in this region to some extent (curve b ) , but absorption is eliminated using the double sheets in the reference beam (curve e ) . Single sheets of this thickness are easily compensated (curve e ) . The spectrum of benzoic acid on polyethylene is shown in Figure 2. For comparison the spectrum in methanol is included. The resolution is of the same order because the two small bands between 270 and 290 mp are shown. These two bands are shifted from their positions in solution. The technique using the polyethylene sheets permits easy observations of substances eliminating solvent effects. Near-Infrared Region (0.8 to 3.0 Microns). The spectrum of poly-
ethylene shows strong bands near 2.3 microns and weaker bands near 1.9 and 2.6 t o 2.8 microns (Figure 3).
MICRONS
10.0
15.0
20.0
00
00
AWO
'
'
m
3500
Figure 5. region
3000
2500
2000
1500
Spectrum of a-isoaminobutyric acid in 3-micron
Single 0.001 5-inch sheet, compensated; repeat runs
524
ANALYTICAL CHEMISTRY
-
1.0
10
-1
25.0
1300
1200
11W
1WO
900
800
700
600
500
400
Figure 6. Spectrum of potassium chlorate in NaCl infrared region Single 0.001 5-inch sheet, compensated
a-isoaminobutyric acid shown in Figure 4 is also presented in Figure 7 (16- to 36micron region). Far-Infrared Region (Above 40 Microns). The spectrum of polyethylene has one band at 143 microns. T h e band in t h e spectrum of Kl3r powder due to the lattice vibration (120 cm.-l) is shown in Figure 8.
5 WAVELENGTH,miCront
Figure 7. Spectrum of cr-isoaminobutyric acid in CsBr infrared region Double polyethylene sheet, 0.001 5-inch sheet
However, these can easily be compensated by using a similar polyethylene sheet in t h e reference beam (Figure 3). Mid-Infrared Region (3 to 40 Microns), T h e spectrum of double sheets of polyethylene (0.0015 inch) is given in Figure 4 in t h e sodium chloride region. The major absorption bands appear near 3.5, 6.8, and 13.6 t o 14.1 microns. Weaker bands are present in 7.5 and 7.6 microns. These bands are easily compensated by using similar pieces of polyethylene sheet in the reference beam. The spectrum of a-aniinoisobutyric acid (Figure 4) shows that even where polyethylene absorbs strongly, usable spectra can be obtained: for example, in t h e 3- and 7-micron regions. The spectrum of this sample compares favorably with spectra made \vith Nujol mulls and KDr disks. Using a single sheet of 0.0015-inch polyethylene with compensation, t h e bands of the amino acid are easily found even in t h e 3.5-micron region where polyethylene absorbs strongly (Figure 5). The spectrum of KC103 is included t o show what type spectra are found with inorganic compounds (Figure 6). The undulations in the background are due to interference fringes, which could be eliminated by crimping the sheet in the reference beam. The bands are marked and are in agreement with those previously published. The spectrum of polyethylene in the CsBr region has no absorption bands. The spectrum of t h e same sample of
DISCUSSION
The powder polyethylene film technique is an easy preparative method. If a particular film is prepared and the spectrum is inadequate, t h e film can be destroyed because the cost of the polyethylene is nominal. There are no interactions between the niatris and sample as have been found lvith potassium halides in preparing disks. This is of particular value when working with inorganic compounds. The region near 3 microns is free for ob5erving the hydrosyl bands. By the use of a compensating sheet in the reference beam, spectra can be obtained in all spectral regions. A single sample-sheet may be uqable in all spectral regions. It is only necessary n hen compen3ation is used to prepare the reference sheet a t the same time as the sample-sheet and to use the reference sheet with this sample-sheet only because the thicknesses and number of esposures niust be kept equal. The amount of sample that adheres to the polyethylene sheet will vary and in some cases may not be sufficient. This technique is an additional one available to the spectroscopist for solving his sampling problems. ACKNOWLEDGMENT
The authors gratefully acknowledge the technical assistance of Evelyn X a y . The spectrum of KBr in the farinfrared region was provided by John Ferraro, Argonne Sational Laboratory, Argonne, 111. LITERATURE CITED
(1) Brash, J. W., Jakobsen, R. J., Spectrochirn. A c t a 20, 1644 (1964). (2) Ferrand, E. F., Jr., A p p l . Specfry. 16, 22 (1962). (3) Lawson, G. J., Purdie, J. W.,Chem. I n d . 1961, p. 508.
Figure 8. Spectrum of potassium bromide in far-infrared region Top, KBr on riiigle polyethylene sheet; bottom, zero line
(4) McDonald, F. R., Cook, 0 . L., .4ppl. Spectry. 15, 110 (1961). (5) hIcKnight, R. T., Moller, K. D., J . Opt. Soc. Am. 54, 132 (1964). (6) May, L., Schwing, K. J., A p p l . Spectry. 17, 166 (1963). ( 7 ) Ogata, Y., Noshiro, T., Sakamoto, K., Kagaku KORyoiki 17,137 (1963). (8) Sands, J. K., Turner, G. S., ANAL. CHEM.24, 791 (1952). (9) Smethurst, B., Steele, D., Spectrochim. Acta. 20, 242 (1964). (10) Steger, E., Herzog, K., Z. Chem. 4. 142 11963). ( I l j Steger, E., Turcu, A., Macovei, V.,Speccrochz’m. A c t a 19, 293 (1963). (12) Sung, C., S o h , I., J . Korean Chem. Soc. 7, 58 (1063); Chem. Abstr. 59, 14738h (1963). (13) Vasko, A,, Srb, I., Czechoslov. J. Phys. 9, 128 (1959). (14) Rillis, H. A., AIiller, R. G. J., Adamu, D. M.,Gebbie, H. A., Spectrochirn. A c t a 19, 1457 (1963). (15) Yoshinga, II., Oetjen,--R. A., J. Opt. SOC.Am. 45, 1085 (19m).
PRESENTED at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1965. This investigation was supported in part by a research grant, GM 10574-03, National Institute of General 1Iedical Sciences, Public Health Science.
VOL. 38, NO. 3, MARCH 1966
525
