LITERATURE CITED
Table I.
Spectral Lines Suitable for Wave Length Calibration above 600 Mk
Element Hydrogen Argon
Mercury
Recommended Mean Wave Length, M p
Suggested Slit, Mm., on Beckman DU
656.3
0.03
751.0 763.5 811.2 841.6 912.3 1014.0 1128.7
0.1 0.1 0.05 0.07 0.1 0.02 0 3
Source Beckman hydrogen lamp, part 2230
Beckman Instruments, Inc., Fullerton, Calif., Beckman Instruction Manual 305A, DU Spectrophotometer, p. 8. (2) Beckman Instruments, Inc., South Pasadena, Calif., Beckrnan Bull. 8Q-E, (1)
p. 3. (3) Bra?, W. R., “Chemical Spectroscopy, 2nd ed., p. 520, Wiley, New York. - .~1945. - - ~ (4) Gibson, K. S., J. Opt. Soc. Am. 21, 564-87 (1931). (5) Gibson, K. S., Natl. Bur. Standards, Circ. 484 (September 1949). (6) Gibson, K. S., Balcolm, M. M., J . Opt. SOC.Am. 37, 595-608 (1947). ( 7 ) Harrison, G. R., “M.I.T. Wavelength Tables,” Wiley, New York, 1939. I
Beckman mercury lamp, part 2260
(8) Mellon, M. G., “Analytical Absorp
with the low pressure lamps (Beckman 2260 is 8 low pressure lamp). I n the high pressure lamps, 253.65 is unusable because of self-reversal, a s t p n g line
at 275.97 appears near 275.28, and the argon lines are so short-lived after start-up as to be unusable. Otherwise, high pressure lamps can be used.
tion Spectroscopy,” Chap. 5, Wiley, New York, 1950. (9) Saidel, A. X., et al., “Tables of SpecDD. 381-2. Verlaz Technik. trum Lines.” .. Berlin, 1955. I
Use of the Beckman DU Spectrophotometer in the 1 .O- to 1.9-Micron Range J. P. Phillips, Chemistry Department, University of Louisville, Louisville, Ky.
the monochromator of the h a n DU functions up to 2ooo mfi, ita phototubes do not respond beyond about loo0 mfi. The nearinfrared contaim characteristic bands for all functional groups and compounds containing hydrogen [Kaye, W., Spectrochim. &a 6, 257 (1954); 7, 181 (1955)], and a simple method of extending the DU range is therefore desirable, especially for laboratories that cannot justify the expense of an allregion spectrophotometer. Most of the suggeeted adaptations of the DU for the near-infrared [Schuler,... W. E., ANAL. CREW 31, 1604 (1959)l have used a lead sulfide photoconductive cell, but the characteristics of these cells offer some complexities, and a simpler and cheaper substitute for manual operation is a germanium photodiode. LTHOUQA
Ak
The germanium photodiode (Nucleonic Products, Inc., Type TP-50) may be used aa a photovoltaic cell having
characteristics and cost essentially similar to those of the familiar selenium cell but with peak rertponse. in the 1.2to 1.6-micron range. The phototube housing of the DU may be removed and replaced with a wooden block having a quarter-inch hole drilled through the center, in which the diode can be positioned to receive the light passed through the monochromator and cell compartment. Connection of the photodiode leads to a low resistance lamp and scale galvanometer or microammeter completes the circuit for transmittancy measurements. Like the lenium cell, approximately linear response of the diode with light intensity is obtained only in a low resistance external circuit, preferably one with less than 700-ohm resistance, according to the manufacturer’s data. Curr e n b as high aa 50 to 100 pa. were obtained a t large slit settings of the monochromator, but smaller slits are desirable, and a 0.2-mm. slit was about the minimum that was useful with the available galvanometer. Drift
of readings over a few hours was considered negligible. Comparison of several spectra from this setup with published curves obtained with a Beckman DK spectrophotometer in the 1 . 0 to 1.8-micron region (Beckman Instrumenta, Inc., Bull. 726, ‘%ear Infrared Spectra of Characteristic Organic Compounds”) shoned somewhat poorer rcsolution due to the larger slits, but still very satisfactory results. With a more sensitive galvanometer to permit smaller slita, performance could probably be improved appreciably. The principal advantages of the germanium diode are its extreme instrumental simplicity and low cost. Although it may be used as a photoconductive cell, this sacrifices simplicity and adds many complications due to dark current, signal-noise ratio, and other factors.
Near-Infrared Spectroscopy with the Beckman DU Spectrophotometer W. E. Shuler, Savannah River Laboratory, E. 1. du Pont de Nemours & Co.,Aiken,
variety of applications of nearinfrared spectroscopy currently appearing in the literature-for example, to the determination of the oxidation states of neptunium in solution (7) or to the concentration of water in organics or in fuming nitric acid-(I) attests t o the analyst’s growing interest in this region of the spectrum. Unfortunately, commercial spectrophotomHE
T
1604
ANALYTICAL CHEMISTRY
eters that cover the pear-infrared region are priced beyond the reach of many laboratories. Use of a small portion of the sensitivr area of the excellent lead sulfide cells now available permits a modification of the widely used Beckman DU spectrophotometer for near-infrared studies a t a . very modest outlay.
s. c. DElECTOR C O N S T R U C T I O N A N D PERFORMANCE
A commercial lead sulfide photoconductive cell is used in a direct current circuit (Figure 1). The direct current output is fed to the amplifier of a recorder, or to a potentiometer. A resolution of 6 to 8 rnp (Figure 2) is obtained with slit widths of the order of 0.05 mm. The weter vapor band shown in Figure 2 was obtained with
