These anhydrous forms are insoluble in non-coordinating solvents and solution of them in methanol or in N,N-dimethylformamide gives the original pale green color. Reflectance spectra could not be obtained because of their extreme hygroscopic nature. The anhydrous forms can however be obtained in KBr disks by heating the mixture under an IR lamp for six hours under a flow of nitrogen and pressing the disk in a hot die. The infrared spectra of the disks confirm the absence of water and the visible spectra [Table I(C)] show bands of 710, 900sh nm for Ni(maq)Clz and at 700, 900sh nm for Ni(maq)Brz. These spectra argcharacteristic of octahedral compounds of nickel(I1) and differ from the spectra of the analogous hydrated complexes. Complexes containing nitrate, perchlorate, and sulfate groups may be subject to exchange with potassium bromide on grinding. Table I(D) shows the visible spectral data of the complexes examined which do undergo exchange as indicat,ed by a change in color on grinding and changes in the infrared. The copper(II)(tmb) complexes exchange to the analogous bromo complex, although a comparison of the spectra with the spectra of Cu(tmb)Br2 or Cu(tmb)nBrz ( 1 1 ) indicates that the exchange is not complete. The COT balt complexes Co(tmb)(N03)yHzO and Co(tmb)nNOr C104 are pink in color and octahedral ( 5 ) . On grinding in potassium bromide, the complexes turn bright blue, The visible spectra of the disks are very similar to that of the
blue tetrahedral Co(tmb)ClZ Table I(A) and suggest that in both cases the exchange product is the blue Co(tmb)Brz. The conversion of octahedral-tetrahedral cobalt in the solid is unusual, especially for Co(tmb)zN03C1O4 which must necessarily lose both a bidentate nitrato group and a molecule of tmb from the coordination sphere on grinding. This suggests that the bis complex is relatively unstable and this is supported by the failure to produce the bis complex Co(tmb)~(NO3),from solution ( 5 ) . The complex Co(tmb)Brz has not previously been prepared.
LITERATURE CITED (1) G. G. Allan. H. Chang, and K. V. Sarkanen, Cbem. Ind. (London), 699 (1967). (2) J. R. Hall, M. R. Litzow, and R. A. Plowman, Anal. Cbem., 35, 2124 (1963). (3) J. R. Hall, M. R. Litzow. and R. A. Plowman. Aust. J. Cbem., 18, 1339 (1965). (4) J. R. Hail. M. R. Litzow, and R. A. Plowman, Aust. J. Cbem., 18, 1331 (1965). (5) J. R. Hall, M. R . Litzow, and R. A. Plowman, Aust. J. Cbem., 19, 201 (1966). (6) M. R. Litzow, L. F. Power, and A. M. Tait. J. Cbem. SOC.A, 275 (1970). (7) M. R. Litzow, L. F. Power, and A. M. Tait, J. Cbem. SOC.A, 3226 (1970). (8) M. R. Litzow. L. F. Power, and A. M. Tait, Aust. J. Cbem., 23, 1375 (1970). (9) M. R. Litzow, L. F. Power, and A. M. Tait, J. Cbem. SOC.A, 2907 (1970). (10) J. King and L. F. Power, Aust. J. Cbem., 25, 1863 (1972). (11) M. R. Litzow, Ph.D. Thesis, University of Queensland, 1964.
RECEIVEDfor review December 2, 1974. Accepted April 21, 1975.
Simple, Inexpensive Monochromator Modification Permitting Dual-Channel Operation D. W. Brinkman and R. D. Sacks’ Department of Chemistry, University of Michigan, Ann Arbor, Mich. 48 104
As the need for lower and lower detection limits has increased in optical spectroscopy, methods for controlling external variables have proliferated. Mechanical or electronic chopping in conjunction with a phase-sensitive lock-in amplifier detection system has become a normal part of many spectroscopic instruments (1-3). When flame sample cells are used, even more sophisticated techniques, using two light sources, have been proposed ( 4 , 5 ) ,including one variation utilizing a vibrating quartz plate (6). However, there are conditions under which none of these methods is feasible. In many actual applications, only micro quantities of sample are available for analysis. This has led to the routine use of a number of flameless atomizer systems for atomic absorption and fluorescence in which the resultant signal is transient in nature. In other cases, the method itself involves a transient signal, such as exploding wire atomic emission ( 7 ) and exploding wire-excited atomic fluorescence (8). Here a simultaneous background correction is necessary. Some attempts a t background correction have been reported using optical multichannel analyzers (9) or a second, totally separate detection system for background monitoring (10). Both of these approaches suffer the obvious disadvantage of large expense and, in the case of the latter method, an increase in the overall complexity. The drying and ashing sample pretreatment recommended for carbon rod atomizers is another attempt a t side-stepping the background problems encountered in non-ideal samples ( 1 1 ) . The instrumental modification described here combines
an inexpensive monochromator with a simple addition to the exit slit assembly to obtain simultaneous line and background or line and reference line detection. Full slit width flexibility is retained, and separation of observation wavelengths is variable up to about 2 nm. The basic component is a quartz plate which is positioned just in front of the exit slit. The plate is placed so that it intercepts only the radiation from the focusing mirror of the monochromator going through the top half of the exit slit. By rotating the plate to some incidence angle CI# with respect to the surface normal, as shown in Figure 1, a different wavelength passes through the top half of the exit slit than through the bottom half. The wavelength separation Ah can be predicted knowing the linear reciprocal dispersion p of the monochromator, the thickness a of the quartz plate, and the incidence angle 4, along with the tabulated index of refraction for quartz in the wavelength region of interest. The wavelength separation Ah in nanometers is expressed as,
where cl is the linear displacement of the refracted and unrefracted rays of a given wavelength. This displacement, however, can be expressed as a function of line segment CD and the angle a between the emerging ray and the plate surface. [ A A ~ ,=
p CD sin
cy
=
p
CD cos 4
(2 )
However,
* Author to whom correspondence should be directed.
-
CD =
BD - BC
=
AD s i n 4 - AC sin e
ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
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i SLIT
0
I 2
LE
Figure 3. Dual-mirror beam-splitter detection system with side cover removed
Figure 1. Schematic representation of the quartz plate wavelength separation system Wavelength A 1 does not pass through the quartz plate, and thus is not in the plane of the flgure. 6,Incidence angle; 8, angle of refraction; a, width of quartz plate; d, linear separation of the upper and lower beam of same wavelength; A,, wavelength of unrefracted light going through bottom of exit slit; A*, wavelength of refracted light going through top of exit slit
WIMLCNGTH. nm
Figure 4. Percent difference in response expected from the two channels for a 6000 K blackbody radiation source as a function of mean wavelength for various wavelength separations
Figure 2. Rotating quartz plate assembly The micrometer head drives the blate through a lever arm. The axis of rotation of the plate is through its center
or,
Here, R is the angle of refraction within the quartz plate with respect to the plate normal. Using the identity cos 6 = (1 - sin2R)lI2and Snell’s law, the quantity CD can be obtained in terms of +. The refractive index of air ~1 here is assumed to be 1.00.
Finally, Equation 5 is substituted into Equation 2 to obtain
= p u sin 4 cos 6
X
Thus, the wavelength separation obtained for a given instrument can be controlled by both the quartz plate thickness and the incidence angle $.
EXPERIMENTAL Apparatus. The basic module is the GCA McPherson Model EU-700, 0.35-m Czerny-Turner monochromator, which has a first1724
ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
order linear reciprocal dispersion of about 2.0 nm/mm. The exit slit assembly is modified by attaching the rotating quartz plate assembly shown in Figure 2. It is fastened to the top of the slit assembly with two thumb screws and requires no modification of the monochromator other than tapping of the screw holes. The assembly is placed such that the radiation from the top half of the focusing mirror of the monochromator passes through the quartz plate just before reaching the exit plane. The 0.5-inch range micrometer head drive moves a lever arm attached to the quartz plate holder, thus rotating the plate. The lever arm and micrometer drive are kept in contact by a spring between the arm and the micrometer drive holder. The micrometer is calibrated empirically and set for the wavelength separation desired. For this study, the plate was a 1.27-cm square of 0.64-cm thick S1-UV quartz (Esco Products). The detection system is shown in Figure 3. A 2-mm thick baffle fits right up to the exit slit to eliminate light diffracted by the edge of the quartz plate. The divided beam then is reflected by a dualmirror beam-splitter made of two 1-mm thick mirrors fastened to a 5.08-cm wide aluminum block shaped like an equilateral triangle with 5.72-cm edges. This beam splitter is placed directly behind the baffle. The distance between the exit slit and the socket assembly for the RCA 1P28 multiplier phototubes is adjustable to allow alignment of the tubes with the reflected radiation. The phototubes are positioned with respect to the mirror surfaces to provide normal illumination of the cathode area. The signal from one of the multiplier phototubes is monitored across a 10 kR load resistor; while that of the second tube is monitored across a load which can be varied from 5 to 15 kR by using a lo-kR, ten-turn potentiometer in series with a 5-kR fixed resistor. The tubes should have fairly similar spectral response. Balancing and Calibration. The output signals are balanced a t each wavelength of interest using a 1600-W xenon arc lamp (Christie Corp., CXL-1600). Stripchart recorders are used to monitor the signals from each half of the slit. Calculated intensity differences, assuming the xenon arc to be a blackbody radiator having a color temperature of 6000 K, are used to determine the appropriate signal ratio for the two tubes for a given AA and mean wavelength.
/ -20
-10
0
INCIDENCE
IO ANGLE,
20 degrees
Flgure 5. Calibration curve for a 0.64-cm thick quartz plate at 253.7
nm Figure 4 shows a family of calibration curves for a 6000 K blackbody source. These curves describe the percent difference expected for the response of the two tubes as a function of wavelength for a family of AX values. This procedure corrects for differences in grating efficiency and gain of the multiplier phototubes a t the two wavelengths. A mercury pen lamp is used as a line source for calibration of the wavelength separation attained for various settings of the micrometer head plate drive. For final balancing, the instrument is set for the wavelength of interest, and the quartz plate is set for the desired separation. The potentiometer then is adjusted until the same signal level is obtained from both channels using a sample blank. Although the transmittance of the quartz was found to be independent of the incidence angle, variations in source intensity, grating efficiency, and spectral response of the multiplier phototubes for large wavelength separations do not allow balancing a t zero deflection with subsequent rotation of the quartz plate.
RESULTS AND DISCUSSION A plot of wavelength separation obtained as a function of the incidence angle 4 is shown in Figure 5 for a 0.64-cm thick quartz plate a t 253.7 nm. No attempt was made to obtain a linear drive mechanism. A comparison of the theoretical values of wavelength separation as obtained from Equation 6 with those obtained experimentally, using the Hg line source, produced very satisfactory agreement. Most of the observed error could be attributed to uncertainty in the determination of both 4 and the separation of the peaks on the stripchart records. With a maximum separa-
tion of 1.8 nm for this particular design and plate thickness, a separation of 1.8 bandpasses between the centers of the two slit images would be obtained for a 500-pm slit width, which is commonly used in atomic fluorescence. Much greater separation in bandpass units would be obtained for the narrow slit widths used in atomic absorption. The reproducibility of the method for calibrating AA was evaluated by repeatedly resetting the micrometer head to obtain an incident angle of 14’. The Hg(1) 253.7-nm line then was scanned, and the distance between the peaks from the top and bottom portions of the slit was obtained from the stripchart record. A relative standard deviation of 1.0% was found. This could be attributed almost completely to variations in the speed of the chart drives of the two recorders. No decrease in signal level was observed as the incidence angle was changed. This permits arbitrary selection of wavelength separation without loss of sensitivity or calibration.
ACKNOWLEDGMENT The authors express their appreciation to N. Johnston and W. Wolf for their help in the design and construction of the apparatus.
LITERATURE CITED (1) D. P. Hubbard and R. G. Michel, Anal. Chim. Acta, 67, 5 5 (1973). (2) R. C. Elser and J. D. Winefordner, Anal. Chem., 44, 698 (1972). (3) W. K. Fowler, D. 0. Knapp, and J. D. Winefordner. Anal. Chem., 46, 601 (1974). (4) T. C. Rains, M. S.Epstein, and 0. Menis, Anal. Chem.. 46, 207 (1974). (5) D. L. Dick, S. J. Urtamo, F. E. Lichte, and R. K . Skogerboe. Appl. Spectrosc., 27, 467 (1973). (6) D. G. Mitchell, J. M. Rankin, and B. W. Bailey, Spectrosc. Lett., 5, 87 (1972). (7) J. A. Holcombe and R. D. Sacks, Spectrochlm. Acta, Pari B, 28, 451 (1973). ( 8 ) D. W. Brinkman and R. D. Sacks, Anal. Chem., 47, 1279 (1975). (9) K. W. Busch, N. G. Howell, and G. H. Morrison, Anal. Chem., 46, 575 (1974). (10) R. L. Sellers, G. W. Lowry. and R. W. Kane, Amer. Lab., March, 1973. (11) J. W. Robinson, G. D. Hindman, and P. J. Slevin, Anal. Chim. Acta, 66, 165 (1973).
RECEIVEDfor review March 6, 1975. Accepted April 28, 1975. This work was supported in part by National Science Foundation Grant GP-37026X.
Guide for Scoring, Stripping, and Spotting Thin Layer Plates Oscar Sudilovsky and Albert Kovach Institute of Pathology, Case Western Reserve University, and University Hospitals of Cleveland, Cleveland, Ohio 44 106
Numerous thin layer chromatography accessories have been designed for each specific application. As a result, there has been an increase in the number of attachments required for the performance of basic chromatographic steps. T o obviate this inconvenience, we have developed a low cost multipurpose guiding support. This instrument effectively holds thin layer chromatography plates in place during the various manipulations, facilitates the preparation of parallel channnels, allows scoring the finishing line, serves as a guide for accurately spotting mixtures a t the or. used in conjuncigin and simplifies reading of R ~ sWhen tion with the suction-scrapper device described previously ( I ) , lateral cross contamination of samples and aspiration of dust (a potential health hazard when working with silicates) are effectively precluded. To our knowledge, no single TLC accessory has been reported that performs all of the functions mentioned above.
DESCRIPTION The device shown in Figure 1,which is made of Plexiglas, consists basically of a supporting unit (A) and a sliding guide (B). The supporting unit rests on four rubber feet (C) and is receded a t both its top and base to accomodate the rectangular bars (D and E). Bar D is fixed to the supporting unit; bar E can be displaced toward the short arm of the sliding guide B by gently pushing it in that direction. When a chromatographic plate is laid between the top bar D and the displaced bar E, it is firmly held in position by virtue of the indirect force exerted on it (through bar D) by the distended lateral springs. The sliding guide B is composed of a short and a long arm, perpendicular to each other. It moves laterally, by means of an adjustable screw (F), along a slit (G) milled in ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
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