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Anal. Chem. 1983, 55,2022-2025
Stray Radiation from Ruled Gratings Wilbur Kaye Beckman Instruments, Inc., P.O. Box C-19600, Irvine, California 92713
Ruled gratings are characterized by ghosts arislng from Imperfections in the ruling process. The bandwidth of each ghost is determlned by the bandwidth of radiation lncldent on the gratlng, and the intensities of these ghosts are strongly influenced by surface plasmon scattering. Unfortunately the rules governing the intensltles and displacements of these ghosts are compllcated, making the convolution test for stray radiation a laborious exerclse. Still the principles Involved in the convolution test help In understandlng performance of Instruments equipped with ruled gratings. An unusual situation can exist when uslng a deuterium source. The 486- and 656-nm atomic line emlsslon from the source can produce line ghosts which superimpose on the UV continuum. The resuitlng sudden fluctuations in stray radlatlon are easily misinterpreted as structural features of sample absorptlon spectra.
Most ruled gratings diffract a small portion of the incident radiation at discrete angles not predicted by wavelength and groove spacing. Rays so diffracted are called ghosts and have been a recognized problem in atomic emission spectrometry for over 100 years (1). They are attributed to imperfections in the ruling process. The pattern of ghosts varies among different rulings, but replicas produce essentially the same ghost pattern. While it is generally recognized that these ghosts influence stray radiation in spectrophotometers designed for absorption spectrometry, the complex character of stray radiation and its variation with wavelength and polarization has only recently been demonstrated (2). New information concerning the impact of ghosts on stray radiation is provided by a convolution test (3). This test has permitted a quantitative measurement of stray radiation in spectrophotometers equipped with holographic gratings (4). Unfortunately its application to ruled gratings is greatly complicated by the ghosts and it has been necessary to resort to approximations. In essence the convolution method treats polychromatic radiation as the summation of its monochromatic components, each of which is described by a slit function. These slit functions in turn are classified as either primary or stray radiation by means of a parameter called the “limit”, L. Unlike other tests for stray radiation, this one provides information at instrument dial settings X where most of the detected radiant power (DRP) arises from primary radiation. In the course of studying slit functions with ruled gratings it has been found that surface plasmon scattering strongly affects ghost intensities. The grating orientations, given by X, at which plasmon scattering is a maximum have been found to be the same for the ruled gratings and for the holographic ones of the same groove spacing earlier studied (4).Ghost intensities are found to be both enhanced and depressed by plasmon scattering. Unusual effects are observed when a strong line ghost from a source such as the deuterium lamp having both line and continuum character is enhanced by plasmon scattering. EXPERIMENTAL SECTION Instrumentation. The equipment used in this study has been described elsewhere ( 3 , 4 ) . All spectra shown here were obtained
Table I. Ghost Location no. K h(656) 1 0.147 560 2 0.156 554 3 0.287 468 4 0.303 457 5 0.381 406 386 6 0.412 7 0.494 332 8 0.515 318 9 0.597 264 10 0.627 245
I(656) 2.2 x 10-5 3.0 10.8 6.3 21.0 11.4 2.2 2.6 3.7 2.1
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h(486) 415 410 347 339 301 286 246 236 196 181
with a high-quality grating ruled and replicated by Bausch and Lomb. It was a 1200 groove/mm grating blazed at 250 nm (blaze angle 8.6O). Slit Function. Figure 1 shows a slit function for the DU-8 spectrophotometer manufactured by Beckman Instruments, Inc., equipped with the ruled grating and mercury source. A single interference filter isolated radiation of 546-nm wavelength. All of the lines shown with expanded ordinate, with the exception of three lines marked “Hg”, arise from the same wavelength (546 nm) and are grating ghosts. The ordinate has been expanded 28500 times so the ghosts are weak relative to the parent or first-order line. Some of the ghosts exhibit a fine structure that is shown with a 10-fold expanded abscissa. Two abscissa scales are identified. The lower one labeled X is simply the monochromator wavelength dial reading and should not be confused with true wavelength X. The upper scale, labeled X - X indicates the displacement of the ghosts from the parent line. RESULTS AND DISCUSSION The ghosts are seen to be symmetrically displaced from the parent line at X = 546 nm and the more intense ghosts appear in pairs numbered for convenience. When monochromatic radiation of other wavelengths is used, it is found that the ghost displacement is a linear function of wavelength.
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One cannot treat these ghosts as arising from some subset of grooves having a spatial frequency less than 1200 per mm. Each ghost has a characteristic K value. Table I lists these K values. It would appear from Figure 1that 546-nm radiation exits from the monochromator only at discrete X values. In truth some 546-nm radiation appears at all orientations of the grating. The intensity of both ghosts and continuum, relative to the parent line, is a function of slit width and source bandwidth. If the slit width is widened, while using a monochromatic source, the continuum intensity increases relative to that for the ghosts. This is a consequence of the pattern of radiation falling on the inside of the exit slit of the monochromator. The DRP from a line image increases linearly with slit width, while that from a continuous band increases quadratically. If the bandwidth of source radiation increases while holding slit width constant, the intensities of both ghosts and continuum increase linearly relative to the parent line. This is illustrated in Figure 2 obtained by using radiation from a tungsten source whose bandwidth is determined by a pair of narrow-band interference filters. Of course the closely spaced ghosts overlap, modifying the relative intensity of the ghosts.
0003-2700/83/0355-2022$01.50/00 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 13,NOVEMBER 1983
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Flgure 1. Slit function, unlpolarized 546-nm line source, S S W 0.1 nm.
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Flaure 3. Slit function. 674.5-nm radiation of 3.2 nm bandwidth, polarized in the S plane,.'
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The continuum is now about 6% of the intensity of the strongest ghost. The intensity of the continuum is almost independent of X and is quite similar to that from a holographic grating. The discontinuities near 260 and 57 0 nm are zero checks which, a t high gain, have to be made while scanning. Surface Plasmon !Scattering. The character of surface plasmon scattering from holographic gratings is discussed in the companion paper ( 4 ) . There is no visually discernible difference in the plasma arc pattern seen when illuminating the two gratings with a HeNe laser. 'The intensity of continuum scatter from the ruled grating is enhanced similar to that from the holographic grating; however, the intensity of ghosts is found to be both enhanced and depresse'd by the plasmon scatter. These effects are evident in Figure 3 where 674.5-nm radiation was chosen to superimpose ghost number 6 on the plasmon maximum. Not only is the intensity of this ghost, relative to the parent line, increased about a factor of 4, compared to Figure 2, but ghost number 8 has virtually disappeared. Intensities of four ghosts relative to the parent line are plotted in Figure 4 as a function of wavelength A. The relative intensities of each ghost are seen to vary about 30-fold as wavelength changes.
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Flgure 4. Normalizedl ghost intensities vs. wavelength for four ghosts.
Effect of Source Bandwidth. Earlier it was shown that stray radiation in an absorption spectrophotometer equiplped with a holographic grating could be quantitatively expressed by a convolution of the slit function with the single beam power spectrum ( 3 ) . The multivariant character of the slit function from ruled gratings renders the convolution test unduly laborious. Nevertheless, the principles can be applied to understand the complex character of stray radiation in instruments equipped with ruled gratings. As the bandwidth of incident radiation increases, the ghosts also widen and ultimately overlap. Such a condition is commonly encountered in UV-Vis spectrophotometers where incident radiation bandwidth is dictated by blocking filters. Plasmon enhancement of ghosts can produce some surprising results. Figures 5 and 6 show single beam power spectra of the EIU-8 spectrophotometeir with the ruled grating and two different blocking filters. In both illustrations high-pass sharp cutoff filters are used. The DRP falls off at long wavelengths because of the spectral sensitivity of the detector. Below the filter
ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983
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Flgure 5. Single beam power spectrum, DU-8 with a Hoya Y-46 blocking filter.
Flgure 7. Single beam power spectrum, DU-8 with an unfiltered deuterium source.
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Flgure 8. Single beam power spectrum, DU-8 with a deuterium source, no blocking filter, chromate sample.
parent line, but the enhancement at the grating angles of plasmon scattering must also be taken into consideration. These angles can be estimated from Figure 4 of ref 4. I t is seen that the enhancement of scatter from 656-nm radiation is a maximum at 411 nm and that from 486-nm radiation occurs at 206 nm. Ghost number 5 from 656 nm falls close to the enhancement angle and this ghost is particularly intense as indicated by the column of Table I headed I(656). Ghosts 8 and 9 from 468-nm radiation are also enhanced. If a line ghost intensity is negligible in comparison with the continuum stray radiation, the line ghost is seldom a problem. The continuum stray radiation will, of course, introduce an absorbance error in a sample spectrum but will not seriously distort band shape. However, if the line ghost is greater than the continuum, the abrupt rise in stray radiation may seriously distort sample band shape and lead to erroneous interpretation of spectra. Figure 8 shows an expanded scale transmittance spectrum of a chromate sample sufficiently concentrated to absorb essentially all of the radiation in the 340-420-nm interval. It
ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983
was obtained with a narrow (0.1 nm) slit, using the deuterium source without a blocking fiiter. Ghost number 5 fronn 656-nm radiation is seen to have an intensity 4 times greater than the stray radiation continuum. Ghost number 6 of 656 nm and ghost number 4 of 486 nm are also prominent. The intensities of these lines relative to the continuum will be influenced by slit width. Blocking filters may or may not eliminate the 656- and 486-nm ghosts. Most commercial spectrophotometers do not use blocking filters at wavelengths shorter than 315 nm and eight of the ghosts in Table I fall in this region. Some instruments use only high-pass blocking filters to eliminate second-order radiation and these will pass the offending wavelengths. Even some band-pass blocking filters may pass ghosts of small displacement (or K). It may appear unimportant to be concerned about the behavior of ruled gratings since most spectrophotometers being produced today use holographic gratingn; however, many older instruments are equipped with ruled gratings andl it is desirable to understand their behavior. Furthermore, ruled gratings still have ceirtain advantages for which they will continue to be used. The blaze angle is better controlled by the ruling process and it is impractical to produce holographic gratings with large grloove spacings. A knowledge of stray radiation is most useful at wavelength settings, X, where most of the DRP arises from primary radiation. This knowledge, obtainable from the convolution test, can be used to correct, absorbance readings for the error introduced by the stray radiation. It would appear from the above experiments that the complexities introduced into the slit functions, by the ghosts render the convolution test impractical. This is not entirely true. If the displacement for the first significant ghost is larger than the bandwidth of the blocking filter, the most important portion of the slit function for the ruled grating is as simple as that for the holographic grating. For the ruled grating used here the blocking filter would have to have a bandwidth less than 50 nm if passing 400-nm radiation and have a bandwidth less than 100 nm at 800 nm in order to use the convolution test. Blocking filters
i!025
of these bandwidths are not currently employed but are feasible. The influence of the ghosts on other tests for stray radiation should be recognized. In the conventional ASTM test (5) the stray radiant power ratio is equated to the transmittance of sharp cutoff materials at X values where the test material is essentially opaque. Ghosts may cause the apparent transmittance to fluctuate below the cutoff in a manner similar to Figures 5 and 6. One might easily misinterpret such a spectrum as indicating the test material to be fluorescent, or the monochromator to have spurious reflections at certain grating orientations, or the reference spectrum to be unusual. In the blocking filter test (3),the DRP at some X below the cutoff is presumed to represent the level of stray radiation at all wavelengths passed by the blocking filter. If ghosts cause this DRP to fluctuate with X, one may be at a loss whilch i( to use. Experience indicates it is best to use the closest IIRP minimum below the cutoff. The narrower the bandwidth of the blocking filter, the smaller the error in this test. It should be emphasized that the ghost “spectra”shown here are distinctive of a particular ruled grating and its replicas. The displacementri and intensities of ghosts from other ruled gratings will be different. One should be particularly concerned with the displacement of the first ghost. It is a wise precaution to measure the slit functions for 486- and 656-nm radiation in any spectrophotometer equipped with a ruled grating in order to predict if any strong ghosts of the deuterium source will appear at values where no blocking filter is in the beam. Registry No. Dz,7782-39-0.
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LITERATURE CITED (1) Quinck, G. Ann. Phys. Chem. 1872, 146 (5), 1. (2) Brown, S.; Tarrsint, A. W. S. Opt. Acta 1978, 25, 1175. (3) Kaye, W. Anal. Chem. 1981, 53,2201. (4) Kaye, W. Anal. Chem. 1983, 55,2018. (5) “Standard Method of Estimating SRE”; ASTM Designation E-307-72; American Society for Testing Materials: Philadelphia, PA.
RECEIVED for review May 16, 1983. Accepted August 8,1.983.
