A Method of Background Correction for Direct Reading Optical Emission Spectroscopic Trace Analysis Using Offset Exit Slits John A. Leys 3M Central Research Laboratories, 3M Center, St. Paul, Minn. 55101
THEPROBLEM of monitoring and correcting for background continuum adjacent to analytical spectral lines in optical emission spectrometry is one which has received considerable attention. Background correction measurements are of particular importance in trace analyses by dc arc excitation because of the generally poor absolute intensity precision of the dc arc. In fact, with the normal direct reading optical emission spectrometer which has no background correction system, the detection limits for analytical lines becomes primarily a function of how reproducibly one can maintain the background intensity from one exposure to another. It is not unusual to experience detection limits by direct reading techniques which are three- to ten fold inferior to those obtained by photographic recording. Commercial instruments are available which correct line intensity data for background and are in basic principle similar to the system described by Weekly and Norris ( I ) . This system 'uses only one photomultiplier tube as :t background monitor. Corrections are made by subtracting an electrical signal from each of the analytical integrating capacitors. The magnitude of this subtracted signal is determined by the total charge accumulated on the background integrating channel capacitor. This method is valid only to the extent that a change in background intensity at the monitoring position is accompanied by a directly proportional background change in other regions of the spectrum where the analytical lines of interest might lie. This relationship often holds very well in the case of spark excitation. In the case of dc arc sample excitation, substantial variations in background are encountered from one spectral region to another and it is often not possible to establish a simple relationship to relate precisely the intensity of one spectral region to another. The improvement in detection limits affordedby the Weekly-Norris system is less than optimum under these conditions. Perhaps the most accurate method of monitoring background intensities with dc arc excitation is through the use of auxiliary exit slits or beam splitters using a separate photomultiplier tubeforeachmeasuremen1 immediately adjacent to the analytical line of interest. With this system a program using many spectral lines necessitates an extremely crowded and complex focal curve and alignment and changes become a major task. The system to be described permits simultaneous measurement of spectral line and background intensities using an offset exit slit and utilizes only one photomultiplier tube for both measurements. EXPERIMENTAL
Offset Exit Slits. The slit used is shown in Figure 1 and consists of two segments, one below the other and slightly offset. If the top segment is aligned to pass the spectral line of interest, then the bottom segment will pass the spectral background adjacent to the line. When the bottom segment is aligned to pass the spectral line of interest the slit can then ~
(1) R. E. Weekley and J. A. Norris, Appl. S'ectrosc.,
396
ANALYTICAL CHEMISTRY
18.21 (1964).
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COPPER VAPOR COATING
'-2 Figure 1. Offset exit slit
be used to measure the background on the left side of the h e if this situation is more desirable. The spacing between the background slit and the line slit is made to exclude any possible neighboring interfering lines. In practice, slits with three different spacings between the top and bottom segmentsnamely, 150,300,and 450 p-have been suitable for all problems. The slits are made quite readily by photoresist techniques similar to the processes used to make printed circuit boards. The three masters with different spacings were made by a photoengraving firm (Buckbee Mears Co., St. Paul, Minn.). With these masters the slits are then made by photoresist techniques. It is possible to use a conventional exit slit for this application by using a quartz refractor plate behind the slit which covers only one half the height of the exit slit. The rotation of this refractor plate can then be set to select the best region for background correction. The Jarrell-Ash slits on the instrument used already have one refractor plate on the slit and a second refractor plate was thought to be somewhat cumbersome. Chopper-Switching System. In order to use this slit configuration with only one photomultiplier a chopper-switching system is used. The chopper shown in Figure 2 rotates at about 150 rpm in front of the entrance slit of the instrument and alternately illuminates the top half and the bottom half of the entrance and exit slits. The line and background phototube signals are separated and stored as shown in Figure 3. When the top half of the entrance slit is illuminated, the top half of the exit slit passes the spectral line of interest which then illuminates the photomultiplier tube. The resulting signal during this time, tl, is stored in the line signal integrating capacitor. As the chopper rotates, the bottom half of the entrance slit becomes illuminated; simultaneously an auxiliary light source and phototransistor are used to switch leads on the reed relay and the background signal is stored during time, t p , in the background signal integrating capacitor. At the completion of the sample burn, the charge on each of the capacitors is determined
SPECTROMETER ENTRANCE SLIT
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ENTRANCE SLIT
CHOPPER
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Figure 2. Chopper
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INTEGRATING CAPACITORS
Figure 3. Schematic of offset exit slit system for background corrections
with the conventional readout system. The amplification provided by the 2N3569 transistor is adequate to drive ten reed relays of the type shown. The IN679 diodes must be selected for impedance of 1000 megohms or greater in the reverse direction. Photomultiplier Tube Calibration. In order to use this background correction system it is first necessary to caiibrate the photomultiplier response and integrating capacitors for the line and background segments of the exit slit. This curve is made readily from data obtained by burning blank samples. Intensity counts for the line segment are plotted opporite the y-axis and intensity counts for the background segment are plotted opposite the x-axis. A straight line relationship is obtained. Sometimes it is necessary to deliberately vary the burning times slightly for the blank samples in order to establish a range for calibration. If it is not possible to burn blank samples for calibration, then a xenon lamp may be used in or behind the arc-spark stand as a source of spectral continuum for calibration. Construction of one such curve is necessary for each element in the program that requires background correction. These calibration curves are used as follows: The data obtained from analysis of samples consists of a line and background reading for each element. The amount of background intensity to be subtracted from each of the analytical line readings is determined by use of the proper line segment us. background segment calibration curve for the element being measured. Correction for background in this manner also results in a photomultiplier dark current correction. The resulting net intensity value is then divided by the internal standard element intensity to obtain the conventional intensity ratiob. These intensity ratios are used then for construction of analytical working curves from standard samples or for the analysis of unknown samples in the normal manner. RESULTS AND DISCUSSION
Analysis of Copper in Silver Nitrate. Figure 4 shows analytical curves for copper in silver nitrate with and without the background correction system. The operating conditions are shown in Table I. Without background correction the curve approaches the x-axis asymptotically because the copper intensity measurement includes the spectral background superimposed on the spectral line. With background correction, the cuwe is straight and approaches
3000
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100 0.1
0.3
300
1.0
3.0
10.0
P A R T S PER MILLION COPPER
Figure 4. Parts per million copper in silver nitrate
theoretical slope. This data is typical of that obtained using this background correction system. In general the detection limits are improved three- to ten-fold and the precision of measurement at lower concentrations is also greatly improved. In many cases the absolute limit of detection is very close to that which can be obtained using photographic recording techniques. Quartz-Chopper Background Correction System. Another useful system investigated for dynamic background corr'ection makes use of the same basic switching circuitry described.
Spectrometer Excitation Exposure Electrodes Analytical gap Analytical lines Slit width
Table I. Operating Conditions Jarrell-Ash 3.4, mcter Ebert convertible 17-amp dc 20 seconds ASTM S-4 lower-85-mg sample charge ASTM C-1 upper 2 mm
Copper, 3247.5 Silver, 3099.12 internal standard Entrance, 30 g Exit and background, 75 g
VOL. 41, NO. 2, FEBRUARY 1969
397
The chopper used, however, is a two-bladed quartz wheel which is mounted behind the entrance slit of the spectrometer and at an angle of approximately 40' to the normal of the light passing through the slit. When one of the quartz chopper blades intercepts the light, it displaces, by diffraction, all of the spectral lines to one side of their respective exit slits. During this time spectral background is being received by the photomultiplier tubes. As the chopper rotates one quarter revolution further the quartz blade passes by and the light from the entrance slit resumes its normal course with the spectral lines again passing through their respective exit slits. This system has the advantages that conventional exit slits may be used and the total time for integrating the spectral line signal is reduced only by a factor of two instead of by a factor of four as in the case with the offset slit system, This reduction of integrating time, however, is generally not of significance in analysis by dc arc because the intensity of the spectrum is usually substantially greater than is necessary for good phototube signal-to-noise ratio. The disadvantage of the quartz-chopper system is that the background for all the spectral lines of interest must be measured on the same side of the line and at about the same distance (depending, of course, somewhat on wavelength) from the line. This system is, therefore, applicable only to programs with comparatively simple spectra having no interfering spectral lines on the same side and lying within 200 p (for the instrument, quartz-chopper thickness and angle used) from the analytical lines of interest in the program. Offset Exit Slit Using Two Photomultiplier Tubes. Frequently an analytical program contains only one or two elements which are difficult to detect and require background correction, In this case the offset slit is used to advantage
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Figure 5. Background monitoring using offset exit slit and post-mounted mirror without the entrance slit chopper and switching system but utilizing two photomultiplier tubes in conjunction with a post-mounted mirror as shown in Figure 5 . The method used to calibrate the line and background photomultiplier tubes is identical to that described under the system using the chopper. The results observed are essentially the same as for the chopper system.
RECEIVEDfor review August 28, 1968. Accepted October 29, 1968.
Small Sample Handling in Laser Raman Spectrometry Stanley K. Freeman International Flavors & Fragrances, R & D Center, Union Beach, N . J. Donald 0. Landon Spex Industries, Znc., Metuchen, N . J.
SEVERAL REPORTS have appeared recently (1-3) pertaining to the generation of Raman spectra by laser excitation of small samples. Bailey, Kint, and Scherer (3) adopted the axial excitation/viewing geometry (Figure 1, left) of Porto ( 4 ) , and reported spectra from as little as 0.04 p1 of CCl4 in specially fabricated capillary cells. However, this system gives poor depolarization ratios for the 218, 314, and 459 A cm-1 bands. Dr. J. R. Scherer recently advised us that the system has been improved and it now yields good results on 0.04-pl sample quantities. Pez (5) obtained Raman spectra of liquids in 1.0-mm bore melting point capillaries by transverse irradiation/viewing (Figure 1, right). Unlike the tube with a single focused hemispherical lens of the type
(1) R. C . Hawes, K. P. George, D. C . Nelson, and R. Beckwith, ANAL.CHEM., 38, 1842 (1966). (2) A. Lau and J. H. Hertz, Spectrochem. Acta, 22, 1935 (1966). (3) G. F. Bailey, S. Kint, and J. R. Scherer, ANAL.CHEM., 39, 1040 (1967).
(4) T. C. Daman, R. C . C. Leite, and S. P. S. Porto, Phys. Reu. Lett., 14, (I), 9 (1965). (5) G. Pez, McMaster University, private communication, 1968. 398
ANALYTICAL CHEMISTRY
employed by Bailey et al., the transverse viewing method allows multipassing of the laser beam. In the former method, a lens at each end of the capillary would be necessary to permit multipassing. To obtain spectra of smaller samples, we have extended Pez's technique to liquids in 0.1-mm bore tubing, as well as to the spectral examination of solids trapped from a gas chromatograph in 0.5-mm bore capillaries. A spectrum of 8 nl of CC14 appears in Figure 2. With this system, the measured depolarization ratios are excellent (pJls and 314: 0.75 + 0.01, p t s s a 0.006 ==I 0.002) Under the same conditions, 8 nl of linalool yielded a usable spectrum (Figure 3). (Note that only a ten-fold signal amplification is required to yield a curve similar to the one obtained on 800 nl of linalool in a 1-nim bore capillary. The intensity difference is due to a difference factor of ten between scattering paths of the two capillaries.) The different spectral backgrounds are attributed to greater fluorescence of the 0. I-mm capillary. As a result of the intense scattering of laser radiation from the walls and windows of capillary microcells, it is essential that a double monochromator be employed to fully realize the capabilities of the micro techniques.
