to 30-lb pressure on the top bolt. In general, a disk made in this manner is clear and transparent. Loosen and remove the top bolt. Scratch a small shallow depression (about 2 to 3 mm diameter) in the center of the disk, but leave the other parts of the disk intact. Remove any loose pieces of KBr into an agate micromortar. With a micropipet, place an appropriate amount of the sample of pesticide to be analyzed in the mortar, and mix the KBr well with the sample. Then transfer the sample matrix to the shallow depression in the disk. Cover the whole disk in the barrel with another 200 mg of KBr, and press it again with bolts and the torque wrench until the pressure is 25 to 30 lb. Remove the bolts, and insert the barrel directly in the sample beam of the spectrophotometer. Prepare a similar KBr disk (without sample) with another Wilks Mini-Press barrel and bolts, and use it in the reference beam. For scanning the spectrum, adjust the gain control, suspension, etc., and set the instrument at 1X or at any other expansion desired. Some spectra obtained by this procedure are shown in Figure 1.
DISCUSSION
If the sample of pesticide is a solution, the solvent must be evaporated completely after the solution is mixed with the KBr in the agate mortar and before it is transferred into the depression in the disk. Therefore, a low-boiling solvent such as spectranalyzed methylene chloride or pentane is preferred in making the sample solution. In subsequent experiments, we have found that this disk sandwich procedure for volatile chemicals can also ,be used with minute amounts of certain samples of pesticidal residues (Figure 2 ) . If the sample is placed entirely in the path of the beam of infrared energy, a spectrum of maximum intensity can easily be obtained. RECEIVED for review October 26, 1971. Accepted December 21, 1971. Mention of a pesticide or a proprietary product in this paper is for identification only and does nct constitute a recommendation or an endorsement of this product by the U.S. Department of Agriculture.
Commercial Tungsten Filament Atomizer for Analytical Atomic Spectrometry Mark Williams and E. H. Piepmeier' Department of Chemistry, Oregon State Uniuersity, Corvallis, Ore. 97331 NON-FLAME ELECTRICALLY HEATED means of atomizing samples for analytical atomic absorption and atomic fluorescence have recently received renewed attention because of their ability to determine picogram amounts of elements in microgram samples. Winefordner and Elser ( I ) have briefly reviewed some of the graphite furnaces, filaments, and wire lamps used in atomic fluorescence. Tantalum foils have been studied (2, 3) and applied ( 4 ) to blood samples. The carbon rod atomizer has been used for clinical samples ( 5 , 6 ) and graphite furnaces for biological samples (7), snow (7), and air samples (8). The importance of correcting for light scattering, particularly when biological samples are used, has been discussed (9). Platinum and tungsten wire loops have been studied (10) and an automated system using a platinum wire loop has been built (11). In this investigation, a rigid spiral-wound tungsten filament from a commercially available light bulb (General Electric 1
Author to whom requests for reprints should be sent.
(1) J. D. Winefordner and R. C. Elser, ANAL.CHEM., 43,(4), 24A (1971). (2) H. M. Donega and T. E. Burgess, ibid., 42, 1521 (1970). (3) J. Y.Hwang, S. D. Smith, and A. L. Malenfant, 10th National Meeting of the Society for Applied Spectroscopy, St. Louis, Mo., Oct. 1971, paper 137. (4) J. Y . Hwang, ibid., paper 71. (5) M. D. Ames, P. A. Bennett, K. G. Brodie, P. W. Y.Lung, and J. P. MatouSek, ANAL.CHEM.,43,211 (1971). ( 6 ) D. P. Sandoz, K. G. Brodie and J. P. MatouSek, 10th National Meeting of the Society for Applied Spectroscopy, St. Louis, Mo., Oct. 1971, paper 136. (7) R. Woodriff, B. Culver and D. Shrader, ibid., paper 139. ( 8 ) R. Woodriff and J. F. Lech, ibid., paper 145. (9) S. R. Koirtyohann and Helen Severs, ibid., paper 134. (10) M. P. Bratzel, Jr., R. M. Dagnall, and J. D. Winefordner, Appl. Spectrosc., 24, 518 (1970). (1 1) S. R. Goode and S. R. Crouch, The Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1971, paper 80. 1342
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
QUARTZ
7
Figure 1. Exploded view of argon sheath and mounting for tungsten filament
No. 1763) was used to atomize 3-111 samples. A method of compensating for background tungsten emission is presented. The nominally 6-V, 4-A filament was driven from a n easily programmed precision power supply (MP-1026, McKeePedersen Instruments, Danville, Calif.). The atomic vapor was observed by atomic absorption. Matrix effects for complex samples might be expected to be more severe for a wire loop system than for a furnace or rod because spectral observations are usually made in the cooler region above the wire loop rather than in the heart of a furnace or hollow rod. Our need was for a system that could measure nanogram amounts of a single element dissolved in a small amount of solution. The precision construction and ready availability of the tungsten filaments together with the simplicity of our sample matrix led to the study of the wire loop system rather
MP-1018 MONOCHROMATOR
HOLLOW CATHODE
Figure 2. Optical system for atomic absorption measurements
LAMP
I
'ARGON
than a furnace or rod that would require large amounts of electrical power. EXPERIMENTAL
Apparatus. A flowing argon gas sheath was built around the filament as shown in Figure 1. A 1-cm i.d. quartz tube was inserted into a brass rod drilled to 1-cm (8/g-in.) i.d. to provide a smooth flow of argon throughout the assembly. The quartz tube rested on a ledge inside the brass rod and two nylon set screws held it firmly in place. The brass rod was shaped to allow passage of the hollow cathode beam through the quartz tube. Slots were drilled in the brass rod and sawed into the quartz tube to allow insertion of the tungsten filament. The sample was inserted by a hypodermic needle through a small hole in the front of the quartz tube. Preliminary work without the quartz tube gave erratic results, probably because of erratic argon flow past the filament. Although the tungsten filaments were essentially identical from one bulb to the next, the mounting flanges varied slightly in their location along the main shaft of the bulbs. A 1-mm compensating adjustment was allowed between the brass block used for the socket and the main mounting angle, Figure 1. This allowed the filament to be readily positioned in the center of the argon flow. The main mounting angle was drilled so that the assembly could be reproducibily mounted in place of a Beckman burner in the atomic absorption instrument. The glass bulb around each filament was removed by placing the bulb in an envelope and gently cracking it between the jaws of a vice. Pliers were used to remove the remaining glass. The optical system for the atomic absorption instrument is shown in Figure 2. Two monochromators (MP-1018 McKeePedersen Instruments) were available. One monochromator was mounted on top of the other, and a quartz plate was used to split the beam. The over and under configuration allows both reflections from the quartz plate to enter the lower monochromator. The two paths from the entrance slit of each monochromator to the first lens were made equal so that an image of the sample vapor could be focused on both slits simultaneously. A 1-mm aperture was placed just outside the quartz tube to limit the region of observation and to minimize the tungsten emission that reached the monochromator. The hollow cathode was focused on the sample by another quartz lens. The optical system was aligned by reverse illumination using a frosted tungsten bulb and tuning the wavelength to green (550 nm). The sampling region was placed 2 cm in front of the green image of the first lens to compensate for chromatic aberration of the quartz lens. The lens was fixed and no attempt was made to relocate the image for each wavelength used. The best results were obtained with the top turn of the tungsten filament centered and located just below the bottom of the green cone of light that resulted when the I-mm aperture was in place. A second green image was located in the center of the hollow cathode. To compensate for chromatic aberration, final adjustment of the position of the hollow cathode lamp was made while photoelectrically observing the emission line of the analyte element, the hollow cathode being positioned where it gave maximum signal. The programmable power supply was a stable operational amplifier with high power output capabilities, operated in the
.... . - - MP-1008 b
Figure 3. Precision power supply for tungsten filament Rib, Rq, Rr, Rq = 100 KnlZ Ra, R3 = 20Kn 1 % C = 10 pF, low leakage
Ria,
inverting mode Figure 3. R5 was connected directly to the terminals of the MP-1026. The rest of the circuit was enclosed in a small aluminum box with 1-ft electrical leads. A MP-1008 millivolt source provided a precise, variable input voltage. When switch S1 was in the HEAT position, 0.45 V was supplied to the filament to evaporate the sample without splattering. In the ATOMIZE position, 4.0 V was supplied to the filament to atomize the sample. Switch S2 was connected to the argon gas flow valve to prevent heating the filament when the argon flow was off. The contact resistance of a switch placed in this position does not influence the precision of the voltage supplied to the filament. An additional voltage increment which rapidly decayed exponentially with a time constant CR2 could be added to the filament by discharging C through RPinto the summing point. This pulse qiiickly heated the filament to an initially high temperature, but it did not improve the analytical results and was subsequently omitted. The two 1P28 photomultipliers were powered by an MP1030 dual power supply set at 900 volts. Each photoanodic current was sent to the summing point of a chopper stabilized high impedance input MP-1031 operational amplifier. The current passed through a feedback resistor and capacitor in parallel to produce an output voltage proportional to the current. The feedback resistor was typically 106 or lo7 ohms. After choosing a filament voltage and argon flow rate that provided good peak heights as discussed below, the feedback capacitor was set to give a 0.1-sec time constant with the feedback resistor. Having experimented with other time constants, we found this one best to minimize the noise level without seriously reducing the peak height. The signal was observed on a recorder with a 0.1-sec response time or a storage oscilloscope. Absorption peaks were on the order of 0.5-sec duration. A significant part of the signal for wavelengths above 3300 A was due to scattered light from the tungsten filament. A 754 CFrning optical filter was used for wavelengths below 4000 A to reduce the scattering signal. Monochromator slit widths were usually set near 200 micrometers. Wide slits were desired to reduce shot noise by allowing a high flux to reach the photomultiplier from the hollow cathode. On the other hand, the slits had to be narrow enough to isolate the hollow cathode analyte emission line as well as to minimize ANALYTICAL CHEMISTRY, VOL. 44, NO. 7 , JUNE 1972
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Table I. Sensitivities and Detection Limits Detection Element Line, A Sensitivity, pg limit, pg 4227 Ca 3 20 3578 Cr 20 70 3247 cu 20 10 3720 Fe 200 400 70 2852 10 Mg 2801 Mn 300 50 Sn 2863 20,000 30,000 the scattered tungsten emission that reached the photomultiplier. The continuous tungsten emission signal increases quadratically with slit width while the hollow cathode emission line signal increases linearly. For slit widths on the order of 200 pm, the 754 optical filter eliminate: the tungsten emission signal when analyte lines below 3300 A were used. To compensate for scattering, the lower monochromator was set a t a nearby wavelength where there was negligible emission from the hollow cathode lamp. With the hollow cathode..beam blocked, the filament was turned on and the voltage signal from the lower channel was coarsely adjusted until it was equal t o the voltage signal from the main analyte channel. Both signals were then sent t o a difference amplifier and the lower channel was finely adjusted until the output was zero. Changes in intensity of the filament then caused no change in the output of the difference amplifier. It is important that the time constants of both channels be the same. Otherwise the signal from the slower channel would lag the faster channel and compensation would not be achieved during the transient signals that are typical of the experiments. Compensation was sufficient to cause no change in signal above the noise level when the filament was suddenly switched on. Replacing the lower monochromator and photomultiplier with a phototube and broadband optical filters directly observing the filament compensated for 80% of the scattered light. The difference signal in this case had transient peaks because the two channels were observing significantly different wavelength regions whose intensities varied disproportionately as the filament heated up. Procedure. A microliter syringe is used t o place a 3-pl sample on the tungsten filament. With the argon flowing at 10 I./min, 0.46 volt is applied to the filament for 20 seconds. The filament does not glow visibly and complete evaporation of the solvent occurs without boiling or splattering. The filament voltage is then switched to 4.0 volts to atomize the sample. The voltage is left on for 60 seconds to allow essentially complete vaporization to occur. RESULTS AND DISCUSSION
Absorption peak heights observed on the oscilloscope increased for voltages up to 4.0 volts and for higher voltages
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
decreased and became erratic. Perhaps at the higher voltages, the flow increase caused by the heated argon gas swept the sample out of the observation region too rapidly and/or nonuniform flow occurred. Even at 4.0 volts, the peaks became erratic when the region of observation was moved 2-mm above the top of the filament. At 4.0 volts, argon flow above and below 10 l./min caused reduced peak heights, probably because at low flow rates the sample was not swept up to the region of observation rapidly enough and a t high flow rates the sample was swept by the region of observation too rapidly. It was found, upon attempting t o increase the size of the droplet used, that one can place upon the filament as large a sample as it can physically support, and still achieve smooth evaporation a t 0.46 volt. Satisfactory results were also obtained by evaporating a first 3-111 sample, placing a second identical droplet and evaporating it, and continuing this process several times before atomizing the residue. In both of the above cases, the absorption peaks increased linearly in height with a n increase in the total amount of the metallic salt present o n the filament at the time of atomization. The formation of an overlapped double peak was observed for copper and manganese. The first peak was ordinarily used for calibration purposes. Nearby non-resonant emission lines from the hollow cathode showed no absorption corresponding to either peak, demonstrating that the double peaking was not due t o matrix scattering or refractive index changes. The double peaks could be due to atomization effects caused by nonuniform heating of the filament. Visual observation of the hot filament through dark filters showed that the central coils of the filament were hotter than the coils near the ends. Table I lists the detection limits (for unity S/N) and sensitivities (in picograms for 1% absorption) for the seven elements that were studied. All solutions were prepared from the chlorides in deionized water except Fe and Sn which were prepared in 10% HC1. The only matrix effect expected in our application would be due to the presence of nitric acid. Use of 10% nitric acid resulted in relative peak depressions of 50-70 depending upon the element. Linear calibration curves were obtained out to 50z T by calculating peak absorbance values. Reproducibility for points significantly above the detection limit was 5 % relative. RECEIVED for review November 22,1971. Accepted February 16, 1972. The authors gratefully acknowledge the support of the Undergraduate Research Participation Program of the National Science Foundation.
