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was the focusing portion of the setup. Once conditions had been established, the system was easy to use. The camera observed approximately 2 mL of the buret. Demagnification will result in an increased observation range but a loss in resolution. If the range greater than 2 mL (or the equivalent) is essential, then one must either use another camera or provide a movement mechanism for the camera. We consider the second more practical since the camera could be used to determine its own position during and after movement by reading the buret or an externally attached scale. It should be noted that a high precision movement mechanism is not essential because of this “self calibration” capability. With the experimental magnification used, the camera was comparable to human visual performance. The low cost, high performance, and ease of use of this device make it far more than a curiosity or teaching tool. Clearly it offers the possibility for automation in environments which previously could not consider the cost of conversion.
ACKNOWLEDGMENT The authors wish to thank Micron Technology for the donation of the MicronEye used in this research.
Exposure time study.
LITERATURE CITED
spectively. Obviously this will depend on the type of buret used. Certain limitations should be mentioned. It is essential that the subject be brightly illuminated. Usually this requires light levels above ambient. In some environments this may not be practical. An advantage accrued from high-intensity illumination is shorter exposure times and larger f-stops (smaller apertures) with an attendant increase in the depth of field. An increase in magnification can also lead to a loss of depth of field. This can make focusing a problem if the scale under observation is long. The major problem with the experiment
Dessy, R. E. Anal. Chem. 1983, 55, 1100A. Dessy, R. E. Anal. Chem. 1983, 55, 1232A. Owens, G. D.; Eckstein, R. J. Anal. Chem. 1982, 5 4 , 2347. Geni, G. J . Electroanal. Chem. Interfacial Necrochem. 1982, f40, 137. (5) Ciarca, S. Byte 1983, 8, 20. (6) Ciarca, S. Byte 1983, 8, 67. (7) Wieland, C. Byte 1983, 8, 316. (8) Weckier, G. fop. Nectronics 1967, 13, 75. (9) Parsons, R. “Statistical Analysis: A Decision Making Process”; Harper and Row: New York, 1974; pp 694-696.
(1) (2) (3) (4)
RECEIVED for review February 25, 1985. Accepted Mas 21, 1985.
Modification of an Inductively Coupled Plasma Radio Frequency Supply for Amplitude Modulation with Complex Wave Forms Ronald Withnell, G. D. Rayson, A. F. Parisi, and G . M. Hieftje* Department of Chemistry, Indiana University, Bloomington, Indiana 47405 Amplitude modulation of an inductively coupled plasma (ICP) has potential both as a means of improving detection 0003-2700/85/0357-2414$01.50/0
limits in atomic spectrometry (1)and as an investigative probe for fundamental studies (24).With such a system, detection 0 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985
limits can be improved and signal-to-noiseenhanced through synchronous detection. Moreover, signal-to-background ratios could be improved (5) by combining the excellent atomization capabilities of a conventional high-power plasma (1.0-1.5 kW) (6-8) with the relatively low background emission observed with a low-power ICP (300-500 W) (9-11). Such features should be available at the peak or trough, respectively, of a modulation cycle. Similarly, the time-dependent energy-flow mechanisms within an ICP can be more easily identified and monitored with an amplitude-modulated plasma than would be possible with a constant-power ICP. Ensman, Carr, and Hieftje (12) have modified an ICP rf supply which enabled it to be amplitude modulated. The modification, which involved modulating of the grid voltage to the first buffer stage of a Plasma-Therm rf power supply, permitted the observation of interesting temporal phenomena involving both excited (2) and ground-state (I) species within the ICP. Unfortunately, although the method was found to work well for sinusoidal modulation, application of a square wave caused substantial distortion in the resulting rf envelope (12). Moreover, insertion of a sinusoidal or more complex wave form a t the buffer stage required the incorporation of an external dc bias source and the removal of the automatic power control circuitry within the rf supply. The attainable depth of modulation was found to be dependent on both the magnitude of the applied external bias potential and the peak-to-peak amplitude of the applied wave form. Adjustment of the external bias supply for maximum modulation depth with minimal distortion often resulted in operation at bias voltages (-85 V) very different from the value utilized with the internal bias supply (-62 V). Hence, serious questions arose concerning the accuracy of the forward power meter on the rf supply. More recently, Farnsworth has employed (3, 4 ) a commercially obtained unit which permits the controlled momentary interruption of the rf carrier signal. Although this scheme enables effectively 100% modulation of the plasma (Le., the rf field is either “ON” or “OFF”), it does not readily lend itself to modulation of the plasma with selected wave forms. Although a power-modulated ICP has been hypothesized to improve analytical utility of an ICP (5), only its use as a diagnostic method has yet been demonstrated (1-4). In those studies, sinusoidal power modulation enabled the investigation of the frequency-response characteristics of the ICP and permitted the examination of energy-transport rates. Similarly, square-wave amplitude modulation of the rf power provides a means of investigation of the temporal response of various species within the plasma. However, square-wave modulation offers a less ambiguous means of following a transient energetic wave as it propagates across the discharge. In the present work, a 5-kW, 27.12-MHz ICP power supply (Plasma-Therm Inc., Kresson, NJ; RF power supply Model HFP 5000D) was modified to incorporate the circuit schematically represented in Figure 1. This circuit, although quite simple, enables the direct modulation of the current to the screen-grid of the supply’s power amplifier tube (PAT).
EXPERIMENTAL SECTION Figure 1A depicts a portion of the unmodified circuitry of the rf power supply associated with the power amplifier tube (PAT). The modulation circuit, schematically represented in Figure lB, is inserted between points A and B of the original circuit and connected to point C (as indicated in Figure 1A). Under normal operation of the power supply, potentiometer R48 is used to set the maximum operating power applied to the ICP by controlling the current at the screen grid of the PAT. Hence, when the modulation circuitry is introduced, the amplitude of the current at the screen grid is controlled by transistor Q1
2415
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(Figure 1B). When a chosen wave form is applied to the base of Q2, the current at the base of Q1 is correspondingly modulated. Therefore, the condition of no current applied to the base of Q2 results in Q1 being in a fully “on” condition, thus allowing operation of the ICP at the power level set by R48 and its associated circuitry. The application of greater than 1 mA to the base of $2 causes Q1 to be in a fully “off”condition. This “off”state is manifested by the delivery of a minimum power level to the ICP, dictated by the voltage level at point C in Figure 1A (approximately 350 W here). The closing of switch S1 effectively disables the circuit and restores the rf power supply to normal operation.
RESULTS AND DISCUSSION The modulation envelope of the rf signal was monitored by two methods. The first involved the use of a pick-up coil placed in the vicinity of the rf load coil; a detectable signal was observed with the coil placed outside the shielded plasma enclosure when the coil signal was fed into the input of an oscilloscope (Tektronix Model 7844, Beaverton, OR). The pickup coil consisted of 5-10 turns of insulated 18-guagewire in a loop approximately 6 cm in diameter. In the second method, the demodulated output of the directional coupler within the rf power supply housing was used to monitor directly the magnitude of forward power applied t~ the load coil. Both methods were judged to provide satisfactory monitoring, although the second is simpler to implement and perhaps more quantitative. Figure 2 depicts the forward power applied to the load coil of the ICP as a function of the amplitude of the applied dc signal. Understandably, the modification described above can vary the resulting forward power only between a set maximum
ANALYTICAL CHEMISTRY, VOL.
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value (adjusted a t the front panel of the power supply) and a minimum value as diaeuased previously. The resulting signal response function is shown in Figure 2 for maximum power settings of 1.0 and 1.5 kW. Readily apparent are the plateau regions when the applied signal is less than 0.5 V and greater than 2.0 and 2.6 V (for maximum power settings of 1.0 and 1.5 kW, respectively). The minimum applied signal corresponds to the bias potential of transistor QZ (Figure 1B). Although this limitation could be avoided by including an internal biasing circuit for Q2, the present arrangement proved convenient. Importantly, a relatively linear response can be seen in Figure 2 to exist for applied signals between 0.5 and 2.0 v. The frequency response of the new circuitry is shown in Figure 3, where the output of the directional coupler was used to indicate the amplitude of the output rf envelope. No degradation of forward power amplitude was observed with modulation frequencies up to 5 kHz; measurable modulation of the rf field (approximately10%) was observed as high as 100 kHz. The cause of the maximum observed at 1.5 kHz is not understwd. With this circuitry, the ICP could be modulated at frequencies as low as 0.2 Hz without destabilization and, from Figure 2, modulation at de is possible. The earlier system (12)was found to yield an unstable plasma at operating frequencies lower than 100 Hz. Several applied wave forms and the resulting rf output envelopes are shown in Figure 4. Distortions observed during the application of triangular (Figure 4B) or square waves (Figure 4C) can be attributed to the frequency limitations (Figure 3) described earlier or to a lack of linearity (Figure 2). The lower trace in each frame of Figure 4 shows the applied wave form in each case. Figure 5 demonstrates the response time of the modified rf power supply to an applied step function. The response
Flpur 4. Response 01 forwardrl power lo me appkalkm of (A) s h e wave, (E) triangular wave. and (GIsq-are wave modulating signals. Lower bace In each frame s me appiiea sagnal: upper hace is the resumng rf envelope
Response of the rf power supply to a single step function pulse. Lower trace is applied signal; upper trace is the resulting rf envelope. Flgure 5.
time of the system was determined to be 25 pa, which agrees well with the frequency response of Figure 3. The secondary traces visible on the rf envelope in Figures 4 and 5 are anomalies arising from a tuning mismatch between the pickup coil and the 27.12-MHz primary wave form.
LITERATURE CITED A. F.: Oteslk. J. W.; Hienje. 0. M. Elevenm Annual Meeling of +Jw Fwhratlon of Analytical Chernisby and Speclmmpy Societies. Philadelphia. PA, Sepl 1984.
(1) Parki.
(2) Oiask, J. W.: Hienje. G. M. 1984 Winter Conference on plasma Specbochemistry. Sa" Diego. CA. Jan 1984.
Anal. Chem. 1985, 57, 2417-2419 (3) Farnsworth. P. B. Eleventh Annual Meeting of the Federationof Analyticai Chemistry and Spectroscopy Societies, Philadelphia, PA, Sept 1984. (4) Farnsworth, P. B. Appl. Spectrosc., in press. (5) Oiesik, J. W., personal communication, 1985. (6) Greenfield, S.; Jones, 1.; Berry, C. Analyst (London) 1984, 89, 7 13-720. (7) Fassei, v. A,; Kniseiey, R. N. Anal. Chem. 1974, 46, 111 0 ~ - 1 1 2 0 ~ . (8) Fassel, V. A.; Kniseley, R. N. Anal. Chem. 1974, 46, 1155A-1164A. (9) Rezaaiyaan, R.; Hieftje, G. M.; Anderson, H.; Kaiser, H.; Meddlngs, B. Appl. Spectrosc. 1982, 36, 627-631. (10) Rezaaiyaan, R.; Hieftje, 0. M. Anal. Chem. 1985, 57, 412-415.
2417
(1 1) Rezaaiyaan, R.; Oiesik, J. W.; Hieftje, G. M. Spectrochim. Acta, Pelf B 1985, 4 0 8 , 73-83. (12) Ensman, R. E.;Carr, J. W.; Hieftje, G. M. Appl. Spectrosc. 1983, 3 7 , 571-573.
RECEIVED for review April 22,1985. Accepted June 10,1985. Supported in part by the National Science Foundation through Grants CHE 82-14121 and CHE 83-20053 and by the Office of Naval Research.
Preconcentration of Atmospheric Organic Compounds by Heat Resorption and Solvent Microextraction Elliot L. Atlas* and Kevin F. Sullivan Department of Oceanography, Texas A&M University] College Station, Texas 77843
C. S. Giam Graduate School of Public Health, University of Pittsburgh, Pittsburgh] Pennsylvania 15260 To analyze organic compounds in atmospheric samples, it is often necessary to concentrate the campounds prior to analysis. Concentration techniques that have been used include cryogenic trapping, liquid impingers, and preconcentration on solid adsorbents. These techniques are well-established and have been reported widely in the literature (1-3). Among the different methods, the use af a solid adsorbent, such as charcoal (4),Tenax-GC (51, Florisil(6),polyurethane foam (a, and others (8),is perhaps the most used preconcentration technique for atmospheric organic compounds. Desorption of the sample for subsequent analysis of organic compounds will depend on the volatility of the analyte and the nature of the adsorbent. Two methods commonly used to remove the adsorbed compounds from the sorbent trap are solvent extraction and heat desorption directly into a gas chromatograph. Each technique has merits and drawbacks. To achieve high sensitivity,solvent extracts of sorbent traps require evaporative concentration prior to gas chromatographic analysis. This step will permit loss of volatile organic compounds. Procedures which do not include solvent concentration are not subject to volatilization losses, but since the analytes are diluted in solvent, only compounds present in high concentration can be measured. Thus, solvent extraction is most suitable for trace analysis of high molecular weight compounds such as PCBs, pesticides, and various petroleum hydrocarbons, or for analysis of high concentrations of organics in air. Direct heat desorption offers increased sensitivity for the analysis of more volatile organic compounds. However, because the entire sample is desorbed for analysis, there is no chance for replicate or multiple analyses of a single sample. Our objective was to develop a simple and sensitive method to screen atmospheric organics which would complement our normal high-volume sampling system. For example, Florisil adsorbent used in our high volume sampling system retains high molecular weight chlorinated hydrocarbons, alkanes less volatile than n-C15,and other moderately volatile compounds (9). We wished to extend the range of our analyses to include more volatile compounds such as chlorobenzenes,>C, alkanes, and naphthalenes in the ambient atmosphere. Furthermore, we did not wish to modify a gas chromatograph solely for analyses by heat desorption techniques. Still, we wanted higher sensitivity than that which could be obtained by using normal solvent desorption procedures, which dilute the ana0003-2700/85/0357-24 17$0 1.50/0
lytes in 1-2 mL of solvent. Finally, it was necessary to develop a method suitable for field use where full laboratory and gas chromatographic facilities would not be available. In this report we describe a novel application of heat desorption and solvent extraction techniques to preconcentrate organics in air into a small (10-15 pL) solvent volume without evaporative concentration. This technique allows multiple injections of a single sample using a conventional capillary inlet system, offers excellent sensitivity, and allows use of different sizes and geometries of sorbent tubes.
EXPERIMENTAL SECTION Apparatus and Materials. Sorbent tubes were prepared from amber-coated 5-mL Kimax pipets. The amber coating was applied to inhibit photooxidation of the Tenax-GC sorbent. After Soxhlet extraction with methanol and petroleum ether, Tenax-GC (60-80 mesh) was packed into the volumetric bulb of the pipet and was held in place by plugs of silanized glass wool. Approximately 0.6 g of Tenax was held in the trap, though traps of other sizes could be used in this system. Charcoal traps (1.5 mg) designed for use in a closed-loop stripping apparatus (IO) were purchased from Tekmar, Inc. (Cincinnati, OH), and were used without modification. A tube oven was constructed from 6 in. X 11J4in. cylindrical ceramic heaters (Thermcraft, Inc., Winston-Salem, NC). The heaters were controlled by a variable voltage controller. An aluminum block with a central hole of sufficient diameter for the sorbent tube (0.475 in.) was placed inside the tube oven. Temperature was monitored with a thermocouple inserted in the block. The apparatus is illustrated in Figure 1. (The tube oven was designed around low-cost materials immediately available in our laboratory; a more sophisticated oven with automatic temperature control would be preferred, but the present design worked well and was adequate for our purposes.) All solvent&were high-purity Distilled-in-Glass(Burdick and Jackson, Inc.; Muskegon, MI) or equivalent. Procedures. Prior to sampling, Tenax tubes are conditioned for a minimum of 8 h at 320 "C under nitrogen flow. The charcoal trap is precleaned immediately prior to use by flushing with several milliliters of solvent and drying with a stream of purified purge gas. Samples are obtained by drawing air through the Tenax-filled tube with a metal bellows or oilless vacuum pump at flow rates of 0.9-1.0 L/min. After the sample is taken, the inlet end of the sample tube is connected to a precleaned charcoal trap with a Cajun Ultra-torr adapter. The Tenax trap is then inserted through the bottom of the oven and connected to purge gas with another Ultra-torr fitting. In this configuration compounds sorbed on 0 1985 American Chemical Society
