Anal. Chem. 1991, 63,159-163 (25) Gardner, M. J.; Gunn, A. M. Fresenius' Z . Anal. Chem. 1988. 330, 103- 106. (26) Olsen, S.; Ruzlcka, J.; Hansen, E. H. Anal. Chim. Acta 1982, 736, 101-112. (27) Tyson, J. F. Analyst 1984, 709, 319-321. (28) Tyson, J. Fresenius' 2.Anal. Chem. 1888, 329, 663-687. (29) Sysouth, S. R.; Tyson, J. F.; Anal. Chim. Acta 1986, 779, 461-486. (30) Thommen, C.; Garn, M.; Glsin, M. Fresenius' 2.Anal. Chem. 1988, 329, 678-684. (31) Baxter, D. C.; Frech, W. Spectrochim. Acta 1987, 428, 1005-1010. (32) Welz, B. Fresenius' 2.Anal. Chem. 1986, 325, 95-101. (33) Tyson, J. F.; Adeeyinwo, C. E.; Appleton, J. M. H.; Bysouth, S. R.; tdris, A. B.; Sarkisslan, L. L. Analyst 1985, 770, 487-492. (34) Stults, C. L. M.; Wade, A. P.; Crouch, S. R. Anal. Chim. Acta 1987, 792, 301-308. (35) Shatkay, A. Anal. Chem. 1968, 4 0 , 2097-2106. (36) Hosklng, J. W.; Oliver, K. R.; Sturman, B. T. Anal. Chem. 1979, 57, 307-310. (37) Hansen, E. H.; Ruzicka, J. Anal. Chim. Acta 1983, 748, 111-125. (38) Hwang, J. D.; Winefordner. J. D. h o g . Anal. Spectrosc. 1988, 7 7 , 209-249. (39) Martens, H.; Naes, T. Trends Anal. Chem. 1984, 3 , 204-210. (40) Naes, T.; Martens, H. Trends Anal. Chem. 1984, 3 , 266-271. (41) Gilbert, P. T. Anal. Chem. 1958, 37, 110-114. (42) Shatkay, A. Appl. Spectrosc. 1970, 2 4 , 121-127. (43) KosElelnlak, P.; Parczewskl, A. Anal. Chim. Acta 1983, 753, 111-119. (44) KosElelniak, P.; Parczewskl, A. Anal. Chim. Acta 1984, 765, 297-301. (45) KosEielnlak, P. Analyst 1986, 7 7 7 , 991-992. (46) Aneva, 2 . Anal. Chim. Acta 1988, 277, 311-316. (47) Wilson, B. E.; Kowalski, B. R. Anal. Chem. 1989, 67, 2277-2284.
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(48) Sperling, M.; Fang, 2.;Welz, B. Paper presented at the XXVIth Colloquium Spectroscopicum Internationale, Sofia, July 2-9, 1989. (49) Muller, H.; Kramer, J. Fresenius' 2. Anal. Chem. 1989, 335, 205-209. (50) Muller, H.; Kramer, J. Fresenius' Z . Anal. Chem. 1989, 335, 210-21 5. (51) Valdrcel, M.; Luque de Castro, M. D. Automatic Methods of Analysk; Elsevier: Amsterdam, 1988. (52) Analytical Methods for Atomic Absorption Spectrometry; Perkin-Elmer Corp.: Norwalk, CT, 1982. (53) Fang, 2.; Weiz, B.; Sperllng, M. J. Anal. At. Spectrom., in press. (54) Israel, Y.; Barnes, R. M. Mkrochlm. Acta 1990, 7 , 17-30. (55) Savitzky, A.; Golay, M. J. E. Anal. Chem. 1964, 36, 1627-1639. (56) Linning, F. J.; Mandel, J. Anal. Chem. 1964, 36, 25A-32A. (57) Fassel. V. A.; Becker, D. A. Anal. Chem. 1868, 41. 1522-1526. (58) Maglll, W. A.; Svehia, G. Fresenlus' 2. Anal. Chem. 1974, 268. 177-1 80. (59) Dickson, R. E.; Johnson, C. M. Appi. Spectrosc. 1966, 2 0 , 214-218. (60) Adeeyinwo, C. E.; Tyson, J. F. Anal. Chim. Acta 1988, 274, 339-347. (61) Fang, 2.; Welz, B.; Schlemmer, G. J. Anal. At. Spechom. 1989, 4 , 91. (82) AlOnso, J.; Bartroli, J.; Del Valle, M.; Escalada, M.; Barber, R. Anal. Chim. Acta 1987. 799. 191. (63) Rios, A.; Luque de Casko, M. D. Tabnta 1989, 3 6 , 812. (64) Zlegier, H. Appl. Spectrosc. 1981, 35, 68-92. (65) Enke, C. G.; Nieman, T. A. Anal. Chem. 1976, 48, 705A-712A. (66) Fry, R. C.; Northway, S. J.; Denton, M. B. Anal. Chem. 1978, 50, 1719-1722.
RECEIVED for review May 22,1990. Accepted October 8,1990.
Microwave- I nduced-Plasma Reflected-Power Detector for Gas Chromatography Rosa M. Alvarez Bolainez and Charles B. Boss*
Department of Chemistry, North Carolina State University, Box 8204, Raleigh, North Carolina 27695-8204
The potentlai usefulness of the reflectlve properties of microwave-induced plasma (MIP) as a gas Chromatographic detector Is reported. The detector operatlon is based on the measurement of the change in reflected power arislng from the Interaction of the anaiyte with an atmospheric pressure argon plasma sustained In the highly efflcient TM,,, resonant cavity. Mkrowave forward power and tangential gas flow are optimized for n-pentane. The lowest microwave powers produced the best signal sensitivities. For the partlcular dlscharge tube employed, a maximum response Is obtained at approximately 1.6 L/mln. The nonlinear callbration curve obtained for n-pentane Is discussed, and its conversion to a linearlzed calibration cwve Is presented. Calculated detection llmits for carbon and hydrogen lie in the upper nanograms of element per second range.
INTRODUCTION A great increase in the popularity of the gas chromatographic technique occurred in 1958 as a result of the development of the flame ionization detector (FID) ( I , 2). The remarkable sensitivity of this detector (3) made it the detector of choice for gas chromatographic analysis. Its nearly nonselective response, however, limited somehow its application. Compounds within the same class differing in their elemental composition cannot be distinguished with the FID. On the
* To whom correspondence should b e addressed. 0003-2700/91/0363-0159$02.50/0
other hand, element-selective detectors such as the thermionic ionization detector and the flame photometric detector can only be used for a very limited range of compounds. The need for an element-selective and -sensitive detector that could also be used in a nonselective mode led to the use of plasmas as gas chromatographic detectors. When organic compounds enter a plasma, molecular breakdown occurs producing emission spectra characteristic of the atoms from the sample. The optically measured emission intensity is proportional to the number of atoms in the plasma since at plasma temperatures, molecular breakdown is considered to be complete. By monitoring the appropriate wavelength the plasma emission detectors can be used as either universal or selective detectors. In 1965, McCormack et al. ( 4 ) published the first article describing the utilization of a microwave-induced plasma (MIP) as an elemental emission detector for the gas chromatographic determination of various organic compounds. Since then, the combination of microwave plasmas with gas chromatographs has been the subject of numerous publications (5-16). Several of these publications report the introduction ( 5 ) and use of commercial plasma-emission detectors (6, 7). In the GC-MIP research field, many investigators are engaged with the optimization of the GC-MIF' system, particularly with the design of GC-MIP interfaces (7-12),the design of plasma tubes (13-16), and the improvement of coupling techniques to resonant cavities (17-21). Coupling is the process of transferring microwave power from a generator into the plasma. The efficiency at which power is transferred to the plasma via the resonant cavity 0 1991 American Chemical Society
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depends upon the degree of coupling of the system (17). The achievement of an efficiently coupled microwave system is very important from a n economical and personnel health point of view. In a n efficiently coupled system, essentially no power is reflected from the resonant cavity t o the generator, thus extending the lifetime of the generator, whose magnetron can be damaged if too much power is continuously reflected. Furthermore, when a microwave-induced plasma is sustained in a n inefficient MIP system, high forward and reflected powers generate large currents in transmission lines and other structures which can generate hazardous levels of environmental electromagnetic radiation. In an MIP system, efficient coupling is desired between the microwave generator and the resonant cavity. Two processes are involved in the accomplishment of a critically coupled system: frequency tuning and impedance matching. The cavity is tuned into resonance with the generator by adjusting its natural resonant frequency to be equal to the frequency of the generator. The performance of a MIP system using a transmission line to deliver power from a generator to a load depends upon the impedance relationship a t both ends of the transmission line. For all practical microwave systems, generators and transmission lines have a characteristic impedance of 50 R resistive. Thus, the cavity acts to transform power from the 50-2 impedance to plasma impedance. When the cavity’s resonant frequency is equal to the frequency of the microwave source and the plasma impedance is equal t o 50 R resistive, the system is defined as critically coupled. Due to nonresonance and impedance mismatches, the reflected power in a noncritically coupled microwave system remains high (1-50 W). Thus, small changes in reflected power (in the order of milliwatts) resulting from perturbing factors, such as the interaction of the plasma with a sample, would not be perceptible. In a critically coupled microwave system, on the other hand, the measurement of small changes in reflected power arising from such perturbing factors is possible, since the “background” level of reflected power can be maintained a t 0.002-0.01 mW. Recently, in this laboratory, it was observed that the power reflected back to the generator from a highly efficient MIP resonant cavity, sustaining a n atmospheric pressure argon plasma, is sensitively affected by the vapors of organic substances. It is the purpose of this paper t o show the potential of this phenomenon for use as a gas chromatographic detector. This plasma ionization detector could be considered similar to the FID. in the sense that the signal is due to fluctuations in the plasma conductivity in the presence of organic vapors. The response from gas chromatographic effluents of normal hydrocarbons will be presented. The performance of the detector with respect to two important operational parameters will be discussed. Also, the shape of the calibration curve with respect t o n-pentane and the limits of detection for carbon and hydrogen will be considered.
EXPERIMENTAL SECTION Gas Chromatographic System. A GOW-MAC 69-100 chromatograph (GOW-MACInstrument Co., NJ) was employed. The gas chromatograph was equipped with a stainless steel 4 f t X in. 0.d. column of 20% silicone DC200 on 80/100 mesh Chromosorb-P. Argon carrier gas flow rates were 31, 29, and 27 mL/min for the tangential gas flow rate optimization experiment, the microwave power optimization experiment, and the evaluation of detector response as a function of mass of analyte experiment, respectively. The column temperature was maintained at 110 “C for all the experiments. Microwave Plasma System. The plasma discharge, initiated by inserting an insulated copper wire into the open end of the tangential flow plasma tube, was sustained in the highly efficient TMoloresonant cavity (19). The cavity has an inner diameter of 8.85 cm and an inner height of 1.00 cm. Tuning the cavity into resonance was achieved by inserting three 8 mm diameter quartz rods through the cavity walls. Matching of the input impedance
of the coupling probe to the output impedance of the generator was accomplished by monitoring the reflected power while the coupling probe was slid through the slot in the cavity lid, from the edge to the center of the cavity. When the reflected power goes through a minimum value, the impedance match is made. The tangential flow torch, designed by Perkins (ZZ), was placed through 0.95-mm holes in the center of the cavity lids. The microwave generator employed (KIVA MPG-4-195, KIVA Instrument Co., Rockville, MD) had a 0-120-W, 2.45-GHz microwave output. The generator was connected to the cavity via two coaxial cables and two General Microwave N710-20 directional couplers (General Microwave Co., Farmingdle, NY). The coaxial cable connecting the generator to the directional couplers had a length of 34 cm. A longer coaxial cable, 130 cm long, was used as a link between the directional couplers and the cavity. In this arrangement, with the reflected power measured closer to the generator than to the cavity, the system was more easily tuned and matched to the generator. Power measurements were made with a Hewlett-Packard 413B power meter (Hewlett-Packard Co., Palo Alto, CA). A Telonic Engineering Corp. band-pass filter (Engineering Co., Laguna Beach, CA) was used to discriminate power at 2.45 GHz from other harmonic modes. Data Collection. The change in reflected power due to the analyte-plasma interaction was registered by a 413B HewlettPackard power meter. The output voltage from the power meter was amplified by a factor of 10 by means of a LF411CN operational amplifier (National Semiconductor Co., Santa Clara, CA). The amplified output was then sent to an Apple I1 Plus computer through a laboratory-built 12-bit A/D (analog to digital) interface card (Analog Devices AD574A, Norwood, MA). A basic program was used to collect the data a t 70.37-ms intervals, to integrate the area of the chromatographic peaks, and to store the data. Interface. The effluent from the column was split by using a l/s-in. tee. The splitter was adjusted so that one-ninth of the total flow was directed to the tangential flow discharge tube. The remaining portion was vented to the atmosphere. Heating of the transfer line, which conducted the portion of the column effluent directed to the plasma discharge tube, was done by wrapping a heating tape around it and adjusting the temperature with a Variac controller. The transfer line was connected to the plasma tube with a in. X in. brass reducing union. Materials. Commercial grade argon was used throughout the experiments described in this paper, as gas chromatographic carrier gas and plasma gas. HPLC grade n-pentane and n-hexane were obtained from Fisher Scientific (Pittsburgh, PA). Highpurity cyclopentane and cyclohexane were obtained from American Scientific Products (McGaw Park, IL). Procedure. A three-component mixture of 3.0 mL of n-pentane, 10.0 mL of n-hexane, and 10.0 mL of cyclohexane was used to evaluate the detector response as a function of mass of analyte. The mass of analyte, n-pentane, was varied in the range 9-27 pg by injecting various volumes of the same mixture and splitting the column effluent. Five replicates were obtained to estimate the precision. The forward microwave power level was set to 12 W. The detector’s response was evaluated by calculating the background-corrected area of the peak corresponding to n-pentane. The experiments evaluating the effect of tangenital gas flow rate and microwave forward power on the detector’s response were carried out by repeatedly injecting 1.0 pL of a three-component mixture containing 1.0 mL of n-pentane, 1.0 mL of n-hexane, and 1.0 mL of cyclohexane. The gas chromatograph column temperature and carrier gas flow rate were maintained constant throughout both experiments. During the investigation of the effect of tangential gas flow rate on the detector’s response, the flow rate was varied over the range approximately 13-2.3 L/min. Evaluation of the detector’s response as a function of microwave forward power was done by adjusting the plasma gas flow rate at 1.8 L/min and varying the input power over the range 6-26
w.
RESULTS AND DISCUSSION When a hydrocarbon was injected into a gas chromatograph and fed into a microwave-induced, atmospheric pressure argon plasma, changes in the color and length of the plasma were observed. Before any interaction with the sample, the plasma appears as a white filament surrounded by a light blue glow.
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30 40 50 $0 70 Time ( s ) Flgure 2. Plot of reflected power vs time. The chromatogram was obtained when the highly efficient TM,,, microwave cavity was deliberately not critically coupled.
As soon as the sample starts interacting with it, the plasma turns green and increases in length. Then, as the concentration of analyte increases, the plasma contracts to a shorter plasma than the initial one. As the analyte concentration decreases the plasma returns to its original size and color. Besides these visually detected changes, a change in the magnitude of one of the electrical properties of the microwave system, the reflected power, was observed. A plot resembling a typical chromatogram is obtained when the change in reflected power, arising from the analyte-plasma interaction, is recorded as a function of time. Figure 1 illustrates a gas chromatogram of a three-component mixture of n-pentane, n-hexane, and cyclopentane. Since the time necessary for the whole chromatogram to be recorded was longer than the data collection time permitted by the program used to gather the data, the recording of the chromatogram in Figure 1 was initiated 1 min after injection to allow acquisition of only the data points corresponding to the peaks of interest. The first peak, at approximately 15 s, corresponds to the change in reflected power produced by 9 pg of n-pentane. Similarly, the second and third peaks correspond to the reflected-power change resulting from the plasma interaction with 32 and 38 pug of n-hexane and cyclohexane, respectively. No air peak was observed. The flat top of the major peaks does not reflect the nature of the plasma-analyte interaction. It is only the result of reaching the limit of the A/D converter employed to collect the data. The increase of the magnitude of the reflected power, due to the hydrocarbon-plasma interaction, indicates that the TMoloresonant cavity becomes less efficiently coupled, momentarily. It is known from microwave principles that when a resonant cavity is critically coupled to a generator, power is transferred with nearly 100% efficiency and essentially no power is reflected back to the generator or lost as heat. When a cavity is not critically coupled, on the other hand, a considerable amount of power is reflected back to the generator. This suggests that the analyte-plasma interaction has an effect on the processes that determine the achievement of an efficiently coupled system. Those processes are frequency tuning and impedance matching. Tuning consists in adjusting the cavity impedance, which is a complex impedance, to be purely resistive. Impedance matching consists of making the resistance of the cavity equal the characteristic impedance of the transmission lines and generator (50 Q resistive). If either the cavity impedance is not purely resistive or it is purely resistive but not equal to 50 0,the cavity becomes less efficiently coupled to the generator. Although, at this stage of our investigation, it is not known with certainty whether the
phenomena illustrated in Figure 1is due solely to a detuning or to a mismatching effect or to a combination of them, a hypothesis to explain such a phenomena can certainly be formulated. Upon ionization of the sample by the plasma, the population of free electrons is modified affecting the plasma conductivity. Changing the conductivity of the plasma causes the coupling efficiency to change, which translates into a change of the magnitude of the reflected power. Figure 1 suggests that, due to the hydrocarbon-plasma interaction, the plasma conductivity is altered in a direction so an increase in conductivity makes the cavity less efficiently coupled. The results shown in Figure 1, obtained when the cavity had been critically coupled prior to injection of the sample, lead us to believe that hydrocarbons add conductivity to the plasma. The proposed theory can also be used to explain the shape of the peaks in the chromatogram shown in Figure 2. The data plotted in Figure 2 were obtained under identical chromatographic conditions and with the same sample as in Figure 1. Notice that the first peak in Figure 2, corresponding to n-pentane, goes down from the baseline, rather than up as in Figure 1. This negative peak evinces a decrease of the reflected power and indicates that the plasma conductivity has been increased by the amount necessary to make the cavity become critically coupled. When a larger sample is ionized, as it is the case for the second and third peaks, the reflected power undergoes a change in the negative direction followed by considerable large increase in the positive direction. The conductivity added, in this case, is so large that the cavity is taken beyond the critical coupling point to a decoupling stage. Optimization. The detector's performance was optimized with respect to two parameters: the tangential gas flow rate and the microwave power setting. Effect of Tangential Gas Flow Rate. Two gas flow rates are used in the discharge tube. There is a central flow and a tangential flow. The central flow carries the organic vapors from the gas chromatograph to the microwave discharge tube. The tangential flow sustains the plasma and keeps it centered in the discharge tube. The emission intensities of various elements have been shown to depend on the tangential gas flow rate through a similar discharge tube (10). Here, to investigate the effect of the tangential gas flow rate, the response of the detector to n-pentane was quantified at various tangential gas flow rates, while a constant central gas flow rate was maintained. Quantification of the detector's response was done in terms of analyte peak area. The tangential gas flow rate was varied from 1.0 to 2.3 L/min. This range was established by the stability of the plasma. At flow rates less than 1.0 L/min, the
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plasma became uncentered, clinging to the walls of the discharge tube. This made the monitored reflected power very unstable, which resulted in a very noisy baseline. At tangential gas flow rates above 2.3 L/min, the plasma became increasingly erratic, again leading to excessive noise. In the middle of this range, between 1.5 and 1.8 L/min, the response was the largest and the plasma was very positionally stable. According to the results shown in Figure 3, for the particular plasma tube utilized in this study, the MIP detector performs best at tangential flow rate settings between 1.5 and 1.8 L/min. A tangential flow rate of 1.8 L/min was used in all subsequent experiments. Effect of Microwave Power Level. In order to investigate the effect of forward power on the detector's performance, the microwave power applied to the argon discharge was varied in the range 6-26 W. Figure 4 shows the detector's response to the hydrocarbon n-pentane as a function of the microwave power setting. The response was maximum at the lowest power level studied and decreased with increasing microwave powers. Although larger responses were obtained at lower microwave power settings, the signal below 10 W was not very stable with respect to fluctuations in forward power. In addition, at the lowest forward power setting, the plasma was prone to be extinguished because of its interaction with the sample. Between 10 and 20 W, the signal was more stable with respect to fluctuations in the reflected power and the response was still reasonably high. A t forward powers above 20 W, the response decreased significantly and the signal, once again,
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became unstable with respect to fluctuations in the reflected power. On the basis of signal intensity, signal stability with respect to forward power fluctuations, and plasma positional stability, 12 W was chosen as the forward power setting a t which to operate the detector. Shape of Calibration Curve. The dependence of the detector's response on mass of analyte was determined for n-pentane. The experimental results are shown in Figure 5 as points with their corresponding error bars. Notice that the error bars a t higher concentrations are larger that the ones a t lower concentrations. At higher concentrations the plasma becomes more positionally erratic, making the monitored reflected power very unstable. According to the data obtained the response does not appear to be a linear function of the mass of analyte. I t is more likely for the response to be a quadratic function of the mass of analyte. To test this hypothesis, the experimental data were regressed to a quadratic function. The results of the regression are shown in Figure 5. It is clear that the calculated curve, represented as a solid line, fits the experimental data very well. This suggests that the response is a function of the square of the mass of analyte. The quadratic shape of the n-pentane calibration curve can be explained on the basis of the quantitative law governing the operation of most chromatographic detectors and the relationship between power and voltage. The signal for most chromatographic detectors is a voltage proportional to the instantaneous quantity of sample eluting through the detector. In this particular MIP detector, the signal being measured is not voltage but power. Since power is a square function of voltage, a quadratic dependence of power on sample mass would be expected. A detector with a quadratic-like response is not convenient for quantitative analysis, since it makes the interpretation of results quite complicated. In order to overcome such inconvenience, a linearized response was attained by calculating the square root of each data point prior to integration of the n-pentane peak. Figure 6 displays the calibration curve obtained by plotting the linearized response as a function of mass of n-pentane. The 95% confidence limits for each data point are shown as error bars. The best straight line, shown as a solid line, was derived from the data presented in Figure 6 by application of the least-squares method. A correlation coefficient of 0.994 was obtained. This confirms the validity of the quadratic response of the detector. Detection Limits. The detection limits, defined as the mass per unit time of analyte required to produce a signal equal to 3 times the standard deviation of the background, was determined for n-pentane. The calculated value was 1
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Goode, Chambers, and Budin (16) have reported a gas chromatographic MIP emission detector which uses a 4.0mm inner diameter tangential flow torch. The detection limits reported by Goode are 1-2 orders of magnitude higher than those acquired with capillary plasma tubes or small laminar flow torches. Compared to Goode's limits of detection, the values reported here are higher by 2 orders of magnitude. The discrepancy between those values may be partially attributed to our larger inner diameter torch. The torch used in the work presented in this paper was 2.0 mm larger than the one used by Goode. Our limits of detection indicate that the reflected power MIP detector is not yet ready for practical applications. It is not the intention of this paper to present a practical gas chromatographic detector but to show that the phenomena studied has a great potential for practical application. A potential improvement of our detection limits of 2-4 orders of magnitude may be achieved by using a torch with smaller inner diameter.
LITERATURE CITED pg of n-pentane per second. Since, in the work presented in this paper, a nonselective detection was performed, the minMcWilliam, I.G.; Dewar, R. A. Nature 1958, 787, 760. Harley, J.; Nel, W.; Pretorius, V. Nature 1958, 787, 177. imum detectable quantity for carbon and hydrogen were P o o h C. F.; Schuette, S. A. Contemporary Practice of Chromatogracalculated in a similar fashion to that employed to compute phy; Elsevier Science Publishing Co. Inc.: New York, 1984; Chapter 3. p 161. FID detection limits for carbon (23). The minimum detectable McCormack, A. J.; Tong, S. C.; Cooke, W. D. Anal. Chem. 1985. 37, quantity of carbon was 950 ng/s and for hydrogen was 190 1470. Baum, R. Chem. Eng. News 1989. 67(Jan 16). 37. ng/s. Compared to the detection limits obtained when a Donkin, P.; Mann, S. V.; Hamilton, E. I. Sci. TotalEnviron. 1981, 79, microwave plasma emission detector (7,9,12,16,18,24,25) 121. or a flame ionization detector (3) was used, the limits of deBrenner, K. S. J. Chromatogr. 1978, 767, 365. Quimby, B. D.; Uden, P. C.; Barnes, R. M. Anal Chem. 1978, 50, tection reported in this work are relatively higher by 2-5 orders 2112. of magnitude. Such poor detection limits may be attributed Estes, S. A.; Uden, P. C.; Barnes, R. M. Anal. Chem. 1981, 53, 1829. to two reasons: (a) high gas flow rates and (b) a large discharge Haas, D. L.; Caruso, J. A. Anal. Chem. 1985, 57, 846. Cerbus, C. S.; Gluck, S. J. Spectrwhim. Acta 1983, 388, 387. tube. The relatively high flow rates, required to keep the Wasik, S. P.; Schwarz, I.P. J. Chromatogr. Sci. 1980. 78, 660. plasma centered in the discharge tube, decrease analyte resBruce, M. L.; Workman, J. M.; Caruso, J. A,; Lahti, D. J. Appl. Spectrosc. 1985, 39, 935. idence time and thus produce a decreased signal intensity. BolbKamara, A.; Codding, E. G. Spectrochim. Acta 1981, 368, 973. Because the inner diameter of the plasma tube (6.0 mm) is Michlewicz, K. G.; Urh, J. J.; Carnahan, J. W. Spectrochim. Acta much larger than the diameter of the plasma (2 mm), a large 1985, 408, 493. Goode. S. R.;Chambers, 6.; Budin, N. P. Appl. Spectrosc. 1983. 37, fraction of the sample does not interact with the plasma. 439. Gas chromatographic ME' emission detection systems have Matus, L. G.; Boss, C. 6.; Riddle, A. N. Rev. Sci. Instrum. 1983, 54, 1667. been extensively reported in the literature. For the most part, Beenaker, C. 1. M. Spectrochim. Acta 1977, 328, 173. in these systems, the plasma is contained in a capillary tube Burns, B. A. Effects on Coupllng Efficiency on Atomic Spectrometry with inner diameter ranging from 0.5 to 1.5 mm. These small wRh a Microwave Induced Plasma. PhD. Dissertation, North Carolina State University, 1987. inner diameter plasma tubes provide plasmas with high Van Dalen, J. P. J.; DeLezenne Coulander, P. A.; DeGalan, L. in analytical sensitivities. Limits of detection (9,18,24-26), Spectrochim. Acta 1978, 338, 545. Haas, D. L.; Carnahan. J. W.; Caruso, J. A. Appi. Spectrosc. 1983, the order of picograms of an element per second have been 37, 82. reported. Perkins, L. D. Development and Characterization of a Low-Power HeComparable limits of detection have been achieved by Bruce lium Microwave-Induced Plasma for Spectrometric Determinatlons of Metals and Nonmetals. PhD. Dissertation, Virginia Polytechnic Instiand Caruso (27) by using a 2.0 mm inner diameter laminar tute and State University, 1989. flow torch. It is believed that the small inner diameter caEttre, L. S.; Zlatkis, A. The Practice of Gas Chromatography; John Wiley 8 Sons, Inc.: New York, 1967; Chapter 5, p 256. pillary plasma tubes offer low detection limits since the plasma Quimby, B. D.; Sullivan, J. M. Anal. Chem. 1990, 62, 1027. is effectively confined to a small volume so complete anaCerbus, C. S.; Gluck, S. J. Spectrochim. Acta 1983, 388, 387. lyteplasma interaction is very likely to occur. In a laminar Bache. C. A.; Lisk, D. J. Anal. Chem. 1965, 37, 1477. Bruce, M. L.; Workman, J. M.; Caruso, J. A,; Lahti, D. J. Appl. Specflow torch, a centered and stable plasma can be produced at trosc. 1985, 39, 942. flows as low as 5 mL/min. These relatively low flow rates allow longer analyte residence times, which results in low limits of detection. RECEIVED for review May 21,1990. Accepted October 15,1990.
