Anal. Chem. 1987. 59, 519-521
2 4 , l i
TOTALS 2 4 . 1
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I
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”
PROCESS STAGE
15.2
9.4 24.6
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15” 0.1
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TOTALS 25.3
151
24.5
6.5
1 6 1
519
outgoing streams. The total quartz flow rate of the incoming streams should equal that of the total in the outgoing streams. Analysis of samples from each stream gave balanced quartz flow rates to within * 5 % relative (Figure 4). The analysis accurately pointed out the contribution of each stage in the beneficiation process and indicated what changes could be made in order to reduce the silica level in the final products.
ACKNOWLEDGMENT The authors wish to express their gratitude to C. A. Cody for his invaluable help and suggestions in preparing this paper. Registry No. Quartz, 14808-60-7; cristobalite, 14464-46-1. LITERATURE CITED
5.7
Figure 4. Quartz flow rates (Ib/min) in a commercial beneficiatlon process. Values were determined by combining measured flow rates, percent solids, density, and quartz content in type A and type C sample slurries.
On the basis of 30 limits of the interpolated background counting rates, the lower limit of detection (LLD) is estimated to be 0.001% for quartz and 0.003% for cristobalite. A figure of 10 times these values (0.01 wt % quartz and 0.03 wt % cristobalite) is taken as an estimate of the lower limit of quantitation. Results for type B and type C samples are typically reported to two significant figures because of the magnitude of the 95% confidence limits. Practical Industrial Application. A complete study of a commercial deganguing process to determine the quartz removal efficiency was performed by using the method described for type A samples. The process consists of routing slurries of clay and gangue mixtures through various cleaning stages that are designed to provide maximum refined clay recovery. Depending on the routing scheme each stage can have more than one incoming stream and/or more than two
( 1 ) Criteria for a Recommended Standard-Occupational Exposure to Crystalline Silica, HEW Publication (NIOSH) No. 75-120; US. Department of Health, Education and Welfare, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health: Cincinnati, OH, 1974. (2) Groves, J. A.; Ellwood, R. P. Ann. Occup. Hyp. 1985, 29, 429-433. (3) Talvitie, N. A. Am. Ind. Hyg. Assoc. J . 1968, 29, 364-370. (4) Talvltle, N. A. Anal. Chem. 1951, 23, 623-626. (5) Rowse, J. B.; Jepson, W. 8. J . Therm. Anal. 1972, 4 , 169-175. (6) Klug. H. P.; Alexander, L. E. X-ray Diffraction Procedures for Polycrystalllne and Amorphous Materials, 2nd ed.; Wiley: New York, 1974; Chapter 7. (7) Klug, H. P.; Alexander. L. E. X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials. 2nd ed.; Wlley: New York, 1974; Chapter 5. (8) McCrone Assoclates, Chicago, IL, private communication. (9) Sahores, J. J. Adv. X-ray Anal. 1972, 16, 186-197. (10) Reynolds, R. C. Am. Mineral, 1963, 4 8 , 1133-1143. (11) Bar-On, P.; Zevln, L.; Lach, S. X-ray Spectrosc. 1981, 10, 57-60. (12) Grim, R. E. Clay Mineralogy, 2nd ed.; McGraw-Hill: New York, 1968; Chapter 7. (13) Grim, R. E.; Guven, N. Bentonltes , Development in Sedhentology 2 4 , Elsevler Scientific: Amsterdam, 1976. (14) Biaedel. W. J.; Iverson, D. G. Anal. Chem. 1976, 4 8 , 2027-2028.
RECEIVED for review May 27, 1986. Accepted September 22, 1986.
CORRESPONDENCE Optoacoustic Detection with a Helmholtz Resonator: Application to Trace Detection of Carcinogens Sir: The first reports of the use of a laser to produce the optoacoustic effect (1-4) have shown that detection of infrared active gases can be carried out with both high sensitivity (at the sub-part-per-billion level) and high selectivity. Although such characteristics immediately suggest application of this effect as a detection method for gas chromatography, in fact, only a few reports of a laser-optoacoustic detector have appeared. Kreuzer (5) described a small volume, cylindrical absorption cell with an external microphone that was excited at a frequency below the first acoustic resonance. Similarly, Zharov and co-workers (6) have developed a laser-optoacoustic detector for infrared active species. Choi and Diebold (7) have recently reported the application of chemical amplification to detection of a number of species eluting from a gas chromatography column. Here, we report the use of a Helmholtz resonator as an optoacoustic detection cell for a gas chromatograph. For this application, a Helmholtz resonator has two important advantages over a nonresonant acoustic cell. First, the micro0003-2700/87/0359-0519$01.50/0
phone can be isolated, in this case, in a chamber some distance from the hot gas that elutes from the chromatograph, thereby eliminating thermal damage to the microphone and its preamplifier. Second, a Helmholtz resonator can be designed to have both a small volume and an acoustic resonance at low frequency. Modulation of the incident laser beam at the resonance frequency thus gives an enhancement of the signal amplitude proportional to the quality factor of the resonance. Since the noise in a conventional microphone-field effect transistor preamplifier combination is dominated by internally generated preamplifier noise, the signal enhancement from the acoustic resonance generally acts to improve the overall sensitivity of the detector. In fact, these two features of the Helmholtz resonator have led to its application in previous experiments where optoacoustic signals are generated at gas-solid interfaces (8-16). A rudimentary theory of the Helmholtz resonator has been given by Morse (17);a detailed analysis of several different resonator designs can be found in papers by Nordhaus and 0 1987 American Chemical Society
520
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987 OVEN
Table I. Detection Limits"
laser line and compound
MICROPHONE CHROMATOGRAPH
Ll REFERENCE
OSCILLATOR
Figwe 1. Diagram of the experimental apparatus used for trace detection. The laser is ampliiude modulated through its power supply.
Pelzl(18) and Pelzl, Klein, and Nordhaus (19). In its simplist form, a Helmholtz resonator consists of a volume of gas which is connected to a length of tubing. The gas in the tube moves as a unit against the restoring force of the gas in the volume, which, in turn, undergoes periodic compression and expansion as a result of the linear motion of gas in the tube. In analogy with a mechanical oscillator consisting of a fixed weight attached to a spring, the frequency of the Helmholtz resonance can be found by determining an effective spring constant for adiabatic compression of the gas and the overall mass of gas in the attached tube. Values for the spring constant and the effective mass are then substituted into the harmonic oscillator expression for the resonance frequency. As shown in ref 18, more complicated cases of Helmholtz resonances can be treated in analogy with electrical circuits. A schematic diagram of the apparatus used in these experiments is shown in Figure 1. The output from a line tunable COz laser (Advanced Kinetics, Model MIRL-50) is directed into the Helmholtz resonator through a NaCl window. The laser is sinusoidally amplitude modulated at the resonance frequency of the acoustic cell by an external oscillator connected to modulate the current in the high-voltage circuit of the laser. The Helmholtz resonator is constructed of brass and consists of a cylindrical volume (0.95 cm radius by 3.3 cm long) with a thin-wall, stainless steel side arm (0.5 cm radius by 10 cm long) attached a t its midpoint. An electret microphone with an internal field effect transistor preamplifier (Archer, Inc., Model 270-092B) is mounted at the end of the side arm. The end of the side arm is cooled with tap water which flows in several turns of copper tubing soldered onto the side arm. A stream of Nz is directed into the end of the side arm to prevent diffusion of the heated eluant to the microphone and to flush gas from the resonator thereby reducing its effective volume. With this configuration, it is possible to heat the body of the detection cell, preventing condensation of the analyte inside the resonator; at the same time, detection of the acoustic signal can be carried out at room temperature. As shown in Figure 1,the signal from the microphone is fed to a lock-in amplifier whose output is displayed on a strip chart recorder. Solid samples of chloronaphthalene and several polychlorobiphenyls (PCB's) (obtained from Analabs, Inc., 99% purity) were dissolved in methylene chloride (Fisher Scientific Co., certified A.C.S. grade) and prepared for injection by successive dilution. Various concentrations of SF, (Matheson, Inc., 99.99% purity), ethylene oxide (Matheson, Inc., 99.7% purity), and vinyl chloride (Ideal Gas Products, Inc., 99.9% purity) were made barometrically by successive dilution in high-purity N P . Several microliters of a number of concentrations of each species were injected onto a chromatograph
detection limit
l-chloro110 pmol (18 ng) naphthalene 3-chlorobiphenyl 170 pmol (31 ng) 2,5,2-trichloro- 170 pmol (45 ng)
sensitivity
power
1.6 pmol/s 10R(12), 8 W
2.4 pmol/s 9R(28), 4 W 2.4 pmol/s 9P(28),4 W
biphenyl
ethylene oxide vinyl chloride
1050 pmol (46 ng) 26 pmol/s 200 fmol (13 pg) 3.4 fmol/s
sulfur
18 fmol (2.7 pg)
0.5 fmol/s
10P(40),1.5 W 10P(22),10 W 10P(20),3 W
hexafluoride The minimum detectable quantities (injected onto the chromatograph) are taken where the signal-to-noiseratio is two with a 1-8 time constant on the lock-in amplifier. The notation for the laser line and power is as follows: the branch (9 or 10 referring to the 9.6 or 10.6pm COBvibrational transition) is followed by the rotational transition (e.g., P(12)) which is followed by the laser power (in watts). (Varian, Inc., Aerograph 90-A-P) with a 3 mm X 1.8m stainless steel column packed with 5% SE-30 on SO/lOO mesh Chromosorb W HP. The column temperature was varied between 50 and 250 "C depending on the species injected; the flow rate of the carrier gas, N2, was set at 25 cm3/min. The infrared absorption spectra of gaseous sulfur hexafluoride, vinyl chloride, and ethylene oxide are well-known and can be found in ref 20 and 21. Selection of the optimum laser wavelength for trace detection can be done by inspection of the absorption profile of the gas in the region that coincides with the laser output. Refinement of the laser wavelength can then be carried out by filling the detection cell with the species of interest and changing lasing transitions to optimize the optoacoustic signal. With the PCB's and chloronaphthalene studied here, no gas-phase spectra appear to have been published. Thus rudimentary absorption spectra were obtained (22)by measuring the amplitude of the optoacoustic signal for fixed injections of the compound of interest at a series of laser wavelengths. While this procedure was tedious, it did not require high vapor pressures of the rather toxic compounds used here. In addition, accurate relative absorption intensities were obtainable a t the wavelengths generated by the laser. T o determine the sensitivity of the apparatus, the Helmholtz resonator was tested initially without the chromatograph. The laser was tuned to the P(20) transition of COPat 10.6 Km with an output power of 4 W. A plot of the lock-in amplifier signal vs. mole fraction of SF6in N2gave a detection limit of 1 part in 1O1O. The limiting factor in the sensitivity was the small, but nevertheless easily detectable, acoustic signal generated by the NaCl windows. As the mole fraction of SF6 was decreased, the analyte signal eventually became comparable in magnitude to the amplitude fluctuations from the window signal. With this limitation, the current in the laser plasma tube was adjusted to give the most stable output. For the laser used here, the most stable operation was found a t output powers in excess of 2 W. In a separate experiment it was found that the acoustic signal amplitude generated by the window was linear in total laser power in the range of 1-8 W. Thus, no advantage in detection sensitivity accrues through the use of high radiation intensities. (This point is valid under the assumption that the laser power is at least large enough to generate an easily detectable signal.) Table I gives the experimentally determined detection limits for the species studied here. A sensitivity is calculated by dividing the detection limit by the elution time (the full width a t half maximum) for the peak in order to account for the differing elution volumes for the various species. No experiments were done to ascertain the selectivity of the optoa-
Anal. Chem. l907, 59, 521-523
coustic detector; however, since sound is generated only when the laser frequency coincides with a molecular absorption, it would appear that the selectivity of this detection should be one of its salient features. In fact, previous workers have noted this property of optoacoustic detectors and have made quantitative estimates of their capability for trace detection in the presence of other gases (3). Although the optoacoustic detection technique described here is general and can be applied, in principle, to any species that possess an infrared absorption, the compounds studied here (with the exception of SF6)have been chosen on the basis of their relevance to topical environmental contamination problems. Ethylene oxide is widely used as a sterilizing agent and is a feedstock for the production of a number of industrial chemicals. It is known to be mutagenic (23)and carcinogenic (24,25). A derivatization technique has been developed (26) for use with gas chromatography that gives a detection limit of 56 ng in a flame ionization detector. PCB’s have been used in transformer oils and hydraulic fluids, and as paint additives, dielectric materials, and plasticizers. They have been found as accidental contaminants in air, water, sediment, and food samples (27,281. PCB’s have acute toxic effects (28,29)and have been found to cause tumors in laboratory animals (30-32). Various methods for trace analysis of PCB’s in environmental samples have recently been reviewed (33, 34); electron capture detection of PCB’s has been reported with a sensitivity of 0.5 ng (34). Vinyl chloride, used extensively in the United States for the production of poly(viny1 chloride), has been documented as mutagenic and carcinogenic (35). The detection limit (36) for vinyl chloride using electron impact ionization mass spectrometry with gas chromatography is 0.1 ng. Polychloronaphthalenes represent a lesser public health hazard than the above species; their toxic and carcinogenic properties have been reviewed in ref 37 and 38. The detection limit (36) for 1-chloronaphthalene using single ion mass spectrometric monitoring of the output of a gas chromatograph is 0.15 ng. SF6 is generally considered to be nontoxic. It can be detected to 0.5 fg with an electron capture detector (36). The work done here with a gas chromatograph serves to point up the contrast between trace detection of concentrations (measured in, for instance, part per billion) and quantities (measured in grams) of a given species. In the former case (e.g., monitoring atmospheric NO concentrations), there are, in general, no restrictions on the sampling volume. As is well-known, the sensitivity of optoacoustic detection is limited by the small signal that emanates from the cell windows. (Note that a few techniques (39-42) have been investigated to reduce this problem.) In the case of trace detection to determine concentrations of analytes, it is possible to use a long cylindrical gas volume to reduce the acoustic energy per unit volume of the window signal without a sacrifice in the acoustic signal amplitude from the analyte. On the other hand, when the sample volume is restricted, as in the case of a detector for gas chromatography, the problem of absorption at the cell windows becomes acute. Despite this limitation, the minimum detectable concentration reported here for the small volume cell is comparable with that previously reported for SF6and a number of other gases (3,43); such sensitivity attained with the use of a relatively
521
straightforward cell design and conventional signal processing techniques gives testimony to the remarkable capabilities of the optoacoustic effect itself.
ACKNOWLEDGMENT The authors are grateful for the support of this research by the National Institutes of Health. LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15)
(16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34)
Kerr, E. L.; Atwood, J. G. Appi. Opt. 1968, 7 , 915. Kreuzer, L. B. J. Appi. fhys. 1971, 4 2 , 2934. Kreuzer, L. B.; Kenyon, N.; Patel, C. K. N. Science 1072, 777, 347. Goldan, P.; Goto, K. J. Appl. fhys. 1074, 45, 4350. Kreuzer, L. B. Anal. Chem. 1978, 5 0 , 597A. Zharov, V. P.; Montanari, S. G.; Tumanova, L. M. Zh. Anal. Khim. 1084, 39,551. Choi, J. G.; Diebold, G. J. Anal. Chem. 1985, 5 7 , 2989. Monahan, E. M.,Jr.; Nolle, A. W. J. Appi. fhys. 1977, 48, 3519. Quimby, R. S.; Selzer, P. M.; Yen, W. M. Appl. Opt. 1077, 16, 2630. Murphy, J. C.; Aamodt, L. C. J. Appl. fhys. 1077, 4 8 , 3502. Fernellus, N. C. Appi. Opt. 1979, 78, 1784. Busse, G.; Herboeck, D. Appi. Opt. 1979, 18, 3959. Shaw, R. W. Appl. fhys. Lett. 1979, 35, 253. Bechthold. P. S.;Campagna, M.; Schober, T. Solid State Commun. 1080. 36. 225. Bechthold, -P. S.; Campagna, M.; Chatzipetros, J. Opt. Commun. 1981. 36. 369. Bechthold, P. S.;Campagna, M. Opt. Commun. 1981, 3 6 , 373. Morse, P. Vibration and Sound; McGraw-Hill: New York, 1936. Pelzl, J. Appl. fhys. 1981, 2 5 , 221. Nordhaus, 0.; Pelzl, J.; Klein, K.; Nordhaus, 0. Appl. Opt. 1082, 2 7 , 94. Lagemont, R. T.; Jones, E. A. J. Chem. fhys. 1051, 79, 534. Hummel, D. 0.; Scholl, F. Infrared Analysis of Polymers, Resins and AWitives: An Atlas; Why-Interscience: New York, 1969; Vol. I . Choi, J. G. Ph.D. Thesis, Brown University, 1985. Ehrenberg, L. Banbury Rep. 1979, 7 , 157. Dunkelberg, H. Br. J. Cancer 1979, 39, 588. Coene, R. F. Vet. Hum. Toxicol. 1981, 2 3 , 439. Nagase, M.; Kondo, H.; Mori, A. Bunseki Kagaku 1938, 3 2 , 633. Horn, E. 0.; Hetllng, L. J.; Tofflemlre, T. J. Anal. N. Y. Acad. Sci. 1070, 320, 591. Safe, S.; Ztko, V. The Chemistry of PCB’s; CRC Press: Hutzinger, 0.; Boca Raton, FL, 1980. Gustafson, G. C. Envlron. Sci. Techno/. 1070, 4 , 814. Kimbrough, R. Ann. N.Y. Acad. Sci. 1979, 320, 415. f C 8 ’ s and f B B ’ s ; Monograph on the Evaluation of The Carcinogen Risk of Chemicals to Humans; Internatlonal Agency for Research on Cancer: Lyons, 1979; Vol. 18. Nlshizumi, M. a n n 1979, 7 0 , 835. Hutzinger, 0.; Sofe, S.; Zltko, V. The Chemistry of PCB’s; Chemical Rubber Co., Press: Boca Raton, FL, 1974. International Agency for Research on Cancer, Monograph 18, 1978; p
65. (35) (36) (37) (38) (39) (40) (41) (42) (43)
Jenssen, D.; Ramel, C. Mutat. Res. 1980, 7 5 , 191. Pelllzzari, E. D., private communication. Sikes, 0.; Bridges, M. Science 1952, 176. 506. McConnell, E. I n Topics In EnvironmentalHea#h;Elsevier: New York, 1980; Vol. 4, Chapter 5. Patel, C. K. N.; Burkhardt, E. G.; Lambert, C. A. Science 1974, 784, 1173. Koch, K. P.; Lahmann, W. Appi. fhys. Lett. 1978, 32, 289. Deaton, T. F.; Depatie, D. A.; Walker, T. W. Appl. fhys. Lett. 1975, 2 6 , 300. Kritchman, E.; Shtrlkman, S.; Slatkine, M. J. Opt. SOC.Am. 1978, 6 8 , 1257. West, G. A.; Barrett, J. J.; Siebert, D. R.; Reddy, K. V. Rev. Sci. Instrum. 1983, 54, 797.
J. G. Choi G. J. Diebold* Department of Chemistry Brown University Providence, Rhode Island 02912 RECEIVED for review April 21,1986. Resubmitted October 16, 1986. Accepted October 16, 1986.
Electrochromatography Using High Applied Voltage Sir: There were a few liquid chromatography experiments where an electric voltage had been applied to columns (1-5).
The term electrochromatography is defined in a book (1) as follows: “Electrochromatography should remain restricted
0003-2700/87/0359-0521$01.50/00 1987 American Chemical Society
