579
Anal. Chem. 1980, 52, 579-580
distribution, using a low nebulization gas flow, is stable in operation, and does not block readily. However, should blockage occur, the nebulizer is sufficiently robust that it can be cleaned with a wire. All alignments are made during construction, and so no further adjustments are necessary during routine operation. In addition, the large-droplet recycling version should prove very valuable when sample volume is restricted, as in much clinical analysis.
maintain a n aspiration rate of 1.2 mL min-', and operating all nebulizers a t 103 kPa pressure differential, the PlasmaTherm crossflow blocked after 10 min, the Meinhard nebulizer blocked after 15 min, and the fixed crossflow design showed no signs of blockage after 60 min of continuous operation. Blockage of the other units was indicated by a rapid drop in nebulization gas flow rate. The units were all operated disconnected from the plasma torch, as experience has shown that aspiration of 20% NaCl solution into an operating plasma over time intervals of the order of 10 min leads to encrustation of salt on the sample injection tube tip. The consequent localized heating can lead to devitrification and structural collapse of the injection tube tip. In certain commercial instrumentation, the Meinhard type of nebulizer is able to run with solutions containing 2&30% salt content by use of wetted Ar gas and a computer controlled switching procedure (19). By this means, the gas passageway is rinsed with 50-100 pL of water following each measurement period. With the fixed crossflow nebulizer, this complicated procedure is unnecessary for routine operation with solutions of high dissolved-solid content. Particulate matter in the sample may, of course, still cause blockage problems for the solution uptake capillary, but this type of blockage can be readily removed with a cleaning wire. Cross contamination effects were also investigated with both Type A and Type B fixed crossflow nebulizers. Type B gave wash-out times comparable to the combination of a variable crossflow nebulizer and a Scott chamber. Type A (large droplet recycling design) required longer wash times at higher solution concentrations, as would be anticipated.
LITERATURE CITED Bastiaans, G. T.; Hieftje, G. M. Anal. Chem. 1974, 7, 901. Mavrodineanu, R.; Boiieux, H. "Fbme Spectroscopy"; Wiley: New Ywk, 1965; Chapter 6. Basset, J. D.; Bright, A . W. J. Aerosol Sci. 1976, 7, 47. Kniseiey, R. N.; Arnenson, H.; Butler, C. C.; Fassel, V. A . Appl. Spectrosc. 1974. 28, 285. Meinhard. J. E. ICP I n f . News/. 1976, 2, 163. Ohls, K.; Koch, K. H.; Grote, H. Fresenrus 2 . Anal. Chem. 1977, 24, 197. Valente, S.E.; Schrenk, W. G. Appl. Spectrosc. 1970, 24, 197. Apel, C. T.; Bieniewski, T. M.; Cox, T. M ; Steinhaus, D. W. Report No. LA-6751-MS, Los Alarnos Scientific Laboratory: Los Alamos, N.M., 1977. Bournans, P. W. J. M.; DeBoer, F. J. Spectrochim. Acta, Part B 1972, 27, 391. Bournans, P. W. J. M.; DeBoer, F. J. Spectrochim. Acta, Part 8 1976, 31, 355. Olson, K. W.; Hass, W. J.; Fassel, V. A. Anal. Chem. 1977, 49,632. Rauterberg. E.;Knippenberg, E. Angew. Chem. 1940, 53, 477. Novak, J. W.; Browner, R. F. Anal. Chem., in press. Monvoisin, J.; Mavrodineanu, R. Spectrochim. Acta 1950, 4, 152. Hartrnann, J.; Troile, 8. J. Sci. Instrum. 1926, 4, 101. Prandtl, L. Phys. 2. 1907, 8, 23. Scott, R. H.; Fassel, V. A,; Kniseley, R. N.; Dixon, D. E. Anal. Chem. 1974, 46, 75. Winge, R. K.; Peterson, V. J., Fassel, V. A . Appl. Spectrosc. 1979, 33, 206. Dahlquist, R. L.; Knoll, J. W. Appl. Spectrosc. 1978, 32, 1.
CONCLUSIONS
RECEIVEDfor review June 6,1979. Accepted December 3, 1979. This material is based on work supported by the National Science Foundation under Grant No. CHE77-07618.
T h e fixed crossflow nebulizer described in this study has most of the characteristics desirable for ICP sample introduction. It produces a spray with a suitably small droplet size
Determination of Low Levels of Krypton in Helium by Gas Chromatography Donald L. Evans and Ani1 K. Mukherji" Materials Analysis Area, Xerox Corporation, Webster, New York
14580
Brunauer, Emmet, and Teller ( I ) used nitrogen as an adsorbate for surface area measurements. Beebe and co-workers ( 2 ) replaced krypton in a krypton-helium mixture as a n adsorbate for these measurements. The surface area measurements depend on the accuracy with which the krypton concentration is known. Generally, gas tanks supplied by Union Carbide provide a nominal value of 0.1% krypton in helium. T h e surface area measurements require, however, that the krypton concentration be known to *0.001% or better. Methods described here provide a fast and sensitive determination of the low concentration of krypton in helium. Carle (3) developed a gas chromatographic method for permanent gases which was designed for the determination of microbial respiratory products. Krypton was used as an internal standard with Porapak Q columns and a thermistor detector. A semimicro chromatographic method for the the separation of krypton from xenon has been described by Janak ( 4 ) . A gas chromatographic procedure has been described by Koch and Grandy ( 5 ) for the separation of krypton isotopes from xenon isotopes in helium streams containing mixed fission gases. Charcoal beds were used for both the sampling and gas chromatographic steps. Thouzeau (6) has used a column of 5A Molecular Sieve activated a t high temperature to separate argon, oxygen, krypton, and nitrogen at 22 and at 35 "C and to separate gaseous fission products. Blackmer 0003-2700/80/0352-0579$01.00/0
and Bremner (7) used an ultrasonic detector and two columns of Porapak Q a t different temperatures to separate NP, 02, Ar, COz, CH4, NzO, and other gases in soil atmospheres. None of these methods indicate the desired reproducibility required for our analyses. We have used Molecular Sieve 5A and Carbon Molecular Sieve as column substrates along with helium and thermal conductivity detectors. It is important that nitrogen be separated from krypton to avoid interference. EXPERIMENTAL Sampling System. Gas sampling valves from Valco Corporation were used. Helium has a tendency to back-diffuse with air at any leak and especially at the sampling valve outlet. To correct for this a 10 f t X in. stainless steel tubing was added to the sampling valve outlet. The system was then thoroughly checked with a Gow-Mac helium detector for leakage. All gas cylinder valves were thoroughly flushed before sampling to ensure elimination of entrapped moisture and air. Columns. Glass coluns (12 f t X in.) were packed with Molecular Sieve 5A 60/80 mesh (Applied Science) or Carbon Molecular Sieve 80/ 100 mesh (Applied Science, Catalog 505958). The Molecular Sieve 5A column was conditioned at 200 O C overnight t o free it from the absorbed moisture. Gas Chromatography and Detectors. (a) A Barber-Colman Series ,75000 gas chromatograph with a helium detector was used. The detector contained 50 mCi tritium in a cell volume of 123 pL; polarizing voltage used was 1420 mV and the cell temperature C ' 1980 American Chemical Society
580
ANALYTICAL CHEMISTRY, VOL. 52, NO. 3, MARCH 1980
Table I. Determination of Krypton with Helium and Thermal Conductivity Detectors and Molecular Sieve Columns
cylinder no.
helium detec torb
LK152854 LK200726 LK63566 LK224454 LK198633 LK237329 LK64297 LK86700 LK244468 a
0.1168
5 %"
0.0348
0.72 5.82
0.0624
1.35
thermal conductivity detectorb 0.1166
0.0347 0.1064 0.0623 0.1203
0.1205 0.1208 0.9733
0,1119
For five determinations.
thermal conductivity detector'
u '7c"
0
0.1164 0.0343
0.17 1.3 0.87 0.63 0.06 0.06
9 0
0.85 1.8 0.31 1.1
0.1070 0.0615
1.14 0.09
1.1
Molecular Sieve 5A column.
Carbon Molecular Sieve column.
values providedd 0.1142 0.035 0.1 0.062 0.1 0.1 0.1 0.1 0.1
By Union Carbide.
i
30,000L
I
II
t
F-
100
200 VOLUME p l
300 KRYPTON
400
I
500
Flgure 1. Standard plots for krypton using a thermal conductivity detector. (a) Molecular Sieve 5A column using disc integration. (b) Carbon Molecular Sieve column using electronic integration
was 100 "C. The sample stream was split to provide assay volumes of 0.038, 0.155, 0.250, and 0.380 mL. Helium flow was set at 11 mL/min. For standardization with pure krypton, the electrometer was set at the range of 1000 and 20x attenuation. Sample loops, 25 mL, were used to assay the sample cylinders. The column was maintained a t 40 "C. (b) A Perkin-Elmer gas chromatograph Model 900 equipped with a thermal conductivity detector was used under the following conditions: detector current 350 mA; temperature 100 "C; Valco six-port zero volume valve with standard volume loops of 0.015,0.025, 0.050, 0.250, and 0.500 and 25 mL was used at room temperature. The molecular sieve column was maintained at 40 "C in a helium flow of 33 mL/min. The electrometer was set at 8X attenuation. The peak areas were measured by a disc integrator, PEP I1 data system (Perkin-Elmer),and Keuffel and Essen ~ 4 2 3 6planimeter. RESULTS AND DISCUSSION Krypton volumes greater than 0.5 mL passing through the helium detector swamp out the signal and produce excessive noise because the massive ion concentrations produced in the cell short the cell electrodes. T o avoid this, a series of calculated splits were produced and placed between the column outlet and the detector. The split ratio was multiplied by the sample loop volume t o obtain the actual volume of krypton passing through the detector. A standard plot of krypton volume in microliters vs. the area under the curve as measured 11.; a planimeter using the helium detector and Molecular Sieve 5.4 column was obtained. I t is a straight-line plot passing throueh the origin. Results presented in Table I show that, except for one cylinder, the results are quite reprodiirible and fall within the C T ~ O of 1.35. Results with a thermal conductivity detector using Molecular Sieve 5A and Carbon Molecular Sieve are also present( I in Tail:? I. T h e thermal conductivity detector must
TIME
-
chromatograms using a thermal conductivity detector. (a) Molecular Sieve 5A column. (b) Carbon Molecular Sieve column Figure 2. Gas
be operated a t a high detector current of 350 mA to obtain an adequate response for nanogram quantities of krypton. The standard curves for krypton using a thermal conductivity detector and molecular sieves are presented in Figure 1. The a % for the thermal conductivity data using the two types of molecular sieves is in the range of 0.06 to 1.8. Figure 2 presents gas chromatograms using the two molecular sieves as column substrates with the thermal conductivity detector. T h e resolution between krypton and nitrogen is better with the carbon molecular sieve column. In conclusion, low levels of krypton in helium can be measured with precision using either a helium or a thermal conductivity detector with Molecular Sieve 5A or Carbon Molecular Sieve columns. LITERATURE CITED ( 1 ) B r u n a u e r , S.; Emmet, P. H.; Teller, E. J . Am. Chem. SOC. 1938, 6 0 , 309. (2) Beebe. R. A.; Beckwith, J. B.; Honig, J. M. J . Am. Chem. SOC. 1945, 67, 1554. (3) Carle. G. C. J . Chromatogr. Sci. 1980. 8 , 551. (4) Janak, J. Collect. Czech. Chem. Commun. 1954, 79,917. (5) Koch, R. C . ; Grandy, G. L. Anal. Chem. 1961, 3 3 , 43. (6) Thouzeau, F. Chromatographia 1976, 9 ,506. (7) Blackmer, A . M.; Brernner, J. M. A m . J . Soil Sci. SOC. 1977, 4 7 , 908.
RECEIVED for review August 17, 1979. Accepted November 12, 1979.
