RVm (1) FWm where R = permeation rate, ng/min; F = diluent gas flow rate, cm3/min; V , = molar volume at 0 "C and 1 atm, 22414 cm3/mol; and W , = molecular weight. For NO2 ( W,,, = 46), Equation 1can be approximated by Concn.ppb
= -
Concn. of NO2, ppb = 500 RIF
(2)
Thus 5 ppb of NO2 can be achieved with a permeation rate of 20 ng/min a t a flow rate of 2000 cm3 'min. At 20 ng/min, Table I shows that the conventional weighing technique would require more than one year to perform the calibration, the Saltzman volumetric apparatus would take 10 days (250 hours), and the electromicrobalance technique about 100 hours; however, the present method would require only 10 hours. In certain applications where lower concentrations or lower diluent gas flow rates are desirable, permeation rates as low as 2 ng/min may be required; the method discussed here is the only practical approach to the absolute calibration of such a low permeation rate. The volumetric technique of Saltzman may be
limited further by competitive reaction with the glass walls and the manometric fluid at low permeation rates, and the electromicrobalance method is limited to condensible gases with exposure to the environment still a problem.
CONCLUSIONS Preliminary results with the permeation wafer and diffusion devices indicated that the new calibration technique will more quickly provide accurate calibration of elution rates of condensible and noncondensible gases from the devices for use as standard reference materials. Various methods of fabricating the devices as well as automatic data collection procedures are currently being evaluated and will be described in a future article. Received for review August 3, 1973. Accepted September 28, 1973. Presented in part at the 166th National Meeting American Chemical Society, Chicago, Ill., August 1973. This work was performed under the auspices of the United States Atomic Energy Commission in contract with the Environmental Protection Agency.
I CORRESPONDENCE Self-Reversal in a Copper Pulsed Hollow Cathode Lamp Sir: During time-resolved studies of atomic line emission profiles from a copper hollow cathode lamp, we have observed extreme self-reversal under pulsed conditions that are similar though not identical, to the intermittent mode used by Cordos and Malmstadt ( I ) , and the pulsed mode used by Dawson and Ellis (2). Of particular interest is the fact that extreme self-reversal was present in spite of a linear relationship between wavelength integrated line intensity and lamp pulse current. A linear or slightly curved relationship of this type has led some investigators to believe that only slight self-absorption might be present. The instrumentation will be explained in detail in a more comprehensive paper to be published later. The purpose of this paper is to present timely results. A Westinghouse copper lamp No. 23042 was pulsed for 5 msec a t a rate of 10 Hz with currents up to 300 mA. The total line intensity (wavelength integrated) for the Cu(1) 324.7-nm line us. lamp pulse current is shown in Figure 1. Measurements were made using an oscilloscope and the scatter of the data points is due primarily to shot noise. The relationship is linear within the expected uncertainty of the data points and passes through the origin. A computer controlled analog-to-digital converter with a time jitter of 17 psec was then used to collect intensity data a t time intervals spaced a t 0.5 msec during the 5 msec pulse starting with the first 21 psec of the pulse. A piezoelectrically scanned Fabry-Perot interferometer with a scanning aperture limited finesse of 23 was used to obtain wavelength resolution. The data resulting from many pulses were time averaged and sorted by the computer and plotted on an X-Y recorder. (1) E. Cordos and H.V. Malmstadt, Anal. Chem., 4 5 , 2 7 (1973). (2) J. 6 .Dawson and D. J. Ellis, Spectrochim. Acta, 23A, 565 (1967).
318
Figure 2 shows the time and wavelength resolved Cu(1) 324.7-nm line for pulses of 100 mA, and 200 mA. Each of the five curves in Figure 2 represents a point in time (with a jitter of 17 psec). The first four or five of a set of ten points in time, spaced a t intervals of 0.5 msec, are shown; the later points are not presented because they are essentially the same as the last point shown. The left hand profile represents the line profile from 4-21 psec following the start of the pulse. There are two lines due to hyperfine splitting ( 3 ) of the 324.7-nm line. The lines are 0.0040 nm apart. Although during the first 21 psec, the two lines show no self reversal, the 200-mA lines are broader than the 100-mA lines. Similar line profiles for 150-mA pulses during the first 21-psec interval show a peak intensity that is about twice as high as those for the 100-mA or 200-mA pulses, and a line width a t half height that lies between the 100-mA and 200-mA line widths. Apparently, therefore, self-absorption causes the broadening and reduced peak intensity of the 200-mA pulse during the first 21-psec interval. By the next point in time, 504-521 psec, the two lines appear to be 4 lines, because of self-reversal. Self-reversal remains strong for succeeding points out to the end of the pulse. Two of these pulse levels, 150 mA and 200 mA, were studied during the first 210 psec of the 5-msec pulse. Points were as closely spaced as possible, 21 psec. The results are shown in Figure 3 for the 200-mA pulse. Intensity scales for these profiles are the same as for those of Figure 2. The profiles taken during the first 210 psec show that the reversal starts early in the pulse and reaches a steady state (no change in intensity or profile with time) after (3) P. Brix and W. Hurnbach, Z.Phys., 128, 506 (1950)
ANALYTICAL CHEMISTRY, VOL. 46, NO. 2, FEBRUARY 1974
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Figure 1. Relative integrated Cu intensity vs. pulse current for the commercial hollow cathode lamD >-
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Figure 3. Time-resolved line profiles of the C u ( l ) 324.7-nm emission doublet. Line profiles taken sequentially 21 p e c apart from the beginning of a 5-msec pulse. Pulse current = 200 m A . ( a ) Time periods 1 through 5, ( b ) time periods 6 through 10
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about 100 psec for the 150-mA pulse and after 200 psec for the 200-mA pulse. The asymmetry of the self-reversed lines is more pronounced earlier in the life of a pulse than later on. Thus, while the wavelength integrated intensity for a pulse increases linearly with pulse current, there remains strong self-reversal in the Cu doublet line, even for long pulses at low pulse rates. This indicates that great care must be taken in the interpretation of data for pulsed systems in which there are large concentration and energy gradients, and where one parameter, such as lamp current, may change not only the sampling rate but also other variables such as the shape or size of the various discharge regions.
G. J . DeJong E. H. Piepmeier Department of Chemistry Oregon State University Corvallis, Ore. 97330
+ WAVELENGTH
Figure 2. Time-resolved line profiles of the C u ( i ) 324.7-nm emission doublet. Line profiles are 0.5 msec apart starting at the beginning of a 5-msec pulse. ( a ) Pulse current = 100 m A , ( b ) pulse current = 200 mA
Received for review July 23, 1973. Accepted October 23, 1973. This work was supported in part by National Science Foundation Grant No. GP-28069.
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 2 , F E B R U A R Y 1974
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