hydrometallurgical technique. A thin layer of copper was deposited on the surface of the beads, followed by a layer of nickel. Two batches, one containing approximately 0.1% and the other 0.2% copper, were each prepared and analyzed carefully by two independent methods. The two materials then were mixed in varying proportions to give a series of 10 to 20 samples. From Figure 3, the relative sampling error for these samples should be no greater than 0.1% if lo5 particles are taken (the most unfavorable mixture has about 1 part of 0.1% to 2 parts of 0.2%). For ZOO-mesh materials, a sampling error below 0.1% is easily achieved with samples of 0.2 gram (Figure 2). If these samples were prepared from pure copper and a diluent such as pure nickel, 1O1O particles (about 20 kg) would be required for each sample analyzed. In summary, the sampling of heterogeneous solids may be a major source of error if the sought-for component is
present in small quantity in discrete particles. Poor precision in trace analysis may be the result of variation in sample composition. In the preparation of reference materials for trace components, the use of inert material plus pure component is not recommended. Other approaches to their preparation, particularly use of homogeneous material or of two materials close together in percentage of the sought-for component, should be used.
ACKNOWLEDGMENT The assistance and cooperation of D. R. Weir and Sherritt-Gordon Mines, Ltd., Fort Saskatchewan, Alberta, in the preparation of the copper-in-nickel reference materials is gratefully acknowledged. Received for review July 31, 1973. Accepted October 4, 1973.
New Method for Calibration of Permeation Wafer and Diffusion Devices Russell N. Dietz, Edgar A. Cote, and James D. Smith Department of Applied Science, Brookhaven National Laboratory, Associated Universities, Inc., Upton, N. Y. 7 1973
Standard reference materials for calibrated sources of gaseous pollutants at ppm and ppb levels are needed to provide improved calibration of current air monitoring instruments. Permeation Teflon (Du Pont) tubes containing liquified gases were developed as such sources and have recently' been standardized for SOz(l a ) . Permeation tubes for condensible gases, originally suggested by O'Keeffe and Ortman ( 2 ) and evaluated for SO2 by Scaringelli, Frey, and Saltzman ( 3 ) , have been calibrated by their weight loss with time (4, 5) and more recently by volumetric displacement (6, 7) and by pressure differential measurements with a capacitance manometer (8). However, the rates are inconveniently high for ambient air applications (1500 ng/min per cm of length), the tubes last only a few months, and the permeation rates for NO2 decline with time because of interaction with moisture. Permeation bottles capped with Teflon disks reduce the rates by about an order of magnitude, but calibration by conventional weight loss measurements is extraordinarily long-about 1 month. Utilization of a recording electrobalance (9) reduces the calibration time, but the total weight of the permeation device is limited. (1) J. R. McNesby and R. Byerly, Jr., "Measures for Air Quality, Annual Report-FY 1971." Nat. Bur. Stand. (U.S.) Tech. Note, 711, January 1972: a. The Sulfur Dioxide Permeation Tube, p 68; b. The Nitrogen Dioxide Permeation Tube, p 70; c. Molecular Complexes of Gaseous Pollutants, pp 29-32. (2) A. E. O'Keeffe and G. C. Ortman, Anal. Cnem., 38, 760 (1966). (3) F. P. Scaringelli. S. A. Frey, and 8. E. Saltzman, Amer. Ind. Hyg. Ass., 28, 260 (1967). (4) F. P. Scaringelli, A. E. O'Keeffe, E. Rosenberg, and J. P. Bell, Anal. Chem., 42, 871 (1970). (5) F. P. Scaringelli, E. Rosenberg. and K. A. Rehme, Environ. Sci. Techno/.. 4.,~~ 924 11970). (6) 8. E. Saitzman. C. R;'Feidmann, and A. E. O'Keeffe, Environ. Sci. Techno/., 3,1275 (1969). (7) B. E. Saltzman, W. R. Burg, and G. Ramaswamy, Environ. Sci. Techno/., 5, 1121 (1971). (8) J. J. McKinley, "A Calibration System for Trace Analyzers," 16th National Symposium, Instrument Society of America, Pittsburgh, Pa.. May 1970. (9) L. J. Purdue and R . J. Thompson, Anal. Chem., 44, 1034 (1972).
Noncondensible pollutant gases such as nitric oxide, methane, and carbon monoxide can permeate through Teflon, but permeation tubes have not been made since the gases cannot be liquified a t ordinary temperatures. Other suitable NO source materials are being investigated including encapsulated gas bubbles, molecular complexing in conjunction with the permeation tube principle, certification of NO-Nz mixtures in cylinders, and quantitative catalytic conversion of NO2 from permeation tubes to NO ( I C ) . Monitoring instruments for the determination of CH4 and CO in the environment ( I O ) are presently calibrated by preparing mixtures in cylinders at about 10 ppm (11); however, there is no way to check the accuracy of these preparations or to determine the loss of calibration with time. McKinley (8) measured permeation rates of noncondensible gases through polymeric membranes (e.g., F E P Teflon tubing) by a pressure differential technique, although many of the experimental details were not given. This paper describes a new pressure differential method for the calibration of permeation wafer and diffusion devices for both noncondensible and condensible gases. Permeation rates less than 5 ng/min can be accurately determined in hours instead of weeks.
EXPERIMENTAL T h e permeation wafer device consisted of a T e f l o n disk h e l d b y compression in a Swagelok tee such t h a t the leg containing the disk was connected t o the p o l l u t a n t gas source a n d the other connections provided t h e diluent gas input a n d o u t p u t flow ports (cf. Figure 1). For calibration, t h e diluent side was connected t o a
~
(10) R. K. Stevens, T. A. Clark, C. E. Decker, and L. F. Ballard. "Field Performance Characteristics of Advanced Monitors for Oxides of Nitrogen, SOz, CO, CH4, and Non-Methane HC." 68th APCA Meeting, Miami, Fla., June 1972. (11) E. E. Hughes and J. K. Taylor, "Standard Reference Materials For Air Pollution and Gas Analysis." 164th National Meeting AEC. New York, N.Y..; Amer. Chem. Soc., Div. Water, Air, Waste Chem., Gen. Pap., 12(2), 238 (1972).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 2, FEBRUARY 1974
315
MONITORING
ARRHENIUS ACTIVATION ENERGY = 14.3 f 0.5 kcolfmole
p
't '4
RATE = 408.9 f 4.5
1_-----1 Figure 1. Permeation calibration and dilution apparatus
TIME, hr
Figure 3. Permeation of NO?through a TFE Teflon wafer ture diffusion and blistering was eliminated since the Teflon wafer was assembled and evacuated in place before admission of the permeating gas. With B specially fabricated 4-way stainless steel hall valve (Whitey No. 43YF52-316) between the permeation wafer device and the quartz pressure gauge, either calibrations could be performed or part per billion concentration mixtures could be withdrawn. The diffusion restrictors used essentially the same apparatus but, in place of the Teflon disk, either a stainless steel capillary tube or a rod within a stainless steel tube provided the diffusion Permeation tubes were also calibrated by the same technique, but a glass chamber of about 50 em3 volume was used to contain the tube. However, the tubes are exposed to the atmosphere during preparation, transportation, and storage and thus can he affected by moisture.
RESULTS Figure 2. Permeation wafer device quartz Bourdon pressure gauge (Texas Instrument, Model 141) having a resolution of 0.01 mm Hg. Temperature was measured at three important locations-the permeation device chamber, the quartz pressure gauge, and the interconnecting line-using mercury thermometers with a resolution of 0.1 "C and, later, digital thermistor thermometers (Digitec, 25 to 45 'C) and digital semiconductor thermometers (Electronic Research Co., -60 to 160 "C) with a resolution of 0.01 "C. The Teflon disk as well as about 50% of the internal volume was temperature controlled with a power proportioning controller to about 0.01 "C. With an accurately measured total internal volume of about 10 cm3, as little as 0.3 $1 (-0.oOo0006 gram) of permeated gas could be detected. Using the correction equations for compressibility inan-ideality) and back-diffusion developed by Saltzman for NO2 permeation (7). the pressure and temperature measurements were converted, with the aid of a programmable computer-calculator, to the mass of permeated gas. For each measurement, the program printed the weight of permeated gas, the time from the start of the run, and, for the case of NOS, the compressibility and reverse diffusion corrections employed; then the least mean square permeation rate, its standard deviation, and the intercept were camputed. The permeation wafer device (cf, Figure 2) was so constructed that it could be used with noncondensible gases such as nitric oxide by attachment to a supply of about the size of a lecture bottle. Any gas could he used in the apparatus provided it was compatible with stainless steel, Teflon, and quartz. As long as dry diluent gases were used in the device, the problem of mois316
-
T h e initial version of the device was assembled using a tetrafluoroethylene (TFE) Teflon wafer 0.0813 cm thick with a surface area available for permeation of about 0.15 cmz. Temperature was measured with mercury thermometers with divisions only every 0.1 "C, and NOz was chosen as the permeating gas. As shown in Figure 3, data obtained at 30.4 and 39.8 "C gave permeation rates of 200.5 a n d 408.9 ng/min, respectively. The calculated activated energy of 14.3 kcal/g-mol agreed very well with that of 14.6, reported by Saltzman (7). The glass chamber apparatus for permeation tbbes was used to determine the rate for a n SOz permeation tube a t temperatures of 30.0, 50.5, and 60.0 "C (cf. Figure 41, giving an activation energy of 12.6 kcal/g-mol. The permeation rate determined at 30 "C (1530 ng/min) compared very favorably with the certified (Analytical Instrument Development Inc.) rate of 1510 ng/min, determined by weight-loss measurements. The positive intercept for the 50.5 "C data indicates t h a t sufficient time was not allowed to achieve steady-state conditions. A preliminary evaluation of the diffusion of methane through a capillary stainless steel tube, which had been rolled t o further reduce the inside diameter, gave diffusion rates a t several pressures as shown in Figure 5. For this tube, the rates were high; for example, a t a pressure differential of 20 psi, the methane diffused at 0.0247 cm3/ min which, with a diluent air flow of 1000 cm3/min, would give 24.1 p p m CHI. The rates were correspondingly
ANALYTICAL CHEMISTRY, VOL. 46. NO. 2, FEBRUARY 1974
ARRHENIUS ACTIVATION
$
?-
8
s
6-
E
5-
/;A
W W
6980 i I80 4-
:EK{ 3-
1 5 3 0 i 5 0 ng/min
00
20
40
80
60
5
IO
TIME, min
I5
20
25
:
30
TIME, rnin
Figure 4. Permeation of SO2 through a Teflon tube
Figure 5. Diffusion of CHI through a capillary tube
Table I. Comparison of Calibration Methods at Minimum Time to Determine Permeation Rate with 2yo Precision
Determining method
Weighing" (microbalance) Volumetricb
Weighingc (electrobalance) This method
Permeation rate, ng/minute
2000 200 20 2000 200 20 2 200 20 2 200 20 2
~
i
Hours
n tirne b
~
~
100 1000
10,000 21/2
25 250 2500 10 100 1000 1 10 100
4 40 400 ,
..
1 10 100 0.4 4 40
A P = 200 psi
g
A P =lOOpsi
0.3
1
9
V
10.00663 0 . 0 0 0 0 6 cc/rnin
0.0301 cc/min
4
0.2
&
I
I /
... 0.4 4
/-I
i
0.41
Days
0.1
I
i
/
-
-
j
AeS~mhg2% precision for a weight change of 0.012 gram. Based on the technique of Saltzman et al. (7). Based on the Purduc and Thompson method (9). '0
higher a t higher pressure. Repetition of this type of restrictor seemed very good; a determination one day later at 20 psi gave 0.0244 f 0.0001 cm3/min-a reproducibility of about 1%. Lower methane diffusion rates were achieved with a Swagelok restrictor, as shown in Figure 6. At 100 psi, a flow of 0.00663 f 0.00006 cm3/min was obtained, corresponding to 6.6 ppm a t 1000 cm3/min total flow. A description of the Swagelok orifice design, which was developed for the Brookhaven 90Sr ozone generator, is given elsewhere (12). Both types of diffusion restrictors appear capable of use as readily-calibrated, standard reference sources.
DISCUSSION The 30.4 "C NO2 run with the permeation wafer device (Figure 3) shows that a 200 ng/min permeation rate can be determined with 2% precision from 5 measurements (12) M. Steinberg and R. N. Dietz, "SrgOOzone Generator for Sub-pprn Concentration Range," Brookhaven National Laboratory, BNL 14199, November 1969.
lb
2b
20
20
20
60
70
TIME, rnin
Figure 6. Diffusion of CHI through a Swagelok restrictor
taken over 2.5 hours. With the use of digital thermometers (0.01 "C resolution) in place of the mercury thermometers (0.1 "C resolution), at least a %fold improvement in the precision should be attained. Table I gives the comparative times to obtain several permeation rates by various techniques. For the conventional weighing method, it was assumed that a minimum weight change of 0.012 gram would be necessary to achieve a 2% precision in the permeation rate. For the electromicrobalance technique, it appeared that a 20 ng/min rate could be calibrated within 2% in four days. And Saltzman indicated that about 30 minutes was required for 2% precision at a rate of 10,000 ngjmin. To obtain calibrated concentrations of NO2 in air near ambient values, about 5 ppb, requires low permeation rates even at moderate flow rates. The concentration of a permeated gas in a diluent gas stream is given by
ANALYTICAL CHEMISTRY, VOL. 46, NO. 2, F E B R U A R Y 1974
317
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 u p 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 d a t a 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
