hours. A picture of the sampling pump and the front panel of the programmable controller is shown in Figure 2. The sample bags were made from Teflon and were approximately 30-1. capacity. The use of various plastic hags for sampling and storing atmospheric gases has been described by other researchers (9-11 ). LITERATURE C I T E D Flgure 2..The dc sampling pump and the front panel of the digital controller
(1) H. H. Westberg, R. A. Rasmussen, and M. Haldren. Aml. Chem., 46, 1852 (1974). (2) H. H. Westberg, E. Robinson, and P. Zimmennan, presented in part at the 67th Meeting of the Air Pollution Control ASSOC.,Denver, Calo., June
..
1974
DumD . . motor is also off: and finally attaches a new samDle
hag for collection of the next air sample. In some field monitorine situations where it was necessary to place the dc sampling pump within three feet of the cycle controller, we found that either a miniature DIP relay or a n opto-isolater should he used to couple the controller outDuts with the inductive motor. This additional isolation inrreasea sampling reliability. Noo-Reactive Pumo a n d Samolr Raes. The Kamhvr sampling pumps employed in our monitoring studies were selected for use in this system because all of their gas-contacting parts are Teflon (8). The current drain for each pump is less than 200 mA and they have proved to he quite reliable for field sampling. Their motor life is about 750
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(3) "XR-2240l2340 Application Note". Exar Integrated systems. In=., SunnyyaIe. Calif.. November 1973. (4) "555 and 556 Timers", Signetin. 811 E. Arques Awe.. Sunnyvale, ,.^'il vo
,,,.. *O?.,
(5) G. Karisson. Anal. Chem.. 46, 1618 (1974). (6) C. Crook. Electronics. 47, 132 (1974). (7) 'DOS/MOS Digital integrated Circuits", RCA Solid State Databaak Se ries, Sommervilie. N.J.. 1973. 18) . . Komhn Teflon Gas SamDiino . . Pumw, . M d e i &ZOO. Science Pump Corp.,.Camden, N.J. (9) A. P. Altshuller, A. F. Wartburg, I. R. Cohen, and S . F. Sieva, ht. J. Air WaterPoilut. 6, 75 (1962). (IO) C. A. Clemons and A. P. Alishdler, J. Air Polia. ConWiAssm.. 14,407 (1964). (11) H. Drasche, L. Funk, and R. Herbabheimer, StaubSleinhaL Luff, 32, 20 (1972).
RECEIVEDfor review November 18, 1974. Accepted April 16,1975.
Technique of Preparing Powder Samples for AES and/or ESCA Analysis G. E. Theriault, T. L. Barry, and M. J. B. Thomas Technical Assistance Laboratory, GTE Sylvanis hcorporat~,Sylvania Lighting Center, 100 Endicon Street, Danvers. Mass. 07923
A clean, rapid, and simple technique for mounting powder samples for Auger Electron Spectroscopy (AES) and Electron Spectroscopy for Chemical Analysis (ESCA), where depths analyzed are about 10 to 20 A, offers distinct advantages in obtaining an uncontaminated representative sample over techniques previously employed. Some prior techniques and their major drawbacks are listed below. 1) Utilization of Double Backed Sticky Tape for Powder Retention. Decomposition of tape by interaction with the excitation source (if electron beam encounters any exposed area) and redeposition of organics on the powder sample being analyzed. Also, a significant vapor pressure which limits the level of vacuum attainahle. 2) Pressing Powder into a Mesh Screen. Contamination of and mechanical work on the powder surface from the mesh material plus the possihility of the irradiating beam hitting the mesh material. 3) Entrap Larger Particles in a Mesh Material Which Will Not Peratit Passage of the Powder Granules. Same as in (2) above except for the mechanical action. 4) Pressed Pellets of the Powder Material. Potential contamination of the surface to be analyzed by the pellet mold or any confining surfaces used and potential structural changes due to the pressure applied. 5) Powder Placed in a Depression in the Sample Holder (Carousel). Potential loss of sample material in obtaining a level of vacuum suitable for analysis (-2.0 X Torr). Because of its drawbacks, this technique is seldom used. 1492 * ANALYTICAL CHEMISTRY, VOL. 47. NO. 8, JULY 1975
6 ) Place Powder Inside a Container and Tilt Container toward the Analyzer. Spilling of sample in vacuum chamher aligning sample with analyzer. This, as is (5) above, is a loose powder technique of little utility. The present technique consists of embedding the powder in a suitable metal foil. An excess of powder is placed in a folded indium strip and hand pressed to embed the powder in the very soft, malleable, and ductile metal. When the fold is open, the excess powder not embedded in the indium can be discarded, and a fresh powder surface which has had only contact with other powder, and not even with the indium substrate, may he presented to the instrument for analysis. Advantages of This Technique. A clean, mechanically unworked, surface which has only had contact with itself can be presented for analysis. The indium metal "sample holder" is less susceptible to charging than any technique utilizing a nonmetallic substrate for mounting. Coverage of the indium surface by powder is usually complete and indium lines do not appear in the resulting spectra. In the event indiun is excited, its spectra are quite simple (would not significantly complicate the sample spectrum) and easily distinguishable. If, for some reason, indium cannot he used with a parhicular sample, tin may be used as an alternative. Physical properties of indium which contribute to its utility are a low melting point (156.17') and a high boiling point (ZOO0 "C). These properties contribute to its soft,
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Figure 1. Photoelectron spectra from a sodium thiosulfate powder employing the described technique The appropriate energy level assignments of the elements are given in the text. No indium peaks from the sample substrate were detected
malleable, and ductile behavior for powder embedding and its utility in ultra high vacuum application (i.e., low vapor pressure). Indium foil also lends itself to ease of mounting of irregular shaped samples and as a mask to delimit a specific area for analysis. Disadvantages or Limitations of the Technique. Should baking of the vacuum chamber be required while samples mounted on indium are enclosed, the bake temperature should not exceed about 150 OC. This is not a major concern as most samples are run at room temperature. This is also a higher temperature than would be permitted if the double backed sticky tape method were used. A typical indium substrate would be 12 X 12 X 0.25 mm. Purity of the indium employed was 99.999% with respect to other metals. The cost of such an indium substrate would be about twenty-five cents per sample. However, considerably smaller pieces may be employed if cost of indium be-
comes excessive. Auger Electron Spectroscopy (AES) analyzes an area of about 0.1 to 0.2 mm while Electron Spectroscopy for Chemical Analysis (ESCA) typically analyzes an area of about 2 to 4 mm. The indium foil cannot easily be reused for these analytical techniques and a new piece with each powder sample is recommended. Other items worthy of note in comparison of this technique with previous techniques are touched on below. There is no organic coating on indium foil as is the case in some of the other techniques. If the electron beam were to hit the organic material, decomposition with redeposition of organics back onto the sample would result. This possible problem is eliminated if indium foil is employed. Although the mounting procedure is not as simple as the double backed sticky tape, this is a very simple procedure and thus ease of handling of the foil is not an item of any significance. The vapor pressure of indium over the temperature range which can be employed is lower than the level of vacuum normally achieved in the system. (For example, at 640 K, vapor pressure of indium is lo-" Torr.) The system runs in the low Torr range to the high Torr range whether indium is involved or not. Low temperature operation would not present any difficulty if the sample is previously embedded into the indium a t around room temperature. If the sample and indium were both required to be a t low temperature, then the technique would be severely limited. Figure 1 gives the spectrum taken from a finely divided powder sample by this technique showing the spectrum due to the powder surface with no contribution from the indium substrate. This technique has been utilized successfully in our laboratory now for a two-year period.
RECEIVEDfor review February 20,1975. Accepted April l l , 1975.
Drift-Compensating Integrator for Measurement of Transient Atomic Absorption Signals Lawrence E. Cox University of California, Los Alamos Scientific Laboratory, Los Alamos, NM 87544
Electrothermal atomization (ETA) is now recognized as a valuable addition to the repertory of techniques available for trace metal analysis. Among its assets are very high sensitivity, freedom from solvent interference, and, in many cases, the ability to analyze directly the components of complex matrices. Until recently, however, atomic absorption spectrophotometers have been designed for use with flame atomizers; hence, they are not well suited for processing the fast absorption signals produced by ETA. Matousek ( I ) has shown that the degree of curvature of the analytical curve arises in part from time constant limitations of both the amplifier and the recording system. Even if sufficiently rapid response is achieved, there is ample evidence that integrated absorbance measurements are more reliable than are those of peak absorbance except for the simplest of matrices ( I , 2). This is particularly true of the Delves cup
method in which the duration of the absorption signal is markedly influenced by the history of the cup (2). Although the merits of signal integration seem to be widely recognized, the majority of workers apparently are still using peak absorption measurements. We suspect that a number of these have employed analog integrators only to find, as we did, that even slight base-line drift produces significant error in integrated absorbance values.
THEORY AND CIRCUIT DESCRIPTION Among the factors contributing to system drift are electronic drift, changes in optical alignment, and changes in hollow cathode intensity. The most widely employed solution to the latter problem is to split the beam into spatially separated sample and reference portions. Since the ratio of sample to reference intensity is unaffected by changes in ANALYTICALCHEMISTRY, VOL. 47, NO. 8, JULY 1975
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