relative volume concentration between sections being determined by monitoring specimen mass thickness via the white radiation ( I , 5 ) . The method described here allows the area elemental density to be determined by initial calibration using atomic absorption spectroscopy. Aluminum and iron films have been used but any other metal which may be evaporated in vacuo (e.g., Ag, Cu, Au, etc.) could equally well be employed. Using the calibration of relative x-ray detection efficiency by the SSD (7), such an area density may be calculated from these curves for any element under investigation. Also, when using a standard of known elemental concentration (e.g., potassium in embedding resin), absolute section thickness may be determined for biological specimens using this technique ( 7 ) .Knowing the section thickness and the area elemental concentration, absolute quantities of elements in subcellular regions or in microareas of physical specimens may then be calculated. 0-
I
2
3
4
5
6
7
8
LITERATURE CITED
SUPERFICIAL DENSITY [g/ p A 2 ( x 1 0 - " j
Figure 5. X-ray count rate vs. superficialdensity
The relationship between x-ray count rate and white radiation is linear over the range shown in Figure 4.The linearity extends until the film reaches a thickness where absorption of characteristic radiation occurs and this depends on the element concerned. I t has been shown (8) t h a t this may be several micrometers in evaporated metal films and greater in biological thin sections ( 7 ) .
(1) T. A. Hall, in "Physical Techniques in Biochemical Research", 2nd ed., Vol. l A , Academic Press, London, 1971, p 157. (2) W. H. Lawson, J. Sci. Instrum., 44, 917 (1967). (3) W. H. T. Davison, J. Sci. Instrum., 34, 418 (1957). (4) S . Tolansky, "Multiple Beam Interferometry", Oxford: Clarendon, 1948. (5) J. A. Chandler, J. Microsc., 98(3)359 (1973). (6) J. A. Chandler, in "Techniques of Biochemical and Biophysical Morphology", Vol. 2, D. Glick and R. Rosenbaum, Ed., John Wiley & Sons, New York, N.Y., 1975, p 308. (7) J. A. Chandler, J. Microsc., 106, (3),(In press) (1976). (8) G. Cliff and G. W. Lorimer, J. Microsc., 103(2), 203 (1975).
DISCUSSION
RECEIVEDfor review July 22,1975. Accepted March 12,1976.
In many analyses of biological ultrathin specimens, it is required to know the area concentration of an element, the
The authors are grateful t o the Tenovus Organisation for generous financial assistance.
Time-Resolved Distribution of Atoms in Flameless Spectrometry: Lead Release in Hydrogen Atmosphere Giancarlo Torsi* and Gin0 Tessari lstituto di Chimica Analitica, Universita di Bari, Via G. Amendola, 173, 70 126 Bari, Italy
The release of lead atoms from a graphite surface in hydrogen atmosphere is shown to follow very well a model already presented for the production and transport of atoms in flameless atomic absorption spectrometry. The kinetic parameters of the process are given; their validity is discussed and tentatively justified on the basis of surface kinetic complications.
In the series (1-3) of papers with this headline, a theoretical model was proposed ( I ) describing the time-dependent number of atoms intercepting the optical beam in the observation volume. This quantity was obtained by formulating the time dependence of the atomic source and the time dependence of the gaseous transport. This approach to the problem appears particularly rewarding because it enables one not only t o follow the fate of the released atoms, but also to get informations on the factors governing the release process. This information is usually encoded in the form of the kinetic parameters: energy of activation EA^^^ and frequency factor A . A simple method to extract this information from the experimental data has been outlined ( 2 ) ,the position of the peak 1318
ANALYTICAL CHEMISTRY, VOL. 48,
NO. 9,
AUGUST 1976
of the absorption curve and the parameters defining the heating schedules being the only needed quantities. Lately the model has been tested (3)in the case of nickel release in hydrogen atmosphere. This system is by no means unique, but it is representative of the behavior of a class of systems for which the following assumptions hold: 1)a t the surface, there is a monolayer or a submonolayer distribution; 2) the release is a first-order kinetic process; 3) only one definite energy value characterizes the bond of the investigated atom a t the surface. The agreement with the model found in the case of nickel ( 3 )was quite satisfactory except for a characteristic tail in the absorbance vs. time curves especially when the slowest thermal perturbation were used. This feature suggested t h a t a mechanism of delayed release should be taken into account. As a tentative explanation, the possibility of diffusion of nickel atoms in the solid graphite was considered. I t was felt that this effect could be minimized by choosing an element: i) with a greater atomic radius; ii) with a lower temperature for atomic release. Along this line, the choice of lead appeared convenient as a probe for further testing the proposed model. T o avoid any complication due to the presence of oxides a t the graphite surface, a reducing hydrogen atmosphere was maintained in
the atomizer, after a preliminary test excluded optical interferences by t h e hydrocarbons produced at the surface. In t h e Experimental section, t h e instrumental conditions and the procedures of operations are only briefly recalled, since they were more thoroughly reported elsewhere ( 3 ) .Vice versa, some more details are provided on a new procedure of data handling. In the following section: 1)the influence of the surface conditions is qualitatively presented; 2) a brief summary of the theory relevant t o the recovery of the release and transport parameters is recalled; 3) the assignment of t h e source parameters from experimental data is presented with some emphasis on the practical difficulties encountered; 4) a comparison of theory with experiments is made in order t o discuss some validation criteria for t h e assigned parameters; 5 ) a critical evaluation of the best fit procedure on single curves is discussed; 6) the influence of the hydrogen flow rate on the peak of the resulting curves is studied; 7 ) the physical meaning of the release parameters is critically evaluated; 8) a comparison between nickel and lead release is made.
EXPERIMENTAL Apparatus. The fast response spectrometer has already been described ( 3 , 4 ) .A minor modification was introduced in the channel measuring the temperature of the graphite rod. Since the release temperature in the case of lead is around 1000 K , a red sensitive photomultiplier (Hamamatsu R 196) having S-1 spectral response peaked at 800 nm. was used. The monochromatizing element was a Wratten filter No. 99 (Kodak). This filter has a broad transmission range and is peaked at about 1000 nm. This range was narrowed by the fast decay of the photomultiplier response towards the longest wavelengths. A plot of the photomultiplier output in logarithmic form vs. 1/T gave a straight line as foreseen by theory. From the slope of this line, a characteristic wavelength of 900 nm could be computed corresponding to the peak of the combined filter. T h e pyrometer was calibrated ( 3 )by simultaneously recording at the equilibrium the response of the pyrometer and the emf of a Pt, Pt Rd (10%)thermocouple positioned in the middle of a graphite tube acting as a heating element during the temperature calibration. T h e deposition of t h e sample was made via an home-made automatic dispenser already described ( 5 ) .This device enables one to control the atmosphere of the atomization chamber during the measurements, since it removes the need of opening the cell for the sample deposition, thus avoiding the possibility of graphite surface contamination by the outer atmosphere. T h e graphite rod shape and the procedures for its mechanical preparation have already been reported ( 3 ) T h e procedure of lead absorption measurements was as follows: i) drying step (of 1~1 volume solutions); ii) ashing step for 10 s a t -800 K in hydrogen atmosphere; iii) flashing step with hydrogen flowing at 0.08 1. mind'. Different chemical compounds were tried. The data reported in this paper were obtained with lead acetate and lead oxalate which decompose in the absence of an oxidizing atmosphere, giving the metal directly (6).Other conditions relevant to the experimensl set-up were collected in Table I. Reagents. All chemicalsused were of analytical grade purity. Stock solutions were prepared with demineralized water and stored in polyethylene bottles. The most dilute solutions were freshly prepared before use. The hydrogen used was certified to contain
