Supply and removal of sample vapor in graphite thermal atomizers

Abstract

3. In the absence of interferents, the precision and detection limits of SWS and DWS are normally comparable. DWS may, however, be more precise than SWS where the latter measurement has substantial associated sample presentation error.In the presence of interferents, DWS precision is markedly reduced because of increase of independent and square root errors. In particular, dark current noise may become significant under these conditions. 5. The use of very small wavelength differences, as in differential spectroscopy, results in poor measurement precision.While the above conclusions are generally valid, it must be stressed that the different functional dependences of errors of different types on AI and D lead to a complex overall dependence which will vary with individual instruments and measurements. T o some extent, therefore, it is appropriate to consider each instrument and each type of measurement 4.as a special case.The timedependent supply and removal of sample atoms have been measured separately for two commercial graphite thermal atomizers. I n either type the release of the atoms from the graphlte wall Is determlned by the wall temperature and descrlbed by an Arrhenlus-type rate constant. With common heatlng rates, the equivalent tlme constant Is about 1 8. The removal of the atoms from the cell depends on the type of atomizer used. I n the Varian Techtron mlnlfurnace, dlffuslon Is the domlnatlng process and the equlvalent time constant is less than 70 ms. I n the larger Perkln-EIPner furnace operated under flow conditions, convection leads to equally small time constants. I f , however, the argon flow is slopped completely, dlffudon and, to some extent, expansion raise the time constant to about 1 s. Theoretical analysis and experimental measurement show that only In the latter case some 25% of the sample can be contained in the cell. Under common operating conditions, this efficlency is less than 10%.

Atomization efficiency of a graphite furnace in atomic absorption spectrometry at high heating rates

“…Also, before comparison can be made, it must be noted that the efficiency values determined in this study were for the HGA-400, and were only applicable under the specific experimental conditions, especially the heating rates used in this work. The most important difference between this work and those of the above authors was that the heating rates used in this work were much higher (Table 2) than those used by the above authors (1,2). Regardless of whether atomization was isothermal or nonisothermal, higher heating rates increased k I values much more than k 2 values, and hence decreased t,;,, values by eq.…”

“…Table 4 shows that most of the atomization efficiencies reported by Sturgeon and Berman (1) are of the same order of magnitude as those of this work. van den Broek and de Galan (2) reported that atomization efficiency of the HGA-72 for Ag -a theoretical value of 0.45 and experimental value of 0.20. These authors (2) defined the atomization efficiency as N,,,/No -the ratio of the maximum number of atoms, N,,, (obtained from the measured peak absorbance), and the amount of analyte injected into the furnace, No.…”

“…Atomization efficiencies of some commercial graphite furnaces have been studied by several authors (1)(2)(3)(4). However, the differences in the published results of different authors indicate that the problem of atomization efficiency still remains unresolved.…”

“…[4] shows that the rate of formation of B is equal to the difference between the rate of conversion of species D into species B and the rate of loss of species B by concentration diffusion. The difference between the extrapolated absorbance and the experimental absorbance at any time t is equal to the concentration of the pre-atomization species D. Equation [2] suggests that a plot of the natural logarithm of this difference vs. time should be linear, with a slope equal to the rate constant for atom formation k , .…”

“…The use of the solution of eq. [2] with the initial concentration of the species D and [Dlo and with no B present initially, gives the following solution of eq. [4]:…”