Atomic absorption, discrepancies

Brugmann et al. [680] compared three methods for the determination of copper, cadmium, lead, nickel, and zinc in North Sea and northeast Atlantic waters. Two methods consisted of atomic absorption spectroscopy but with preconcentration using either freon or methyl isobutyl ketone, and anodic stripping voltammetry was used for cadmium, copper, and lead only. Inexplicable discrepancies were found in almost all cases. The exceptions were the cadmium results by the two atomic absorption spectrometric methods, and the lead results from the freon with atomic absorption spectrometry and anodic scanning voltammetric methods. 

The focus of this research and other mass balance studies has been on trace elements (1,2,3). However, in future studies on speciation it will be necessary to know the concentrations of the elements present in amounts above 1%. Therefore, analyses of the oil shale and spent shale samples were performed for these elements. Atomic absorption and colorimetry were used for many of these analyses. Some major element results also were obtained by the broad-range instrumental analysis surveys. The comparison of the results obtained by the different techniques shows large discrepancies. 

A multilaboratory program is in progress to compare lead values obtained by ASV with the thin film with those determined by isotope dilution. Initial intercomparisons with samples of Southern California coastal waters were disheartening but were tentatively traced to procedural errors and contamination. Recent efforts have produced some values which are within a factor of two of the standardized lead value (10). Our own initial efforts to compare copper concentrations obtained using the system described herein with those obtained by extraction on Chelex 100 and subsequent analysis by atomic absorption showed ASV values to be considerably higher. This discrepancy was traced to interference by silver from the reference electrode. Recent work with a noncontaminating reference electrode yielded copper values that were essentially identical to those obtained by extraction followed by atomic absorption (11). 

Since atomic absorption spectroscopy utilizes the ground state atom population for its measurements, it would appear that atomic absorption has a great advantage over flame emission in terms of detection limits and sensitivities of detection. An inspection of Appendix VIII, where detection limits are given for a number of elements for flame emission and atomic absorption, indicates this is not true. The reason for this apparent discrepancy lies in the relative stabilities of ground state and excited state atoms. An excited atom has a lifetime of the order of 10 -10 sec, and thus emits its energy very quickly after being excited. The usual flame emission source has an upward velocity of from 1 to 10 m/sec, so the excited atom will move only about 10 -10 m between the time of excitation and emission. 

The RQDO /-values conform with the recommended values by Wiese et al. [18] for the 3p ( P) 4s-3p ( P) 4p transitions in C//better than those of the much more complex theoretical procedure of Ojha and Hibbert [17], who used large multiconfiguration expansions in the atomic structure code CIVS. On the other hand, the discrepancies between the length and velocity CIVS oscillator strengths are not negligible. Wiese et al. [18] remark that, for the case of the 4s " P-4p multiplet, the stronger lines measured by Bengtson et al. [16] seem to be affected by self-absorption (Table 2). 

Porter et al flashed I2-NO mixtures and calculated a third-order rate coefficient for recombination of iodine atoms by nitric oxide of A = 3 x 10 P.mole. sec At very high NO pressures, the recombination rate falls off concurrently, a new transient intermediate appears in the absorption spectrum which is considered to be NOI. Porter et al found nitric oxide to be 20 times more effective than I2 in the recombination of iodine atoms. This is about a factor of ten less than observed by Engleman and Davidson but no account of possible NOI absorption was included in Engleman and Davidson s work. This could account for part of the discrepancy but not all of it. Because of the fall-off of the apparent third-order rate coefficient at high NO pressures, it is apparent that this mechanism is valid only at very low pressures. For high pressures bimolecular reactions of NOI must become rate controlling. 

X 10 cm molecule" s" is obtained, and discrepancies in the previously reported values are probably due to neglect of the reverse processes taking place.For the corresponding state in atomic fluorine, diode laser measurements of the absorption cross-section of the "Pi - transition at 404cm" yields a radiative lifetime of 660s for the upper spin-orbit state. ... 

---