Theoretical and experimental factors affecting the accuracy and precision of atomic absorption measurements Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/4m90dz73p

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  • A procedure for evaluating the precision of atomic absorption measurements is presented and applied to 21 elements under varying instrumental conditions. The experimental relative standard deviations (RSD's) in absorbance obtained from repetitive measurements made with a high resolution voltmeter and on-line computer, are compared to those predicted by a recently proposed theoretical equation. Good agreement between theory and experiment is obtained which indicates the usefulness of the theoretical equation and evaluation procedure. The study reveals that, under the conditions used for atomic absorption measurements for the 21 elements, the noise associated with the lamp (i.e., signal shot noise and source flicker noise) and with the flame (i.e., flame transmission flicker noise) limit the precision at small absorbances (i.e., A < 0.1 ). Flame transmission flicker noise generally increases with the flame absorbance. Over most of the analytically useful range of absorbances (i.e., 0.1 > A > 1.0-1.5) analyte absorption flicker limits the precision to a value near 1% RSD for one second integration periods. The precision can be improved by a factor of two to three for any absorbance region with ten second integration periods, although not much better than this with any longer integration periods. Precision at high absorbances (i.e., A > 1.0-1.5) is limited by emission noises (analyte and background). For longer wavelengths, the analyte emission noises become more significant. The limiting noises' frequency dependence is obtained with noise power spectra and analyte absorption, emission, and fluorescence flicker are shown to all have a similar 1/f (inverse frequency) character, as well as a value of about 1% for the RSD for Cu. Abbreviated procedures for evaluation of noise in atomic emission (AE) and atomic fluorescence measurements are presented. Other studies into the source of analyte absorption noise are presented, and the conclusion is drawn that this flicker noise is probably due to nebulization fluctuations, non-fundamental in nature. The study of these various noise sources' dependence on the instrumental parameters (e.g. burner position, slit height and width, flame type and stoichiometry, and lamp current) suggest how to optimize conditions or concentrations so that an analysis can be carried out with maximum precision.
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