External drift correction

These three phenomena can be corrected, to a degree, by the use of internal standardisation techniques (see Chapter 4). For matrix-induced signal drift, an external drift correction technique may also be used, whereby the drift is corrected by use of drift correction standards run at intervals during the analysis. Spectral interferences are discussed in detail in Chapters 4 and 8. The effects of most of these problems can be minimised through careful instrument set-up and/or selection of an appropriate instrument configuration or sample introduction components. 

Cheatham, M. M., SangreyW. M., and White, W. M. (1993). Sources of error in external calibration ICP-MS analysis of geological samples and an improved non-linear drift correction procedure. Spectrochim.Acta, Part B 48B(3), E487. 

Conventional external calibration uses pure standard solutions (single- or multi-element) and is therefore unable to compensate for matrix effects, fluctuations or drifts in sensitivity. To some extent drifts can be compensated for by regularly repeating the calibration or by repeated measurement of one standard, which allows a mathematical drift correction to be applied. Matrix effects can be compensated for by using matrix-matched calibration solutions, which means... 

Equation (10.6) for the mobility in the two-state model implicitly assumes that the electron lifetime in the quasi-free state is much greater than the velocity relaxation (or autocorrelation) time, so that a stationary drift velocity can occur in the quasi-free state in the presence of an external field. This point was first raised by Schmidt (1977), but no modification of the two-state model was proposed until recently. Mozumder (1993) introduced the quasi-ballistic model to correct for the competition between trapping and velocity randomization in the quasi-free state. 

To be able to produce quantitative data, an internal standard and external standards are required. Internal standardization corrects for possible instrument drift or changes in the efficiency of the ablation and thus improves the... 

The error terms might be correlated with each other or with an external parameter (i.e., time, injection sequence). It is always a good idea to plot the residuals vs. time and injection sequence to evaluate possible correlation effects. Detector drift, carry-over or other elfects can be easily detected with these plots. For a correct evaluation of the independence of the error, however, the calibration solutions should be injected in a randomized manner and correction for heteroskedasticity and/or curvilinearity should be applied on the calibration model. 

The dominant forces that determine deviations from ideal behaviour of transport processes in electrolytes are the relaxation and electrophoretic forces [16]. The first of these forces was discussed by Debye [6, 17]. When the equilibrium ionic distribution is perturbed by some external force in an ionic solution, electrostatic forces appear, which will tend to restore the equilibrium distribution of the ions. There is also a hydrodynamic effect. It was first discussed by Onsager [2, 3]. Different ions in a solution will respond differently to external forces, and will thus tend to have different drift velocities The hydrodynamic (friction) forces, mediated by the solvent, will tend to equalize these velocities. The electrophoretic ( hydrodynamic) correction can be evaluated by means ofNavier-Stokes equation [18, 19]. Calculating the relaxation effect requires the evaluation of the electrostatic drag of the ions by their surroundings. The time lag of this effect is known as the Debye relaxation time. 

Standard addition calibration is more robust and reliable than conventional external calibration, but is more time consuming and costly if it is applied separately for each sample. A major advantage of standard addition is the correction of multiplicative matrix effects, for example alteration of nebuli-sation efficiency. The intensities of all samples (and spiked samples) change by the same factor, which leads to an altered cahbration slope. However, for additive effects, such as interferences caused by the matrix, the calibration line is shifted parallel and the intercept changes, which results in biased analyte concentrations. In some cases, this bias can be avoided (or indeed identified) by choosing another isotope and comparing the results for each. Standard addition has no inherent compensation for instrumental drift in the ICP-MS system. However, reduction of the drift, whieh limits the applicability of standard addition for ICP-MS, has been achieved by applying a chemometric method (a bracket approach, where the spiked sample is measured between two different measurements of the sample). ... 

External calibration One isotope free of spectroscopic interference per analyte Matrix matching and/or correction for drift (using internal standardization or frequent calibration) Good aoouraoy and preoision... 

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