Drift correction

Table 5.26. Drift correction during ICP-MS measurements time (in seconds) and mass-138 intensity (in Mcps) data.

The random-access sampler can go to any sample cup position, any number of times, at any time during a run. This abihty to sample cups in any order and to return to sample cups more than once, allows system automation to be greatly extended. It saves time and work by allowing automatic re-run of sample(s) following off-scale peaks and also the automatic dilution and re-analysis of off-scale samples. The sampler also saves cup positions, allowing more samples and longer unattended runs. For example, one set of standards provides initial cahbration, drift correction, carry-over correction and periodic quality control. In addition, samples or standards can be sampled in repHcate form from a single cup. The random-access sampler can be controlled and either the operator or the computer can make the decision as to which cup the sampler must go to. 

Chemometric techniques have gained enormous significance in the treatment of spectral information by virtue of their ability to process the vast amount of data produced by modern instruments over short periods of time with a view to extracting the information of interest they contain and improving the quality of the results. In some cases, the operator is unacquainted with the chemometric techniques (spectral smoothing, baseline drift correction) embedded in the software used by the instrument in others, the chemometric tools involved are inherent in the application of the spectroscopic technique concerned (e.g. in NIR spectroscopy) and thus indispensable to obtaining meaningful results. 

Expected change per year (after drift correction in program) -0.0002%... 

The most common way to deal with the problem of stochastic drift is to modulate the exposure of the analyte to the sensor and to synchronously detect the sensor response. When the analyte is off (i.e., the sensor is zeroed ), the sensor signal can be recorded as the baseline value. Drift-corrected signals can be obtained by subtracting the baseline signal from that recorded when the analyte is on. If the frequency of the on/off modulation is much higher than the frequency of the baseline drift, then this scheme results in dramatically improved stability in the measured data. An implicit requirement in this measurement strategy is that the response kinetics of the sensitive film/analyte combination be sufficiently fast to allow on/off modulation at the desired frequency. 

Analytical standard Calibration standard Drift correction standard Primary standard... 

There are analytical methods where it is impracticable to carry out a full calibration after every set of measurements. The analyst will need to demonstrate that the calibration has not significantly changed before carrying out another set of measurements. The analyst can prepare drift correction standards to carry out this check. A drift correction standard is a standard solution of known concentration used to monitor the calibration of an instrument. 

The analyst uses ICP-OES (inductively coupled plasma, optical emission spectroscopy) to measure twenty different metal ions in solution. To fully calibrate the instrument requires the preparation and measurement of 100 individual calibration standards (five point calibration per element). It would be impracticable for an analyst to calibrate the instrument daily. The instrument is calibrated at regular intervals (say fortnightly) by the analyst. In the intervening time, the calibration for each metal ion is checked by the use of a set of drift correction standard solutions. Minor corrections can then be made to the calibration to allow for day-to-day drift. 

The examination of several individual cut-and-paste cycles gives a drift corrected accuracy of about 11 nm, which is due to the length of the involved spacers (see reference [39]). Besides these small structures, large arrays of molecules can be created. In reference [26], we assembled 10 p sized structures with more than 5000 units and with a loss in transport efficiency of less than 10%. 

Fig. 8.4. Drift correction of the sensor TGS 2620 by multiplicative factor estimated by ethanol measurements...

Artursson, T., Eklov, T., Lundstrom, I., Martersson, P., Sjdstrdm, M., Holmberg, M. Drift correction for gas sensors using multivariate methods. Journal of chemometrics 14, 711-723... 

Baseline drift correction is an indispensable part of a good chromatographic data processing procedure. The following questions have to be answered ... 

What is the integration variance after drift correction ... 

A close look at Equation 48 leads to the following conclusion concerning the effect of uncorrected linear drift. The estimated ACF contains two systematic components, each proportional to a and b respectively, and two stochastic components, proportional to a and b. A final conclusion can be derived from the formulae a considerable error in the estimation of the ACF and derived quantities can be expected if baseline drift is not corrected. This leads us to the remaining question, the determination of the integration variance after baseline drift correction. 

The length of the integration intervals are Tg, and T, respectively. The baseline drift corrected integral is ... 

Figure 9. Standard deviation of the integrated noise after drift correction versus the integration time, with the correction interval width as a parameter.

If a drifting baseline is present and possibly corrected, the formulae become rather complicated and are not directly usable in daily practice. An extension to other kinds of noise, i.e. the more realistic 1/f or flicker noise leads to even more complicated formulae. Nevertheless, it is possible to determine quantitatively detection limits in case of the application of some specific baseline drift correction procedure, if the measurement conditions are well defined and stable and good models of the noise and drift are known from an a priori analysis. Moreover, a better insight in several effects influencing the remaining uncertainty after drift correction is obtained. 

Fig. 60. Atomically resolved STS measuiements of the (V7 x >/7) R19.1 ° structure. A,B,C positions refer to Sfcc, Pd and Shcp atoms, respectively. I U) curves (left). Drift corrected I image at Uq = —0.1 V (middle). (d/d(/)/((/) curves (ri t) [209]. Reprinted with permission from S. Speller et al., Phys. Rev. B 61,7297 (2000), 2000, The American Physical Society.

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