Drift correction standard

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. 

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. 

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. 

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

The isotope ratios may need to be corrected due to mass bias, detector counting inefficiencies, mass drift, or other causes. The isotope ratio results for a given correction standard is used to correct QC samples or samples in the same concentration range. If the correction standard results for 235U/238U are 0.00725, or for are 0.000055,... 

FIGURE 7.4 Plot of mass spectral ion current as a function of time showing effect of drift correction by internal standardization. 

FIGURE 7.5 Calibration curve using internal standard drift correction. 

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... 

In a performance-based approach to quality assurance, a laboratory is free to use its experience to determine the best way to gather and monitor quality assessment data. The quality assessment methods remain the same (duplicate samples, blanks, standards, and spike recoveries) since they provide the necessary information about precision and bias. What the laboratory can control, however, is the frequency with which quality assessment samples are analyzed, and the conditions indicating when an analytical system is no longer in a state of statistical control. Furthermore, a performance-based approach to quality assessment allows a laboratory to determine if an analytical system is in danger of drifting out of statistical control. Corrective measures are then taken before further problems develop. 

For PyMS to be used for (1) routine identification of microorganisms and (2) in combination with ANNs for quantitative microbiological applications, new spectra must be comparable with those previously collected and held in a data base.127 Recent work within our laboratory has demonstrated that this problem may be overcome by the use of ANNs to correct for instrumental drift. By calibrating with standards common to both data sets, ANN models created using previously collected data gave accurate estimates of determi-nand concentrations, or bacterial identities, from newly acquired spectra.127 In this approach calibration samples were included in each of the two runs, and ANNs were set up in which the inputs were the 150 new calibration masses while the outputs were the 150 old calibration masses. These associative nets could then by used to transform data acquired on that one day to data acquired at an earlier data. For the first time PyMS was used to acquire spectra that were comparable with those previously collected and held in a database. In a further study this neural network transformation procedure was extended to allow comparison between spectra, previously collected on one machine, with spectra later collected on a different machine 129 thus calibration transfer by ANNs was affected. Wilkes and colleagues130 have also used this strategy to compensate for differences in culture conditions to construct robust microbial mass spectral databases. 

Many instruments utilize a double beam principle in that radiation absorbed or emitted by the sample is automatically compared with that associated with a blank or standard. This facilitates the recording of data and corrects for matrix effects and instrumental noise and drift. Instrumentation for the generation of radiation is varied and often peculiar to one particular technique. It will be discussed separately in the relevant sections. Components (b) and (c), however, are broadly similar for most techniques and will be discussed more fully below. 

Drifts of migration times can partly be compensated by calculating the mobility for analyte identification and using corrected PAs or internal standards for quantitation. 

Equilibrium Dialysis Standard ED High-throughput, 96-well format. Long incubation time - compound instability and plasma degradation. Issues of pH drift and osmotic volume shifts (can be corrected for). Membrane adsorption/non-specific binding. [30-32]... 

It is also possible to use an internal standard to correct for sample transport effects, instrumental drift and short-term noise, if a simultaneous multi-element detector is used. Simultaneous detection is necessary because the analyte and internal standard signals must be in-phase for effective correction. If a sequential instrument is used there will be a time lag between acquisition of the analyte signal and the internal standard signal, during which time short-term fluctuations in the signals will render the correction inaccurate, and could even lead to a degradation in precision. The element used as the internal standard should have similar chemical behaviour as the analyte of interest and the emission line should have similar excitation energy and should be the same species, i.e. ion or atom line, as the analyte emission line. 

First the responses Rq are measured for the sample. Thereafter K is determined by fitting the changes in the concentrations of the analytes in the sample, brought about by the standard additions, to the changes in the responses. Once all elements in the calibration matrix, K, have been determined, the concentration vector of the analytes in the sample is calculated. The method has been successfully applied to absorption spectrophotometry , anodic stripping voltametry and ICP-atomic emission spectrophotometry Attractive features of the method are that automation is very easy and automatic drift compensation is possible . A drawback is that all interferents should be known and be corrected for. 

An advantage of ICP-MS compared to all other atomic mass spectrometric techniques including TIMS is that usually only simple sample preparation (e.g., by microwave induced digestion of solid samples) is necessary. Sample preparation steps for ICP-MS analyses are similar to those of ICP-OES. Concentrated solutions are analyzed after dilution with high purity water only. In order to correct mass drifts of the instrument, an internal standard element like In or Ir with known concentration (e.g., I Op.g 1) is added. The solution is then acidified with HN03 to stabilize the metal ions in aqueous solution. 

CH4 and mass range, 33-500 at one scan/s. Decachlorobiphenyl (10 ng/pL) was added as an internal standard prior to derivatization and p-chlorobenzophenone was added (20 ng/pL) just prior to injection on the GC-MS to correct for any changes during sample handling and to normalize drift in MS sensitivity. These steps permitted the determination of changes in the concentrations of detected components. 

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