Recalibration procedures

The standardised sequence for measuring calibration solutions and solutions to be analysed is (1) Measuring of calibration solutions in order of increasing concentration. (2) Measuring of sample solution, twice. (3) Measuring of calibration solutions in sequence of decreasing concentration. 

Measuring the calibration solution in order of decreasing concentration, after the solution to be analysed has been measured, has no influence on the results for the sample solutions. It is better to calibrate in order of increasing concentration before measuring the sample solutions and to check the calibration values by measuring in order of decreasing concentration memory-effects can then be recognised when calibration points deviate. 

If a measurement has to be interrupted because of time, or if the same problem occurs again after a few hours or the next day (same element in the same matrix, e.g., A1 in steel), then the calibration curve may be stored. With the spectrometer switched to stand-by, data are preserved in the micro- 

When analysing low concentrations, in the range of a few jug ml-1, or even less, it has to be remembered that solutions of very low concentrations are not very stable. It is essential that fresh solutions are prepared for calibration as well as for recalibration. Thus, it is recommended to prepare a solution of a higher concentration, such as a stock solution, since it will remain stable longer. 

Just before measuring the calibration solutions are diluted to their nominal concentrations with the appropriate solutions. 

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Because of the extreme accuracy expected of many of these products, some include internal test weights which can be used to recalibrate regularly and to adjust for nonlinearity. Some balances monitor changing conditions and initiate the recalibration procedure as needed. 

Another issue is that of transferability of the calibration model among instruments. This has been a significant obstacle to more widespread use of NIR methods. Transferability is especially important to multisite facilities, because it is needed to avoid time-consuming recalibration procedures. Calibration errors may occur among instruments because of slight differences in instrument response, especially if full-spectrum multivariate models are used. Shenk and Westerhaus addressed the problem and proposed a standardization algorithm, which was modified by others. ... 

Workflow management systems provide lists of the possible workflow activities that the user can perform next. The user can take responsibility for the execution of a pending workflow activity or can initiate computer-based work to execute the activity by selecting it from the work list. Several laboratory activities have to be executed at regular intervals (e.g., recalibration procedures, stability studies) that can be handled by the AWM using time-based triggers. 

In most of the encountered case studies, this approach is not satisfactory because one would like to avoid the recalibration procedure. However, there are some cases where this approach can be recommended. Firstly, all calibration samples should be perfectly stable over time, otherwise the new calibration model will account for both calibration information and sample aging. If this model is applied on new samples, erroneous predictions will be obtained since the new samples will not be properly located in the calibration domain because of sample aging. Another important point is the size of the recalibration subset. If the number of samples to be retained to cover all sources of variation is large, the use of the subset recalibration will be very time-consuming and the use of a smaller subset for standardization will be a better alternative. However, if the relevant spectral information can be easily summarized in a few calibration samples (yielding a parsimonious and simple model), this approach can be recommended, since the amount of work for the recalibration will be similar to the one for instrument standardization. 

The samples to be added are analyzed by the primary reference method and the new population of calibration samples resubmitted to the entire calibration procedure. After the best equation for each constituent has been selected, continue to monitor the analysis of this new equation. Depending on the spectral variation of the new population to be predicted, the calibration, monitoring, and recalibration procedure many need to be repeated a number of times. 

The development of a calibration model is a time consuming process. Not only have the samples to be prepared and measured, but the modelling itself, including data pre-processing, outlier detection, estimation and validation, is not an automated procedure. Once the model is there, changes may occur in the instrumentation or other conditions (temperature, humidity) that require recalibration. Another situation is where a model has been set up for one instrument in a central location and one would like to distribute this model to other instruments within the organization without having to repeat the entire calibration process for all these individual instruments. One wonders whether it is possible to translate the model from one instrument (old or parent or master. A) to the others (new or children or slaves, B). 

The most common and intuitive method for the determination of this number of eigenvalues is called Cross Validation. The idea is to remove one (or several samples) from the calibration set, use what is left for the computation of a new calibration, and use it to predict the quality of the removed sample(s). Each prediction is compared with the actual quality that is known as the removed sample really is part of the total calibration set. In a loop all samples are removed either one by one or in groups and after recalibration with the reduced calibration set their qualities are predicted and compared with the true values. In order to determine the best number of eigenvectors this procedure is repeated in a big loop systematically trying all numbers of eigenvectors. This complete procedure is called Cross Validation. 

Any mass spectrometer requires mass calibration before use. However, the procedures to perform it properly and the number of calibration points needed may largely differ between different types of mass analyzers. Typically, several peaks of well-known m/z values evenly distributed over the mass range of interest are necessary. These are supplied from a well-known mass calibration compound or mass reference compound. Calibration is then performed by recording a mass spectrum of the calibration compound and subsequent correlation of experimental m/z values to the mass reference list. Usually, this conversion of the mass reference list to a calibration is accomplished by the mass spectrometer s data system. Thereby, the mass spectrum is recalibrated by interpolation of the m/z scale between the assigned calibration peaks to obtain the best match. The mass calibration obtained may then be stored in a calibration file and used for future measurements without the presence of a calibration compound. This procedure is termed external mass calibration. 

Step 5 Off-line method or analyzer development and validation This step is simply standard analytical chemistry method development. For an analyzer that is to be used off-line, the method development work is generally done in an R D or analytical lab and then the analyzer is moved to where it will be used (QA/ QC lab, at-line manufacturing lab, etc.). For an analyzer that is to be used on-line, it may be possible to calibrate the analyzer off-line in a lab, or in situ in a lab reactor or a semiworks unit, and then move the analyzer to its on-line process location. Often, however, the on-line analyzer will need to be calibrated (or recalibrated) once it is in place (see Step 7). Off-line method development and validation generally includes method development and optimization, identification of appropriate check samples, method validation, and written documentation. Again, the form of the documentation (often called the method or the procedure ) is company-specific, but it typically includes principles behind the method, equipment needed, safety precautions, procedure steps, and validation results (method accuracy, precision, etc.). It is also useful to document here which approaches did not work, for the benefit of future workers. 

Procedure for mineral soils of pH less than 5.0. Add 20 ml of double strength buffer solution to the soil suspension retained from the pH determination, and stir for 5 min. Mix 25 ml water and 20 ml double strength buffer with pH adjusted to between 6.9 and 7.1, and use to recalibrate the pH meter to read 7.00. Read the pH of the stirred sample. 

An electrode verification procedure should be performed as frequently as possible since electrodes are known to drift with time. Recalibrate if the electrode response has drifted outside the value specified (see the limit cited above). 

Requires shielding and special procedures to avoid exposure of personnel to radiation Frequent recalibration is required to compensate for source decay over time Electrical power and power line transient protection required... 

In addition to inspection and calibration of instrumentation carried out as part of an SAT, the need for recalibration of critical instruments prior to IQ, OQ, and PQ should be reviewed and the decision documented in the respective qualification report. All site calibration activity should be conducted in accordance with quality standards and the respective engineering procedures. Any remedial work should be undertaken under document control, and where necessary, evaluated under change control. 

Describe in detail the procedure to be followed for recalibrating an instrument that is found to be out of calibration when tested. 

There may be a significant time lapse between the OQ and PQ phases, and as a result, consideration must be given to whether any control and monitoring instrumentation needs to be recalibrated. It is advisable to recalibrate critical instrumentation under the site calibration procedures and so guarantee correct calibration prior to commencing PQ. 

Normal operation—The computer system is maintained in accordance with the planned preventative maintenance schedule. Typical activities include recalibrating field instrumentation and computer I/O cards in accordance with site calibration procedures, running system diagnostics, checking operator logs for any abnormalities, and planning service visits by the system supplier. 

On-line measurements produced with in situ sensors are difficult to validate. The usual procedure for evaluating the quality of a measurement is restricted to calibration/checking prior to and after a cultivation. A few sensors such as the pC02- or the Cranfield/GBF-glucose sensor [42] allow removal (at least of measuring buffer and also of the transducer itself) and, therefore, recalibration of the transducer during a cultivation (Fig. 23). 

Recalibration must be carried out to agreed upon standard procedures using calibration test equipment that is traceable back to national standards. All calibration tests must be fully documented, the results recorded, and the sheets signed off by an authorized person. Calibrated instruments must be provided with a full calibration certificate that details the test results and their limits of uncertainty. A detailed account of the calibration life-cycle processes can be found in the GAMP Good Practice Guide on Calibration ManagementP... 

Typical activities associated with the periodic review phase for instramentation inclnde establishing a recalibration program and conducting rontine maintenance activities as part of a plaimed preventive maintenance scheme. All maintenance activities mnst be carried out under a formal change control procedure and any associated testing mnst be fnlly documented nsing test record sheets. 

Shunt calibration A procedure of transducer testing when a resistor with a known value is connected to one leg of the bridge. The output should correspond to the voltage specified in the calibration certificate. If it does not, something is wrong and the transducer needs to be inspected for possible damage or recalibrated. 

The example is illustrated by the results of Table 10.5. The Raman shift range from 400 to 2000 cm was calibrated with the 4-acetamidophenol shift standard, and the calibrated spectrum was recorded and stored on disk. Then calcium ascorbate was observed, with and without recalibration between spectra. Finally, spectra of calcium ascorbate were obtained approximately daily (each after recalibration) over a period of 2 months. The 769- and 1582 cm peaks were chosen for analysis, and their peak frequencies were determined by a center-of-gravity criterion included in the data analysis software (GRAMS 32). It is important that these qualification spectra duplicate the instrumental conditions to he used for real samples, at least as far as optical geometry, sampling mode, and calibration procedure. The objective is to provide an accurate indication of instrument performance in the intended application. 

The first line of Table 10.5 indicates short-term repeatability. The second line indicates the reproducibility of the calibration procedure, based on six recalibrations over a 10-day period. Long-term drift is indicated by the third line, over a period of time selected by the user, with recalibration. These results may be used to verify specifications claimed by the manufacturer and to provide a baseline for future measurements. Table 10.5 provides an assessment of the Raman shift accuracy of a particular instrument, obtained using samples and procedures that should be similar to the intended application. 

Recalibration of the instrument response function reduces or eliminates most of the instrumental factors that lead to relative intensity variations over time. For example, a luminescent standard could be used at the beginning of each session as described in Section 10.3.3. Use of the same standard and correction procedure during qualification could establish the true value of one or more peak ratios for future reference. Table 10.9 shows results for this approach applied to the example of calcium ascorbate. The ratio of the 767- and 1587 cm" peak intensities was monitored after calibration of the response function with a luminescent standard. The standard deviations listed in Table 10.9 for the 767/1582 peak height ratio provide indications of the reproducibility of the response correction and sample spectra. 

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