Instrument calibration, importance

Thus, while a well-maintained instrument is important for any chemical/physical measurement, in NIR, without a concurrent standard for comparison, it is critical that the instrument be continuously calibrated and maintained. Since the major manufacturers of equipment have worked with the pharmaceutical industry, this has been formalized into what is called IQ/OQ/PQ, or Instrument Qualification, Operational Qualification, and Performance Qualification. The first is routinely performed (at first) by the manufacturer in the lab/process location, the second in situ by the user with help from the manufacturer, and the third is product/use dependent. These formal tests apply to all instruments in any industry. 

Kenneth Johnson is a Senior Scientist at the Monterey Bay Aquarium Research Institute. His research interests are focused on the development of new analytical methods for chemicals in seawater and application of these tools to studies of chemical cycling throughout the ocean. His group has developed a variety of analytical methods for analyzing metals present at ultratrace concentrations in seawater. His expertise lies in trace metal analysis and instrumentation. The creation of reference materials to calibrate these instruments is important for the production of long-term, high-precision datasets. Dr. Johnson has participated on the NRC Committee on Marine Environmental Monitoring and the Marine Chemistry Study Panel. 

Standardization The instrument response function can vary from analyzer to analyzer. If calibration transfer is to be achieved across all instrument platforms it is important that the instrument function is characterized, and preferably standardized [31]. Also, at times it is necessary to perform a local calibration while the analyzer is still on-line. In order to handle this, it is beneficial to consider an on-board calibration/standardization, integrated into the sample conditioning system. Most commercial NIR analyzers require some form of standardization and calibration transfer. Similarly, modem FTIR systems include some form of instrument standardization, usually based on an internal calibrant. This attribute is becoming an important feature for regulatory controlled analyses, where a proper audit trail has to be established, including instrument calibration. 

The interface provides efficient transfer of samples into the Merlin, and, most importantly, a rapid flush-out there is no hold up of mercury (which is a feature of the commonly used atomic absorption techniques). To aid the transfer of mercury vapour, the tin(II) chloride regime is used, together with a gas/liquid separator designed for this task. Mercury is sparged from the reaction vessel into the Merlin Detector. Full automation is provided by using a simple standard DIO card fitted into an IBM compatible computer system with the PSA Touchstone software. This is an easy-to-use menu-driven system which controls the modules used in the instrumentation, calibrates the system, collects, collates and reprints the results, and which finks to host computer systems. 

IDMS is based on measurements of masses and isotope ratios only. Some important advantages, compared with other calibration strategies, such as external calibration and standard additions, are that instrumental instabilities such as signal drift and matrix effects will have no influence in the final concentration in the sample, high accuracy and small measurement uncertainties are enabled, possible loss of substance of the isotope-diluted sample will have no influence on the final result and there is no need to resort to an external instrumental calibration or standard additions to the sample. 

Two important components of quantitative analysis of environmental samples are the determination of method detection limits and instrument calibration. Understanding how they contribute to data quality will enable us to make decisions related to data validity during the assessment phase of the data collection process. 

Table 5.4 summarizes the acceptance criteria for instrument, calibration, and method blanks. No contaminants of concern should be present in method blanks above the laboratory PQL. Equally important is that instrument blanks show no memory effects. If these conditions are not met, a possibility for false positive sample results becomes real. For decision on sample data with contaminated method blanks the chemist may rely on the following rules of the Functional Guidelines ... 

Evaluation of the calibration uncertainty component is the most important component the uncertainty of results depends on the uncertainty value of the CRMs used for the calibration and the quantitative relationships between uncertainty and traceability, which are two fundamental concepts of metrology which are intimately linked. In this way the traceable instrument calibration is an important step in assuring the traceability of spectrochemical results. 

In this approach, calibration uncertainty is an important component of the traceability chain and uncertainty of results depends on the uncertainty of the certified values of RMs used for the calibration. Thus, the results are traceable to the standards used for the instrument calibration. The traceability of certified values of RMs is as important as that of spectrometric measurements. Therefore, it is necessary to use the spectrometric RMs that are characterized in a metrological manner. In this framework, the uncertainty and traceability, as two fundamental metrological concepts, are intimately linked. 

Another factor which influences the speed in performing an analysis is calibration of the instrument. Calibration is especially time-consuming in cases where different elements are run on every analysis but even in cases where the same elements are determined time after time, the frequency of instrument calibration required to maintain a desired level of accuracy is an important consideration. Since manual data collection is not feasible in multielement determinations, the ideal system would undoubtedly be computerized. The computer would handle all data collection steps, the construction of calibration curves by mathematical curve-fitting methods, and the calculation of concentrations from these curves. 

Traceability is one major factor that can be achieved via CRMs as main means in the held of chemical metrology. In general, CRMs are applied for the validation of analytical methods. Standard solutions are then used for instrument calibration. Nonetheless, CRMs should not be understood as the solution for all problems in chemical measurement. It goes without saying that the matrix of a CRM should match the analytical problem as exactly as possible. It is clear that there are not CRMs available for all matrices and analytes. Thus, it is important to have the best matrix match. 

The first of the separation techniques to be used in process measurement was gas chromatography (GC) in 1954. The GC has always been a robust instrument and this aided its transfer to the process environment. The differences between laboratory GC and process GC instruments are important. With process GC, the sample is transferred directly from the process stream to the instrument. Instead of an inlet septum, process GC has a valve, which is critical for repetitively and reproducibly transferring a precise volume of sample into the volatiliser and thence into the carrier gas. This valve is also used to intermittently introduce a reference sample for calibration purposes. Instead of one column and a temperature ramp, the set up involves many columns under isothermal conditions. The more usual column types are open tubular, as these are efficient and analysis is more rapid than with packed columns. A pre-column is often used to trap unwanted contaminants, e.g. water, and it is backflushed while the rest of the sample is sent on to the analysis column. The universal detector - thermal conductivity detector (TCD)-is most often used in process GC but also popular are the FID, PID, ECD, FPD and of course MS. Process GC is used extensively in the petroleum industry, in environmental analysis of air and water samples" and in the chemical industry with the incorporation of sample extraction or preparation on-line. It is also applied for on-line monitoring of volatile products during fermentation processes" ... 

As with any instrument, calibration is important for thermocouples. Although manufacturers supply accuracy data for typical thermocouples, calibration can be used to check that these data apply to the specific instrument being used in an experiment. The accuracy can be improved by calibrating the specific thermocouple being used. More importantly, a thermocouple calibrator (see Figure 5.5) can be used to check for any wiring problems or any setup mistakes in the data acquisition system. A thermocouple that has been wired and connected to the data acquisition system can be inserted into the calibrator to check for problems. For example, if a Type R thermocouple is actually being used but it... 

In this section, we shall explore some of the procedures needed for the recording of reliable diffusion data including instrument calibration, sample preparation and equilibration requirements. We shall also address some of the most important factors that can lead to the collection of inaccurate data, something to which diffusion measurements can be especially prone. Indeed, the set-up of a diffusion sequence is no more demanding than most other 2D experiments but the potential for erroneous results is considerably greater in most cases and, worst still, is not always readily apparent if one is not aware of the potential pitfalls and how to avoid them. 

APECS is used in studies where determining the true shape of the XPS or Auger peaks free from background subtraction errors or overlapping features is important, such as for compihng standard reference spectra, instrument calibration, and verification of theoretical models of photoelectron and Auger emission. 

This chapter introduces the most important aspect of TEQA for the reader After the basics of what constitutes good laboratory practice are discussed, the concept of instrumental calibration is introduced and the mathematics used to establish such calibrations are developed. The uncertainty present in the interpolation of the calibration is then introduced. A comparison is made between the more conventional approach to determining instrument detection limits and the more contemporary approaches that have recently been discussed in the literature (1-6). These more contemporary approaches use least squares regression and incorporate relevant elements from statistics (7). Quality assurance/quality control principles are then introduced. The chapter ends with a comparison of the performance from two hypothetical labs. Every employer wants to hire an analyst who knows of and practices good laboratory behavior. 

It is very important and the most important task for the analyst who is responsible for operation and maintenanee of analytical instrumentation. Calibration is followed by a verifieation proeess in which specifications can be established and the analyst ean evaluate whether or not the calibration is verified or refuted. A calibration that has been verified can be used in acquiring data from samples for quantitative analysis. A calibration that has been refuted must be repeated until verification is achieved, e.g., if, after establishing a multipoint calibration for benzene via a gas chromatographie determinative method, an analyst then measures the concentration of benzene in a certified reference standard. The analyst expects no greater than a 5% relative error and discovers to his surprise a 200% relative error In this case, the analyst must reconstruct the calibration and measure the certified reference standard again. Close attention must be paid to those sourees of... 

As with any experimental techniques, it is important to ensure that the theological evaluation is carried out correctly and the results are adequately inteqireted. Experimental errors such as deficient instruments calibration, for those that require a calibration, are lo be avoided. Other type of errors that have to be prevented arc viscous heating, end efTects. and secondary flow. 

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