Calibration and standardization

Analytical chemists make a distinction between calibration and standardization. Calibration ensures that the equipment or instrument used to measure the signal is operating correctly by using a standard known to produce an exact signal. Balances, for example, are calibrated using a standard weight whose mass can be traced to the internationally accepted platinum-iridium prototype kilogram. 

Under many experimental conditions, the mass spectrometer functions as a mass-sensitive detector, while in others, with LC-MS using electrospray ionization being a good example, it can behave as a concentration-sensitive detector. The reasons for this behaviour are beyond the scope of this present book (interested readers should consult the text by Cole [8]) but reinforce the need to ensure that adequate calibration and standardization procedures are incorporated into any quantitative methodology to ensure the validity of any results obtained. 

These problems are not as significant In the macroenvironment, where many standards can be used for both calibration and standardization, since photodegradation of the sample Is usually not significant, running around 5-10%. However, If measurements of the highest accuracy or of small differences are required, these uncertainties... 

The trade-offs between direct calibration and standard addition are treated in Ref 103. The same recovery as is found for the native analyte has to be obtained for the spiked analyte (see Section 3.2). The application of spiking to potentiometry is reviewed in Refs. 104 and 105. A worked example for the application of standard addition methodology to FIA/AAS is found in Ref 106. Reference 70 discusses the optimization of the standard addition method. 

BRET [31, 32]), lock-in detection techniques exploiting optical switches [33], and schemes for alternating D/A excitation (ALEX [34]). The increased attention to quantitative FRET imaging encompasses the use of polarization [35-39], the perennial issue of calibration and standards [40-44], and practical guides to operational principles and protocols ([45, 46] and other references above). The fundamental distinctions between the requirements for live and fixed cell imaging cannot be overemphasized, as is exemplified in a report of erroneous FRET determinations with visible fluorescent proteins (VFPs) in fixed cells [47],... 

California Department of Health. Air and Industrial Hygiene Laboratory. Calibration and Standardization of Continuous Photometric Analyzers of Atmospheric On-dants. Recommended Method No. 5-B. Berkel California State Department of Health 1970. 18 pp. 

If proper attention is given to calibration and standardization, a well-designed, constructed and maintained automatic analyser will operate reproducibly over long periods in the hands of a trained operator. 

Some examples of calibration and standardization are presented below. 

In the above discussion, standard reference materials (SRMs) were mentioned often. A reference material (RM) is a material or substance suitable for use in calibrating equipment or standardizing solutions. A certified reference material (CRM) that a vendor indicates, via a certificate, is an RM. A standard reference material (SRM) is one that is distributed and certified by a certifying body, such as NIST. The SRM is the material to which all calibration and standardization materials should be traceable. A standard material becomes one when it is compared to or prepared from another. Ultimately, it all rests on the SRM — meaning all standard materials are traceable to an SRM (see Figure 5.10). 

Reference materials are often real samples that have been carefully prepared and analyzed by many laboratories by many different methods. In this way, their known value and accompanying confidence limits are determined. The regular use of reference materials not only provides for calibration and standardization, but it also can demonstrate an analyst s proficiency with a method. It should be noted that SRMs are expensive, however, and are not often used for routine calibration and standardization work. Usually, primary and secondary standards are used for that. Another important fact about RMs is that they are considered to have a finite shelf life and cannot be confidently used as RMs after a certain period of time. 

Figure 5.10 All calibration and standardization materials should be traceable to a SRM.

Calibration and standardization. Quantitative data are obtained by comparison with dose-response curves obtained using standard preparations of host-derived protein antigens. Since these preparations are mixtures of poorly defined proteins, a standard preparation is prepared and calibrated by a suitable protein determination method. This preparation is stored in a stable state suitable for use over an extended period of time. 

The terms test, calibration, and standardization are interrelated. Each term has a special meaning, but there is some overlap of the terms. 

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. 

Internal standards could be used in external calibration, matrix-matched external calibration, and standard addition calibration [2], However, the use of internal standards in LC-MS quantitative methods should not be confused with internal calibration in which an internal standard is employed as a calibrant and the concentration of a unknown sample is calculated from the concentration of this internal standard and its analyte/IS signal ratio, i.e., the concentration of the unknown sample is calculated without the need for a calibration curve [3], The use of internal standards in most LC-MS quantitative methods belongs to signal-ratio calibration or internal standardization [2,4], In fact, the majority of bioanalytical LC-MS methods use matrix-matched signal-ratio external calibration. 

The analysis of As in tuna bsh in Figure 7.25 shows a similar picture. External calibration and standard addition calibration were applied. It is evident that the calibration strategy did not lead to a signibcant difference in the average laboratory performance. 

Calibration and Standardization Inject the Standard Preparation into the chromatograph. Calculate the p-pheneti-dine factor (F) by the formula... 

Calibration and standardization are often confused and used interchangeably. Calibration refers to the process of adjusting an instrument or instrument s scale to a known physical standard [22], Wavelength scales are calibrated by comparing the instrument s wavelength scale to a spectral line whose value is known from first... 

Errors in calibration and standardization result in poor accuracy, and are considered in Section 2.5. 

Another unglamorous, but vitally important, aspect of oceanographic analysis is the area of calibration and standards. Standard reference materials appropriate for most of the constituents in seawater simply do not exist, and considering the low concentration levels usually involved and the unstable nature of natural seawater samples, the way to approach the problem is by no means obvious. Calibration of laboratory methods... 

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