Measures of Instrument Performance

This chapter describes and discusses the measures of instrument performance, the types of information that can be acquired, and the methodologies and strategies available. The discussions are with respect to both the multiple types of instruments manufactured and the various classes of compounds that can be analyzed. The range of compounds makes defining the types of data that can be obtained somewhat problematic. Therefore, small molecules are dealt with in Sections 3.2 and 3.3, while biopolymers are covered in Section 3.5, although the differentiation is somewhat artificial. For instance, the determination of accurate mass data described in Section 3.1.3 is relevant to both small molecules and the analysis of the peptides derived from proteins. 

SPONTANEOUS FISSION See Fission (Nuclear). STABILITY The measure of instrument performance through variations in temperature, line voltage, count rate, time, or other variables. 

Regular calibration and verification ensures that the parameters measured by a particular instrument can be related to a recognized standard. The frequency of instrument calibration may be quite varied, depending largely on the application. If, during the verification of instrument performance, it has been shown that the instrument stays in calibration for about three months, the calibration would be repeated at approximately two-monthly intervals. However, verification (system suitability) will be carried out each time samples are analysed. For some critical analyses, calibration may be performed for each batch of samples or, in an extreme case, for each separate sample. 

Experimental Determination of the Number of Plates in a Column The number of theoretical plates, N, and the plate height, H, are widely used in the literature and by instrument manufacturers as measures of column performance. Figure 30-12 shows how N can be determined from a chromatogram. Here, the retention time of a peak t and the width of the peak at its base W (in units of time) are measured. It can be shown (see Feature 30-4) that the number of plates can then be computed by the simple relationship... 

With enzyme determinations, standards for calibration purposes and for checking of instrumental performance make use of separately prepared solutions of one of the reactants or the products of the enzyme-catalyzed reaction. For instance, a solution of phenol may be standardized and thereafter used itself as the standard in determinations of acid and alkaline phosphatase, in methods employing phenyl phosphate as substrate and depending on the measurement of the amount of phenol liberated. This standard phenol solution, however, cannot be taken through all the steps in the determination of phosphatase, and a separate control solution must be used to check the performance of the overall technique if this control were omitted, it would be possible, for instance, for a buffer to be incorrectly prepared and for erroneous levels of phosphatase activity to be found. There is no substitute, therefore, in the control of enzyme determinations for the inclusion of a sample (of serum, urine, etc.) previously investigated for its level of enzyme activity. For long-term monitoring of an enzyme method, the repeated analysis of aliquots of a standardized enzyme preparation is most useful, provided... 

A better testing of model predictions requires determination of particle size distributions in raw waters and throughout treatment plants. These data will also provide needed understanding of treatment processes and more useful measures of plant performance. Existing instrumentation can be used for particle sizes larger than 1 fim. New techniques are needed for submicron particles. 

Mayhew T.P. and Rothstein J.M. 1985. Measurement of muscle performance with instruments. In J.M. Rothstein EA.), Measurement in Physical Therapy,pp 57-102. New York, Churchill Livingstone. 

The evaluation of new video endoscopic equipment is also difficult because of the lack of objective standards for performance. Purchasers of equipment are forced to make an essentially subjective decision about image quality. By employing virtual instrumentation, a collaborative team of biomedical engineers, software engineers, physicians, nurses, and technicians at Hartford Hospital (Hartford, CT) and Premise Development Corporation (Avon, CT) have developed an instrument, the EndoTester , with integrated software to quantify the optical properties of both rigid and flexible fiberoptic endoscopes. This easy-to-use optical evaluation system allows objective measurement of endoscopic performance prior to equipment purchase and in routine clinical use as part of a program of prospective maintenance. 

Kroemer K.H.E. 1991. A taxonomy of dynamic muscle exertions. J. Hum. Muscle Perform. 1 1—i. Mayhew TP. and Rothstein J.M. 1985. Measurement of muscle performance with instruments. In J.M. Rothstein (Ed.), MeasurementinPhysicaTTherapy,pp 57 102. New York, ChurchillLivingstone. Miller P.J. 1985. Assessment of joint motion. In J.M. Rothstein (Ed.), Measurement in Physical Therapy, pp. 103-136. New York, Churchill Livingstone. 

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