taken to make a quantitative measurement of one unknown specimen by reference to an instrument calibration standard (see Chap. 10). This, in fact, involves the measurement of two specimens: the unknown and the instrument standard. The purpose of the instrument standard is to test for drift in the instrument’s sensitivity. By ratioing the intensity measured on the unknown to that measured on the stan¬dard, the effects of long-term drift are approximately canceled (see Sec. 11.2.5). Thus, long-term drift is defined as the component or components of drift that are significant over a period of time that is long compared to the time taken to make a quantitative measurement of one unknown specimen by reference to an instrument calibration standard. By definition, the ratio method cannot correct for short-term drift. Consequently, long-term drift can be compensated for by operator technique, whereas short-term drift cannot.The short- and long-term drift characteristics vary from instrument to instru¬ment, and the relative importance of the various causes of drift is a complex function of instrument design and maintenance. Where the drift characteristics are thought
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