Response of an instrument

There are two uses of chemical standards in chemical analysis. In the first place, they may be used to verify that an instrument works correctly on a day-to-day basis - this is sometimes called System Suitability checking. This type of test does not usually relate to specific samples and is therefore strictly quality assurance rather than quality control. Secondly, the chemical standards are used to calibrate the response of an instrument. The standard may be measured separately from the samples (external standardization) or as part of the samples (internal standardization). This was dealt with in Section 5.3.2. 

Calibration Operation that establishes the relationship, obtained by reference to one or more measurement standards, between the response of an instrument and the values of the standards. 

A procedure which enables the response of an instrument to be related to the mass, volume or concentration of an analyte in a sample by first measuring the response from a sample of known composition or from a known amount of the analyte, i.e. standard. Often, a series of standards is used to prepare a calibration curve in which instrument response is plotted as a function of mass, volume or concentration of the analyte over a given range. If the plot is linear, a calibration factor... 

We mentioned in Section 6.2 that the response of an instrument used for chemical analysis is proportional to the concentration of the analyte in a solution. This proportionality can be expressed as follows ... 

According to scalar diffraction theory (Section 4.4) the scattering amplitude in the forward direction is proportional to the cross-sectional area of the particle, regardless of its shape, and is independent of refractive index. To the extent that diffraction theory is a good approximation, therefore, the radius corresponding to the response of an instrument that collects light scattered near the forward direction by a nonspherical particle is that of a sphere with equal cross-sectional area. The larger the particle, however, the more the... 

If a calibration function is used with coefficients obtained by fitting the response of an instrument to the model in known concentrations of calibration standards, then the uncertainty of this procedure must be taken into account. A classical least squares linear regression, the default regression... 

The standard recognizes (section 5.6.1) that the traceability requirements should apply to aspects of the method that have a significant influence on the result of the measurement. For an analytical chemistry laboratory, as well as for the reference materials used for calibrating the response of an instrument, balances will need to be calibrated from time to time, and appropriate certification of the traceability of glassware and thermometers must be available. 

When the relative response of an instrument to analyte and standard remains constant over a range of concentrations, we say there is a linear response. For critical work, this assumption should be verified because it is not always true."... 

Figure 3.7. Unit-step responses of an instrument (a) and of a high-frequency filter (b) in the same frequency range 1 - no filter 2 - with a filter 3 - ideal filter characteristic 4 - approximation by a second order Butterworth filter.

In principle, one could calculate an absolute Raman cross section from the response of an instrument calibrated with a standard radiometric source. This approach is difficult but has been used to provide the cross sections in Table 2.2. If the relative response function is calibrated accurately, however, it is much simpler to determine cross sections by comparison to standards. Provided the sample positioning and optics permit quantitative Raman signal reproducibility, cross sections of liquids may be determined by comparing the response-corrected peak area to a band with known absolute cross section, such as the benzene 992 cm band. For response-corrected spectra, the ratio of the peak areas under identical experimental conditions equals the ratio of the absolute cross sections. 

Bias A systematic error in a measurement system. (Section 1.7) Calibration The process of establishing the relation between the response of an instrument and the value of the measurand. (Section 5.2)... 

Calibration is at the heart of chemical analysis, and is the process by which the response of an instrument (in metrology called indication of the measuring instrument ) is related to the value of the measurand, in chemistry often the concentration of the analyte. Without proper calibration of instruments measurement results are not traceable, and not even correct. Scales in supermarkets are periodically calibrated to ensure they indicate the correct mass. Petrol pumps and gas and electric meters all must be calibrated and recalibrated at appropriate times. 

A real instrument delivers yr(T) for the interval [0, L/c], where L is the maximum path difference which depends on the maximum displacement of the moving mirror of the FTS. The L-dependent instrumental function, that is the response of an instrument to a monochromatic incident light E(t)-coscobt, can be written as... 

Elxercise 5.5. Calibration is a very important procedure in instrumental analytical chemistry. In calibration, one attempts to find a quantitative relation between the response of an instrument and the concentration of a chemical species one wishes to determine. Normally, this relation is determined by means of the least-squares method. Consider the matrices... 

Production of a calibration model or calibration function is defined as the verification of the response of an instrument to a material of known properties and, if necessary, correction by a factor to take the instrument to the corresponding mark. Frequently, this term is used interchangeably with the concept of standardization, although that is a distinct idea. Standardization means to characterize the response of an instrument according to the known properties of the material, and this is usually done by the calibration curve (which should be called a standardization curve ). Indeed most present-day instruments consist of different systems and their complexity makes it difficult truly to calibrate them in a strict sense. In fact, what is usually done is to standardize the response by means of a series of samples of known concentrations. ... 

Sensitivity Ratio of the change in the response of an instrument with a corresponding change in the concentration of a chemical species. 

---