ABSORBANCE, A, is represented by the equation:
A = log T = log (1/T) or A = log (I/R)
in which T and R are the transmittance and the reflectance, respectively.
BACKGROUND SPECTRUM is also referred to as a reference spectrum or background reference. A ratio of this spectrum to that of the sample radiation intensities produces a transmittance or reflectance spectrum. For example, in reflectance measurements, a highly reflective standard reference material is used.
CALIBRATION MODEL is a mathematical expression to relate the response from an analytical instrument to the properties of samples.
DIFFUSE REFLECTANCE is that portion of radiated light penetrating the sample surface, interacting with the analyte material, and being reflected back to the detector. This is the component of the overall reflectance that produces the absorbance spectrum of the sample.
DURBINWATSON is a method of testing the linearity of a calibration by comparing the sum of squares of successive calibration residuals to the sum of squares of the calibration residuals around their mean. The expected value of the DurbinWatson statistic for random, independent, normally distributed residuals is two.
FIBER-OPTIC PROBES consist of two components: optical fibers, which may vary in length and in the number of fibers, and a terminus, which contains specially designed optics for examination of the sample matrix.
INSTRUMENT BANDWIDTH is a measure of the ability of a spectrometer to separate radiation of similar wavelengths.
MULTIPLE LINEAR REGRESSION is a calibration algorithm used to relate the response from an analytical instrument to the properties of samples. The distinguishing feature of this algorithm is the use of a limited number of independent variables. Linear-least-squares calculations are performed to establish a relationship between these independent variables and the properties of the samples.
OPERATIONAL QUALIFICATION is the process by which it is demonstrated and documented that the instrument performs according to specifications, and that it can perform the intended task. This process is required following any significant change such as instrument installation, relocation, major repair, etc.
PARTIAL LEAST SQUARES (PLS) is a calibration algorithm used to relate instrument responses to the properties of samples. The distinguishing feature of this algorithm is that, while similar to PCR, this algorithm includes data concerning the properties of the samples used for calibration in the calculation of the factors used to describe the instrument responses.
PERFORMANCE QUALIFICATION is the process of using one or more well-characterized and stable reference materials to verify consistent instrument performance. Qualification may employ the same or different standards for different performance characteristics.
PHOTOMETRIC LINEARITY, also referred to as photometric verification, is the process of verifying the response of the photometric scale of an instrument.
PRINCIPAL COMPONENT REGRESSION (PCR) is a calibration algorithm used to relate the response from an analytical instrument to the properties of samples. This algorithm, which expresses a set of independent variables as a linear combination of factors, is a method of relating those factors to the properties of the samples for which the independent variables were obtained.
REFERENCE SPECTRUMSee Background Spectrum.
REFLECTANCE is described by the equation:
R = I/Ir
in which I is the intensity of radiation reflected from the surface of the sample; and Ir is the intensity of radiation reflected from a background reference material and its incorporated losses due to solvent absorption, refraction, and scattering.
ROOT-MEAN-SQUARE (RMS) NOISE is calculated by the equation:
in which N is the number of points per segment; Ai is the absorbance for each data point; and bar(A) is the mean absorbance over the spectral segment.
SPECTRAL REFERENCE LIBRARY is a collection of spectra of known materials used for the purpose of comparison with unknown materials. The term is commonly used in connection with qualitative methods of spectral analysis (e.g., identification of materials).
STANDARD ERROR OF THE LABORATORY (SEL) is a calculation based on repeated readings of one or more samples to estimate the precision and/or accuracy of the reference laboratory method, depending on how the data was collected.
STANDARD ERROR OF PREDICTION (SEP) is a measure of accuracy of an analytical method based on applying a given calibration model to the spectral data from a set of samples different from but similar to those used to calculate the calibration model. The SEP is the standard deviation of the residuals obtained from comparing the values from the reference laboratory to those from the method under test, for the specified samples. The SEP provides a measure of the accuracy expected when measuring future samples.
SURFACE REFLECTANCE, also known as specular reflection, is that portion of the radiation not interacting with the sample but simply reflecting back from the sample surface layer (sample-air interface).
TRANSFLECTANCE is a transmittance measurement technique in which the radiation traverses the sample twice, the second time after being reflected from a surface behind the sample.
TRANSMITTANCE is represented by the equation:
in which I is the intensity of the radiation transmitted through the sample; I0 is the intensity of the radiant energy incident on the sample and includes losses due to solvent absorption, refraction, and scattering; and A is the absorbance.