Page:NIOSH Manual of Analytical Methods - 3800.pdf/11

ORGANIC AND INORGANIC GASES by FTIR Spectrometry: METHOD 3800, Issue 1, dated 15 March 2003 - Page 11 of 47 APPENDIX A. Terminology.

absorption cell—a structure which contains a fluid sample, but allows light to pass through a sample at known temperature, pressure, and absorption path length.

absorption band—a contiguous wavenumber region of a spectrum (equivalently, a contiguous set of absorbance spectrum data points) in which the absorbance passes through a maximum or a series of maxima.

absorption pathlength—the distance, measured in the direction of propagation of the beam of radiant energy, between the surface of the specimen on which the radiant energy is incident and the surface of the specimen from which it is emergent.

absorbance (units: abs)—in terms of the incident and transmitted intensities I$0$ and I, the absorbance A is given by A = -log(I/I$0$). From a pair of FTIR single beam spectra A (the background spectrum) and B (the sample spectrum), the sample absorbance for each wavenumber value (with index i) in the spectra is approximated by A$i$ = -log(B$i$/A$i$).

absorbance linearity—a characteristic of (ideal) absorbance spectrum; for such a spectrum, the measured absorbance is described by Beer's Law (Equation C1).

absorptivity—a measure of the fraction of the incident infrared radiation that is absorbed by a particular compound per molecule and per absorption pathlength; see equation C1.

analytical region—a contiguous wavenumber region (equivalently, a contiguous set of absorbance spectrum data points) used in the quantitative analysis for one or more analytes. Note: The quantitative result for a single analyte may be based on data from more than one analytical region.

analyst—a person familiar with and experienced in performance of all aspects of this FTIR-based method. Analysts m ay perform any portion(s) of the method, and must perform certain portions of the method (see also "operator").

analyte—a compound whose concentrations in a sample is of interest and must be accurately quantified (see also "interferant").

aperture—an optical device which physically restricts the diameter of the optical beam.

apodization—modification of the interferogram through its multiplication by a weighing function whose magnitude varies with the position of the interferometer's moving element.

background spectrum—the single beam spectrum obtained with all system components and without sample present (or in the presence of a non-absorbing gas replacing the sample).

baseline—any line (or smooth function of wavenumber) drawn on an absorption spectrum to establish a reference point that represents a function of the radiant power incident on a sample at a given wavelength.

Beer's Law—the direct proportionality of the absorbance of a compound in a homogeneous sample to its concentration. See Equation C1, which also describes the more general case of gas mixtures.

calibration transfer standard (CTS) gas—a gas standard of a compound used to measure the sample absorption pathlength; see Step 7, Step 11, Appendix B (Section 1), and Appendix D, (Section 5).

cm$-1$—see wavenumber

compound—a substance possessing a distinct, unique molecular structure.

concentration—the quantity of a compound contained in a unit quantity of sample. The unit "ppm" (number, or mole, basis) is recommended, and is equivalent to the volume basis for ideal gases.

concentration-path length product (CCP)—the mathematical product of concentration of the species and the NIOSH Manual of Analytical Methods, Fourth Edition