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ORGANIC AND INORGANIC GASES by FTIR Spectrometry: METHOD 3800, Issue 1, dated 15 March 2003 - Page 12 of 47 absorption pathlength. For reference spectra, this is a known quantity; for sample spectra, it is the quantity directly determined from Beer's Law. The units "ppm-meters" (ppm-m) are recommended.

data quality objectives—parameters pertaining to a certain application of this method, including the estimated LOD values for each compound.

de-resolve—to form spectra of lower resolution (higher FW HM) from spectra of higher resolution (lower FW HM); see Reference 2 (Appendix K) and Reference 11 for de-resolution procedures and programs.

detector linearity—a characteristic of an (ideal) IR detector; for such a detector, the measured detector output voltage, when plotted against the total IR in a broad-band IR signal incident on the detector, would form a straight line.

double beam spectrum—a transmittance or absorbance spectrum derived by dividing the sample single beam spectrum by the background spectrum.

Note: The term "double-beam" is used elsewhere to denote a spectrum in which the sample and background interferograms are collected simultaneously along physically distinct absorption paths. Here, the term denotes a spectrum in which the sample and background interferograms are collected at different times along the same absorption path.

extractive—the type of spectroscopy which includes extracting and transporting a sample stream from gases at a certain location to an absorption cell, and isolating the sample in the absorption cell for analysis. Other types of spectroscopy in which the sample is not isolated in an absorption cell include "remote", "open path", and "local open path" techniques.

filter—(1) A device made of inert materials which physically removes solid and liquid phase particles from a gas stream. (2) An optical device which transmits some fraction of the radiation incident on it; "neutral density" and "mesh" filters transmit an approximately constant fraction of the incident radiation at all wavelengths over a specified wavelength range.

FFT (Fast Fourier transform)—a discrete (digital) approximation to an FT (Fourier transform; see below) involving the factoring of the original data into sparse matrices containing mostly zeros.

FT (Fourier transform)—the mathematical process for converting an analytical (non-discrete) amplitude-time function to an amplitude-frequency function, or vice versa.

FTIR (Fourier transform infrared) spectrometer—an analytical system that employs a source of mid-infrared radiation, an interferometer, an enclosed sample cell of known absorption pathlength, an infrared detector, optical elements that transfer infrared radiation between components, and a computer system. The time-domain detector response (interferogram) is processed by a Fourier transform to yield a representation of the infrared power vs. infrared frequency. See Figures C1 and C2.

FTIR spectrometry—use of an FTIR system to perform quantitative measurements.

FTIR system—the combination of an FTIR spectrometer and a sample interface.

FTIR system configuration—the set of parameters required to reproduce, as closely as possible, results from a particular FTIR system at a later time. This set includes (at least) the nominal MIL, the absorption pathlength, the apodization function, the gas temperature, the gas pressure, the zero filling factor, the measured wavenumbers of specific water absorption bands, the sources of the reference library spectra, the integration time, the detector type and serial number, the detector gain (including hardware and software settings).

FTIR system response time—the minimum time required for the output of an FTIR system to accurately reflect a sudden change in the sample gas composition; see Appendix B, Section 5.

frequency,—v the number of cycles per unit time; for light, $$\scriptstyle{v=s\backslash\lambda}$$, where's is the speed of light and λ is the light's wavelength. Unlike the speed and wavelength, which are medium-dependent, the frequency of light is independent of the medium through which the light travels. Th e term "frequency" is often used to NIOSH Manual of Analytical Methods, Fourth Edition