Page:NIOSH Manual of Analytical Methods - 3800.pdf/13

ORGANIC AND INORGANIC GASES by FTIR Spectrometry: METHOD 3800, Issue 1, dated 15 March 2003 - Page 13 of 47 denote the wavenumber (w, cm$-1$) in FTIR spectroscopy because (in a given medium) the wavenumber is proportion al to the frequency v. (See Appendix C, Section 4, and "wavenumber" in this Appendix.)

full-width-half-maximum (FWHM)—for a single, symmetric absorption band, the full width of the band (in cm$-1$) between its 50% relative maxim um absorption levels.

impinger—a device constructed of inert materials which passes a gas stream through a liquid phase.

infrared source—a device which emits a pattern, stable in intensity and wavelength profile, of infrared radiation over a wide range of infrared wavelengths. High temperature filaments or ceramic elements, in conjunction with appropriate focusing optics, are often employed.

infrared detector—a device which (ideally) produces a voltage proportional to the total infrared power incident on it. Examples are 1) the mercury-cadmium -telluride (MCT) detector, which requires cooling (and is often cooled to liquid nitrogen temperature), and 2) the deuterated triglycine sulfate (DTGS) detector, often operated at ambient temperature.

interferogram—record of the IR detector's response to the modulated component of the interference signal measured as a function of retardation.

interferometer—device that divides a beam of radiant energy into two or more paths, generates an optical path difference between the beams, and recombines them in order to produce repetitive interference maxima and minima as the optical retardation is varied.

integration time—the total time over which the interferometric results of single scans are averaged to produce an interferogram (and its subsequent single- and double-beam spectra). Most software packages allow selection of the number of scans rather than the integration time. The integration time is approximately equal to (but is always less than) the time interval over which the selected number of scans is actually executed.

interferant—a compound whose presence in a sample spectrum must be taken into account to accurately determine one or more analyte concentrations, but whose concentration need not be accurately determined.

least squares fitting (LSF) algorithm—a calculation whereby one or more com pound concentrations are estimated from a sample spectrum by minimizing the squared error in Beer's Law within a defined analytical region (see Equations C1 through C6).

limit of detection (LOD, ppm)—for a defined FTIR system configuration and sample matrix, an estimate of the lowest detectable concentration of a specific analyte based on the FTIR system's RSA and the analyte's integrated absorbance for a selected analytical region.

line—see absorption band

linewidth—see full-width-half-maximum (FWHM) and minimum instrumental linewidth (MIL).

metering valve—a gas valve allowing reproducible adjustments of a gas stream flow rate on the order of 2% of the valves full flow rate.

mid-infrared (MIR)—the region of the electromagnetic spectrum from approximately 400 to 5000 cm$-1$.

minimum instrumental linewidth (MIL)—for a given FTIR spectrometer and FTIR system configuration, the minimum measured FWHM for any absorption band. In wavenumbers, the MIL is often estimated as the reciprocal of the retardation expressed in centimeters. The MIL depends on the choice of apodization function, and is often larger than the MIL estimated from the retardation.

multi-pass cell—an absorption cell which uses mirrors to pass the infrared radiation through the gas sample more than once; this leads to an absorption pathlength larger than the physical length of the cell (see also "White cell").

mm Hg—a measure of pressure difference which results in the vertical displacement of a column of liquid NIOSH Manual of Analytical Methods, Fourth Edition