Page:NIOSH Manual of Analytical Methods - 3800.pdf/14

ORGANIC AND INORGANIC GASES by FTIR Spectrometry: METHOD 3800, Issue 1, dated 15 March 2003 - Page 14 of 47 mercury by one millimeter. A pressure difference of one atmosphere (atm) is equal to 760 mm Hg, to 1.01 × 10$5$ Pascals (Pa), and to 14.7 pounds per square inch (psi).

operator—a person familiar with and experienced in performance of only some aspects of this FTIR-based method. Operators may perform many portions of this method, but specific portions of the method (see above) must be performed by an "analyst".

peak—see absorption band

qualitative analysis—examination of sample spectra to determine the presence or absence of particular compounds in a sample.

quantitative analysis—estimation of the actual concentrations of a specific set of compounds using a specific set of analytical regions.

reference spectra—absorption spectra of gases with known chemical compositions, recorded at a known absorption pathlength, which are used in the quantitative analysis of gas samples.

relative wavenumber accuracy (RWA, %)—the percent difference in wavenumber values measured by an FTIR spectrometer relative to a reference library standard. The RWA is estimated through spectral comparisons of the wavenumbers of two isolated water vapor absorption. See Appendix B, Section 3.

residual squared area (RSA)—a measure of the noise (random and systematic) and/or spectral artifacts for an absorbance spectrum in some analytical region; see Appendix B, Section 2 for a mathematical definition. The RSA can be used to estimate the LOD for a given compound measured with a given FTIR system configuration.

retardation—optical path difference between two beam's in an interferometer; also known as "optical path difference" or "optical retardation." In the case of a standard Michelson interferometer, the retardation is simply twice the distance moved by a mirror in the interferometer during a scan.

rotameter—a device indicating the volumetric flow rate of a gas by the vertical displacement of an object suspended by the gas stream.

sampling location—the point in space at which sample gases enter the sample interface.

sample interface—that part of the FTIR system which comes in contact with the sample and/or calibration gas es. It includes the sample probe, sample filter, sample line, sample pump, gas valves, internal surfaces of the absorption cell, pressure gauge, sample rotameter, the vent lines, and the calibration components (gas cylinders, regulators, and rotameters).

scaling—application of a multiplicative factor to the absorbance values in a spectrum.

scan—digital representation of the detector output obtained during one complete motion of the interferometer's moving assembly or assemblies.

single beam spectrum—Fourier-transformed interferogram, representing the relative detector response vs. wavenumber.
 * Note: The term "single-beam" is used elsewhere to denote any spectrum in which the sample and background interferograms are recorded on the same physical absorption path; such usage differentiates such spectra from those generated using interferograms recorded along two physically distinct absorption paths (see "double-beam spectrum" above). Here, the term applies (for example) to the two spectra used directly in the calculation of transmittance and absorbance spectra of a sample.

system zero (SZ) spectrum—the absorbance spectrum of a non-absorbing gas (nitrogen or zero air) acquired using those portions of the sampling interface used to acquire actual sample gases.

transmittance, T—the ratio of radiant power transmitted by the sample to the radiant power incident on the NIOSH Manual of Analytical Methods, Fourth Edition