Page:NIOSH Manual of Analytical Methods - 3800.pdf/16

APPENDIX B . System Tests.

These procedures must be conducted at least once on new or significantly altered (by replacement of components, dis- and reassembly, etc.) systems. The tests described in Sections B2 and B4 must also berepeated during either the pre-test preparations or quality assurance procedures. In all cases, activate the FTIR system and allow sufficient time for the infrared source, the infrared detector, and (if required) the temperature control systems to stabilize before proceeding.

B1 Absorption pathlength

Obtain one or more absorbance spectra of a calibration transfer standard (CTS) gas (ethylene at 200 to 300 ppm-meters is recommended) at the gas temperature and pressure of a reference CTS spectrum of the same compound. For each spectrum, calculate the indicated absorption pathlength according as

${\displaystyle {L_{S}}={\frac {{L_{R}}{P_{R}}{A_{S}}}{{P_{S}}{A_{R}}}}}$ Equation B1

where
${\displaystyle L_{S}}$ = pathlength indicated by the sample CTS spectrum (meters).
${\displaystyle L_{R}}$ = pathlength of the reference CTS spectrum (meters).
${\displaystyle A_{S}}$ = area of the reference CTS spectrum (abs cm^-1).
${\displaystyle A_{R}}$ = area of the reference CTS spectrum (abs cm^-1).
${\displaystyle P_{S}}$ = pressure of the sample CTS spectrum (mm Hg).
${\displaystyle P_{R}}$ = pressure of the reference CTS spectrum (mm Hg).

When multiple CTS spectra are available, assign to ${\displaystyle L_{S}}$ the mean of the single-spectrum ${\displaystyle L_{S}}$ results. The reference CTS spectrum pathlength and concentration used must be based on multiple, high-quality gas standards and physical length measurements (see Appendix D, Section 5). The analyst must document criteria for the selection of the analytical region and any baseline correction procedures employed.

B2 Residual Squared Area

Note: If the following calculations are performed during testing or as part of the QC procedures (steps 14 through 17), perform them using a workspace air spectrum instead of the “water vapor (absorbance) spectrum” described in the two paragraphs below.

Use the integration time selected for field testing in recording the spectra described below. Record a background spectrum of dry N₂ gas or zero air. Using a suitable impinger, humidify the N₂ or zero air stream and record a single-beam spectrum at an absolute pressure between 725 and 795 mm Hg. Form the absorbance spectrum of this water vapor sample from the single beam spectra. Assign the spectrum a unique filename and save it for the calculations described below.

From this water vapor spectrum, subtract a scaled spectrum (see Appendix A) formed from the water vapor reference spectrum to be used in subsequent quantitative analyses. The scaling factor may be varied to minimize the absorbance in the resulting difference spectrum in the various analytical regions to be used in the analyses. Subtract a constant offset, a linear function, or a quadratic function from the difference spectrum in each analytical region to form a residual spectrum R for each region. For each residual spectrum R with discrete absorbance value ${\displaystyle R_{i}}$ for i = p to q, in the wavenumber range ${\displaystyle w_{p}}$ to ${\displaystyle w_{q}}$, the residual squared area (RSA) is defined as: