Tobias TBX - US Environmental Protection Agency.doc

DISCUSSION
of Procedure
This procedure outlines the method of light transmission analysis using the Tobias TBX-10 Densitometer.
Principle
This instrument is used to measure optical density (.):
. = log10 1
T
where T = light transmitted through a particle filter. The instrument is adjusted so T = (. = ) when no filter is present.
Measurement Interferences and Their Minimization
Densitometer readings are subject to measurement biases due to:
•Inhomogeneous deposits: the densitometer measures a small area in the center of each filter. Collection of samples using an in-line filter holder with insufficient collimation of the air flow before the filter will result in a heavier deposit in the center of the filters. Such inhomogeneities are avoided by using properly designed samplers and utilizing open face filters with sufficient collimation.
•Filter media thickness: variations among filters of a given type of substrate are insignificant, as the absorbance of the filter deposit is determined as a difference between initial and final OD readings. However, different filter media have various inherent absorbencies. For example, quartz fiber filters have a higher absorbance than Teflon membrane filters. The primary effect of such baseline differences is to push absolute absorbance readings higher. Such an effect may pose a problem if deposit absorbencies approach the detection limit of the instrument(. Densitometer). The use of Teflon membrane filters, with lower baseline absorbencies, will avoid such problems.
Ranges and Typical Values of Measurements
Ranges and typical values of . measurements can only be defined on a project-by-project basis, as the . values obtained depend on the density of deposit material (μg/cm2) on the filter substrate. The deposit density in turn is dependent upon the type of samples collected (ambient or source), concentrations of material sampled (μg/m3), total a