S.O.P. No. MLD 034 - Determination of Elemental Concentrations in
Ambient Air by
Energy-Dispersive X-Ray Fluorescent Spectroscopy
This document describes a non-destructive method for the determination of concentrations of elements in air particulates. All non-volatile elements with atomic numbers from 11 to 92 may be detected by the Air Resources Board’s current XRF instrument. The XRF method does not distinguish oxidation states; hence only total elemental concentrations are measured.
SUMMARY OF METHOD
A sample of air is drawn through thin Teflon® membrane filters over a period of twenty-four hours. The sampled volume is about 17 m3 for low-volume total suspended particulate or “total-metal” samples and about 10 m3 for PM2.5 samples.
The suspended particles collected by the filters are placed under vacuum in the XRF spectrometer and irradiated with x-rays of various energies. These (primary) x-rays generate fluorescent x-rays, which have characteristic energies for each element in the sample. The fluorescent x-rays, along with primary x-rays scattered from the sample, are individually detected and converted to electrical pulses. These electrical pulses are stored and utilized. Net counts of the characteristic fluorescent x-rays are assumed proportional to elemental surface concentrations (i.e., surface densities in units of µg/cm2). The sample is assumed to be distributed uniformly on the filter.