Principles of Sampling and Analysis - Particulates

Particulate monitoring is usually accomplished with manual measurements and subsequent laboratory analysis. A particulate matter measurement uses gravimetric principles. Gravimetric analysis refers to the quantitative chemical analysis of weighing a sample, usually of a separated and dried precipitate. In this method, a filter-based high-volume sampler (a vacuum- type device that draws air through a filter or absorbing substrate) retains atmospheric pollutants for future laboratory weighing and chemical analysis. Particles are trapped or collected on filters, and the filters are weighed to determine the volume of the pollutant. The weight of the filter with collected pollutants minus the weight of a clean filter gives the amount of particulate matter in a given volume of air. Chemical analysis can be done by atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), inductively couple plasma (ICP) spectroscopy, and X-ray fluorescence (XRF) spectroscopy.

Figure:Atomic absorption spectrometry

Atomic Absorption Spectrometry (AAS) has been the most used method for the determination of single elements in analytical samples. AAS is a sensitive means for the quantitative determination of more than 60 metals or metalloid elements. This technique operates by measuring energy changes in the atomic state of the analyte. An analyte is a substance whose chemical composition is to be determined by chemical analysis. For example, AAS is used to measure lead in particulate monitoring. After lead particles are collected by gravimetric methods in a Teflon (PTFE) filter, lead is acid-extracted from the filter. The aqueous sample is vaporized and dissociates into its elements in the gaseous state. The element being measured, in this case lead, is aspirated into a flame or injected into a graphite furnace and atomized. A hollow cathode or electrodeless discharge lamp for the element being determined provides a source of that metal's particular absorption wavelength. The atoms in the unionized or "ground" state absorb energy, become excited, and advance to a higher energy level. A detector measures the amount of light absorbed by the element, hence the number of atoms in the ground state in the flame or furnace. The data output from the spectrometer can be recorded on a strip chart recorder or processed by computer. Determination of metal concentrations is performed from prepared calibration curves or read directly from the instrument.