Chemical Analyses of Particle Filter Deposits* - Aerosols: Measurement, Dosimetry, and Health Effects - page 187

a circular jig over the broad bandwidth incandescent light source that accommodates the ilter diam-
eter. The jig includes a stop for the light detector arm so that the photodetector is positioned at the
same distance above the ilter deposit, and the detector is calibrated with neutral density ilters
(Kodak, Rochester, NY). A new transmissometer (Sootscan OT21, McGee Scientiic Instruments,
Berkeley, CA) measures 47 mm ilters at 370 and 880 nm wavelengths simultaneously. Emissions
from the smoldering phase of biomass burning have higher absorption eficiencies at ∼350 nm than
those from higher temperature combustion (e.g., diesel engine exhaust have higher absorption efi-
ciencies at ∼880 nm), and the ratios of transmittance through the ilter at these wavelengths have
been used to distinguish contributions from pollution sources (Sandradewi et al., 2008). The optical
density (OD = ln(I b /I l ), where I b is the light intensity transmitted through a blank ilter and I l is the
light intensity transmitted through the loaded ilter) is converted to b abs (in inverse megameters,
Mm −1 ). This is not a true measure of light absorption in the atmosphere due to multiple scattering
within the ilter and changes in particle shape and orientation after ilter collection (Horvath, 1993),
but it is highly correlated (correlation R > 0.8) with BC (Chow et al., 2010c).
7.4.3 x-r ay F luorescence a nalysis For e leMents
Elements are important source markers, and some of them are believed to be toxic. XRF (Dzubay
and Nelson, 1974; Giauque, 1973; Grennfelt et al., 1971) irradiates the thin aerosol deposit on the
Telon membrane with photons of 1,000-30,000 eV generated by an x-ray tube. This removes inner
shell electrons, and a characteristic x-ray photon is emitted as an outer-shell electron drops to the
vacant lower-energy level. The XRF analyzer is calibrated with thin-ilm standards (Baum et al.,
1977; Billiet et al., 1980; Dzubay et al., 1977; Greenberg, 1979; Heagney and Heagney, 1979).
Characteristic peaks for each element must be separated from background and overlapping peaks;
and corrections must be made for x-ray absorption with the particles, deposit matrix, and ilter
(Watson et al., 1999). Telon membranes are preferred for these analyses because the deposit does
not penetrate as deeply into the ilter as with ibrous ilters.
Older XRF analyzers used a broad range of excitation energies (Watson et al., 1999), but newer
units (e.g., PANalytical Epsilon 5, Almelo, The Netherlands) use secondary targets to generate
excitation energies close to the absorption edges of four or ive elements. For the Epsilon 5, a side
window x-ray tube with dual scandium (Sc)/tungsten (W) anodes excites secondary x-rays from up
to 11 secondary targets (i.e., Al, Ca, Ti, Fe, Ge, Zr, Mo, Ag, Cs, Ba, and Ce), or an aluminum oxide
(Al 2 O 3 ) Barkla target (Heckel, 1995), which in turn emits polarized x-rays used to excite elements
in the sample. The luoresced photons are detected by a solid-state silicon (Si) or germanium (Ge)
detector. Each photon that enters the detector generates an electrical charge with magnitude propor-
tional to the photon's energy. Electrical signals from the detector are sorted into energy channels,
counted, and displayed. Table 7.1 shows an example of excitation conditions optimized for different
sets of elements. Analysis times, primary x-ray voltage and currents, and secondary targets can be
selected to minimize background and peak overlaps.
7.4.4 i nductively c ouPled P lasMa -M ass s PectroMetry
a nalysis For e leMents and i sotoPes
Inductively coupled plasma-mass spectrometry (ICP-MS) (e.g., Thermo Elemental X-7, Waltham,
MA) (Herner et al., 2006; Komarek et al., 2008; Ohno and Hirata, 2004; Polak et al., 2006) com-
plements, rather than replaces XRF, by obtaining lower detection limits for certain elements and
isotopes identiied in the caption of Figure 7.2. A portion of the acid-digested sample is nebulized
into a plasma torch, which ionizes the dissolved elements. These ionized components are directed
to a quadrupole MS where they are sorted by their mass-to-charge ratios. Ions are detected by an
electron multiplier and tallied for the elements sought. The electron multiplier response is calibrated
against known solution standards.