Chemical Analyses of Particle Filter Deposits* - Aerosols: Measurement, Dosimetry, and Health Effects - page 189

7.4.6 a utoMated c oloriMetry a nalysis For a MMoniuM
Automated colorimetry (AC) (Fung et al., 1979; Sandell, 1959) has long been used to quan-
tify water-soluble anions and cations by reacting them with reagents that create a colored solu-
tion. Light at a characteristic wavelength is directed through the solution and detected by a
photomultiplier. Beer's Law relates the liquid's absorbance to concentration of the ion in the
sample as determined by reactions with known solution standards. Modern ACs (e.g., Astoria
Analyzer Automated Colorimetric System, Astoria Paciic, Clackamas, OR) are automated to the
extent that solution vials can be placed in an autosampler from which the instrument retrieves
the desired volume, mixes it with appropriate reagents, quantiies the absorbance, and applies
the appropriate calibration curve. The sample can be split among different analysis modules to
obtain a variety of anions and cations, including Cl − , NO 3 − , SO 4 = , and NH 4 + . The method is espe-
cially applicable to large numbers of samples (Mueller et al., 1983).
As a complement to anion IC, AC applies the indophenol method (Bolleter et al., 1961; Rommers
and Visser, 1969) in which water-soluble NH 4 + is reacted with phenol and alkaline sodium hypo-
chlorite to produce indophenol, a blue dye. The reaction is catalyzed by the addition of sodium
nitroprusside, and absorbance is measured at 630 nm. The system determines carryover by analysis
of a low concentration standard following a high concentration. Formaldehyde has been found to
interfere with measurements when it is present in an amount that exceeds 20% of the NH 4 + content.
H 2 S interferes with measurements when it is present in concentrations that exceed 1 mg/mL. NO 3 −
and SO 4 = also potentially interfere when present at levels that exceed 100 times the NH 4 + concentra-
tion. These levels are rarely exceeded in ambient samples. Precipitation of the hydroxides of heavy
metals such as magnesium (Mg) and calcium (Ca) is prevented by addition of a sodium citrate/
sodium potassium tartrate buffer.
7.4.7 a toMic a bsorPtion s PectroPHotoMetry a nalysis For M onoatoMic c ations
Atomic absorption spectrophotometry (AAS) (Butler et al., 2009; Fernandez, 1989) has long been
used for individual elemental analysis, but the capability of ICP-MS makes it more attractive for
multi-element applications. Modern AAS units (e.g., SpectrAA 880 Double Beam AAS, Atomic
Absorption Spectrometer, Varian, Palo Alto, CA), however, are sensitive and cost-effective for a
small number of elements, such as water-soluble Na + , Mg ++ , K + , and Ca ++ . Water extracts of 1-2 mL
are aspirated into an air/acetylene lame at approximately 0.5 mL/min. A hollow-cathode lamp
emits wavelengths appropriate for each species: 589 nm for Na + , 285.2 nm for Mg ++ , 766.5 nm for
K + , and 422.7 nm for Ca ++ . Transmitted light is detected by a photomultiplier with and without the
presence of the sample, and the reduction of transmitted light absorbed at each characteristic wave-
length is related to the concentration via Beer's Law and quantiied by similar absorption for known
solution standards.
7.4.8 t HerMal /o Ptical a nalysis by r eFlectance and t ransMittance
(tor and tot) For o rganic and e leMental c arbon
OC and EC are important components of most combustion sources (Chow et al., 2004b, 2011a;
Sahu et al., 2011; Watson et al., 1994, 2001a). More than 20 methods (Watson et al., 2005) sepa-
rate OC from EC fractions by heating the sample to various temperatures, oxidizing the evolved
carbon to carbon dioxide (CO 2 ), and quantifying evolved CO 2 directly with an infrared absorp-
tion detector, or reducing it to methane (CH 4 ) for more sensitive carbon detection by a lame
ionization detector (FID). Although these methods produce comparable results for total carbon
(TC = OC + EC) (Watson and Chow, 2002), they provide different values for the division between
OC and EC (Schmid et al., 2001). Since EC is typically the smaller of the two, it is the least com-
parable among the methods.