Chemical Analyses of Particle Filter Deposits* - Aerosols: Measurement, Dosimetry, and Health Effects - page 186

to room temperature, and diluted to 50 mL with high-purity DDW. The samples are then capped,
shaken for 10 s, and aged for 12 h prior to metal analyses.
7.4 LABORATORY ANALYSES
Laboratory analyses designated in Figure 7.2 are briely described in the following subsections.
The methods described here were selected because they can be practically and cost-effectively
implemented for a large number of samples using commonly available sampling systems. Each
one is implemented following a standard operating procedure (SOP) that includes (1) summary of
measurement method, principles, expected accuracy and precision, and the assumptions for valid-
ity; (2) materials, equipment, reagents, and suppliers; (3) individuals responsible for performing
each part of the procedure; (4) traceability path, primary standards or reference materials, toler-
ances for transfer standards, and schedule for transfer standard veriication; (5) start-up, routine,
and shutdown operating procedures and an abbreviated checklist; (6) data forms; (7) routine main-
tenance schedules, maintenance procedures, and troubleshooting tips; (8) internal calibration and
performance testing procedures and schedules; (9) external performance auditing schedules; and
(10) references to relevant literature and related SOPs. Although not mentioned in every subsec-
tion, each method requires periodic analyses of calibration standards, performance standards, audit
standards, solution blanks, ilter blanks, and sample replicates. These are used to identify issues that
arise during analysis and to quantify the accuracy, precision, and validity of the individual sample
concentrations.
7.4.1 F ilter w eigHing g raviMetry For M ass
The mass of the deposit is determined as the weight difference between the exposed and unexposed
ilter. Filter weighing should be performed on an electro-microbalance (e.g., Mettler XP6; Toledo,
OH) with ±0.001 mg sensitivity (Allen et al., 2001; Feeney et al., 1984; Lawless and Rodes, 1999;
Swanson and Kittelson, 2008; Yanosky and MacIntosh, 2001). Unexposed and exposed Telon-
membrane ilters are equilibrated at 21.5°C ± 1.5°C and 35% ± 5% RH for a minimum of 24 h prior
to weighing (Brown et al., 2006; Kajino et al., 2006; McInnes et al., 1996), as speciied by U.S. EPA
(1997) for PM 2.5 compliance monitoring. These equilibration conditions appear to reduce variability
to acceptable levels (Chow et al., 2006), although there is evidence that liquid water is still retained
when water-soluble compounds dominate the PM deposit. The charge on each ilter is neutralized
by exposure to a polonium source (Po 210 ) for 60 s prior to the ilter being placed on the balance pan
(Engelbrecht et al., 1980; Swanson and Kittelson, 2008; Tsai et al., 2002). The balance is zeroed
and spanned with class 1 weights (50, 100, and 200 mg) prior to weighing each batch of ilters. After
every 10 ilters are weighed, the calibration and tare are rechecked. The ilters should be reweighed
by a separate technician. Since the initial weight can never be recovered, 100% reweighing of the
unexposed ilters is a good insurance policy. An unexposed ilter should be reweighed when the
deviation exceeds the tolerance of ±10 μg/ilter for a 47 or 37 mm diameter ilter. For exposed ilters,
30% reweighing is recommended, and an entire batch should be reweighed when deviations exceed
±15 μg/ilter. If pre- and postexposure weighing are performed at locations with different elevations,
there must be accounting for the change in ilter buoyancy, which is ∼20 μg for a 1524 m (5000 ft)
elevation change (Rasmussen et al., 2010).
7.4.2 l igHt t ransMission /a bsorPtion ( b abs ) a nalysis as a b lack c arbon s urrogate
Due to the potential effects of BC on climate (Chow et al., 2010d; Jacobson, 2002), the b abs measure-
ment is being used more frequently as a surrogate for BC measurements. This is most conveniently
done as an accompaniment to the pre- and postexposure Telon-membrane ilter weighing. A pho-
tography densitometer (e.g., Tobias TBX-10, Ivyland, PA) can be adapted to this purpose by afixing