Appen dix B: Gra vimetric An alysis Proc edure
Airmetrics
73
Becaus e the sam ple filters (both clean and expose d) are "co nditioned " in
the sam e room as the con trol filters, we assum e that the rela tive hum idity
affects the sample filters to the same extent as the control filters.
Note : the Cah n C-32 E lectrobala nce ma ss units are in milligram s. All
filter masses are, therefore, recorded in milligrams.
3.
Handle filter (sam ple filters:
F
) with stainless forceps and spatula. Pass
filter through field of static eliminators for a few seconds, then center on
balance pan. C lose door.
4.
Read and record the clean sa mple filter weight (clea n masses:
F
c
) only
when microbalance reado ut remains steady for a minimum of 20 seconds.
If filters have been equilibrating and the room relative humidity and
temperature have been fairly constant for a few hours, this should not
require over a minute or two. If the readout continues to drift for
excessive time, it may be necessary to put off weighing until equilibration
of the filters is complete or the room conditions stabilize.
5.
After recording the weight, load the filter in a holder above a drain disc,
fasten a numbered label to the holder, then cover with a sandwich bag
topped b y a plastic c ap. Loa ded hold ers are store d upright in racks to
prevent contamination.
Filters to be shipped or stored in bulk for sp ecial projec ts are loade d into
Petrislides with two numbered labels attached. W hen these filters are
loaded into holders, one label is attached to the holder as above, and the
other rem ains with the Petrislide so the exp osed filter ca n be return ed to
the correct slide.
6.
Genera lly, filters are se nt to the field lo aded into a tackle bo x fitted with
quick disconnect fittings. Field sampling reports or data sheets are sent
with each filter or set of filters (Figure 3).
7.
Filter heads or Petrislides received from the field are first checked against
the accompanying field sampling report for correct numbers, site and flow
information, then the plastic bags are removed and holders are placed
upright on racks for equilibration. The receiving date is logged on the
field report.
8.
After a m inimum of 24 hou rs equilibra tion, weig h each e xposed sample
Airmetrics MiniVol Users Guide
74
Airmetrics
filter (exposed ma sses:
F
e
) as in 1, 2, and 3 above, making certain the
three control filters for each pertinent lot are reweighed (control filter
masses be fore weighing exposed sam ple filters:
C
1ea,2ea,3ea
, and after
sampler filter weigh ing:
C
1eb,2eb,3eb
) at the same time.
9.
Store weighed samples in 47 mm Millipore® Petrislides with ID number
attached.
C.
Calculations and Reporting
1.
The gross m ass of particulate m atter on the samp le filter (
W
g
) is found by
subtracting the exposed filter mass from the clean filter mass:
The change in the weighs of the control filters between the clean and
exposed sample filter weighings (
)
C
) is calculated:
where
n
is the number of control filters used. While it is recommended
that three control filters be used, on occasion a control filter will become
unusable (soiled, torn, or otherwise damaged). In these instances the
number of available control filters will be less than three.
The net m ass of particulate m atter on the filter (
W
n
) is obtained by correc t-
ing the gross mass for the mass change o f the control filters:
These calculations and
W
n
, the filter sample or loade d weight, are
recorded in the appropriate filter weighing log book.
2.
The field s ampling report is use d to calcu late the air vo lume an d samp le
time using the lab portion below the dashed line. Start and stop air flows
are read from the latest calibration plot for the orifice used for the vacuum
