some loss of white phosphorus.
ticles could be used rather than the single 5.46-mg
Systematic error may occur when samples need
particle used in this study.
dilution. We found that the large dilution (by a
factor of 10,000) required to analyze these samples
Low concentration performance evaluation
introduced some systematic error, the magnitude
samples
of which varied with the manner in which the di-
Several methods were tried to produce a sample
with a stable low concentration (around 0.001 g/
lutions were performed. The most accurate results
were obtained using serial dilution (1 to 100, 1 to
g) of white phosphorus. Initially we tried to simu-
100) using 1.00-mL pipets and 100-mL volumetric
late the mechanism by which soils are contami-
flasks. (Calibration standards were also made by
nated under field conditions, that is, by exposure
serial dilution.) Direct dilutions using microliter
to water containing dissolved or colloidal white
syringes were precise, but not accurate, and re-
phosphorus. The first method attempted was the
sulted in an overestimation of concentration. For
spiking of wet soil with an aqueous solution of
example, if 1 L of solution was measured in a
white phosphorus. This solution was produced by
glass microliter syringe and added to 10.0 mL of
agitating reagent grade water with solid pieces of
solvent by simply depressing the plunger of the
white phosphorus. The concentration of white
syringe, the resulting concentration was 108% of
phosphorus in the solution was determined by
the expected concentration. However, if the
isooctane extraction and gas chromatography to
plunger of the syringe was withdrawn and de-
be approximately 0.5 mg/L. Despite a total of three
pressed such that the needle was rinsed into the
weekly additions of spike solution, for a total of
1.5 g of white phosphorus per sample, white
diluted solution, the resulting concentration was
217% of the expected concentration. This large er-
phosphorus was not detectable after one week of
ror shows the importance of proper technique
storage at room temperature in 120-mL jars. Loss
when performing dilutions.
of white phosphorus may have been due to vola-
Since serial dilutions require large amounts of
tilization or chemical reaction. To eliminate
solvent, laboratories may chose to do direct dilu-
volatilization, another set of wet soils were spiked
tions. If so, the magnitude of the systematic error
and sealed in ampoules with minimal headspace.
should be evaluated by each laboratory. We per-
This method slowed the rate of loss (Table 2a), yet
formed direct dilutions for all the samples in these
after two weeks of storage the mean concentra-
experiments, and multiplied the result by a cor-
tion of white phosphorus was only 2% of the ini-
rection factor based on the estimates obtained by
tial mean concentration.
a serial dilution of a subset of samples.
Next we added a 12-g piece of solid white phos-
At the last sampling time, the white phospho-
phorus to a kilogram of wet soil and gently agi-
rus particles were isolated for visual examination
tated the sample for several days. The piece of
from the left-over samples. The particle diameters
white phosphorus was removed, and then the wet
were unchanged, but the surfaces of the particles
soil dispensed, either into 22-mL vials (no
were not longer translucent. Rather, the surfaces
headspace) or 10-mL flame-sealed ampoules
were dull white, indicating that some surface oxi-
(minimal headspace). This method resulted in
dation had occurred. Surface oxidation occurs
white phosphorus soil concentrations much
when solid white phosphorus is stored under
greater than desired, and greater than what would
water (Nikandrov and Smirnov 1983).
be expected if the water in the samples was satu-
Particles were sliced to examine the interior of
rated with white phosphorus (Table 2b and 2c).
the particles. Inside the very thin coating, the par-
These high concentrations indicate that small
ticles were translucent, and showed no evidence
particles of white phosphorus may have formed
of change. Loss of WP in only the outer 0.01 mm
due to abrasion from the soil particles during agi-
would be equivalent to the loss of 0.2 mg, which
tation. The heterogeneity between subsamples on
is similar to the magnitude of the loss in some
day 0 also indicates that small particles of white
samples. Once formed, a surface oxidation layer
phosphorus may have been formed. After storage
should slow any additional changes.
at room temperature, white phosphorus concen-
When used for quality assurance, the concen-
trations declined in these samples. Still detectable
trations of performance evaluation samples should
after over 200 days of storage, the remaining white
be unknown to the analyst. To vary the concentra-
phosphorus concentration was less than 1% of the
tion of samples spiked with white phosphorus
mean day 0 concentration (Table 2b and 2c).
particles, various size particles or multiple par-
After of the first two methods of spiking failed
4
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