2. 120-mL (Nalgene or equivalent) plastic
9.0 QUALITY CONTROL
9.1 GENERAL: The accuracy and precision of
3. Volumetric pipets: one each--10, 8, 6, 4, 2
this method are subject to the common errors as-
sociated with poor-quality measurements of
4. Volumetric flasks: 100-mL (two).
weights and volumes. There are a few sources of
5. 100-mL graduated cylinder.
error specific to this method. The binding and re-
6. 10-mL graduated cylinders (two).
lease of picrate in the solid-phase extraction (SPE)
7. Disposable eyedroppers or pasteur pipets
tube is flow-rate dependent. The flow rates speci-
(one for sulfuric acid and one for each
fied in the method (Procedure Steps 5 and 10)
should be implemented carefully. The removal of
8. Millex SR filter units, 0.5 m, two per
residual water from the spectrophotometer cu-
vettes (Procedure Step 17) is critical for successful
9. Glass cuvettes, 1-cm path length.
application of the background correction calcula-
10. Disposable plastic syringes: 60-mL (one
tion. The manufacturer's instructions for the SPE
per sample), 10-mL (two per day for acidi-
tubes call for a conditioning rinse. This step is not
fied acetone and rinses).
necessary. The tubes may be used directly from
11. 22-mL glass vials with caps (one per
9.2 INITIAL DEMONSTRATION OF PERFOR-
12. Supelclean Alumina-A, solid-phase ex-
MANCE: An initial calibration curve for the
traction (SPE) tubes, 3 mL (Supelco
method should be performed as directed in CALI-
57082u) (one per sample).
BRATION AND STANDARDIZATION. The cali-
13. Teflon-coated stir bar (10 15 mm).
bration curve should be linear, with a zero inter-
cept and a slope (response factor) of 90 15. The
14. 500-mL squirt bottles (three).
linear range of the method is 0.069 g/g.
15. Bottle-top dispenser for acetone (highly
9.3 ASSESSING PERFORMANCE: A method
16. Timer or wristwatch to measure flow rates.
detection limit (MDL) analysis should be per-
formed. A 150-g sample of blank soil is spiked
with 6.0 mL of the 125-mg/L working standard to
produce a picric acid concentration of 5 g/g. The
7.0 REAGENTS AND STANDARDS
soil is homogenized as completely as possible.
Picric acid (SARM or reagent grade).
Seven 20-g subsamples are processed according
Acetone, commercial grade.
to the method. The concentration of picric acid in
Methanol, reagent grade.
the subsamples is determined using the initial cali-
Concentrated sulfuric acid, reagent grade.
bration curve. The standard deviation (SD) of the
seven determinations is calculated. The MDL =
3.14 SD and should be about 1.3 g/g.
The recovery of the spiked picric acid should
be between 75 and 125%.
8.0 SAMPLE COLLECTION,
9.4 ASSESSING ANALYTE RECOVERY AND
PRESERVATION AND STORAGE
DATA QUALITY: Blank and spiked soil samples
This method may be used with field-moist or
should be analyzed daily as directed in CALIBRA-
dried soil samples.
TION AND STANDARDIZATION and records
A soil sample is mixed as thoroughly as pos-
kept. The background-corrected absorbance of a
50-g/g soil spike should be above 85% of the
sible and a 20-g subsample added to a 250-mL
plastic bottle containing three 1/4-inch steel ball
bearings. The bottle is capped until extraction is
according to the directions for the daily calibra-
conducted. The samples should be keep cold (4C)
tion of response factor. The blank soil should pro-
and in the dark until extraction takes place.
duce a background-corrected absorbance less
Samples should be analyzed the same day they
than 0.01 absorbance units. The relative stan-
dard deviation of daily matrix spikes should be
less than 15%.