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SOILS VOC ANALYSIS – COMPARISON OF EPA
METHODS 8260C & 8261A - NEW
Ref: http://www.epa.gov/esd/chemistry/vacuum/reference/pubs.htm
Hiatt, M., “The Role of Internal Standards and their Interaction with Soils Impact Accuracy of Volatile Organics Determinations,"
Int.J.of Environ.Anal.Chem.,2010 90:8 591-604.
IMPORTANT CONCLUSIONS
- Equilibration
of internal standards (IS)with the soil is NOT required in both the
methods.
- Adding IS to the soils prior to adding water to the
sample, gives more accurate
determination but less
reliable quality control.
- With increase in Total Organic Carbon (TOC) and no
equilibration of IS with the matrix – results are less likely to be
accurate.
- Extending times for equilibration of internal standardsimproves
accuracy but is
conducive to analyte degradation not normally observed during analyses.
- Soil-matrix effects on a given analyte can be greatly
understated using a single internal standard as described in Method
8260C,
while the use of multiple ISs as described in Method 8261A is more
accurate.
- Method 8261A’s reporting error when spiking soils
before adding water provides confidence intervals with accuracy near the
experimental rule (75.2, 95.7 and 99%) with the exception of two
analytes that
require overnight equilibration.
- “Vacuum distillation had been found to be more efficient in
extracting volatile analytes from soil than headspace, ambient
purge-and-trap,
and heated purge-and-trap techniques [10]”.
EXPERIMENTAL CONDITIONS
GC/MS:
- The Vacuum Distiller is interfaced to a GC/MS so that
the vacuum distillate is transferred directly to the GC/MS for analysis
after
distillation.
- GC/MS used was Thermo DSQ mass spectrometer and Trace
GC.
- The GC capillary column was a 30 m x 0.25 mm i.d., 1.5
µm film VOCOL (Supelco, Bellefonte, PA)
- The GC operating conditions were 2.5 min at -20C,
40C/min ramp to 60C, 5C/min ramp to 120C and held at 120C for 1 min,
20C/min
ramp to 220C and held for 12 min resulting in a GC run time of 34 min.
- The injection was split 60:1 with a constant flow rate
of 1.4 ml/min.
- The mass spectrometer scanned between 35 and 300 amu at
1 scan/sec
Vacuum Distiller:
- A Cincinnati Analytical Instruments Model VDC1012
vacuum Distiller (Indianapolis, IN) performed the distillations in this
study.
- Samples were vacuum distilled for 7.5 min with a 2.5
min transfer to the GC/MS through a transfer line held at 200C.
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