6 research outputs found
U.S. Geological Survey External Quality-Assurance Program Results Reported to the National Atmospheric Deposition Program/National Trends Network and Mercury Deposition Network for 2005-06
The U.S. Geological Survey (USGS) used seven distinct
programs to provide external quality-assurance monitoring
for the National Atmospheric Deposition Program /
National Trends Network (NTN) and Mercury Deposition
Network (MDN) during 2005-06. Overall variability of NTN
data was estimated using a co-located-sampler program.
Variability and bias of NADP chemical analysis data were
estimated using two separate interlaboratory-comparison
programs, one for each network. Bias in MDN sample
analysis data for total mercury (Hg) was evaluated using
a blind-audit program. A separate blind-audit study was
conducted in 2006 for the NTN chemical analysis data to
evaluate laboratory analytical detection limits. The sensitivity
of NADP measurements was evaluated using a field-audit
program for NTN and a system blank program for MDN.
Overall measurement sensitivity and laboratory analysis
sensitivity are evaluated by comparison to data-quality
objectives (DQOs) for each network. Field-audit results for
2005-06 indicate that DQOs for NTN overall measurement
sensitivity were met during 2005-06. Network maximum
contamination levels (NMCLs) have been increasing
in NTN samples since 2005 for all constituents except
magnesium, sulfate, and hydrogen ion, but increases in
3-year moving average NMCLs between 2002 and 2006
were less than the 10 percent. NMCLs determined from
the field-audit data are compared to analytical detection
limits to evaluate sample analysis sensitivity. Although
the DQO for NTN sample analysis sensitivity was not
attained for magnesium, ammonium, chloride, nitrate,
and sulfate in selected 3-year time periods prior to 2005,
the DQO was met for all constituents during 2005-06.
Interlaboratory-comparison program results indicate
variability and bias in NTN data are low relative to data
from other participating laboratories. Central Analytical
Laboratory (CAL) sulfate and hydrogen-ion data were
slightly positively biased during 2005, and CAL potassium
and hydrogen-ion data were slightly positively biased during
2006. CAL analyses of deionized-water samples indicated
possible low-level potassium contamination in NTN samples
during 2005-06. CAL data were within statistical control
during at least 90 percent of 2005-06 with few exceptions.
CAL analyses of synthetic wet-deposition solutions were
within +10 percent of most probable values (MPVs) except
for selected sodium and potassium analyses. A blind-audit
study, in which the CAL analyzed quality-control samples
disguised as real samples, confirmed CAL’s reported detection
limits. In fact, CAL detection limits for nitrate and
sulfate might actually be lower than reported by CAL. ...
Three pairs of co-located samplers were used to
estimate overall variability of NTN wet-deposition
measurements in terms of median absolute error (MAE).
MAEs were less than 15 percent for nitrate and sulfate
concentrations, specific conductance, and collector catch
for water year 2005 (WY05), whereas MAEs for these
analytes were less than 8 percent for WY06. Consistent
with co-located sampler data collected during previous
water years, MAEs for cations were higher than for
anions. Median absolute difference (MAD) values for the
co-located samplers indicated that NADP DQOs for uncertainty
were met for most analytes during WY2005-06.
For the MDN system-blank program, the median
system-sample minus bottle-sample Hg concentration difference
was 0.027 nanogram per liter (ng/L), which is 21 percent
of the HAL method detection limit (MDL). The calculated
NMCL for the combined 2005 and 2006 system-blank
differences is approximately 0.42 ng/L, which is an order of
magnitude higher than the 2004 NMCL of approximately
0.04 ng/L. This implies that contamination levels in MDN
samples might have increased during 2005-06 as compared
to 2004. In response, USGS submitted followup diagnostic
samples to either confirm or reject this conclusion. The diagnostic
results failed to confirm an increase in MDN sample
contamination. The NMCL (0.42 ng/L) is less than the 10th
percentile of all 2004-06 MDN data, and thus the draft DQO
for Decision Rule 1 for sensitivity has been met. The NMCL
is more than 2 times the MDL (0.13 ng/L) reported by the
HAL for 2006. Therefore, the sensitivity of the HAL analytical measurements is acceptable per the draft DQO Decision
Rule 2. DQO decision rules are described in the report.
For the MDN interlaboratory comparison program,
HAL reported data with the lowest variability among
the participating laboratories during 2005 but had the
second highest variability during 2006. The median difference
between HAL-reported concentrations and MPVs
was small (-0.25 ng/L) during 2005, and no significant
(α=0.05) bias was detected. During 2006, however, the
HAL data were negatively biased. Control charts were
used to show that HAL reported four values (less than
4 percent of all results) outside statistical control.
The 2005 median Hg concentration for HAL blanks
(0.94 ng/L) is approximately 11 percent of the median
result of 8.80 ng/L for all valid 2005 MDN samples associated
with measurable wet deposition. The 2006 median Hg
concentration for HAL blanks (0.28 ng/L) is approximately
3 percent of the median result of 9.50 ng/L for all valid 2006
MDN samples associated with measurable wet deposition.
The MDN blind-audit program officially began
during 2006 to evaluate accuracy of HAL Hg analyses.
The median total Hg percent recovery for 13 blind-audit
samples was 97 percent with an f-pseudosigma (nonparametric
standard deviation analogue) of 11 percent. Sample
volume, which affects how much Hg mass is available for
analysis, did not correlate with Hg percent recovery.
Overall, the external quality-assurance program
results indicate that HAL analytical performance meets
MDN DQOs. Overall MDN measurement sensitivity
also meets DQOs and is adequate to distinguish
between measurement noise and environmental signals.published or submitted for publicationis peer reviewe
U.S. Geological Survey External Quality-Assurance Project Report to the National Atmospheric Deposition Program/National Trends Network and Mercury Deposition Network, 2007-08
The U.S. Geological Survey (USGS) used six distinct programs to provide external quality-assurance monitoring for the National Atmospheric Deposition Program / National Trends Network (NTN) and Mercury Deposition
Network (MDN) during 2007-08. The field-audit program assessed the effects of onsite exposure, sample handling, and shipping on the chemistry of NTN samples, and a system-blank program assessed the same effects for MDN. Two interlaboratory-comparison programs assessed the bias and variability of the chemical analysis data from the Central Analytical Laboratory (CAL), Mercury (Hg)
Analytical Laboratory (HAL), and 12 other participating laboratories. A blind-audit program was also implemented for the MDN to evaluate analytical bias in HAL total Hg
concentration data. A co-located-sampler program was used to identify and quantify potential shifts in NADP data resulting from replacement of original network instrumentation with new electronic recording rain gages (E-gages) and prototype precipitation collectors. The results indicate that NADP data continue to be of
sufficient quality for the analysis of spatial distributions and time trends of chemical constituents in wet deposition
across the U.S. NADP data-quality objectives continued to be achieved during 2007-08. Results also indicate that retrofit of the NADP networks with the new E-gages is not likely to create step-function type shifts in NADP precipitation-depth records, except for sites where annual precipitation depth is
dominated by snow because the E-gages tend to catch more snow than the original NADP rain gages. Evaluation of prototype precipitation collectors revealed no difference in sample volumes and analyte concentrations between the original NADP collectors and modified, deep-bucket collectors, but the Yankee Environmental Systems, Inc. (YES) collector
obtained samples of significantly higher volumes and analyte concentrations than the standard NADP collector.published or submitted for publicationis peer reviewe
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