Environmental Engineering Reference
In-Depth Information
sampling/analytical procedure may have an accuracy of 95%. This means
that repeated measurements indicate that the measured value deviates from
the true value by -5%, or 5% < true value. Another procedure may have an
accuracy of 110%;
i.e., on average the concentration is +10%, or 10% > true
value. A good sampling/analytical procedure will have an accuracy within
±10% of the true value.
Sampling/analytical procedures should also be rela-
tively precise. Precision in the scientific (rather than dictionary definition)
sense is the reproducibility of measured results. Precision indicates the rel-
ative variation around the mean. It is reported as a ± percentage around the
mean of multiple values determined from measuring the same known con-
centration. It is determined by calculating the coefficient of variation
b.
Precision.
---
CV
=
(
100
)
(9.1)
where
σ
= standard deviation
x = mean
Sampling/analytical procedures should have both high accuracy (within
±10% of the true value) and high precision. Acceptable precision values
depend on the technique employed. For many instrumental techniques,
±10% is desirable. For techniques such as gas sampling tubes and passive
samples, a precision of ±25% is generally acceptable.
Though accuracy and precision are scientifically well-defined concepts,
they are used interchangeably by the lay public as well as by technically
trained individuals. As a consequence, it is often difficult to communicate
their scientific meaning and relevance in environmental sampling.
In addition to acceptable accuracy and precision, it is
important to use sampling/analytical procedures that are sufficiently sensi-
tive to measure contaminant concentrations expected. Sensitivity is deter-
mined from reported limits of detection (LODs) for different sample sizes
and durations. The LOD varies with different analytical procedures. It can
often be extended (within limits) by increasing the volume of air sampled
into/onto sorbing media by increasing sampling duration. This cannot be
done on direct-read, real-time instruments.
c.
Sensitivity.
In most, but not all cases, a procedure should be specific
for the contaminant under test. This is especially true for gases and vapors.
Nonspecific techniques have diminished accuracy when two or more con-
taminants with similar chemical characteristics are present. Measured con-
centrations may be higher than they actually are. Such results can be char-
acterized as positive interference. In other cases they will be lower; therefore,
interference is negative. Interference with measured concentrations can also
d.
Specificity.
