Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
clearly presented as part of the QA/QC procedures. The term "lower
limit of detection" (LLD) may refer to very different concepts. For
example, the instrumental detection limit is defined as a multiple of
the noise level of the analytical instrument, usually a factor of 3.
However, when sampling and analysing environmental media, the
instrumental detection limit constitutes only a fraction of the true
limit of detection. The LOD takes into consideration the response of
the analytical instrument to the specific analyte in standards of
known concentration or, preferably, a matrix similar to that of the
sample. Since the response of the instrument to repeated analysis of
the same analyte concentration varies, the LOD is estimated as a
function of the standard deviation (SD) of the repeated analysis,
typically as t n-0.01 × SD, where t is the value of the Student
t distribution with n-1 degrees of freedom, alpha = 0.01 is the
type I error and n the number of repeated analyses, usually 7.
However, sampling methods can have a background of one or more of
the target analytes. For example, when using solid sorbent for air
sampling, the sorbent may contain varying levels of background
contamination across samples (e.g., varying benzene concentrations in
Tenax). The method detection limit (MDL) includes the background
contamination in the sampler. It is determined using a similar
statistical approach as the LOD, but using field and/or laboratory
blanks instead of the known analyte concentrations. The MDL can be
defined as the analyte concentration at which we have a given
certainty (e.g., 1-alpha = 0.99) that the sample concentration differ
from background. Finally, the lower quantification limit (LOQ) is
defined as 10 SD, where SD is derived from the LOD determination. LOD,
MDL and LOQ differ across laboratories using the same methods. More
importantly, they vary over time for the same laboratory.
The treatment of values below the MDL in statistical analysis of
the data can have a strong impact on estimates of exposure
distribution parameters. The reader may refer to Gilbert (1987), for
more specific information. There also are upper limits of detection,
defined by the range of linear response of the analytical method. In
that case the detector may be overloaded with the result that it is
not working in its linear detection range.
11.5.2 Internal quality control
The purpose of internal QC is to document that the method is in
statistical control and without systematic errors so that the observed
sampling results consistently fall within established control limits.
Internal QC procedures are used primarily for the control of
analytical precision (Taylor, 1988).
11.5.2.1 Control charts
Sometimes changes in the analytical performance are not abrupt
but take place gradually ("drift"). Such gradual changes are difficult
to perceive from a single central result, but many become evident with
time if the results of control samples are graphically displayed with
respect to time or sequence of measurements.
Control limits are used as criteria for signalling the need for
action, or for judging whether a set of data does or does not indicate
a state of statistical control. Control limits can be based entirely
on the data from the samples, in which case the chart illustrates
whether the method or procedure is repeatable. If the control chart
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