Thirty one laboratories of various EU Member States have participated
in two interlaboratory comparisons in order to assess errors of personal
sampling methods associated with both the sampling and the analytical
steps. In contrast to conventional quality control schemes, this project
particularly focuses attention on the sampling and identification step; it
is executed by means of sampling exercises and has included discussions on
potential sources of error. In a sampling exercise, participants come to a
central facility and perform measurements on synthetic workplace air in a
laboratory installation. Concentration levels of formaldehyde, acrolein,
glutaraldehyde and acetaldehyde between 0.1 and 2 times the limit value
for workplace air were prepared at various humidity levels and with
acetone, occasionally, as interferent. Sampling times varied from
1–4 h. The related analytical work is performed at the
analyst’s own laboratory. The intention is for each participant to
determine the observed value of the delivered standard atmosphere using
the sampling method of his own choice. Trueness (bias), precision and
relative overall uncertainty of each method–laboratory combination
is calculated and verified towards compliance with EN 482, which outlines
minimum performance criteria. The first challenge involved the precise gas
phase generation of the selected analytes in high air flows (up to 300 l
min
-
1
) and calculating the true value only by
direct reference to primary standards. This was accomplished by modifying
the capillary dosage injection technique so that reactive compounds, like
low molecular mass aldehydes, could be dosed with the same accuracy and
precision as unreactive solvents. A permeation tube with high emission
rate was developed for formaldehyde. Up to ten different sampling
techniques were evaluated. The measurement methods used by the majority of
the participants were based on pumped sampling on silica cartridges (or
tubes) and glass fiber filters, coated with 2,4-dinitrophenylhydrazine. It
was observed that for formaldehyde, and in some cases for acetaldehyde and
glutaraldehyde, the majority of the method–laboratory combinations
complied with an overall uncertainty of 30%. The results for acrolein,
however, indicated a systematic negative bias, often larger than minus 50%
of the true value, caused by the decomposition of the acrolein DNPH
derivative in the presence of excess acid and excess DNPH.
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