The Directive describes
the requirements for assessment and monitoring of benzene in air.
The position paper
related to benzene (see web section below) describes the assessment of benzene
in air in Europe presently, sources, risks and measurement methods.
Monitoring methods are
generally based on gas chromatography, with either active (pumped) or passive
(diffusive) sampling and thermal or solvent desorption
prior to analysis. Automated instruments with active (pumped) sampling and
thermal desorption are available, which allow for
quasi continuous monitoring. Advantages and disadvantages as well as typical
uncertainties of the different methods are given. Pumped sampling on tubes
followed by GC analysis including calibration is recommended as basis for a
reference method. A detailed reference method will be worked out by CEN.
Standard measurement
methods are being developed by CEN (under approval, as of December 2004):
·
prEN 14662-1: Pumped sampling followed by thermal desorption and gas chromatography;
·
prEN 14662-2: Pumped sampling followed by solvent desorption and gas chromatography;
·
prEN14662-3: Automated
pumped sampling with in situ gas chromatography;
·
prEN 14662-4: Diffusive sampling followed by thermal desorption and gas chromatography;
·
p2EN 14662-5: Diffusive sampling followed by solvent desorption and gas chromatography.
Brief description of
methods:
On-line gas
chromatographs are available either as BTX-monitors (Benzene, Toluene, Xylene), measuring benzene,
toluene, ethyl benzene and xylenes or more capable of
measuring C2-C10 hydrocarbons. These instruments are based on the same
principle.
Canister sampling is
performed in two ways either as grab sampling or as pumped sampling. The grab
sampling is carried out by opening an evacuated canister, which instantaneously
fills the canister with ambient air up to ambient pressure. Alternatively, air
can be pumped into the canister over time in order to obtain an integrative
sample. The canisters are then brought to the laboratory where they are
analysed by gas chromatography (GC).
Pumped sorbent tube sampling is performed by pumping ambient air
through a tube filled with a sorbing material. The
trapped benzene is removed by solvent extraction or thermal desorption
followed by GC analysis.
Diffusive sampling of
benzene is performed by placing benzene adsorbent in a glass or metal tube. The
sampler collects benzene by diffusion (following Fick's
first law) due to the gradient established between ambient air and the
adsorbing material. Benzene is removed from the sampler by solvent extraction
or thermal desorption and in both cases benzene is
analysed by GC.
DOAS (Differential
Optical Absorption Spectroscopy) is an open path.
Optical
measuring technique applicable for a number of gases which includes benzene. This method has so far not been through a standardization
procedure.
Quality control of
monitoring data
Annex VIII of the 1st
and 3rd Daughter Directives (see web links below) specify the
required accuracy of measurement data, as well as the minimum data capture
(parts of the year the measurement data must be available from a monitoring
station). These requirements are laid down as a guide to what quality-control
and –assurance (QA/QC) programmes that monitoring network operators need to
follow to comply with the quality objectives. QA/QC procedures are dealt with
in the Topic How to secure the quality of the monitoring data? Quality assurance (QA) and Quality Control (QC) systems and procedures..
|