Chemistry Reference
In-Depth Information
Whereas most of the relevant inorganic elemental
species are commercially available (e.g., selenite and
selenate), there is often a lack of organoelemental spe-
cies such as selenosugars. There is also a total lack of
isotopically labeled elemental species needed for the
species-specifi c isotope dilution technique, which
contains the elemental species to be determined in a
different isotopic composition from that of the sam-
ple. The synthesis of the corresponding spike must
be carried out in the laboratory. The spiked calibrant
is added to the sample before all analytical sample
treatment steps. This method will allow coping with
any loss of substance, because the isotopic ratio of the
isotope diluted elemental species does not change. It
also guarantees a similar response to that of the ana-
lyte from the sample. When no spiked calibrant is
available, external calibration in ICP-IDMS is still very
interesting with a species-unspecifi c standard added
continuously just before the detection. Such a spike
does not allow for any losses during the separation
procedure, but it allows correction of any variation in
response of the analyte in, for example, the eluent of
the LC. Because the amount of spike is constant, any
variation in response of the spike will be used to recal-
culate the response of the analyte from the sample.
This assumes that detector systems working at high
temperatures, such as those with inductively coupled
plasma excitation, should not show signifi cant differ-
ences in the detector response for various elemental
species (Heumann
et al.
, 1998).
For determining total element and even some ele-
mental species in matrices of toxicological importance,
a relatively wide choice of CRMs is available. There are
several serum, blood, urine, tissue, water, working place
dust, and environmental matrices issued by internation-
ally recognized bodies. The complete inventory of these
products can be found on the web pages of these insti-
tutes: EVISA (European Virtual Institute for Speciation
Analysis), IAEA (International Atomic Energy Agency),
IRMM (Institute for Reference Materials and Measure-
ment of the European Commission), NIST (National
Institute for Standards and Technology of the USA),
NRCC (National Research Council Canada), NIES
(National Institute for Environmental Studies in Japan),
and VIRM (Virtual Institute for Reference Materials).
When choosing a reference material, care should be
taken to not only use a matrix that is the same or very
similar to the matrix of the sample to be monitored but
also that the concentration of the analyte in the RM is
within the concentration range expected during the
analysis of the actual sample. For example, a liver ref-
erence material with a concentration of an element in
the mg/kg
−1
range is not suitable as a reference mate-
rial for analysis of blood or urine where the concentra-
tion of the analyte is in the
µ
g/L
−1
range.
13 QUALITY ASSURANCE
Because the analyst provides scientifi c evidence for
taking important decisions, he or she has to assure that
all actions have been taken to ensure that any factor that
has an infl uence on the fi nal result has been considered
and reported in a permanent record (Prichard, 1995).
This should cover the several steps related to procedures
before analysis (e.g., sampling, avoidance of contamina-
tion or losses during sampling, storage of the sampled
material, that is, preanalytical quality assurance). Chem-
ical changes of a substance after sampling may cause
problems if the specifi c chemical form of a substance is to
be monitored. Recording and treatment of data are to be
included in both the preanalytical and analytical phase.
It must also be the policy to deliver analytical results
that are fi t for purpose. This implies a thorough discussion
with the end user of the results before starting the analy-
sis. It is vital to understand why the work is being done,
what will happen to the results, and what decisions will
be taken, depending on the reported numerical values.
It goes without saying that the analyst should make the
measurements to the best of his or her abilities. The high-
est level of precision is, however, not always required.
The result should be precise and accurate enough for the
intended purpose. Every result should carry an estimate
of the uncertainty of the value.
12 REFERENCE MATERIALS
The use of reference materials is the major monitor-
ing tool for the performance of all analytical work. The
ISO (International Organisation for Standardisation)
Guide 30 gives the following defi nitions for reference
materials (ISO, 1991).
Reference material (RM)
: a material or substance
one or more of whose property values are
suffi ciently homogeneous and well established
to be used for the calibration of an apparatus,
the assessment of a measurement method, or for
assigning values to materials.
Certifi ed reference materials (CRM)
: a reference mate-
rial, accompanied by a certifi cate, one or more
property values are certifi ed by a procedure
which establishes traceability to an accurate
realization of the unit in which the property
value is accompanied, and for which each
certifi ed value is accompanied by an uncertainty
at a stated level of confi dence.
