Prior
to the early 1990s, attempts were made to quantify odour using
dynamic olfactometry. However, the methods available at that
time for estimating odour concentrations were generally regarded
as being too subjective. Since then, the introduction of improved
instrument calibration, the adoption of n-butanol as a reference
material, and improved panel screening procedures have made
it possible to measure odour concentration objectively. The
performance of an odour laboratory now can be assessed in terms
of measurement accuracy (bias and repeatability) in relation
to an agreed reference material such as n-butanol.
Accuracy of a measurement method
is described by two terms, "trueness" and "precision" (ISO 5725):
- Trueness
(bias) is defined as the closeness of agreement between a
test result and an accepted reference value and may be investigated
by comparing an accepted reference value with the level of
the results given by the measurement method.
- Precision
(repeatability and reproducibility) involves the random errors
inherent in every measurement procedure. Precision describes
how close repeated measurements are to each other. While the
term "repeatability" is used to describe precision in the
same laboratory under repeatability conditions, the term "reproducibility"
is used to describe precision between laboratories.
At standard
conditions for olfactometry, the reference value corresponds
to a butanol concentration of 0.040 mmol/mol or 40 ppb(w/w).
The overall sensory quality criteria for accuracy are: the bias
must be equal to or less than 0.217; the repeatability shall
be not greater than 0.477. This will result in repeatability
limit of 3 (=10 to the power of 0.477). The repeatability limit,
expressed as the difference between two single measurements
performed on the same testing material in one laboratory under
repeatability conditions, should not be larger than a factor
3 in 95% of cases.
An interlaboratory
comparison study of olfactometry in Europe was undertaken in
1996. The study demonstrated that individual laboratories following
the methods specified in the draft CEN standard for odour concentration
measurement (CEN/TC264/WG2) can achieve quality requirements
in terms of bias and repeatability as specified in the standard.
To assess compliance with the overall sensory quality requirements
of the standard a reference value of one odour unit corresponding
to 123 mg n-butanol evaporated in one cubic metre of neutral
gas has been set.
Over the
last 12 month (April 99 - April 98), it was reported that a
has achieved a bias of 0.182 and a repeatability limit of 2.3
over more than 50 butanol samples tested with an averaged butanol
threshold of 61.6 ppb.
