Response surface modelling is proposed as an approach to the
estimation of uncertainties associated with derivatisation, and is compared
with a kinetic study. Fatty acid methyl ester formation is used to
illustrate the approach, and kinetic data for acid-catalysed methylation
and base-catalysed transesterification are presented. Kinetic effects did
not lead to significant uncertainty contributions under normal conditions
for base-catalysed transesterification of triglycerides. Uncertainties for
acid-catalysed methylation with BF3 approach significance, but
could be reduced by extending reaction times from 3 to 5 min. Non-linearity
is a common feature of response surface models for derivatisation and
compromised first-order estimates of uncertainty; it was necessary to
include higher order differential terms in the uncertainty estimate.
Simulations were used to examine the general applicability of the approach
and to study the effects of poor precision and of change of response
surface model. It is concluded that reliable uncertainty estimates are
available only when the model is statistically significant, robust,
representative of the underlying behaviour of the system, and forms a good
fit to the data; arbitrary models are not generally suitable for
uncertainty estimation. Where statistically insignificant effects were
included in models, they gave negligible uncertainty contributions.
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