  The second edition of ISO 19229 expands the guidance in its predecessor in
two ways. Firstly, it provides more support and examples describing possible
experimental approaches for purity analysis. A novelty is that it describes how
the beta distribution, or some other suitable probability distribution can be
used to approximate the distribution of the output quantity, i.e., the fraction
of a component. It also provides guidance on how to report coverage intervals
in those cases, where the usual approximation from the GUM (Guide to the
expression of Uncertainty in Measurement) to use the normal or t distribution
is inappropriate because of vicinity of zero. Coverage intervals play an
important role in conformity assessment, and it is also customary to report
measurement uncertainty in the form of a coverage interval, notwithstanding
that ISO/IEC 17025 does not explicitly require it. ISO 6141, which sets
requirements for certificates of calibration gas mixtures, does require the
statement of an expanded uncertainty, which has been interpreted that in the
case of a non-symmetric output probability distribution, a coverage interval
should be stated, along with the value and the standard uncertainty. This paper
gives a brief background to the choices made and examples in ISO 19229.
