Sample preservation prior to analysis - Determination of Anions in Natural and Treated Waters - page 776

9 96 0.34 0.025 −0.0035 −0.010 −0.006 −0.012

9 96 0.08 0.025 −0.002 0.07† 0.0008 0.065†

10 2

48 0.022 0.025 −0.0005 −0.001 −0.0005 −0.0013

10 a

48 0.112 0.025 0.0001 0.0026 −0.0027 −0.0046

11 31 0.014 0.025 −0.0005 −0.0034 0.002 0.006

11 31 0.015 0.025 0.004 0.009 0.023 0.028*

*exceeds target (either mean change or max poss change)
†exceeds target but inadequate power in test
a Laboratory 10 performed the second analysis at time T/2
Time T was chosen in each laboratory to be the maximum time between sampling and analysis
which is routinely encountered
Source: Reproduced with permission from the Water Research Centre, UK [5]

The results of the tests are shown in Tables 18.1 and 18.2. No precautions, other than
refrigeration (employed by all laboratories except Laboratory 2) were taken to preserve
the samples. The samples were analysed to establish the initial determinand concentration
within 1h of being collected. Each laboratory then performed an analysis after times T/4
and T hours, where T was chosen as the maximum time between sampling and analysis in
that laboratory. For some determinands, tests were performed on two samples in a given
laboratory. All laboratories used sample containers of glass except Laboratory 6, which
used polyethylene bottles.

Total oxidised nitrogen
From a possible 21 sample/storage time combinations there were five instances,
involving four samples, of failure to demonstrate adequate stability. Of these, one was for
a mean change in concentration; one was for maximum possible instability greater than
the target (with adequate power); and the remaining three were instances where the
power of the test was insufficient.
The indication is that, in most cases, samples are adequately preserved. The observed
change in concentration observed for the unrefrigerated sample (Laboratory 2) did not
exceed the target.

Nitrite
From a possible 26 sample/storage time combinations, there were three instances of
failure to meet the target. Of these, two were almost certainly due to lack of power in the
tests and the remaining one was marginal.
The evidence is that sample preservation for nitrite is satisfactory though the results for
the unrefrigerated sample suggest that refrigeration is desirable.

18.5 Phosphate

There are two approaches to the preservation of samples for phosphate analysis.