Sample preservation prior to analysis - Determination of Anions in Natural and Treated Waters - page 775

10 a

48 4.0 0.20 −0.019 −0.056 −0.021 −0.050

11 31 1.9 0.10 −0.003 −0.010 0.12* 0.175*

11 31 5.3 0.26 0.15 −0.32† 0.13 −0.30†

*exceeds target (either mean change or λmax poss change)
†exceeds target but inadequate power in test
a Laboratory 10 performed the second analysis at timeT/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]

A study has been reported [5] of stability tests of solutions containing nitrate and nitrite.
Briefly, the tests involved the analysis of a sample on three occasions—one as soon as
possible after sampling and subsequently, after intervals chosen to represent the likely
periods for which the sample may be transported and stored.
The tests of sample stability were designed with the objective of demonstrating that
any change in determinand concentration between sampling and analysis should not be
greater than a target maximum instability. These targets were set as half of the target for
maximum possible bias in analytical results which had been agreed for each determinand.
The results of these tests have been evaluated with respect to the following guidelines.
The maximum (95% confidence) instability, or change in determinand concentration,
observed during the period of storage under consideration should not be greater than the
stability target. In the case of total oxidised nitrogen and nitrite, this target is 0.025mg N
L −1 or 5% of the determinand concentration in the sample, whichever is the greater.

Table 18.2 Results of stability tests for nitrite nitrogen Results in mg N L −1

Change in sample concentration

after T/4 after T

Lab
No .

Time T
(hours)

Sample
concn

Stability
target

Mean (Max 95%

confidence
limit)

Mean (Max 95%

confidence
limit)

2 30 0.06 0.025 −0.017 −0.025 −0.015 −0.021

3 24 0.00 0.025 0.00 0.00 0.0095 0.0106

3 24 0.025 0.025 0.00 0.00 0.0015 0.0043

6 24 0.022 0.025 0.002 0.006 0.005 0.013

6 24 0.047 0.025 0.002 0.003 0.003 0.003

8 48 0.013 0.025 −0.002 −0.002 0.0002 0.0007

8 48 0.08 0.025 0.0007 0.0015 −0.0025 −0.004