Sample preservation prior to analysis - Determination of Anions in Natural and Treated Waters - page 772

Dionex Corporation [2] have issued an ion chromatographic method capable of resolving
the following phosphorus-containing anions in natural waters: phosphate, pyrophosphate,
tripolyphosphate and tetrapolyphosphate with retention times, respectively, of 3, 7, 10
and 14min.
The advantage of this approach is that by analysing a sample at timed intervals after
the sample was taken it is possible to gain information on any changes occurring in the
composition of the sample, to observe particular anions decreasing in concentration and
possibly to observe an increase in concentration in new anions produced by time-
dependent reactions in the sample. By plotting observed concentrations versus time
interval since sampling it may be possible to deduct the concentration of a particular
anion at time zero, ie at the moment the sample was taken.
Very often such considerations as these do not apply and the analysis can be carried
out within a few days of the sample being taken without interference effects occurring.
This would apply in the case of nitrite, nitrate, chloride, fluoride, phosphate, sulphate,
perchlorate, borofluoride, ferrocyanide, silicate and borate; also many metal-containing
anions (molybdate, uranate, vanadate, titanate, tellurate, germanate and tungstate) and
also many of the organic anions.
Anions which can change during sample storage by, for example, aerial oxidation by
dissolved oxygen in the sample include iodide, bromide, bromate, iodate, polysulphide,
sulphide, dithionate, polythionates, cyanate, thiocyanate, arsenite and possibly carbonate
and bicarbonate. In general for the analysis of these anions the original paper should be
referred to, and certainly, the interval between taking the sample and carrying out the
analysis should be minimal. The original paper should also discuss the need or otherwise
for a preserving agent to be present in the bottle at the moment of sampling.
Unfortunately many published methods do not publish this detail and in some instances
are thereby invalidated.
It is necessary to go to the leading workers in the field to obtain reliable
recommendations regarding sample preservation. Thus, the US Environmental Protection
Agency [3] has published a list of sampling preservation and holding times for several
common anions, when sampled into scrupulously clean glass or polyethylene bottles.

Analite Preservation Holding time

Chloride None required 28 days

Fluoride None required 28 days

Nitrate Cool to 4°C 48h

Nitrite Cool to 4°C 48h

o-phosphate Filter and cool to 4°C 48h

Sulphate Cool to 4°C 28 days

Available information on the preservation of particular anions is reviewed below.