Sample Preservation

Sample Preservation

The aim of correct environmental sampling is to maintain the integrity of the sample preservation, from it being taken in the field to being analysed by the laboratory. In order to achieve this it is necessary to preserve in the field for some analytes as they can degrade and change during the transport period. If samples are not presented in the correct containers, with preservatives if applicable, the result may not be accredited.

Sample preservation in soils

Chemical preservation of soil is not usually advised. Sampling into the correct containers and cooling is the best approach. Plastic tubs are recommended for sampling soils for inorganic analysis, metals, pH, cyanide, sulphide, sulphate. Amber jars are recommended for organics. The amber glass protects from the UV light which can chemically degrade some species.

Waters

The chemical preservation of waters is recommended for some analytes together with the use of correct containers and cooling.

Cyanide

The sampling bottle for cyanide contains a sodium hydroxide pellet to keep the water alkaline and the cyanide in solution. If the water is not preserved and is slightly acidic, the cyanide may convert to hydrogen cyanide gas and be lost from the sample.

Sulphide

Sulphide oxidises to sulphate as soon as it meets the oxygen of the atmosphere. It is preserved with a SAOB (sulphide anti oxidant buffer) which fixes the sample for transportation.

Dissolved Metals (filtered in the field only)

Sample into nitric acid after filtration to prevent hydrolysis to insoluble oxides. Our methodology also allows accreditation for samples filtered and acidified in the laboratory.

Ferrous iron

Sample into a bottle containing hydrochloric acid to prevent the ferrous iron oxidising to ferric iron. Total iron can also be taken from this bottle.

Manganese II

This requires filtering to remove insoluble MnIV compounds before adding to a bottle containing hydrochloric acid. The acid prevents oxidation of MnII to insoluble MnIV. BOD/COD/Dissolved oxygen (dissolved oxygen should ideally be measured in the field) Our methodology includes filling one litre to the brim to exclude any oxygen, as well as keeping the sample cool to prevent uptake of oxygen and reduce bacterial activity. We then return the sample to the laboratory as soon as possible.

Organic analytes

Generally no preservation is required but it is necessary to sample into amber glass bottles and keep cool. Sample volatiles (VOCs, BTEX, GRO, C5-C10 of CWG) are placed into a 40ml vial with no headspace.

Inorganic analytes

Apart from the analytes detailed above, these can be sampled into plastic bottles with no preservation. Our results, no matter how accurate and precise the testing, depends entirely on correct field sampling, packaging and returning to the laboratory as quickly as possible after the sampling event, which why we place such high importance on sample preservation.