Sample storage

Selected examples of recommended storage conditions.

Draft: Published 22nd January 2019

Introduction

Often there is no choice but to store samples. It is a requirement of many experiments especially where the analysis cannot be carried out straight away. Unfortunately, if you store the sample incorrectly it will come out of storage different to how it was when it went it. All samples will change over time—even if just through evaporation—meaning that you are effectively measuring a different sample than the one that went into storage.

The mechanism of change during storage might be physical, chemical or biological. It might be time dependent or instantaneous. It is dependent on the nature of the sample to be analysed and sometimes hard to predict. This all means that sample storage is an enormous and complex topic and I only intend to touch on a few key aspects here.

The three most important points are:

  1. Plan your storage!
  2. Plan your storage!
  3. Plan a back-up!

Mechanism of sample change

There are various mechanicsm by which samples change during storage. Depending on your sample some or all of these might be relevant. The mechanisms you need to think about will be dependent on:

  1. What you sample is (solid, liquid, gas, organic, inorganic, etc.)
  2. What you are trying to measure
  3. When you plan to measure it
  • (Micro)biological activity: Bacteria, algae and other organisms can consume certain constituents present in the samples; they can also modify the nature of the constituents to produce new constituents. This biological activity affects for example the contents of dissolved oxygen, carbon dioxide, nitrogen compounds, phosphorus and sometimes silicon.
  • Oxidation: Certain compounds can be oxidized by the dissolved oxygen contained in the samples or by atmospheric oxygen (for example organic compounds, iron(II), sulfides).
  • Precipitation: Certain substances can precipitate out (for example calcium carbonate, metals and metallic compounds such as Al(OH)3, Mg3(PO4)2)
  • Evaporation: Certain substances can be lost to the vapour phase (for example oxygen, cyanides, mercury). Otherwise, water can be lost from a sample over time (even when stored at 5ºC)
  • Absorption of CO2: The pH, conductivity, carbon dioxide content, etc. can be modified by the absorption of carbon dioxide from the air.
  • Adsorption: Metals dissolved or in a colloidal state as well as certain organic compounds can be adsorbed or absorbed irreversibly on the surface of containers or solid materials contained in the samples.
  • Depolymerisation: Polymerized products can depolymerize; conversely, simple compounds can polymerize.

Options for storage

Table 1 shows some examples of how to get around these potential issues for a selection of analytes in aqueous samples. These recommendations come from the British Standard and are generally conservative.

Selected examples of recommended storage conditions.
Table 1: Selected examples of recommended storage conditions. (BS EN ISO5667-3:1996, BS 6068-6.3:1996). Assumes analysis takes place in laboratory. 1 defined as having passed through 45μm filter.

Table 2 shows the main preservation techniques for aqueous samples and what analytes can be preserved in each. This table does not include methods for preservation of material for microbiological analysis (e.g. use of ultra-cold (-80oC) storage). Or the use of some form of “poison” (e.g. copper chloride) to prevent microbiological changes to the solution (e.g. bacterial oxidation of nitrite to nitrate).

A selection of preservation techniques and what analytes they are suitable (or unsuitable) for
Table 2: A selection of preservation techniques and what analytes they are suitable (or unsuitable) for. (BS EN ISO5667-3:1996, BS 6068-6.3:1996

Further reading

More information on aqueous sample preservation can be found in these references:

  • BRITISH STANDARD, BS EN ISO5667-3:1996, BS 6068-6.3:1996, Water quality —Sampling —Guidance on the preservation and handling of samples
  • Munro Mortimer, Jochen F. Müller, and Matthias Liess,  Sampling Methods in Surface Waters, in Handbook of Water Analysis (2014).
  • ISO 18512:2007, Soil quality — Guidance on long and short term storage of soil samples
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