Environmental samples comprise a wide range of matrices, each presenting the laboratory with analytical challenges. Analytical techniques and guidance documents exist to direct the laboratory to use the most appropriate sample preparation techniques, but it is also crucial that the end user of the analytical data generated is aware of the preparation techniques which may be employed, and those that may be inappropriate for the sample matrix or test. 

There is a continual need from the industry for laboratories to achieve lower and lower limits of detection while offering cost effective analysis. This drive has led to laboratories investing in high quality scientific instruments, which enable high throughput and routine low limits
of detection. 

There is also increasing requirement for those taking samples to understand and take into account recommended guidance on deviating samples with respect to sample containers, holding times and conditions, and the use of chemical preservatives.

“most VOC methods define a VOC as volatile in a static headspace which is sampled in the range of 70 to 80°C”

An area which has come to the foremost in terms of interest recently is the sampling and analysis of volatile organic compounds (VOCs) in soil samples. There are a plethora of sampling and preservation methods which can then be used with a range of analytical methods and it’s important to understand the reasoning behind the use of each so that the appropriate techniques are chosen for any given scenario. 

Firstly, it is important to define what is considered to be a VOC. These compounds are volatile up to a given temperature, beyond this temperature the compounds are defined as semi-volatile (SVOCs). Most VOC methods define a VOC as volatile in a static headspace which is sampled in the range of 70 to 80°C.

Sampling and sample preservation

Sample preservation from sampling to analysis is key to maintaining sample integrity of an appropriately taken sample. As these compounds are volatile at a relatively low temperature, care must be taken to ensure the samples are kept cool right throughout the sampling process all the way through to extraction at the laboratory.

A range of sampling equipment is available such as corers, a review of options available on the market reveal a whole range of tools available. Once sampled, it’s key that sample integrity is maintained.

A good preservative for most types of analysis, for both soil and water samples, is cooling. Laboratories typically provide cool boxes or purpose designed boxes, which hold frozen ice packs, and are used for sample storage and shipping. These help to provide a stable cool environment for shipping. Some purpose built boxes are also designed to reduce the risk of container breakage and sample cross contamination, however it is important that the containers are correctly sealed and are relatively free of surface contamination beforehand. 

Additionally, there may be further preserving measures that are appropriate for VOC soil samples:

  • • Sample into a sealed coring tube which is submitted to the laboratory sealed
  • • Transferred to an analysis vial containing an appropriate preservative which may be:
  • – Methanol
  • – Sodium bisulphate
  • • Transferred or sampled directly in to a sampling container with a PTFE liner

All of which require the sample to be shipped cool using the low temperature preservation method to ensure minimal volatile loss. 

Each of these has benefits and disadvantages to use, each of which are briefly considered in turn.

Sample into a sealed coring tube which is submitted to the laboratory sealed 
This is where a sample is taken directly and the core is sealed and sent directly to the laboratory. In the lab, the seal is broken and a sub sample taken for analysis which is immediately added to an analysis vial for the analytical method of choice.

Transferred to an analysis vial containing an appropriate preservative
With this method, the sample is taken in the field and an accurately determined weight of sample is transferred to an analysis vial containing an accurately determined volume/mass of preservative. Different preservatives may be used. Methanol is one option and sodium bisulphate another. The latter acts to inhibit degradation by lowering the sample pH (the same can be accomplished with water samples by using sulphuric acid as a preservative). The former is a solvent for which the organic compounds to be determined have a great affinity and this is the mode of preservation. Both methods of preservation require a separate sample to be submitted to determine moisture content and the utmost cleanliness and accuracy in the preparation of the sample on site (the label cannot even be written on as this changes the vial weight); something which may be challenging. The final analysis is carried out by the method of choice, usually headspace gas chromatography. However, with methanol preserved samples, it should be noted that the limits of detection for analytes may be severely compromised – being up to 100-1,000 times higher compared to a sample without methanol used.

Transferred or sampled directly in to a sampling container with a PTFE liner
Without a chemical preservative, there are two key elements of physical sample preparation that need to be strictly adhered to. The sample container must be filled right to the top so that when the container is sealed the sample is effectively “headspace tight” and there is minimal vapour (and indeed analyte) loss; and secondly the sample must be chilled to ensure the analytes of interest are maintained within the soil rather than in the vapour phase. The final analysis is achieved by preparing the sample in an analysis vial under controlled conditions and analysing the sample by the method of choice.

“it is critical to note that samples must be taken in an appropriate manner into containers which are fit for the analysis being undertaken on those samples”

It is critical to note that samples must be taken in an appropriate manner into containers which are fit for the analysis being undertaken on those samples. The types of containers required are typically advised by the laboratory based on the sample type and analysis requirements. Each laboratory will provide the appropriate sample containers for their analytical techniques and will have undertaken trials to ensure the containers used are appropriate. All methods require the sample to be cooled and correctly cared for. For volatile analytes and where chemical preservation is not carried out, there is evidence to suggest temperature control is sufficient to maintain sample integrity; however where the temperature is “cycled” between upper and lower control limits over time, then the sample may be compromised and some more volatile analytes lost. 

There are a number of areas of warning to anyone wishing to sample for volatile analysis. Firstly, the background levels of volatiles need to be minimal (field background, sampling kit background, even background from the sampler). The sampling process should obtain a representative homogenous sample for testing. This may be considered to be less important for methanol preserved samples; however, for these samples the analysis of moisture blanks and duplicates is a mandatory part of the process and therefore homogeneity is critical. And finally the sampling protocol and final analysis method should be appropriate for the contaminant levels of interest.

Quality assurance

Taking a lead from the USEPA sampling and sample integrity guidelines, the use of trip blanks, spikes and duplicates is a good way to ensure the samples are adequately preserved and shipped. Trip blanks are typically a “clean” sample, provided by the laboratory. This sample is not opened in the field and is simply re-tested on receipt at the laboratory. This is not to be confused with a field blank, which is opened in the field and exposed to any ambient cross contamination. The use of a spiked sample is a good way of checking samples have not been degraded. A sample is spiked with a known concentration of an analyte/analytes of interest, and then treated the same as a trip blank, but the analytical data generated is checked against the known concentration at the beginning and assessed with regards the acceptable method bias. Sample duplicates are used routinely, less to ensure sample integrity, more to ensure sample homogeneity and that the sample data is representative. 

“taking a lead from the USEPA sampling and sample integrity guidelines, the use of trip blanks, spikes and duplicates is a good way to ensure the samples are adequately preserved and shipped”

A more common approach used widely is the use of duplicate samples. This is essentially one sample submitted to the laboratory as two – ideally split in the field. These are then analysed blind by the laboratory. The data is then compared for statistical agreement to validate both the sample integrity and the analysis quality. 

Soil gas and sampling

As the main reason for analysing for VOCs in a soil sample is to determine the risk of exposure through an inhalation pathway, an approach which is seeing more and more use is the determination of soil gas using a variety of sampling techniques. Instead of sampling a soil directly, the soil gas is sampled, sent to the laboratory, and analysed. This technique is often much more sensitive and gives a direct indication of compounds in the vapour phase at various locations on site. This is a specialised area of analysis in itself and warrants a whole separate narrative; however, it is worthy of consideration here as many traditional site investigations are moving away from the sampling and analysis of soils for VOCs and towards soil gas instead. 

Summary

It is clear that there are a variety of approaches that can be taken in the sampling and preservation of environmental soil samples for volatile analytes. These different approaches all have positive and negative considerations which need to be taken in to account when planning a site investigation. Care should be given when preservatives are used in the field with both the site investigator and laboratory clear about their areas of responsibility. Information on specific protocols (e.g. the use of Methanol) must be fully understood and instructions followed in order to maintain sample integrity; after all, a poorly followed protocol can do more damage to the samples and their data than the use of a general protocol without sample preservatives used. 

There is a plethora of different reference methods available from different standards publishing houses such as CEN, BSI, USEPA and DIN and the intention of this article is simply to highlight broadly the different approaches available. 

Finally, and perhaps most importantly, it is key that consultants or contractors understand how their laboratory processes samples for analysis. From this understanding, fit for purpose sampling techniques and strategies can be developed to produce robust and defensible data.