Setting the standard in soil sampling

New standards when sampling for volatiles in soil will demand new processes and new equipment according to Vincent Van Walt

If you are involved in sampling for volatile compounds in soils – things have changed with new regulations and standards coming into force in the USA (USA-EPA Guideline 5035a_r1) and with similar in preparation in Europe through the ISO. In summary these new standards recommend that less soil (16ml ~ 25g) is taken for laboratory analysis and that any soil to be investigated should be sampled with a limited amount of disruption during the extraction process in order to minimise VOC losses prior to sample preparation and determination in the laboratory. This usually means collecting the soil sample with a single transfer to an air tight container that will be used for storage and preparation for analysis. In addition, acceptable sampling devices, procedures, preservatives and techniques are required to be recorded and observed.

Previously samples of soils for the analysis of very volatile components such as benzene, toluene and chlorinated hydrocarbons were normally obtained directly from a borehole. The stainless steel sample tubes containing the soil were closed completely and cooled for further transport to the laboratory for analysis. The new regulations recommend that a small, accurately measured sub-sample be taken above ground from a larger sampler or auger. 

For environmentalists in the field, this new closed-system purge-and-trap process for the analysis of volatile compounds in solid materials like soils, sediments, and solid waste requires different equipment. For a low soil method a hermetically-sealed sample vial is required, the seal of which is never broken from the time of sampling to the time of analysis and since the sample is never exposed to the atmosphere after sampling, the losses of VOCs during sample transport, handling, and analysis should be minimised. The applicable concentration range of the low soil method is dependent on the determinative method, matrix, and compound. However, it will generally fall in the 0.5 to 200μg/kg range.

As with any preparative method for volatiles, samples should be screened to avoid contamination of the purge-and-trap system by samples that may contain very high concentrations of purgeable materials. In addition, because the sealed sample container cannot be opened to remove a sample without compromising the integrity of the sample, multiple samples should be collected to allow for screening and reanalysis.

However, not all closed-system purge-and-trap equipment employed for low concentration samples is appropriate for soil samples preserved in the field with methanol. Precautions should be taken when preserving a soil by this method since certain compounds within the olefins, ketones, esters, ethers, and sulfides classes may react under low pH conditions and possibly not be representative of the material sampled. Additionally, acidification of solid wastes may evolve toxic gases that may be harmful to field and laboratory personnel. It is therefore recommended that when collecting wastes of unknown composition, preliminary screening and characterisation of potential sample contents should be performed prior to use of acidification as a means to chemically preserve samples designated for determinative analyses.

After a fresh surface of the solid material to be analysed is exposed to the atmosphere, the sub-sample collection process should be completed as quickly as possible in order to minimise the loss of VOCs due to volatilisation. Removing the sub-sample from a material should be done with the least amount of disruption (disaggregation) as possible. Additionally, rough trimming of the sampling location’s surface layers should be considered if the material may have already lost VOCs (been exposed for more than a couple of minutes) or if it may be contaminated by other waste, different soil strata, or vegetation. Removal of surface layers can be accomplished by scraping the surface using a clean spatula, scoop, knife, or shovel.

Sub-sampling should be carried out using an appropriate device. The new standards recommend a smaller coring device that will help to maintain the sample structure during collection and transfer as do their larger counterparts used to retrieve subsurface materials. When inserting a clean coring tool into a fresh surface for sample collection, air should not be trapped behind the sample. If air is trapped, it could either pass through the sampled material causing VOCs to be lost or cause the sample to be pushed prematurely from the coring tool.

After an undisturbed sample has been obtained by pushing the barrel of the coring tool into a freshly exposed surface and then removing the corer once filled, the exterior of the barrel should be quickly wiped with a clean disposable towel. The next step varies, depending on whether the coring device is used for sample storage and transfer or solely for transfer. If the coring tool is used as a storage container, cap the open end after ensuring that the sealing surfaces are cleaned. If the device is to be solely used for collection and not for storage, immediately extrude the sample into a VOA vial or bottle. The volume of material collected should not cause excessive stress on the coring tool during intrusion into the material, or be so large that the sample easily falls apart during extrusion.

Obtaining and transferring a sample should be done rapidly (<10 seconds) to reduce volatilisation losses. If the vial or bottle contains methanol or another liquid, it should be held at an angle when extruding the sample into the container to minimise splashing. Just before capping, a visual inspection of the lip and threads of the sample vessel should be made, and any foreign debris should be removed with a clean towel, allowing an airtight seal to form.

Following the new standards, a site-specific Sampling and Analysis Plan should clearly list the required sample collection equipment necessary to ensure that the loss of volatile constituents is minimised during the sample collection process. As with all environmental sampling applications, the analytical data usability and representativeness will be affected by improper sample collection techniques.

Sampling personnel will be responsible for ensuring that VOA vials are sealed properly using a septum of sufficient thickness without any punctures. The improper vial sealing (i.e. due to excess sample retained on the vial threads) and tightening of caps are the primary factors in the loss of volatiles due to sample collection activities. Care should also be exercised in the selection of approved pre-cleaned and certified VOA vials absent of burrs on the glass.

Procedures should be in place for the selection and appropriate use of sample collection devices (i.e. bailer, coring tool, etc.) along with the required decontamination measures.  It is also recommended to store a control sample when collecting volatile samples in order to assess possible field induced contamination.

Whichever method is most appropriate for the volatile and soil conditions to be sampled the equipment chosen should be easy to decontaminate so a high grade stainless steel is ideal.  Aluminium lined seals will prevent sorption and diffusion and, the mechanism of the corer should aim to eliminate headspace and therefore loss of VOCs from the soil.

The new standards will improve the quality of results obtained when sampling for volatiles in soil however the problem of obtaining the sample should not be understated.  The problem lays in the field not the laboratory so the soil sampler used, the time of exposure prior to sub-sampling, wind conditions, temperature and the type of volatile being tested for will all impact on the potential losses.

There is no doubt with the new standards preservation of the volatiles is optimised and freezing of the sample mitigates volatilisation but it is still the first steps in the process that remain the Achilles heel - but who am I to argue with the esteemed authors of guidelines or standards?