Comparison of active sampling methods TO-15 and TO-17 for subsurface vapour intrusion assessment
Since the adoption of the NEPM by regulators there has been an increase in soil vapour measurement to ascertain risk and potential human exposure. SGS has fifteen years’ experience across various differing sampling techniques and has been involved in many technically challenging projects.
Vapour intrusion is the migration of volatile chemicals from the subsurface into overlying buildings. There has been a shift towards the use of subsurface gas data because of the significance of this exposure pathway for vapour intrusion. Subsurface vapour intrusion investigations can be sampled using either a passive sampling method or an active sampling method.
Passive sampling involves the use of sorbent media which is left in the subsurface for a known period of time, then desorbed either thermally or in solution and analysed via Gas Chromatography – Mass Spectrometry (GC-MS) in the laboratory. It is a simple and inexpensive tool used to gather site contamination data prior to spending large sums of money investigating soil and groundwater. The detection limits, desorption processes and analytical suites differ for each passive sampler and results vary between types.
Active sampling involves the collection of discrete soil vapour samples from targeted depths by actively drawing the sample out of the formation. The two most preferred active sampling methods are TO-15 (evacuated canisters) and TO-17 (thermal desorption tubes). Active sampling is the preferred method for obtaining vapour intrusion data, particular for health-based risk assessments.
OVERVIEW OF THE TO-15 METHOD
The TO-15 method uses evacuated (vacuum) canisters which are supplied with a negative pressure (approximately -30”Hg), ranging in size from 1.4L to 6L and glass vacuum bottles from 0.02L to 1L. The canisters have a chromatographic grade internal coating which renders the surface less chemically reactive.
Canisters are supplied with a pre-set flow restrictor/controller and particulate filter, noting that the flow controller can be adjusted by SGS prior to the supply of canisters. For soil vapour sampling, the flow rate is 200ml/min or less. Once connected to the sample point, gas is drawn into the canister by opening the valve.
Laboratory analysis is completed using GC-MS with results presented in μg/m3. Multiple analyses and high concentration samples can be diluted by transferring a small volume to another canister.
OVERVIEW OF THE TO-17 METHOD
The TO-17 method uses sorbent tubes that are packed with multiple beds of carbonised molecular sieve sorbents, separated by glass wool. A range of sorbents are available for target compounds. The tubes range in size with the Type 2 tube which is approximately 9cm long and 6mm diameter is one of the most versatile tubes. Distributed volume pairs are used at each sample location so that two samples with different volumes are collected. This ensures robust data quality.
A measured volume is drawn through the sample media via a sampling pump and a calibrated, flow regulating device. The flow rate can be adjusted in the field if required. VOCs are absorbed to the media as the sample slowly passes through the sorbent tubes.
VOCs are thermally desorbed from the tubes at temperatures up to 320°C using an inert carrier gas and analysed in the laboratory using GC-MS with results presented in μg/m3 (and a 5 μg/m3 detection limit for a 1L sample). Once the sample is thermally desorbed, traditionally it was only possible for one analysis per tube, however, latest advancements allow for a portion of the sample to be recollected to perform additional analysis if required.
FACTORS TO CONSIDER WHEN DECIDING BETWEEN TO-15 AND TO-17
The factors to consider include costs, laboratory analysis, sampling requirements and compounds of interest.
|Practicable quantification limit||Low practicable quantification limit||Low practicable quantification limit|
|Holding time||Approximately 30 days||Approximately 30 days|
|Replication of analysis||Multiple analyses possible||Multiple analysis per tube possible|
|Sample preservation||Not required||Tubes need to be capped and cooled at less than 4°C during transport|
|Stability of VOCs in humid conditions||Good, however potential loss of polar and high molecular weight compounds (naphthalene)||Minimal influence from high humidity conditions|
|Decontamination and cleaning||Labour intensive and lengthy||Easy to clean|
|Screen for unknown compounds||Yes, scan mode||Yes, scan mode|
|Power required to sample||No||Yes, pump with an internal battery|
|Adjustment of flow rate in the field||No, flow rate is set by the analytical laboratory and cannot be adjusted in the field||Yes, flow rate can be adjusted and calibrated over a wide range in the field|
|Measurement of vacuum during sampling||Not as standard practice however SGS equipment does measure vacuum||Vacuum is measured during sampling and purging|
|Difficulty in sampling||Less complex methodology (trained staff required)||More complex sample methodology (requires trained staff and sampling equipment)|
COMPOUNDS OF INTEREST
|Standard analysis||62 compounds||56 compounds|
|Analysis of additional VOCs||Expensive to purchase standards. If making gas standards internally, only a single point calibration can be achieved (30 day life)||Suites can be modified more easily and accurately – 5 point calibration|
|Hydrocarbon assessments||Performs well when analysing lighter compounds <C10||Performs better for a greater range of hydrocarbon chains C2-C18|
|Chlorinated solvent VOCs||Performs well||Performs well|
|General gases||Capable of providing general gases data from the canister||An alternative method is required to obtain general gases data|
Both USEPA TO-15 and USEPA TO-17 are reliable and proven to be accurate within their working range, and can even be used in as complementary rather than competing methods. For environmental consultants it is important to understand the differing technologies and when a particular sampling technique may be more suited to a specific application.
SGS is able to provide site specific recommendations to ensure data quality and successful project outcomes.
Download brochure here