TO-15 FAQ

Why do 8260 and TO-15 sample results differ?

Should I purge sample tubing before collecting a soil gas sample in a Summa Can?

Yes, it is always recommended to purge the “dead space volume” prior to collecting any type of soil vapor sample. Specifically, in the case where Summa canisters are being used it is recommended by the DTSC/RWQCB to default to three (3) purge volumes. If a purge test has been previously conducted and site conditions are consistent from the present and/or previous sampling event then the purge volume yielding the highest concentration for the compound(s) of greatest concern detected during the purge test should be used.

How do I calculate purge volume?

Please feel free to use our purge volume calculator to help calculate your purge volume.

Calculation of the purge volume (a.k.a. “dead space volume”) is accomplished by summation of three components: 1.) the void space of the sand pack around the probe tip; 2.) the annular space in the dry bentonite; and 3.) the internal volume of the tubing used. More discussion on purge volumes can be located through the following link purge volume calculator.

How do I connect a Summa to the sample probe?

Connecting a Summa canister to a sample probe if unfamiliar with the process can be tricky. Improper connection of the Summa canister to the sample probe yields high hits of the tracer gas and/or diminished recoveries on target analytes. It is therefore essential to properly connect the Summa canister to the sample probe.

Depending on the type of Summa canister you have and how the soil vapor probe was installed a variety of procedures and/or instructions could be relevant. We recommend using either PTFE Teflon or Nylaflow tubing, stainless steel Swagelok fittings, Watt quick connect fittings, and Cole-Parmer luer valves. An in-line connection of a differential pressure gauge is also recommended to demonstrate the sampling train does not have a leak. It is highly recommended to consult with the laboratory providing the Summa canister in order to follow their specific instructions for connecting the Summa canister to the soil vapor probe. Next, a dry run should be conducted to practice how the Summa canister will be connected to the soil vapor probe.

What flow rate should I use when collecting a soil gas sample?

The recommended flow rate is 200 mL/min or less. Here, maintaining a consistent sampling rate for all samples during a soil vapor investigation is key for generating consistent analytical data. That being said site specific conditions (e.g., low-flow situations or large diameter tubing) may necessitate deviation from a consistent sampling rate.

Flow regulators are used to maintain a consistent flow rate when sampling soil gas samples via Summa canister. Flow regulators are pre-set by the laboratory at the desired flow rate.

What flow rate should I use when collecting an ambient air sample?

The size of the Summa canister and the duration of the sampling event will determine the desired flow rate.

Typical sizes for Summa canisters are 1- and 6-liters. 1-L Summa canisters are recommended for soil gas sampling and “short” (1- and 2-hour) indoor/ambient air sampling events. 6-L Summa canisters are typically utilized for 8- and 24-hour indoor/ambient air sampling events. It is not recommended to use 6-L Summa canisters for soil gas sampling, especially shallow soil gas samples.

The duration of the sampling event is pertinent to indoor/ambient air sampling. Typical sampling events are 1-, 8-, and 24-hours.

The desired flow rate is determined by calculating the total volume (capacity of the Summa canister) divided by the total time of the sampling event. For example, the flow rate for a 6-L Summa canister over a 24-hour sampling event would be:

[6 L x (1000mL/1L)] / [24 hours x (60 minutes/1 hour)] = 4.17 mL/min.

What size Summa should I use to collect my sample?

The size of the Summa canister is determined by the following parameters: 1.) the location of the sample; 2.) the duration of sampling event; and 3.) the required detection limits.

The location of the sample will be either sub-surface (i.e., soil gas samples) or above surface (i.e., ambient/indoor samples). Soil gas samples should be collected with 1-L or smaller Summa canisters. Ambient/indoor samples are typically collected with 6-L Summa canisters unless the sampling duration is 2 hours or less.

Soil gas samples are not gauged by their sample duration but rather by their sampling rate. It is recommended to sample soil gas samples at flow rates of 200 mL/min or less and with 1-L or less Summa canisters. Conversely, ambient/indoor air samples do have a fix sampling time. Typical sampling events are 1-, 8-, and 24-hours. Unless the sampling event is 2 hours or less, it is recommended to use 6-L Summa canisters for ambient/indoor samples.

1-L Summa canisters can routinely meet shallow soil gas residential CHHSLs and 6-L Summa canisters can routinely meet or exceed indoor air residential CHHSLs.

What is the hold time on a sample collected in a Summa?

Samples, be it either soil gas or ambient/indoor samples, which are collected via Summa canisters have a recommended hold time of 14- and 30-days from the time of collection as designated by DTSC/RWQCB and EPA, respectively.