First and foremost, there may be specifics required by regulators in your state...by in general...
Silicone tubing is more reactive than harder tubing, which is to say that Silicone, and softer tubing generally, is more prone to absorbing and outgassing organic compounds. Greg Ouellette’s 2004 Soil Vapor Sampling and Analysis – Lessons Learned, DOE/PERF Soil Vapor Workshop, Brea, CA Jan. 27-29, is one of several reports that demonstrates the higher reactivity of soft tubing, this is out of context for our application. Ouellette drew vapors through 50 ft of Tygon tubing, which has approximately 85 square inches in contact with soil gas. In the case of the Vapor Pin®, the only contact between soft tubing (Silicone) and soil gas is the cross sectional area between the slab and the bottom of the pin, which is 0.11 square inches. I think we also agree that while Silicone would absorb some VOCs, the tubing would reach equilibrium at some point and stop absorbing vapors.
We test the Silicone tubing when it arrives by drawing ambient air through the entire 50 ft lengths with a PID, and record the readings. The tubing does contain a few ppm of vapors in the first few days, specifically siloxanes, which are not on any vapor intrusion list. We continue to test for several days until the entire length of tubing contributes less than 1 ppm of VOCs. We then cut the tubing into short lengths and let them continue to air before sending them.
They are relatively obscure issues such as what kind of helium to use during leak testing, or whether to use barbed versus compression fittings, when most of the problems boil down to a few issues:
1) Run a shut-in leak test on the assembled canister & regulator, as described in ASTM D7663-11, before going to the field. Leak test the assembled sample train on site, except for the connection to the Vapor Pin® sampling device or equivalent point with a hand-held vacuum pump or peristaltic. Last, connect the sample train to the sub-slab point and leak test it the point. Helium works, but we use distilled water. That’s a problem if the point leaks, but I’ve never seen it with the Vapor Pin® sampling device. We also check the regulator flow rate prior the shut-in test, but an improper flow rate is more likely to result in no sample than an invalid sample. We’ve developed our own techniques if you’re interested. Also, documenting your test results and presenting them to the lab improves your credibility and keeps them on their toes. Even the best labs occasionally provide faulty equipment.
2) Minimize the lengths of soft tubing by butting up the harder tubing or hardware against each other as close as possible. We find compression fittings more likely to leak, not less.
3) Avoid incompetent labs if you have a choice.
4) Collect an adequate number of samples to evaluate spatial heterogeneity. This may be the most important part. At least when working near sources, we’ve repeatedly found that the primary source isn’t where we expected it to be, and our best guess would have provided concentrations that are orders of magnitude lower than maximum. The only solution is to drill lots of holes, which is practical with the Vapor Pin®. I’ve personally installed as many as 56 in a day, and my younger coworkers have hit 90. In many cases you can screen them with a PID and collect a select few for lab analysis.
Please refer to our White Papers under resources for additional information. One with the Michigan DEQ in which we installed four Vapor Pin® sampling device next to their “conventional” sub-slab points (Swagelok fittings set in cement). They used a variety of labs, sample containers, and analytical methods over the course of months. But any way we cut it, when we plot the Vapor Pin® against conventional points, we get a good correlation. The second white paper was done in conjunction with H&P labs in San Diego. Their sub-slab points, as described in the CA guidance, amount to miniature monitor wells, complete with sand packs and screens. They collected 10 sample pairs, and again, the correlation is excellent. In spite of all the issues concerning the Silicone sleeves, the Tygon used to make connections, and others issues we haven’t touched on, the fact that we get the same results over a broad range of concentrations and compounds proves that the Vapor Pin® sampling devices does what it’s designed to do.