How much soil gas should be evacuated from the Vapor Pin® sampling device prior to sampling?
The applicable guidance document might specify a particular number of purge volumes, but two or three volumes are commonly recommended. For the Vapor Pin® sampling device, one purge volume is equal to: 0.83 milliliters (ml) for the Vapor Pin®, plus 5.0 ml for each inch of the hole beneath the Vapor Pin® sampling device, plus 0.42 ml for each inch of tubing, if using 1/4-inch OD nylaflow LM, plus the internal volume of any other fittings or hardware in the sample train.
If you wish to calculate purge volume on the basis of equipment dimensions, the Vapor Pin® has a total length of 3.25 inches, of which 2 inches are inside the 5/8-inch hole in the slab, and 1.25 inches are above the 5/8-inch hole. The inner diameter (ID) of the Vapor Pin® sampling device is 0.187 inches. The nylaflow LM tubing has an outer diameter (OD) of 0.25 inches and an ID of 0.187 inches. The tygon™ tubing has an OD of 0.312 inches and an ID of 0.187 inches. Bear in mind that the tygon™ tubing will expand in use, and because it should be used in short lengths to minimize contact with soil gas, its ID will typically not be part of the purge-volume calculation.
Vapor Pin® prefers to purge soil gas with a combination photo-ionization detector (PID) and oxygen (O2) meter and collects the sample after the PID and O2 levels stabilize, which indicates the presence of soil gas. VOCs and O2 typically stabilize after purging 50 ml to 100 ml of soil gas, which, depending on slab thickness and tubing length, generally equals around three purge volumes.