Leak check instruments are dependent on what leak check compound is use in the field. Some commonly used liquid leak check compounds are n-propanol, 1,1-difluroethane (1,1-DFA), and pentane. Commonly used gaseous tracer compounds include helium and sulfur hexafluoride. In the field, these compounds can be analyzed by either a mobile laboratory or field instrument. Typically, n-propanol, 1,1-DFA and pentane are evaluated with a mobile laboratory; however, helium and sulfur hexafluoride can be evaluated with hand-held instruments. When collecting samples in passive stainless steel canisters, leak check compounds can be quantified by the stationary laboratory; however, sample quantification usually occurs after the field crew has demobilized. If the sample contains the leak check compound above the sample rejection criteria, that sample is compromised and resampling can only occur upon remobilization. Accordingly, Section 4.2.2.3 of the ASGI recommends collecting a sample in the field for leak compound screening purposes and analyzing the sample before the field crew demobilizes. Qualitative devices, such as a hand-held photoionization detector (PID), should not be used to determine whether the sampling system has a leak.