Bag Sampling - Active Air Sampling
Personal Monitoring Pump Bag Sampling
An industrial hygienist faces many challenges in the routine task we call air sampling. The myriad of air borne contaminants seems never-ending, and every target substance does not readily collect on a conventional filter, sorbent tube or impinger. Some substances, usually gases, require collection in a gas-sampling bag for laboratory or portable instrument analysis. While less common than filter or sorbent tube sampling, the use of gas sampling bags is no less important, and it does require a slightly different approach in order to maintain the integrity of the sample.
First and foremost, in any sampling situation, always follow an established sampling method whenever available. This is true not just for bag sampling but for any air sampling. A sampling method published by agencies like NIOSH, OSHA or EPA has been validated in laboratory studies to work correctly within the limitations set forth in the method. It will list acceptable flow rates, minimum and maximum sample volumes and the correct capture media. Veering outside of these guidelines is an invitation for problems to occur. For example, if a sampling method calls for a flow rate of 50 to 200 cc/minute, don’t try to run it at 5 LPM. The capture media may not work at the higher flow rate. Similarly if a method calls for a Tedlar bag, don’t try to use polyethylene.
Bag sampling poses issues that are unique to bag sampling. First is the issue of choosing the correct bag material. Gas sampling bags may be made of materials with trade names like Tedlar, Mylar or Teflon. Each material has its own unique qualities, which include resistance to attack from specific chemicals or chemical groups. Because of the large surface area of a sampling bag, this becomes an even bigger issue than in other types of sampling. If the target gas is not compatible with the bag material, the two can react and consume the target gas before it can be analyzed by the laboratory. Following an established sampling method will prevent choosing the wrong material. If an established method does not exist for the target gas, consult the bag manufacturers for a chemical compatibility chart. Such charts are also available on the Internet.
Another issue with bag sampling is one of sampling logistics. Sampling pumps normally draw a sample through a capture device from the vacuum side of the pump. Bags are often filled using the pressure side of the pump. Most monitoring pumps on the market have a means of converting the pump to a bag-fill mode by placing an air boss at the pump’s exhaust side. This is acceptable for target gases that are compatible with the pump’s internal path materials (e.g., vinyl tubing, silicone valves, etc.), but for target gases that are not compatible an indirect sampling method is preferred. An indirect sampling method places the bag inside a container, and the pump is used to evacuate the area inside of the container but outside of the bag. The bag will inflate like an artificial lung.
Sampling rate and time become an important issue in bag sampling, because over-inflating of the bag is a potential problem. Attention must be given to the flow rate and sample time with respect to the capacity of the bag. If a short-term sample is required, a two-liter bag may be filled in two minutes with a pump set at 1 LPM. In that short period, the inflation of the bag is easily monitored by sight. If a long-term sample is required, we simply must do the math. For example, if a five-liter bag is used for a four-hour sample, the desired flow rate can be calculated by dividing the bag size in cc by the time in minutes. In this example 5000 cc divided by 240 minutes equals 20.8 cc/minute. If the flow rate is set higher than 20.8 cc/minute, the bag will over-inflate within the desired sample time. A flow rate below 20.8 cc/minute must be chosen to prevent the bag from inflating to capacity too soon.
The personal monitoring pump has traditionally served as the most versatile sampling tool in the industrial hygienist’s toolbox. Knowing how to conduct bag sampling properly fills yet another loop in the tool belt.