Preparation of a New Sample Bag for Use, or Recycling Used Bags
Tedlar bags from most sources are clean when they arrive. However, a trace of dirt or grease, or absorption of gas into the surface of the bag material, can cause a new bag to give erroneous results. For best results, we recommend flushing, scrubbing, and seasoning, which are really independent operations.
Flushing a New Bag, or a Recycled Old One
For many applications, e.g, permanent gases, only flushing is needed, and amounts to nothing more than partially filling a bag with clean diluent and evacuation (complete deflation) with a vacuum pump. For concentrations in the low PPM or PPB range, however, a little more care is wise.
Evacuate the bag thoroughly, and then run a small amount of diluent gas into the bag, e.g., 4 liters into a 40 liter bag. Evacuate again, and repeat about three times. It is much better to use several partial fillings with small volumes than a single filling with a large volume. It is also faster. Flushing can be improved further if, on the final filling, the bag is left overnight with the flush gas inside, giving any contaminants time to desorb from the bag materials.
Scrubbing
'Scrubbing' a sample bag means adding a gas that aggressively attacks and destroys traces of contaminants inside. The ideal scrub gas is ozone in 1% to 10% concentration, which requires the luxury of an ozone generator. Filling a bag with ozone and leaving from 2 to 24 hours removes most organic and many inorganic residues, such as sulfides, unsaturated organics, etc. In a pinch, 100 ppm nitrogen dioxide can be used, but it is not as effective on organic compounds.
Both ozone and nitrogen dioxide are hazardous to handle, even in open air. A fume hood must always be used.
If there is a problem with scrubbing, it's that it can render the bag 'too clean', ie, with absorptive and reactive sites on the bag polymer exposed and easily contaminated by new compounds. To prevent that, we can 'season' the bags.
Seasoning
'Seasoning' is the process of prior exposure of the bag to the gas to be measured, in order to occupy adsorption sites inside the bag. This is particularly important with low concentrations of volatile organic compounds, which will partially permeate any bag material.
Often, the same concentrations of a gas are used repeatedly in experiments. If a given bag is labeled and used only for a specific concentration of a gas, the most reliable results will be obtained.
