Method 6850 calls for filtering in the field with a 0.2 micron filter. The laboratory prep calls for a 0.45 micron filtration. Is the 0.2 micron filter a requirement? Can we use a 0.45 micron filter? If no filtration is performed in the field, should the sample be collected with or without headspace? The method requires headspace.
Whenever possible, field filtration of water samples using sterile 0.2 μm PTFE filter membranes is recommended to remove perchlorate‐reducing bacteria (PRB) that could potentially introduce low bias due to biodegradation, as well as to remove filterable organic carbon (an electron donor) that may act as a nutrient for PRB under anoxic conditions. The 0.45 μm filtration criteria are to both ensure that there is no potential for occluding the high‐performance liquid chromatography (HPLC) column with suspended material and to generate a sample whose suspended solids content is representative of water quality within an underground aquifer composed of fine‐grained materials. Although not specifically required by the method, also consider cooling the samples to <4°C to further inhibit microbial growth.
If not field‐filtered, the sample should be filtered as soon as possible following receipt at the laboratory. There is some ambiguity here regarding whether to filter to 0.2 or 0.45 μm; typical filter media used in the laboratory prior to analysis are 0.45 μm syringe filter cartridges. If the samples are not analyzed relatively soon following receipt, the laboratory should filter using a 0.2 μm filter.
In any case, the samples should be collected with residual headspace in the containers, since this (ambient air) will help minimize growth of the anaerobic PRB.