Instrumentation: Oil/Water Testing Guidelines

By Stephen Kennedy

• Compliance with a new federal regulation can be achieved using a methanol rinse in EPA Method 1664A, says water analysis specialist, the Hach Company.

On Jan. 16, 2009, the U.S. Environmental Protection Agency’s Office of Water issued a memorandum pertaining to Modifications to Method 1664A for measuring oil and grease in water.

The memo details three types of 1664A modifications: 1) allowable changes that are within the current flexibility of 1664A and don’t require prior contact with the EPA before implementation; 2) a modification that’s only allowed by EPA Region 8 and does require prior contact before implementation; and, 3) use of alternate solvents or co-solvents, which aren’t allowed.

HEM (hexane extractable material, i.e., oil and grease) is a method-defined analyte and the only solvent which may be used to extract the analytes of interest is n-hexane (85% minimum purity, 99.0% min. C6 isomers, residue less than 1 mg/L). Use of any co-solvents, alternate solvents or any substance that can introduce the target analyte into the final extract isn’t allowed per 40 CFR Part 136.6. Unfortunately, much confusion pertaining to the use of solvents in Method 1664A-SPE applications still exists, and especially with systems which can use methanol or acetone to condition the SPE filter and also use them as rinse solvents as well.

Allowable Modifications

Use of methanol to condition an SPE filter is acceptable due to the subsequent filtration of the 1 L sample (methanol goes to aqueous waste) and the air dry of the SPE filter prior to extraction with hexane, the only approved 1664A extraction solvent. Also acceptable is the rinsing of glassware with a polar solvent (methanol, acetone, etc) for glassware cleaning purposes.

Additionally, a methanol rinse may be allowed to remove water residual if all of the following conditions are met:

1. The methanol rinse is immediately discarded to waste.

2. The SPE filter is sufficiently air dried with vacuum to remove any traces of methanol remaining in the filter so as to ensure that no methanol will introduce, collocate or be collected with the n-hexane extractions: This point is much more difficult to satisfy as SPE filters adsorb liquid (water, methanol, hexane, etc.) into their interstitial pore structure. Although the filter may “appear” dry after the bulk flow of liquid through it has ceased, the filter still retains liquid in its pore structure and typically weighs 2-5 times heavier than its dry weight before filtration.

Even after a 2-3 minute vacuum air dry step, the liquid weight gained can still be as much as double the weight of the dry filter. After these initial 2-3 minutes, the SPE filter will continue to lose additional liquid weight on a gradual basis depending on the strength and free air flow of the vacuum, SPE filter diameter (surface area), materials of construction and density of the SPE filter.

To ensure that no methanol will collocate with the hexane in the SPE filter and potentially introduce any polar HEM’s into the sample, the three air dry steps below may need to be incorporated into the modified SPE method for this methanol “rinse” use to be allowable:

a.) The 1st air dry step removes any residual water from the SPE filter prior to the 1st n-hexane rinse
b.) The 2nd air dry step removes any residual hexane from the SPE filter prior to the 1st methanol rinse
c.) The 3rd air dry step removes any residual methanol from the SPE filter prior to any final hexane rinses

3. Laboratories demonstrate and document the appropriate operating conditions of 1 & 2 above – One potential way to demonstrate the removal of retained liquid from the filter is to weigh a “dry” filter, run a blank, and then vacuum air dry the “wet” SPE filter until the “wet” weight is equal to the “dry” SPE filter weight for the appropriate step in the process. This experimentation should be performed by the lab and documented as these operating conditions will be different between labs, automated SPE equipment and SPE filters.

Besides meeting all 1664A QC performance criteria, to completely satisfy point 3, the laboratory should at a minimum, perform and document a GC analysis for methanol (or by other equivalent means) of any collected layer(s) to ensure that methanol is not present in the collection vessel in order to meet the special exception use conditions above for using methanol to remove residual water from a wetted SPE filter as listed above.

Conclusion

Although labs can use methanol and other polar solvents in modified 1664A-SPE methods if they meet all of the requirements above, this practice is generally not recommended by the EPA. As such, affected labs with automated SPE equipment may find it easier and faster to regain compliance with EPA Method 1664A by simply implementing an all hexane SPE method.