Checking extraction and recovery

Method B assumes complete retention of the analytes by the silica cartridge and complete recovery upon elution with HPLC eluent.

It is, however, good practice to evaluate the performance of the cartridges from batch to batch and the performance of the method with strongly oxidized oils in which the more polar compounds may be retained in preference to teh anlytes on the cartridge.

1. Checking extraction

Test the cartridge for retention efficiency following the procedure described in clause 9.1.2 but retaining the n-pentane eluates. Re-filter the eluate through a second, new cartridge, extract the cartridge and analyse by HPLC.

The chromatogram of this second extract must indicate the absence of any of the five analytes.

If teh chromatogram indicates the presence of analytes during the analysis of strongly oxidized oils, it can be due to the presence of large amounts of polar compounds. In such a case the amount of mineral oil to be analysed shall be reduced.

If there are doubts about the performance of the cartridge batch, check the extraction procedure using a standard solution of analytes in unused mineral oil.

2. Checking recovery

To ensure that all the analytes retained in the cartridge have been recovered with HPLC eluent proceed as follows:

Once the cartridge has been eluted with HPLC eluent, dry it with air for at least 10min, elute with pure methanol and analyse by HPLC.

The chromatogram must indicate the absence of any of the analytes. A positive result indicates that the amount of water in the mixture with HPLC eluent is excessively high or the cartridge is unsuitable for this method because the silica is excessivedly activated.

A more rapid procedure consists of collecting sequentially two separate 2ml portions from the same extraction. This procedure gives information only about the irregular activity of the silica.