ISO 6468:1996 pdf download – wWater quality – Determination of certainorganochlorine insecticides, polychlorinated biphenyls and chlorobenzenes – Gas chromatographic method after liquid-liquid extraction.
7.5 Clean-up and separation
Applying the procedure described in 7.2 may lead to coextraction of relatively polar and/or other undesired substances, which are likely to interfere by the appearance of unknown peaks overlapping the pesticide peaks.
NOTE 10 Treatment by column chromatography may help to eliminate some of the substances. However, this method cannot be considered as an absolute system.
Use one or both of the following procedures:
— clean-up on an alumina-alumina/silver nitrate column, for purification to remove polar compounds
— clean-up on a silica gel column, for separation of PCB from most insecticides (7.5.2).
NOTE 11 The quality of each batch of columns should be checked with standard solutions.
7.5.1 Cleanup on alumina-alumlnalsllver nitrate column
Carry out the purification on an alumina-alumina/silver nitrate column as described in and If interference persists, the additional procedure described in annex A may be carried out.
NOTE 12 Some compounds, for example endosulfan, may be retained on the column. Preparation of the column
Place 15 ml ± 1 ml of the extraction solvent (4.2) in the column (5.10), then add 1,0 g ± 0,2 g of alumina/silver nitrate (4.7) and allow to settle while tapping gently. Then add 2,0 g ± 0,2 g of alumina (4.6) and again allow to settle while tapping gently. Add a sufficient amount of sodium sulfate (4.3) to produce a 5 mm layer on top of the column. Prepare the column immediately before use. Purification
Prepare an alumina-alumina/silver nitrate column as described in Run oft the surplus of the extraction solvent (4.2). When the solvent level reaches the top of the column, add the concentrated sample extract (see 7.3). Wash the sample vessel with 2 ml ± 0,5 ml of extraction solvent and add the washings to the column. Elute the column with 30 ml ± 1 ml of extraction solvent. Collect and concentrate the extract as described in 7.3 and then perform the gas chromatographic analysis according to 7.4.
During addition to the column, do not allow the meniscus of the solvent (4.2) to fall below the surface of the and repeat the purification. II total blackening is a common occurrence, larger columns may be used but additional solvent will be required for elution.
7.5.2 Clean-up on silica gel Preparation of the column
Choose a chromatography column (5.12) as shown in figure F.1 in annex F. [Initially without the solvent reservoir (figure E.2) attached.] Plug the column temporarily with a rubber cap at the lower end, and fill it with extraction solvent (4.2).
Insert a plug of glass wool (4.15) close to the lower end.
Suspend 1 g of silica gel (4.8) in the extraction solvent (4.2) in a small beaker.
Transfer the suspension to the chromatography column with the aid of a pipette.
Let the silica gel settle down during constant vibration of the column, to produce a dense layer. Otherwise, the sodium sulfate which is placed onto the silica gel will move into the silica gel layer.
Remove the rubber cap.
Carry out the following steps, including the steps described in, without interuption as soon as the column starts dripping continuously.
Place 0,2 g of sodium sulfate (4.3) onto the layer of silica gel. Attach the solvent reservoir to the column and rinse the system with 5 ml of solvent (4.2).
Once again, remove the solvent reservoir as soon as the level of solvent has moved down to the column section of the apparatus and follow the steps described in immediately.
NOTE 13 AlternatIvely, dry packed and/or commercially available disposable columns may be used, 11 they are found to be equally suitable. Clean-up and separation
Add 100 il of the sample extract onto the column with the aid of a 100 p.1 syringe, just before the meniscus of the solvent has reached the sodium sulfate layer.