FIELD DOCUMENTS - Laserfiche WebLink

Home Browse Search

4 / ORGANIC ANALYTICAL METHODS - GC material falls directly into the alcohol without contacting the neck of the flask. 5.3.3 Transfer the stock standard solution into a Teflon -sealed screw -cap bottle. Store at 4' C and protect from light. 5.3.4 Prepare fresh standards weekly for those compounds %,those boiling point is less than or equal to 30' C. All other standards must be replaced after 1 month, or sooner if comparison with check standards indicates a problem. 5.4 Secondary dilution standards: Using stock standard solutions, prepare secondary dilution standards in methyl alcohol that contain the compounds of interest, either singly or mixed together. The secondary dilution standards should be prepared at concentrations such that the pre- pared aqueous calibration standards will completely bracket the working range of the analytical system. Secondary dilution standards must be stored with zero headspace and should be checked frequently for signs of degradation or evaporation, especially just prior to preparing calibration standards from them. Quality control check standards, available from the EPA's Environmental Monitoring and Support Laboratory in Cincinnati, can be used to determine the accuracy of calibration standards. 5.5 Calibration standards: In order to prepare accurate aqueous standard solutions, the following precautions must be observed. 5.5.1 Do not inject more than 20 pl of alcoholic standards into 100 ml of reagent water. 5.5.2 Use a 25-p1 Hamilton 702N microsyringe or equivalent. (Variations in needle geometry will adversely affect the ability to deliver reproducible volumes of methanolic standards into water.) 5.5.3 Rapidly inject the alcoholic standard into the filled volu- metric flask. Remove the needle as fast as possible after injection. 5.5.4 Mix aqueous standards by inverting the flask three times only. 5.5.5 Discard the contents contained in the neck of the flask. Fill the sample syringe from the standard solution contained in the expanded area of the flask. 5.5.6 Never use pipets to dilute or transfer samples or aqueous standards. 5.5.7 Aqueous standards are not stable and should be discarded after one hour unless preserved, stored, and sealed according to 6.1 and 6.3.