Sample Preparation

samples cannot be held for 14 days but must be extracted immediatelyafter collection and preservation: carboxin, diazinon, disulfoton, disulfotonsulfoxide, fenamiphos, and terbufos. Sample extracts may be stored at 4 C for up to 30 days after sample extraction.”Q16. From the chemistry described in sections 8.2 and 8.3 of the EPA method, whatmight be the cause of pesticide degradation before extraction?Standards, Spikes and SurrogatesNote: A discussion of standards, spikes and surrogates will also be covered in themethod validation section. This section will highlight their roles in sample preparation.Take a look at the following excerpt from the EPA Method.“7.8 Fortification Solution of Internal Standards and Surrogates -- Prepare aninternal standard solution of acenaphthene-d 10 , phenanthrene-d 10 , andchrysene-d 12 , in methanol, ethyl acetate, or acetone at a concentration of 500µg/mL of each. This solution is used in the preparation of the calibrationsolutions. Dilute a portion of this solution by 10 to a concentration of 50µg/mL and use this solution to fortify the actual water samples (see Section11.1.3 and Section 11.2.3). Similarly, prepare both surrogate compoundsolutions (500 µg/mL for calibration, 50 µg/mL for fortification). Surrogatecompounds used in developing this method are 1,3-dimethyl-2-nitrobenzene,perylene-d 12 , and triphenylphosphate. Other surrogates, for examplepyrene-d 10 may be used in this solution as needed (a 100 µL aliquot of this50 µg/mL solution added to 1 L of water gives a concentration of 5 µg/L ofeach internal standard or surrogate). Store these solutions in an amber vial at4 C or less. These two solutions may be combined or made as a singlesolution.”Q17. What is a surrogate compound?Q18. What assumptions must be made when choosing a surrogate?Q19. When is the surrogate added during the sample preparation (i.e. before extractionor after) and why is it added then?Take a look at the following tables taken from the EPA Method 525.2.