EPA method 525.2
We're using EPA method 525.2 to confirm the absence of Benzo(a)Pyrene and Pentachlorophenol in drinking water. Both compounds are analyzed simultaneously in 1L samples. Sample prep is the following :
-Acidification to a pH lower than 2 using HCl 1N
-Addition of internal standards (Phenanthrene-D10 for PCP and Benzo(b)fluoranthen for B(a)P)
-Addition of surrogate standards (PCP-C13 for PCP and Benzo(e)pyrene for B(a)P)
-Addition of analytes of interest in laboratory fortified matrix samples
-Addition of 5mL Methanol
-Agitation for 1 minute
Compounds are extracted by SPE using C18 disks (3M 2215) :
-Disks are rinced using CH2Cl2
-Disks are activated using MeOH
-Sample (1L) is extracted
-Disks are dried 5 minutes
-Analytes are extrated using 3x5mL CH2Cl2
-Samples are dried using Na2SO4
-Samples are evaporated under nitrogen
-Samples are reconstituted using Toluene for analysis by GC-MS
In the 13 years I've been here, we've never once reported a result that wasn't lower than the reporting limit for a routine sample. The method is accredited and PT results have been satisfactory for many years now.
We're facing two recurring problems however :
1. Surrogate and matrix sample recovery for Pentachlorophenol is often above 130%
2. Benzo(a)Pyrene recovery in fortified matrix samples is often low (10-20%) while surrogate recovery is acceptable
Therefore, I have 3 questions :
1. What could account for the high recovery for Pentachlorophenol and should we be worried about this? Has anyone ever had an auditor question high recoveries when confirming the absence of analytes in a sample?
2. Is our low recovery of B(a)P merely matrix effect (since we don't have recovery problems for the surrogate) and if this is the case, do we have to take any action, or is the low recovery caused by sample preparation?
3. We've always added the internal standard during sample prep and are using a recovery standard added at the end of the analysis. Since surrogates standards are also used, has anyone tried placing the I.S. just before injection (thus replacing the recovery standard) without question by an auditor?
I would post a copy of our method but it's in French, so I apologize for the lack of details. Please note that we're situated in Canada, so we're not rigidly tied to the EPA method. Changes are acceptable if validation data is provided.