EPA Method 331

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EPA Method 331:
Perchlorate in water by LC/EMI/MS. Official Name: Determination of perchlorate in water by liquid chromatography electrospray ionization mass spectrometry

Water samples are collected in the field using a sterile filtration technique. Prior to analysis, isotopically enriched perchlorate (Cl18O4-) is added to the sample as an internal standard. The sample is injected without cleanup or concentration onto a chromatographic column (Dionex IonPak@ AS-21 or equivalent), which separates perchlorate from other anions and background interferences. Perchlorate is subsequently detected by negative electrospray ionization mass spectrometry. A remote controlled valve is used to divert early eluting cations and anions to waste. Prior to the elution of perchlorate, the valve is switched sending the chromatographic eluent to the mass spectrometer. This diversion helps prevent unnecessary fouling of the electrospray source. Perchlorate is quantified using the internal standard technique.

This method is used for the determination of perchlorate in raw and finished drinking waters. This method can be used to acquire data using either selected ion monitoring (LC/SIM) or multiple reaction monitoring (LC/MRM) detection. The SIM and MRM mass spectrometry conditions described in this method were developed using a conventional LC system.

Wendelken, S.C., Munch, D.J., Pepich, B.V., Later, D.W., and Pohl, C.A., 2005, Determination of perchlorate in drinking water by liquid chromatography electrospray ionization mass spectrometry: U.S. Environmental Protection Agency Report EPA/815/R-05/007, Revision 1.0, 34 p.

All glassware must be meticulously washed and rinsed with reagent water. Disposable high-density polyethylene equipment may be used. If background contamination is a problem, detergents used in glassware cleaning should be investigated as perchlorate has been detected in some common laboratory detergents.Method interferences may be caused by contaminants in solvents, reagents, sample bottles and caps, and other sample processing hardware. All laboratory reagents and equipment must be routinely demonstrated to be free from interferences under the conditions of analysis. Subtracting blank values from sample results is not permitted.Matrix interferences may be caused by contaminants that are in the sample. The extent of matrix interferences will vary considerably from source to source. Samples high in organic carbon or dissolved solids may lead to elevated chromatographic baselines or interfering peaks. Hydrogen sulfate that is formed from the 34S isotope of sulfur is a common interference.Equipment used for sample collection and storage has the potential to introduce interferences. These interferences must be investigated during the Initial Demonstration of Capability (IDC) by preparing and analyzing a Laboratory Reagent Blank (LRB). This procedure should be repeated each time that a new equipment is used to ensure that background contamination does not interfere with the identification and quantitation of perchlorate. Lastly, it must be demonstrated that solutions containing a working level of the internal standard do not contain unlabeled perchlorate at concentrations greater than 1/3 of the MRL. The percent of 18O enrichment of the internal standard may vary between manufacturers. Poor isotopic enrichment of the internal standard may lead to sample contamination by Cl16O-4 and is initially confirmed during the analysis of the LRB during the IDC and is monitored in each Analysis Batch.

QC Requirements:
Quality control requirements include conducting an initial demonstration of capability, and the on-going analysis of laboratory reagent blanks, fortified blanks, fortified sample matrices and internal standards, and conducting continual calibration checks and quarterly instrument performance checks.

Maximum Holding Time:
28 days



Method report does not expressly state range; however, it is noted that the calibration procedures for this method are designed to cover the full scanning range of the instrument.

The precision and accuracy data presented are based on the MRM detection method of perchlorate in reagent water.

The detection limit is dependent on sample matrix, fortification concentration, and instrument performance; it is based on the analysis of at least seven replicate laboratory fortified blank samples. The DL listed in NEMI applies to the LC/MRM mode.

Revision Number:
Rev. 1.0, January 2005