EPA-NERL: 502.2 (by ELCD):  VOCs in Water by GC/PID/ELCD

  • Summary
  • Analytes
  • Revision
  • Data and Sites
Official Method Name
Volatile Organic Compounds in Water by Purge and Trap Capillary Column Gas Chromatography with Photoionization and Electrolytic Conductivity Detectors in Series
Current Revision
Revision 2.1, 1995
Gas Chromatography with Electrolytic Conductivity Detection
Method Subcategory
Method Source
  Methods for the Determination of Organic Compounds in Drinking Water - Supplement III (EPA/600/R-95-131)
Brief Method Summary
Inert gas is bubbled through a water sample to purge highly volatile organic compounds with low water solubility. Purged sample compounds are trapped in a tube containing suitable sorbent materials. When purging is complete, the sorbent tube is heated and backflushed with helium to desorb the trapped compounds onto a capillary gas chromatography (GC) column. The concentrations of compounds from the trap are measured using a capillary column GC system equipped with a photoionization detector (PID) and an electrolytic conductivity detector (ELCD) placed in series.
Scope and Application
This method determines purgeable volatile organic compounds (including some disinfection by-products) in finished drinking water, raw source water, or drinking water in any treatment stage.
Applicable Concentration Range
Approximate range is 0.02 - 200 ug/L. Range is analyte and instrument dependent.

(A) Contamination: Major contaminant sources are volatile materials in the laboratory and impurities in the inert purging gas and in the sorbent trap. The use of non-polytetrafluoroethylene (PTFE) plastic tubing, non-PTFE thread sealants, or flow controllers with rubber components in the purging device should be avoided since such materials out-gas organic compounds which will be concentrated in the trap during the purge operation. Analyses of laboratory reagent blanks provide information about the presence of contaminants. When potential interfering peaks are noted in the laboratory reagent blanks, the analyst should change the purge gas source and regenerate the molecular sieve purge gas filter. Subtracting blank values from sample results is not permitted. Also, when traps containing combinations of silica gel and coconut charcoal are used, residual water from previous analyses collects in the trap and can be randomly released into the analytical column. Therefore, as preventative maintenance, the trap should be reconditioned after each use.

(B) Memory Interferences: Carryover may occur whenever high and low concentration samples are analyzed in sequence. For prevention, rinse sample syringe between samples with solvent. Also, unusually concentrated samples should be followed by the analysis of a solvent blank.

Quality Control Requirements

Initial demonstration of laboratory capability, followed by determination of surrogate compound recoveries in each sample and blank, monitoring internal standard peak area or height in each sample and blank, analysis of laboratory reagent blanks (LRBs), laboratory fortified matrices, laboratory fortified blanks (LFBs), and QC samples. A MDL for each analyte must also be determined.

Sample Handling

Dechlorinate samples using 3 mg of sodium thiosulfate or 25 mg of ascorbic acid per 40 mL of sample to the sample bottle prior to collection. Additional amounts of sodium thiosulfate or ascorbic acid should be added per each 5 mg/L of residual chlorine.

All samples should be collected in duplicate. Overfill all bottles, making sure to leave no bubbles or headspace. Adjust the pH of all samples to less than 2 at the time of collection, but after dechlorination with two drops of 1:1 HCl for each 40 mL of sample. When sampling for THM analysis only, acidification may be omitted if sodium thiosulfate is used to dechlorinate the sample. If sample foams vigorously after addition of HCl, discard the sample, collect a set of duplicate samples, but do not acidify them. Store samples at 4oC until analysis.

Maximum Holding Time
14 days.
Relative Cost
$201 to $400
Sample Preparation Methods