Purgeable Organic Compounds in Wastewater by Purge and Trap with Gas Chromatography/Mass spectrometry using EPA Method 624.1

Wastewater is essentially used water. It can contain a wide variety of substances including sewage, runoff, food waste, surfactants, solvents, a variety of other chemicals, and petroleum products. Wastewater sources may include homes, industries, and businesses, or any other facility that uses water. Municipal and industrial wastewater must be treated or cleaned before it is released back into the environment, and while nature can handle small amounts of water wastes and pollution, it would be overwhelmed if humans didn’t treat the billions of gallons of wastewater produced every day around the world. Treatment plant effluent, also known as discharge, must be treated properly to protect human health and the environment.

In 1977 the Clean Water Act (CWA) was put into effect allowing the USEPA to study and, if necessary, regulate 65 priority pollutants. Many test procedures and results for monitoring of municipal and industrial wastewater fall under the Act. Associated with this is the National Pollutant Discharge Elimination System (NPDES), which regulates sources that discharge pollutants to our water. Effluent limitations serve as the primary mechanism in NPDES permits for controlling discharges into receiving waters. When developing effluent limitations, limits are based on both the technology to control the pollutants and limits protecting water quality standards. The CWA promulgates test procedures for the analysis of pollutants, which fall under NPDES permitting. These are known as the 600 series for analytical methods for organic pollutants.

There are various categories of water pollutants which include organic pollutants, pathogens, nutrients and agricultural runoff, suspended solids and sediments, inorganic pollutants, thermal pollution, and radioactive pollutants. A subset of the organic compounds is volatile organic compounds (VOC) which are compounds that have a high vapor pressure at room temperature and low boiling point. Many VOCs are dangerous to human health or cause harm to the environment. In order to regulate organic compounds in the effluent from municipalities, industry, and waste treatment plants, it is necessary to measure the concentration of certain compounds.

EPA Method 624.1 is a method for the determination of purgeable organic pollutants in various discharges and environmental samples with concentration by purge and trap (P&T) and detection by gas chromatography with mass spectrometry (GC/MS). Analytes of interest are those required by a regulatory authority, in a permit, or by a client. If a list of analytes is not specified, the analytes listed here must be determined at a minimum. The list may be expanded to compounds depending upon permitting.

Many of the listed compounds have poor purging efficiencies and/or poor chromatography. The original method itself was developed and validated through inter-laboratory studies more than 29 years ago. The purge and trap parameters were restricted to purging the sample for 11 minutes at 40 ml/minute at ambient temperature and desorbing for 4 minutes. The method also had a limited analyte list. Many of the water-soluble compounds respond better with heated purge, and the 4 minute desorb time is problematic since it transfers more water to the GC/MS than a shorter desorb time. The method was updated in 2017 and is a performance-based method. New technology such as capillary columns, better purge and traps, optimized instrument parameters, and more sensitive GC/MS instruments will enable laboratories to achieve better calibrations and lower reporting levels.

The OI Analytical 4760 purge and trap along with the 4100 sample processor automates the analysis of samples for VOCs. The 4760 has innovative components such as the Cyclone Water Management system, Infra-Sparge heating, heated sparger during the bake to reduce carryover, and direct resistance heating of the trap at >1000 degrees C/minute. The efficiency of the water management system allows for a more accurate analysis of polar compounds and less system downtime.

Left - Soil Mode, Right - Water Mode

The 4100 has two sampling stations; one for water and one for soil. In the water mode vials containing water are lifted up to a sampling needle that aspirates the sample to fill a loop which then transfers the sample to the purge and trap while adding internal standards and surrogates. In the soil mode vials containing samples are lifted to engage the probe which injects a programmable amount of water with internal standards and surrogates. The sample is heated and stirred while inert gas purges the sample in the vial to deliver the VOCs to the purge and trap. The 4100 has two 3-ml reservoirs which are electronically controlled for high-speed injection of internal standards and surrogates.

Samples for Method 624.1 often contain surfactants that can foam over when purging using a frit sparger. The 4760 has a foam sensor that detects foaming and stops the run to prevent foam from getting into the sample pathway. Another option that can be used especially for very “soapy” samples is to purge the sample in soil mode. Less foaming is produced because the sample is purged in a 40 ml vial rather than a frit sparger. Running this way is also better for samples that contain a lot of sediment or suspended solids which may clog the water needle.

Method performance is significantly improved with the allowance of better P&T parameters, such as a 0.5 minute desorb and heating the sample during the purge, as well as many instrument improvements made over the years. OI Analytical offers state of the art instrumentation as well as superior customer support.


OI Analytical 4100 & 4760 Purge and Trap