Purge Trap with Colorimetric technique
This is an on-line system that can give a reading every 4 hours or more.
In this method the instrument extracts THMs from the water sample by P/T on an adsorbent material and the concentrated THMs are subsequently desorbed into a proprietary reagent mixture that generates a colored product.
Because of the cost of reagents the system is very expensive to run (tens of thousands of dollars per year for the reagents) and usually it is not feasible to have a measurement every hour.
The method also means that the instrument has an inherent complexity and many different components, which can lead to expensive repair and ongoing issues.
Purge/Trap – Gas Chromatography – Surface Acoustic Wave Detector
This is a laboratory instrument which has been adapted for on-site application.
The monitor is an integrated purge-and-trap system connected to a compact gas chromatograph column and a surface acoustic wave (SAW) detector. The system requires Helium gas, an external manual calibration and needs to be operated by skilled technicians.
Also, because of the nature of the technology it is impossible to use it as an on-line system: the operator will have to go on site, take a sample, wait 30 minutes and get the result.
Membrane Permeation – Gas Chromatography – Electron Capture Detector
This is an experimental method, not yet widely accepted in industry
In this kind of system, THMs are extracted from a water sample into a carrier gas. This is achieved by forcing the water through a permeable membrane, then adsorbing the THMs onto a trap, followed by separations onto a GC column. Finally the Trihalomethanes are detected by an Electron Capture detector. Using a gas chromatographer for online monitoring generates stability problems as retention times, calibration and system validation.
Extraction through a semi-permeable membrane has a huge impact on sample matrix including ionic strength, pH and temperature. Fouling is a problem that could arise in this kind of systems. The use of the radioactive element 63Ni in the detector is not desirable and such a system would require a carrier gas and be quite expensive.
Fluorescence Detector with Membrane Permeation and Chemical Reaction:
This is an experimental method, not widely accepted in the industry.
In this method, the Trihalomethanes move from the water into the chemical reaction mixture across a semi-permeable membrane in a continuous flow of reagents, followed by measurement of the fluorescence generated from the reaction of the THMs with alkaline nicotinamide. This method requires a lot of reagents. Extraction through a semi-permeable membrane has a huge impact on the sample matrix including ionic strength, pH and temperature. Also fouling is a problem that could arise in this kind of systems.
Process gas chromatograph (GC) with TID (Thermal Ionization Detector) and PID (Photo Ionization Detector)
This system is a traditional process gas chromatograph with TID and PID technology. This technology is quite accurate and allows for speciation. Given the complexity of the instrument it demands a high purchasing cost and frequent recalibration (once per month). Also, in order to work, a source of compressed air is needed not to mention a skilled system’s technician.