Problem: Sample matrices are known to create analytical obstacles during routine Total Organic Carbon (TOC) analysis. Until now, interferences from sample matrices have caused calibration curve stability to be sacrificed. Some common aggressive matrices require frequent maintenance and weekly, or even daily, calibration.

Supercritical Water is formed once pressure can no longer be increased to maintain the liquid phase. Beyond 374 °C and 218 atm (3200 psi), the gas and liquid phases merge to form another phase of matter.

Solution: By re-engineering the sample flow path and the oxidation technique, one manufacturer has developed a robust TOC analyzer to hold a calibration curve for up to six months for even the most troublesome brine matrices.

The process used is Supercritical Water Oxidation (SCWO). The novel approach to this wet chemical oxidation technique employs both heat and pressure. The increased pressure within the reaction cell dramatically increases the efficiency of the oxidation process, thereby offering better recovery for difficult matrices. Unlike combustion techniques, this process completely removes all oxidation byproducts from the sample flow path between every sample run.

When in a supercritical state, water exhibits the characteristics and benefits of both a liquid and a gas. The SCW has a density closer to that of a liquid, but can still diffuse like a gas. Organic material and gases become highly soluble in SCW and, conversely, inorganic salts become insoluble. These conditions are ideal for SCWO reactions.

When measuring TOC, several techniques are used to oxidize the organic carbon in the sample to form carbon dioxide (CO2). Once the CO2 is formed, it can be detected and quantified. The main problem facing TOC analysts is ensuring efficient oxidation of the organic carbon. Using a wet chemical oxidation technique, the process seeds the solution with an oxygen donating reagent. Systems such as GE’s Sievers InnovOx employ a 30 percent weight/volume solution of sodium persulphate as the oxidizer. It then heats the sample and oxidizer in a sealed reactor past the critical point and SCWO is achieved.

When water reaches a supercritical state, organic material and gases become highly soluble in SCW, while inorganic salts become insoluble. This is very important, since salts will typically scavenge the oxidizer, resulting in an incomplete organic carbon to carbon dioxide conversion. This SCW medium is the perfect environment for efficient sample oxidation.

By utilizing the fundamental supercritical properties of water, wet chemical oxidation has proven to be significantly more reliable and robust than the original combustion technique. The 375 °C and 218 atm (3200 psi) combination have allowed the process to achieve ultra-efficient conversion of organic carbon to carbon dioxide.

Controlling the purging of reaction by-products and matrix impurities between each analysis, SWCO systems offer something not available before: long-term system integrity. Each analysis starts with a clean sample path that ensures data accuracy, calibration robustness, and extended periods of time between routine system maintenance activities.

SWCO appears to be a very innovative way to perform TOC analysis on what were once considered very difficult or impossible sample matrices.

For more information, go to www.geinstruments.com