This instrument is capable of measuring the amount of carbon in a sample. Carbon can be present in water in two forms: organic and inorganic. Inorganic carbon includes carbonate, bicarbonate, and carbon dioxide. Organic carbon includes hydrocarbons and carbohydrates.

Initially, the total carbon concentration in the sample is determined by catalytic oxidative combustion. This process uses high temperature to decompose all carbon containing molecules present to the most fundamental and detectable form: carbon dioxide. Then, the instrument measures the amount of carbon dioxide present via an infrared detector. Inorganic carbon is measured by lowering the pH of the sample so that all the inorganic carbon is decomposed to carbon dioxide. The carbon dioxide is then purged from the sample to the infrared detector for measurement. The inorganic carbon value is subtracted from the total carbon value to determine the total organic carbon present: TOC=TC-IC.

Fundamental to developing a treatment scheme for a wastewater is knowing the elemental composition of the water matrix. The EPA routinely screens wastewater treatment effluent using ICP-OES for the presence of regulated metals. This is done to ensure treatment technologies remove these metals to acceptable concentrations for the protection of human and environmental health. Similar to GC-MS, ICP-OES incorporates an analyte separation mechanism (ICP) with a detection mechanism (OES). First, the sample is introduced to a plasma, at several thousand degrees Celsius, vaporizes and dissociates the sample into its elemental components.

The high energy of the plasma excites electrons in each element to a higher energy state. After excitation, they are relaxed to their ground state and, as a result, emit light at wavelengths unique to that element. This characteristic light strikes a detector, generating an electrical signal proportional to the element’s concentration.

Often times in water treatment, it’s advantageous to know the exact compounds that are in the water to be treated so that treatment can be tailored to problematic compounds. In contrast to bulk measurements, like TOC, GC-MS analysis can determine the mass of individual compounds, providing another identification tool over GC-FID.

Like GC-FID analysis, the sample passes through a column (I.E. the stationary phase) and compounds are separated based on size, weight, and overall charge. Then, the column is slowly heated and compounds are carried off the column in the carrier gas (I.E. the mobile phase) to the mass spectrometer when the column reaches the boiling temperature for each compound. After leaving the column, the compounds enter the mass spectrometer and are ionized via bombardment with electrons from a charged filament. Once charged, they can be further separated and manipulated by electromagnetic fields before striking a detector, where their mass and abundance is determined by the magnitude of the electric signal generated. 

A valuable aspect of GC-MS is that when the compounds are ionized, they often fragment before striking the detector. The mass of each of these fragments strikes at the same time, creating a “mass spectrum”. This spectrum can be compared against a database of thousands of known spectra for compound identification. Otherwise, it is left to the analyst to piece the fragments together to identify unknown compounds.