TOC Theory
TOC is a popular analytical technique in water quality testing, as seen in many official analytical methods today. The United States Pharmacopoeia (USP), European Pharmacopoeia (EP) and Japanese Pharmacopoeia (JP), recognizes TOC as a required test for purified water and water for injection (WFI). As stated in USP method <643> on Total Organic Carbon:

"TOC is an indirect measure of organic molecules present in pharmaceutical waters measured as carbon. Organic molecules are introduced into the water from the source water, from purification and distribution system materials, and from biofilm growing in the system. TOC can also be used as a process control attribute to monitor the performance of unit operations comprising the purification and distribution system."

TOC has also found wide acceptance in the biotechnology industry to assist in the validation cleaning procedures, especially clean-in-place (CIP). TOC concentration levels can be used to track the success of these cleaning procedures.
Since the relationship between Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD), and TOC was established in the 1970s, TOC analyzers have become an analytical backbone in many water treatment and quality control laboratories worldwide. In environmental applications, TOC is an essential environmental parameter used to measure wastewater from municipal and industrial sources.

TOC has a long history as being a world-renowned analytical technique to measure water quality during the drinking water purification process. Before source water is treated for disinfection, TOC provides an important role in quantifying the amount of natural organic matter (NOM) in the water source. Many researchers have determined that higher levels of NOM in source water during the disinfection process will increase the amount of carcinogens, called disinfection by-products (DBPs), in the processed drinking water. Today, Environmental Protection Agencies regulate the trace limits of DBPs in drinking water. In recent methods, such as USEPA method 415.3, D/DBP rule, the amount of NOM is regulated to prevent the formation of DBPs in finished waters.

TOC Overview
TOC analyzers can measure:
• Total carbon (TC)
• Total organic carbon (TOC)
• Inorganic carbon (IC)
• Purgeable organic carbon (POC)
• Nonpurgeable organic carbon (NPOC)

TOC measurement involves:
• Oxidizing organic carbon in a sample
• Detecting and quantifying the oxidized carbon (CO2)
• Presenting the result in units of mass of carbon per volume of sample

Total Carbon (TC) – The measure of all the carbon in the sample, both inorganic and organic, as a single parameter. Generally, the measurement is made by placing the sample directly into the analyzer without pretreatment.
Total Organic Carbon (TOC) - The sum of all the organic carbon in the sample.
Direct TOC Measurement – In the direct approach, inorganic carbon is first removed by acidification and sparging, and the remaining carbon is measured as TOC. Inorganic and purgeable organic carbon are not recovered for further analysis in this approach. However, since POC generally represents 1% or less of total carbon in a sample, it is considered negligible.
TOC measurement by Difference - This approach requires two analyses: one to measure TC, and one to measure IC. The difference between these two measurements is rigorously TOC.
TOC Measurement by Sum - This approach measures nonpurgeable organic carbon (NPOC) and purgeable organic carbon (POC). The sum of these measurements is rigorously TOC.
Inorganic Carbon (IC) - Includes carbonate, bicarbonate, and dissolved carbon dioxide. IC is analyzed in liquid samples by acidifying with an inorganic acid to pH 3 or lower, and then sparging with a stream of inert gas. The acidification converts carbonates and bicarbonates to carbon dioxide, which is then removed along with dissolved CO2 by the gas stream, and measured to provide an IC value.
Purgeable Organic Carbon (POC) - Volatile and semi volatile organic materials sparged from a sample. However, these materials are generally less than 1% of total carbon in a sample.

NDIR Pressurization Overview
Pressurized detection, or static read, is a technique used for concentrating the CO2 produced by the oxidation of the sample. The technique can be used for both qualitative and quantitative analysis. In this section the basics of the pressurization technique will be described.

Using the traditional use of NDIR technology, these measurements are performed by oxidation of the specific carbon component by UV/Persulfate oxidation to create CO2, which is swept through an NDIR detector. In this technique, the adsorption of the infrared light is measured over time as the CO2 is swept through the detector. The resulting measurement correlates to a peak, which can be integrated and correlated to a concentration.

The use of a pressurized detection scheme, or static read, allows for the specific carbon component to be oxidized and the resultant carbon dioxide swept into the detector using a non-interfering, inert gas, which is metered by a mass flow controller. A valve located at the outlet of the detector prevents the escape of any of the CO2 from the detector. A single measurement can be made to determine the amount of CO2 in the detector cell. The reading correlates directly to the concentration of the carbon contribution from the sample.

An inherent advantage of this technique is that all of the CO2 is in the cell at the same time for the detector measurement. With all of the CO2 in the cell, the sensitivity of the analysis is significantly increased.

Another advantage of this application is that there is one measurement made that represents the concentration of CO2 in the cell versus multiple measurements made in flow-through designs over time that result in a peak. Since this technique is a static read, it eliminates the inherent error that is associated with time delays between measurements with traditional flow-through technology. These time delays add error to the integration of the CO2 peak. The elimination of this error allows for lower detection limits and increased precision.

The static read of the detector is accomplished by pressurizing the detector cell with carrier gas, which contains the CO2 from the oxidized sample. The pressure required for static read is generally between 30-60psig. Tekmar’s recommended pressurization setting is 50psig.