Total organic carbon (TOC) is the amount of carbon in an organic compound and is often used as a non-specific indicator of water quality or cleanliness of pharmaceutical manufacturing equipment. Organic substances are monitored by oxidizing them and detecting the resulting oxidation products. By convention, the measurement is expressed as total organic carbon (TOC) and reported in ppb or µg/L.
A number of different approaches can be used to determine TOC.
Off-line Measurements made on water samples that are collected into containers and transported to an instrument are considered to be off-line. Off-line measurement is not recommended when the TOC specification for purified water is below 50 ng/g (ppb) because the potential for contamination of the sample by organic and inorganic substances in ambient laboratory air, transfer systems, and containers will likely introduce significant error.
Containers should be validated as suitable for TOC determination by measuring blanks.
Containers should be closed immediately after filling and their closures should be protected from contamination that could enter the sample when they are opened (e.g., by placing the containers in plastic bags). Certified, precleaned containers are available with a secondary cover for their closures. If samples will not be analyzed within 24 hours, they should be protected from light and stored in a refrigerator but not frozen.
On-line For on-line measurements, the instrument is connected directly to the purified water stream. On-line instruments permit continuous or semicontinuous determination of organic contamination. On-line measurement is recommended for TOC specification levels of <50 ng/g (ppb) and highly recommended when TOC specification levels are <20 ng/g (ppb).
On-line measurements use a variety of sampling designs appropriate to the instrumentation, accuracy, and frequency of monitoring desired. The instrument can be connected to the main water stream by means of a connection designed to prevent contamination and to divert a sample portion of the stream for continuous or intermittent analysis. Depending on instrument design and other factors, the oxidized water sample can be directed to waste or recycled through an upstream stage of the water purification system to remove contaminants introduced by oxidation. Alternatively, a portion, or all, of the main water stream may pass through the measuring cell and continue on with the main stream, in which case there should be a subsequent repurification step to remove impurities introduced by oxidation.
2- Oxidation of Organic Molecules
Purified water can contain a wide variety of organic compounds. Some organic molecules are oxidized more easily than others. Oxidation does not take place instantaneously, and the kinetics of oxidation depend on the nature of the organic material present and the oxidation conditions. The following techniques have been successfully used in various instruments for oxidation of organic species in purified water samples:
185 nm and 254 nm UV light;
185/254 nm or 300 to 400 nm UV light combined with a catalyst;
persulfate and 185/254 nm UV light at room temperature or 90 °C;
persulfate at 100 °C;
high temperature (680 to 1050 °C) catalytic oxidation.
As a general rule, complete oxidation of organic molecules contributes to the accuracy of a TOC result. Some instruments oxidize for a fixed period of time that has been qualified to ensure that all organic molecules likely to be in the water sample are fully oxidized. Other instruments use dynamic techniques to determine when oxidation is complete by, for example, waiting until no more CO2 is produced or the resistivity of the sample reaches a constant level. Some instruments are designed to partially oxidize the organic contamination in a sample.
3- Instruments That Determine TOC
Instruments that determine TOC are designed to detect CO2 selectively; their detectors respond minimally, if at all, to the other products of organic oxidation.
TC (total carbon) is the total concentration of carbon (organic and inorganic) in a sample.
TOC (total organic carbon) is the total concentration of carbon contained in organic molecules in a sample. Elemental carbon is included as organic carbon.
TIC (total inorganic carbon) is the total concentration of carbon in the form of carbonate (CO3=), bicarbonate (HCO3–), and dissolved CO2 in a sample.
POC (purgeable organic carbon) is the concentration of carbon that escapes the sample in the gas phase during the process of sparging the sample to remove inorganic carbon prior to measuring the organic carbon. POC depends on the instrument design and operating conditions as well as the nature of specific organic molecules present.
NPOC (nonpurgeable organic carbon) is the concentration of organic carbon remaining after sparging a sample to remove inorganic carbon. NPOC depends on the instrument design and operating conditions as well as the nature of specific organic molecules present that can be lost as POC.
Nondispersive Infrared (NDIR) Detection NDIR detectors measure CO2 in a dry gas phase and are specific for CO2. TOC instruments that use NDIR detectors can be designed to make two measurements, TIC and TC. First, the water sample is acidified (pH 2) to convert TIC to CO2, which is sparged by carrier gas through the NDIR detector in a recirculating loop. Because NDIR detectors are specific for CO2 molecules, the concentration of TIC can be determined, even though other inorganic gases and POC may be present in the recirculating loop. After the TIC has been determined, the recirculating gas loop is kept closed, the sample is oxidized, and the TC is determined. There is no loss of POC, because both NPOC and the recirculated POC are oxidized. Therefore, subtracting TIC from TC will equal TOC. As the TIC concentration increases with respect to TC, the uncertainty of TOC calculations will increase, because two independent measurements, each with an associated error, are being subtracted. Traces of moisture and impurities in the gas phase, the purity of the acidifying and oxidizing reagents, and the relative volume of the gas phase can also affect the limits of detection and accuracy.
When water purification systems produce water with a resistivity approaching that of pure water, and measurements are made on-line to prevent CO2 absorption from air, TIC will be very low and, depending on the TOC limit required, may be negligible and the step of an analysis that is intended to calculate, or remove, TIC can be omitted. In this special case, there will be no loss of POC, and instruments with this design will determine TOC.
Resistivity Detection Resistivity detectors cannot be specific for CO2 unless ions other than CO3=, HCO3– and H+ are excluded. Interposing a membrane that is selective for small gas molecules such as CO2 (e.g. AF, or similar) between a sample chamber and a resistivity cell can achieve the necessary selectivity. TOC instruments that use resistivity detectors combined with such membranes can be operated in much the same way as NDIR detectors. Some of these instruments split samples into two essentially matched paths to achieve greater stability. In one path, the sample is acidified to determine TIC, and in the other path the sample is oxidized and acidified to determine TC.
TOC detection is an important measurement because of the effects it may have on the environment, human health, and manufacturing processes. TOC is a highly sensitive, non-specific measurement of all organics present in a sample. It, therefore, can be used to regulate the organic chemical discharge to the environment in a manufacturing plant. In addition, low TOC can confirm the absence of potentially harmful organic chemicals in water used to manufacture pharmaceutical products. TOC is also of interest in the field of potable water purification due to disinfection of byproducts. Inorganic carbon poses little to no threat.
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