Total organic carbon (TOC)

Total organic carbon (TOC) is the amount of carbon bound in an organic compound and is often used as a non-specific indicator of water quality or cleanliness of pharmaceutical manufacturing equipment.
TOC measures both the total carbon (TC) present as well as the inorganic carbon (IC). Subtracting the inorganic carbon from the total carbon yields TOC. Another common variant of TOC analysis involves removing the IC portion first and then measuring the leftover carbon. This method involves purging an acidified sample with carbon-free air or nitrogen prior to measurement, and so is more accurately called non-purgeable organic carbon (NPOC).
Whether the analysis of TOC is by TC-IC or NPOC methods, it may be broken into three main stages:
Acidification
Oxidation
Detection and Quantification
The first stage is acidification of the sample for the removal of the IC and POC gases. The release of these gases to the detector for measurement or to the air is dependent upon which type of analysis is of interest, the former for TC-IC and the latter for TOC (NPOC).
Prepared samples are combusted at 1,350o C in an oxygen rich atmosphere. All carbon present converts to carbon dioxide, flows through scrubber tubes to remove interferences such as chlorine gas, and water vapor, and the carbon dioxide is measured either by absorption into a strong base then weighed, or using an Infrared Detector. Most modern analyzers use non-dispersive infrared (NDIR) for detection of the carbon dioxide.
A manual or automated process injects the sample onto a platinum catalyst at 680o C in an oxygen rich atmosphere. The concentration of carbon dioxide generated is measured with a non-dispersive infrared (NDIR) detector.
Photo-Oxidation (UV Light)
Ultra-violet light alone oxidizes the carbon within the sample to produce CO2. The UV oxidation method offers the most reliable, low maintenance method of analyzing TOC in ultra-pure waters.
UV/Chemical (Persulfate) Oxidation
UV light is the oxidizer but the oxidation power of the reaction is magnified by the addition of a chemical oxidizer, which is usually a Persulfate compound. The mechanisms of the reactions are as follows:
Free radical oxidants formed
S2O82- ---hv----> 2SO4–
H2O ------> H+ + OH
SO4– + H2O ----> SO42– +OH + H+
Excitation of organics:
R----> RX
RX + SO4– + OH ----> nCO2 + …
UV/chemical oxidation method offers a relatively low maintenance, high sensitivity method for a wide range of applications. However, there are oxidation limitations of this method. Limitations include the inaccuracies associated with the addition of any foreign substance into the analyte and samples with high amounts of particulates. Performing "System Blank" analysis, which is to analyze then subtract the amount of carbon contributed by the chemical additive, inaccuracies are lowered. However, analyses of levels below 200 ppb TOC are still difficult.
Thermo-Chemical (Persulfate) Oxidation
Thermo – Chemical Oxidation is known as heated Persulfate, the method utilizes the same free radical formation as UV Persulfate oxidation except uses heat to magnify the oxidizing power of Persulfate. Chemical oxidation of carbon with a strong oxidizer, such as Persulfate, is highly efficient, and unlike UV, is not susceptible to lower recoveries caused by turbidity in samples. The analysis of system blanks, necessary in all chemical procedures, is especially necessary with heated Persulfate TOC methods because the method is so sensitive that reagents cannot be prepared with carbon contents low enough to not be detected.
Persulfate methods are used in the analysis of wastewater, drinking water, and pharmaceutical waters. When used in conjunction with sensitive NDIR detectors heated Persulfate TOC instruments readily measure TOC at single digit parts per billion (ppb) up to hundreds of parts per million (ppm) depending on sample volumes.

Detection and Quantification
Accurate detection and quantification are the most vital components of the TOC analysis process. Conductivity and non-dispersive infrared (NDIR) are the two common detection methods used in modern TOC analyzers.
Conductivity
There are two types of conductivity detectors, direct and membrane. Direct conductivity provides an inexpensive and simple means of measuring CO2. This method has good oxidation of organics, uses no carrier gas, is good at the parts per billion (ppb) ranges, but has a very limited analytical range. Membrane conductivity relies upon the same technology as direct conductivity. Although it is more robust than direct conductivity, it suffers from slow analysis time. Both methods analyze sample conductivity before and after oxidization, attributing this differential measurement to the TOC of the sample. During the sample oxidization phase, CO2 (directly related to the TOC in the sample) and other gases are formed. The dissolved CO2 forms a weak acid, thereby changing the conductivity of the original sample proportionately to the TOC in the sample.

Non-dispersive infrared (NDIR)
The non-dispersive infrared analysis (NDIR) method offers the only practical interference-free method for detecting CO2 in TOC analysis. The principal advantage of using NDIR is that it directly and specifically measures the CO2 generated by oxidation of the organic carbon in the oxidation reactor, rather than relying on a measurement of a secondary, corrected effect, such as used in conductivity measurements. A traditional NDIR detector relies upon flow-through-cell technology, the oxidation product flows into and out of the detector continuously. A region of adsorption of infrared light specific to CO2, usually around 4.26 µm (2350 cm-1), is measured over time as the gas flows through the detector. A new advance of NDIR technology is Static Pressurized Concentration (SPC). The exit valve of the NDIR is closed to allow the detector to become pressurized. Once the gases in the detector have reached equilibrium, the concentration of the CO2 is analyzed. This pressurization of the sample gas stream in the NDIR, a patent-pending technique, allows for increased sensitivity and precision by measuring the entirety of the oxidation products of the sample in one reading.

Combustion
In a combustion analyzer, half the sample is injected into a chamber where it is acidified, usually with phosphoric acid, to turn all of the inorganic carbon into carbon dioxide as per the following reaction:
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32-
This is then sent to a detector for measurement. The other half of the sample is injected into a combustion chamber which is raised to between 600–700°C, some even up to 1200°C. Here, all the carbon reacts with oxygen, forming carbon dioxide. It's then flushed into a cooling chamber, and finally into the detector. Usually, the detector used is a non-dispersive infrared spectrophotometer. By finding the total inorganic carbon and subtracting it from the total carbon content, the amount of organic carbon is determined.

Chemical Oxidation
Chemical oxidation analyzers inject the sample into a chamber with phosphoric acid followed by Persulfate. The analysis is separated into two steps. One removes inorganic carbon by acidification and purging. After removal of inorganic carbon Persulfate is added and the sample is either heated or bombarded with UV light from a mercury vapor lamp. Free radicals form from the Persulfate and react with any carbon available to form carbon dioxide. The carbon from both determination (steps) is either run through membranes which measure the conductivity changes that result from the presence of varying amounts of carbon dioxide, or purged into and detected by a sensitive NDIR detector. Same as the combustion analyzer, the total carbon formed minus the inorganic carbon gives a good estimate of the total organic carbon in the sample. This method is often used in online applications because of its low maintenance requirements.

Total Carbon (TC) – all the carbon in the sample, including both inorganic and organic carbon
Total Inorganic Carbon (TIC) – often referred to as inorganic carbon (IC), carbonate, bicarbonate, and dissolved carbon dioxide (CO2); a material derived from non-living sources.
Total Organic Carbon (TOC) – material derived from decaying vegetation, bacterial growth, and metabolic activities of living organisms or chemicals.
Non-Purgeable Organic Carbon (NPOC) – commonly referred to as TOC; organic carbon remaining in an acidified sample after purging the sample with gas.
Purgeable (volatile) Organic Carbon (POC) – organic carbon that has been removed from a neutral , or acidified sample by purging with an inert gas. These are the same compounds referred to as Volatile Organic Compounds (VOC) and usually determined by Purge and Trap Gas Chromatography.
Dissolved Organic Carbon (DOC) – organic carbon remaining in a sample after filtering the sample, typically using a 0.45 micrometer filter.
Suspended Organic Carbon – also called particulate organic carbon (PtOC); the carbon in particulate form that is too large to pass through a filter.