Total organic carbon

A typical analysis for total carbon (TC) measures both the total organic carbon (TOC) present and the complementing total inorganic carbon (TIC), the latter representing the amount of non-organic carbon, like carbon in carbonate minerals. Subtracting the inorganic carbon from the total carbon yields TOC. Another common variant of TOC analysis involves removing the TIC 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).

Measurement

Since all TOC analyzers only actually measure total carbon, TOC analysis always requires some accounting for the inorganic carbon that is always present. One analysis technique involves a two-stage process commonly referred to as TOC differential method. It measures the amount of inorganic carbon (IC) evolved from an acidified aliquot of a sample and also the amount of total carbon (TC) present in the sample. TOC is calculated by subtraction of the IC value from the TC of the sample. Another method directly measures TOC in the sample by acidifying the sample to a pH value of two or less to release the CO₂ gas by decomposition of the carbonates and vent these gases to the air by a purge step. The remaining non-purgeable organic carbon (NPOC) contained in the liquid aliquot is then oxidized releasing the CO₂ gases. These gases are then sent to the detector for measurement. This method is also referred to as the direct TOC method. A further variant employs acidification of the sample to evolve carbon dioxide and measuring it as inorganic carbon (IC), then oxidizing and measuring the remaining non-purgeable organic carbon (NPOC). This is called TIC-NPOC analysis. TC oxidizes in a combustion chamber at 1000 degrees Celsius; if no supporting catalyst is used to allow full combustion at temperatures lower than 1000 degrees C; while the combustion chamber for IC heats only to 150 degrees Celsius. The reason for this is because inorganic is decomposed at lower temperatures than organic carbons.

Whether the analysis of TOC is by TC-IC or NPOC methods, it may be broken into three main stages:

1. Acidification
2. Oxidation
3. Detection and Quantification

Acidification

Addition of acid and inert-gas sparging allows all bicarbonate and carbonate ions to be converted to carbon dioxide, and this IC product vented along with any purgeable organic carbon (POC) that was present.

Oxidation

The second stage is the oxidation of the carbon in the remaining sample in the form of carbon dioxide (CO₂) and other gases. Modern TOC analyzers perform this oxidation step by several processes:

• High temperature combustion
• High temperature catalytic oxidation (HTCO)
• Photo-oxidation alone
• Thermo-chemical oxidation
• Photo-chemical oxidation
• Electrolytic oxidation

High temperature combustion

Prepared samples are combusted from 1000 up to 1200 degrees 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. Compared to the conventional high temperature catalytic oxidation, the great benefit of the combustion-method is the high oxidation power, so that oxidation-promoting catalysts are superfluous.

High temperature catalytic oxidation

A manual or automated process injects the sample onto a catalyst in a combustion tube operated from 680 up to 950 degrees C in an oxygen rich atmosphere. The concentration of carbon dioxide generated is measured with a non-dispersive infrared (NDIR) detector.

Oxidation of the sample is complete after injection into the furnace, turning oxidizable material in the sample into gaseous form. A carbon-free carrier gas transports the CO₂, through a moisture trap and halide scrubbers to remove water vapor and halides from the gas stream before it reaches the detector. These substances can interfere with the detection of the CO₂ gas. The HTCO method may be useful in those applications where difficult to oxidize compounds, or high molecular weight organics, are present as it provides almost complete oxidation of organics including solids and particulates small enough to be injected into the furnace. The major drawback of HTCO analysis is its unstable baseline resulting from the gradual accumulation of non-volatile residues within the combustion tube. These residues continuously change TOC background levels requiring continuous background correction. Because aqueous samples are injected directly into a very hot, usually quartz, furnace only small aliquots (less than 2 milliliters and usually less than 50 - 100 microliter, with a maximum of approximately 300 - 400 micro-liters under special conditions; such as repetitive injections) of sample can be handled making the methods less sensitive than chemical oxidation methods capable of digesting as much as 10 times more sample. Also, the salt content of the samples do not combust, and so therefore, gradually build a residue inside the combustion tube eventually clogging the catalyst resulting in poor peak shapes, and degraded accuracy or precision, unless appropriate maintenance procedures are followed. The catalyst should be regenerated or replaced as needed. To avoid this problem the manufacturing industry has developed several concepts, such as matrix separation, ceramic reactors, better process control or methods without catalysts.

Photo-oxidation (ultraviolet light)

In this oxidation scheme, ultraviolet 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.

Ultraviolet/persulfate oxidation

Like the photo-oxidation method, 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:

${\displaystyle {\mathrm{S}}_2{\mathrm{O}}_8^{2-}\underset{hv}{⟶}2{\mathrm{S}\mathrm{O}}_4^{-\bullet }}$

${\displaystyle {\mathrm{H}}_2\mathrm{O}\underset{hv}{⟶}{\mathrm{H}}^{+}+{\mathrm{O}\mathrm{H}}^{\bullet }}$

${\displaystyle {\mathrm{S}\mathrm{O}}_4^{-\bullet }+{\mathrm{H}}_2\mathrm{O}⟶{\mathrm{S}\mathrm{O}}_4^{2-}+{\mathrm{O}\mathrm{H}}^{\bullet }+{\mathrm{H}}^{+}}$

Excitation of organics:

${\displaystyle \mathrm{R}\underset{hv}{⟶}{\mathrm{R}}^{\ast }}$

Oxidation of organics:

${\displaystyle {\mathrm{R}}^{\ast }+{\mathrm{S}\mathrm{O}}_4^{-\bullet }+{\mathrm{O}\mathrm{H}}^{\bullet }⟶n{\mathrm{C}\mathrm{O}}_2+\dots }$

The 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.

Thermochemical persulfate oxidation

Also 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 all-encompassing approach of measuring CO₂. This detection method 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 filtering of the CO₂ prior to measuring it with a conductivity cell. 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. Conductivity analyses assume that only CO₂ is present within the solution. As long as this holds true, then the TOC calculation by this differential measurement is valid. However, depending on the chemical species present in the sample and their individual products of oxidation, they may present either a positive or a negative interference to the actual TOC value, resulting in analytical error. Some of the interfering chemical species include Cl⁻, HCO₃⁻, SO₃²⁻, SO₂⁻, ClO₂⁻, and H⁺. Small changes in pH and temperature fluctuations also contribute to inaccuracy. Membrane conductivity analyzers have improved upon the direct conductivity approach by incorporating the use of hydrophobic gas permeation membranes to allow a more “selective” passage of the dissolved CO₂ gas and nothing else. This provides a more precise and accurate measurement of the organics that were converted to CO₂.

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, where the oxidation product flows into and out of the detector continuously. A region of absorption 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 second reference measurement that is non-specific to CO₂ is also taken and the differential result correlates to the CO₂ concentration in the detector at that moment. As the gas continues to flow into and out of the detector cell the sum of the measurements results in a peak that is integrated and correlated to the total CO₂ concentration in the sample aliquot.

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 patented technique, allows for increased sensitivity and precision by measuring the entirety of the oxidation products of the sample in one reading, compared to flow-through cell technology. The output signal is proportional to the concentration of CO₂ in the carrier gas, from the oxidation of the sample aliquot. UV/ Persulfate oxidation combined with NDIR detection provides good oxidation of organics, low instrument maintenance, good precision at ppb levels, relatively fast sample analysis time and easily accommodates multiple applications, including purified water (PW), water for injection (WFI), CIP, drinking water and ultra-pure water analyses.