What is total organic carbon?

What is Total Organic Carbon?

All organic molecules have carbon in them: Total organic carbon (TOC) is the measure of the concentration of organic carbon in a substance and is considered to be the best indicator of contamination, or purity, of a substance. It is also possible to measure the level of inorganic carbon (TIC), total carbon (TC) and even purgeable and non-purgeable organic carbon (POC and NPOC).

Why is measuring TOC important?

As an analytical technique, TOC enables companies across a range of industries to know whether the water they are using is clean enough for their processes and purposes and will not lead to environmental damage once it leaves their site.

TOC can also test the different cleaning methods used in various industries, helping ensure that systems which need non-contaminated water can do so with confidence.

Why Does Water Purity Matter?

Water has numerous applications in various industrial processes. For example, it is used in the pharmaceutical industry, in the manufacture of semiconductors, and in generating power. Drinking water must also be free of potentially harmful contaminants.

These, and other applications, depend on water being pure. If it is contaminated or polluted then it can lead to very serious issues and challenges for both industries and their customers or consumers.

For example, in the pharmaceutical industry, high purity water is one of the essential components in several production processes. Bacterial contamination can severely compromise product quality.

Contaminants in water can be due to filtration systems failing, or problems with storage or other components or processes.

The types of water which typically require testing in this way are:

  • Public drinking water supplies
  • Industrial wastewater
  • Water in power plants
  • Water used by pharmaceutical manufacturers
  • Water use in electronics manufacturing

The two main reasons water needs to be tested are for either process, or quality control, or to comply with specific legislation. By testing water for its purity, and measuring levels of contaminants, industries can help to protect their processes, products and consumers.

What is the methodology of a TOC analysis?

Any TOC analysis involves three stages:

  • Sampling
  • Oxidation
  • Detection

Sampling

Samples can be introduced into the analyser either manually or by the use of an autosampler.

Most TOC analysers use a flow-injection sample-introduction technique where a syringe pump pulls an aliquot of sample thought a 6/8 port valve into a reservoir, switched the valve and then adds diluent before injecting the sample into the combustion furnace or UV cell. Some, more advanced, systems do not require a reservoir.

An alternate sample-introduction approach is to use a direct-injection sample-introduction where the sample is directly injected onto the catalyst inside the furnace.

Flow-injection is a technique normally used for very clean samples as both the 6/8 port valve and syringe pump are susceptible to particulates either blocking the flow or damaging the seals. The direct-injection approach can handle far ‘heavier and dirty’ samples as it has no valves or seals that can get blocked.

Oxidation

There are various oxidation methods with the three most common being

  • Photocatalytic oxidation – using UV light
  • Chemical oxidation – using a combination of sodium persulfate and UV light
  • High temperature combustion – using a combination of an oxidation catalyst and hight temp.

Simple photocatalytic oxidation instruments are less common these days, and it is important that a high-temp system has a high-enough range to ensure all the carbon bonds can be broken.

Detection

There are two types of detection systems that TOC analysers use:

  • Non-dispersive infrared (NDIR)
  • Conductivity detectors.

NDIR measures the level of carbon dioxide by analysing the absorption spectrum of CO2 in the infra-red region of the spectrum. The sample-generated CO2 passes between an infra-red source and a detector; differences in absorption profiles indicate the different levels of CO2 in the sample

A conductivity detector measures the level of dissolved carbon dioxide in a sample as this dissolved CO2 forms a (very) weak acid. The differences in conductivity indicate the different levels of CO2 in the sample.

NDIR detectors are more commonplace; they are more robust, are less prone to interferences from ions and can usually work over a wider dynamic range.

Conductivity detectors are cheaper and don’t require carrier gas, but they have a more limited dynamic range and are far more susceptible to interferences.

What types of carbon can a TOC analyser measure?

Using TOC analysis, you can measure different forms of carbon in a sample:

  • Total organic carbon (TOC)
  • Total carbon (TC)
  • Total inorganic carbon (TIC)
  • Purgeable organic carbon (POC)
  • Non purgeable organic carbon (NPOC)
  • Dissolved organic carbon (DOC)

Although TOC is by far the most prevalent parameter to be measured different industries, and applications, sometimes need to know what levels are present of other carbon forms, so it is very important that the TOC analyser can measure all the different carbon species above.

How do you calculate TOC?

TOC will determine the sum of all organic compounds within a sample, but it is a non-specific form of measurement. This means it cannot determine which individual organic compounds are present.

Most compounds are complex combination of thousands of different organic carbon compounds.

In a TOC analysis there are two common calculations/methods that are used to determine the TOC value. They are the differential method and the NPOC method.

The differential method requires two analytical measurements. One measurement is for the total carbon in the sample and the other is for the total inorganic carbon (the sample is acidified and the liberated inorganic carbon is converted to carbon dioxide and then measured). The TOC value is then calculated using the simple TOC = TC – TIC formula.

The NPOC method is performed by acidifying the sample and purging for several minutes to drive off the purged carbon. An aliquot is then analysed and the NPOC value is taken to be the same as the TOC value.

The differential method has the advantage that the final answer includes volatile organic carbon but if the level of inorganic carbon is particularly high in the original sample then this method is not so suitable. As it takes two aliquots the overall measuring time is also longer.

The NPOC method works better for samples that have high levels of TIC, but the disadvantage of this approach is volatile organic carbon compounds are not measured.

Most TOC analysers allow either method to be used, and some instruments will even give the TIOC value when using the NPOC method

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