What You Need to Know about TOC analysis

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All TOC analysers perform two functions:

Oxidising organics to form CO2 and then measuring the CO2  produced.

What makes each TOC analyser different is how these two functions are performed.

These differences determine the accuracy, application, reliability, cost of ownership of the analysers. It is important to understand the differences in order to ensure the best solution to your analytical needs.

For ease of explanation, we will deal with each technology separately and go into some basic detail to explain the pros and cons of each technique for you to maximise your understanding.

Firstly, let's look at the Oxidation process:

There are four oxidation methods used commercially in TOC analysers. These are:

  • High Intensity UVUses a high intensity UV lamp to oxidise carbon to CO2. This is a long time proven method used by TOC analysers. The UV light will oxidise samples ranging from less than 1ppb (µg/l) to 2500ppb (2.5ppm or mg/L). This is an ideal method for monitoring purified water in industries such as pharmaceutical manufacturing, semiconductor and power generation. 
  • UV/PersulphateAgain uses high intensity UV but also utilises a chemical oxidant (usually ammonium persulfate) to enhance the oxidation process. The advantage of the UV/Persulphate method is the extended analytical range of the analyser which is less than 1ppb TOC to 50,000ppb (50ppm). This also aproven method used for decades. It is suitable for the same industries as above but also ideal for drinking water, ground water, treated water, treated sewerage and effluent monitoring. Although this is probably the most popular, robust and flexible flexible oxidation method, its only real downside is that it cannot tolerate water samples with high chloride ion levels such as those found in Coal Seam Gas (CSG) mines that have high salt (NaCl) concentrations. This is due the the chloride ion being a "persulphate scavenger". The chloride ion consumes the oxidiser before it has a chance to oxidise the organic carbon. The generally accepted limit for chloride ions is 250ppm. If you sample has these sorts of salt levels then you would need to consider either diluting your sample or an alternative oxidation process such as the ones described below.
  • Combustion - As the name suggests, this technique literally burns the sample in a furnace/combustion tube in conjunction with a catalyst (usually platinum) to convert organic carbon to CO2. It is another proven method and suitable for higher level samples of up to 50,000ppm (50g/L). However this method is rather poor at detecting low concentrations of TOC and contrary to some published information and brochures, this technique loses both accuracy and precision below 500ppb TOC. This technique does have some tollerance to salt laden samples if they are run infrequently. However frequent salt samples will eventually destroy the combustion tube and poison the catalyst - both of which will require replacement after somewhere between 20 and 100 samples - depending on the brand of TOC and the concentration of the salt.  An advantge of combustion oxidation is that it is the only method that will allow you to run solid samples using a combustion boat accessory. This oxidation method is also capable of tollerating particulates - so there is no need to filter your sample. The final thing to consider with combustion which is not covered in any brochure is that though they are capable of measuring samples from 500ppb to 30,000ppm, you cannot just switch from one sample concentration to another. The system will run lower levels (~500ppb or a bit less relatively reliably, but the moment you run say a high level sample of say 1000ppm, the analyser becomes contaminated and need decontamination before you can run a low level sample again. These analysers will suit environmental samples, solids, effluent, treated water, drinking water and salt water applications (though limited).

  • Super Critical Water Oxidation - Is the latest oxidation technique which utilises the chemistry of supercritical fluids to oxidise the sample. In this case, the analyser uses sodium persulphate oxidiser but instead of a UV lamp, it utilises a small furnace tube and heats the sample to 375C. At this temperature, the water sample becomes a supercritical fluid and the water becomes non-polar. The salts precipitate out and the organics are oxidised in an enhanced environment. Ideal for the Coal Seam Gas industry where ground salts are a problem, this method has an extended range wider than combustion - 500ppb to 50,000ppm. It's big advantage over combustion is that it can run salt samples all day long without issue. It can also tollerate particulates and has the advantage of very low running costs. This method has bee available for around 6 years for TOC analysis eventhough the chemistry behind supercritical fluids has been around for decades. It is the most recent and innovative advancement in TOC analysis. It has been adopted by many traditional combustion TOC users that have had problems with running slat laden samples. These analysers will suit drinking water, treated water, effluent, ground water, reclaimed water, brines, any salt water application.

Now, let's look at the measurement process:

Again, TOC analysers differentiate themselves by using one of three types of measurment techniques. Depending on your application, one of these processes will be best suited for you...

  • Conductometric - This process uses a direct conductivity measurement of the oxidised water sample. This method uses two assumptions. Firstly, it assumes that the CO2 formed in the oxidation process dissolves in the water to form carbonic acid (HCO3¯) - which it does. Secondly, it assumes that there are NO other ions present. If the two assumptions hold, then a conductivity measurement is a direct reading of the CO2. However, the measuremnt falls apart if there are any other ions present. If there are, then you end up with a false high result. Because of this, this measurement technique is ONLY suitable for ultra pure water measurement such as in the semiconductor industry and some pharmaceutical applications. It is a reliable method ONLY if the second assumption holds true.
  • Membrane Conductometric - One of the most reliable and robust measurement technologies. Was developed for NASA for TOC analysers used in space (both in shuttle missions and on the space stations). It utilises a gas permeable membrane to separate the CO2 from any possible ion contamination. When the CO2 forms carbonic acid in water, the effect of any ions are removed and the conductivity measurement is a direct reading of the CO2 present. This method has no ion interference in the measurement and has been proven to be both precise and accurate over the range of 0.03ppb to 50ppm. It is suitable for the semiconductor, pharmaceutical, process waters, drinking water, ground water, treated effluent water applications.  A distinct advantage of this method is that the calibration is stable for over 12 months and thus requires little maintenance.
  • NDIR (Non-Dispersive Infrared) - This too is a reliable and robust technology. It uses the fact that CO2 absorbs certain infrared wavelengths. An infrared source emiits the particular wavelegth at a set intensity which is absorbed by the CO2. A detector then measures the amount of infrared that was absorbed (which is proportional to the amount of CO2 present) and calculates the concentration of CO2 accordingly. The advantageof this measurement technique is that it can cover a range up to 50,000 ppm TOC. The disadvantage is that it has a lower limit of around 10 to 250ppm depending on the instrument manufacturer.  Another consideration with NDIR measurement systems is that, depending on the brand of analyser, stability of the calibration could be anywhere from 1 week to 6 months.

Now that we have covered the fundamentals, you may be thinking that all this information is confusing and overwhelming or you may have (hopefully) found some answers to your questions. However, in reality, TOC analysis is a simple science based on fundamental chemistry principles.

Your choice of TOC analyser really comes down to three fundamental questions:

  1. What your expected TOC levels will be, 
  2. Your application and 
  3. What your sample matrix consists of

This is where our TOC Selection Guide helps take the confusion out of choosing the right analyser. Whether you need an online prcess TOC, a Laboratory TOC, or a portable TOC the guide will help you pick the right analyser for your application. Of course, if you have any questions, please feel free to contact us and we would be more than happy to help you, run a sample for you, or even arrange an onsite demonstration.

One other last bit of information for you to consider.... You may have heard the terms TOC, DOC, NPOC and POC. All of these acronyms refer to organic carbon but each one is slightly different from the other. 

  • All water samples contain the following: TC (Total Carbon), IC (Inorganic Carbon) and TOC (Total Organic Carbon)
  • Total carbon is the sum of all carbon species in the sample: TC = TOC + IC
  • TOC is ALL the organic carbon present (including dissolved ions and solid particulates)
  • DOC refers to only Dissolved Organic Carbon (no particulates)
  • TOC is a generic term used nowadays to describe dissolved organics in water
  • TOC is made up of NPOC and POC: TOC = NPOC + POC
  • NPOC is Non Purgeable Organic Carbon (such as dissolved fertilisers, humics, pesticides - basically non-volatile organics)
  • POC is Purgeable Organic Carbon (volatile organics that may be present such as some hydrocarbons)
  • Thus in a water sample (assuming particulates are not of interest): TOC = DOC = NPOC + POC

Some analysers will allow you to measure multiple parameters such as TOC, TC, IC and NPOC. You need to know what parameters are of interest to you. Our consultants will be more than happy to explain things further and in more detail should you require it. We have extensive knowlege regarding TOC and would be happy to share it with you. Just use our contact form at the top right hand corner of this page