A Close Look at the Specialists: TOC and TNb in Every Matrix

For several decades, the TOC parameter has stand for the total of all organic contaminants in water samples, soils, sediments and construction rubble. Since it is often impossible or simply unnecessary to record the many possible organic compounds separately in a single substance analysis, the TOC sum parameter provides a quick and cost saving alternative for recording a sample's total organic contamination. For this purpose the TOC sum parameter uses the common property of all organic compounds – carbon atoms represent an integral part of every organic compound and can be quantitatively converted into carbon dioxide under defined oxidation conditions and selectively detected. This is done either according to the principle of catalytic high temperature combustion in the oxygen flow or via wet chemical UV oxidation with the help of oxidation agents. Optional solids modules also enable through a robust catalytic-free high temperature combustion the combination of solid and liquid TOC in one device.
The TOC has successfully made its way into a number of application fields. In the field of environment analytics, ground, drinking, surface water (river, lake and sea water) and waste water as well as waste and soil eluates are analysed. But also in several industries, the TOC is used as a process control parameter. In the field of power plants, for example, TOC monitoring is vital in the controlling of the corrosion effects of boiler feed water and cooling water circuits. In pharmaceutical industry, TOC threshold values are established for ultrapure water, water for injection purposes (WFI) and for cleaning process control in pharmaceutical manufacturing (cleaning validation). On top, it is indispensable in the chemical and electroplating industry for the contamination control of acids, base, salts and brines where the ruggedness of the instrument technology is of special interest.

What is the difference between common TOC analysers? Are all devices suitable for this wide range of application fields?
Beside price and quality of the equipment components, the biggest differences consist in the area of sample feed to the reactors as well as in digestion and detection technology. However, it is important to choose the right solution because the differences in these details are crucial. If you want a tailor-made TOC analyser for the specific application, you need a specialist in his field.
The variety of application and benefits of the TOC and TNb sum parameters as well as the applied analyser technology are shown by the selected examples below.
Waste water and solids using the suspension method
The standard DIN EN 15936 ‘Sludge, treated biowaste, soil and waste – Determination of total organic carbon (TOC) by dry combustion’ published in November 2012 with the suspension method contained in informative annex C bring new challenges for liquid TOC analysers.
Both sample matrices have in common that they are solids suspensions with sometimes high contents of particles. DIN EN 1484 defines the TOC as being the sum of dissolved and particle-bound organic compounds. Thus, in the case of TOC analysis it is of special importance to transfer the sample with its solid contents into the reactor in a representative way.
In addition to an effective sample homogenisation on the sample tray of the autosampler, this notably requires a sample introduction technique where particles neither "get lost" on the way nor do very large particles lead to system failure due to blockage or increased wear is to be expected.
In the case of multi N/C® analysers, this is ensured by standard large inner diameters of the sample introducing components as well as by specific rinsing techniques. The principle of direct injection in the case of multi N/C® 2100 especially convinces by its short sample paths without any hoses and valve technology. The wastewater specialist realises the most direct transfer of the sample into the combustion system in combination with a septum-free pneumatic furnace head.
The following original measurement table shows the TOC/TNb results as they occur each day in municipal treatment plants at the measurement of inflows and outflows. Special emphasis was put on the particle handling capacity of the measuring device as well as the recovery of the many included control standards.

Table 1. Display of the measured NPOC/TNb values after triplicate determination with the corresponding relative standard deviation

TOC in the field of power plants
The challenge consists in the formation of corrosive organic acids and carbonic acid in the boiler feed water out of organic compounds, which are easily oxidised under high pressure and high temperature. TOC monitoring in the boiler feed water is crucial for controlling this process. This is a matter of trace contamination in the range below 1 ppm. Salts are often added as oxygen scavengers.

The wet chemical UV digestion technology is especially suitable due to its extraordinary measurement sensitivity and the gentle digestion method compared to the catalytic high temperature combustion. The large injection volume of samples up to 20 ml makes the multi N/C® UV HS one of the most sensitive TOC devices on the market. The multi N/C® UV HS especially stands out through its specifically designed UV reactor, which not only uses the common UV radiation of 254 nm but also the high energy short-wave radiation at 185 nm in order to oxidise organic compounds. This brings the advantage that no addition of the oxidation agent sodium peroxodisulphate, which is burdened by a TOC blank value, is required for the complete oxidation of ultrapure water. The UV radiation and the dissolved oxygen in the sample itself are sufficient to convert all organic compounds into CO2. This allows extremely reliable TOC measurement results, unspoiled by blank values, in the boiler feed water and in other ultrapure waters up to drinking water analysis.
Table 2. The multi N/C® UV HS: lowest concentration at highest precision.
TOC and TNb in smallest sample volumes
Especially in the field of research, when it comes to the determination of contaminations in glacial ice, connate water, soil extracts or rainwater, only small sample volumes (often no more than a few millilitres) are available for the TOC measurement, due to other different parameters to be done.
However, there is a very high number of samples to be analysed in the case of such environmental analysis in order to have an interpretable data base. This calls for automated TOC analysers that do not use the majority of the sample for the rinsing of hoses and valves but for measurement.
The determination of proteins according to Ph.Eur. 2.5.33 in the pharmaceutical industry, whether in vaccines or pollen suspensions, also has this requirement towards a modern analysis system.
The specialist in this field is definitely the multi N/C® 2100 direct injector which allows multiple determinations even with a sample volume of less than 1 ml due to its 100µl syringe. On top of that the 112 sample tray even provides magnetic stirring for an optimal sample homogenisation in the 2 ml vials.

Summary
As can be seen from the examples, many TOC/TNb applications require the special capabilities of a field specialist to be able to do stable and successful analysis in the long term. The multi N/C® series by Analytik Jena AG provides you with the right analyser for each application. With the built-in Self Check System the devices are designed for maximised operation security and ease of use in unattended 24 hour operation. The associated high system availability essentially increases productivity in your laboratory and significantly contributes to the reduction of running costs.