Carbon and nitrogen analysis using HiPerTOC/TNb

Figure 1. The Thermo HiPerTOC and Total Nitrogen analyser in the Physical Geography laboratories

The HiPerTOC/TNb (Figure 1) analyser is used to measure total carbon (TC), total organic carbon (TOC), total inorganic carbon (TIC) and total nitrogen (TN) in water samples.

Within the HiPerTOC/TNb are four main analytical methodologies for analysing carbon and nitrogen in water samples:

- a high temperature reactor oxidizes carbon to CO2 at either 680oC with a catalyst or 1000oC without a catalyst.

- a UV-persulphate method whereby sodium persulphate is used to create hydroxyl radicals which, under the influence of UV light decompose carbon

- an ozone assisted UV-persulphate method decomposes samples includes the use of ozone in the decomposition of carbon compounds.

- an ultrapure UV method, in which no reagent is added and hydroxyl molecules are formed from oxygen under the influence of UV-light.

The choice of method for analysis of TOC depends on the sample matrix. High temperature combustion is particularly suitable for non-saline samples with highly resistant carbon. UV-persulphate method is recommended for non-saline dissolved carbon samples. The ozone assisted UV persulphate method is recommended for samples with high salt content to prevent chloride interference with the reaction. The ultrapure UV method is used where very low levels of carbon are analysed.

We use the HiPerTOC/TNb analyser to analyse carbon and nitrogen in a wide range of samples, including:

• estuarine samples• marine porewater samples • small volume (< 200 l) samples• groundwater and drinking water samples• soil porewaters

Dr Kate Heppell is investigating the transport of organic carbon during storm flows in a small upland catchment in Exmoor. Preliminary results show that dissolved organic carbon (DOC) transport varies considerably over storm events (Figure 3). This can give us valuable clues to the sources of DOC and transport paths through the catchment.

Projects using the HiPerTOC/TNb

Methods for TOCTOC analysis

Examples of research at QMUL

TN analysis Nitrogen is also released from organic matter decomposition using the high temperature combustion technique. Gases leaving the infrared detector are fed into a nitrogen detector.

TIC analysisTotal inorganic carbon (TIC) can be analysed by adding hydrochloric acid to samples within the UV reactor. This converts carbonates to CO2 which is measured as above.

TC/TOC analysisTotal carbon (TC) can be analysed by any of the four techniques above. To analyse TOC hydrochloric acid must first be added to samples to samples to remove any inorganic carbon.

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Figure 2. HiPerTOC software showing 50 mg l-1 carbon peak

Figure 3. R. Holne, dissolved organic carbon concentrations during a storm event (October 2006).

After oxidation, carbon dioxide is measured using two infrared detectors (one x10 more sensitive than the other). Figure 2 shows a typical carbon signal measured using the ozone assisted UV persulphate method.