Dumas high-temperature combustion technology has advantages for the measurement of TSN for several reasons (McGill and Figueiredo 1993). For example, all forms of soil N can be included without pre-treatments; it can be adapted for 15N tracer studies when linked to an isotope ratio mass spectrometer; it eliminates the need for strong-acid digestions; requires little laboratory bench space, and has multi-element capability. Disadvantages include occasionally lower apparent TSN concentrations in soils with high levels of fixed NH4-N, and the reverse (due to incomplete combustion resulting in the formation of methane rather than CO2) in some organic-rich soils (Bremner 1996).
Nowadays, several manufacturers produce high-temperature combustion analysers suited to the measurement of TSN across a range of concentrations (e.g. 0.02 to >0.5% N). Typically, the dry, finely-ground sample is subjected to high-temperature combustion (e.g. 900–1350°C) in a stream of purifed O2. An aliquot of the gases produced by combustion is carried by helium gas to a thermal conductivity cell for measurement of any N2 generated, a process typically taking 3–5 min. A heated Cu catalyst reduces NOx to N2. The example methodology described is based on the automated LECO™ CNS 2000 analyser.
Finely ground (<0.5 mm), air-dry (40°C) soil samples are required. These are accurately weighed into ceramic boats and then combusted. Water and CO2 are subsequently absorbed in an anhydrone tube, leaving the N2 to be measured by thermal conductivity. Practical quantitation limits depend on the amount of sample used but are typically around 0.01–0.02% N. At all times, workplace health and safety procedures need to be in place and followed, given the use of high temperature combustion and pressurised O2 gas. Have a fire blanket and an appropriate fire extinguisher close to hand.
LECO™ CNS–2000 analyzer or equivalent, plus essential gases and other accessories.
Ethylenediaminetetraacetic Acid Reference Standard (EDTA)
Use dry (105°C for 2 h), high-grade EDTA (C10H16N2O8), calibrated against EDTA certified by the instrument manufacturer. When fully dry, this contains 9.586% N.
Other Reagents
Use those specified by the instrument manufacturer.
Set up and maintain the high-temperature combustion analyser in accord with the manufacturer’s Operation and Procedures’ Manual. This includes performing door-maintenance and a combustion-leak check.
Run three separate ceramic ‘boats’ of EDTA Reference Standard (9.586% N) to stabilise the detectors, noting that irritating, toxic NOx is released when EDTA is heated to its decomposition temperature of 240°C. Next combust three empty ceramic ‘boats’ as blanks, using 0.200 g as the weight, to set the instrument blank from these results. Follow this by weighing into ceramic ‘boats’ and analysing three separate replications of EDTA Reference Standard. Use the two closest results to perform a drift correction. Confirm the instrument setup and calibration by analysing at least one internal LCS for quality assurance purposes, using a weight between 0.3 and 0.75 g depending on the expected concentration.
If the LCS sample/s test within its/their accepted concentration/s, proceed to analyse unknown samples. Should the analyser be ‘out-of-range’, analyse another EDTA Reference Standard portion of known weight. Again perform a drift correction. Follow this with the reanalysis of another portion of the LCS sample/s. If the expected result/s is/are still out of specification, the instrument, gas lines and detectors should be double-checked before proceeding any further (see Note 1).
When optimum analytical performance specifications are confirmed, prepare a known weight (e.g. 0.5–0.75 g of finely ground (<0.5 mm) air-dry soil) and proceed to analyse all samples. If results are ‘out-of-range’, adjust sample weights as necessary. Include a LCS, followed by an EDTA Reference Standard portion of known weight to check instrumental drift about every 25 samples. Finally, include a further LCS and an EDTA Reference Standard portion of known weight at the end of the samples’ ‘run’, then finish with 2 blanks to enable the gas blank to be reset if necessary.
At the end of the run, go back and check that the LCS values are within their accepted range. If not, use the next measured EDTA Reference Standard value to ‘drift correct’, then recalculate the results. In general, recalculate the results halfway back to the last in-range LCS.
TSN(%N) = [a × MF]
where:
a = N concentration in air-dry sample (%N)
MF = air-dry moisture to oven-dry moisture ratio
Report TSN (%) on an oven-dry basis (see Note 2).
1. Tiessen and Moir (1993) warn that in routine use at high temperatures, hairline cracks may form in the ceramic tube of the combustion furnace, causing leaks and consequential low recoveries.
2. If C and N are the only tests to be undertaken, it is acceptable to place the samples in a small oven for 1 h at 105°C prior to analysis, and weigh the sample directly from the oven. (This is in lieu of analysing air-dry samples and correcting the results using the relevant air-dry to oven-dry moisture ratio – the MF). Refer to Method 2A1 for further guidance on air-dry to oven-dry moisture calculation.