Total soil S, inclusive of sulfate and sulfide, can be determined by catalysed, high temperature combustion, simultaneously with measurement of TOC and TSN. This method is based on the detection of S, after the combustion of the sample in a stream of O2, with the S released and converted to sulfur dioxide (SO2). The use of vanadium pentoxide (V2O5) as an accelerant ensures the decomposition and release of inorganic S into the gas stream. The method is both rapid and convenient but does require reliable instrumentation, now available from a range of manufacturers.
From a workplace health and safety perspective, laboratories that employ O2-enriched, high temperature combustion methods must take precautions to prevent burns and fires, and also while handling compressed gases. As O2 is highly inflammable, avoid open flames and sparks and have a fire blanket and an appropriate fire extinguisher close to hand.
The method described (as an example) is based on use of a LECO™ CNS-2000 multi-functional analyser. A sample is weighed and accelerant added before being ignited in a stream of O2 gas at 1350°C. Water vapour is removed and the combustion gases are collected and equilibrated. The amount of S released is measured quantitatively (as SO2) using IR detection. The instrument is calibrated against a reference standard (in this case sulfamethazine), with the total S reported as % S. No local measurement performance data are available.
Sulfamethazine (2-4-aminobenzenesulfonylamino-4:6-dimethylpyrimidine; C12H14N4O2S)
Calibration standard, LECO Part No. 502–298. Pure dry sulfamethazine contains 51.78% C, 20.13% N and 11.52% S.
Vanadium Pentoxide Accelerant
LECO Part No. 501–636.
Anhydrone (a.k.a. – Magnesium Perchlorate)
LECO Part No. 501–171.
Lecosorb
LECO Part No. 502–174.
Compressed Oxygen
Industrial grade >99.5% purity.
Compressed Helium
Ultra High Purity >99.99% BOC Part No. 220.
Compressed Air
Medical grade, filtered to be free of oil and water.
Combustion Boats
LECO Part No. 528–203
Instrument Preparation
Turn on and log onto the total S, high-temperature combustion analyser, and select the relevant total S method to initiate recommended operating parameters. The furnace will also commence heating to its operating temperature.
While the furnace is heating up, clean and repack the Anhydrone tube at the base of the furnace, then from the instrument’s Front Panel, reset the Anhydrone counter to zero and save changes.
After checking all gas connections and confirming there is sufficient carrier gases, turn on each of the required gases, including back-flow valves. Check the gas regulators and set/check gas flow rates.
To check that the furnace has reached operating temperature, go to the MAINTENANCE page and access the AMBIENT MONITOR. (Accessing the AMBIENT MONITOR will also turn on the instrument gas flow, if this has not been done). Observe the status of the other parameters listed. A tick indicates that the value shown is within the accepted range.
From MAINTENANCE, access the DIAGNOSTICS page and perform leak checks on the Helium, Ballast and Combustion systems by selecting each icon in turn. When all have passed, then the LECO CNS-2000 is ready to start analysis.
Calibration and Analysis
Follow manufacturer’s instructions to calibrate the analyser and to prepare it for total S analysis. Instrument calibration involves analysing the one standard (sulfamethazine) many times at varying weights, using each result to calculate the best-fit curve and store in memory. The calibration (graph/curve) should not change but the standard values obtained in any set of analysis may drift from the certified value. Once a trend is confirmed, a Drift Correction can be made and, if needed, previous samples recalculated: see Note 1.
Routinely for a 0.2 g soil sample, allocate ‘blanks’ to a nominated weight of 0.2 g. (When the Blank ID is selected, press Manual Weight, type in 0.200 and then hit enter. Don’t use a boat or V2O5 for blanks but remember to leave a space in the Autoloader rack.)
Weigh (typically) 0.2 g finely-ground (<0.5 mm) of air-dry (40°C) soil into a combustion boat then combine with two standard scoops (≈1.0 g) of V2O5 as an accelerant, taking care not to create any dust, then place in the Autoloader rack for analysis. Repeat for remaining soil samples.
Analyse 3 blanks to warm up the system and to check the IR cell baseline voltage (peak value), then analyse 5 standards (Sulfamethazine: ≈0.05 g) to establish and check the existing calibration. After analysing the fifth standard, run another 2 blanks.
Commence analysing soil samples when all instrument operating parameters are within specifications. As a further check on analytical performance, analyse one sample in duplicate for each batch of 12 to 15 samples. In addition, analyse 3 calibration standards followed by 2 blanks between each batch of unknowns, and at the end of each batch.
Note that when each analysis is complete, the result is momentarily displayed on the instrument’s screen and the data printed out. The Autoloader then removes the finished sample from the furnace, loads up the next sample and begins the analysis sequence again. The LECO™ CNS-2000 will continue to analyse samples without further prompting, so long as there are weights present in the Weight Stack and the sample in the 49th position has not been analysed.
Once the sample in the 49th position has been analysed, that sample boat is removed from the furnace and all further analysis is halted. The operator must touch Analyse to recommence the analysis process. At run’s end, turn all the gases off at the bench top, cylinder and back-flow valves. Empty the boat bucket under the Autoloader with care, as it may still be HOT.
Report total S (%) on an oven-dry basis. Apply an air-dry moisture to oven-dry moisture conversion (Method 2A1).
1. If samples have been analysed between the standards used for the drift correction, then the values for these can be recalculated by selecting the RECALCULATE RESULTS icon. In the same manner as done for the standards, choose the element(s) for correction and then highlight each sample that needs re-calculation. Press Process Results Key when done and save changes.