Introduction
The instrument may be operated in either CHNS or O mode. For CHNS, combustion and reduction happen in one tube.
- Tubes are packed in-house using reagents purchased from CE Elantech or EA Consumables.
- The reagents in the CHNS reduction tube are copper oxide wires and electrolytic copper.
- The reagents in the O reduction tube are nickel plated carbon and quartz turning.
- Quartz tubes are reused when possible; new tubes are purchased from CE Elantech.
- Pressed tin capsules (9mm x 5mm) are used for most samples.
- Silver capsules (9mm x 5mm) are used for halogenated samples and samples prepared for O analysis.
- For volatile, liquid, or air-sensitive samples, we have a capsule sealer, which uses smooth-walled tin capsules.
- BBOT is typically used as the calibration standard.
- Vanadium pentoxide is used as a combustion aid, especially for samples that require accurate sulfur determination.
- UHP grade oxygen is used to support combustion.
- UHP grade helium is used as the carrier gas.
Instrument Preparation
The system is checked for leaks whenever a tube is replaced, or just before starting a run.
CHNS Operation
Normal operating procedure includes running a bypass, two blanks, three standards, and two "checks" (standards run as unknowns) before running submitted samples. 2,5-Bis(5-tert-butyl-benzoxazol-2-yl)thiophene (BBOT) is used as the standard for most studies. Acetanilide (ace) is another good standard, but does not contain S. The normal run sequence is:
| Run # | Compound | Run as... | Purpose |
|---|---|---|---|
| 1 | BBOT (unweighed) | bypass | condition the system |
| 2 | empty tin capsule | bypass | ensure no BBOT remains in system |
| 3 | empty tin capsule | blank | set initial blank values |
| 4-6 | BBOT (2 - 3 mg) | standard | generates calibration curve |
| 7-8 | BBOT (2 - 3 mg) | unknown | "checks" to ensure good mass % |
| 9 | empty tin capsule | bypass | ensure no BBOT remains in system |
| 10-? | user samples | unknown | unknown samples |
| ? | BBOT (2 - 3 mg) | unknown | optional: addt'l "checks" during run |
| 200 | change tube | combustion tube good for 200 samp |
Parameters
CHNS (Left Furnace)/Oxygen (Right Furnace)
(Oven at 100°C for Oxygen)
Data Processing
After a run is complete, a calibration curve and chromatograms can be viewed for data processing. The calibration curve is based on the three standards and is what all unknown sample results are based off of. This should be checked whenever a new table is started to ensure accurate results. The following example calibration curve should be expected:
The results are summarized into a table to make comparison easy between each sample run. When comparing the results for a sample, it is important to look at the % relative standard deviation. This should remain below 3% for results to be reliable, and if it is not, or the samples seem off, then the chromatograms and component tables can be adjusted to try to resolve this issue. Chromatograms are used to view the peak window for each element in which the system integrates to determine the results. An example chromatogram and its component table looks like the following:
The verticle red lines represent the peak window and can be adjusted in the component table to adjust integration.
It is critical that duplicates be run in order to identify those instances of poor precision; whenever possible a third replicate is then run and the outlier (always lower than the others) is excluded from the average.