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Introduction
The CHNS procedure measures nitrogen and total carbon (inorganic plus organic). Organic carbon content is then calculated by using the inorganic carbon value from coulometric analysis. Hydrogen and sulfur can also be analyzed on the CHNS.
General Safety
The user should be familiar with the hazards related to the use of this system before beginning work (see CHNS User Guide for more information). High-pressure gases, high temperatures, and toxic chemicals are all present in CHNS work.
Sample Preparation
Samples are freeze-dried, crushed, and homogenized using a mortar and pestle or electric mill and weighed into a tin sample cup (crucible). If sulfur is being analyzed, vanadium pentoxide is also added, acting as a catalyst. The crucibles are then closed (referred to as "wrapping" the sample) for instrumental analysis.
The following amounts are used:
- Standards: 3, 5, 10, 15, 20 and 40 mg
- Unknown samples: 12–15 mg
- Vanadium pentoxide (if used): ½ small spatula for blanks, standards, and unknowns.
Worklist Generator
Use the Worklist Generator application to identify the samples, weighed on the Cahn balance, to be run on the CHNS Analyzer. Go through the samples very carefully. Look for missing container numbers, duplicate container numbers, duplicate entries and anything out of ordinary. Get everything perfect and ready to run. Go through the container numbers several times and make sure every container number is there and that there are no duplicates. Sometimes the software saves a weighing twice. Only select one of those. Once the worklist is ready to be converted into a sample table, export it to the current Expedition's folder. This file will be imported in the CHNS software as the sample table.
Instrument Operation
Run the program Eager Xperience. Select Analyzer #1.
Instrument Parameters
Helium pressure: 260 kPa
Left furnace: 950°C
TCD on: ~1400 µV
Oxygen pressure: 300 kPa
Oven: 60°C
Combustion Column (a.k.a. reactor tube)
- A combustion column will last approximately 100–120 samples. The life of a combustion column can be extended by using a quartz insert.
- To condition a new column, run a standard 2 times as an Unk (unknown). Put a scoop of standard in a tin cup without weighing it. Wrap the sample as normal. Enter mass 1. Verify that you see a peak for nitrogen and carbon if using Buffalo River standard. Below is a sample chromatogram from a Buffalo River check standard run. With conditioning we are only verifying that we get a "normal" looking chromatogram with peaks. The height of the peaks is unimportant at this point.
Typical Buffalo River check standard chromatogram
Sulfur and Hydrogen
- If analyzing for sulfur, add vanadium pentoxide to the sample cups.
- If analyzing for hydrogen, remove the magnesium perchlorate trap. Run time will be around 1200 seconds.
Calibration
The instrument is calibrated at the beginning of each sequence. The calibration factors will first need to be reset:
Recalculation > Reset Calibration Factors
Immediately after the standards have been run, according to the sample table, check the calibration:
View > View Calibration Curve (select the element of interest)
If the calibration is not acceptable, stop the sequencing, redo the standards, and start the sequence again, so no samples will have to be re-weighed.
Calibration is verified during the sequence by running the standard as an unknown.
Running Samples
- Upload the LIMS sample table (Figure 2) to the instrument:
Edit > Sample Table
Edit Sample > Import sample table from LIMS (select the file exported from Worklist Generator)
The sample/sequence table should look similar to the one above.
- Column A: Act signifies which is the current sample
- Sample name:
- BYPASS
- BLANK
- TEXT ID
- Filename: A unique name. Worklist Generator creates the names for the samples
- Type:
- Bypass (empty slot in autosampler)
- Blank (empty tin capsule)
- Std (calibration standard)
- Unk (sample or standard)
- Standard name: only the calibration standards will have this field filled in.
- Weight: Values from Worklist Generator are automatically filled in for standards and samples.
2. With the zero slot in the drop position, load the autosampler tray with the standards and samples; leave empty slots for BYPASSes. Make sure that the lid is on the tray. Advance the zero slot to slot 1.
3. Run samples:
a. Click the green icon.
b. Monitor acquisition status at View > View Sample Being Acquired. Keep an eye on the autosampler and add samples as necessary. A good time window to add samples is after the peaks have come out in a chromatogram and before the end of the run. Remember to put the lid back on after adding samples. Any samples analyzed while the lid is not on will have to be reweighed and rerun.
Analyzing Data
- Go to Recalculation > Summarize results. Select weight percent results.
- Go to File > Export to Excel. Save as "site-date-filename-wtpct".
- Make a working copy of the wtpct file. Look for any negative nitrogen values and make them BDL (below detection limit). Also, if you have sulfur values show up when you didn't analyze for sulfur, make those 0. Go through all the data very carefully and tidy it up before uploading. Editing afterward will be very difficult.
- Copy the file created in Step 3 to the MUT upload directory ("Data in" folder on desktop). Save the raw data file - all files will go on Data1.
5. In MUT, select Upload to open a window as shown above in Figure 4. Uncheck all files that you are not uploading. Do not leave any bypasses or blanks checked. Check standards do get uploaded.
Click on :
a. Select Method: Choose your EA method file.
b. Select Summary: Choose the summary file from the sequence just run.
c. Select Configuration: Choose your method configuration file
d. The OK button will then become active. Click on it. The rows will turn green as they upload.