Soil sample prep for EA-IRMS analysis
This procedure describes the grinding and loading procedure for soils to be run in an EA-IRMS (Elemental Analyzer coupled to an Isotope Ratio Mass Spectrometer). This analysis gives carbon and nitrogen concentration (percent by weight) and the C and N stable isotope ratios (^13^C/^12^C, and ^15^N/^14^N) of a soil sample. Before samples can be loaded into this analyzer they must be very finely ground and homogenized, and small amounts (2-20mg) must be weighed into small tin capsules. These capsules are then loaded into the analyzer by SIRFER staff.
Ball mill grinding
- Rough ground original soil or organic samples (in yellow topped plastic sample cups).
- Steel grinding capsules, with balls (borrow from Ehleringer lab)
- Clean glass subsample vials.
- Labeling tape
- Inventory sheet (Greg's)
- SIRFER Lab supplies - you will be doing this in the SIRFER lab weighing area.
- After selecting a sample from the pile, label a clean, glass subsample vial with the site code, plot number, and sample type from the original sample cup. Always make sure the all label information follows a sample from the original container to the subsample container.
- Using a lab spoon, mix the sample in the cup, then scoop it into a steel grinding capsule. Fill the capsuleabout 2/3 to the top, unless the original sample is very small. Try to leave at least a bit of original sample in its cup for archiving.
- Add a steel grinding ball to the capsule, then put the steel lid on the capsule, and wrap labeling tape all the way around the lid.
- Label the grinding capsule with the sample information (again - all info follows the sample).
- Take the labeled capsules upstairs to the Ehleringer ball mill and securely fasten 2 capsules into the holders.
- Run the capsules on the ball mill for at least 1 minute at 25+ cycles per second.
- Empty the now powdered subsample into its labeled glass vial. A plastic funnel is useful here. Use a clean funnel between each sample so that you don't cross contaminate samples while transfering them.
- These powdered samples can now be further processed, or weighed into tin capsules.
Acid treating soil samples
Soils that have, or may have, carbonates present should be acid treated to remove carbonate carbon, as this does not come directly from organic matter in the soil. Such soils include alkaline soils or dry soils, but could also include soils that derived from carbonate rocks.
Weighing samples into tin capsules
- Milled soil or organic samples (in glass subsample vials - see grinding procedure above).
- Inventory sheet (Greg's)
- Sample loading sheet from SIRFER
- Tin capsules
- Sample tray (96 well "Nunc" type)
Samples must be carefully weighed into pressed tin capsules, then folded up and placed in the wells of a sample tray. Be sure to keep samples organized as you measure them, and clean all tools and work surfaces before each sample.
- Turn on the balance, clean off the tray with the vacuum, and calibrate it (press F1, or instructions are on the wall).
- Using tweezers, take a new tin capsule from the box and place it on the tray of the scale. Press TARE.
- When the scale reads 0.000 again, open the door, remove the tin capsule, and place it on the glass work surface in front of you.
- Open the sample vial and use a small scoop to transfer the sample to the capsule. Put the capsule back on the balance and then add or remove sample as needed to meet the target weight on the inventory sheet.
- Once you are close to the target weight (±5%), close up the capsule. The best way to do this is to use the flat ended tweezers to "clamp and roll over" the opening to the tin capsule. You can then pin down the rolled edge with the other tweezers and squeeze together the corners, forming a squared off bundle. This takes practice.
- Once you have a solid looking package, pick it up with tweezers and tap it on the glass a couple times and look for leaks. If it is not leaking, brush it off and weigh it on the balance a final time.
- Don't forget to record the weight on the inventory sheet AND the SIRFER loading template (with its sample ID)
- Clean your tools and work surface with a Kimwipe and alcohol before moving to the next sample.
- Remember to skip (leave empty) all wells labeled with COND or standard codes on the SIRFER loading template. They will be filled by SIRFER staff.
- You may mix 2-3 repeated unknowns into each SIRFER run.