Sample preparation for carbon, nitrogen, and oxygen isotopic analysis of solid organic matter involves weighing small (1-5 mg) samples of finely ground, homogenized sample into tin capsules for carbon and nitrogen analysis and silver capsules for oxygen analysis.
Because isotope ratio mass spectrometers measure matter in gaseous form, solid samples must be heated to high temperatures and converted to gas by an inlet device attached to isotope ratio mass spectrometer. For solid organic carbon and nitrogen analysis, samples are heated to 1700-1800∘C and combusted in the presence of oxygen in an elemental analyzer. This forms CO2 and N2, the forms in which carbon and nitrogen, respectively, are measured in an isotope ratio mass spectrometer. If configured correctly, the elemental analyzer can measure the carbon and nitrogen concentration of the sample before it is introduced into the isotope ratio mass spectrometer for measurement of stable isotopic composition.
For solid organic oxygen analysis, samples are subjected to high temperature conversion (pyrolysis) at 1400∘C forming CO, which is then entered into the isotope ratio mass spectrometer for oxygen isotopic composition.
This protocol involves the weighing of ground solid samples for isotopic analysis. See the sample preparation-grinding protocol for methods on grinding and homogenizing samples prior to weighing.
- Sartorius CP Series Micro Balance Model# CP2P
- Metal Spatula
- Carbon Plate
- Tin Capsules 3.5x5mm
- Data Sheet
- Well Plate and Cover
- Kim Wipe
- Desiccators with Desiccant
Units, terms, definitions
1. Preparations for First Use
Remove cover from Sartorius Precision weighing balance and plug in the power cord. Power ON using the blue power button located on the bottom left of the display screen. Tare the balance by pressing one of the yellow Tare buttons located on the left and right of the display screen. Prepare samples for isotopic analysis by laying a paper towel in front of the balance. Using gloves and a Kim Wipe with alcohol gently cleanse the carbon plate and set it on the paper towel. Also, clean the tweezers and metal spatula with alcohol and a Kim Wipe and place on paper towel. Prepare a well plate by labeling the lid with the date, Lab, and sample description and set aside. Fill in the data sheet with the date, your name, and sample description.
2. Weighing and Rolling Samples
Using tweezers put a tin capsule on the balance and close the door. Tare the tin capsule so that the display screen shows 0.000mg. Remove the tin capsule and place it in one of the holes in the carbon plate half way down and spread the upper half onto the carbon plate so that it fans out. Using the metal spatula, scoop or scrape the sample material into the tin capsule. With the tweezers, grab the part of the tin capsule that is fanned out and place it on the balance. Remove or add any sample material to reach the desired amount. Record in notebook: sample ID, sample #/description, and weight. Record on data sheet: sample ID and weight. The reason for this is the Sample ID will always go in ascending order starting at #1 while the sample #/description may not. This keeps it orderly for the person running the actual analysis. Now carefully remove the tin capsule from the balance being sure to not spill any material. Using the tweezers, squeeze the top half of the capsule (part that is fanned) together and fold the top twice so that the material will not fall out. Now, using the tweezers, the capsule can be rolled into a tight ball and placed in the well plate. Numbers on the well plate go from A1-12 all the way to H1-12, be sure to place samples in the well plate across and not down so that they are in the correct order pertaining to the data sheet. Between each sample, wipe the spatula with a Kim Wipe and alcohol to prevent cross-contamination of samples. Tape the well plate lid to the well plate to prevent any accidental dropping and mixing of rolled samples and place in the Desiccator until analysis.
C/N Isotope Analysis Weights
Sugar 1-2 mg
Leaves 3-5 mg
Litter 5-6 mg
Soil 12-14 mg
O Isotope Analysis Weights
Approximately 1 mg (0.8 – 1.2 mg)
3. Clean Up
Power the balance OFF by pressing the blue power button on the bottom left of the display screen and put the protective cover on top. Clean carbon plate, tweezers, and metal spatula with Kim Wipe and alcohol and place in cleaned box next to balance.