In preparing samples there are a couple of fundamental requirements to be met if your are to obtain good results. The first, the simplest and the most important is cleanlingness. As you are measuring very small quantities, contaimination can easily occur. Further to this you need to meet some instrumental requirements.
The mass spectrometer is designed to measure an isotopic ratio. Consequently, as part of this design it is optimized to work only in a given range of ion currents. This means that when measuring your sample to fit the MS, you need to focus not on the weight of your sample, but focus on the amount of elemental material in your sample that will create the correct ion flow for the MS. For example if you are measuring the 15N in your sample, you first need to adjust the amount of sample in your capsule, so that it contains the correct amount of Nitrogen to get the correct 15N ion flow in the MS. This is the reason that a weight of a sample is not specified directly but the specimen requirements are given as XX ug of elemental N or XX umols of N. The XX will vary with the sensitivity and the type of preparation unit on the MS, but ultimately you will need to focus on the amount of N.
For the mass spectrometer that we use the following levels are required in each capsule;
100 ug to 200 ug of elemental Nitrogen
800 ug to 1000 ug of elemental Carbon
Fine grinding of plant and soil material is needed to ensure good capsule combustion and a homogeneous representative sample. Ideally finely grounding to approximately the consistency of fine flour is excellent, although as long as you have a representative sample within your capsule then slightly coarser is tolerable. A disc mill is a good choice to achieve the fine grinding of samples, and eliminate the possibilities of cross contamination between samples (NOTE: re drying of materials, especially plant materials for a short time immediately before grinding is recommended).
All samples should be dried before weighing. Be aware that drying at too high a temperature can volatilise what you may require to measure., and drying at too low a temperature may allow biological activity occur and also alter the sample composition, this is especially true for Nitrogen compounds.
For the mass spectrometer samples should be dried at
Soils approximately --- 70 to 80 Celsius
Plant material approximately --- 60 C
Biologically active material ---- Freeze dried
Standards especially those contain ammonium --- 40 C
Weights are required if you wish to obtain a more accurate estimate of the element % of the Nitrogen or Carbon in your sample.
If only the isotopic ratio is required, then the sample weights do not need to be recorded HOWEVER you must still have your sample weight the required range to provide the correct amount of elemental material. Accurate and precise weighing will help achieve better results, by eliminating variations in gas flow and combustion.
Arranging your sample capsules in rows of ten within the micro-litre tray is better suited to incorporating your samples into the MS run. A five figure balance is required to obtain the required weigh precision, for plant material you will be aiming for something in the order of 2 mg to 4 mg, and you will need to record the weight of the contents, NOT the weight of the capsule and contents.
When sealing your sample in the tin capsules, take each capsule in turn squeeze the top with blunt tweezers and fold the top 1mm tightly. Using two blunt tweezers compress the capsules into compact balls. Important Note: If the balls are not able to roll in the sample carousel, then it is liable to jam the carousel and ruin the measurements on many of your samples. .....Charging will still be applied for these samples......
Encapsulation is best carried out on a clean glazed surface (mirror tile) so that any breakage and spillage of material from within the capsule can be seen. If loss of material from the capsule is observe discard the capsule and reweigh the sample in another capsule. Avoid touching the capsule with your bare hands as they will contain N and C.
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