Pre-weighed Sample Preparation
Some of our customers prefer to pre-weigh their own samples. Samples should be weighed into tin capsules (available from OEA Labs) using a 6 decimal place balance. The tops of the capsules should be folded over and the capsules formed into roughly spherical shapes using forceps. This will avoid jamming in our autosamplers. Care must be taken not to rupture the capsule to avoid loss of sample and consequential contamination in our systems.
Inorganic carbon, if present, in the form of carbonates will interfere with the measurement of organic 13C/12C. The inorganic carbon can be removed by weighing the samples into silver capsules (tin capsules can decompose), slightly wetting with water and arranging in 96 well trays. The entire tray can be placed in a desiccator containing a beaker of concentrated sulphurous acid and leaving at room temperature for 48 hours. Samples should then be dried at 60’C and then sealed. Alternatively the whole silver capsule (which can become brittle after acid treatment) can be placed into a new tin capsule and sealed.
The sample weights should be catalogued (emailed Excel spread sheets are preferred when many samples are submitted) and indexed into 96 cell wells prior to submission. Capsules in various sizes and sample trays are available from us. Please enquire.
The optimum sample weight will depend on the type of material submitted for analysis. Our ideal target sample weights would contain 100 micrograms of nitrogen for 15N, 400 micrograms of carbon for 13C, 100 micrograms of oxygen for 18O or 30 micrograms of sulphur for 34S absolute. In practice there are ‘safe’ working limits on either side of these values. Please contact us if you have any questions.
For practical reasons maximum sample weights should not exceed 60mg because they may not fit our autosamplers. If samples fit loosely into a 96 cell tray (6mm diameter x 10mm deep) they will run on our systems. Samples containing more than 7mg absolute of carbon submitted for sulphur or nitrogen analysis should be avoided because the amount of available oxygen in our elemental analyser prep systems might be exceeded. In practice we can work outside of this range where necessary by the use of a large autosampler or oxygen dosing loop.
It is very useful for us to have a two sets of pre-weighed samples to allow us to run random QC check samples. This also ensures that we can keep our sample turn-around on schedule should we have any problems with our instrumentation or encounter other unforeseen failures.
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