Analytical Services
for Stable Isotope Analysis
Stable Isotope Analysis (SIA)
With two continuous-flow isotope ratio mass spectrometers both interfaced with configured elemental analysers, OEA Labs routinely analyse the stable isotope ratios of carbon (13C/12C), nitrogen (15N/14N), oxygen(18O/16O) and sulfur (34S/32S) on solid or liquid samples.
In stable isotope analysis, milligram amounts of samples are combusted or pyrolysed at high temperature in a helium carrier gas. After suitable preparation the measurable gases (N2, CO2, CO or SO2) are separated on a chromatography column. A small proportion of this gas is fed into an evacuated ion source where the gas molecules are accelerated into a strong magnetic field. The gas species of different masses are subsequently deflected across a collector array where the individual masses are monitored and quantified against known references.
Sample Delivery Address
OEA Laboratories Ltd.
Unit 1A-1D
North Road Industrial Estate
Okehampton
EX20 1BQ
Sample submission forms:
- SI analysis
Although we prefer to weigh your samples we are able to accept pre-weighed encapsulated samples for this service. We monitor our IRMS total beam values to ensure that the weight criteria is met and will inform you if there are any discrepancies.
Dual 15N & 13C
Both d15N and d13C can be analysed simultaneously if the sample can meet certain criteria. In order to achieve accurate results it is necessary to work within the linear response range of our isotope ratio mass spectrometers (IRMS). This necessitates having an absolute weight of 20 to 200 micrograms of nitrogen and 40 to 800 micrograms of carbon in a single weighed sample. If you know the elemental composition and the %C:%N ratio is less than 40 then it should be possible to weigh a sample within range. If this cannot be achieved then it is our practice to run nitrogen and carbon isotope analysis as two separate analysis runs.
Precision (as average standard deviation) on well prepared samples will generally be better than 0.3‰ for nitrogen and 0.2‰ for carbon. We report nitrogen results referenced to air and carbon to Vienna Pee Dee Belemnite (VPDB) and if required the %N and %C obtained from our total beam values.
15N
Samples are precisely weighed to provide 20 to 200 micrograms of nitrogen for d15N analysis. Precision (as average standard deviation) on well prepared samples will generally be better than 0.3‰. We report nitrogen results referenced to air and if required the %N obtained from our total beam values.
13C
Samples are precisely weighed to provide 40 to 800 micrograms of carbon for d13C analysis. Precision (as average standard deviation) on well prepared samples will generally be better than 0.2‰. We report carbon results referenced to Vienna Pee Dee Belemnite (VPDB) and if required the %C obtained from our total beam values.
18O
Samples are precisely weighed to provide 20 to 200 micrograms of oxygen for d18O analysis. Precision (as average standard deviation) on well prepared samples will generally be better than 0.3‰. We report oxygen results referenced to Vienna Standard Mean Ocean Water (VSMOW) and if required the %O obtained from our total beam values.
34S
Samples are precisely weighed to provide 20 to 100 micrograms of sulphur for d34S analysis. Precision (as average standard deviation) on well prepared samples will generally be better than 0.3‰. We report sulphur results referenced to Vienna Canyon Diablo Troilite (VCDT) and if required the %S obtained from our total beam values.
Sample Preparation
Because stable isotope analysis techniques typically use sample weights in the order of 1 to 5 milligrams, considerable attention should be given to sample preparation. To achieve reliable data samples must be of acceptable homogeneity and be representative of the whole. Samples should be dehydrated by freeze drying or oven drying at 40’C to 70’C (depending on material type) then finely ground using a mortar & pestle or a ball mill. Inorganic carbon in the form of carbonates can interfere with the measurements of organic 13C in soils or sediments. If inorganic carbon is present it can be eliminated by careful direct acidification with dilute hydrochloric acid or indirectly by exposure to concentrated sulphurous acid vapour. We are able to prepare or pre-treat your samples at additional cost. Please contact us to discuss any aspects of sample preparation that you may require.
Most of our customers submit dried and prepared samples for analysis in vials or bottles ready for us to weigh and run on our systems. In this case we will determine the workable weight ranges best suited for a particular analysis. Generally, if the %C:%N is less than 40, it is possible for us to determine 15N/14N and 13C/12C simultaneously on a single weighed sample. Please contact us to discuss your requirements.
Samples submitted for analysis should be catalogued (emailed Excel spread sheets are preferred when many samples are submitted).
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.
