IRMS is a technique used to measure the relative abundance of isotopes in a sample, and the calibration of the data is a crucial aspect of any IRMS measurement routine. When calculating delta values, the ratio of heavy to light isotope of the sample of interest is compared to that in a standard, the correct nomenclature for which is reference material. Delta values are calculated as shown in equation 1; the results are expressed in parts per thousand, or per mil.
These delta values are then calibrated via a multipoint calibration to international reference materials of known isotopic value, the reference materials used should be matrix matched to the sample and should encompass the range of delta values one expects to measure. An example of a good calibration is shown in figure 1.
Choice of reference material used to calculate delta values
Sercon instruments give ultimate flexibility in the choice of using either a reference gas or a reference material when calculating delta values. If a reference gas is used, this should itself be calibrated to known international reference materials, and should not be relied upon to give final delta values. A multipoint calibration should always be carried out with matrix matched reference materials.
If a reference gas is not used, a reference material may be used at regular intervals within the sample run to calculate delta values. This reference material may also be used to tune the source, by using Sercon’s trapping loop technology. Using the trapping loop, the reference material is treated in the same way as a sample, held in a trapping loop, and bled into the source slowly for source tuning. This novel technique avoids the need for a reference gas, and may be especially useful in environments where gas storage is restricted, for example when SO2 cylinders are not allowed inside the lab.
Choice of reference materials used for multi-point calibration
Reference materials must be matrix matched, i.e. of a similar material to that of the sample, and encompass the range of delta values which are expected in the samples to be analysed. Sercon provide a range of certified reference materials for both dual inlet and compound specific IRMS analysis. We provide both organic and inorganic reference materials, solid, liquid and gas, covering a broad range of delta values. Our interactive table allows these reference materials to be filtered by element and delta value. For more information on any of the reference materials available from Sercon please contact firstname.lastname@example.org or click below to access our interactive table.