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, 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 standards of known isotopic value, the standards 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 standard used to calculate delta values
Sercon instruments give ultimate flexibility in the choice of using either a reference gas or a standard when calculating delta values.
If a reference gas is used, this should itself be calibrated to known international standards, and should not be relied upon to give final delta values. A multipoint calibration should always be carried out with matrix matched standards.
If a reference gas is not used, a standard may be used at regular intervals within the sample run to calculate delta values. This standard may also be used to tune the source, by using Sercon’s trapping loop technology. Using the trapping loop, the standard 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 standards used for multi-point calibration
Standards 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 standards for both dual inlet and compound specific IRMS analysis. We provide both organic and inorganic standards, solid, liquid and gas, covering a broad range of delta values. Our interactive table allows these standards to be filtered by element and delta value. For more information on any of the standards available from Sercon please contact firstname.lastname@example.org or click below to access our interactive table.