Isotopes and the Isotope Internal Standard Method

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons; for example, carbon has ¹²C and ¹³C, hydrogen has ¹H and ²H (deuterium), and nitrogen has ¹⁴N and ¹⁵N. In nature, most elements exist as a mixture of several isotopes in fixed proportions; these natural abundances leave traces on mass spectra: compounds containing one carbon atom will exhibit an M+1 peak of approximately 1.1 per cent, whilst compounds containing chlorine or bromine will display characteristic double peaks due to the near 1:1 and 1:1 ratios of ³⁵Cl/³⁷Cl and ⁷⁹Br/⁸¹Br, respectively. By analysing the relative intensities of these isotopic peaks, it is possible to deduce the molecular formula; this is a fundamental skill in mass spectrometry identification.

Stable isotope tracers are a powerful tool for quantitative analysis. By replacing the compounds naturally present in a sample with ‘heavy isotope versions’—for example, adding deuterated nicotine (d₄-nicotine) to a tobacco extract—the chemical properties remain virtually identical, yet a fixed mass shift appears in the mass spectrometry spectrum, thereby occupying a ‘dedicated channel’ in the spectrum. By measuring the response ratio between the sample peak and the internal standard peak, the true concentration of the original substance can be calculated.

The core advantage of the isotopic internal standard method is ‘synchronous compensation’. Losses during sample preparation, fluctuations in ionisation efficiency, and ion suppression caused by matrix effects affect the light and heavy versions of the same compound almost identically; these effects can be cancelled out by dividing one by the other. This is precisely why the use of internal standards is essential for quantification in complex matrices (tissues, blood, flue gas).

Relevant to New Ceyuan products: The ProC series of online time-of-flight mass spectrometers, when performing multi-component quantification, and the MSI series, when performing imaging quantification, can both incorporate stable isotope internal standards, elevating absolute quantification from ‘semi-quantitative estimation’ to ‘reliable concentration’. Simultaneous acquisition of the internal standard peak and the target peak is also a key method for ensuring the comparability of long-term online monitoring data.

It should be noted that the internal standard must be structurally similar to the analyte and must not overlap with it in the mass spectrum; the choice of labelling site also affects the compensation effect, and these details determine whether the quantitative results are reliable.

There is also a common misconception: treating natural isotope peaks as ‘impurity peaks’ and ignoring them. In fact, the relative intensities of M, M+1 and M+2 provide independent evidence for verifying the molecular formula; when cross-referenced with high-resolution mass accuracy, they enhance the reliability of identification. Conversely, if the measured isotope pattern does not match the theoretical one, this often indicates the presence of isomeric co-elution or addition interference. The internal standard method addresses ‘quantitative accuracy’, whilst the isotope pattern addresses ‘identity’; the two operate on different dimensions and are both indispensable. For quantitative workflows using ProC and MSI, it is standard practice to first use the isotope pattern to screen out misidentifications, and then use an internal standard to determine the concentration.