Capture Recapture

The elegance of isotope dilution mass spectrometry (IDMS) is its simplicity. The technique’s accuracy is deeply rooted in advanced instrumentation, particularly within the finely tuned parameters of mass spectrometry (MS). Interestingly, this now broadly implemented technique was in use some 18 years before Joseph John Thomson was able to prove the existence of isotopes at the beginning of the 20th century.

In fact, the origin of isotope dilution analysis is based on an ecological study of sea fish by Carl Georg Johannes Petersen, who used the capture-recapture method for estimating plaice population (1). It was a simple idea: you capture a group of individuals, mark them and then release them back into their habitat. After sufficient time for marked and unmarked individuals to mix, the fish are captured once more. This time the catch contains marked and unmarked individuals and by calculating their ratio, you can estimate the population of the unmarked individuals relative to marked fish. And so yes, the approach provides good estimates for biological measurements, but it also earned George de Hevesy the 1943 Nobel Prize in Chemistry for his work in radiochemical isotope dilution.

Before the 1980s, IDMS was restricted to nuclear, geochemical and metrological applications. With the introduction of inductively coupled plasma (ICP) as an ion source for MS, the technique was liberalized with a pioneering application of post-column IDMS by Klaus Heumann’s group in 1994 (2). That breakthrough led to rocketing use of IDMS in many diverse branches of chemistry.

Only 21 of all the pre-1940 elements in the periodic table are monoisotopic, which means that the window of opportunity for IDMS is wide open. In my view, we should all consider that opportunity by shifting slightly off our predefined paths to enrich our common knowledge. I believe that our own scientific struggle makes us innovative and broadens our capabilities for greater understanding; however, we must also take good care that our struggles do not kill our desire to know more.

In the trace elements speciation group at Aberdeen, we quantify chemical species present within various environmental matrices. It can be challenging. One of the main problems is recovering the analyte. How many of you have also faced the loss of volatile analytes during sample preparation or its incomplete liberation from the matrix during derivatization? Honest answers only, please!

And what about incompatibility between calibration standards matrix with the matrix of the sample? Like many before me, I struggled with these issues during my PhD research. However, I was also fortunate enough to work with mercury, which has seven stable isotopes. IDMS was the light at the end of the tunnel.

Quantitative extraction of mercury species from animal tissue, however, proved to be rather challenging and when the list of tested possible variables was exhausted, I decided to seek out species-specific IDMS. The basic principle of this technique, which highlights its superiority, is that once the isotope ratio between the endogenous and enriched species is altered, the quantification is virtually independent of the derivatization and extraction efficiency. In other words, whether you derivatize or extract 30 or 90 percent of the species of interest, the altered isotope ratio is going to be the same. Moreover, as the isotope ratio is the only measured variable when using IDMS, species quantification is relatively easy.

The evolution from radiochemical to triple-spike IDMS revolutionized research, but we should not forget that its origin is not in chemistry. Therefore, when you next face big challenges in research, try to reach out to other scientific disciplines. As the question we are trying to answer increases in its complexity, we often need to cross our own boundaries.