Isotope dilution

Sciencewoman has an interesting article enumerating the barriers to people in general, and women in particular, who wish to be professional researchers. There has been some interesting discussion on this issue. I would like to contribute. Unfortunately, before I can do so, I need to explain isotope dilution.

Isotope dilution is a way of getting very accurate concentration numbers from mass spectrometers. One of the problems with mass spectrometry is that you generally don’t know what proportion of the total sample you introduce actually gets to the detector. This depends on a number of different things, including the ionization efficiency and transmission efficiency of the instrument. In general, the relative efficiencies of different elements are not equal and not constant. As a result, tricks must be employed to account for this.

Isotope dilution is one of these tricks. In isotope dilution, you add a known amount of a single isotope of the element you wish to measure to the sample. The isotope you add is called the spike. The action of adding it is called spiking. Since you know how much spike you added (this is measured very carefully), the ratio of spike sampled to spike detected gives the detection efficiency of the instrument, and the ratio of spike to natural isotope gives the concentration of that isotope.

Spikes are often, but no always, short-lived radioactive isotopes not found in nature. For example, ²³³U is commonly used to spike U solutions for measuring U concentrations. Because separating and/or creating spikes via nucleosynthesis is difficult, they are often expensive, and the radioactive ones can be a potential radiation hazard.

Obviously there are all sorts of further improvements on the process- double spiking to determine mass bias, changing dilutions to determine detector linearity, etc. But the basic idea is the same. You add a known amount of something unique to account for the screwy processes that occur inside the machine.