Mass Spectrographs and the Birth of Stable Isotope Geochemistry

I’ve been working on a little project that has required me to dig a little into the history of stable isotope geochemistry. Today I learned about the first mass spectrometers. I’ve found the results interesting and thought I might share them here. This is research in progress…

The First Mass Spectrographs

The original mass spectra were used to learn about the relative sizes of different elements. Much work was done late In the late 19th century into the early 20th century, researchers like J.J. Thompson (1897) and W. Wein (1898) (Budzikiewicz and Grigsby, 2006) developed various methods to attempt to determine the masses of individual elements. Much of this was done by ionizing pure gasses into ‘cathode rays,’ that we know now are ion beams with positive charges. Cathode rays were deflected in a vacuum using electric and magnetic fields, where the electric field was held constant and the magnetic field was changed stepwise, resulting in a spectrum. The intensity and position of the resultant rays were recorded on photographic plates (a mass spectrograph), or viewed as patches on a phosphorescent screen or as voltages in a Faraday cup (a mass spectrometer). Peaks showed where the different masses were.

Discovery of isotopes

When mass spectrometry first developed, the peaks were broad, but with advances in detection and vacuum, they became sharper. With improvements, Thompson (1911) worked with F.W. Aston to build a mass spectrograph that revealed the mass spectrum for carbon dioxide, with peaks for CO₂, CO, O, and C. Further improvements showed that the cathode ray for neon actually had two peaks, one much smaller than the other (Thompson, 1913). We know now that what he observed were the two isotopes of neon, mass 20 and mass 22.

Modern mass spectrometry

In 1918, A.J. Dempster developed a new type of mass spectrometer which is very similar to the ones in today’s laboratories (Dempster, 1918). Rather than using canal ray guns to ionize the gasses, he used thermal desorption or electrical ionization. Like modern mass spectrometers, ions of the gasses were accelerated away from the ion source using a controlled electrical potentials. The resultant beams passed through a slit into the analyzer. This new instrument had much higher resolution than the original systems.

Stable Isotopes

F.W. Aston went on to receive the Nobel Prize in Chemistry in 1922 in part for showing that elements can have more than one stable isotope (see Aston, 1921). His work in developing the modern mass spectrometer and identifying various isotopes helped lay the ground work for what is now modern mass spectrometry.

Paleothermometry with oxygen isotopes from carbonates

The implications and possible applications of study of stable isotopes became clear in 1950, when McCrea published his paper on the relationship between oxygen isotopes and temperature in carbonates (McCrea, 1950). He showed that calcium carbonate precipitated from water of a known oxygen isotopic ratio and of a particular temperature had a predictable isotopic value. This meant that if we could make assumptions about the oxygen isotopic ratio of ancient waters, we could possible determine paleotemperature using isotopic analysis. In this seminal work McCrea (1950) helped us understand the temperature dependence of fractionation of isotopes, and in the process, he developed the methods that we still use today for isotopic analysis of carbonates.

Modern isotope work

By 1953, researchers were already using McCrea’s (1950) relationship to learn about ancient environments. Among the first papers was one by Emiliani and Edwards (1953) where foraminiferans (single-celled aquatic organisms with a calcite shell) were collected from ocean cores and analysed using McCrea’s (1950) methods. This work showed a temperature change over time from which they inferred a change in the mixing of ocean waters during glacial times. This was among the very first times that isotopes were used to examine past climate. Today, isotopes are an essential part of any paleoclimate study.