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October 2001
Vol. 10, No. 10, p 7.
 
 
 
For Openers
Mass spectrometry

“I feel sure that there are many problems in chemistry which could be solved with far greater ease by this than any other method.”

—J. J. Thomson, speaking on mass spectrometry in 1903

I always enjoyed working with analytical instruments, especially mass spectrometers. Sometimes I miss being in a lab where I can operate them. There is something so very satisfying about making an instrument like a mass spectrometer function properly, knowing that the data produced represents the best of both instrument and analyst.

The first mass spectrometer I used was a Hitachi RMU-6E, a big clunky contraption that took up half the instrumentation lab in what was then a brand new chemistry building at the University of Virginia. If memory serves, this instrument had at least two mercury diffusion pumps and a panoply of roughing pumps. To tune the instrument, caliper dials were manually rotated to open and close knife-edge ion slits. Solid samples were introduced by probe; gas samples were introduced through a complicated glass assembly. No gas chromatograph input system was available. Mass spectra were recorded by oscillographic light-pen recorders on rolls of Kodak photosensitive paper, and ions were identified by manual counting. Since there were few reference libraries or computers to search them, spectra were decoded one-by-one like a complex puzzle. I loved it.

I think the reason that many of us like mass spectrometry is because instrument operation is so literal. With an oscilloscope, or even with some of today’s computer displays, you can convince yourself that you’re almost visually seeing the ions produced.

As an analytical tool, mass spectrometry began with the experiments of J. J. Thomson at the University of Cambridge a century ago. He found that electrical discharges produced ions that could be deflected with an electromagnetic field onto a photographic plate. He was the first to show that the amount of an ion’s deflection depended on the mass-to-charge ratio of the specie produced in the discharge. Thomson’s student, Francis Aston, adapted and improved the instrument’s resolution, thereby allowing him to discover elemental isotopes. Thomson was awarded the Nobel Prize in Physics in 1906 for his work in discovering and defining the electron using his “spectrograph”. Aston received the Nobel Prize in Chemistry in 1922 for his work on isotopes.

Since that time, mass spectrometry has steadily advanced as an analytical technique, used to identify and quantify both organic and inorganic chemicals. Until the 1960s, an analyst had to have milligram or more quantities of the analyte. Now we work in quantities of femtograms and attomoles, and we’re to the point that we can get a signal from a single molecule. On the other end of the scale, mass spectrometry is also one of the ways that large quantities of fissionable uranium is separated from its less lethal counterparts.

The mass spectrometers used in the typical laboratory today, based on quadrupole, time-of-flight, and ion-trap mass analyzers, are much smaller than their magnetic counterparts. Such units are one of the analytical mainstays of environmental protection and drug discovery. There are now instruments with multiple mass analyzers, serially bolted together, that allow one to generate a stream of ions that themselves can be broken down into their constituents. Thus proteins can be broken down into peptides that can be further analyzed for their individual amino acids.

The ion source used in a mass spectrometer ranges from the traditional 70 electron volt source used for organic analysis to the spark source used for inorganic analyses, now superseded by the inductively coupled plasma mass spectrometer used in trace metals determinations. We have other soft ionization techniques for gently removing one electron from a labile chemical, thus keeping it from breaking into fragments and allowing an analyst to identify the molecular ion and thereby the molecular weight, the most fundamental of chemical properties.

It’s difficult to imagine an analytical technique with more applications than mass spectrometry, though I suspect that our chromatography and light-bending spectroscopy brethren might disagree. Nevertheless, the quote of J. J. Thomson’s, transcribed decades ago from some book I can no longer find, to me seems more true than he might ever have imagined.

James F. Ryan

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