Chronologies sequence significant developments in mass spectrometry (MS), although there is always some imprecision in determining the "proper" date for the conceptualization or adoption of an idea. Because hindsight offers a better focus, significant advances and the publications that describe them are sometimes more easily recognized years after they first appear. But it's not necessarily a singular date that is important. Instead, a clear description and execution of a new approach, and its subsequent adoption, often in an unexpected arena, can result in a transformative development in the practice of MS. This column describes some central transformative themes and their impact in modern analytical MS.
Central ThemesIsotopic Analysis
We have discovered all of the stable isotopes and measured their masses and abundances. In MS, the distribution of isotopes in atomic and molecular ions, in fragment ions resulting from dissociations, or in supramolecular ions, can be predicted from natural occurrence and has been mapped as a function of differential reaction rates, or specifically designed artificial manipulation. When we measure the mass of an ion, we use that mass to deduce atomic and empirical formula composition, using our knowledge about the mass and abundance distribution of isotopes. We can also use our knowledge about isotopes to serve as a quality check on our measured data, both in terms of the measured mass and the measured abundance. For example, for a given number of 12C atoms in an ion, there is a known and predictable number of 13C atoms. Ions with distinctive isotopic envelopes (such as chlorine or bromine) are visually evident in a mass spectrum. Isotopic analysis, and our knowledge of isotopic distribution, underlies every facet of modern MS. If the isotopic distribution varies from what we expect, either our measurement is inaccurate or we are looking at the end result of a differential phenomenon.
To repeat (with a slightly different emphasis), we have discovered all of the stable isotopes, and we have measured their exact masses and their exact abundances. Those measurements are accurate (true) and precise (reproducible) to extraordinary tolerances. They are "exact," and ongoing research is focused on making them more so. Exact measurements at the limit requires sophisticated and specialized instruments, and "exacting" protocols for sample collection, preparation, and analysis. Measurement performance has steadily improved with each generation of MS instrumentation. Accurate mass measurement has become more accessible across the board. Dynamic ranges are extended with better electronics. As a consequence, instrumental sensitivity increases, and consequently, limits of detection and quantitation trend lower and lower. Better analytical performance places new demands on sample collection and sample storage protocols, with the result that the overall error is often determined by noninstrumental factors.
MS is inherently a multidimensional and data-rich analysis method. Even when light-beam oscillographs were used to record the data, the three concurrent traces (for example, 1×, 10×, and 100×) provided an automated means of scaling intensity measurements. When film was used as the detection method, as in a focal plane spark-source mass spectrometer, a scanning densitometer (and subsequent processing) converted film exposure as a function of plate position into intensity as a function of mass. Digitization of the analog output of a detector allowed for data system recording of data, comparison of data, and archiving and sharing of data. Higher digitization rates and higher-bit transformation meant more and more data. But importantly, after the data was in a digital format, it could be archived, shared, processed with statistical methods, assessed with information theory, and displayed in more meaningful ways. Effective information storage and processing is rapidly becoming a concern across-the-board in modern MS applications.