While definitely not a new technology, mass spectrometry (MS) has seen incredible growth over the past 25 years. Mass spectrometry has rapidly evolved to the forefront of analytical techniques; its ability to analyze proteins is the major driving force in the field of proteomics. MS instrumentation has increased approximately 5-fold in sensitivity every three years. The level of performance that is achievable with MS today allows scientists to study proteins in ways that were inconceivable a quarter century ago. This review of the history of MS over the past 25 years is timely in that it encompasses two of the biggest developments, electrospray and matrix-assisted laser desorption/ionization (MALDI), which have enabled many of the uses of this technology today.

Introduction

While it has the appearance of taking the analytical world by storm, mass spectrometry (MS) has existed in one form or another for over a century. Many scientists, however, remember the mass spectrometer as that big, user-unfriendly instrument in the cellar of their chemistry department. Most undergraduate scientists’ familiarity with MS was limited to a single laboratory demonstration and no real hands-on experience. Very few courses taught the fundamentals of MS and most textbooks devoted little space to this technology. How times have changed! Mass spectrometry has now become an essential analytical tool in a variety of disciplines, including organic and inorganic chemistry, protein science, biochemistry, physics, and more. Most large private and academic institutions now have large programs that are centered on MS or rely heavily on this technology. Over the past quarter century, MS has moved from the basement to the penthouse.

Why did it take so many years for the power of MS to be recognized? The reasons can be traced to key technological developments that have occurred in the past 25 years. The most important development was the invention of new ionization techniques, specifically electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI).

Discovery of ESI and MALDI

While ESI's early practical history began with the work of Malcolm Dole, who described it in studies of ions from synthetic polymers in 1968 (1), it wasn't until 1988 that the recognition of ESI's impact on biochemical studies was first presented. At the annual meeting of the American Society for Mass Spectrometry, John Bennet Fenn presented the ESI mass spectra of a number of proteins (2). Subsequent to this presentation, Fenn published the ESI mass spectra of monomeric and dimeric forms of BSA (3). These results showed that it was now possible to analyze high molecular weight proteins using MS.

Around the same time, laser desorption was being developed as an effective method to volatilize and ionize small molecular weight compounds, including peptides. In 1987 Koichi Tanaka described the use of glycerol polymer mixtures containing fine metallic particles as a laser-absorbing matrix for the analysis of high molecular weight (i.e., 22,000 Da) polymeric molecular ions (4). In 1988, Tanaka published results of using soft laser desorption combined with time-of-flight MS to measure the proteins lysozyme (14306 Da) and chymotrypsin (25717 Da) (4). Both Fenn and Tanaka were awarded the 2002 Nobel Prize in Chemistry for their development of soft desorption/ionization methods for MS analysis of biological macromolecules. Another key step toward the effective use of laser desorption/ionization for the analysis of proteins, and one that many have argued should also have been recognized with a Nobel Prize, was Franz Hillenkamp's use of organic matrices to obtain mass spectra of proteins with masses of greater than 10,000 Da (5). Developed over the last 25 years, these new ionization techniques, along with existing ones, have rendered just about every compound class analyzable through MS. The development of ESI and MALDI had an enormous impact on biochemical analysis and it is difficult to imagine the inception of proteomics without these two methods.

Development of New Mass Analyzers

With the ability to analyze almost every class of molecule, the rapid growth and use of mass analyzers quickly followed. A commercial quadrupole ion trap, the Finnigan MAT, which traps and mass-analyzes ions using a three-dimensional (3-D) quadrupolar radiofrequency electric field, was introduced in 1983. The popularity of the quadrupole ion trap is primarily due to George Stafford Jr.'s discovery of the mass-selective axial instability scan (6). This discovery converted a simple ion storage device into a quadrupole ion trap mass spectrometer that has applications across a wide variety of areas. Earlier work using quadrupole ion traps was based either on mass-selective stability for mass analysis or on its use as ion transmission or storage devices. The fundamental difference between the mass-selective axial instability mode and previous methods is that with the new method all ions created over a specific time period are captured within the ion trap and sequentially ejected onto the detector. The second major breakthrough from Stafford's group was the discovery that adding helium gas to the trap greatly improves the mass resolution and sensitivity of the instrument (6). The ion trap quickly became the primary instrument for conducting proteomics because of its ability to conduct tandem MS (MS/MS) analysis of complex mixtures of peptides, generated by enzymatic digestion of proteome samples such as cell lysates. Ion traps are almost ubiquitous in analytical laboratories worldwide and serve as both GC and LC downstream MS detectors.