What Is an Electrospray Mass Spectrometer?

A mass spectrometer (MS) is an electronic instrument used to identify chemical structure. In most mass spectrometric procedures, molecules are electrically bombarded, resulting in ionization with fragmentation. The fragments are then accelerated magnetically toward detection and recording devices, resulting in specific peaks and intensities researchers can study as a kind of "molecular fingerprint." The electrospray mass spectrometer (EMS) functions differently — not leading to fragmentation. This makes it invaluable in the study of large species, or macromolecules.

If simple chemical bonding is all that needs to be determined, the use of an electrospray mass spectrometer will probably not be required. For larger molecules such as peptides, however, molecular shape and molecular folding — even molecular interaction with surrounding molecules — are every bit as important. In such instances, it is essential that the molecule remain unfragmented. The delicacy needed mandates the use of an electrospray mass spectrometer, which does not require the use of either high temperatures or a vacuum.

When using an electrospray mass spectrometer, a pure macromolecular specimen is first dissolved in a solvent system, which is next injected via narrow-bore needle into a high-voltage electric field. The solvent rather than the solute receives the brunt of the bombardment. As the liquid reaches a critical level of charge, the solution breaks apart violently into aerosol-sized droplets, their charge causing them individually to repel one other. Soon the droplets evaporate, depositing their multiple charges upon the still intact molecules, which through intermolecular repulsion become extended. In this state their structure, even at high levels of complexity, may be studied and determined.

The first successful intact protein spectrum was produced in 1989 by researchers at Yale University in Connecticut. Advancement in EMS technique was rapid, and in 1996 chemist Carol Robinson detected spectral peaks that could be associated, not just with a single structure, but with a complex of protein with coenzyme. One major improvement since then is the coupling of the electrospray mass spectrometer with time-of-flight (TOF) analysis. Collisional cooling takes even that improvement a step further by reducing the fragmentation of immense structures produced by heat.

One difficulty experienced in electrospray mass spectrometer determinations is that introduced by elemental isotopes. This is because peaks are dependent upon mass-to-charge ratio. The mass of a fragment or a molecule, divided by the number of discrete charges it carries, determines location. Different elemental isotopes contribute different masses, perhaps the most critical variance being that between carbon-12 and carbon-13. For this reason, samples of complex molecules should be monoisotopic if possible.