Electron Ionization Sources: The Basics

Jun 30, 2006

The sensitivity of a mass spectrometer (the source combined with the remainder of the instrument, but sometimes also the source itself) is defined strictly in units of C/μg, where C represents the charge in coulombs carried by ions that can be created from 1 μg of sample introduced to the source. The sensitivity of any MS measurement is predicated first upon instrument sensitivity (including factors such as source performance, mass analyzer transmission, and ion detection efficiency), but also expands to include sample preparation, signal-to-noise discrimination, and matrix effects in real samples. The central position of source performance in this scheme should be clear. For EI sources, source design and operational details directly affect source performance. It is useful then to broadly consider the 50-year history of the EI source in terms of sensitivity, and its optimization, but also to include all the other factors that go into the design and construction of an ion source.

We begin with the essential elements of the EI source. Essential components of an EI source assembled into the basic layout can be found in schematics in introductory texts or manufacturer's literature on the web. Often it seems that short shrift is given to the EI source, and details of design and optimization are overlooked in the fact that the EI source is so common. Lack of current research literature on the design of the EI source reflects the long history of successful development of this source for organic analysis, the tendency for design studies to be completed now in conjunction with instrument manufacture (and therefore to be proprietary), and the attention given to new ionization methods. However, more specialized applications of the EI source often require revisiting these fundamental design parameters; examples of these applications include portable instruments and instruments for analysis in extreme environments. Importantly, an appreciation of the factors considered in the design of the EI source, the balances reached in accord with the transfer of sample molecules into the source, and the transfer of ions out of it informs our designs and evaluations of other ionization sources. In this column, we begin a brief overview of some of these factors and balances; then in the next column, we revisit some of the "older" scientific literature and hopefully enlighten the understanding of source design for mass spectrometers.