Instrumentation ionization techniques

A discussion of the motivation behind doing sputtered neutral analysis versus SIMS, plus a description of the first prototype SALI instrument. A well written introduction for someone without previous surface analysis experience it also includes an historical overview of the various post-ionization techniques. 

R. W. Odom and B. Schueler. Laser Microprobe Mass Spectrometry Ion and Neutral Analysis, in Lasers and Mass Spectrometry (D. M. Lubman, ed.) Oxford University Press, Oxford, 1990. Presents a useful discussion of LIMS instrumental issues, including the post-ablation ionization technique. Several anal)n ical applications are presented. 

On account of having the same number of electrons and protons, isotopes of a given element have identical chemical properties and reactivity. However, since they differ in the number of neutrons, they have different atomic masses. As MS measures and discriminates mass, isotopes are detected and they are present in every mass spectrum, independent of the ionization technique used, instrumentation, etc. 

There are many reviews on specific MS or ionization techniques that will be referenced in the following sections, but electrospray is one of the newest ionization methods that are fast becoming an important technique in inorganic and organometallic chemistry. A review (17) on electrospray applied to inorganic and organometallic chemistry appeared in 1995 and the rapid growth of this area probably warranted another review (18). A useful book entitled Electrospray Ionization Mass Spectrometry, Fundamentals, Instrumentation, and Applications has... 

A.P. Bruins, Atmospheric pressure ionization-mass spectrometry I. Instrumentation and ionization techniques, Trends Anal. Chem., 13 (1994) 37-43. 

Over the last decade or so, new ionization techniques such as ES and MALDI have been introduced and have increased still further the use of mass spectrometry in biology. Identification of proteins and characterization of their primary structure is a rapidly growing field in the postgenomic era. ES ionization was the first method to extend the useful mass range of instruments to well over 50 kDa (Mano and Goto... 

Recent advances in instrumentation and ionization techniques are described briefly. Several sections have been rewritten and expanded for greater clarity. The useful Table of Formula Masses (four decimal places) has been shortened by eliminating entries that are unlikely in the present context. The Table is convenient for selecting tentative molecular formulas and fragments on the basis of unit-mass peaks. Given high-resolution peaks, specific molecular or fragment formulas can be selected. 

TLC-MS-MS has been developed using either manual or instrumental approaches for sample introduction into the MS-MS section. For manual TLC-MS-MS, the spot or zone containing the analyte is removed from the plate after suitable concentration and, if FAB or LSIMS is going to be the ionization technique, it is suspended in a liquid matrix such as glycerol. The resulting mixture is then applied to the probe tip to be introduced into the spectrometer in the usual way. Although this approach involves manipulation of the sample and thus the potential for loss of some of the spatial resolution obtained by the TLC separation, it is readily implemented and requires no additional equipment or interface. 

Most mass spectrometers equipped for electrospray ionization can be converted to APCI, and many commercial LC-APCI-MS instruments are equipped with both ionization techniques. During APCI, ionization takes place in an atmospheric pressure chamber when the sample molecules collide with solvent ions formed in a continuous corona discharge. Unlike electrospray, the needle used to spray the HPLC effluent is not at high voltage. 

The theory and instrumentation of Fourier transform mass spectrometry (FTMS) have been discussed extensively in this book and elsewhere [21-23]. All experiments were performed on a Nicolet prototype FTMS-1000 Fourier transform mass spectrometer previously described in detail [24] and equipped with a 5.2 cm cubic trapping cell situated between the poles of a Varian 15 in. electromagnet maintained at 0.85 T. The cell was constructed in our laboratory and utilizes two 80 transmittance stainless steel screens as the transmitter plates. This permits irradiation with a 2.5 kW Hg-Xe arc lamp, used in conjunction with a Schoeffel 0.25 m monochromator set for 10 nm resolution. Metal ions are generated by focusing the beam of a Quanta Ray Nd YAG laser (either the fundamental line at 1064 nm or the frequency doubled line at 532 nm) into the center-drilled hole (1 mm) of a high-purity rod of the appropriate metal supported on the transmitter screen nearest to the laser. The laser ionization technique for generating metal ions has been outlined elsewhere [25]-... 

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