Alternative Ionization Methods

There are numerous other ionization methods, but they have limited applications. Fast atom bombardment (FAB), also known as liquid secondary ion mass spectrometry (LSIMS), was one of the early methods developed for the ionization of polar molecules. FAB is based on bombarding analytes in a matrix of low volatility, such as glycerol, with accelerated energetic neutral atoms (argon or xenon) or ions (cesium) that wiU sputter [M + H]+ ions from the surface. Although of major importance during its heyday, FAB has been superseded by ESI. 

Once ions have been formed, either outside the mass spectrometer using API methods or within the vacuum system by El, Cl, or MALDI, the ions must be separated according to their m/z ratios. There are several types of mass analyzer with significantly different modes of operation, but all separate ions according to their miz ratios, so that these ratios and their intensities can be recorded by the detector. Current mass analyzers include quadrupole (Q), quadrupole ion traps (QIT, LIT), Fourier transformed based (FT), time-of-flight (TOF), and to a much lesser extent, magnetic field (B). 

It is now common to combine two or more analyzers within a single instrument (MS/MS) to improve and extend analytical capabilities. The combinations may involve similar analyzers (tandem), as in a TOF/TOF system, or be of mixed types (hybrid) as in a QTOF instrument. 

---

FigureBl.7.2. Schematic representations of alternative ionization methods to El and PI (a) fast-atom bombardment in which a beam of keV atoms desorbs solute from a matrix (b) matrix-assisted laser desorption ionization and (c) electrospray ionization.

As explained above, the continuous radioactive source used in our IMS also produces small, and sometimes troublesome, amounts of impurities within the ion source. In an effort to eliminate this problem, we are presently exploring the use of other source designs by which less radiation damage is done to the source gas. One of these alternative ionization methods is based on the photoemission of electrons from the back side of a gold foil. This ion source offers an attractive advantage for the study of negative IM reactions in that it can be shut off during most of the IMS duty cycle. 

The resolution of this latter problem passed through the development of the so-called soft ionization methods, in which ions are directly produced from the solid or liquid state. Field ionization (FI) and field desorption (FD) were two of the first alternative ionization methods. A few years later, other techniques were developed. Examples of these include desorption/chemical ionization (D/CI), secondary ion mass spectrometry (SI-MS), and fast atom bombardment (FAB). 

Current developments in MS have been directed to the application of alternative ionization methods for sensitive molecules, to the use of tandem systems (Durand et al. 1992), and to the development of interfaces with chromatographic systems (Arpino 1990 Niessen et al. 1991 Arpino et al. 1993). Some examples include the following. 

Organophosphonium salts are a general class of compounds which are difficult to analyse via conventional EI-MS Thus several reports have appeared in the literature which describe the use of alternative ionization methods to analyse this important class of compounds " Early studies, which centered around the use of field desorption (FD), demonstrated that the intact phosphonium ion is often the most abundant ion. A comparison was made between the use of FD and FAB as ionization methods for the analysis of the diphosphonium salt FD produces high abundances of the intact phosphonium ions [M - Br ] and [M - 2Br ], whereas these ions are only minor peaks in the FAB mass spectrum using glycerol as the matrix. Instead, the fragment ion 35 is the base peak in the FAB spectrum. 

An absolute method for molecular weight determination is matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF) (Kona et al., 2005 Creel, 1993 Nielsen, 1999 Cho et al., 2001). The sample is dispersed in a UV-absorbing matrix (e.g., trans -cinnamic acid or 2,5-dihydroxybennzoic acid). Irradiation with a UV laser induces evaporation of ionized polymer chains, which are then detected using TOF. The technique requires relatively narrow MWD samples. Alternative ionization methods have been employed, such as electrospray ionization mass spectrometry (ESI-MS), which may have advantages for certain polymer end groups (Vana et al., 2002). IFFF and MALDI-TOF can be coupled to analyze polydisperse samples and polymer mixtures (Kassalainen and Williams, 2003). 

In the past several years, a number of new ionization methods in mass spectrometry have been introduced. These new techniques have extended mass spectrometric analysis to a wide variety of labile (thermally unstable), highly polar, and higher molecular weight materials. Field ionization (FI) and field desorption (FD) are two of the pioneering techniques in this list of alternative ionization methods. FI-MS, which was introduced for organic molecules in 1954, was the first soft ionization method. (Soft ionization refers to processes that produce high relative abundances of molecular, or quasimolecular, ions.) FD-MS, which was invented in 1969, was the first desorption/ionization method. (Desorption/ionization refers to processes in which die vaporization/ desorption, and ionization steps occur essentially simultaneously.)... 

Electron impact (El), the method outlined in Section 9.6.1, is the conventional method of ionizing samples for MS. However, El is not free from disadvantages, such as inability to provide molecular weight information from thermally labile samples, difficulty for study of low-volatility materials, complex spectra arising from interference between molecular and fragment ions, etc. In order to overcome these problems, a large number of alternative ionization methods, collectively known as soft ionization methods have been developed. Reviews of the available techniques have been compiled by Milne and Lacey (1974) and Games (1978). Chemical ionization (Cl), field ionization (FI) and field desorption (FD)... 

As an alternate ionization method to ESI, the atmospheric pressure chemical ionization (APCI) has been applied to the MEKC-MS system. In MEKC-APCTMS, an SDS micellar solution can be introduced directly into the interface without a severe decrease in MS intensity. For highly sensitive analysis of environmental pollutants, an application study of sweeping to MEKC hyphenated with MS using an APCI interface has been reported. 

---