Field desorption sources

LMI sources with needle emitters operate in essentially the same way as field ionization or field desorption sources. The filament is resistively heated to melt the metal film and/or promote its flow to the tip of the emitter. Typically, the emitter or anode is positively biased 3-5 kV with respect to its counter electrode, the cathode the actual operating voltage is determined... 

These direct ion sources exist under two types liquid-phase ion sources and solid-state ion sources. In liquid-phase ion sources the analyte is in solution. This solution is introduced, by nebulization, as droplets into the source where ions are produced at atmospheric pressure and focused into the mass spectrometer through some vacuum pumping stages. Electrospray, atmospheric pressure chemical ionization and atmospheric pressure photoionization sources correspond to this type. In solid-state ion sources, the analyte is in an involatile deposit. It is obtained by various preparation methods which frequently involve the introduction of a matrix that can be either a solid or a viscous fluid. This deposit is then irradiated by energetic particles or photons that desorb ions near the surface of the deposit. These ions can be extracted by an electric field and focused towards the analyser. Matrix-assisted laser desorption, secondary ion mass spectrometry, plasma desorption and field desorption sources all use this strategy to produce ions. Fast atom bombardment uses an involatile liquid matrix. 

Burlingame s group installed the collision cell in the El source located just at the exit of the field desorption source on a modified AEI MS 902 instrument (conventional geometry). This installation enabled them to study the decompositions of high-molecular-weight molecules (> 600), as we will show later. 

Describe the difference between gaset>us held ioni/uition st>urces and field desorption sources. 

Anbar, M. and St. John, G.A. (1976), Field Ionization— Field desorption source for nonfragmenting mass spectrometry. Anal. Chem., 48,198. 

The main difference between field ionization (FI) and field desorption ionization (FD) lies in the manner in which the sample is examined. For FI, the substance under investigation is heated in a vacuum so as to volatilize it onto an ionization surface. In FD, the substance to be examined is placed directly onto the surface before ionization is implemented. FI is quite satisfactory for volatile, thermally stable compounds, but FD is needed for nonvolatile and/or thermally labile substances. Therefore, most FI sources are arranged to function also as FD sources, and the technique is known as FI/FD mass spectrometry. 

For nonvolatile or thermally labile samples, a solution of the substance to be examined is applied to the emitter electrode by means of a microsyringe outside the ion source. After evaporation of the solvent, the emitter is put into the ion source and the ionizing voltage is applied. By this means, thermally labile substances, such as peptides, sugars, nucleosides, and so on, can be examined easily and provide excellent molecular mass information. Although still FI, this last ionization is referred to specifically as field desorption (FD). A comparison of FI and FD spectra of D-glucose is shown in Figure 5.6. 

Recent attention has focused on MS for the direct analysis of polymer extracts, using soft ionisation sources to provide enhanced molecular ion signals and less fragment ions, thereby facilitating spectral interpretation. The direct MS analysis of polymer extracts has been accomplished using fast atom bombardment (FAB) [97,98], laser desorption (LD) [97,99], field desorption (FD) [100] and chemical ionisation (Cl) [100]. 

Field desorption (FD) was introduced by Beckey in 1969 [76]. FD was the first soft ionization method that could generate intact ions from nonvolatile compounds, such as small peptides [77]. The principal difference between FD and FI is the sample injection. Rather than being in the gas phase as in FI, analytes in FD are placed onto the emitter and desorbed from its surface. Application of the analyte onto the emitter can be performed by just dipping the activated emitter in a solution. The emitter is then introduced into the ion source of the spectrometer. The positioning of the emitter is cmcial for a successful experiment, and so is the temperature setting. In general, FI and FD are now replaced by more efficient ionization methods, such as MALDI and ESI. For a description of FD (and FI), see Reference 78. 

Hogg, A.M. Payzant, J.D. Design of a Field lonization/Field Desorption/Electron Impact Ion Source and its Performance on a Modified AEIMS9 Mass Spectrometer. Int. J. Mass Spectrom. Ion Phys. 1978, 27, 291-303. 

Heinen, H.J. Hotzel, C. Beckey, H.D. Combination of a Field Desorption Ion Source With a Quadrapole Mass Analyzer. Int. J. Mass Spectrom. Ion Phys. 1974,13, 55-62. 

Figure 5. Field desorption ion source showing the position oj the emitter during the...

Field desorption mass spectra were obtained on a Varian MAT 731 instrument (Florham Park, NJ) fitted with the combined EI/FI/FD ion source. Emitters were prepared in the Varian apparatus according to Schulten and Beckey (3J, or were pretreated before activation by soaking in a saturated salt solution (9 ). 

A LMI emitter s smallness makes it possible to attach it to the ion source of almost any magnetic sector or quadrupole mass spectrometer. Ion sources which have already been designed for fast atom bombardment (FAB) or field desorption (FD) are ideally suited to modification for LMI/SIMS operation. 

The application of molecular SIMS as a sensitive ionization source for nonvolatile and thermally labile molecules compares favorably with other new ionization methods in mass spectrometry such as field desorption (FD), Californium-252 plasma desorption (PD), fast heavy ion induced desorption (FHIID), laser desorp-... 

Ionization Methods/Processes. The recent development of several new ionization methods in mass spectrometry has significantly improved the capability for the analysis of nonvolatile and thermally labile molecules [18-23]. Several of these methods (e.g., field desorption (FD), Californiun-252 plasma desorption (PD), fast heavy ion induced desorption (FHIID), laser-desorption (LD), SIMS, and fast atom bombardment (FAB) or liquid SIMS) desorb and ionize molecules directly from the solid state, thereby reducing the chance of thermal degradation. Although these methods employ fundamentally different excitation sources, similarities in their mass spectra, such as, the appearance of protonated, deprotonated, and/or cationized molecular ions, suggest a related ionization process. 

Electron-spin resonance (ESR) indicated free radicals in a brown polymer (mean ca 1 kDa by field-desorption MS) from a glucose-4-chloroaniline model system.185 Such free radicals (see Chapter 2) could be a source of visible colour. Exposure of the melanoidin to nitric oxide, a radical-trapping agent, diminished the ESR signal by 48% and changed the colour to red-brown. 

A different, but no less severe problem, is encountered with the analysis of unknown biological substances by field desorption, laser desorption, and/or laser assisted field desorption mass spectrometry. A priori, it is not known at what point in time a component of interest is desorbed. This makes it difficult, if not impossible to time a scan or several scans in such a way that a representative spectrum is obtained. Again the answer to the problem is found by integrating the ion output over the entire sample or repeatedly over portions of the sample profile as it emerges from the ion source. 

One of the greatest open questions in the field of fullerenes is the understanding of how such compact, closed structures form so efficiently from carbon vapor. Under appropriate conditions, as much as 20% of the carbon vaporized in an arc or a laser desorption source is incorporated into fullerenes however, no laboratory chemical synthesis has succeeded It is known from an isotope-scrambling experiment that the fullerenes are formed by the condensation of carbon atoms in the hot carbon vapor, as opposed, for example, to small sheets of graphite torn from the electrodes, and subsequently curling up.[Me90b]... 

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