Ionization field desorption

Field desorption was eclipsed by the advent of FAB (next section). Despite the fact that the method is often more useful than FAB for nonpolar compounds and does not suffer from the high level of background ions that are found in matrix-assisted desorption methods, it has not become as popular as FAB probably because the commercial manufacturers have strongly supported FAB. 

Field (desorption) ionization (FDI) was first described by Beckey in 1969 [4]. In FDI, the sample solution is deposited on a lO-qm-diameter FDI emitter, which is activated to provide for whiskers or microneedles on the surface [5, 6]. The emitter is kept at a high potential ( 5 kV) in the high-vacuum ion source, mostly of a sector instmment (see Fig. 7.1). By passing through a current, slow heating of the emitter is achieved. As heating of the emitter continues, nonvolatile analytes can be desorbed and ionized by various mechanisms. High local electrical fields at the 

Because of the experimental difficulty of the technique and because more user-friendly and to some extent more powerful alternatives have become available, FDI is not frequently applied anymore, except for some specific applications. In this respect, an important development is liquid injection field desorption ionization (LIFDI), which enables sample application to the emitter without breaking the vacuum (see Fig. 7.1) [7, 8]. The specific applications where FDI and LIFDI are still applied comprise the analysis of some oiganometallic compounds [9,10], ionic liquids [11], and compound classes, such as (cyclo)paraffins, aromatic hydrocarbons, and nonpolar sulfur compounds (thiophenes) [7, 12-14], not readily amenable to ESI or MALDI. For such nonpolar analytes, mainly molecular ions M+ are observed, whereas for some more polar compounds, [M+H]+ and/or sodiated molecules ([M-l-Na] ) may be observed, e g., for glycosides (Sect. 7.5.2), lipids (Sect. 7.5.4), and peptides (Sect. 7.5.5). A detailed overview on technology and applications of FDI-MS was provided by Schulten et al. [15, 16]. 

---

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. 

Schaub, T.M. Hendrickson, C.L. Qian, K. Quinn, J.P. Marshall, A.G. High-Resolution Field Desorption/Ionization Fourier Transform ion Cyclotron Reso-... 

Liquid Injection Field Desorption Ionization Mass Spectrometry 545... 

Figure 14.2. Arrangement of an activated emitter in a liquid injection field desorption ionization (LIFDI) system. Courtesy of Linden ChroMasSpec GmbH, Leeste, Germany.

Fu, J., Klein, G. C., Smith, D. F., Kim, S., Rodgers, R. R, Hendrickson, C. L., and Marshall, A. G. (2006a). Comprehensive compositional analysis of hydrotreated and untreated nitrogen-concentrated fractions from syncrude oil by electron ionization, field desorption ionization, and electrospray ionization ultrahigh-resolution FT-ICR mass spectrometry. Energy Fuels 20,1235-1241. 

Griep-Raming, J., and Linden, H. B. (2005). Fully automated liquid injection field desorption/ ionization (LIFDI) mass spectrometry for high throughput screening of compounds with various polarity. 38. Annual DGMS Conference, Rostock, Germany. 

Gross, J. H., Nieth, N., Linden, H. B., Blumbach, U., Richter, F. J.,Tauchert, M. E.,Tompers, R., and Hofmann, P (2006). Liquid injection field desorption/ionization of reactive transition metal complexes. Anal. Bioanal. Chem. 386, 52-58. 

Schaub, T. M., Rodgers, R. R, Marshall, A. G., Qian, K., Green, L. A., and Olmstead, W. N. (2005). Speciation of aromatic compounds in petroleum refinery streams by continuous flow field desorption ionization FTICR mass spectrometry. Energy Fuels 19,1566-1573. 

Burlingame s group [254] adopted as an alternative, field desorption ionization combined with BjE linked scan mode, which led to a better resolution ( 1 amu). Biologically important molecules were studied with this technique. 

Fig, 54. Field desorption of electronegative adatom A (/ — >> 0). V (x)—atomic potential at distance x E/D—activation energy for field desorption. Ionization occurs at xe for adatoms sufficiently energetic to reach this point. 

In view of the ionization of samples of low volatility, desorption chemical ionization (DCI) is one of the most promising new techniques. The sample is placed on a heatable probe tip or on a field desorption emitter probe and introduced directly into the plasma of a chemical ion source. It was shown that by such an exposure of the sample to the ion plasma, mass spectra can be obtained at much lower temperatures than usually required . This extraordinarily simple sample handling and the apparently more intense and better reproducible ion beam than in field desorption ionization have obviously induced a rapidly growing field of applications of DCI. This development is also supported by a growing number of DCI probe devices available from different manufacturers at relatively reasonable prices. Although many applications, for instance to underivatized peptides , to cyclic adenosine monophosphate, guanosine, and thiophosphoric acid pesticides , as well to creatinine and arginine have been reported, the analytical potential of DCI is not yet exhausted and will obviously result in many new possibilities in analytical research (for recent publications see ). 

T. M. Schaub, C. L. Hendrickson, K. Qian, J. P. Quinn, and A. G. Marshall, High-resolution field desorption/ionization FT ICR mass analysis of nonpolar molecules. 

Fig. 7.1 Pieture of a liquid injection field desorption ionization probe tip (LIFDI-MS). (Reprinted from [9] with kind permission from Springer Seienee and Business Media)...

Linden HB. Liquid injection field desorption ionization a new tool for soft ionization of samples including air-sensitive catalysts and non-polar hydrocarbons. Fur. J Mass Spectrom. 

Smith DF, Schaub TM, Rodgers RP, Hendrickson CL, Marshall AG. Automated liquid injection field desorption/ionization for Fourier transform ion cyclotron resonance mass spectrometry. Anal Chem. 2008 80 7379-82. 

Gross JH. Liquid injection field desorption/ionization-mass spectrometry of ionic liquids. J Am Soc Mass Spectrom. 2007 18 2254-62. 

---