Ions/ionization atmospheric gases

Two models can explain the events that take place as the droplets dry. One was proposed by Dole and coworkers and elaborated by Rollgen and coworkers [7] and it is described as the charge residue mechanism (CRM). According to this theory, the ions detected in the MS are the charged species that remain after the complete evaporation of the solvent from the droplet. The ion evaporation model affirms that, as the droplet radius gets lower than approximately 10 nm, the emission of the solvated ions in the gas phase occurs directly from the droplet [8,9]. Neither of the two is fully accepted by the scientific community. It is likely that both mechanisms contribute to the generation of ions in the gas phase. They both take place at atmospheric pressure and room temperature, and this avoids thermal decomposition of the analytes and allows a more efficient desolvation of the droplets, compared to that under vacuum systems. In Figure 8.1, a schematic of the ionization process is described. 

Figure 22-17 (a) Pneumatically assisted electrospray interface for mass spectrometry. (fc>) Gas-phase ion formation. [Adapted from E. C. Huang, T. Wachs, J. J. Conboy, and J D. Henlon, "Atmospheric Pressure Ionization Mass Spectrometry. Anal. Chem. 1990, 62. 713A. and P Kebarte and L Tang, Rom Ions in Solution to Ions in the Gas Phase, Anal. Chem. 1993,65.972A ] (c) Electrospray from a silica capillary. [Courtesy R. D. Smith. Pacific Northwest Laboratory, Richland, WAJ... 

DESI Desorption electrospray ionization (DESI) is a recently developed technique that permits formation of gas-phase ions at atmospheric pressure without requiring prior sample extraction or preparation. A solvent is electrosprayed at the surface of a condensed-phase target substance. Volatilized ions containing the electrosprayed droplets and the surface composition of the target are formed from the surface and subjected to mass analysis (Takats et al., 2005 Wiseman et al., 2005 Kauppila et al., 2006). 

As is mentioned in Sect. 2.2, a discussion of de-ionization processes in the Earth s atmosphere would be incomplete without a mention of the r le of aerosols. The attachment of ions to aerosols in the stratosphere and troposphere has been considered by several workers213. It is clear that their presence will enhance the loss of ions from the gas phase at a rate dependent on the nature, size and number density of the particles, and so this process, which could be the dominant ionization loss process, must be considered along with gas phase ionic recombination in detailed atmospheric de-ionization rate calculations. 

Mobile-phase volatility is required in API LC/MS due to the need to produce gas-phase ions, whether through electrospray or chemical ionization. Atmospheric pressure chemical ionization can require higher solvent gas-phase volatility than electrospray due to its ionization mechanism. Low-surface-tension solvents also perform better due to improved nebulization properties [44-46]. Solvents such... 

APCI is a gas phase ionization process that creates ions at atmospheric pressnre. A sample solution flows through a heated tube (>400°C), where it is volatilized and sprayed into a corona discharge with the aid of nitrogen nebulization. Ions are produced in the discharge and extracted into the mass spectrometer as depicted in Figure 4.16. 

Chemical ionization is an ionization mechanism that allows the formation of protonated or deprotonated molecules via a gas-phase ion—molecule reaction. It exists under two different forms one under vacuum (Cl) and the second one at atmospheric pressure referenced as atmospheric pressure chemical ionization (APCI). The principal difference between Cl and El mode is the presence of reagent gas, which is typically methane, isobutene, or ammonia (Mimson, 2000). The electrons ionize the gas to form the radical cations (in the case of methane, CH4 -I- e CH4 -I- 2e ). In positive chemical ionization (PCI), the radical cations undergo various ion—molecule reactions to form CHs and finally lead to the formation, after proton transfer (CHs + M [M + H] ), of protonated molecules. Negative chemical ionization (NCI) (Budzikiewicz, 1986), after proton abstraction, leads to the formation of deprotonated molecules [M — H] . Negative ions can be produced by different processes such as by capture of low-energy electrons present in the chemical ionization... 

Which chromatography-mass spectrometry interface, electrospray or atmospheric pressure chemical ionization, creates new ions in the gas phase and which just introduces existing solution-phase ions into the gas phase ... 

Kauppila TJ, Talaty N, Salo PK, Kotiaho T, Kostiainen R, Cooks RG (2006) New surfaces for desorption electrospray ionization mass spectrometry porous silicon and ultra-thin layer chromatography plates. Rapid Commun Mass Spectrom 20 2143-2150 Kebarle P, Tang L (1993) From ions in solution to ions in the gas phase - the mechanisms of electrospray mass spectrometry. Anal Chem 65 972A-986A Laiko W, Taranenko NI, Berkout VD, Musselman BD, Doroshenko VM (2002) Atmospheric pressure laser desorption/ionization on porous silicon. Rapid Commun Mass Spectrom 16 1737-1742... 

Desorption electrospray ionization (DESI) may serve as an example of the maiy atmospheric-pressure surface ionization technique that has recently been introduced [63, 76]. In DESI, the high-velocity spray of charged microdroplets from a (pneumatically assisted) electrospray needle is directed at a surface, which is mounted in front of the ion-sampling orifice of an API source (see Fig. 7.6). Surface constituents are released fiom the surface and ionized. These gas-phase ions can be introduced to and observed by MS [77]. In this way, DESI-MS enables for instance the analysis of dmgs in tablets or natural products in plant parts withont extensive sample pre-treatment or prior separation. In addition, DESI-MS and some of its related snrface ionization techniqnes enable chemical imaging of surfaces such as thin-layer chromatography (TLC) plates and tissue sections [78]. 

The ESI source apparently suffers from the limitation that it cannot accept more than 40—50 xL/min of the LC mobile phase. These flow rates are compatible with 1 mm ID LC columns. Or, the effluent from a conventional 4.6 mm ID LC column can be partially diverted by a split device to the ESI source. As the ESI-MS arrangement is a concentration-sensitive detector, diverting only a fraction of the LC mobile phase does not affect sensitivity. Another way of overcoming the problem of coupling LC with 4.6 mm ID conventional columns is that of inducing analyte ionization by gas-phase ion-molecule reactions under APCI conditions. Reactant ion formation is achieved by the introduction of electrons from a corona discharge located in the chamber at atmospheric pressure. In this way, reversed-phase LC effluents can be handled as high as 2 mL/min. 

---