Atmospheric-pressure chemical ionization mechanism

One of the first successful techniques for selectively removing solvent from a solution without losing the dissolved solute was to add the solution dropwise to a moving continuous belt. The drops of solution on the belt were heated sufficiently to evaporate the solvent, and the residual solute on the belt was carried into a normal El (electron ionization) or Cl (chemical ionization) ion source, where it was heated more strongly so that it in turn volatilized and could be ionized. However, the moving-belt system had some mechanical problems and could be temperamental. The more recent, less-mechanical inlets such as electrospray have displaced it. The electrospray inlet should be compared with the atmospheric-pressure chemical ionization (APCI) inlet, which is described in Chapter 9. 

A number of analytical techniques such as FTIR spectroscopy,65-66 13C NMR,67,68 solid-state 13 C NMR,69 GPC or size exclusion chromatography (SEC),67-72 HPLC,73 mass spectrometric analysis,74 differential scanning calorimetry (DSC),67 75 76 and dynamic mechanical analysis (DMA)77 78 have been utilized to characterize resole syntheses and crosslinking reactions. Packed-column supercritical fluid chromatography with a negative-ion atmospheric pressure chemical ionization mass spectrometric detector has also been used to separate and characterize resoles resins.79 This section provides some examples of how these techniques are used in practical applications. 

Atmospheric-pressure chemical ionization (APCI) is another of the techniques in which the stream of liquid emerging from an HPLC column is dispersed into small droplets, in this case by the combination of heat and a nebulizing gas, as shown in Figure 4.21. As such, APCI shares many common features with ESI and thermospray which have been discussed previously. The differences between the techniques are the methods used for droplet generation and the mechanism of subsequent ion formation. These differences affect the analytical capabilities, in particular the range of polarity of analyte which may be ionized and the liquid flow rates that may be accommodated. 

Atmospheric pressure chemical ionization (APCI) interface for LC-MS became a reality at the end of the 1980s [42]. Its sensitivity and robustness were immediately impressive in comparison with all the other interface mechanisms present on the market. 

The advent of atmospheric pressure chemical ionization (APCI) is a relatively recent development, in which the same processes occur as in CI, outlined previously, but at atmospheric pressure. By a very similar mechanism to CI, the reagent gas (water) becomes protonated and can act as an acid towards the analyte, leading to the addition of a proton. Once again the species formed in positive ion mode is [M + H]. In the case of negative ion mode, the reagent gas acts as a base towards the analyte, and deprotonation occurs leading to the formation of [M—H], Once ions have been formed, they are funnelled towards the analyser inlet of the MS instrument by the use of electric potentials. APCI is also employed in LC-MS systems (see Section 5.6). 

Thanks to gas chromatography and atmospheric-pressure chemical-ionization mass-spectrometry analyses, a mechanism of nPr-BTP degradation has been proposed. The first step is assumed to be the attack at one CH2 group on the a position of the triazine rings to form a nitro compound observed by 15N NMR. In the second step, the compound is degraded into an alcohol (which is the main degradation product observed in the absence of acidic aqueous phase) or into a ketone (which is the main degradation product observed after the hydrolysis of nPr-BTP by molar nitric... 

Liquid chromatography-mass spectrometiy (LC-MS) based on atmospheric-pressure ionization (API) was demonstrated as early as 1974 (Ch. 3.2.1). However, it took until the late 1980 s before API was starting to be widely applied. Today, it can be considered by far the most important interfacing strategy in LC-MS. More than 99% of the LC-MS performed today is based on API interfacing. In this chapter, instrumentation for API interfacing is discussed. First, vacuum system for MS and LC-MS are briefly discussed. Subsequently, attention is paid to instrumental and practical aspects of electrospray ionization (ESI), atmospheric-pressure chemical ionization (APCI), and other interfacing approaches based on API. The emphasis in the discussion is on commercially available systems and modifications thereof. Ionization phenomena and mechanisms are dealt with in a separate chapter (Ch. 6). Laser-based ionization for LC-MS is briefly reviewed (Ch. 5.9). 

In the current LC-MS interfaces, i.e., ESI and atmospheric-pressure chemical ionization (APCl), interfacing and analyte ionization are closely interrelated. The column effluent is nebulized and ionization takes place in the aerosol generated, either with or without a primary external source of ionization. Ionization mechanisms of ESI, APCl, and atmospheric-pressure photoionization (APPl) are discussed in this chapter. 

Meurer, E. C., Santos, L. S., Pilli, R. A., Eberlin, M. N. Probing the Mechanism of the Petasis Olefination Reaction by Atmospheric Pressure Chemical Ionization Mass and Tandem Mass Spectrometry. Org. Lett. 2003, 5, 1391-1394. 

The need for an ionization source that provided both softer ionization, i.e., less fragmentation of the molecular ion, and a convenient interface with liquid chromatography (at ambient pressure) to mass spectrometry (at high vacuum) helped spur the creation of atmospheric pressure ionization. Two techniques fall under the heading of API, electrospray and atmospheric pressure chemical ionization (APCI), and the technical aspects of each are discussed individually. However, many of the fundamental principals that describe these ionization mechanisms can be applied to both electrospray and APCI sources. 

Atmospheric pressure chemical ionization, like electrospray ionization, is a mass spectrometer ionization source in which ionization occurs not in a vacuum but at atmospheric pressure. In contrast to electrospray ionization, in which the ionization process occurs in solution phase, atmospheric pressure chemical ionization is a gas-phase ionization process whereby gas-phase molecules are isolated from the carrier solvent before ionization [6]. Because the ionization mechanisms of APCI and electrospray are fundamentally different (gas-phase and liquid-phase ionization, respectively) the two methods have the potential to provide complimentary analyte characterization. To generalize, electrospray ionization is more... 

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... 

Commercial pSFC instruments are easily coupled to electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) mass spectrometry with only a very minor modification. Pure CO2 does not produce ions under ESI or APCI conditions. Furthermore, CO2 does not play any direct role in ion formation due to the lack of CO2-derived primary ions. Therefore, it is essential to add postcolumn modifiers (e.g., MeOH or other solvents) into the CO2 stream. This is especially true with pSFC/ESI-MS to introduce a solution chemistry mechanism where (de)protonated or adduct-based molecular ions of the analytes are produced. 

A comparison of various LC-MS systems for the analysis of complex mixtures of PAHs showed that (1) the moving belt interface was mechanically awkward and is compatible only with a limited range of mobile phases, (2) particle-beam interface had low sensitivity, and the response was nonlinear, (3) a heated nebulizer interface that uses atmospheric pressure chemical ionization (APCI) was the preferred procedure (Anacleto et al. 1995). 

Ostman, R, Jantti, S., Grigoras, K. et al. (2006) Capillary liquid chromatography-microchip atmospheric pressure chemical ionization-mass spectrometry. Lab Chip, 6 (7), 948-953. Chang, Y.Z., Yang, M.W. and Wang, G.J. (2005) A new mass spectrometry electrospray tip obtained via precise mechanical micromachining. Anal Bioanal Chem, 383 (1), 76-82. 

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... 

Meurer, E.C., Santos, LS., PiUi, R.A., and Eberlin, M.N. (2003) Probing the mechanism of the petasis olefination reaction by atmospheric pressure chemical ionization mass and tandem mass spectrometry. Org. Lett., 5, 1391-1394. 

Electrospray ionization and atmospheric pressure chemical ionization are popular as ionization techniques, for qualitative and quantitative LC—MS analysis of lipids [63—65]. Based on flieir ionization mechanisms, ESI is more suitable for ionization of polar and ionic compounds and is capable of ionizing both small and large biomolecules. APCI can ionize less polar and neutral compounds more efficiently than ESI. Consequently, APCI—MS coupled to LC is the most used tool for TAG identification, because of the full compatibility with common NARP LC conditions, easy ionization of nonpolar TAGs, and the attainment of both protonated molecules [M + H]+ and fragment ions [M - - H — RzCOOH]. On the other hand, ESI is usually employed for the more-polar phospholipids. However, ESI or matrix-assisted laser desorption—ionization (MALDI) have been used for TAGs, as well [66,67]. 

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