Atmospheric-pressure chemical ionization suitability

Atmospheric Pressure Chemical Ionization (APCI)/MS APCI/MS is used to analyze compounds of intermediate molecular weight (100-1,500 da) and intermediate polarity and is particularly useful for the analysis of biochemicals such as triacylglycerides, carotenoids, and lipids (Byrdwell, 2001). For volatile, nonpolar compounds of low molecular weight, GC/MS is preferred to APCl/MS whereas APl-electrospray/ MS provides better results for larger, more polar materials. The selection of APCl/MS over GC/MS or APl-electrospray/MS depends on the compounds to be analyzed. Many LC/MS instruments can be easily switched between APCl/MS and APl-electrospray/MS so that it can be rapidly determined which ionization process is more suitable to a given chemical. Additional manipulations such as pre and postcolumn derivatization reactions (Nagy et al., 2004 Peters et al., 2004) or coulometric oxidation (Diehl et al., 2001) can make the chemicals of interest more amenable to detection by APCI. 

Figure A.3A.1 Flow chart illustrating the selection of a suitable ionization technique for the mass spectrometric analysis of a sample. Abbreviations APCI, atmospheric pressure chemical ionization Cl, chemical ionization El, electron impact FAB, fast atom bombardment MALDI, matrix-assisted laser desorption/ionization.

They are still the workhorses of coupled mass spectrometric applications, as they are relatively simple to run and service, relatively inexpensive (for a mass spectrometer), and provide unit mass resolution and scanning speeds up to approximately 10,000 amu/s. This even allows for simultaneous scan/ selected ion monitoring (SIM) operation, in which one part of the data acquisition time is used to scan an entire spectrum, whereas the other part is used to record the intensities of selected ions, thus providing both qualitative information and sensitive quantitation. They are thus suitable for many GC-MS and liquid chromatography-mass spectrometry (LC-MS) applications. In contrast to GC-MS with electron impact (El) ionization, however, LC-MS provides only limited structural information as a consequence of the soft ionization techniques commonly used with LC-MS instruments [electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI)]. Because of this limitation, other types of mass spectrometers are increasingly gaining in importance for LC-MS. 

Most often positive ESI and only to a small extent positive atmospheric pressure chemical ionization (APCI) were used as ion source (interface) to generate desol-vated free ions of TTA or QTA suitable for MS or MS/MS detection. TTA were detected as their proton adducts [M+H]+, whereas QTA were simply monitored as the original cations [M]+ thus not requiring adduct formation (Table 9). 

The dynamic development of mass spectrometry has had a huge impact on lipid analysis. Currently, a variety of suitable mass spectrometers is available. In principal, a mass spectrometer consists of an ion source, a mass analyzer, and an ion detector. The typical features of each instrument (Fig. 2) result mostly from the types of ion source and mass analyzer. To date, the ionization techniques apphed to lipid analysis include Electrospray Ionization (ESI or nano-ESI), Atmospheric Pressure Chemical Ionization (APCI), Matrix-Assisted Laser Desorption/Ionization... 

Various types of HSCCC-MS have been developed using frit fast-atom bombardment (FAB) including continuous flow (CF) FAB, frit electron ionization (El), frit chemical ionization (Cl), TSP, atmospheric pressure chemical ionization (APCI), and electrospray ionization (ESI). Each interface has its specific features. Among those, frit MS and ESI are particularly suitable for directly interfacing to HSCCC, because they generate low back-pressures of approximately 2 kg/cm, which is only one-tenth of that produced by TSP. 

Nearly all known ionization methods of mass spectrometry (including electron impact, laser desorption and fast atom bombardment) were already successfully applied to lipids. However, many ionization techniques are not very suitable for the analysis of complex PL mixtures as they provide considerable amounts of fragment ions. Therefore, only three soft-ionization methods play nowadays a major role in lipid analysis. Beside atmospheric pressure chemical ionization (APCI) (Byrdwell 2001), electrospray ionization (ESI) (Pulfer and Murphy... 

Suitable analytes exhibit high electron capture capacity or high electron affinity. Cl is the technique of choice for the analysis of isomers in environmental samples. In APCI (Atmospheric Pressure Chemical Ionization) solvent evaporation and analyte ionization are two separate processes. 

Ionization of condensed-phase analytes occurs by mixing a sample in a suitable matrix and bombarding the matrix-analyte mixture with an energetic beam made of either laser photons as in MALDI, high-energy fission particles as in Cf plasma desorption, or high-energy fast atoms or ions (FAB or liquid SIMS). When an analyte is present in a solution, such as an effluent from a separation device, it can be ionized via thermospray ionization, atmospheric-pressure chemical ionization, atmospheric-pressure photoionization, or electrospray ionization. Desorption electrospray ionization and direct analysis in real time are new modes of ionization that are accomplished in ambient air. 

Most of the mass spectrometry analyses are conducted under vacuum environment. However, ambient mass spectrometry is a rapidly growing field that provides fast and direct analysis of solid sample surfaces or liquid samples introduced on a suitable surface (Alberici et al. 2010 Weston 2010 Huang et al. 2010 Chen et al. 2010). For that, different ambient ionization MS methods, such as atmospheric pressure desorption/ionization on porous silicon (AP-DIOS) (Huikko et al. 2003), desorption electrospray ionization (DESI) (Takats et al. 2004), direct analysis in real time (DART) (Cody et al. 2005), desorption atmospheric pressure chemical ionization (DAPCI) (Takats et al. 2005), and desorption atmospheric pressure photoionization (DAPPI) (Haapala et al. 2007), have been successfully used in the direct analysis of compounds fi"om various samples, such as body fluids (Cody et al. 2005 Chen et al. 2006), finiits, plant leaves (Luosujarvi et al. 2010), milk (Yang et al. 2009), banknotes (Cody et al. 2005), textiles (Cody et al. 2005 Chen et al. 2007), and pharmaceutical formulations (Ifa et al. 2009 Gheen et al. 2010), just to mention a few, without any sample pretreatment. 

The MS ion source which has been almost exclusively used for the ionization of dmgs and metabolites is electrospray (ESI). This source gives better analytical performances, however it suffers the problem of matrix effect, a phenomenon in which the presence of coeluting substances causes suppression or enhancements of the ionization signal of analytes. This is particularly evident with more complex samples, such as wastewater influents and effluents, than with cleaneT samples, such as river or lake water samples. The atmospheric pressure chemical ionization source (APCf) is considered to be less susceptible to matrix interferences, however, it is not suitable for all substances, in particular, the more polar ones (morphine, morphine-3 p-D-glucuronide, and ecgonine metltyl ester). 

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]. 

ELSD), but in this case, careful calibratioD is required for quantification. A variety of MS detectors (quadrupole, ion trap, time-of-flight) using different ionization techniques (electrospray [ESI], atmospheric pressure chemical ionization [APCI]) have also been used for analyzing fatty acids (Lima and AbduUa, 2002). With suitable derivatives, detection by chemiluminescence (Ohba, Kuroda, and Nakashima, 2002) and under appropriate conditions electrochemical detection (Kotani, Kusu, and Takamura, 2002) are also possible. 

Atmospheric pressure chemical ionization. LC-MS with acetonitrile/water/ammo-nium acetate or even columnless injection of acetonitrile solutions of APE gives molecular positive ion peaks or pseudomolecular adducts for all oligomers in the sample, limited only by the mass range of the spectrometer. Identification and quantification of individual peaks is difficult and requires experience to differentiate PEG from the surfactant (61,62). Positive molecular ions characteristic of NPE are of the form [44x + 220]", while those for PEG are [44x + 18] (18). Plomley et al. report that APCI is superior to electrospray ionization for determination of APE in the environment, because fewer interfering compounds are ionized (79). APCI is also more suitable for use with normal phase LC. 

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