Atmospheric-pressure-chemical-ionization adducts

Cl is not the only ionization technique where this aspect of interpretation must be considered carefully fast-atom bombardment, thermospray, electrospray and atmospheric-pressure chemical ionization, described below in Sections 3.2.3, 4.6, 4.7 and 4.8, respectively, all produce adducts in the molecular ion region of their spectra. 

Some combinations of IC with MS detection are available, including ICP-MS (element-specific detection), particle beam MS and MS with atmospheric pressure ionization (API) operated in either electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) modes that give information on molecular ions or adducts. 

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). 

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. 

Hoberg AM, Otteneder M, Mamett LJ, Poulsen HE. Measurement of the malondial-dehyde-2 -deoxyguanosine adduct in human urine by immuno-extraction and liquid chromatography/atmospheric pressure chemical ionization tandem mass spectrometry. J Mass Spectrom 2004 39 38-42. 

Goto, T. Shibata, A. lida, T. Mano, N. Goto, J. Sensitive mass spectrometric detection of neutral bile acid metabolites. Formation of adducts with an organic anion in atmospheric pressure chemical ionization. Rapid Commun. Mass Spectrom. 2004,18 (19), 2360-2364. 

The basic function of the mass spectrometer is to measure the mass-to-charge ratios of analyte ions, and the various designs of mass spectrometers have been described in detail in the literature. The HPLC-MS system has four main components consisting of a sample inlet, an ion source, a mass analyzer, and finally an ion detector. The sample introduction system vaporizes the HPLC column effluent. The ion source produces ions from the neutral analyte molecules in the vapor phase. Several designs of ion sources have been used over the past years including electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), thermospray ionization (TSP), continuous flow fast atom bombardment (FAB), and atmospheric pressure photoionization (APPI). The inductively coupled plasma (ICP) is a hard ionization source and is used specifically for the detection of metals and metals in adducts or in organometallic compounds. Generally, ICP-MS is used for elemental speciation analysis with HPLC, which has been described elsewhere in... 

Atmospheric pressure chemical ionization. LC-MS analysis with an APCI interface can be used in either the positive or negative ion mode. The positive ion mode is much more sensitive, but negative ion mode gives less interference from nonionic surfactants. Pseudomolecular ions of Na adducts predominate in the positive ion spectra, with molecular ions predominant in the negative ion spectra (21). 

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. 

A detailed description of sources used in atmospheric pressure ionization by electrospray or chemical ionization has been compiled.2 Atmospheric pressure has been used in a wide array of applications with electron impact, chemical ionization, pressure spray ionization (ionization when the electrode is below the threshold for corona discharge), electrospray ionization, and sonic spray ionization.3 Interferences potentially include overlap of ions of about the same mass-charge ratio, mobile-phase components, formation of adducts such as alkali metal ions, and suppression of ionization by substances more easily ionized than the analyte.4 A number of applications of mass spectroscopy are given in subsequent chapters. However, this section will serve as a brief synopsis, focusing on key techniques. 

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