Atmospheric pressure chemical mass analyzers

A liquid chromatography-mass spectrometry (LC-MS) method that can quantitatively analyze urinar y normal and modified nucleosides in less than 30 min with a good resolution and sufficient sensitivity has been developed. Nineteen kinds of normal and modified nucleosides were determined in urine samples from 10 healthy persons and 18 breast cancer patients. Compounds were separ ated on a reverse phase Kromasil C18 column (2.1 mm I.D.) by isocratic elution mode using 20 mg/1 ammonium acetate - acetonitrile (97 3 % v/v) at 200 p.l/min. A higher sensitivity was obtained in positive atmospheric pressure chemical ionization mode APCI(-i-). 

Figure 2.1 Mass spectrometric approach. Dl, direct inlet GC, gas chromatography HPLC, high performance liquid chromatography CZE, capillary zone electrophoresis El, electron ionization Cl, chemical ionization ESI, electrospray ionization DESI, desorption electrospray ionization APCI, atmospheric pressure chemical ionization MALDI, matrix assisted laser desorption ionization B, magnetic analyzer E, electrostatic analyzer...

Various analyzers have been used to analyze phenolic compounds. The choice of the MS analyzer is influenced by the main objective of the study. The triple quadrupole (QqQ) has been used to quantify, applying multiple reaction monitoring experiments, whereas the ion trap has been used for both identification and structure elucidation of phenolic compounds. Moreover, time-of-flight (TOF) and Fourier-transform ion cyclotron resonance (FT-ICR) are mainly recommended for studies focused on obtaining accurate mass measurements with errors below 5 ppm and sub-ppm errors, respectively (Werner and others 2008). Nowadays, hybrid equipment also exists, including different ionization sources with different analyzers, for instance electrospray or atmospheric pressure chemical ionization with triple quadrupole and time-of-flight (Waridel and others 2001). 

In atmospheric pressure chemical ionization (APCI) ion-molecule reactions occurring at atmospheric pressure are employed to generate the ions, i.e., it represents a high-pressure version of conventional chemical ionization (Cl, Chap. 7). The Cl plasma is maintained by a corona discharge between a needle and the spray chamber serving as the counter electrode. The ions are transferred into the mass analyzer by use of the same type of vacuum interface as employed in ESI. Therefore, ESI ion sources can easily be switched to APCI instead of an ESI sprayer, a unit comprising a heated pneumatic nebulizer and the spray chamber with the needle electrode are put in front of the orifice, while the atmospheric pressure-to-vacuum interface remains unchanged. [48,138]... 

HPLC - Beckman 125 binary gradient pumps, 168 diode-array detector, 507 autosampler MS - Ion-trap mass spectrometer Finnigan LCQ equipped by APCI (atmospheric pressure chemical ionization), data analyzed in negative mode, spectra confirming found compounds were obtained from tandem mass spectromectry (MS/MS). 

In IC-MS systems, the core of the equipment is the interface. In fact, inside the interface evaporation of the liquid, ionization of neutral species to charged species and removal of a huge amount of mobile phase to keep the vacuum conditions required from the mass analyzer take place. Two main interfaces are used coupled to IC, namely electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). In the ESI mode, ions are produced by evaporation of charged droplets obtained through spraying and an electrical field, whilst in the APCI mode the spray created by a pneumatic nebulizer is directed towards a heated region (400°C-550°C) in which desolvation and vaporization take place. The eluent vapors are ionized by the corona effect (the partial discharge... 

Atmospheric Pressure Chemical Ionization (APCI) is coupled to Tandem Mass Spectrometry (MS/MS) for the purpose of analyzing vapor phase perfume mixtures. Air-borne fragrances are analyzed directly by APCI/ MS/MS without the need for time consuming and potentially adulterating trapping and chromatography steps. Volatile fragrance chemicals have been rapidly identified by this novel technique as they emanate from vials or directly from skin. 

At least one commercial detector responds to an increased range of analytes by simultaneous electrospray and atmospheric pressure chemical Ionization. Ions produced by electrospray are routed directly to the mass analyzer. Neutral molecules left after electrospray then undergo atmospheric pressure chemical Ionization.26... 

MAI,PI was introduced in the late 1980s and is one of the most successfully developed MS soft ionization techniques that uses the matrix assists laser ablation of sample-coated target to vaporize gas-phase ions for injection into a mass spectrometer. The advantage of MALDI is its gentleness compared with ESI and Atmospheric Pressure Chemical Ionization (APCI) and its ability to analyze the polar, nonvolatile, and large molecules. It has been very successfully used for the analysis of both biopolymers compounds and small molecular organic compounds (<1,500 Da). 

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... 

In atmospheric-pressure chemical ionization (APCI), a nebulized stream of droplets leaves the sample inlet tube and travels toward the entrance to the mass analyzer. During this passage, ions are produced, but the yield is rather small. By introducing electrodes across the flow of sample material at atmospheric pressure, a discharge can be started, which is essentially of a corona type. 

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