Atmospheric pressure chemical ionization process

Another popular and efficient inlet system for the LC/MS combination is the atmospheric pressure chemical ionization process. This system has some similarity to the electrospray interface and can also cope with flow rates of up to 2 ml/min. and thus the total column eluent can be utilized without splitting the flow. 

In addition to the dissolved elements and compounds in the oceanic water column, a wide variety of water column chemicals are found in marine organisms and organic detritus. For example, a milliliter of surface seawater can contain on the order of 10 million viruses, 1 million bacteria, 100,000 phytoplankton, and 10,000 zooplankton [9]. With the advent of soft ionization processes for mass spectrometry systems, scientists have been able to study these marine organisms at molecular level. The use of electrospray ionization (ESI see Section 2.1.15), atmospheric pressure chemical ionization... 

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. 

Atmospheric pressure chemical ionization (APCI) is a gas phase ionization process based on ion-molecule reactions between a neutral molecule and reactant ions [31]. The method is very similar to chemical ionization with the difference that ionization occurs at atmospheric pressure. APCI requires that the liquid sample is completely evaporated (Fig. 1.12). Typical flow rates are in the range 200-1000 xL min , but low flow APCI has also been described. First, an aerosol is formed with the help of a pneumatic nebulizer using nitrogen. The aerosol is directly formed in a heated quartz or ceramic tube (typical temperatures 200-500 °C) where the mobile phase and the analytes are evaporated. The temperature of the nebulized mobile phase itself remains in the range 120-150 °C due to evapo-... 

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). 

In 1995, Taylor and co-workers also described the use of an open-access LC/MS system for routine structure confirmation, featuring atmospheric pressure chemical ionization (APCI). This system featured dual personal computers (PCs) for automated instrument control and sample log-in. A system-PC is responsible for running the Windows NT for Workgroups operating system and interfaces with the network for instrument control. A separate log-in PC, isolated from the LC/MS system, is used by the synthetic chemist to enter details about the samples. The analyst prepares the sample in an autosampler vial in one of several solvent options. The system specifies where to place the sample vial in the autosampler, and following analysis with a standard method, spectra are automatically processed and printed without any chemist intervention. 

Strege summarized the technique of high-performance liquid chromatography-electrospray ionization mass spectrometry (HPLC-ESI-MS) in dereplication of natural products. In contrast to earlier electron impact ionization (El), ESI technique is applicable to virtually any ion in solution with a soft ionization process. A comparison of ESI with fast atom bombardment (FAB), matrix assisted laser desorption ionization (MALDI), atmospheric pressure chemical ionization (APCI) and other techniques demonstrates its superior sensitivity, compatibility and reliability when coupled with HPLC [51]. 

G. J. Lehr, T. L. Barry, G. Petzinger, G. M. Hanna, S. W. Zito, Isolation and identification of process impurities in trimethoprim drug substance by high-performance liquid chromatography, atmospheric pressure chemical ionization liquid chro-matography/mass spectrometry and nuclear magnetic resonance spectroscopy, J. Pharm. Biomed. Anal. 19 (1999), 373-389. 

Modern LC/MS systems employ two ionization processes that are accomplished at atmospheric pressure, termed electrospray (ESI) and atmospheric pressure chemical ionization (APCI). In a typical ESI source, the eluent from the HPLC column composed of liquid mobile phase and analyte molecules passes through a stainless steel capillary with a high positive or negative potential applied to the end (3-5 The electric-field causes instantaneous... 

Capillary electrokinetic chromatography (CEKC) with ESI-MS requires either the use of additives that do not significantly impact the ESI process or a method for their removal prior to the electrospray. Although this problem has not yet been completely solved, recent reports have suggested that considered choices of surfactant type and reduction of electro-osmotic flow (EOF) and surfactant in the capillary can decrease problems. Because most analytes that benefit from the CEKC mode of operation can be effectively addressed by the interface of other separations methods with MS, more emphasis has until now been placed upon interfacing with other CE modes. For small-molecule CE analysis, in which micellar and inclusion complex systems are commonly used, atmospheric pressure chemical ionization (APCI) may provide a useful alternative to ESI, as it is not as greatly affected by involatile salts and additives. 

While the ESI technique has revolutionized the analysis of biomolecules such as peptides, proteins, polysaccharides, and nucleotides, the constraints imposed by its dependence on solution chemistry for ionization limit its applicability in the realm of small- to medium-sized nonpolar molecules (i.e., weakly basic or neutral compounds).120 Atmospheric pressure chemical ionization (APCI), on the other hand, takes advantage of gas-phase processes... 

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... 

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