Ionization, atmospheric pressure

Atmospheric pressure ionization (API) was the first technique to directly connect solution phase analyte supply with a mass analyzer [29]. In API, a solution of the analyte is injected at atmospheric pressure into a stream of hot nitrogen ( 200°C) to rapidly evaporate the solvent. The vapor passes through a p-emitter where electrons from radioactive (Eig. 12.2). 

Beginning with the ionization of N2, consecutive ion-molecule reactions eventually lead to the formation of [(H20) -i-H] cluster ions  

Here only the first reaction step of [(H20)n+H] cluster ion formation is explicitly formulated. As the PA of H2O (697 kJ mor ) is actually below that of any analyte (Table 2.6), water cluster ions protonate the analyte molecules by a chemical ionization process at atmospheric pressure [29-31]  

Note Reaction sequences analogous to API do occur whenever ionization is performed in a moist atmosphere. The initiating source of ionization can be delivered by a corona discharge as in atmospheric pressure chemical ionization (APCI, Chap. 12.8), by UV photons as in atmospheric pressure photoionization (APPI, Chap. 12.9), or by a glow-discharge in helium or argon as used in direct analysis in real time (DART, Chap. 13.5). 

In atmospheric pressure ionization sources (API) the ions are first formed at atmospheric pressure and then transferred into the vacuum. In addition, some API sources are capable of ionizing neutral molecules in solution or in the gas phase prior to ion transfer to the mass spectrometer. Because no liquid is introduced into the mass spectrometer these sources are particularly attractive for the coupling of liquid chromatography with mass spectrometry. Pneumatically assisted electrospray (ESI), atmospheric pressure chemical ionization (APCI) or atmospheric pressure photoionization (APPI) are the most widely used techniques. 

API offers unique opportunities for the implementation of new sources or to develop new applications. Atmospheric pressure matrix assisted laser desorption (AP-MALDI) [21] can be mounted on instruments such as ion traps which were originally designed only for electrospray and LC-MS. New API desorption techniques such as desorption electrospray (DESI) [22] or direct analysis in real time (DART) [23] have been described and offer unique opportunities for the analysis of surfaces or of solid samples. 

Nowadays, ESI is the leading member of the group of atmospheric pressure ionization (API) methods and the method of choice for liquid chromatography-mass spectrometry coupling (LC-MS, Chap. 12). [10-13] Currently, ESI and MALDI (Chap. 10) are the most commonly employed ionization methods and they opened doors to the widespread biological and biomedical application of mass spectrometry. [5,10,11,13-17] Moreover, ESI serves well for the analysis of ionic metal complexes [18,19] and other inorganic analytes. [20-22] 

Note Although the range up to m/z 3000 is normally employed for the detection of ions generated by ESI, ions of much higher m/z can be formed. [23,24] Even ions at m/z 85,000 have been observed. [25] 

Besides AP-MALDI, already described earlier, electrospray (ESI), atmospheric pressure chemical ionization (APCI), atmospheric pressure photoionization ionization (APPI), DESI and DART are other examples of atmospheric pressure ionization (API) sources. 

Such sources ionize the sample at atmospheric pressure and then transfer the ions into the mass spectrometer. An atmospheric pressure interface is then used to transfer ions into the high vacuum of the mass analyser. The problem lies in coupling an atmospheric pressure source compartment with an analyser compartment that must be kept at a very low pressure or at a very high vacuum (10 5 Torr). 

1th stage 2nd stage 3rd Vacuum stage Focalization Analyser 

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Chemical ionization and atmospheric-pressure ionization are covered in Chapters 1 and 9, respectively.) The corona discharge is relatively gentle in that, at atmospheric pressure, it leads to more sample molecules being ionized without causing much fragmentation. 

For a more detailed description of the ionization process inherent in electrospray, please see Chapter 9, which discusses atmospheric pressure ionization (API), The reader also should compare electrospray with thermospray (see Chapter 11). 

The advent of atmospheric-pressure ionization (API) provided a method of ionizing labile and nonvolatile substances so that they could be examined by mass spectrometry. API has become strongly linked to HPLC as a basis for ionizing the eluant on its way into the mass spectrometer, although it is also used as a stand-alone inlet for introduction of samples. API is important in thermospray, plasmaspray, and electrospray ionization (see Chapters 8 and 11). 

A solution of an analyte in a solvent can be sprayed (nebulized) from an electrically charged narrow tube to give small electrically charged droplets that desorb solvent molecules to leave ions of the analyte. This atmospheric-pressure ionization is known in various forms, the one most relevant to this section being called electrospray. For additional detail, see Chapters 8, 9, and 11. 

The nebulization and evaporation processes used for the particle-beam interface have closely similar parallels with atmospheric-pressure ionization (API), thermospray (TS), plasmaspray (PS), and electrospray (ES) combined inlet/ionization systems (see Chapters 8, 9, and 11). In all of these systems, a stream of liquid, usually but not necessarily from an HPLC column, is first nebulized... 

This method is still in use but is not described in this book because it has been superseded by more recent developments, such as particle beam and electrospray. These newer techniques have no moving parts, are quite robust, and can handle a wide variety of compound types. Chapters 8 through 13 describe these newer ionization techniques, including electrospray, atmospheric pressure ionization, plasmaspray, thermospray, dynamic fast-atom bombardment (FAB), and particle beam. 

Practical inlet systems for attaching a high-pressure liquid chromatography (HPLC) column to a mass spectrometer utilize atmospheric-pressure ionization (see Chapters 8 and 11). 

The atmospheric-pressure ionization inlets produce a stream of cations or anions that have been formed at about room temperature and have little excess of thermal energy. Consequently, these ions are very stable and exhibit little or no fragmentation (see Chapter 10). 

Atmospheric-pressure ionization. Chemical ionization performed at atmospheric pressure. 

AIR. (atmospheric) air, a standard for nitrogen and chlorine isotopes APCL atmospheric-pressure chemical ionization, also called plasmaspray API. atmospheric-pressure ionization... 

Another big advance in the appHcation of ms in biotechnology was the development of atmospheric pressure ionization (API) techniques. There are three variants of API sources, a heated nebulizer plus a corona discharge for ionization (APCl) (51), electrospray (ESI) (52), and ion spray (53). In the APCl interface, the Ic eluent is converted into droplets by pneumatic nebulization, and then a sheath gas sweeps the droplets through a heated tube that vaporizes the solvent and analyte. The corona discharge ionizes solvent molecules, which protonate the analyte. Ions transfer into the mass spectrometer through a transfer line which is cryopumped, to keep a reasonable source pressure. 

Anacleto, J. F., QuiUiam, M. A. and Boyd, R. K., Analysis of fuUerene soot extracts by liquid chromatography-mass spectrometry using atmospheric pressure ionization , in Proceedings of the 41st ASMS Conference on Mass Spectrometry and Allied Topics, San Francisco, CA, May 30-June 4, 1993, p. 1083. 

Aerospray An atmospheric-pressure ionization technique in which droplets are formed from a liquid stream by a combination of heat and a nebulizing gas and ions are formed by ion evaporation ramer man ion-molecule reactions. 

Atmospheric-pressure ionization (API) A general term used for all forms of ionization mat take place at atmospheric pressure. 

The enforcement methods provided by the applicants give basic information about appropriate cleanup steps and specific determination procedures. Typically, direct use of this developmental work occurred when a GC multi-residue method was found appropriate. Owing to the recent developments in the field of MS/MS with atmospheric pressure ionization, an alternative approach for those compounds that can be analyzed by liquid chromatography (LC) will soon be possible. It is important that some fundamental considerations for such method(s) should be agreed at the outset. Considerations include the most suitable extraction solvents and cleanup steps and some standard HPLC conditions. 

The method for chloroacetanilide soil metabolites in water determines concentrations of ethanesulfonic acid (ESA) and oxanilic acid (OXA) metabolites of alachlor, acetochlor, and metolachlor in surface water and groundwater samples by direct aqueous injection LC/MS/MS. After injection, compounds are separated by reversed-phase HPLC and introduced into the mass spectrometer with a TurboIonSpray atmospheric pressure ionization (API) interface. Using direct aqueous injection without prior SPE and/or concentration minimizes losses and greatly simplifies the analytical procedure. Standard addition experiments can be used to check for matrix effects. With multiple-reaction monitoring in the negative electrospray ionization mode, LC/MS/MS provides superior specificity and sensitivity compared with conventional liquid chromatography/mass spectrometry (LC/MS) or liquid chromatography/ultraviolet detection (LC/UV), and the need for a confirmatory method is eliminated. In summary,... 

As with GC, the combination of MS and MS/MS detection with LC adds an important confirmatory dimension to the analysis. Thermospray (TSP) and particle beam (PB) were two of the earlier interfaces for coupling LC and MS, but insufficient fragmentation resulted in a lack of structural information when using TSP, and insufficient sensitivity and an inability to ionize nonvolatile sample components hampered applications using PB. Today, atmospheric pressure ionization (API) dominates the LC/MS field for many environmental applications. The three major variants of API... 

Specifically for triazines in water, multi-residue methods incorporating SPE and LC/MS/MS will soon be available that are capable of measuring numerous parent compounds and all their relevant degradates (including the hydroxytriazines) in one analysis. Continued increases in liquid chromatography/atmospheric pressure ionization tandem mass spectrometry (LC/API-MS/MS) sensitivity will lead to methods requiring no aqueous sample preparation at all, and portions of water samples will be injected directly into the LC column. The use of SPE and GC or LC coupled with MS and MS/MS systems will also be applied routinely to the analysis of more complex sample matrices such as soil and crop and animal tissues. However, the analyte(s) must first be removed from the sample matrix, and additional research is needed to develop more efficient extraction procedures. Increased selectivity during extraction also simplifies the sample purification requirements prior to injection. Certainly, miniaturization of all aspects of the analysis (sample extraction, purification, and instrumentation) will continue, and some of this may involve SEE, subcritical and microwave extraction, sonication, others or even combinations of these techniques for the initial isolation of the analyte(s) from the bulk of the sample matrix. 

HPLC, ConstaMetric 3500 MS and ConstaMetric 3200 MS Mass spectrometer, TSQ 7000 with atmospheric pressure ionization (API) electrospray interface Robotcoupe, Model RSI 25 Syringes, Luer lock, 10-mL... 

Hewlett-Packard 1100 Series LC-MSD equipped with an atmospheric pressure ionization (API) source (APcI or ESI)... 

TSQ 7000 mass spectrometer (Finnigan MAT) with atmospheric pressure ionization (API) interface (ESI mode) or equivalent... 

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