Atmospheric pressure ion source

In one instrument, ions produced from an atmospheric-pressure ion source can be measured. If these are molecular ions, their relative molecular mass is obtained and often their elemental compositions. Fragment ions can be produced by suitable operation of an APCI inlet to obtain a full mass spectrum for each eluting substrate. The system can be used with the effluent from an LC column or with a solution from a static solution supply. When used with an LC column, any detectors generally used with the LC instrument itself can still be included, as with a UV/visible diode array detector sited in front of the mass spectrometer inlet. 

When mass spectrometry was first used as a routine analytical tool, El was the only commercial ion source. As needs have increased, more ionization methods have appeared. Many different types of ionization source have been described, and several of these have been produced commercially. The present situation is such that there is now only a limited range of ion sources. For vacuum ion sources, El is still widely used, frequently in conjunction with Cl. For atmospheric pressure ion sources, the most frequently used are ES, APCI, MALDI (lasers), and plasma torches. 

A. L. Gray. Mass-Spectrometric Analysis of Solutions Using an Atmospheric Pressure Ion Source. Analyst, 100(1975) 289-299. 

APCI. The column effluent is nebulised into an atmospheric-pressure ion source. Through a corona discharge, electrons initiate the reactant gas-mediated ionisation of the analytes. Proton transfers are typical reactions generating [M + H]+ or [M — H] ions, although radical ion formation is possible as in high vacuum chemical ionisation (Cl). The ions formed are injected into the high vacuum of the mass spectrometer. APCI typically accepts flow rates of up to 2 mL min-1. 

Ion guides are needed to transfer ions of low kinetic energy from one region to another without substantial losses, [118] e.g., from an atmospheric pressure ion source (ESI, APCI) to the entrance of a mass analyzer (Chap. 11). 

Tsuchiya, M. Atmospheric Pressure Ion Sources, Physico-Chemical and Analytical Applications. Adv. Mass Spectrom. 1995, 13,333-346. 

The previous assay was applied to the analysis of SDZ in salmon tissue, with some modifications. The re-extraction with phosphoric acid solution was replaced by SPE on an SCX cartridge preconditioned with MeCN and phosphoric acid solution. The cartridge was washed with MeCN, and SDZ was eluted with an MeCN phosphoric acid mixture. The eluate was injected directly into the chromatographic system, followed by postcolumn derivatization under similar conditions to the previous assay. The derivatization time was 1.2 min, and the fluorescence intensity was approximately a quarter of that for optimal conditions. However, the postcolumn derivatization was found to be considerably less labor intensive and was easily reproducible (recoveries 83-85% CV < 7%). A significant improvement in the LOD value was obtained (0.2 yug/kg) (160). The SDZ residues from incurred salmon tissue were confirmed by MS detection however, the sample cleanup should be improved due to the lack of sensitivity of MS. Therefore, SDZ residues were eluted from the SCX SPE cartridge with phosphoric acid, and the eluate was concentrated on a C18 SPE cartridge preconditioned with MeOH and water. The residues were eluted with MeCN, and the eluate was evaporated to dryness and reconstituted prior to the analysis. The column effluent was delivered into the atmospheric-pressure ion source, and SIM was chosen for positive ions at m/z 251, 158, 156, and 108, respectively (161). 

Covey TR, Thomson BA, Schneider BB (2009) Atmospheric pressure ion sources. Mass Spectrom Rev 28 870-897... 

An inductively-coupled plasma (ICP) is an effective spectroscopic excitation source, which in combination with atomic emission spectrometry (AES) is important in inorganic elemental analysis. ICP was also considered as an ion source for MS. An ICP-MS system is a special type of atmospheric-pressure ion source, where the liquid is nebulized into an atmospheric-pressure spray chamber. The larger droplets are separated from the smaller droplets and drained to waste. The aerosol of small droplets is transported by means of argon to the torch, where the ICP is generated and sustained. The analytes are atomized, and ionization of the elements takes place. Ions are sampled through an orifice into an atmospheric-pressure-vacuum interface, similar to an atmospheric-pressure ionization system for LC-MS. LC-ICP-MS is extensively reviewed, e.g., [12]. 

Atmospheric-pressnre Cl can be performed in an atmospheric-pressure ion source, i.e., atmospheric-pressure chemical ionization (APCl) (Ch. 6.4). 

A TOF mass analyser requires a pulsed ion introduction. In an electrospray-TOF combination, the duty cycle is an important issue. A significant improvement in the duty cycle can be achieved in an ion-trap-TOF hybrid instmment the ions from a continuous ion source are accumulated in the ion trap between two ion introduction events. An ion-trap-TOF hybrid instrument was first described by the group of Lubman [68-69]. The system consists of an atmospheric-pressure ion source with a vacuum interface, a set of Einzel lenses, an ion-trap device, and a reflectron time-of-flight mass analyser. The system was applied for fast analysis in combination with a variety of separation techniques [70]. 

Immediately after their production, the ions in the humid atmospheric-pressure ion source will attract solvent molecules by ion-dipole interactions. These solvated ions must be desolvated prior to entering the mass analyser. This is achieved by ion-molecule collisions in the transition region, especially between the ion-sampling aperture and the skimmer. A small potential difference between the nozzle and the skimmer is applied to enhance declustering. 

The interest in ESI nebnlization in LC-MS results from the work of Perm and coworkers [11-13]. In an ESI interface for LC-MS, the coluttm effluent is nebulized into an atmospheric-pressure ion source. The nebuhzation is due to the application of a high electric field resulting from the 3-kV potential difference between the narrow-bore spray capillary, the needle , and a surrounding counter electrode. The solvent emerging from the needle breaks into fine threads which subsequently disintegrate in small droplets. Analyte ions are generated from these droplets by a variety of ionization process (Ch. 6.3). 

One of the most undesirable processes that can occur during atmospheric pressure mass spectrometric analysis is a nonlinear decrease of ionization by sample or mobile phase. This ion suppression, or ionisation suppression, is an effect whereby the extent of ionization for an analyte is decreased due to competition between analyte and sample matrix components within the atmospheric pressure ion source. Studies have shown ion suppression to be a somewhat proportional effect [19]. That is, a quasilinear relationship is observed between the amount of salt present in a sample and the loss of analyte molecular ion signal until a limiting amount of salt is reached, whereby the response is constant with increasing... 

Ding, L., Sudakov, M., Brancia, F.L., Giles, R., and Kumashiro, S. (2004). A digital ion trap mass spectrometer coupled with atmospheric pressure ion source, J Mass Spectrom., 39,471 148. 

Gray, A.L. (1975) Mass-spectrometric analysis of solutions using an atmospheric pressure ion source. Analyst, 100, 289-299. 

Figure 7.10 Atmospheric pressure ion source for LC-MS. (Reproduced by permission from VG...

Fig. 10. Mass spectrum of 3 mg/1 silver solution obtained by an atmospheric pressure ion source"...

Gray, A. L. Trace analysis of solutions using an atmospheric pressure ion source. In Dynamic mass spectrometry. Price, D., Todd, J. F. J. (eds.). London Heyden 1976, pp. 153-162... 

The combination of HPLC and mass spectrometry was impossible for many years due to the difficulty in removing the solvent prior to the analyte entering the high vacuum of the mass spectrometer. However, with the advent of the atmospheric pressure ionization techniques discussed previously, the two instruments have been combined to give the relatively new technique of LCMS. The eluant from the HPLC is passed into one of the atmospheric pressure ion sources, where ions are produced and most of the solvent is rapidly evaporated. The ion cloud then passes through a small aperture into the vacuum of the mass spectrometer, where the solvent is totally removed, and the ions can be focused and mass analyzed in the normal manner. 

The column effluent from the LC is nebulized into the atmospheric-pressure ion source region. Typical solvents used are mixtures of water and methanol or acetonitrile, containing up to lOmmoll electrolyte, such as formic acid or ammonium acetate. Nebulization can be performed either by means of a strong electrical field, or by a combination of the strong electric field and pneumatic nebulization. The latter is sometimes named pneumatically assisted electrospray or ionspray . The pure electrospray process is limited to flow rates up to 10 pi min Pneumatically assisted electrospray enables the introduction of higher flow rates, up to Imlmin. Since ion production mechanisms between the two modes are identical, the term electrospray is used here throughout. 

Next to ESI, APCI is an alternative, especially for less polar compounds. The APCI interface consists of a different solvent inlet probe, the so-called heated nebulizer, and a discharge electrode, but uses exactly the same atmospheric-pressure ion source and vacuum transition region as in ESI (see Figure 1). In a heated nebulizer, the LC column effluent is first pneumatically nebulized into an aerosol, which subsequently is evaporated in a heated vaporizer zone. Solvent and analyte vapor meet the discharge needle, where a potential of a few kilovolts initiates a continuous discharge that results in the ionization of solvent molecules, which in turn ionize the analyte molecules by gas-phase ion-molecule reactions (common chemical ionization processes). 

Figure 5 Principle of electrospray ionization inside an atmospheric pressure ion source.

The use of electrospray nebulization and ionization in MS is based on atmospheric pressure ionization. The column effluent from a reversed-phase LC, i.e., a solvent mixture of methanol or acetonitrile and up to 10 mmol 1 aqueous buffer, is nebulized into an atmospheric-pressure ion source region. Because pure electrospray nebulization can only be achieved at low flow rates, i.e., typically up to lOplmin , pneu-... 

After the first demonstration of multiply charged gas-phase proteins ions, all major instrument manufacturers developed atmospheric-pressure ion sources, equipped with electrospray interfaces for both protein characterization and LC-MS applications. Within 5 years, electrospray interfacing became the method of choice in LC-MS coupling. It led to a large increase in the use of MS for the characterization and identification of labile and polar analytes as well as to routine quantitative analysis. The advent of electrospray ionization for peptide and protein analysis stimulated further development and analytical application of existing and new mass analysis approaches, such as quadrupole ion traps, Fourier-transform ion-cyclotron resonance MS, and quadru-pole-time-of-flight hybrid instruments. It opened new application areas, such proteomics. LC-MS... 

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