API source

Since ions and neutral molecules are formed close together in an API source, many ion/molecule collisions occur as in Cl, and so the ion evaporation process also has impressed upon it the characteristics of Cl. Therefore, API is usually thought to involve a mix of ion evaporation and chemical 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. 

Several other interface designs were introduced over this period, including continuous flow fast atom bombardment (CFFAB)" and the particle beam interface (PBI)," but it was not until the introduction of the API source that LC/MS applications really came to the forefront for quantitative analysis. Early work by Muck and Henion proved the utility of an atmospheric pressure interface using a tandem quadrupole mass spectrometer. 

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

Ionisation in an API source can take place in a variety of ways depending on the type of applications, namely by gas-phase ionisation, liquid- and plasma-based ionisation. At present, there are three major application areas of API-MS air or gas analysis (industrial emissions), on-line LC-MS (largest commercial application), and ICP-MS. A wide variety of sample introduction devices are available for gas analysis by API-MS. For use in ICP-MS, ions are sampled directly from the inductively... 

Figure 6.11 Schematic diagram of a typical API source, as for instance used in LC-MS. After Niessen [124]. Reprinted from W.M.A. Niessen, in Encyclopedia of Spectroscopy and Spectrometry (J.C. Lindon, ed.), Academic Press, pp. 18—24, Copyright (2000), with permission from Elsevier...

ToFs can also be used in combination with other mass analysers. Both hybrid sector and quadrupole systems are available. oaToF-MS has been interfaced to a quadrupole mass filter and hexapole gas collision cell, such as to allow recording of mass spectra and product ion spectra with good mass resolution (ca. 10000), high sensitivity, high mass range (ca. 10 000 Da) and high mass accuracy (<5ppm) [177,178]. QqToFMS may be fitted with API sources with flow-rates from nL... 

A limitation in the use of API sources results from the frequent application of mobile-phase composition programming in pSFC. Pinkston el al. [411] have compared electrospray and electron impact for open-tubular and packed-column SFC-MS. Direct on-line coupling of SFC to FAB/MS (as well as SFC-ELSD) is also very promising to detect components which give no response in a UV detector [412]. 

The advent of the atmospheric pressure ionization (API) source in the early 1990s allowed direct coupling of LC to MS. By the mid-1990s, this technology was a common in drug metabolism laboratories. The enhanced selectivity of tandem mass spectrometry (MS/MS) experiments reduced the need for exhaustive chromatographic separations prior to detection and this feature was exploited to significantly reduce analysis times. 

The significant enhancement of ion formation by a corona discharge as compared to a Ni source has already been implemented in early API sources. [139,140] The nature of the APCI plasma varies widely as both solvent and nebulizing gas contribute to the composition of the Cl plasma, i.e., APCI spectra can resemble PICI, CECI, NICI, or EC spectra (Chap. 7.2-7.4) depending on the actual conditions and ion polarity. This explains why APCI conditions suffer from comparatively low reproducibility as compared to other ionization methods. 

With the advent of API sources, LC/MS/MS allows the facile development of quantitative methods that are sensitive, selective, robust, and amenable to the rapid analysis of a majority of small molecules. In order to achieve high-throughput bioanalysis in support of pharmacokinetic studies, many approaches have been reported utilizing automated sample preparation and reducing analysis time by pooling samples, parallel analysis, and fast chromatography. 25,26,152,153... 

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. 

Ionization takes place in a modified API source, as illustrated in Figure 8.15. 

API The atmospheric pressure ionization (API) source is the most common category of source for LC-MS analysis, in which ionization is performed outside of the high-vacuum region of the mass spectrometer. Electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) sources are both examples of API sources. 

Quantitative Bioanalysis with High Mass Resolution Prior to the introduction of the API sources for LC-MS, GC-MS was the dominant format for mass spectrometry. Within GC-MS, mass analysis at high resolution using magnetic sector instruments was relatively common, especially in the central mass spectrometry facilities of major corporations and universities. Uses of these instruments included quantitation by GC-HRMS for improved specificity and sensitivity. 

While highly successful, the API interfaces are not optimal for all small molecules. For cases where an analyte appears to ionize and vaporize with poor efficiency, a fundamental change in the analyte can be made prior to the API source. The use of coulometric assisted ionization is described in Chapter 8. In this method, electrochemistry techniques are used to alter the analyte. Improvements in specificity and sensitivity can result. 

Manufacturing has very well-defined project planning and the vagaries of politics and regulations can severely affect the timelines and cost. So if the final synthetic pathway is not established or the API sourcing comes from distant lands it may be better to choose another opportunity. 

Our recent API negative ion mass spectrometry studies (4) have been carried out with a source (Figure 1) having the physical dimensions of a commercially available electron capture detector. With an electrode in place, and with a nickel-63 foil in place, the source chamber functions as an electron capture detector. A very small nickel-63 foil is used in the aperture region. The gas stream from a gas chromatograph is split before entering the source, so that detection is achieved in three ways. The split stream is directed in part to a standard electron capture detector and in part to the API source, which allows both a source electron... 

Figure 1. This API source is designed to serve both as an electron capture detector and as a source for the mass spectrometer (A).

The stream from a gas chromatograph is split, with one segment directed to a commercial electron capture detector and the other to this source. Three responses can be observed (I) an electron capture response from the usual gas chromatograph-electron capture detector combination (2) an electron capture response from the API source, which should duplicate the normal response and (3) a mass spectrometric response based on ions generated under API conditions. 

Figure 2. The responses due to negative ion formation from benzil are (EC) the GC-electron capture response (EC-API) the corresponding response from the API source and (API-SIM) the selected ion response for the M ion of benzil (A), (benzil, CeH5-CO-CO-C6Hs, Ar/CH, 240°C, PZ179 column)...

Most instruments with an API source offer a choice of APCI or ESI. Detailed descriptions of these ionization methods are provided elsewhere a 3). Both rely on the formation of ions at atmospheric pressure in a source region separated from the high vacuum mass analyzer. Ions are transported to the analyzer through one or more differentially pumped skimmers or a heated capillary. Both APCI and ESI require aerosolization of the liquid eluent from the liquid chromatography (LC) column. The major difference lies in the method and phase of ionization. 

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