Atmospheric Pressure Ionization Sources

There are two major types of ionization sources that operate at atmospheric pressure, ESI, and atmospheric pressure Cl (APCI). A modified version of the ESI source is the ion spray source. These sources are described in detail in Section I3.I.6.I, because they are used to interface EC with MS for the separation and mass spectrometric analysis of mixtures of nonvolatile high MW compounds, especially in the fields of pharmaceutical chemistry, biochemistry, and clinical biomonitoring. ESI will be described briefly so that its use may be demonstrated, but more detail will be found in Chapter 13. 

1 Direct Analysis in Real Time (the DART Source) 

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However, phosphate salts are not volatile. We must constantly remember that mass spectrometry is a gas-phase experiment. Materials to be examined by mass spectrometry must ultimately be made gaseous. Figure 19.14 shows the atmospheric pressure ionization source chamber of a mass spectrometer after infusion of a 20 mM potassium phosphate-containing mobile phase into the instrument for a few hours. The accumulation of phosphate salts on the striker plate is evident. Visual evidence of salt accumulation is also apparent on the back wall of the source chamber, above the striker plate. The overall haziness of the image is not the result of poor photography, but rather due to the coating of dust on the inner walls of the chamber and all surfaces within. 

A number of interfaces such as thermospray (TSP), ionspray (IS), atmospheric chemical ionization (APCI) and electrospray (ES) can tolerate much higher flow rates without requiring that the flow be split at the end of the LC column. Ions that are produced in atmospheric pressure ionization sources are moved directly into the mass spectrometer through small apertures. 

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. 

Electrospray (ESI) is an atmospheric pressure ionization source in which the sample is ionized at an ambient pressure and then transferred into the MS. It was first developed by John Fenn in the late 1980s [1] and rapidly became one of the most widely used ionization techniques in mass spectrometry due to its high sensitivity and versatility. It is a soft ionization technique for analytes present in solution therefore, it can easily be coupled with separation methods such as LC and capillary electrophoresis (CE). The development of ESI has a wide field of applications, from small polar molecules to high molecular weight compounds such as protein and nucleotides. In 2002, the Nobel Prize was awarded to John Fenn following his studies on electrospray, for the development of soft desorption ionization methods for mass spectrometric analyses of biological macromolecules. ... 

In the last decade, LC-MS system manufacturers have commercialized hybrid instruments that can operate in the ESI or the APCI mode with just a few simple modifications. Such instrumentation, based on Bajic s first prototype [49], is able to protonate compounds in the ESI mode together with those yielding characteristic APCI signature. Castoro [50] and Fischer et al. [51] first provided the utility of the dual atmospheric pressure ionization source. 

Over the past two decades, QMF-based quantification assays have become the technique of choice for quantification of drug candidates and their metabolites. Combining a mass spectrometer with LC provides an additional degree of selectivity and makes the combined technique the method of choice for quantitative bioanalysis of drugs and metabolites. Among the mass spectrometer types, QMF are ideal for coupling with LC and atmospheric pressure ionization sources (ESI, APCI, APPI, DART, DESI, etc.) because QMFs have the lowest voltage requirements and vacuum requirements. 

A miniature cylindrical ion trap mass spectrometer with APCI and ESI capabilities was developed [22], The system includes a three-stage, differentially pumped vacuum system and can be interfaced to many types of atmospheric pressure ionization sources. 

Coupling Using Atmospheric Pressure Ionization Sources... 

Figure 32F-1 Block diagram of an LC/MS system. The effluent from the LC column is introduced to an atmospheric pressure ionization source, such as an electrospray or chemical ionization. The ions produced are sorted by the mass analyzer and detected by the ion detector.

Another convenient way to classify ionization sources, rather than from the perspective of odd- or even-electron ion generation, is in relation to where the ions are created relative to the vacuum system that is, either generated at atmospheric pressure or in a vacuum. The two most common atmospheric pressure ionization sources, electrospray and atmospheric pressure chemical ionization, are arguably the most common ionization techniques applied in quantitative mass spectrometry today. However, discussion of earlier ionization sources is useful, as many of these techniques are still commonplace and their understanding provides a framework for appreciation of atmospheric pressure ionization technology and what it has to offer the pharmaceutical industry. 

Figure 3 Schematic of a generic atmospheric pressure ionization source showing a differential pumping system.

Table 1 lists a number of ionization sources which produce ions at either atmospheric pressure or under vacuum conditions. For atmospheric pressure ionization sources a suitable interface is required which allows a controlled leak of ions into the vacuum region of the mass spectrometer. Vacuum ionization techniques likewise require a controlled leak, or mechanical introduction, of neutral molecules into the vacuum chamber, followed by ionization. 

Siegel, M.M. Tabei, K. Tong, H. Lamber, R Candela, L. Zoltan, B. Evaluation of a Dual Electrospray Ionization/ Atmospheric Pressure Ionization Source at Low Flow Rates ( 50 /xL/min) for the Analysis of Both Highly and Weakly Polar Compounds, J. Am. Soc. Mass Spectrom. 9, 1196-1203 (1998). 

Peiris, D.M. et al., Distinguishing N-oxide and hydroxyl compounds Impact of heated capillary/heated ion transfer tube in inducing atmospheric pressure ionization source decompositions, J. Mass Spectrom., 39(6), 600, 2004. 

MALDI ionization mostly takes place in vacuum in spite of the recent development of atmospheric pressure ionization sources. Therefore, coupling a microfluidic system to MALDI-MS implies working under vacuum on the chip or performing the MS analysis off-line, i.e. introducing the microchip in the MALDI-MS once the fluidic operations are finished. Consequently, the first miniaturized developments for MALDI-MS analysis only concerned the... 

The most common instrumentation for the analysis of biomarkers includes microbore and capillary reversed-phase chromatography coupled to a triplestage quadrupole (TSQ) mass spectrometer or ion trap, with an atmospheric pressure ionization source such as electrospray ionization (ESI), nanospray ionization (NSI), or atmospheric pressure chemical ionization (APCI). Ion trap mass spectrometers provide higher sensitivity in full-scan mode, which is useful for product ion identification of a metabolite however, TSQs are used most often due to improved sensitivity for quantification in multiple reaction... 

The complexity of analyte matrixes and the low level of selenium compoimds even in em-iched samples make speciation difficult, but the combination of separation processes with selenium-specific detection is a powerful approach. High sensitivity is vital, and MS with an atmospheric pressure ionization source, such as the ICP, has proved successful for HPLC detection. Selenium presents problems due to moderate ionization efficiency and isobaric interferences, although these can be partly overcome with high-resolution mass spectrometers or dynamic reaction cell (DRC) technology. Significant isotopic overlap from " °Ar2 on the most abundant isotope Se (49.6%) may necessitate measurement of the less abundant isotopes Se (8.6%) or Se (7.6%) of total selenium. 

Pena Quevedo has reported the detection of TATP using direct analysis in real time (DART). The method utilizes an atmospheric pressure ionization source coupled to a TOFMS to observe ammonium adducts of TATP [55]. Wilson et al. have reported the real-time detection of TATP using selected-ion flow tube mass spectrometry [56]. 

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