Interfaces for atmospheric-pressure ionization

Liquid chromatography-mass spectrometiy (LC-MS) based on atmospheric-pressure ionization (API) was demonstrated as early as 1974 (Ch. 3.2.1). However, it took until the late 1980 s before API was starting to be widely applied. Today, it can be considered by far the most important interfacing strategy in LC-MS. More than 99% of the LC-MS performed today is based on API interfacing. In this chapter, instrumentation for API interfacing is discussed. First, vacuum system for MS and LC-MS are briefly discussed. Subsequently, attention is paid to instrumental and practical aspects of electrospray ionization (ESI), atmospheric-pressure chemical ionization (APCI), and other interfacing approaches based on API. The emphasis in the discussion is on commercially available systems and modifications thereof. Ionization phenomena and mechanisms are dealt with in a separate chapter (Ch. 6). Laser-based ionization for LC-MS is briefly reviewed (Ch. 5.9). 

The design of the vaeuum system plays an important role in the development of API interfaees [1-3], 

In the initial development of LC-MS, the gas load to the vacuum system was a serious concern. A mobile-phase flow of 1 ml/min corresponds to a gas flow between 0.3 and 2.1 Pa mVs, depending primarily on the molecular mass of the solvent used. The effective pumping speed at the El ion-source housing of a differentially pumped MS system is between 0.3 and 0.7 mVs, which allows the introduction of ca. 2 pl/min of water, which is oidy ca. 0.2% of the typical flow-rate of a conventional 4.6-mm-ID LC column. In order to introduce the complete effluent of a 4.6-mm-ID LC column into the ion-source housing, a substantial increase of the effective pumping efficiency at the ion-source housing is required. This can be done in various ways  

Analyte-enrichment interfaces for LC-MS show similarities with the interfaces used in packed colunm GC-MS, such as the jet separator, the Watson-Biemann fritted-glass, or the membrane interface. Various analyte-enrichment interfaces have been developed for LC-MS application, as discussed in Ch. 3.3.2. In API, gas-phase analyte-emichment is performed by means of a molecular-beam system. The vaporized column effluent is sampled from an atmospheric-pressure spray chamber via a differentially pumped expansion chamber system [3]. 

This is the most versatile approach to LC-MS, as it keeps most of the mobile phase ont of the high-vacnnm region. API interfaces are special gas-phase analyte enrichment interfaces, becanse analyte ionization takes place already in the spray chamber, and the ions generated are preferentially sampled into the vacuum system. 

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Today, the two most common LC/MS interfaces are atmospheric pressure ionization interfaces, electrospray (ESI) and ion spray (ISI). Electrospray (Fig. 15.8) and its subtype, nanospray, are recommended for use with proteins and highly polar or ionized compounds. They are very soft ionization, concentration-dependent techniques that result in very little fragmentation and often produce multiply charged molecular ions. 

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. 

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... 

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. 

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... 

Sakairi, M. and Kato, Y., Multi-atmospheric pressure ionization interface for liquid chromatography-mass spectrometry, ]. Chromatogr. A. 794, 391, 1998. 

A.P. Bruins, T.R. Covey and J.D. Henion, Ion spray interface for combined liquid chromato graphy/atmospheric pressure ionization mass spectrometry, Anal. Chem., 59,2642 2646 (1987). 

The mass spectrometer is a very sensitive and selective instrument. However, the introduction of the eluent into the vacuum chamber and the resulting significant pressure drop reduces the sensitivity. The gas exhaust power of a normal vacuum pump is some 10 ml min-1 so high capacity or turbo vacuum pumps are usually needed. The gas-phase volume corresponding to 1 ml of liquid is 176 ml for -hexane, 384 ml for ethanol, 429 ml for acetonitrile, 554 ml for methanol, and 1245 ml for water under standard conditions (0°C, 1 atmosphere). The elimination of the mobile phase solvent is therefore important, otherwise the expanding eluent will destroy the vacuum in the detector. Several methods to accomplish this have been developed. The commercialized interfaces are thermo-spray, moving-belt, electrospray ionization, ion-spray, and atmospheric pressure ionization. The influence of the eluent is very complex, and the modification of eluent components and the selection of an interface are therefore important. Micro-liquid chromatography is suitable for this detector, due to its very small flow rate (usually only 10 p min - ). 

With external ion sources it became feasible to interface any ionization method to the QIT mass analyzer. [171] However, commercial QITs are chiefly offered for two fields of applications i) GC-MS systems with El and Cl, because they are either inexpensive or capable of MS/MS to improve selectivity of the analysis (Chap. 12) and ii) instruments equipped with atmospheric pressure ionization (API) methods (Chap. 11) offering higher mass range, and some 5-fold unit resolution to resolve isotopic patterns of multiply charged ions (Fig. 4.47). [149,162,172,173]... 

Although there was an initial success of El in coupling LC and mass spectrometry, it was rapidly outpaced by atmospheric-pressure ionization techniques (API), and the assumption that El is only suited for GC-amenable compounds drove all of the research efforts into the direction of developing soft ionization-based interfaces. Nowadays, the situation is changing and interest is arising among researchers for the unique advantages of EL... 

At present, the most powerful and promising interfaces for drug residue analysis are die particle-beam (PB) interface that provides online EI mass spectra, the thermospray (TSP) interface diat works well with substances of medium polarity, and more recently the atmospheric pressure ionization (API) interfaces that have opened up important application areas of LC to LC-MS for ionizable compounds. Among die API interfaces, ESP and ISP appear to be the most versatile since diey are suitable for substances ranging from polar to ionic and from low to high molecular mass. ISP, in particular, is compatible with the flow rates used with conventional LC columns (70). In addition, both ESP and ISP appear to be valuable in terms of analyte detectability. These interfaces can further be supplemented by preanalyzer collision-induced dissociation (CID) or tandem MS as realized with the use of triple quadrupole systems. Complementary to ESP and ISP interfaces with respect to the analyte polarity is APCI with a heated nebulizer interface. This is a powerful interface for both structural confirmation and quantitative analysis. 

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