Atmospheric ionisation sources

Recently, LC/UV/MS using atmospheric ionisation sources (API) has become ubiquitous wherever large numbers of samples require automated separation and identification analysis. This is especially the case within the pharmaceutical industi where its universal adoption has coincided with the paradigm shift in the way drug discovery is performed. Today, LC/UV/MS is the front-line technique for the analysis of combinatorial libraries and as an introduction it is useful to examine the development of the technique alongside combinatorial synthesis to understand the synergies that make them good bed fellows . 

Modem mass spectrometers within the pharmaceutical industry are more usually fitted with atmospheric pressure ionisation sources that are ideally suited to be connected to HPLC equipment. They are very robust which enables them to be used unattended for many weeks without the need for source cleaning or routine maintenance. There are two types of atmospheric ionisation sources, namely Electrospray Ionisation (ESP) or Atmospheric Pressure Chemical Ionisation (APCl) [15]. Both ionisation modes provide soft ionisation which favours quasi-molecular ion production with little or no fragmentation. Most typically MH ions are observed but MNa, MNHj and MK may also be produced. 

Undoubtedly, mass spectrometric detection has a substantial role to play in condensed-phase chromatographic analyses of toxic impurities. As in GC/MS, it can be highly sensitive, although this is probably more analyte-specific than in GC/MS. Selectivity can be gained by SIM on single quadrupoles or, if necessary, SRM on MS/MS instruments. What must be considered is the appropriate ionisation mode to be used in LC/MS. Most modern instruments use atmospheric pressure ionisation sources, including electrospray ionisation (ESI), atmospheric pressure chemical ionisation (APCI) and more recently atmospheric pressure photoionisation (APPI). 

Figure 2.34 Schematic diagram of an atmospheric pressure chemical ionisation source (Figure used by kind permission of Dr Paul Gates, School of Chemistry, University of Bristol, UK),...

There has been an explosion of new developments in mass spectrometry in recent years. One of the major ones is the reduction in size of the instruments so that many can now be used on the benchtop. This has resulted in prices coming down - initial and running costs used to be prohibitive in many cases. The development of atmospheric pressure ionisation sources is relatively new and has made hyphenation of MS with liquid separation techniques such as HPLC much more feasible. The selection of mass analysers... 

The interface must be able to get rid of the liquid mobile phase, convert the relatively involatile and/or thermally labile analytes into a gas and transfer the gas from atmospheric conditions to a high vacuum. Compared to GC-MS, different ionisation methods must be used for these kinds of analytes (liquid phase, nonvolatile, thermolabile) as El and Cl are not suitable. However, in most cases the capillary is inserted directly into the ion source and, in this case, the ionisation source becomes the interface suitable ionisation sources are described below. 

On-line mass spectrometry has been implemented in pharmaceutical processes for monitoring raw materials andproducts ". In this particular application, dilution of the samples is carried out by a membrane interface coupled directly to the atmospheric pressure chemical ionisation source of a quadrupole mass spectrometer for real-time analysis. Continuous online MS has also been used for monitoring fermentation processes in the brewing industry. ... 

Fig. 10.2 Principle of ionisation source and at atmospheric pressure. The charged aerosol is mechanism of gaseous ion formation in ESI- evaporated due to Coulomb explosions to MS. The sample solution is admitted through a smaller droplets which finally result in desol-small capillary from which the spray is formed vated macro-ions.

Gas chromatography is a most favourable case for interfacing to a mass spectrometer, as the mobile phases commonly used do not generally influence the spectra observed, and the sample, being in the vapour phase, is compatible with the widest range of mass-spectral ionisation techniques. The primary incompatibility in the case of GC-MS is the difference in operating pressure for the two hyphenated instruments. The column outlet in GC is typically at atmospheric pressure, while source pressures in the mass spectrometer range from 2 to... 

LC-APCI-MS is a derivative of discharge-assisted thermospray, where the eluent is ionised at atmospheric pressure. In an atmospheric pressure chemical ionisation (APCI) interface, the column effluent is nebulised, e.g. by pneumatic or thermospray nebulisation, into a heated tube, which vaporises nearly all of the solvent. The solvent vapour acts as a reagent gas and enters the APCI source, where ions are generated with the help of electrons from a corona discharge source. The analytes are ionised by common gas-phase ion-molecule reactions, such as proton transfer. This is the second-most common LC-MS interface in use today (despite its recent introduction) and most manufacturers offer a combined ESI/APCI source. LC-APCI-MS interfaces are easy to operate, robust and do not require extensive optimisation of experimental parameters. They can be used with a wide variety of solvent compositions, including pure aqueous solvents, and with liquid flow-rates up to 2mLmin-1. 

Table 8.62 shows the main characteristics of ICP-MS, which is widely used in routine analytical applications. The ICP ion source has several unique advantages the samples are introduced at atmospheric pressure the degree of ionisation is relatively uniform for all elements and singly charged ions are the principal ion product. Theoretically, 54 elements can be ionised in an ICP with an efficiency of 90 % or more. Even some elements that do not show ionic emission lines should be ionised with reasonable efficiency (namely, As, 52 % and P, 33%) [381]. This is one of the advantages of ICP-MS over ICP-AES. Other features of ICP-MS that make it more attractive than ICP-AES are much lower detection limits ability to provide isotopic ratio information and to offer isotope dilution capabilities for quantitative analysis and clean and simple spectra. The... 

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. 

A heated pneumatic nebuliser is used to produce the aerosol in APCI and the ions are produced by ion-molecule reactions initiated by corona discharges in the ion source region. White et al. (1998) found atmospheric pressure ionisation MS and LC-ICP-MS to be complementary techniques. 

In thermospray interfaces, the column effluent is rapidly heated in a narrow bore capillary to allow partial evaporation of the solvent. Ionisation occurs by ion-evaporation or solvent-mediated chemical ionisation initiated by electrons from a heated filament or discharge electrode. In the particle beam interface the column effluent is pneumatically nebulised in an atmospheric pressure desolvation chamber this is connected to a momentum separator where the analyte is transferred to the MS ion source and solvent molecules are pumped away. Magi and Ianni (1998) used LC-MS with a particle beam interface for the determination of tributyl tin in the marine environment. Florencio et al. (1997) compared a wide range of mass spectrometry techniques including ICP-MS for the identification of arsenic species in estuarine waters. Applications of HPLC-MS for speciation studies are given in Table 4.3. 

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... 

Imoto et al. by high performance liquid chromatography combined with atmospheric pressure ionisation mass spectrometry (API-MS). The crude saponin isolated from the leaves was chromatographed on octadecyl silica column and eluted with an aqueous methanol solution containing ammonium acetate. The fractions thus separated were directly introduced into an atmospheric pressure ionisation mass spectrometer connected with the liquid chromatograph by an interface consisting of a nebulizer and a vaporizer through a PTFE tube (Hitachi, Japan). The vaporized sample and solvent molecules at 300°C were introduced into the ion source of the atmospheric pressure ionisation system. 

Electrospray ionisation (ESI) is a technique that takes place at atmospheric pressure and is considered to be a soft ionisation process. It is very useful for liquids. Unlike hard processes, the molecule is not normally fragmented and so the resulting mass spectrum is much simpler, the principal peak of which will be the pseudo-molecular ion, i.e. a pro-tonated or sodiated peak. It is therefore much easier to decipher the molecular weight of a compound from an ESI source but there is less structural information given about the molecule, if any. 

These sources are described in Section 2.1.5 on mass spectrometry. They are particularly suitable for the analysis of mixtures of nonvolatile and relatively polar molecules. Additionally, they get rid of mobile phase solvents by the use of a drying gas and some heat. These ionisation chambers also make the transition from atmospheric to very low pressure an easy one. 

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