Atmospheric pressure ionisation techniques

Lately, electrospray ionisation technique (ESI-MS) which is compatible with RP-HPLC has been routinely used. This allows labile molecules to be studied intact. Sample molecules are simultaneously nebulised and ionised at atmospheric pressure in the presence of several thousand volts. The resulting ions can be multi-protonated (multiply charged) and relatively stable. This mode of ionisation has recently been used in the development of RP-HPLC coupled with positive ion ESI-MS and ion-trap MS protocols for the identification and... 

The ion source produces ionisation of the analyte as it enters the mass spectrometer. Ionisation can be brought about in a number of ways. However, the so-called soft ionisation techniques used in LC-MS systems are electrospray ionisation (ESI), atmospheric pressure chemical ionisation (APCI), and, more recently, atmospheric pressure photoionisation (APPI). 

Table 6.10 reports the main areas of application of the various ionisation methods and the principal ions detected. A breakdown of MS techniques applied to various types of analytes is as follows thermally stable, low-MW Cl, El thermally instable, low-MW APCI (FLA, LC-MS), ESI and high-MW DCI, FD, FAB, LD, ESI (FLA, LC-MS, CZE-MS). Soft ionisation techniques such as FL, FAB and LD are useful for the detection of non-volatile, sometimes oligomeric, polymer additives. Recent developments in ionisation techniques have allowed the analysis of polar, ionic, and high-MW compounds, previously not amenable to mass-spectrometric analysis. Figure 6.4 shows the applicability of various atmospheric pressure ionisation techniques in terms of molar mass and polarity. 

Figure 6.4 Applications of various atmospheric pressure ionisation techniques...

Principles and Characteristics Ion mobility spectrometry (IMS) is an instrumental technique for the detection and characterisation of organic compounds as vapours at atmospheric pressure. Modern analytical IMS was created at the end of the 1960s from studies on ion-molecule chemistry with mass spectrometers and from ionisation detectors for vapour monitoring. An ion mobility spectrometer (or plasma chromatograph in the original termininology) was first produced in 1970 [272],... 

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... 

A range of MS ionisation techniques are available. Atmospheric pressure chemical ionisation (APCI) and electrospray ionisation (ESI) are becoming the methods of choice for the analysis of low molecular weight additives of mass/ charge (m/z) ratio <3,000. 

In off-line coupling of LC and MS for the analysis of surfactants in water samples, the suitability of desorption techniques such as Fast Atom Bombardment (FAB) and Desorption Chemical Ionisation was well established early on. In rapid succession, new interfaces like Atmospheric Pressure Chemical Ionisation (APCI) and Electrospray Ionisation (ESI) were applied successfully to solve a large number of analytical problems with these substance classes. In order to perform structure analysis on the metabolites and to improve sensitivity for the detection of the various surfactants and their metabolites in the environment, the use of various MS-MS techniques has also proven very useful, if not necessary, and in some cases even high-resolution MS is required. 

These problems have largely been solved by the development of a wide variety of powerful LC-MS interfaces (reviewed in Refs. [1-3]). In the following paragraphs, the two most widely used atmospheric pressure ionisation (API) systems, namely atmospheric pressure chemical ionisation (APCI) and electrospray ionisation (ESI), are briefly described, along with the older technique of thermospray ionisation... 

Several years later, the next step in the application of MS-MS for mixture analysis was developed by Hunt et al. [3-5] who described a master scheme for the direct analysis of organic compounds in environmental samples using soft chemical ionisation (Cl) to perform product, parent and neutral loss MS-MS experiments for identification [6,7]. The breakthrough in LC-MS was the development of soft ionisation techniques, e.g. desorption ionisation (continuous flow-fast atom bombardment (CF-FAB), secondary ion mass spectrometry (SIMS) or laser desorption (LD)), and nebulisation ionisation techniques such as thermospray ionisation (TSI), and atmospheric pressure ionisation (API) techniques such as atmospheric pressure chemical ionisation (APCI), and electrospray ionisation (ESI). 

Whilst these methods are informative for the characterisation of synthetic mixtures, the information gained and the nature of these techniques precludes their use in routine quantitative analysis of environmental samples, which requires methods amenable to the direct introduction of aqueous samples and in particular selective and sensitive detection. Conventionally, online separation techniques coupled to mass spectrometric detection are used for this, namely gas (GC) and liquid chromatography (LC). As a technique for agrochemical and environmental analyses, high performance liquid chromatography (HPLC) coupled to atmospheric pressure ionisation-mass spectrometry (API-MS) is extremely attractive, with the ability to analyse relatively polar compounds and provide detection to very low levels. 

MS techniques have met this need in the analysis of involatile, polar surfactants after coupling techniques of liquid chromatographic methods with MS became available. Different types of interfaces for off-line and on-line coupling of liquid chromatography (LC) and MS in the analyses of surfactants had been in use [7,16] while the methods applied at present were performed predominately with soft-ionising atmospheric pressure ionisation (API) interfaces [16-19],... 

Fenn published work in 1989 [7-9] showing ionisation of large molecules by electrospray ionisation (ESI). Fenn built on the early work of Malcolm Dole [10] but Fenn used a counter current gas to assist with desolvation of the droplets and aid the formation of the ions. In the early 1990s, experiments with atmospheric pressure ionisation (API) showed promise and in a short space of time the first commercial systems utilising the new techniques of ESI [11] and Atmospheric Pressure Chemical Ionisation (APCI) began to appear on analysts benches. The sensitive, reliable and easily operated LC-MS system had arrived. 

ESI and APCI [12, 13] are the dominant techniques today for LC-MS in pharmaceutical analysis. These both occur at atmospheric pressure hence the phrase atmospheric pressure ionisation (API). 

Ion mobility spectrometry (IMS), which has the ability to separate ionic species at atmospheric pressure, is another technique that is useful for detect and characterising organic vapours in air [97]. This involves the ionisation of molecules and their subsequent drift through an electric field. Analysis is based on analyte separations resulting from ionic mobilities rather than ionic masses. A major advantage of operation at atmospheric pressure is that it is possible to have smaller analytical units, lower power requirements, lighter weight and easier use. 

Other MS-fingerprinting techniques that are in commercial development are based on atmospheric pressure ionisation (API), resonance-enhanced multiphoton ionisation (REMPI) TOE and proton-transfer reaction (PTR). They are rapid, sensitive and specific and allow measurements in real time and may play an increasingly important role in the future development of electronic noses and tongues. 

Therefore, application of direct-inlet MS for monitoring complex mixtures of VOCs requires using ionisation techniques which produce little or no fragmentation (soft ionisation). Chemical ionisation in combination with a quadrupole mass filter, either in atmospheric pressure chemical ionisation MS (APCI-MS) [188, 189] or in PTR-MS [193-195], have been successfully applied to VOC analyses. The advantages and limitations of direct-inlet MS with soft-ionisation approaches have been discussed [196]. 

Several of the more recent applications of mass spectrometry involve the determination of elements and of unstable, polar biomolecules (M > 2 000 Da). These applications have become possible with the advent of atmospheric pressure ionisation (API) techniques that permit the analysis of solutions. 

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. 

White, S., Catterick, T., Fairman, B. and Webb, K. (1998) Speciation of organo-tin compounds using liquid chromatography atmospheric pressure ionisation mass spectrometry and liquid chromatography inductively coupled plasma mass spectrometry as complementary techniques./. Chromatogr. A, 794, 211-218. 

Ofrier soft ionisation techniques include field desorption, field ionisation, atmospheric pressure ionisation, and fast atom bombardment and each increases the rmolecular weight but because of the production of cluster ions such as (M + Na), (M + 2Na), 2M, etc., they are not used in the investigation of unknowns. 

These additives are essentially high boiling point liquids and so the most appropriate technique to use is liquid chromatography (LC-MS). A range of synthetic plasticisers such as phthalates, adipates, mellitates and sebacates can be detected using the atmospheric pressure chemical ionisation (APCl) mode. Process oils are hydrocarbon mineral oils and require either the atmospheric pressure photoionisation (APPl) head (which can ionise nonpolar species) or, where the oil contains sufficient aromatic character, the use of in-line UV or fluorescence detectors. A fluorescence detector is particularly sensitive in the detection of polyaromatic hydrocarbon (PAH) compounds in such oils. 

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