Electrospray chemical ionisation

EBS Elastic backscattering ESCI Electrospray chemical ionisation... 

Fast atom bombardment (FAB) Plasma desorption (PD) Liquid secondary-ion mass spectrometry (LSIMS) Thermospray (TSP)/plasmaspray (PSP) Electrohydrodynamic ionisation (EHI) Multiphoton ionisation (MPI) Atmospheric pressure chemical ionisation (APCI) Electrospray ionisation (ESI) Ion spray (ISP) Matrix-assisted laser desorption/ionisation (MALDI) Atmospheric pressure photoionisation (APPI) Triple quadrupole (QQQ) Four sector (EBEB) Hybrid (EBQQ) Hybrid (EB-ToF, Q-ToF) Tandem ToF-ToF Photomultiplier... 

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). 

Detection of M2D-C3-0-(E0)n-CH3 was possible by both positive ion mode atmospheric pressure chemical ionisation (APCI) and electrospray ionisation (ESI) MS methods, with good response down to absolute injections of 0.1 ng. However, ionisation in the negative ion mode was negligible at all concentrations analysed, as the polyether-modified structure has no sites capable of adducting with anions, nor has it any moieties capable of cleavage to yield anionic species. 

The qualitative determination of anionic surfactants in environmental samples such as water extracts by flow injection analysis coupled with MS (FIA-MS) applying a screening approach in the negative ionisation mode sometimes may be very effective. Using atmospheric pressure chemical ionisation (APCI) and electrospray ionisation (ESI), coupled with FIA or LC in combination with MS, anionic surfactants are either predominantly or sometimes exclusively ionised in the negative mode. Therefore, overview spectra obtained by FIA—MS(—) often are very clear and free from disturbing matrix components that are ionisable only in the positive mode. However, the advantage of clear... 

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). 

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. 

The thermospray, particle beam electrospray (ES) (Fig. 4.4) and atmospheric pressure chemical ionisation (APCI) have been used in HPLC-MS-MS. The ES and APCI are both atmospheric pressure ionisation systems. The column effluent is nebulised and ionised in the atmospheric pressure region and the ions are then... 

Figure 4. APCI (left) and ESI spectra of isopropyl ethylphosphonic acid, illustrating the lower abundance of adduct ions with APCI ions at mlz, 143 and 185 are adduct ions with MeOH mlz 175 is [M + Na]+. (Reprinted from Journal of Chromatography A, 794, R.M. Black and R.W. Read, Analysis of degradation products of organophosphorus chemical warfare agents and related compounds by liquid chromatography-mass spectrometry using electrospray and atmospheric pressure chemical ionisation, pp. 233-244 (1998), with permission from Elsevier)...

R.M. Black and R.W. Read, Analysis of degradation products of organophosphorus chemical warfare agents and related compounds by liquid chromatography-mass spectrometry using electrospray and atmospheric pressure chemical ionisation, J. Chromatogr., A, 794, 233-244 (1998). 

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. 

Rather limited use has been made of mass spectrometry in the study of organotin compounds,23-24 though MS linked to gas-liquid chromatography is now being used for the identification of organotin compounds, particularly in environmental studies. Most of the early work involved electron ionisation (El), but in recent years, other techniques such as chemical ionisation (Cl),25 fast atom bombardment (FAB),26, 27 field desorption,28 surface ionisation,29 and, particularly, electrospray (ES),30 31 have been used. 

Sjoberg, J.R. Markides, K.E. Capillary Column Supercritical Fluid Chromatography-Atmospheric Pressure Ionisation Mass Spectrometry Interface Performance of Atmospheric Pressure Chemical Ionisation and Electrospray Ionisation,", 7. Chromatogr. A 855, 317-327 (1999). 

Rosenberg, E. The potential of organic (electrospray- and atmospheric pressure chemical ionisation) mass spectrometric techniques coupled to liquid-phase separation for speciation analysis. J Chromatogr A 2003, 1000, 841-889. 

Mass spectroscopy [electron ionisation (El), chemical ionisation (Cl), electrospray ionisation (ESI), fast atom bombardment (FAB), matrix-associated laser desorption ionisation (MALDI), inductively coupled plasma-mass spectrmetry (ICP-MS, cf and ), etc]... 

Sjoberg, P.J. and Markides, K.E., Capillary column supercritical fluid chromatography-atmospheric pressure ionisation mass spectrometry interface performance of atmospheric pressure chemical ionisation and electrospray ionisation, J. Chromatogr. A, 855(1), 317, 1999. 

Most examples in the literature exploit electrospray ionisation but some research groups have reported the use of atmospheric pressure chemical ionisation (APCI) on-chip. In one such example, the chip was composed of two wafers, one made of silicon wafer one made of pyrex glass . The silicon wafer contained the nebuliser gas inlet, vapouriser channel and a nozzle. The sample inlet from the LC column was directly connected to the... 

The most recent significant advance in plant hormone analysis has been the use of combined HPLC-MS for the analysis of GA conjugates, lAA conjugates and cytokinins. A number of interfaces have been developed for HPLC-MS, including thermospray, atmospheric pressure chemical ionisation, electrospray, particle beam, continuous flow fast atom bombardment (FAB) and frit-FAB (see reference [94]). GA standards have been analysed by HPLC-MS with a thermospray interface [95], an atmospheric pressure chemical ionisation interface has been used with GA conjugates [96] and cytokinins [97] while ion spray and plasma spray have been used to analyse ABA and its metabolites [98]. There are, however, many more reports on the use of frit-FAB HPLC-MS for the analysis of not only standards, but also endogenous hormones and their isotopically-labelled metabolites [18-23,99-101]. 

Bailac S, Barron D, Sanz-Nebot V, Barbosa J, Determination of fluoroquinolones in chicken tissues by LC-coupled electrospray ionisation and atmospheric pressure chemical ionisation, J. Sep. Sci. 2006 29(1) 131-136. 

Probably the most common separation systems used in the laboratory today require the sample to be in solution (e.g. HPLC, CE). The solvent may be aqueous or solvent based. However, onemL of such solution yields far too much vapour (1-2L) to be accommodated by a mass spectrometer s vacuum system. Thus the aim of a sample introduction system for such solutions would require the sample to be ionised and the solvent to be separated from these sanple ions. In addition the interface must maintain the integrity of the chromatography. The chromatographic separation must be maintained as well as allowing sufficient analyte through to generate a mass spectmm. A number of methods have been developed to do this, but the two main techniques used today are electrospray and atmospheric pressure chemical ionisation (APCI for short). These are described below under ionisation techniques. 

Separation is more usually carried out in the liquid, rather than gas phase. Neither El nor Cl can deal very effectively with samples in solution and a number of techniques to overcome this have arisen over the last few years. The first widely used method was thermospray [16], However, this has been largely superseded by atmospheric pressure chemical ionisation (APCl). The other commonly applied technique is electrospray. These techniques are described in detail below. 

---