Atmospheric pressure ionisation interfaces

The two chapters that were selected for this topic one on GC-ion trap mass spectrometry, by SabUer and Fujii and the other by Schroder on LC-MS in environmental analysis give an excellent contribution to the application of GC-MS and LC-MS to environmental analysis. Both chapters include many practical aspects and examples in the environmental field and also cover the historical perspective of the techniques and show the perspective on ionisation and scanning modes. Advances achieved in GC-ion trap by the use of external ion sources and GC/MS/MS possi-bihties are discussed. The LC-MS chapter provides an overview of the first applications of LC/MS interfacing systems, such as moving belt, direct Uquid introduction (DLI) and particle beam (PB), and then on the more recent soft ionisation techniques, like thermospray and atmospheric pressure ionisation interfacing systems. 

The non-atmospheric pressure ionisation interfaces (non-API) PBl and TSP besides the API interfaces ESI and APCI were applied for the analysis of the N-methylcarbamate pesticides (methomyl, aldicarb, aldicarb sulfoxide, aldicarb sul-... 

Mass spectrometry can be the ideal detector for liquid chromatography because it offers both universality and the required selectivity for complex analytical problems. The two most popular types of interface for the benchtop single quadmpole LC-mass spectrometer are the particle beam interface and the atmospheric pressure ionisation interface. The particle beam interface is used to generate electron ioinsation spectra and the API interface is used to generate either electrospray or atmospheric pressure chentical ioinsation. This comprehensive article supplies a detailed analysis of a study in which organic extractable from polypropylene was used as the model analyte to demonstrate how the information obtained separately tfom El and API can be complementary. Seven compounds were separated and detected by both MS detectors. 35 refe. 

Earlier LC-MS systems used interfaces that either did not separate the mobile-phase molecules from the analyte molecules (DLI, TSP) or did so before ionisation (PB). The analyte molecules were then ionised in the mass spectrometer under vacuum, often by traditional El ionisation. These approaches are successful only for a very limited number of compounds. On the other hand, in atmospheric pressure ionisation, the analyte... 

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... 

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],... 

Owing to the anionic character of LAS, an electrospray ionisation (ESI) interface operated in negative ion mode is particularly attractive for the mass spectrometric detection of this surfactant type. Consequently, a great part of the atmospheric pressure ionisation-mass spectrometry (API-MS) work on LAS is devoted to the application of (— )-ESI-MS. 

Polycyclic aromatic hydrocarbons (PAHs) have been separated by MEKC using a double alkyl chain di(2-ethylhexyl) phosphate as anionic micellar pseudostationary phase. Phospholipids have been separated from soya lecithins by MEKC, deoxycholic acid being used for micelle formation. Separation was tested according to solvent polarity and column temperature, with a high n-propanol concentration and a column temperature of 15 °C being ideal. Online MEKC/MS with electrospray ionisation, and atmospheric-pressure Cl, interfaces have been used in the separation and detection of standard compounds, including tetraphenylphosphonium chloride. ... 

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). 

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. 

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. 

Zhou, S. and Hamburger, M. (1996) Application of liquid chromatography-atmospheric pressure ionisation mass spectrometry in natural product analysis, evaluation and optimization of electrospray and heated nebulizer interfaces. J Chromatogr. A 755, 189—204. 

GC/MS with capillary columns has been the gold standard for more than 20 years, but LC/MS has become a complementary method due to the success in interface development with atmospheric pressure ionisation (API) for low molecular weight compounds and the appHcation to biopolymers. For many areas of analytical chemistry, LC/MS has become indispensible due to its advantages over GC/MS for polar and thermolabile analytes. A Hmiting factor for LC/MS has been the incompatibility between the hquid eluting from the LC and the mass spectrometer vacuum. This could be overcome in electrospray ionisation with the use of a nebuliser gas ( ion spray ) or additional heated drying gas ( turbo ion spray ) (70, 71]. Due to its high sensitivity and selectivity, APl-MS has become a standard tool for the stracture elucidation of analytes from complex mixtures. 

Right from the outset of the 1990s, a selection of those interfaces that could be adapted to a routine LC-MS analysis was observable. This trend had been initiated by pharmacological and pharmaceutical research, although it had the TSP interface at its disposal, which was a well-adapted and reliable type of interface that had shown its fiiU capacity in manifold appliances. The sample material, being available only in very limited quantities for such research, and improved separation techniques, as, for example, capillary electrophoresis (CE) or capillary zone electrophoresis (CZE) necessitated different types of interfaces that could be operated with considerably smaller amounts of sample than the TSP interface, which reached its optimized sensitivity with flow rates of about 2 mL min. Such a desirably lower sample demand is guaranteed by atmospheric pressure ionisation (API) interfaces, atmospheric pressure chemical ionisation (APCI) and electrospray ionisation (ESI) interface. 

In parallel with the routine application of TSP the improvement in the handling of the atmospheric pressure ionisation (API) techniques for couphng LC with MS in environmental analysis proceeded. During the last decade of the last century, the dissemination of this interfacing technique grew tremendously with the up-com-ing demand for low detection Hmits in the analysis of polar organic pollutants of concern present in the environment in very low concentration ranges. 

Yu and co-workers [26] discussed LC interfaces for bench-top single quadruple LC-MS. The two most popular interfaces are particle beam and atmospheric pressure ionisation types. The system was applied to the analysis of additives in PP. Dilts [27] used a photodiode array detector coupled with particle beam LC-MS to characterise degradation of Irganox 1010, Irganox 1076 and Irgafos 16S in polyolefins. 

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. 

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. 

For sensitive quantification in LC-MS analysis of non-ionic surfactants, selection of suitable masses for ion monitoring is important. The nonionic surfactants easily form adducts with alkaline and other impurities present in, e.g. solvents. This may result in highly complicated mass spectra, such as shown in Fig. 4.3.1(A) (obtained with an atmospheric pressure chemical ionisation (APCI) interface) and Fig. 4.3.2 (obtained with an ESI interface). 

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. 

Methyl, nitro and chlorophenols (EPA priority pollutants) LC-MS (atmospheric pressure chemical ionisation and ion spray interfaces) ppt [18]... 

Polar pesticides - LC-MS atmospheric pressure chemical ionisation interface sub ppb [31]... 

Atmospheric pressure chemical ionisation (APCI) is a technique that also creates gas phase ions from the liquid sample. It too takes place at atmospheric pressure and uses a similar interface to that in ESI. As in ESI, the sample solution is mixed with a nebulising gas and the sample arrives in the spray chamber as a fine mist of droplets or spray. In APCI, an extra component - a corona discharge - is used to further ionise the analyte droplets in a manner similar to straightforward Cl (Figure 2.34). While a small amount of fragmentation may occur, the technique is still considered to be a soft ionisation one. The gas-phase ionisation in APCI is more effective than ESI for analysing less polar species. ESI and APCI are complementary methods. 

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