Atmospheric pressure ionisation-mass compounds

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. 

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. 

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. 

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... 

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). 

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. 

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. 

P Dugo, L. Mondello, E. Sebastian , R. Ottana, G. Eirante and G. Dugo, Identification of minor oxygen heterocyclic compounds of citi us essential oils by liquid chromatography-atmospheric pressure chemical ionisation mass specti ometiy , J. Liq. Chromatogr. 22 2991-3005 (1999). 

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],... 

Tablets of famotidine, an anti-ulcer compound, were subjected to stress conditions in pack. Figure 9.34 indicates the profile obtained from analysis of an extract from the stressed tablets by LC-atmospheric pressure chemical ionisation mass spectrometry (APCIMS). The structure of famotidine is shown in Figure 9.35.

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). 

Due to the recognized importance of mass spectrometry in the structure elucidation of alkylpyridinium compounds, a detailed MS analysis of synthetic cyclostellettamines H-L (60-63) has recently been reported [46], by using powerful analytical techniques such as high resolution electrospray ionisation (ESI)-MS and with fragmentation experiments carried out by ESI atmospheric pressure ionization-collision induced dissociation (ESI-API-CID)-MS measurements. This analysis allowed a reliable identification of the alkyl chain lengths of cyclostellettamines, and is proposed as a suitable systematic methodology... 

Parcerisa, J., Casals, I., Boatella, J., Codony, R., and Rafecas, M. 2000. Analysis of Olive and Hazelnut Oil Mixtures by High-Performance Liquid Chromatography — Atmospheric Pressure Chemical Ionisation Mass Spectrometry of Triacyglycerols and Gas-Liquid Chromatography of Non-Saponifiable Compounds (Tocopherols and Sterols). J. Chromatogr. A. 881 149-158. 

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. 

In atmospheric pressure chemical ionisation (APCI), a corona discharge is used to ionise the analyte in the source of the mass spectrometer [31]. Complementary to ESI, which is especially suitable for charged, basic or polar analytes, APCI can be used for analysis of uncharged or low-polarity compounds (e.g. 

---