Detection systems ionisation

In LC-MS, specific ionisation conditions can be required for different types of species. This means that in LC-MS studies on extractable additives, it is necessary to use a range of experimental conditions to cover detection of all types of potential species. Depending on instrument type, it is also possible to isolate ions in complex matrices and obtain positive identifications by further unique fragmentation of these ions (by MS-MS or MSn). Quantitative methods based on this secondary ionisation can be employed. The mass accuracy of LC-MS detection systems continues to improve. Accurate mass measurement improves the certainty of identification. Advanced systems are typically offering 1-2 ppm (mass dependent) mass accuracy. 

Other types of atmospheric pressure ionisation detectors are available including APPI (photon ionisation) and MALDI (matrix-assisted laser desorption ionisation). More conventional detection systems for LC include UV absorption, fluorescence and evaporative light scattering. Developments in column packings,... 

The reproducibility of the results from the standards is clear from the narrow spread of intensity ratios in Table 2. Readers are reminded, however, that the k-values will vary somewhat from one instrument to another because they are dependent not only upon the appropriate ionisation cross sections and fluorescence yields, but also upon the efficiency of the detection system. Values that are tabulated in commercial software packages should be used with caution. 

Gas chromatograph with a flame ionisation detection system, equipped with a capillary column (25 m, 0.22 mm inner diameter, i.d., 0.25 pm stationary phase, 100% dimethyl siloxane). 

A mass spectrometer may be conveniently divided into five basic components sample inlet system, ionisation, ion separation, ion detection and amplification, and presentation of results (Fig. I). 

The experiment can readily be extended to other systems, for example, a nitric acid/nitrogen oxide vapour produced by adding concentrated nitric acid to copper. The quantitative exercise could simply be carried out with flame ionisation detection however, if both detection systems are available this forms the basis for a study and comparison of relative detector response using both total ion and selected ion monitoring. 

Two other detection systems which have been used for the quantitation of phosphohpids are flame ionisation detectors (Kiuchi et al.,... 

Tatara, T., Fuji, T., Kaware, T. et al (1983) Quantitative determination of tri-, di-, monoleins and free oleic acid by thin layer chromatography-flame ionisation detection system using internal standards and boric acid impregnated chromarods. Lipids, 18, 732-6. 

The GC was a Hewlett-Packard 5790-A (Hewlett-Packard, USA) equipped with flame ionisation and nitrogen-phosphorus detection systems. The chromatograms were recorded with a Hewlett-Packard 3392-A integrator. A BP-5 fused-silica column (25 m x 0.2 mm id) from SGE (Australia) was used and the carrier gas was helium at a flow rate of 1 ml/min. The oven temperature was kept at 35 °C for 1 minute after injection, then raised to 240 "C for 20 minutes. The injector temperature was 220 °C, the detector temperature 280 "C, and the volume injected 0.1 pi. 

Sandra and co-workers [32] present an overview of the use of MS as a detection system for liquid chromatography. Enjalbal and co-workers [33] characterised soluble polymer-supported organic compounds by liquid chromatography/electroscopy ionisation mass spectrometry. 

Carroll, D.I., Dzidic, L, Stillwell, R.N. and Homing, E.C. (1974), Subpicogram detection system for gas phase analysis based upon atmospheric pressure ionisation (API) mass spectrometry. Anal. Chem., 46,706. 

In the deformulation of PE/additive systems by mass spectrometry, much less fragmentation was observed with DCI-MS/MS using ammonia as a reagent gas, than with FAB-MS [69]. FAB did not detect all the additives in the extracts. The softness and the lack of matrix effect make ammonia DCI a better ionisation technique than FAB for the analysis of additives directly from the extracts. Applications of hyphenated FAB-MS techniques are described elsewhere low-flow LC-MS (Section 7.3.3.2) and CE-MS (Section 7.3.6.1) for polar nonvolatile organics, and TLC-MS (Section 7.3.5.4). 

The principle of MS/MS for direct analysis of a multicomponent system is shown in Figure 6.18, in which the first mass spectrometer (MS I) operates with soft ionisation (FI, FD, Cl, LD), and thus produces an ensemble of molecular ions (M + H+, M — H+, or adducts). For identification of molecule ABC only ABC+ is allowed to enter an interface or fragmentation zone for excitation by collisional activation, laser radiation or surface-induced dissociation. Within the time of one vibration (10-13s), ABC+ dissociates into fragments characterising the original molecule. They are separated and detected by MS II [226]. Soft ionisation with FI/FD produces low ion yields, which may be insufficient for MS/MS LVEI (typically at 20 V) can be an alternative. Complete analysis of a multicomponent system is carried out in some 20 min. 

LC-MS is now a nature technology and operation of an LC-MS system is no longer the realm of an MS specialist. The proper choice of the LC-MS mode to be used in a specific situation depends on analyte class, sample type and problem (detection, confirmation, identification). On-line LC-MS is used more for specialised applications than for general polymer or rubber compound analysis. This derives from the fact that LC-MS method development (column, solvent system, solvent programme, ionisation mode) is rather time consuming. LC-MS (in particular with API interface) enables analysis of a wide range of polar and nonvolatile compounds which cannot be analysed by GC (icf. Scheme 7.7). 

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