Ionisation of compounds

Quaternary ammonium cationic surfactants, such as DTDMAC, were determined in digested sludge by using supercritical fluid extraction (SFE) and FIA-ESI-MS(+) after separation by normal phase LC. Standard compounds—commercially available DTDMAC— were used to check the results. The DTDMAC mixture examined showed ions at m/z 467, 495, 523, 551, and 579 all equally spaced by A m/z 28 (-CH2-CH2-) resulting from the ionisation of compounds like RR N (CH3)2 X (R = / RO as shown in Fig. 2.12.11(a) and (b) [22],... 

Ion suppression is a technique used to suppress the ionisation of compounds (such as carboxylic acids) so they will be retained exclusively by the reversed phase retention mechanism and chromatographed as the neutral species. Column packings with an extended pH range are needed for this application as strong acids or alkalis are used to suppress ionisation. In addition to carboxylic acids, the ionisation of amines can be suppressed by the addition of a base to the mobile phase, thus allowing chromatography of the neutral amine. 

While the removal of solvent by vaporization and its subsequent ionisation was the principle of MBI that came to be followed up later by PBI in the FAB process, a quite different ionisation technique, direct ionisation of compounds from a target, is the principle of the FAB technique. 

Ion suppression is a technique used to suppress the ionisation of compounds (such as carboxylic acids) so they will be retained exclusively by the reversed-phase retention mechanism and chromatographed as the neutral species. 

The continued effectiveness of the nitronium ion in relatively dilute solutions has been indicated by comparing the dependence of the rates on the concentration of sulphuric acid, with the acidity-dependence of the ionisation of model compounds. The (formerly or Cq) acidity... 

There is increasing evidence that the ionisation of the organic indicators of the same type, and previously thought to behave similarly, depends to some degree on their specific structures, thereby diminishing the generality of the derived scales of acidity. In the present case, the assumption that nitric acid behaves like organic indicators must be open to doubt. However, the and /fp scales are so different, and the correspondence of the acidity-dependence of nitration with so much better than with Hg, that the effectiveness of the nitronium ion is firmly established. The relationship between rates of nitration and was subsequently shown to hold up to about 82 % sulphuric acid for nitrobenzene, />-chloronitrobenzene, phenyltrimethylammonium ion, and p-tolyltrimethylammonium ion, and for various other compounds. ... 

It is found in practice that for a number of compounds reacting ma the predominant species an almost horizontal plot is obtained. For compounds presumed to be nitrated via the free bases, such as 2,6-lutidine i-oxide and 3- and 5-methyl-2-pyridone, slopes of approximately unity are obtained. Since this type of plot allows for the incomplete ionisation of nitric acid, it can be used at higher acidities than plots using — ( H + logio Hjo) which break down when the condition is no longer true. 

The reactor coolant pH is controlled using lithium-7 hydroxide [72255-97-17, LiOH. Reactor coolant pH at 300°C, as a function of boric acid and lithium hydroxide concentrations, is shown in Figure 3 (4). A pure boric acid solution is only slightly more acidic than pure water, 5.6 at 300°C, because of the relatively low ionisation of boric acid at operating primary temperatures (see Boron COMPOUNDS). Thus the presence of lithium hydroxide, which has a much higher ionisation, increases the pH ca 1—2 units above that of pure water at operating temperatures. This leads to a reduction in corrosion rates of system materials (see Hydrogen-ION activity). 

With concentrated sulfuric acid, anthraquinone forms oxonium salts, thus falling into a class of compounds known as " oxygen bases" (15). In an aqueous solution, anthraquinone has an ionisation constant equal to 7 x 10 and a of 7.4 (16). 

Improvements in the rate of the condensation reaction have been claimed with the use of co-catalysts such as an ionisable sulphur compound and by pre-irradiation with actinic light. ... 

Electron capture detector. Most ionisation detectors are based on measurement of the increase in current (above that due to the background ionisation of the carrier gas) which occurs when a more readily ionised molecule appears in the gas stream. The electron capture detector differs from other ionisation detectors in that it exploits the recombination phenomenon, being based on electron capture by compounds having an affinity for free electrons the detector thus measures a decrease rather than an increase in current. 

FID Flame ionisation detector Burning of compound in flame... 

Recently, Lattimer et al. [22,95] advocated the use of mass spectrometry for direct analysis of nonvolatile compounding agents in polymer matrices as an alternative to extraction procedures. FAB-MS was thus applied as a means for surface desorption/ionisation of vulcanisates. FAB is often not as effective as other ionisation methods (El, Cl, FI, FD), and FAB-MS is not considered particularly useful for extracted rubber additives analysis compared to other methods that are available [36], The effectiveness of the FAB technique has been demonstrated for the analysis of a live-component additive mixture [96]. 

Different options are available for LC-MS instruments. The vacuum system of a mass spectrometer typically will accept liquid flows in the range of 10-20 p,L min-1. For higher flow-rates it is necessary to modify the vacuum system (TSP interface), to remove the solvent before entry into the ion source (MB interface) or to split the effluent of the column (DLI interface). In the latter case only a small fraction (10-20 iLrnin ) of the total effluent is introduced into the ion source, where the mobile phase provides for chemical ionisation of the sample. The currently available commercial LC-MS systems (Table 7.48) differ widely in characteristics mass spectrometer (QMS, QQQ, QITMS, ToF-MS, B, B-QITMS, QToF-MS), mass range m/z 25000), resolution (up to 5000), mass accuracy (at best <5ppm), scan speed (up to 13000Das-1), interface (usually ESP/ISP and APCI, nanospray, PB, CF-FAB). There is no single LC-MS interface and ionisation mode that is readily suitable for all compounds... 

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... 

LC-TSP-MS without tandem mass capabilities has only met with limited success for additive analysis in most laboratories. Thermospray ionisation was especially applied between 1987 and 1992 in combination with LC-MS for a wide variety of compound classes, e.g. dyes (Fig. 7.31). Thermospray, particle-beam and electrospray LC-MS were used for the analysis of 14 commercial azo and diazo dyes [594]. No significant problems were met in the LC-TSP-MS analysis of neutral and basic azo dyes [594,595], at variance with that of thermolabile sulfonated azo dyes [596,597], LC-TSP-MS has been used to elucidate the structure of Basic Red 14 [598]. The applications of LC-TSP-MS and LC-TSP-MS in dye analysis have been reviewed [599]. 

Even HALS compounds which absorb weakly at 337 nm can be analysed directly without matrix assistance, with the exception of the high-MW Hostavin N 30 (ca. 1500 Da), which fragments by direct laser desorption ionisation of intact molecules occurs only in the presence of a (dithranol) matrix. Direct laser desorption leads only to noncharacteristic, low-MW fragments. Hostavin N 20 leads to [M + H]+, [M + Na]+, [M + K]+ and some fragmentation peaks. MALDI-ToFMS of Tinuvin 765, which consists of a mono- and bifunctional sterically hindered amine, only shows the adduct peaks of the bifunctional amine apparently, the monofunctional amine is not ionisable. 

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