Nebulisation

Although APDC complexes are soluble in many organic solvents, it is found that 4-methylpent-2-one (isobutyl methyl ketone) and heptan-2-one (n-pentyl methyl ketone) are, in general, the most satisfactory for direct nebulisation into the air/acetylene flame used in atomic absorption spectroscopy. 

When the AAS measurements have been completed, aspirate de-ionised water for several minutes to ensure thorough cleaning of the nebuliser-burner system. 

The sample, usually in the form of a solution, is carried into the hot plasma by a nebuliser system similar to that employed for flame methods (see Section 21.5) although for ICP a much slower flow rate of 1 mLmin-1 is used. 

The most widely used nebuliser system in ICP is the crossed-flow nebuliser shown in Fig. 20.12. The sample is forced into the mixing chamber at a flow rate of 1 mL min 1 by the peristaltic pump and nebulised by the stream of argon flowing at about 1 Lmin-1. 

Another kind of nebuliser, the Babington type, is used for handling slurries that can contain up to 10 per cent solids. This design of nebuliser is less likely to suffer from blockage. 

Sequential instruments. The diagram of the light path of the Thermo Electron-200 ICP spectrometer is shown in Fig. 20.14. The plasma is located in the upper centre of the instrument just above the nebuliser, which is powered by a computer-controlled peristaltic pump. Communication with the instrument takes place on a video display, which not only guides the operator through the use of the system, but also provides graphics to simplify methods development. 

The purpose of the nebuliser-burner system is to convert the test solution to gaseous atoms as indicated in Fig. 21.2, and the success of flame photometric methods is dependent upon the correct functioning of the nebuliser-burner system. It should, however, be noted that some flame photometers have a very simple burner system (see Section 21.13). 

The function of the nebuliser is to produce a mist or aerosol of the test solution. The solution to be nebulised is drawn up a capillary tube by the Venturi action of a jet of air blowing across the top of the capillary a gas flow at high pressure is necessary in order to produce a fine aerosol. 

Only 5-15 per cent of the nebulised sample reaches the flame (in the case of the pre-mix type of burner) and it is then further diluted by the fuel and oxidant gases so that the concentration of the test material in the flame may be extremely minute. 

Samples which are viscous (e.g. oils, blood, blood serum) require dilution with a solvent, or alternatively must be wet ashed before the sample can be nebulised. 

Thus, for example a solution containing potassium ions at a concentration of 2000 mg L "1 added to a solution containing calcium, barium, or strontium ions creates an excess of electrons when the resulting solution is nebulised into the flame, and this has the result that the ionisation of the metal to be determined is virtually completely suppressed. 

A -Benzoyl-i V-phenyl hydroxylamine 440 Nebuliser-burner system 785 Neocuproin 178... 

Figure 9.5 Scheaatic diagraa of the aonodlsperse aerosol generation Interface (MAGIC) for LC/MS with Insert providing a nore detailed view of the eluent nebuliser and particle beae oaentun separator.

A wealth of other data can be obtained from the use of US as an analytical method. Sonoelectrochemical analysis of trace metals [220] and organic compounds [221] has been reported. Ultrasonic atomisation [222] is used in many fields where a dispersion of liquid particles is required. Ultrasonic nebulisation (USN) is used for analysis of organic solutions in conjunction with ICP-AES/MS [223,224] and MIP-AES [225],... 

For non-volatile sample molecules, other ionisation methods must be used, namely desorption/ionisation (DI) and nebulisation ionisation methods. In DI, the unifying aspect is the rapid addition of energy into a condensed-phase sample, with subsequent generation and release of ions into the mass analyser. In El and Cl, the processes of volatilisation and ionisation are distinct and separable in DI, they are intimately associated. In nebulisation ionisation, such as ESP or TSP, an aerosol spray is used at some stage to separate sample molecules and/or ions from the solvent liquid that carries them into the source of the mass spectrometer. Less volatile but thermally stable compounds can be thermally vaporised in the direct inlet probe (DIP) situated close to the ionising molecular beam. This DIP is standard equipment on most instruments an El spectrum results. Techniques that extend the utility of mass spectrometry to the least volatile and more labile organic molecules include FD, EHD, surface ionisation (SIMS, FAB) and matrix-assisted laser desorption (MALD) as the last... 

C, is one of the most critical parameters in TSP operation, and should be optimised for different samples, wherever possible. This is considered to be a considerable drawback in routine operation of unknown polymer/additive extracts. Too low a vaporiser temperature results in the solute and solvent spraying into the ionisation source in their liquid form, without formation of gas-phase ions. Too high a vaporiser temperature causes premature evaporation of the solute and solvent before the outlet of the capillary is reached. This causes an unstable, pulsing ion beam. As ion formation in TSP operation depends very critically on the extent of desolvation and the energy of the nebulised droplets, it is clear that an inappropriate vaporiser temperature will cause loss of sensitivity. 

ES ionisation can be pneumatically assisted by a nebulising gas a variant called ionspray (IS) [129]. ESI is conducted at near ambient temperature too high a temperature will cause the solvent to start evaporating before it reaches the tip of the capillary, causing decomposition of the analyte during ionisation and too low a temperature will allow excess solvent to accumulate in the sources. Table 6.20 indicates the electrospray ionisation efficiency for various solvents. 

Heated nebuliser Atmospheric pressure chemical ionisation... 

The obvious alternative for the in-line flow-through cell in HPLC-FTIR is mobile-phase elimination ( transport interfacing), first reported in 1977 [495], and now the usual way of carrying out LC-FTIR, in particular for the identification of (minor) constituents of complex mixtures. Various spray-type LC-FTIR interfaces have been developed, namely, thermospray (TSP) [496], particle-beam (PB) [497,498], electrospray (ESP) [499] and pneumatic nebulisers [486], as compared by Som-sen et al. [500]. The main advantage of the TSP-based... 

As the vast majority of LC separations are carried out by means of gradient-elution RPLC, solvent-elimination RPLC-FUR interfaces suitable for the elimination of aqueous eluent contents are of considerable use. RPLC-FTTR systems based on TSP, PB and ultrasonic nebulisa-tion can handle relatively high flows of aqueous eluents (0.3-1 ml.min 1) and allow the use of conventional-size LC. However, due to diffuse spray characteristics and poor efficiency of analyte transfer to the substrate, their applicability is limited, with moderate (100 ng) to unfavourable (l-10pg) identification limits (mass injected). Better results (0.5-5 ng injected) are obtained with pneumatic and electrospray nebulisers, especially in combination with ZnSe substrates. Pneumatic LC-FI1R interfaces combine rapid solvent elimination with a relatively narrow spray. This allows deposition of analytes in narrow spots, so that FUR transmission microscopy achieves mass sensitivities in the low- or even sub-ng range. The flow-rates that can be handled directly by these systems are 2-50 pLmin-1, which means that micro- or narrow-bore LC (i.d. 0.2-1 mm) has to be applied. 

A group of techniques employing differential selection of solute ions relies on nebulisation and ionisation of the eluent, with some discrimination of ion selection in favour of the solute. Main representatives are APCI [544] and thermospray [545]. In a thermospray interface a supersonic jet of vapour and small droplets is generated out of a heated vaporiser tube. Controlled, partial vaporisation of the HPLC solvent occurs before it enters the ion source. Ionisation of nonvolatile analytes takes place by means of solvent-mediated Cl reactions and ion evaporation processes. Most thermospray sources are fitted with a discharge electrode. When this is used, the technique is called plasmaspray (PSP) or discharge-assisted thermospray. In practice, many... 

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. 

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