Nebulisers desolvating

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. 

Advances in TIMS-techniques and the introduction of multiple collector-ICP-MS (MC-ICP-MS) techniques have enabled the research on natural variations of a wide range of transition and heavy metal systems for the first time, which so far could not have been measured with the necessary precision. The advent of MC-ICP-MS has improved the precision on isotope measurements to about 40 ppm on elements such as Zn, Cu, Fe, Cr, Mo, and Tl. The technique combines the strength of the ICP technique (high ionization efficiency for nearly all elements) with the high precision of thermal ion source mass spectrometry equipped with an array of Faraday collectors. The uptake of elements from solution and ionization in a plasma allows correction for instrument-dependent mass fractionations by addition of external spikes or the comparison of standards with samples under identical operating conditions. All MC-ICP-MS instruments need Ar as the plasma support gas, in a similar manner to that commonly used in conventional ICP-MS. Mass interferences are thus an inherent feature of this technique, which may be circumvented by using desolvating nebulisers. 

A flame, where the solution of the sample is aspirated. Typically, in FAAS the liquid sample is first converted into a fine spray or mist (this step is called nebulisation). Then, the spray reaches the atomiser (flame) where desolvation, volatilisation and dissociation take place to produce gaseous free atoms. Most common flames are composed of acetylene-air, with a temperature of 2100-2400 °C, and acetylene-nitrous oxide, with a temperature of 2600-2900 °C. 

Fig. 9.8 ESI MS plot after 90 min reaction and list of possible cyclopentyl silsesquioxanes with 1

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. 

Cross contamination encountered with desolvation systems has been greatly reduced by using a concentric sheath to prevent deposits on tube walls. It is important to note that nebulisers and spray chambers operate interactively and must be optimised as a unit rather than individually. There are, however, certain parameters that need to be considered in relation to the spray chamber ... 

The membrane desolvator is very effective in reducing the solvent loading further when used in conjunction with the ultrasonic nebuliser. This will allow a range of solvents to be used for ICP-AES that would otherwise quench the plasma by almost totally removing the solvent from the sample and only allowing the dried particles containing the elements of interest to enter the source. 

The behaviour of solvents for the analysis of metal ions is important because the determination of the correct concentration is paramount to whether the ICP-OES can handle a solvent or not. The journey from liquid to nebulisation, evaporation, desolvation, atomisation, and excitation is governed by the physical nature of the sample/solvent mixture. The formation of the droplet size is critical and must be similar for standards and sample. The solution emerging from the inlet tubing is shredded and contracted by the action of surface tension into small droplets which are further dispersed into even smaller droplets by the action of the nebuliser and spray chamber which is specially designed to assist this process. The drop size encountered by this process must be suitably small in order to achieve rapid evaporation of solvent from each droplet and the size depends on the solvent used. Recombination of droplets is possible and is avoided by rapid transfer of the sample droplets/mist to the plasma torch. The degree of reformation depends on the travel time of the solution in the nebuliser and spray chamber. For accurate analysis the behaviour must be the same for standards and samples. 

To be detected by AAS, the analyte must be presented to the optical beam of the instrument as free atoms. The process of converting analyte ions/molecules, dissolved in a suitable solvent, to gaseous atoms is accomplished by the nebuliser flame assembly. The nebuhser (from the Latin nebula meaning cloud) creates an aerosol (a fine mist) of the hquid sample which is mixed with an oxidant gas and a fuel gas (to support the flame combustion). The mixture is ignited above the burner assembly. The liquid droplets are desolvated, the resulting microcrystals are melted and vaporised and finally the gaseous products are thermally dissociated to produce free atoms. The combustion speed of most flames is such that the conversion from liquid droplet to free atoms must be accomplished within a few milhseconds. 

Electrospray ionisation is achieved at atmospheric pressure, the mass analyser, however, operates under high vacuum. A special interface is therefore necessary to transfer the ions from the ionisation chamber into the mass spectrometer. A schematic of such an interface is shown in Fig. 4.14. Usually a zone of intermediate pressure separates the ionisation chamber and the mass analyser. The liquid sample together with a curtain or nebulising gas is introduced into the heated ionisation chamber. An electrospray is generated by applying a potential difference between the needle and the opposite interface plate. A small proportion of the desolvated analyte ions exit the ionisation chamber through a submillimeter orifice and enter the zone of intermediate pressure. The analyte ions then pass via another small orifice into the mass analyser. This is usually a quadrupole which is operated under high vacuum. 

Matrix effects include nebulisation interference, transfer and desolvation interference, chemical or ionisation interference, and atomisation and volatilisation interference. Although the primary indication is a change in the emission intensity, it is often difficult to determine the origin of the interference. 

Poitrasson, F. and Dundas, S.H. (1999) Direct isotope ratio measurement of ultra-trace lead in waters by double focusing inductively coupled plasma mass spectrometry with an ultrasonic nebuliser and a desolvation unit. 

Low frequency noise is below 10 Hz and is largely generated by the sample introduction system (peristaltic pumps, nebulisation). High frequency noise is above 10 Hz and is related to the gas dynamics of the ICP and desolvation in and before the plasma or to interference noise from the AC line voltage. White noise represents the background in a noise amplitude spectrum and occurs for all possible frequencies. A noise amplitude spectrum shows the frequency composition of the noise for the entire spectrometric system (noise amplitude versus frequency). Other fluctuations can be drift effects, which affect sensitivity and mass discrimination. 

NEBULISERS, SPRAY CHAMBERS AND DESOLVATION SYSTEMS - OVERVIEW... 

In ICP-MS, the desolvation of an aerosol can be beneficial in several cases (i) when using efficient nebulisers such as ultrasonic nebulisers or thermospray devices, (ii) when analysing samples containing organic solvents, (hi) when an increase in the sensitivity is required and (iv) when trying to remove polyatomic interferences. 

With the standard sample introduction system, consisting of a pneumatic nebuliser and spray chamber, only droplets < 10 xm in diameter are permitted to reach the ICP. This selection results in an analyte introduction efficiency of only 1-2%, but is required to maintain plasma stability and ensure efficient desolvation, atomisation and ionisation in the ICP. Additionally, the total concentration of dissolved solids is usually limited to a maximum of 2 g/L only, to prevent clogging of the nebuliser, torch injector tube and/or sampling cone and skimmer orifices, and also to limit signal suppression and long-lasting memory effects. Of course, when using ETV for sample... 

Kim et alP described a sample pretreatment system for online determination of Pu using magnetic sector-ICP-MS with isotope dilution. They used a computer-controlled online sample preparation system (the PrepLab, Thermo Electron Corp., Winsford, UK) with Sr-Spec and TEVA-Spec resin micro-columns for Pu preconcentration. These resins, obtained from Eichrom Industries Inc. (Darien, Illinois, USA), were highly selective for radionuclide elements. To reduce the effect of the U H interference on Pu, a microconcentric nebuliser with integrated desolvation system (the MCN 6000, Cetac, Omaha, Nebraska, USA) was used as the sample introduction method. Using this approach, the authors were able to determine less than 10 pg of Pu in soil samples. 

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