Solutions nebulisers

Light collection from the plasma (in the transversal or longitudinal direction) is carried out as a function of the element depending on whether an ionic or atomic line is used for measurement. The temperature varies from 9 000 to 2 000 K between the bottom and the top of the torch. It is colder in the axis and at the solution nebulisation site. 

To fully understand the limitations of practical sample introduction systems, it is necessary to reverse the normal train of thought which tends to flow in the direction of sample, i.e. solution-nebuliser-spray chamber-atomiser, and consider the sequence from the opposite direction. Looking at sample introduction from the viewpoint of the atomiser, the choice of procedure will cling on to what the atomiser can accept. Different properties of temperature, chemical composition, solvent(s), interferences, etc., and an introduction procedure must be selected that will result in rapid breakdown of species in the atomiser irrespective of the sample matrix. 

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 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. 

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 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],... 

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. 

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... 

Hyphenation in capillary electrophoresis is still in its infancy. Critical aspects of CE hyphenation include the minute volumes of sample injected (typically a few nL) and small flow-rates (in the order of nLmin-1). Interfaces are not commercially available. CZE-UV can be used for the analysis of higher polyamide oligomers in HF1P solution [859]. A solvent elimination design with nebuliser has been described for CE-FTIR and CEC-FTIR coupling absolute detection limits are hundreds of pg [860]. An advantage of CE-FTIR is that analytes may be detected and identified without derivatisation. CE(C)-NMR [861-863] is advancing rapidly. 

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. 

Salbutamol is a selective (32 agonist that may be administered by inhalation from either a pressurised aerosol delivering 100 pg per puff (standard dose 1-2 puffs) or a powder inhaler (metered dose 500 pg). The duration of action is 4-6 hours. In patients unable to use a pressurised aerosol, salbutamol-containing solution may be nebulised in a stream of oxygen using a specially designed face mask. Similarly, salbutamol-containing solution can be nebulised and introduced into the inspiratory limb of a mechanical ventilation system. 

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. 

For a given ICP-OES instrument, the intensity of an analyte line is a complex function of several factors. Some adjustable parameters that affect the ICP source are the radiofrequency power coupled into the plasma (usually about 1 kW), the gas flow rates, the observation height in the lateral-viewing mode and the solution uptake rate of the nebuliser. Many of these factors interact in a complex fashion and their combined effects are different for dissimilar spectral lines. The selection of an appropriate combination of these factors is of critical importance in ICP-OES. This issue will be addressed in Chapter 2, where experimental designs and optimisation procedures will be discussed. Many examples related to ICP and other atomic spectrometric techniques will be presented. 

Physical (transport) interferences. This source of interference is particularly important in all nebulisation-based methods because the liquid sample must be aspirated and transported reproducibly. Changes in the solvent, viscosity, density and surface tension of the aspirated solutions will affect the final efficiency of the nebulisation and transport processes and will modify the final density of analyte atoms in the atomiser. 

The thermal device used to elevate the temperature consists of a burner fed with a gaseous combustible mixture or, alternatively, in atomic absorption, by a small electric oven that contains a graphite rod resistor heated by the Joule effect. In the former, an aqueous solution of the sample is nebulised into the flame where atomisation takes place. In the latter, the sample is deposited on the graphite rod. In both methods, the atomic gas generated is located in the optical path of the instrument. 

Instruments that have burners and require nebulisation of dilute aqueous sample solutions generally have low background noise in the signal. With graphite furnaces, incomplete atomisation of the solid sample at elevated temperatures can produce interfering absorptions. This matrix effect does not exist in an isolated state and thus cannot be eliminated by comparison with a reference beam. This is notably the case for solutions containing particles in suspension, ions that cannot be readily reduced and organic molecules, all of which create a constant absorbance in the interval covered by the monochromator. 

When trace analysis is performed, well-established protocols have to be followed to obtain reliable results. The production of free atoms in the flame or furnace can be modified according to the composition of the matrix in which the sample is found. Therefore, salts and mineral or organic reagents are often introduced in the solutions to be nebulised. 

An atomic fluorescence spectrometric determination of selenium was first reported by Dagnall et al. [185] using a dispersive spectrometer equipped with an air-propane flame, giving a detection limit of 0.25 xg/ml of selenium on aspiration of aqueous solutions using a pneumatic nebuliser. Fluorescence from the 204 nm selenium resonance line was observed when the flame was irradiated by radiation from a selenium electrodeless discharge lamp, the optical axis of which was aligned at 90 °C to the optical axis of the monochromator. 

In this method the soil sample is dried overnight at 85 °C and ground into an homogeneous mixture. A 1 g soil sample is placed into a beaker and 10 ml of concentrated nitric acid added. The solution is heated to dryness and 5 ml of concentrated nitric acid is added. The uranium is redissolved in 5 ml of 8 N nitric acid and diluted to 25 ml with distilled water. The inductively coupled plasma mass spectrometry system used was an ELAN Model 250. The ion source consists of a modified plasma Thermal Model 2500 control box. The forward power was set at 1200 W with the plasma flow, auxiliary flow and nebuliser pressure set at 131/min, 1.0l/min and 0.27 MPa, respectively. The focusing lenses B, El, P and S2 are set at +5.3 V, -12.5 V, -18.0 V and -7.6 V, respectively. The m/z238 ion was monitored for two sec-... 

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