Nebulisers spray chambers

Several authors [386,387] have discussed the spectroscopic and nonspectroscopic (matrix) interferences in ICP-MS. ICP-MS is more susceptible to nonspectroscopic matrix interferences than ICP-AES [388-390]. Matrix interferences are perceptible by suppression and (sometimes) enhancement of the analyte signal. This enhanced susceptibility has to be related to the use of the mass spectrometer as a detection system. In fact, since both techniques use the same (or comparable) sample introduction systems (nebulisers, spray chambers, etc.) and argon plasmas (torches, generators, etc.), it is reasonable to assume that, as far as these parts are concerned, interferences are comparable. The most severe limitation of ICP-MS consists of polyatomic... 

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. 

An additional problem is that known elements in the periodic table, e.g. boron (B), tungsten (W) and molybdenum (Mo) tend to stick in the transport fine, nebuliser, spray chamber and torch causing memory effects in atomic spectroscopy. Elements that stick cause problems with quantification, and detection Emits and it is important that methods of reducing these are rigorously applied in analysis of these elements. However, memory effects in ICP-OES are not as pronounced as they are with graphite furnaces for refractory elements but they are present to some extent, and must be reduced or removed. 

Analysis of organic compounds using ICP-OES requires higher radio frequency power, a suitable nebuliser spray chamber and solvent resistant pump tubing for transporting the sample solution using a peristaltic pump. 

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

Generally, a liquid sample introduction system suitable for ICP-MS consists of two main components (i) a nebuliser that turns the liquid bulk into an aerosol and (ii) a spray chamber that selects the maximum drop size that will be introduced into the plasma. The most often used nebuliser-spray chamber combination for ICP-MS is depicted in Figure 5.1. It consists of a pneumatic concentric nebuliser coupled to a double pass spray chamber. 

These drawbacks have driven research into and development of new nebuliser/spray chamber systems, as described in the following section. 

In ICP-AES and ICP-MS, sample mineralisation is the Achilles heel. Sample introduction systems for ICP-AES are numerous gas-phase introduction, pneumatic nebulisation (PN), direct-injection nebulisation (DIN), thermal spray, ultrasonic nebulisation (USN), electrothermal vaporisation (ETV) (furnace, cup, filament), hydride generation, electroerosion, laser ablation and direct sample insertion. Atomisation is an essential process in many fields where a dispersion of liquid particles in a gas is required. Pneumatic nebulisation is most commonly used in conjunction with a spray chamber that serves as a droplet separator, allowing droplets with average diameters of typically <10 xm to pass and enter the ICP. Spray chambers, which reduce solvent load and deal with coarse aerosols, should be as small as possible (micro-nebulisation [177]). Direct injection in the plasma torch is feasible [178]. Ultrasonic atomisers are designed to specifically operate from a vibrational energy source [179]. 

The on-line interface of flow manifolds to continuous atomic spectrometric detectors for direct analysis of samples in liquid form typically requires a nebuliser and a spray chamber to produce a well-defined reproducible aerosol, whose small droplets are sent to the atomisation/ionisation system. A variety of nebulisers have been described for FAAS or ICP experiments, including conventional cross-flow, microconcentric or Babington-type pneumatic nebulisers, direct injection nebuliser and ultrasonic nebulisers. As expected, limits of detection have been reported to be generally poorer for the FIA mode than for the continuous mode. 

A further study by the Olesik group [138] used an interface with a laminar flow in the direction of the detector. The interface was a stainless-steel tee with the capillary threaded through the colinear ends of the tee. A sheath electrolyte was delivered through the lower arm of the tee with a peristaltic pump. Both a high efficiency nebuliser (HEN) and a concentric glass nebuliser were used in the study the former was used with a conical spray chamber and the latter with a Scott double-pass spray chamber. Increasing the sheath electrolyte flow-rate enabled the laminar flow to be eliminated, therefore improv-... 

There are several different types of nebulisers available from local instrument suppliers (Figure 2.9). They are expensive due to the inert material used and precise engineering required to make them. The size of the hole for the gas outlet must be big enough to sustain the very high pressure required to force the sample solution to move violently and rapidly throughout the spray chamber and small enough to create a very high pressure. The two most commonly used nebulisers are pneumatic and ultrasonic. 

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 sample transport system, nebuliser and spray chamber are designed to ensure the maximum amount of sample reaches the atomisation source without quenching it. Only a few solvents can be used that are compatible with direct injection to ICP-OES (see Table 3.5) and these solvents have been studied as part of nebulisation efficiency. 

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. 

---