Atomisation Devices

For multi-element analysis, the significantly more energetic plasma sources are therefore by far superior to flames in most regards. Today, flames are only used for the determination of alkali metals, as these can be excited at low temperatures and give simple spectra free of interferences. The determination of these metals is especially important for the analysis of biological fluids, so that emission in acetylene-air flames is still routinely used in highly automated, and simplified systems based on single or multiple interference filters and photomultiplier detection (see Fig. 12.23). The systems often also include automatic addition of an internal standard and dilution [39]. 

The sample enters the plasma as an aerosol through the irmer tube at a flow rate of about 1 L min and has a residence time of about 2 ms in the plasma at temperatures between 4000 and 8000 K after which it enters the observation zone above the core. The carrier flow shapes the plasma into the characteristic toroidal form. The intermediate gas flow is optional and can be employed for example for the analysis of samples in organic solvents to prevent soot deposition on the torch. 

The nebuUser or carrier gas flow affects not only the residence time in the plasma and the plasma conditions, but also the size of the aerosols produced. 

With conventional nebuhsers, the aerosol size increases at a low nebuhser gas flow, reducing the transport efficiency and decreasing the emission from all lines. However, lower flow rates also increase the residence time and the excitation temperature enhancing the emission of ionic lines. For atomic lines, the excitation is improved either by the increased residence time. On the other hand also the ionisation rate is increased, resulting in a net decrease of atomic hne emission. 

CMPs are operated at higher powers (0.5—3 kW) than MIPs, while the same frequency is used. A waveguide system transfers the energy into a plasma tube, where the plasma is ignited via a spark discharge. While excitation temperatures between 4900 and 8200 K, similar to MIPs, are achieved, gas temperatures between 4500 and 5700 K can be reached due to the higher power, so that CMPs are better suited for direct solution analysis. 

---

Figure 14.9—Thermoelectric atomisation device, a) Graphite furnace heated by the Joule effect b) example of a graphite rod c) temperature program as a function of time showing the absorption signal. The first two steps of this temperature program are conducted under an inert atmosphere (argon scan).

Besides plasmas, which are at the forefront of thermal atomisation devices, other excitation processes can be used. These methods rely on sparks or electrical arcs. They are less sensitive and take longer to use than methods that operate with samples in solution. These excitation techniques, with low throughputs, are mostly used in semi-quantitative analysis in industry (Fig. 15.2). Compared to the plasma torch, thermal homogeneity in these techniques is more difficult to master. 

The following are general conditions for the electrothermal atomisation device. 

The most common technique for the determination of mercury in environmental samples is cold vapour atomic absorption spectrometry (CV-AAS) due to its simplicity and sensitivity. The flameless procedure was investigated by Hatch and Ott (1968) with a view to simplifying the apparatus required and improving the sensitivity of the method. The method is based on the unique properties of mercury. Elemental mercury has an appreciable vapour pressure at ambient temperature and the vapour is stable and monatomic. Mercury can easily be reduced to metal from its compounds. The mercury vapour may be introduced into a stream of an inert gas and measured by atomic absorption or atomic fluorescence of the cold vapour without the need of atomiser devices. 

Aqueous solutions of calcium acetate and diammonium hydrogenphosphate were used together with nitric acid and ethanol to provide additional energy to the flame required for chemical reaction. The precursor solution was injected via an atomisation device into the propane combustion pilot flame. The atomised reactive solution produced a second flame, called main flame, owing to the combustion of the ethanol-containing solution. The energy of the main flame provides the energy required for the chemical reaction, that is dissociation of... 

When a nasal spray is supplied in a squeeze bottle instead of one with a pump atomizer, a viscous spray might be difficult to nebulise. So, in the design of a spray formulation, viscosity has to be considered together with the intended type of container and atomising device. 

The performance of a spray dryer or reaction system is critically dependent on the drop size produced by the atomiser and the manner in which the gaseous medium mixes with the drops. In this context an atomiser is defined as a device which causes liquid to be disintegrated into drops lying within a specified size range, and which controls their spatial distribution. 

Concerning AFS, the atomiser can be a flame, plasma, electrothermal device or a special-purpose atomiser e.g. a heated quartz cell). Nowadays, commercially available equipment in AFS is simple and compact, specifically configured for particular applications e.g. determination of mercury, arsenic, selenium, tellurium, antimony and bismuth). Therefore, particular details about the components of the instrumentation used in AFS will not be given in this chapter. 

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. 

Thermoelectric atomisation. A flameless device without nebulisation, known as a graphite furnace, uses a graphite rod with a cavity that can hold a precise quantity of sample (a few mg or pi deposited using an automatic syringe). This rod, oriented parallel to the optical axis, is heated according to a four-step cycle (Fig. 14.9). The atomisation period is relatively short. 

Figure 14.16—Elements determined by AAS or FES. Most elements can be determined by atomic-absorption or flame emission using one of the available atomisation modes (burner, graphite furnace or hydride formation). Sensitivity varies enormously from one element to another. The representation above shows the elements in their periodic classification in order to show the wide use of these methods. Some of the lighter elements, C, N, O, F, etc. in the figure can be determined using a high temperature thermal source a plasma torch, in association with a spcctropholometric device (ICP-AbS) or a mass spectrometer (1CP-MS).

If the sample is a conductor, it is possible to use it as the cathode of a spectral lamp whose principle of operation is identical to that described for hollow cathode lamps (cf. section 14.5 and Fig. 15.3). The device must be sealed before it can be used, which represents a technical constraint. The advantage of this process, which is commonly used for surface analysis, is that it produces spectra with narrow emission lines because atomisation is made at a lower temperature than with the electrical arc method. 

Using a solution-spray technique,124 an aqueous solution of HAuCU and titanium tetrachloride was atomised by an ultrasonic device to produce a mist without separation of the components this was then calcined, and the fine particles collected on a glass filter at the outlet. Samples of 1 wt.% Au/TiC>2 contained 4 nm particles when the spray reaction temperature was... 

The technology is now available for many more instrument functions to be selected both on the main instrument and peripheral devices such as an electrothermal atomiser or autosampler. Programmes for instrument setting and data processing can be stored, for example, on magnetic cards. Although, as already indicated, the actual speed of analysis may not be vastly improved, the advantages lie in the better reliability and accuracy obtainable and in the possibility of more efficient use of the time of a skilled analyst. 

The dead-space above the sample in the dart-like tip is much reduced by the use of the type of micropipette which uses fine capillary tips. The Oxford Ultramicro-sampler is an example. Use of this device completely overcomes the solvent expulsion problem. The principal disadvantage from the point of view of electrothermal atomisation with a graphite furnace is that its maximum capacity is 5 pi. This may be too little for the sensitivity of some elements. The tip material provided with this syringe is still prone to droplet formation, but this can be replaced by PTFE capillary tubing of the correct dimensions, e.g. Polypenco size TW 24, which appears to overcome the problem completely. Good precision should be attainable with tips made from this material, provided the tips are cut across at 90° to the tube axis. Chamfered tips give rise to a variable position of the meniscus with consequent loss of reproducibility. 

Electrothermal atomisers fall into two classes, i.e. filaments and furnaces. The former category includes all devices in which an electrically heated filament, rod, strip or boat is used and where the atomic vapour passes into an unconfined volume above the viewing area on the other hand, furnaces usually consist of an electrically heated carbon tube into which the sample is injected. The optical axis of the hollow-cathode lamp light beam passes through the centre of this tube. Electrothermal atomisers are connected to a programmable power supply such that the sample can be dried, ashed and atomised at preset temperatures and times. 

Electrothermal atomisers come in many shapes and sizes, ranging from tube furnaces to metal ribbons. At present, the most popular form is the graphite tube furnace of which a number of designs are commercially available. The electrothermal devices vary markedly in their atomisation characteristics and a method suitable for one design will not necessarily work on another without some modification. 

It is usual with electrothermal atomisers to pipette between 5 and 100 pi samples into the device using a micropipette. With petroleum samples dissolved in organic solvents this may be a problem. Due to the low surface tension of many of these solvents they do not pipette easily and often dry irregularly in the atomiser, both factors giving rise to poor reproducibility. The problem of poor drying characteristics may be overcome with many solvents by pipetting into a pre-heated atomiser at approximately 80°C. The solvent is removed immediately, leaving the analyte on a reproducible spot each time. This technique, however, requires some care so as not to melt or contaminate the pipette tip. 

Sample contamination from sampling apparatus, storage media or by introduction of foreign elements during analytical procedures must be given consideration. This is a particular pro Hem in those determinations carried out using electrothermal atomisers, due to the high sensitivity of these devices. 

Standards for the analysis may be prepared from organometallic standards, analysed samples or the NBS (GM-5) Heavy Oil Standard. The most satisfactory results are likely to be obtained using the second or third options. The sensitivity available is critically dependent on the electrothermal device to be used. This and the size of aliquot chosen for injection into the atomiser (normally 5—100 pi) will determine the selection of the concentration ranges chosen for the standards. Refer to the manufacturers information on Ni and V sensitivity and linear range and prepare calibration standards accordingly. Always prepare a blank solution and at least three standards to cover the chosen range. 

Instead of flames, atomic absorption spectrometers sometimes employ graphite furnaces or (relatively speaking) cold quartz tubes as atomisers these devices are not normally required for the purpose of qualitative analysis (unless the volume of sample available is very small), and will not be discussed here. 

Pipetting can release up to 10 000 droplets of a diameter of 1-10 pm. The expulsion of liquid from nozzles is used routinely in devices such as the Collison spray and in two fluid atomisers as a means of generating fine aerosol clouds. 

To avoid contamination with nasal Uquid, the squeeze bottle should be kept pressed in until it is removed from the nose. This is not needed with the pump atomiser. Both devices have to be cleaned by rinsing or wiping. 

---