Nebulisers concentric

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. 

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. 

The extension of inductively coupled plasma (ICP) atomic emission spectrometry to seawater analysis has been slow for two major reasons. The first is that the concentrations of almost all trace metals of interest are 1 xg/l or less, below detection limits attainable with conventional pneumatic nebulisation. The second is that the seawater matrix, with some 3.5% dissolved solids, is not compatible with most of the sample introduction systems used with ICP. Thus direct multielemental trace analysis of seawater by ICP-AES is impractical, at least with pneumatic nebulisation. In view of this, a number of alternative strategies can be considered ... 

A. Woller, H. Garraud, J. Boisson, A. M. Dorthe, P. Fodor and O. F. X. Donard, Simultaneuous speciation of redox species of arsenic and selenium using an anion-exchange microbore column coupled with a micro-concentric nebuliser and an inductively coupled plasma mass spectrometer as detector, J. Anal. At. Spectrom., 13, 1998, 141-149. 

Figure 7.4—Light diffusion detector. Using nitrogen gas, the mobile phase is nebulised at the end of the column with a device of varying geometry. When a compound elutes from the column, the droplets under evaporation are transformed into fine particles that can diffuse light from a laser. This is called the Tyndall effect (it is similar to what is observed for a car when its lights are diffused by fog). The signal, detected by a photodiode, is proportional to the concentration of the compound. This detector can only be used for compounds that cannot be vaporised into the gas phase in the heated zone.

The EOF and make-up flow were combined at the exit of the capillary before nebulisation. In this way, no suction was observed and band broadening was not significant. Alkali, alkaline-earth and heavy metal ions were analyzed at concentrations of 2-100... 

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-... 

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-... 

Both Cr111 and Cr concentrations in natural water samples were measured by flame AAS after pre-concentrations of the chromium species on microcolumns packed with activated alumina (acidic form) (Sperling et al., 1992). An FI manifold was used in this work to obtain conditions for species-selective sorption and subsequent elution of the chromium species directly to the nebuliser of the spectrometer. In this procedure, water samples were maintained at a safe pH of 4 prior to analysis. Analytical conditions of pH 2 and 7 were attained by adding buffers on-line only fractions of a second before the corresponding chromium species was sorbed into the column. In this manner, any risk of losses of analytes and/or shifts in equilibria between the species at pH 2 and 7 were minimised. The detection limits were 1.0 and O.Smgdm 3 for Cr111 and Cr, respectively. 

The acid concentration should be as low as possible. The nebuliser fitted as standard may have a stainless steel capillary tube which will be attacked by acid over a period of time. Solutions containing more than 5% mineral acid should be nebulised using a nebuliser having a platinum/iridium capillary. A corrosion-resistant nebuliser having a plastic throat is required for solutions containing hydrofluoric acid. Additionally, in this instance, a teflon impact bead must replace the standard glass one where these are employed. 

This is the simplest technique but is prone to contamination or element loss by evaporation. Also the sample matrix may become too concentrated to pass through the nebuliser and burner without deposition. Since the matrix is also concentrated the final sample aspirated may suffer from matrix interference. This should be investigated and if necessary a method of standard additions used see section V. 

In general, ferrous alloys are difficult to dissolve with acids so that a fusion, for example with sodium/potassium carbonate, is recommended. The resulting high salt concentration can produce difficulties (viscosity, nebuliser/ burner system), as is discussed in Chapter 3. Once sample solutions are available, there is no difference in analysis from the methods for iron and steel. 

Clearly the most popular separation and preconcentration technique for atomic absorption analysis is solvent extraction. In this case it is easy to identify extraction systems which will remove a broad range of impurities from matrices such as acids, bases and alkali metal salt solutions. In addition to the advantages to be gained from separation, especially valuable in furnace work, and the concentration factors available, solvent extraction confers an additional advantage (typically a factor of 3—5) in flame analysis arising from the favourable nebulisation characteristics of several organic solvents. 

To achieve accurate results, it is advisable to minimise the difTcrcnccs in nebulisation flow rate that may be caused by differences in surface tension or viscosity between the standard solutions and the samples. These differences are due. for example, to variations in the total salt content and in acid concentration levels. An increase in the viscosity of the solution leads to a fall in the rate of introduction into the burner and a reduction in absorption. 

There are several types of nebulisers, for example the concentric quartz system (Fig. 3.9) or cross-flow system (Fig. 3.10) the tips of which are made from synthetic material or precious stones. The first system can be subject to blockage with solutions containing as little as 0.1% of dissolved salts due to its small capillary inlet. The second system is less fragile and is resistant to corrosion from solutions containing hydrofluoric acid. 

Figure 3,9 Concentric quartz nebuliser (after G. L. Moore, Introduction to Inductively Coupled Plasma. Atomic Emission Spectrometry, p, M2. 1989 with the permission of Elsevier Science, Amsterdam).

Chemical interference is practically non existent as a result of the high temperature of the plasma. On the other hand, physical interference may be observed. This stems from variations in the sample atomisation speed which is usually due to changes in nebulisation efficiency caused by differences in the physical properties of the solutions. Such effects may be caused by differences in viscosity or vapour tension between the sample solutions and the standards due, for example, to differences in acidity or total salt content. The technique most commonly used to correct this physical interference is the use of internal standards. In this technique a reference element is added at an identical concentration level to all the solutions under analysis, standards, blank and samples. For each element, the ratio of simultaneous measurements of the lines of the element and the internal standard is then determined in order to compensate for any deviation in the response of the plasma. If the internal standard behaves in the same way as the element to be determined, this method can be used to improve the reliability of the result by a factor of 2 to 5. It can also, however, introduce significant errors because not all elements behave in the same way. It is thus necessary to take care when using it. Alternatives to the internal standard method include incorporating the matrix into the standards and the blank, sample dilution, and the standard addition method. 

Melted snow and ice samples are nebulised at atmospheric pressure and transported into the plasma by an Ar flow. Ions formed in the high temperature plasma are extracted by a special interface with two concentric cones which allow ions to be accelerated and separated by a mass spectrometric analyser and finally detected by a secondary electron multiplier. 

Physical interferences are caused by matrix effects, which can change the physical properties of the solution being nebulised. An example of the suppression of calcium by proteins and fats in semm by high concentration of barium, chromium, cobalt and zinc is evident when a concentration of 10 gL 1 of each element is added to the same... 

---