Ionisation suppressor

Another type of interference that can arise in the atomiser is called ionisation interferences . Particularly when using hot atomisers, the loss of an electron from the neutral atom in metals with low ionisation energy may occur, thus reducing the free atom population (hence the sensitivity of the analyte determination, for which an atomic line is used, is reduced). These interferences can be suppressed in flames by adding a so-called ionisation suppressor to the sample solution. This consists in adding another element which provides a great excess of electrons in the flame (he. another easily ionisable element). In this way, the ionisation equilibrium is forced to the recombination of the ion with the electron to form the metal atom. Well-known examples of such buffering compounds are salts of Cs and La widely used in the determination of Na, K and Ca by FAAS or flame OES. 

For the majority of elements commonly determined in water by AAS, an air—acetylene flame (2300°C) is sufficient for their atomisation. However, a number of elements are refractory and they require a hotter flame to promote their atomisation. Because of this, a nitrous oxide—acetylene flame (3000° C) is used for the determination of these elements. Refractory elements routinely determined in water are aluminium, barium, beryllium, chromium and molybdenum. Chromium shows different absorbances for chromium(III) and chromium(VI) in an air-acetylene flame [15] but use of a nitrous oxide-acetylene flame overcomes this. Barium, being an alkaline earth metal, ionises in a nitrous oxide—acetylene flame, giving reduced absorption of radiation by ground state atoms, however in this case an ionisation suppressor such as potassium should be added to samples, standards and blanks. 

Weigh out accurately 0.7 g of dried cement into a 250 cm beaker. Disperse in water, add 10 cm cone. HCl and 150 cm hot water. Bring to the boil and keep hot for 5 minutes. Cool and transfer quantitatively to a 250 cm volumetric flask and make up to the marie. Dissolve in a 100 cm aliquot 0.191 g purest KCI and in another 100 cm aliquot 0.254 g purest NaCl. These chlorides are added as ionisation suppressors. Use the atomic spectrophotometer in the emission mode and measure the readings for the two solutions, using the manufacturer s recommended operating conditions. Use calibration standards to find [K] and [Na]. Calculate from the results of spectrophotometry the percentages of oxides of the elements determined. 

The ESI-MS is nowadays the most commonly used LC-MS coupling device for the analysis of LAS and SPC. But also for the analysis of these anionic analytes the most serious drawbacks of this ionisation technique are matrix effects. A crucial role is played by mobile phases containing relatively high salt concentrations or ion-pair reagents that lead to signal instability or even, in some cases, to plugging of the orifice plate. For successful removal of alkaline salts, a suppressor was incorporated between the LC column and the mass spectrometer [24]. With this set-up it was possible to reduce a spiked sodium concentration of 15 mg L-1 in the HPLC eluent before the suppressor by more than 99.8% at the entry into the MS. 

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