Ionisation interference

Fig. 2.5.12. APCI-FIA-MS(+) overview spectra of industrial surfactant blends used as pure blends or mixtures in the examination of ionisation interferences, (a) C13-AE, (b) cationic (alkyl benzyl dimethyl ammonium quat) surfactant, (c) amphoteric C12-alkylamido betaine, and (d) non-ionic FADA all recorded from methanolic solutions. 

Obstacles of ionisation interferences in the quantitative determination of the N-containing surfactants from industrial blends dissolved together with AE compounds in methanol applying FIA-MS can be minimised or even eliminated if quantification was performed in the standard addition mode. So the standard deviations (SD) observed now reached a maximum of 7% for N-containing compounds whereas AE could be quantified with a SD of 4%. In parallel, the time investment for FIA-MS quantification in the standard addition mode, however, increased considerably and reached a factor of 3-4. 

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. 

Interferences encountered in AAS can be separated into the following categories (A) Spectral. (B) Flame emission. (C) Chemical. (D) Matrix. (E) Non-specific scatter. (F) Ionisation. The majority of difficulties that the analyst can expect to encounter arise from chemical, matrix, light-scattering and ionisation interferences. 

To understand ionisation interferences, it is necessary to appreciate what is occurring in the flame during the aspiration of a sample. The flame is being used as a source of energy to convert elements in the solution droplets created by the nebuliser into ground-state atoms. 

Many determinations require the use of the nitrous oxide—acetylene flame and it is usually under these conditions that ionisation interferences occur. They arise from the energetic nature of the flame which gives ground-state atoms but also excites some atoms to such an extent that one or more electrons are lost and ionisation occurs. 

For strontium determinations samples, standards and blanks are made to 1% (m/v) in lanthanum to prevent the formation of refractory compounds. Analyses for strontium also suffer from ionisation interferences which are discussed later. 

When the energy supplied is sufficient to excite and ionise some of the atoms, the atomisation process docs not necessarily come to an end with the atoms in their ground state. Ions absorb at different wavelengths so the atomic absorption at the resonance wavelength is reduced. Consequently, when an excessive amount of energy eliminates atoms in their ground state, there is ionisation interference. 

Ionisation interference is normally eliminated by adding a large excess of a more easily ionisable element, which creates a large quantity of free electrons in the flame and eliminates... 

The sample and reference solutions have the same concentration level of HCI with the addition of an excess of caesium (I 000 mg/1) to correct ionisation interference. 

The method of standard additions will correct for physical and minor chemical interferences which are independent of concentration (i.e., interferences which influence the slope of the cahbration plot only). Concentration dependent chemical interferences and ionisation interferences cannot be ehminated since both influences result in cmwed cahbration plots (which are difficult to extrapolate back to zero concentration). Additionally, this cahbration technique cannot correct for background interferences since the two components of the net signal (true analjde absorbance and interference) cannot be separated. A deuterium background correction system can be used in combination with this technique to correct for background interferences. 

Ionisation interferences result when moderate (0.1-1% m/m) amounts of a matrix ion change the analyte signal. This effect, which usually reduces the analyte signal, also is known as suppression. The use of an adapted internal standard is suitable to correct for this type of suppression. 

P.M. Radmore, P.L. Knight, Two-photon ionisation Interference and population trapping, Phys. Lett. A 102 (1984) 180. 

Matrix effects include nebulisation interference, transfer and desolvation interference, chemical or ionisation interference, and atomisation and volatilisation interference. Although the primary indication is a change in the emission intensity, it is often difficult to determine the origin of the interference. 

Metal content was measured by atomic absorption spectrophotometry using a Pye-Unicam SP9 spectrophotometer. Samples (0.05 g) were ashed for 6 hours at 600 °C and the residue dissolved in aqua regia. Any remaining carbon residue was filtered off and the volume made up to 100 cm with hydrochloric acid (0.1 mol dm ) containing lanthanum chloride (equivalent to 1% w/v La) to control ionisation interference. 

Another type of interference that can arise in the atomiser is called ionisation interference . Particularly when using hot atomisers, the loss of the 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... 

To prevent ionisation interferences, cesium is added to act as an ionisation buffer. 

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