Easily ionisable element

ICP-AES and ICP-MS analyses are hampered in almost all cases by the occurrence of sample matrix effects. The origins of these effects are manifold, and have been traced partly to physical and chemical aerosol modifications inside sample introduction components (nebulisation effects). Matrix effects in ICP-AES may also be attributed to effects in the plasma, resulting from easily ionised elements and spectral background interferences (most important source of systematic errors). Atomic lines are usually more sensitive to matrix effects than are ionic lines. There exist several options to overcome matrix interferences in multi-element analysis by means of ICP-AES/MS, namely ... 

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. 

Note to Table 1 All sensitivities and detection limits were obtained using an air—acetylene or nitrous oxide—acetylene flame unless otherwise noted. Potassium was added to the aqueous standards to suppress ionisation for easily ionised elements. 

An equally important effect arises when an easily ionised element is being determined in the presence of another. There will be an enhancement of sensitivity compared with pure aqueous standards. This arises from the presence of excess free electrons which suppress further ionisation. 

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 design of the DCP-OES allows the use of both aqueous and most non-aqueous solvents, providing standards and samples are prepared under similar conditions. It is more expensive to operate than AAS but cheaper than ICP-OES. The limitation of DCP-OES is the susceptibility to excitation interferences and increased signals from easily ionisable elements (EIEs). It has lower limits of detection and wider linear range for most elements but not as good as ICP-OES. 

Electrolytic solutions used for extra-renal infusion and dialysis contain metal chlorides of Na, K, Ca and Mg salts at concentrations that are critical for effective treatment. These solutions also contain dextrose, citrate and lactate additives as part of this special formulation. The analysis for these metals must be precise and accurate and this can be achieved with ICP-OES using yttrium or scandium as internal standard to correct for matrix affects. The method of standard addition may also be used with similar success but is a more tedious method. The ability to dilute the sample several fold due to the high concentrations of metals reduces/eliminates the effect of EIE (easily ionised elements) caused by other elements in the same solution. The dilution and the ease of detection and corrections with an internal standard using the multi-element capability make this an excellent method. 

Easily ionised elements have many influences ... 

It can thus be concluded that in the presence of an easily ionisable element B in the flame the degree of ionisation of the analyte A is reduced and thereby its absorption is increased. Note, that at higher flame temperatures ionisation is increased which may counterbalance the increase in atomisation. Thus, a hotter flame does not necessarily result in an improved sensitivity of AAS measurements. 

The addition of an easily ionisable element at relatively high concentration as an ionisation buffer allows one to reduce the effect of shifts in ionisation equihbria. The ionisation buffer (often an alkali metal salt such as potassium chloride) creates a high concentration of electrons in the flame, resulting in suppression of the ionisation of the analyte. 

Easily ionisable elements, such as alkali and alkaline earth metal elements, can alter the emission intensity and may cause an enhancement or a depression. This problem is more serious with DCPs, MIPs, and CMPs, while less important with ICPs. Several mechanisms have been suggested to explain these changes, for example lateral diffusion, changes in thermal conductivity, an altered volatihsation rate, ambipolar diffusion, or shifts in the ionisation equilibrium and the collisional processes. 

For conventional analysis by ICP or DCP, liquid samples are used, which are either easily prepared or commercially available. Interference problems are reduced to a minimum if the cahbration solutions are matched to the samples with respect to their content of acids and easily ionisable elements (see above). Calibration curves obtained with sparks, arcs, and laser ablation systems are usually curved so that 8—15 calibration samples or more are needed to define a suitable calibration. In the case of liquid analysis by DCP and ICP, fewer cahbration samples can be used due to the better linearity and dynamic range and absence of selfabsorption effects. With the introduction of hquids, the spray chamber is the major source of flicker noise due to aerosol formation and transport. While shot noise can easily be compensated by longer integration times, the flicker noise is of multiplicative nature so that any element can be used as an internal standard provided that a true simultaneous measurement of the analyte and internal standard line intensity is possible. 

This effect mostly occurs with alkali and alkaline earth metals. The low ionisation potentials of these elements cause them to be readily ionised in the flame with a resultant lowering of the population of ground state atoms and a suppression of sensitivity. The technique used to overcome this is to add an easily ionised salt such as potassium chloride to samples and standards. This ionises in preference to the analyte in the flame and enhances sensitivity. As an example, strontium, barium and aluminium are subject to ionisation in the flame. In water analyses, this is suppressed by adding potassium to the samples and standards so that the solution contains 2 000 mg l-1 potassium. 

The alkali and alkaline earth elements are easily ionised in the flame. It is therefore necessary to add an ionisation suppressant when analysing for these elements. The potassium salt of naphthasulphonic acid or a commercially available potassium standard solution must be added to give a final potassium concentration of approximately 1000/igml-1 in both samples and standards. 

When the plasma is sufficiently energetic, atoms may be ionised. The degree of ionisation depends on the temperature of the plasma and the ionisation energy of the considered element. In particular, for easily ionisable species, ionic spectra also contribute to the emission spectra observed in a plasma to a great extent The ionisation of atoms o of a particular element into ions i with liberation of electrons e is an equilibrium reaction. 

The elements become more reactive down groups 1 and 2. The atoms of the elements lower down the groups lose electrons more easily than those higher up, as illustrated by the trends in ionisation energies of the elements. 

The release of Kr is an indication for the particle failure, because this fission gas is closed up in the coated particles. This release can easily be monitored by means of an ionisation chamber. The fuel elements are crushed and densified until the cylindrical space of the pressure vessel is completely filled. The tests showed that particle failure depends on the coating design and the bum-up. However, the failure fraction observed remained below 1% (see FIG. 11) /4/. Coated particle... 

The alkali metals are easily determined by flame photometric methods which are always less time-consuming than chemical methods and offer the same order of accuracy. The precise procedure used will depend on the type of equipment which is available but certain generalisations can be made which are applicable to all instruments. These elements are easily excited and have relatively low ionisation potentials so a low temperature flame is always indicated and air-coal gas, air-propane and air-hydrogen are most convenient. As the resonance lines appear in the visible region of the spectrum filter instruments can be used so long as spectral interferences are negligible or can be corrected for. If a monochromator instrument is used such corrections are more easily and accurately made. 

We look for a large jump in the value of the ionisation energy. This occurs between the removal of the 6th and 7th electrons. So, six electrons have been removed comparatively easily. The removal of the 7th electron requires about three times the energy required to remove the 6th electron. So, there must be six electrons in the outer shell of X. So, element X must be in Group 16 of the Periodic Table. 

Significance.—(1) A Transport Element.—Chlorides of the biochemical metals are easily soluble in water and readily ionised. Chlorine thus provides a transport partner for the absorption, distribution, and excretion of other radicles. 

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