Easily ionized element

The material evaporated by the laser pulse is representative of the composition of the solid, however the ion signals that are actually measured by the mass spectrometer must be interpreted in the light of different ionization efficiencies. A comprehensive model for ion formation from solids under typical LIMS conditions does not exist, but we are able to estimate that under high laser irradiance conditions (>10 W/cm ) the detection limits vary from approximately 1 ppm atomic for easily ionized elements (such as the alkalis, in positive-ion spectroscopy, or the halogens, in negative-ion spectroscopy) to 100—200 ppm atomic for elements with poor ion yields (for example, Zn or As). 

A second type of interference is ionization interference. Certain elements, particularly the alkali metals in high temperature flames, become partially ionized in the flame. This event causes a decrease in the number of neutral atoms and hence, a decrease in the sensitivity. For example, an appreciable fraction of sodium atoms will be ionized. Now if another easily ionized element such as potassium is added to the sodium solution, it will contribute free electrons to the flame and cause the equilibrium for the sodium ionization to shift toward the formation of a larger fraction of neutral atoms. This is, therefore, a positive interference. It can be overcome by adding the same amount of interfering element to the standard solution. Or, more simply, a large amount of an ionizable element such as potassium (200 to 1000 ppm) can be added to both sample and standard solutions this will effectively suppress ionization to a small and constant value and at the same time increase the sensitivity. 

For many elements, the atomization efficiency (the ratio of the number of atoms to the total number of analyte species, atoms, ions and molecules in the flame) is 1, but for others it is less than 1, even for the nitrous oxide-acetylene flame (for example, it is very low for the lanthanides). Even when atoms have been formed they may be lost by compound formation and ionization. The latter is a particular problem for elements on the left of the Periodic Table (e.g. Na Na + e the ion has a noble gas configuration, is difficult to excite and so is lost analytically). Ionization increases exponentially with increase in temperature, such that it must be considered a problem for the alkali, alkaline earth, and rare earth elements and also some others (e g. Al, Ga, In, Sc, Ti, Tl) in the nitrous oxide-acetylene flame. Thus, we observe some self-suppression of ionization at higher concentrations. For trace analysis, an ionization suppressor or buffer consisting of a large excess of an easily ionizable element (e g. caesium or potassium) is added. The excess caesium ionizes in the flame, suppressing ionization (e g. of sodium) by a simple, mass action effect ... 

Differing amounts of easily ionizable elements in real samples cause varying ionization suppression and hence the possibility of interference (see Section 2.4.2). 

Also called vapour-phase interferences or cation enhancement. In the air-acetylene flame, the intensity of rubidium absorption can be doubled by the addition of potassium. This is caused by ionization suppression (see Section 2.2.3), but if uncorrected will lead to substantial positive errors when the samples contain easily ionized elements and the standards do not. An example is when river water containing varying levels of sodium is to be analysed for a lithium tracer, and the standards, containing pure lithium chloride solutions, do not contain any ionization suppressor. 

The capacitatively coupled microwave plasma is formed by coupling a 2450 MHz magnetron, via a coaxial waveguide, to metal plates or a torch where the plasma is formed. Considerable problems have been encountered with this low-cost plasma, particularly from easily ionizable elements which cause dramatic changes in the excitation temperature in the plasma. 

Flame atomization produces ions as well as atoms. Since only atoms are detected, it is important that the ratio of atoms to ions remain constant for the element being analyzed. This ratio is affected by the presence of other elements in the sample matrix. The addition of large amounts of an easily ionized element such as potassium to both the sample and standards helps mask the ionization interference. 

In atomic absorption spectroscopy (AAS) both ionization and chemical interferences may occur. These interferences are caused by other ions in the sample and result in a reduction of the number of neutral atoms in the flame. Ionization interference is avoided by adding a relatively high amount of an easily ionized element to the samples and calibration solutions. For the determination of sodium and potassium, cesium is added. To eliminate chemical interferences from, for example, aluminum and phosphate, lanthanum can be added to the samples and calibration solutions. 

It is relatively easy to overcome ionization interferences. A large excess of an easily ionizable element such as potassium or caesium is added, which maintains the electron concentration constant. The substance added is known as an ionization buffer. Ionization interferences are, as might be expected, substantially worse in the nitrous oxide-acetylene flame than in the air-acetylene flame. It is a common misconception that an ionization buffer totally suppresses determinant ionization, but this is not strictly true. It buffers the degree of ionization at a fixed, reduced level. 

The ICP-AES and ICP-MS techniques may also suffer from matrix effects, such as spray chamber effects caused by the different viscosity of the samples and the calibration standards. The careful choice of internal standards can reduce this problem. The effects caused by high amounts of easily ionized elements may be solved by internal standardization or by the use of matrix-matched calibration curves. An additional specific problem with ICP-AES is the risk of spectral overlaps. 

The presence of species in the flame other than those of the analyte msy alter the emitted intensities of analyte lines through chemicai interactions. Thus, easily ionized elements in hot flames will suppress the ionization of the... 

The worst offenders, as far as ionization interference goes, are the easily ionizable alkali metals. For example, in the determination of calcium, the absorbance is increased when sodium is added to the mixture since the ionization of sodium represses the ionization of calcium. Obviously, the apparent concentration of calcium will increase as the sodium content increases. The customary method of minimizing this effect is to swamp the system with an easily ionizable element. Typically, both samples and standards are prepared so that the flnal solutions will... 

Matrix interferences are often associated with the sample introduction process. For example, pneumatic nebulization can be affected by the dissolved-solids content of the aqueous sample, which affects the uptake rate of the nebulizer and hence the sensitivity of the assay. Matrix effects in the plasma source typically involve the presence of easily ionizable elements (EIEs), e.g. alkali metals, within the plasma source. 

Potassium, rubidium, and cesium possess especially low ionization potentials and at the temperature of the commonly used air-acetylene flame, for instance 30-70% of the total number of these atoms may be ionized (Dll, F6). The degree of ionization of an alkali metal, however, is reduced by the presence of other easily ionized elements. The admixture of such elements affords one means of controlling this type of interference. 

Substances that alter the ionization of the analyte also cause ionization interferences. The presence of an easily ionized element, such as K, can alter the extent of ionization of a less easily ionized element, such as Ca. In flames, relatively large effects can occur unless an easily ionized element is purposely added to the sample in relatively large amounts. These ionization suppressants contain elements such as K, Na, Li, Cs, or Rb. When ionized in the flame, these elements produce electrons, which then shift the ionization equilibrium of the analyte to favor neutral atoms. 

With alkali metal elements the free atom concentrations in the flame can decrease as a result of ionization, which occurs particularly in hot flames. This leads to a decrease of the absorbances for the alkali metal elements. However, it also may lead to false analysis results, as the ionization equilibrium for the analyte element is changed by changes in the concentration of the easily ionized elements. In order to suppress these effects, ionization buffers can be added. The addition of an excess of Cs because of its low ionization potential is most effective for suppressing changes in the ionization of other elements, as it provides for a high electron number density in the flame. 

With the so-called current-free or transferred plasma, the observation zone is situated outside the current-carrying zone. A source such as this can e.g. be realized by the use of a supplementary gas flow directed perpendicular to the direction of the arc current and by the observation zone being in the tail-flame. In this observation zone no current is flowing. This type of plasma reacts significantly on cooling as no power can be delivered to compensate for temperature drops. Therefore, it is fairly insensitive to the addition of easily ionized elements. They do not cause a temperature drop but only shift the ionization equilibrium and give rise to ambipolar diffusion, as discussed previously. 

Accordingly, an over-population of the argon metastable levels would explain both the over-ionization as well as the high electron number density in the ICP. Indeed, it could be accepted that argon metastables act both as ionizing species and at the same time are easily ionized [385]. This could explain the fairly low interferences caused by easily ionized elements and the fact that ion lines are excited very effi-... 

When working with the SCP, the plasma is so well established between the electrodes, that it is possible to insert a sample into the plasma, without moving it away. This opens up the possibility of drying sample aliquots on a quartz rod and then etching them from the rod by inserting it into the plasma (Fig. 102) [435], Both with Ar and with He, for Pb, Mg, Cd and Cu detection limits in the 100-600 pg range are obtained for 20 gL samples, however, with considerable interferences from easily ionized elements. 

For the easily ionized elements working at so-called cool plasma conditions has been shown to be very successful. From the calculation of the degrees of ionization... 

Kalnicki D. J., Fassel V. A. and Kalnicky R. N. (1977) Electron temperatures and electron number densities experienced by analyte species in ICP with and without the presence of an easily ionized element, Appl Spectrosc 31 137-150. 

---