Ionization elements

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). 

As illustrated in Fig. 3.41, several laser schemes can be used to ionize elements and molecules. Scheme (a) in this figure stands for non-resonant ionization. Because the ionization cross-section is very low, a very high laser intensity is required to saturate the ionization process. Scheme (b) shows a simple single-resonance scheme. This is the simplest but not necessarily the most desirable scheme for resonant post-ionization. Cross-... 

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. 

Isobaric interferences (especially those arising from the plasma itself, e.g., ArO+ on Fe) can be eliminated using cool-plasma conditions, sometimes in combination with a shield torch. This option is not suitable for seawater samples because a cool plasma, in the presence of a heavy matrix, cannot fully ionize elements with high first ionization potentials, notably Zn, Cd, and Hg. Protocols have thus been established for analysis of 10-fold diluted seawater on instalments with sufficiently high resolution to separate most of the affected isotopes from their isobaric interferences [1], To circumvent the issue entirely, others have used online chemical extraction to separate analytes of interest... 

The efficiency of Ar plasma ionization can introduce some unwanted isobars from the production of doubly ionized elements, as well as from the ionization of atmospheric gasses, the Ar plasma gasses, and formation of molecular ions such as metal oxides. Such unwanted molecular isobars can produce difficulties for some elements such as Ca from " Ar+, Fe from " Ar 0+, or Se from ( " Ar " Ar)+. 

In either case the material is passed into a chamber where an ionizing element, often 63Ni, a radioactive isotope that produces /3 particles (electrons), converts the molecules in the chamber to ions, the same technique used in many household smoke detectors. Newer designs sometimes use 241 Am, which decays in a particles and y rays. To avoid the regulatory inconvenience of radioactive material, several electronic ionizing techniques have also been proposed. 

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. 

At this early nebular phase the continuum has dropped and neutral or weakly ionized elements are no more observed, with the exception of 0 I 844.6 nm (probably excited by Hydrogen Lyman coincidence). Lines belonging to the following... 

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. 

One obvious way to improve ionization efficiency is to make sure the sample is as clean as possible. A heated filament provides a constant amount of energy, and any devoted to evaporating or forming ions of contaminant species is lost to the desired process. Sodium, potassium, calcium, and other readily ionized elements are bad actors the fact they are also ubiquitous makes the problem just that much more difficult. Every element presents its own challenges, and much effort has been invested in purifying target elements of interest. Loading a chemically pure sample on the filament is one way to improve ion emission. 

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. 

From Eq. (4.3), wo see that as the pressure is reduced at constant temperature, the dissociation becomes greater, until finally at vanishing pressure the dissociation can become complete, even at ordinary temperatures. This is a result of importance in astrophysics, as has been pointed out by Saha. In the solar atmosphere, there is spectroscopic evidence of the existence of rather highly ionized elements, even though the temperature of the outer layers of the atmosphere is not high enough for us to expect such ionization, at ordinary pressures. However, the pressure in these layers of the sun is extremely small, and for that reason the ionization is abnormally high. 

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. 

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