Ionic lines

Different analytical techniques are used for detection of the elemental composition of the solid samples. The simplest is direct detection of emission from the plasma of the ablated material formed above a sample surface. This technique is generally referred to as LIBS or LIPS (laser induced breakdown/plasma spectroscopy). Strong continuous background radiation from the hot plasma plume does not enable detection of atomic and ionic lines of specific elements during the first few hundred nanoseconds of plasma evolution. One can achieve a reasonable signal-to-noise ra-... 

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 ... 

A small quantity of sample is vaporised as an aerosol when submitted to sparks generated under argon or helium between 20 and 50 keV. The high current density under an inert atmosphere creates an atomic gas that can be excited by collisions in the plasma. Spark-induced optical emission leads to ionic line spectra. 

Very large statistics - all available atomic and ionic lines, different frequency ranges, different redshifts (epoch/distances). 

Self-absorption, which has a critical influence on AES sensitivity, could not be accurately quantified by using an equation relating the emission intensity of a resonance line affected by this phenomenon to the sputtering rate of the analyte concentration in the matrix [196]. Subsequent studies revealed that, for Cu and V, ionic lines are more sensitive than atomic lines — the latter proved independent of self-absorption but were strongly influenced by the type of plasma used (Ar or Ne) [197]. 

From the above it can be concluded that there exists for each spectral line emitted by a radiation (plasma) source an optimum temperature at which its emission intensity reaches a maximum. This so-called standard temperature depends on the energy of ionisation and excitation of this element, and on the electron pressure and temperature in the plasma. The standard temperatures for many atom lines are around 4000 K, while the standard temperature of ionic lines is often around 10000 K. 

With conventional nebuhsers, the aerosol size increases at a low nebuhser gas flow, reducing the transport efficiency and decreasing the emission from all lines. However, lower flow rates also increase the residence time and the excitation temperature enhancing the emission of ionic lines. For atomic lines, the excitation is improved either by the increased residence time. On the other hand also the ionisation rate is increased, resulting in a net decrease of atomic hne emission. 

When the sum of the metal analyte s ionization and excitation energies is below the ionization potential for Ar (i.e., 16 eV), ionic lines become the most sensitive. The following table summarizes those elements that yield the... 

Elements whose ionic lines are most sensitive... 

Elements whose atomic and ionic lines have similar sensitivities... 

The dissociation of molecular plasma gases or analyte molecules within the radiation source is an equilibrium reaction. Accordingly, highly stable radicals or molecules are always present in a radiation source. They emit molecular bands which are superimposed on the atomic and ionic line spectra in the emission spectrum. Radicals and molecules may also give rise to cluster ions which may be detected in mass spectra. Common species in plasma gases are CN, NH, NO, OH. and Nt (or N2). From the analytes, highly stable oxides may persist (e.g.. AlO, TiO", YO ). A thorough treatment of molecular spectra is available [17], [18]. 

Quantitative AES. In quantitative AES, the intensity of an elemental atomic or ionic line is used as analytical signal. As AES is a relative method, calibration has to be performed. 

Accordingly, an overpopulation of the argon metastable levels would explain both the overionization as well as the high electron number density in the ICP. Indeed, it may be that argon metastables act as ionizers, but at the same time are easily ionized [236]. This could explain the rather low interference caused by easily ionized elements, and the fact that ionic lines are excited very efficiently, despite the fact that their standard temperatures are much higher than the plasma temperatures. However, discrepancies are not encountered to such a great extent when the temperatures from the Saha equation are used in the calculations [237]. Nevertheless, a number of processes may be considered to predominate in well-defined zones of the plasma, as indicated by spatially resolved measurements of various plasma parameters [238]. 

The nebulizer gas flow and power, and the observation height are also important parameters to optimize. Such multivariate optimizations can be done by single-factor studies or Simplex optimization [240], [241]. The nebulizer gas influences the aerosol droplet size, nebulizer efficiency, and plasma temperature. For each element and line there is a rather sharp optimum for the aerosol gas flow where the SBR is maximum. The power determines mainly the plasma volume and is optimum for soft lines (atomic and ionic lines of elements with low ionization potential) at rather... 

The detection limits for most elements are at the 0.05 ng/mL (Mg, Ca, etc.) to lOng/mL level (As, Th, etc.). Especially for elements which have refractory oxides (Cr, Nb, Ta, Be. rare earths, etc.), fairly low ionization potential, and sensitive ionic lines, the detection limits are much lower than in AAS. For P, S, N, O, and F, the most sensitive lines are at vacuum wavelengths. 

The first stage is actually characterized by a broad emission originating from the Bremsstrahlung of the free electrons and electron-ion recombination and it has duration of a few hundred nanoseconds. Weak lines show up on the strong continuum and they are mostly identified as ionic lines of the plume constituents. This time domain was considered as not suitable for analytical applications and minimum delay time of 100-500 ns was recommended to remove the Bremsstrahlung. 

At the later end of temporal plasma history, after long delay times very important analytical information was revealed. For example, spectra from diatomic molecules of halogens and alkali-earth elements were detected after a 25 ps delay these spectra enable fluorine and chlorine detection much more easily than the traditional F and Cl ionic lines. After a delay of more than 100 ps, sometimes plasma-induced luminescence was detected it is very effective for rare-earth elements identification. 

Fig. 9.1 Highly resolved LIBS spectra of glassy and CaMo04 powellite crystalline phases of a Mo-rich borosilicate glass-ceramic in the near UV range (368-398 nm) atomic lines are labeled with I and ionic lines are labeled with 11 ...

As in inductively coupled plasma optical emission (ICP-OES) spectra, in addition to atomic lines, intense ionic lines are also observed, the use of an ICP as an ion source for MS seemed logical, but overcoming the difference in pressure between the ICP (generated at atmospheric pressure) and the mass spectrometer (10 —10 mbar) proved difficult and had to be accomplished via the use of a two-cone interface. Despite the advantages that double-focusing sector field mass spectrometers (higher mass resolution) and TOP analyzers (high data acquisition speed) can offer, approximately 90% of the ICP-MS units used worldwide are equipped with a quadrupole filter for mass analysis. 

Types of transitions The terminology for X-ray transitions in ions is the same as that of optical transitions in an atom. The three main types of transitions observed in an ionic line spectrum are 1) resonance transitions, 2) intercombination transitions, and 3) satellite transitions. 

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