Polyatomic ions: theoretical aspects of excited-state properties calculation
Firstly, let us discuss what mass spectrometers with double focusing are and what problems exactly mass spectrometry with inductively coupled plasma experiences in connection with polyatomic ions. Inductively coupled plasma mass spectrometry (ICP/MS, ICP/MS) has developed into one of the most successful techniques in atomic spectroscopy due to its high sensitivity and the possibility of providing the multi-element analysis. However, from the very first days of the existence of ICP/MS, the Achilles feet of this method was a large number of spectroscopic and non-spectroscopic interferences that limited its analytical advantages. Spectroscopists have considered many methods of reducing the effects of these interferences, however, none of them was able to solve the entire problem. All of these methods either have limited impact on some specific interference or are applicable only to a limited selection of items. The only common method of overcoming the limitations caused by spectroscopic interference is mass spectrometry of high resolution, which necessarily requires the use of double-focusing instruments that combine magnetic and electrostatic analyzers. This is determined by the main difference from the low-resolution devices, which cost more than a cheap and simple quadrupole analyzer. Although ICP/MS high-resolution instruments have been appearing on the market of analytical equipment since 1988, they have not been widely adopted because of their high cost. More recently, the price of this equipment has fallen dramatically with the advent of second-generation devices. Excellent performance of these devices have given a significant impetus to the development of analytical applications of mass spectrometry with high resolution and inductively coupled plasma.
What spectroscopic interference truly is? Since the beginning of the use of inductively coupled plasma for elemental analysis, the ionization method has evolved into the most successful of all approaches tested. Firstly, it was used as the method of excitation coupled with emission spectroscopy. In the last 15 years, it is widely used as a source of ions for mass spectrometry. Certain technical problems associated with the selection of polyatomic ions from plasma have been successfully resolved nowadays and the combination of the source of inductively coupled plasma with mass spectrometer begins to spread widely. In inductively coupled plasma polyatomic ions are generated at the atmospheric pressure while a mass spectrometer usually operates at a pressure less than 10-5 mbar. Between ISP and MS spectroscopists use the interface as a bottleneck through which ions are drawn from the plasma and the differential pressure is carried out. At the beginning of the development of ICP/MS elongated spout hole with the diameter of only 50-70 mm and cooled with water was simply used as the interface. A problem with this design lies in the fact that cold boundary layers in front of the cone contribute to the generation of large amounts of extraneous polyatomic ions. This problem has been overcome by increasing the diameter of the inlet to 1 mm so that boundary layers are pushed off and ions enter into the mass spectrometer directly from the plasma. This technique is known as a continuous selection of the sample and, therefore, the cone is called the sample cone.
Polyatomic ions and spectroscopic interferences
There are, mainly, two variations of spectroscopic interferences that are important for a researcher, namely:
- Spectroscopic interference that is caused by molecular or polyatomic ions having the same nominal mass as the isotope of the element analyzed. The resulting interfering signal can distort or completely block a true analytical signal, thus determining the accuracy as well as the element detection limit. The sources from which comes interferences are unlimited. So far there is not any uniform model for the explanation of all the factors contributing to the interference, however the fact that the interface plays an important role in the emergence of molecular interferences is recognized everywhere and by virtually all scientists
- The spectroscopic interferences can be subdivided into isobaric atomic ions, multiply charged ions, polyatomic ions imposing large signals and polyhydric ions of different origin. Isobar overlaps exist because the different isotopes of the same elements coincide in their nominal masses. For each element, except for Indium, there can be found at least one isotope-free isobaric overlay, but the trouble is that these isotopes are not the most intense. Multiply charged ions are in the mass spectrum according to the m/z value. The contribution to the mass spectrum is given, mainly, by doubly charged ions, which are main components of the matrix and multiply charged ions generated in the charge process with argon. The signals coming from neighboring ions with very high intensity (for example, from those originating from the elements of the matrix) make a significant distortion in the analytical signal by imposing tails on neighboring peaks each time when isotopic sensitivity is insufficient. Polyhydric ions may consist of argon atoms and impurities plus solvent and template components.
Of all of these different groups of spectroscopic interferences polyhydric or polyatomic ions create the most serious problems. The interference of polyatomic ions can be caused by the analyte. For example, oxides can remain undisturbed after passing through the hot zone of plasma due to the fact that the energy gap of connections is very large. They can be introduced as impurities on the chemical sample preparation processes or from a plasma gas and the air entrained by plasma. In principle, this type of spectroscopic interferences can be separated from the analyzed isotope using high-resolution mass spectrometry.
Polyatomic ions and their impact on resolution
One of the most widely discussed examples of the spectral interference is 56Fe and 40Ar16O+. The latter occurs due to an interaction of an argon ion with oxygen contained in the solvent. In this example, as an alternative to the measurement of iron, we can use isotopes 54Fe, 57Fe and 58Fe, although 58Fe is superimposed by isobaric interference from the 58Ni isotope. At the same time, there are other interferences, such as 40Ar14N+ or 40Ar16O1H+, which makes 57Fe the best alternative choice. However, its natural prevalence equals only 2.2% and the detection limit for this isotope is very bad when using a low-resolution instrument. Thus, resolution of less than 2500 is sufficient to separate the analyte from the spectral interference of isotope with the mass m/z= 56. Even more problematic is the analysis of 75As in the case where chlorine is present in the sample. Arsenic is a monoisotopic element, so any alternative isotope cannot be selected for the measurement, and the need for tuning out interference from the resolution should be increased to 7800. However, resolution 3000 is sufficient to get rid of 90% of the interferences caused by polyatomic ions.
Because the gas flow that goes through the sample cone is much longer than it used to be previously (when the apertures with a smaller diameter were popular), the pressure should be reduced by using differential vacuum pumping in two or more stages. For this reason, the second cone was installed on the gas flow path whereas the space between the cone and the sample cone was pumped with a forepump featuring high pumping speed. Since there is a large pressure difference between the source of the inductively coupled plasma and the first pumping stage, the polyatomic ions are sucked into the interface space and accelerated to supersonic speeds. Taking this into account, in order to avoid turbulence at the second cone, it is produced with sharp edges so that the ions from the supersonic beam can be cut, and therefore, this cone is called the skimming cone. The structure consisting of a sample cone and a skimming cone with the diameters of about 1 mm is called the interface. Creating the interface meant a breakthrough in the ICP/MS technology, as it provided more efficient extraction of ions, improving the transmission of ions, and therefore, the sensitivity of the method – which also reduced spectral interference on more than an order of magnitude. However, spectral interference still remains one of the major limitations of the method of elemental analysis.
Finally, high mass spectral resolution is not a panacea for all types of spectroscopic interferences. Most of the isobaric interferences can not be resolved with the use of commercially available devices. For example, 58Fe, 58Ni and some polyatomic ions and polyhydric compound with argon, oxides, hydrides require the resolution, which lies at the very limit of the technically feasible today, or even resolution so high that it can not be obtained principally.
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