Interferences polyatomic molecular

Different gases such as CH4, NH3 and H2 have be used in collision cells to reduce isobarie or polyatomic molecular interferences (Niemela et al. 2003). H2 or He is typically used with the ICP-Q-MS employed in the present study (i.e. a Varian 820 ICP-MS). In this instrument, the flow of gas is introduced into the plasma with a double-walled skimmer cone. While the eoUision/reaetion takes place in the plasma, high amounts of electrons are available for electron-ion dissociative recombination to attenuate molecular interferences (Abdelnour and Murphy 2004). All samples were measured both in standard mode (without He) and with the aid of He as a collision gas in conjunction with the ID-ICP-Q-MS for comparative purposes. 

Effects that impact the measurement of specific isotope ion currents in the mass spectrum are identified as spectrometric interferences. In general, these types of interferences result in a positive error on the analyte ion current measurement. There are four basic types of mass spectrometric interferences isobaric spectral overlap, polyatomic molecular ion overlap, multiple charged species (usually doubly charged ions), and background contribution to the measurement of the ion current. Each of these types of interferences is described in this section. 

Another correction approach that is somewhat more straightforward is to compensate for polyatomic molecular spectral overlap interferences on trace element analyses is by making a correction in the concentration domain. This correction is accomplished by measuring the equivalent analyte concentration of the interference in the absence of the analyte (i.e., C aiyte 0).This measurement is made for a known concentration in the sample of the interfering component of the molecular ion. Typically, a standard solution (known concentration) of the interferent, with no analyte present, is measured under the same conditions as the calibration and analysis of the analyte element. The apparent analyte concentration for this interfering molecular ion is computed from the analyte calibration curve (or regression function). An interferent correction constant (K) is calculated by the following equation ... 

Although this technology has demonstrated real merit in solving the polyatomic molecular ion interference problems for some selected applications, there is still much research to be accomplished before its universal applicability is understood. This is particularly true for multielement analyses because the operating conditions and choice of parameters appear to be specific for the solution of each analytical problem. 

The table below lists some common spectral interferences that are encountered in inductively coupled plasma mass spectrometry (ICP-MS), as well as the resolution that is necessary to analyze them.1 The resolution is presented as a dimensionless ratio. As an example, the relative molecular mass (RMM) of the polyatomic ion 15N160+would be 15.000108 + 15.994915 = 30.995023. This would interfere with 31P at a mass of 30.973762. The required resolution would be RMM/8RMM, or 30.973762/0.021261 = 1457. One should bear in mind that as resolution increases, the sensitivity decreases with subsequent effects on the price of the instrument. Note that small differences exist in the published exact masses of isotopes, but for the calculation of the required resolution, these differences are trivial. Moreover, recent instrumentation has provided rapid, high-resolution mass spectra with an uncertainty of less than 0.01%. 

In environmental analysis, flame photometry is most widely used for the determination of potassium, which emits at 766.5 nm. It is also often used for the determination of sodium at 589.0 nm, although spectral interference problems (see Chapter 3) then may be encountered in the presence of excess calcium because of emission from calcium-containing polyatomic species. Molecular species are more likely to be found in cooler flames than in hotter flames. Some instruments use single, interchangeable filters, while others have three or more filters, for example for the determinations of potassium, sodium and lithium,... 

At high concentrations especially, a number of elements produce significant concentrations of polyatomic species in flames. Such species absorb, and may therefore cause spectral interference. However the molecular absorption spectra are very wide compared with the atomic spectral lines. Figure 6, for example, shows how the presence of CaOH species in flames may interfere in the determination of barium by AAS. The formation of any solid particles in the flame causes scatter, which also causes an apparent broad band absorption, especially at lower wavelengths. 

Monoatomic interferents natural abundance polyatomic species maximum possible abundance calculated as the product of the natural abundances of the two isotopes forming the molecular ion divided by 100. 

Molecular Interferences High abundance of ions is likely to lead to the formation of polyatomic or molecular ions with the same mass as the elements of interest [51, 52]. Polyatomic interferences arise mainly from various combinations of Ar, C, Ca, Na, Cl, N, O, and S matrix, elements, which are present in sample matrices, plasma gas, and reagents used in the digestion of samples [53]. Arsenic has serious problems from polyatomic interferences arising from Ar plasma gas and matrix constituents (C and Cl). The use of correction equations, the addition of N2 or the use of a DRC or CC can minimize and sometimes eliminate these interferences [36-40, 54]. The use of correction equations should be avoided, especially if the As mass counts are small compared with the interference present. Correction equations suffer from inherent problems in that the masses used to correct for interferences may themselves suffer from interferences. 

For many types of electron spectroscopies there are still comparatively few studies of SOC effects in molecules in contrast to atoms, see, e.g., [1, 2, 3, 4, 5, 6, 7] and references therein. This can probably be referred to complexities in the molecular analysis due to the extra vibrational and rotational degrees of freedom, increased role of many-body interaction, interference and break-down effects in the spectra, but can also be referred to the more difficult nature of the spin-orbit coupling itself in polyatomic species. Modern ab initio formulations, as, e.g., spin-orbit response theory [8] reviewed here, have made such investigations possible using the full Breit-Pauli spin-orbit operator. 

In addition, the occurrence of isobaric interferences of analyte ions with isobaric polyatomic ions can hamper the accuracy and precision of isotope ratio measurements (see also Section 6.1.3). The main factors affecting the accurate and precise determination, for example, of using ICP-MS, are the isobaric interference of the molecular ion on 236pj+ analyte ions, and... 

ICP-MS elevated cost (although less than most molecular MS instrumentation), possibly troublesome polyatomic interferences, low ionization efficiency for biologically important elements (Se, S, P, halogens), and, in most commercially available instruments, sequentially scaiming nature of mass analyzer and detection system. 

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