Interferences polyatomic

Isobaric and polyatomic interferences (10000 mass resolution separates interferences)... 

Although this technology is effective in resolving a wide range of polyatomic interferences, the increased cost associated with this type of instrumentation (more than twice the price of a quadrupole instrument) limits its use in most routine laboratories, hence alternative methods of interference reduction have been sought for. The use of chemical extraction and chromatography (in order to separate the analyte from the matrix prior to analysis) or the operation of the ICP-MS under so-called cool plasma conditions, allows the elimination of... 

The ICP-MS has several analytical attractions including very low detection limits [parts per billion to parts per trillion (ppb to ppt) levels], a large linear dynamic range, relatively simple spectra, excellent stability, multielement determination capability, and ability to measure isotopic abundances. Disadvantages are mainly due to the formation of polyatomic interferences from the plasma gas, entrained gases, and matrix elements such as Cl [16]. 

A. Krushevska, M. Kotrebai, A. Lasztity, R. M. Barnes, D. Amarasiriwardena, Application of tertiary amines for arsenic and selenium signal enhancement and polyatomic interference reduction in ICP-MS analysis of biological samples, Fresenius J. Anal. Chem., 355 (1996), 793D800. 

The choice of the appropriate mineral acid and oxidant depends on the final analytical technique to be used. For instance, HC1 is not recommended for furnace analysis as it can cause Cl interferences, while H2S04 is not desirable with ICP-AES or ICP-MS because of transport interferences derived from its viscosity. With ICP-MS HNO3 and H202 are preferred since the effect of polyatomic interferences is minimal as compared with HC1, HC104 and H2S04, which introduce polyatomic ions such as C10+ and SO+ [28-32]. In any case, in order to minimize corrosion of the metal sampler and skimmer cones with ICP-MS, final sample solutions should not contain high acid concentrations (e.g., above 10 percent for HNO3). 

To overcome polyatomic interferences in Q-ICP-MS a host of physical approaches (i.e., optimization of plasma parameters, addition of supplementary plasma gases, the use of alternative plasma gases or collision/dynamic reaction... 

The difference between a DRC and a CC ICP-MS is that a reaction cell uses a Q mass spectrometer with a defined stability region, allowing the rejection of a defined mass region from the ion beam, whereas a CC uses a hexapole or octapole mass spectrometer as an ion guide. DRC have no limit on the gas type used, whereas CCs can only use inert gases or a simple reactive gas such as He or H2. Limits of detection (LoDs) for As are 0.01 ng l-1. However, LoDs can only be achievable with careful control of contamination and minimization of polyatomic interferences. Arsenic LoDs may be improved by the introduction of the element into the ICP-MS in the gaseous form, for example, by formation of ASH3, which markedly improves the LoD as nearly 100 percent of the analyte in solution reaches the mass detector. 

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. 

ICP-MS can detect nanogram per liter concentrations of As, but suffers from polyatomic interferences generated from the sample matrix. Efficient digestion of samples is required to eliminate C enhancement of As signals within the plasma. HG coupled to ICP-MS should not be used for total As measurements unless As species in digests have been degraded to inorganic As, as total As concentrations will be underestimated. 

---