Mass interferences

Molecular ion mass interferences are not as prevalent for the simpler matrices, as is clear from the mass spectrum obtained for the Pechiney 11630 A1 standard sample by electron-gas SNMSd (Figure 4). For metals like high-purity Al, the use of the quadrupole mass spectrometer can be quite satisfiictory. The dopant elements are present in this standard at the level of several tens of ppm and are quite evident in the mass spectrum. While the detection limit on the order of one ppm is comparable to that obtained from optical techniques, the elemental coverage by SNMS is much more comprehensive. 

Under Cs bombardment the matrix effect can be significantly reduced by using the MCs" ion signals for quantification of species M. The detection limit is increased, i.e. the detection power deteriorates, by two or more orders of magnitude, but sometimes even standard-free quantification has been reported [3.51]. MCs" ions have high masses this is a disadvantage because many mass interferences occur in this mass range. 

High-mass resolution is needed to separate mass interferences of molecular and atom ions. Because of the mass defect of the binding energy of the nucleus, atomic ions have a slightly smaller mass than the corresponding molecular ions. To observe this typical mass resolutions between 5000 and 10000 are necessary. 

In practice image quality is also reduced by use of high mass resolution and energy offset. Often, therefore, mass interference cannot he avoided. Determination of element distributions is possible by use of image processing tools for classification of mappings of different masses [3.53]. 

Ion implantation is a method commonly used for doping semiconductors. Because the concentrations of the dopants (mostly B and P) are very low, a dynamic range of more than five orders of magnitude is often necessary. Measurement of is more difficult than that of B, because of the mass interference of °Si H. High mass resolution of m/Am = 5000, or an energy offset of 300 V, is necessary. 

Advances in TIMS-techniques and the introduction of multiple collector-ICP-MS (MC-ICP-MS) techniques have enabled the research on natural variations of a wide range of transition and heavy metal systems for the first time, which so far could not have been measured with the necessary precision. The advent of MC-ICP-MS has improved the precision on isotope measurements to about 40 ppm on elements such as Zn, Cu, Fe, Cr, Mo, and Tl. The technique combines the strength of the ICP technique (high ionization efficiency for nearly all elements) with the high precision of thermal ion source mass spectrometry equipped with an array of Faraday collectors. The uptake of elements from solution and ionization in a plasma allows correction for instrument-dependent mass fractionations by addition of external spikes or the comparison of standards with samples under identical operating conditions. All MC-ICP-MS instruments need Ar as the plasma support gas, in a similar manner to that commonly used in conventional ICP-MS. Mass interferences are thus an inherent feature of this technique, which may be circumvented by using desolvating nebulisers. 

TABLE 11.2. Major Potential Mass Interferences on Pd Isotopes... 

As already discussed above, Q-ICP-MS may still require matrix removal and preconcentration because the Q mass analyzer is unable to resolve most of the spectral interferences affecting the analytical masses of PGEs. Moreover, the detection power might be inadequate for the performance of reliable measurements in some samples. PGEs determination is hampered by mass interferences and requires high analytical selectivity and sensitivity. Double-focussing SF-ICP-MS with mass resolution capabilities higher than those afforded by Q instruments (currently up to 10,000) can properly deal with most mass interferences that have... 

An appropriate RF storage voltage is applied to the ring electrode to trap and accumulate ions with an m/z above a low-mass cut-off value. The ion-injection and trapping process may be influenced, e.g., by filtering low-mass interferences in the RF-only ion guide prior to injection into the trap [40], or by the application of auxiliary RF waveforms to the end caps [41]. 

Other lactate analyzers use lactate oxidase (LOD). In the YSl 23 L instrument (USA) LOD is immobilized between a cellulose acetate membrane and a polycarbonate membrane [368], the latter serving to exclude high-molecular mass interferents. Lactate measurement in whole blood pipetted immediately after withdrawal into the phosphate buffer of the analyzer yields the following correlation with values obtained with deproteinized blood [369] ... 

In order to investigate the interfacial chemistry and the possible formation of metal-polymer complex, molecular ions containing metal atom and PET fragments are researched. However these peaks may interfere with PET peaks already present and, as shown in Table 1, the isotopic mass resolution of the gwadrupole spectrometer does not allow to resolve the mass interferences for... 

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