Interferences ICPMS

By for the most simple acid to work with in ICPMS is nitric acid. This has minimal spectral interferences and in concentradons under 5% does not cause excessive wear to the sample cones. Other acids cause some spectral interferences that often must be minimized by dilution or removal. When HF is used, a resistant sampling system must be installed that does not contain quartz. 

Detection limits in ICPMS depend on several factors. Dilution of the sample has a lai e effect. The amount of sample that may be in solution is governed by suppression effects and tolerable levels of dissolved solids. The response curve of the mass spectrometer has a large effect. A typical response curve for an ICPMS instrument shows much greater sensitivity for elements in the middle of the mass range (around 120 amu). Isotopic distribution is an important factor. Elements with more abundant isotopes at useful masses for analysis show lower detection limits. Other factors that affect detection limits include interference (i.e., ambiguity in identification that arises because an elemental isotope has the same mass as a compound molecules that may be present in the system) and ionization potentials. Elements that are not efficiently ionized, such as arsenic, suffer from poorer detection limits. 

Another type of interference in ICPMS is suppression of the formation of ions from trace constituents when a large amount of analyte is present. This effect depends on the mass of the analyte The heavier the mass the worse the suppression. This, in addition to orifice blockage from excessive dissolved solids, is usually the limiting factor in the analysis of dissolved materials. 

Full quantitation is accomplished in the same manner as for most analytical instrumentation. This involves the preparation of standard solutions and matching of the matrix as much as possible. Since matrix interferences are usually minimized in ICPMS (relative to other techniques), the process is usually easier. 

ICP-OES is a destructive technique that provides only elemental composition. However, ICP-OES is relatively insensitive to sample matrix interference effects. Interference effects in ICP-OES are generally less severe than in GFAA, FAA, or ICPMS. Matrix effects are less severe when using the combination of laser ablation and ICP-OES than when a laser microprobe is used for both ablation and excitation. 

Althoi h nonspectral interference effects are generally less severe in ICP-OES than in GFAA, FAA, or ICPMS, they can occur. In most cases the effects produce less than a 20% error when the sample is introduced as a liquid aerosol. High concentrations (500 ppm or greater) of elements that are highly ionized in the... 

ICP-OES is one of the most successful multielement analysis techniques for materials characterization. While precision and interference effects are generally best when solutions are analyzed, a number of techniques allow the direct analysis of solids. The strengths of ICP-OES include speed, relatively small interference effects, low detection limits, and applicability to a wide variety of materials. Improvements are expected in sample-introduction techniques, spectrometers that detect simultaneously the entire ultraviolet—visible spectrum with high resolution, and in the development of intelligent instruments to further improve analysis reliability. ICPMS vigorously competes with ICP-OES, particularly when low detection limits are required. 

Higher mass resolution is becoming more common in MC-ICPMS technology. The result will be a reduction in the hindrances of isobaric interferences. With judicious use of narrow entrance slits and improvements in ion optics, even smaller radius instruments can resolve 50Ti from Mg+, for example. However, at this writing most studies have made use of low-mass-resolution instruments, and even with high mass resolution, care must still be taken to avoid changes in instrumental fractionation due the presence of elements other than the analyte in the plasma. 

In another recent application, a polymer-based ISE was used for the determination of Cu2+ in drinking water [18]. The optimal detection limit was reported as 2 x 10-9 M which deteriorated after first week of use by about 0.5 logarithmic units. In the long term, however, it remained at < 10-8 M over 55 days. The LOD in water samples containing the most common interferences was 1 x 10 7M which was much better than the required 2 x 10 6M (1/10 of EPA action level), so the electrode was applied to four different drinking water samples. The obtained results deviated by 30% from the values obtained by ICPMS as the reference method. 

Darrouzes, J., Bueno, M., Lespes, G., Holeman, M., Potin-Gautier, M. Optimisation of ICPMS collision/reaction cell conditions for the simultaneous removal of argon based interferences of arsenic and selenium in water samples. Talanta 71, 2080-2084 (2007)... 

Sources of errors that were detected were mainly due to calibration errors, high blanks explaining high results and uncontrolled interferences (e.g. in ICPMS). 

Ar-interferences with and " Fe were likely the cause for high standard deviations in ICPMS for potassium and iron, respectively. 

Possible problems of interferences in ICP-MS were highlighted as well as the need to carry out a preconcentration step. Spectral interferences may occur for copper, leading to high results caused by spectral overlap from unidentified polyatomic species at mass 65. The existence of this species in natural samples was stressed in the certification of an estuarine water reference material by a mass spectrum obtained by high resolution ICPMS, although its exact composition could not be determined. 

Spectra such as that in Figure Il-I5b led early workers in the field of ICPMS to have hopes of an interference-free method. I Infortunaiely. this hope was not realized in further studies, and serious interference... 

Like ICP mass spectra, spark source mass spectra are much simpler than atotnic emission spectra, consisting of one major peak for each isotope of an element as well as a few weaker lines corresponding to multiply charged inns and ionized oxide and hydroxide species. The presence of these additional ions creates the potential for interference just as in ICPMS. 

In spite of the high selectivity of the technique, quadrupole-based ICPMS is not totally free from spectral interferences. The occurrence of polyatomic ions that show the same nominal mass-to-charge ratio as the analytes of interest is a well-documented problem. In the last decade, the introduction of colUsion/reaction cells has helped to minimize the importance of spectral interferences [30]. These devices most often make use of selective ion-molecule chemistry in such cells, although it is also possible to profit from selective... 

However, although the techniques mentioned above are able to provide results of sufficient quality in many analytical situations, there are still some other instances where specific problems may hamper their use for example, analysis of a liquid sample, the requirement of very low LODs, the lack of calibration standards, or the occurrence of interferences. In such instance, electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICPMS) may constitute a versatile alternative, overcoming these and other inconveniences. As will be shown in this chapter, the unique characteristics of ETV-ICPMS provide unique capabilities so that difficult analytical situations can be addressed... 

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