Mass spectrometry interference

C Molina, P Grasso, E Benfenati, D Barcelo. Automated sample preparation with extraction columns followed by liquid chromatography-ionspray mass spectrometry. Interferences, determination and degradation of polar organophosphorus pesticides in water samples. J Chromatogr A 737 47-58, 1996. 

Chan E, Sandhu M, Benskin JP, Ralitsch M, Thibault N, Birkholz D, et al. Endogenous high-performance liquid chromatography/tandem mass spectrometry interferences and the case of perfluorohexane sulfonate (PFHxS) in human serum are we overestimating exposure Rapid Comm Mass Sped 2009 23(10) 1405—10. 

Gammelgaard, B., and Joens, O. (1999). Determination of selenium in urine by inductively coupled plasma mass spectrometry Interferences and optimization./A a/./If. Spectrom. 14(5), 867. 

The nebulization concept has been known for many years and is commonly used in hair and paint spays and similar devices. Greater control is needed to introduce a sample to an ICP instrument. For example, if the highest sensitivities of detection are to be maintained, most of the sample solution should enter the flame and not be lost beforehand. The range of droplet sizes should be as small as possible, preferably on the order of a few micrometers in diameter. Large droplets contain a lot of solvent that, if evaporated inside the plasma itself, leads to instability in the flame, with concomitant variations in instrument sensitivity. Sometimes the flame can even be snuffed out by the amount of solvent present because of interference with the basic mechanism of flame propagation. For these reasons, nebulizers for use in ICP mass spectrometry usually combine a means of desolvating the initial spray of droplets so that they shrink to a smaller, more uniform size or sometimes even into small particles of solid matter (particulates). 

Gold is a useflil caUbration standard for this method (see Radioactive tracers). Whereas similar sensitivities can be achieved by inductively coupled plasma mass spectrometry (qv), the latter requires more extensive sample preparation to overcome interference by other metals such as copper (64). 

The requirements regarding commodities which are difficult to analyze are also not very clear. The listed crops do not cause difficulties in each kind of determination [e.g., brassica or bulb vegetables in gas chromatography/mass spectrometry (GC/MS)]. On the other hand, different species of the same crop may have different interference peaks, which may or may not affect quantitation. Presumably, the easiest approach is to perform additional validations, even if the final extracts are not difficult to analyze. In the author s experience, validations should generally include hops and tobacco, if the pesticide is used in these crops. 

LC/MS/MS. LC/MS/MS is used for separation and quantitation of the metabolites. Using multiple reaction monitoring (MRM) in the negative ion electrospray ionization (ESI) mode, LC/MS/MS gives superior specificity and sensitivity to conventional liquid chromatography/mass spectrometry (LC/MS) techniques. The improved specificity eliminates interferences typically found in LC/MS or liquid chro-matography/ultraviolet (LC/UV) analyses. Data acquisition is accomplished with a data system that provides complete instmment control of the mass spectrometer. 

For method tryout, run a control sample and two fortifications from each site. One fortification should be done at the LOQ and the other at the highest expected residue level, perhaps 1000 x LOQ. If the recoveries are within the acceptable range of 70-120% and there are no interferences, proceed with the method validation. If interferences are present which prevent quantitation of the analyte, try additional cleanup steps with SPE or use a more selective detection method such as liquid chromatography/mass spectrometry (LC/MS). 

An alternative to the bridge technique was recently reported for thorium analysis in silicate rocks for which both Th and Th are measured on a single lon-counting detector (Rubin 2001). With careful chemistry and mass spectrometry, °Th/ Th ratios of igneous rocks can be measured with this technique with a precision that is similar to the bridge method. The disadvantage of this technique is that °Th ion-count rates are extremely low (around 10 cps) with normal silicate thorium ratios and are therefore subject to perturbations from background variation and low-level isobaric interferences in normal samples. 

Enhanced molecular ion implies reduced matrix interference. An SMB-El mass spectrum usually provides information comparable to field ionisation, but fragmentation can be promoted through increase of the electron energy. For many compounds the sensitivity of HSI can be up to 100 times that of El. Aromatics are ionised with a much greater efficiency than saturated compounds. Supersonic molecular beams are used in mass spectrometry in conjunction with GC-MS [44], LC-MS [45] and laser-induced multiphoton ionisation followed by time-of-flight analysis [46]. 

Mass spectrometry is the only universal multielement method which allows the determination of all elements and their isotopes in both solids and liquids. Detection limits for virtually all elements are low. Mass spectrometry can be more easily applied than other spectroscopic techniques as an absolute method, because the analyte atoms produce the analytical signal themselves, and their amount is not deduced from emitted or absorbed radiation the spectra are simple compared to the line-rich spectra often found in optical emission spectrometry. The resolving power of conventional mass spectrometers is sufficient to separate all isotope signals, although expensive instruments and skill are required to eliminate interferences from molecules and polyatomic cluster ions. 

Table 8.60 shows the main features of GD-MS. Whereas d.c.-GD-MS is commercial, r.f.-GD-MS lacks commercial instruments, which limits spreading. Glow discharge is much more reliable than spark-source mass spectrometry. GD-MS is particularly valuable for studies of alloys and semiconductors [371], Detection limits at the ppb level have been reported for GD-MS [372], as compared to typical values of 10 ppm for GD-AES. The quantitative performance of GD-MS is uncertain. It appears that 5 % quantitative results are possible, assuming suitable standards are available for direct comparison of ion currents [373], Sources of error that may contribute to quantitative uncertainty include sample inhomogeneity, spectral interferences, matrix differences and changes in discharge conditions. 

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