Systematic investigation of ion suppression and enhancement effects of fourteen stable‐isotope‐labeled internal standards by their native analogues using atmospheric‐pressure chemical ionization and electrospray ionization and the relevance for multi‐analyte liquid chromatographic/mass spectrometric procedures

In clinical and forensic toxicology, multi‐analyte procedures are very useful to quantify drugs and poisons of different classes in one run. For liquid chromatographic/tandem mass spectrometric (LC/MS/MS) multi‐analyte procedures, often only a limited number of stable‐isotope‐labeled internal standards (SIL‐ISs) are available. If an SIL‐IS is used for quantification of other analytes, it must be excluded that the co‐eluting native analyte influences its ionization. Therefore, the effect of ion suppression and enhancement of fourteen SIL‐ISs caused by their native analogues has been studied. It could be shown that the native analyte concentration influenced the extent of ion suppression and enhancement effects leading to more suppression with increasing analyte concentration especially when electrospray ionization (ESI) was used. Using atmospheric‐pressure chemical ionization (APCI), methanolic solution showed mainly enhancement effects, whereas no ion suppression and enhancement effect, with one exception, occurred when plasma extracts were used under these conditions. Such differences were not observed using ESI. With ESI, eleven SIL‐ISs showed relevant suppression effects, but only one analyte showed suppression effects when APCI was used. The presented study showed that ion suppression and enhancement tests using matrix‐based samples of different sources are essential for the selection of ISs, particularly if used for several analytes to avoid incorrect quantification. In conclusion, only SIL‐ISs should be selected for which no suppression and enhancement effects can be observed. If not enough ISs are free of ionization interferences, a different ionization technique should be considered. Copyright © 2010 John Wiley & Sons, Ltd.     

Wake flow past a plate with spoiler II: Gravity effects

Summary The effects of transverse gravity on steady flow past a split plate are investigated, by adopting the wake model proposed in the preceding paper (I). The existence and uniqueness of the solution as well as the convergence of an iteration process involving the free streamlines are proved for large Froude numbers by means of the Banach contraction mapping principle using Lipschitz norms.

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Sunto Si studiano gli effetti dovuti ad un campo di gravità uniforme trasversale sul flusso con scia attorno ad un profilo sottile con spoiler usando un modello proposto nel precedente lavoro (I). Si dimostra l'esistenza, unicità, e la convergenza di un procedimento iterativo sulle linee di corrente libere, per grandi numeri di Froude, mediante il principio di contrazione in spazi di Banach.