Declustering

Finally, the beam — composed mainly of single substrate and solvent molecules and very small clusters — is passed through a heated wire grid, where the last declustering and desolvation occurs, leaving a beam of substrate molecules. 

Chovan et al.30 described a system that integrates different components of bioanalysis including automatic in vitro incubation, automatic method development (mainly SRM transitions for LC/MS/ MS analysis), and a generic LC method for sample analysis to minimize human intervention and streamline information flow. Automaton software (Applied Biosystems) was used for automatic MS method development. Flow injection was used instead of a HPLC column to decrease run time to 0.8 min per injection. Two injections were performed. The first was performed to locate the precursor ion and optimal declustering potential (DP). The second injection was performed to locate the product ion and optimal collision energy (CE). 

If aU other terms are constant, AKis Hnked principally to AQ]3(Teg) and to changes in effective reduced mass, Afi. The effects of the clustering—declustering process lead to changes in K of approximately 2—8%. Negative a functions arise from... 

Notes DP, declustering potential CE, collision energy. The CE values were optimized in such a way that the sensitivity of the multiple reaction monitoring signal was at the maximum. 

The spectrum shown in Figure 13.13 demonstrates how a PEG spectrum can be dominated by doubly charged ions ([M + 2 NH4]2"1") if the wrong conditions are chosen. In this case the concentration of ammonium acetate in the reference solution was too high (5 mM ammonium acetate is the maximum that should be used) and the declustering potential is too low. Doubly charged ions can be... 

In summary, ions that exhibit a positive dependence of K on E/N can be understood to undergo clustering and declustering events as shown in Fig. 8. During a low-field portion of the waveform, ions are not electrically heated or are slightly over thermal energies, and associations with water and supporting atmosphere can occur with lifetimes proportional to the heats of association. These associations result in increases in flD and decreases... 

Fig. 8. Model for positive alpha with increasing electric field. During die high-field part of die duty cycle (Ei), ions are declustered, leading to an increase in mobility (Wh) due to a decrease in ft. During die low-field part of the duty cycle (E2)> ions are re-clustered, leading to a decrease in mobility (J, Kh > K ) due to an increase in ft(fth < ftj). With this model, a positive AK may be expected.

The mechanism of clustering and declustering for ions with positive alphas was developed and supported by results from the study of the changes in alpha functions with moisture in the gas atmosphere in the DMS analyzer [37,38], Gas-phase protonated... 

Declustering seems to be also possible by the application of appropriate catalyst supports. For tris-allyl-neodymium Nd(r/3- C3H5P dioxane the rate constants for monomer consumption are determined for supported as well as for unsupported catalysts. Under comparable experimental conditions the respective rate constants differ by a factor of 100. This observation can be explained by differences in Nd efficiency or by differences in the concentration of active Nd sites [408]. 

Fig. 9. a Electrospray mass spectrum of a 10 4 mol l"1 solution of HC1 in H20 under mild declustering conditions, showing the presence of different clusters [(H30)(H20)n]+, the clusters with n=l, 2, and 3 being most prominent, b CID threshold curve (collision gas Ar), showing the dependence of the dissociation cross section against collision energy, and fit to Ar-mentrout s threshold function... 

Immediately after their production, the ions in the humid atmospheric-pressure ion source will attract solvent molecules by ion-dipole interactions. These solvated ions must be desolvated prior to entering the mass analyser. This is achieved by ion-molecule collisions in the transition region, especially between the ion-sampling aperture and the skimmer. A small potential difference between the nozzle and the skimmer is applied to enhance declustering. 

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