Skimmer potential

All mass spectra were obtained on a VG Quattro triple-quadrupole mass spectrometer (Micromass Inc., Altrincham, U.K.). Peptides were ionized with electrospray ionization under the following conditions mobile phase, methanol/water (50/50 v/v) needle voltage, 2.8 kV high voltage lens (counter electrode), 0.05 kV and skimmer potential, -12 V. The flow rate of the mobile phase for the spectra of the whole library was 100 pl/min rather than 10 pl/min, which was used for the samples after binding. All samples were dissolved in methanol. Either 5 or 10 pi of sample was injected. Two injections were performed for each sample. Data were acquired in Multichannel Analysis... 

Smith et al. [26-28] demonstrated that by a further increase of the nozzle-skimmer potential difference the internal energy of the ions can be increased and fragmentation of the multiple-charge protein ions can be induced as a result of colhsion-induced dissociation (CID). This is called in-source CID in this text. 

Studies on the analysis and characterization of proteins by ESI-MS generally involve the direct infusion of a protein solution. This enables the determination of the molecular weight of the proteim Initially, the influence of a variety of experimental parameters, e.g., solvent composition, temperature, and influence of nozzle-skimmer potential, was studied for model proteins [10-13]. Given the diverse nature and properties of proteins, general statements on optimum solvent conditions... 

Other mass spectrometric experiments that can be useful for studying phosphorylation include postsource decay, precursor ion scan, neutral loss scanning, and stepped skimmer potential. In order to... 

So-called in-source CID occurs in the vicinity of the skimmer cone in API interfaces (e.g. Figure 6.40(b)) where the background pressure is comparable to that in a q collision cell (mtorr range). A potential is normally applied to the skimmer cone to permit gentle collisional activation that leads to stripping of clustered solvent molecules from the analyte ions (see Section 5.3.3a), but by increasing this potential (referred to as the cone potential or skimmer potential ) it is possible to increase the... 

In some cases it is observed that, under the experimental conditions used (mobile phase composition, ionization and API interface parameters), more than one ionized form of the intact analyte molecule is observed, i.e. adduct ions of various kinds (see Section 5.3.3 and Table 5.2). An example is shown in Figure 9.6, in which a well known anticancer drug (paclitaxel, Figure 9.6(a)) was analyzed by positive ion ESI-MS (infusion of a clean solution). The first spectrum (Figure 9.6(b)) shows four different adducts (with H+, NH, Na+ and K+). Adjustment of the cone (skimmer) potential (Section 5.3.3a), to lower values in this case, enabled production of the ammonium ion adduct to dominate the MS spectrum (Figure 9.6(c)) in a robust fashion, and this ion yielded a useful product ion spectrum (that appeared to proceed via a first loss of ammonia to give the protonated molecule) which was exploited to develop an MRM method that was successfully validated and used. It is advisable to avoid use of analyte adducts with alkali metal ions (commonly Na+ and to some extent K+) since, when subjected to colli-sional activation, these adducts frequently yield the metal ion as the dominant product ion with only a few low abundance product ions derived from the analyte molecule. However, when feasible, both the ammonium adduct and protonated molecule should be investigated as potential precursor ions at least until it becomes clear that one will provide superior performance (sensitivity/selectivity compromise) than the other. 

Set the orifice potential or skimmer potential of the mass spectrometer to 80 V. Set the mass resolution to mlAm 1000. Acquire data to disk from 400 to 2000 u at 0.25 u/step and 1 msec/step. Set the mass defect to 50 mmu/100 u. 

Set the mass resolution to w/Aw 1000. Set mass spectrometer for multiple ion scans. Scan for three carbohydrate-fragment masses 204 (HexNAc ), 292 (NeuAc ), and 366 (Hex-HexNAc" ). Use a scan width of 6 u for each ion and set the orifice or skimmer potential for each ion at 160 V. Set the fourth mass window for miz 1200 with a scan width of 1600 (to scan from 400 to 2000)... 

The spectrum (Figure 1.16) was obtained at low nozzle (spray capillary) to skimmer potential, so that there was little collisional activation of the protein. At a high potential, multiple loss of HI was observed (see Figure 1.17). All the peaks that contained Na +1 pairs have disappeared and were replaced by peaks with unpaired Na adducts. 

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