Forward mass scan

A typical scan function, or scan table, for a forward mass scan performed by the DIT is shown in Figure 12.6. The scan function differs substantially from that used in QIT experiments. Time is reported on the abscissa, while the period (T) of the... 

FIGURE 12.6 A typical scan table for a forward mass scan of the digital ion trap. 

The interface properties can usually be independently measured by a number of spectroscopic and surface analysis techniques such as secondary ion mass spectroscopy (SIMS), X-ray photoelectron spectroscopy (XPS), specular neutron reflection (SNR), forward recoil spectroscopy (FRES), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), infrared (IR) and several other methods. Theoretical and computer simulation methods can also be used to evaluate H t). Thus, we assume for each interface that we have the ability to measure H t) at different times and that the function is well defined in terms of microscopic properties. 

As reversible ion transfer reactions are diffusion controlled, the mass transport to the interface is given by Fick s second law, which may be directly integrated with the Nernst equation as a boundary condition (see, for instance. Ref. 230 232). A solution for the interfacial concentrations may be obtained, and the maximum forward peak may then be expressed as a function of the interfacial area A, of the potential scan rate v, of the bulk concentration of the ion under study Cj and of its diffusion coefficient D". This leads to the Randles Sevcik equation [233] ... 

Scans based on resonant ejection may either be carried out in a forward, i.e., from low to high mass, or a reverse manner. This allows for the selective storage of ions of a certain m/z value by elimination of ions below and above that m/z value from the trap. Thus, it can serve for precursor ion selection in tandem MS experminents. [156,158] Axial excitation can also be used to cause collision-induced dissociation (CID) of the ions as a result of numerous low-energy collisions with the helium buffer gas that is present in the trap in order to dampen the ion motion. [150,156] A substantial increase of the mass range is realized by reduction of both the RF frequency of the modulation voltage and the physical size of theQIT. [154,159,160]... 

Example Tandem mass spectrometric experiments in quadmpole ion traps are performed by combining the techniques of resonant ejection, and forward and reverse scanning to achieve an optimum in precursor ion selection, ion activation, and fragment ion scanning (Fig. 4.45). [156]... 

Doroshenko, V.M. Cotter, R.J. Losses of Ions During Forward and Reverse Scans in a QIT Mass Spectrometer and How to Reduce Them. J. Am. Soc. Mass Spectrom. 1997, 8,1141-1146. 

In the type of linear-sweep voltammetry discussed thus far, the potential is changed slowly enough and mass transfer is rapid enough that a steady state is reached at the electrode surface. Hence, the mass transport rate of analyte A to the electrode just balances its reduction rate at the electrode. Likewise, the mass transport of P away from the electrode is just equal to its production rate at the electrode surface. There is another type of linear-sweep voltammetry in which fast scan rates (1 V/s or greater) are used with unstirred solutions. In this type of voltammetry, a peak-shaped current-time signal is obtained because of depletion of the analyte in the solution near the electrode. Cyclic voltammetry (see Section 23D) is an example of a process in which forward and reverse linear scans are applied. With cyclic voltammetry, products formed on the forward scan can be detected on the reverse scan if they have not moved away from the electrode or been altered by a chemical reaction. 

The three steps of an MS/MS experiment are performed by DIT using an approach substantially different from that employed in 3D IT or linear ITs (Ding, 2004). In those cases, the precursor ion isolation is performed by applying one or more dipole excitation waveforms, with a maximum isolation resolution of -1300 (expressed as the isolation mass divided by the baseline width of the isolation window). In the case of DIT, ion isolation is performed by sequential forward and reverse scans, so as to eject all ions with m/z values lower and higher than that of interest, respectively. This method can provide precursor ion isolation with a resolution >3500. 

Linked-field scan at constant B/E. This scan mode is used to obtain a product-ion spectrum on forward- and reverse-geometry inslruments [37]. In this scan, the value of V is fixed and B and E are both scanned while the ratio BIE is held constant. Fragmentation occurs in the first FFR. The mass resolution of the product-ion analysis is much higher selection of the precursor ion is at a lower resolution. 

Fig. 1.5 Methanol oxidation peak currents as a function of Pt coverage (4 nmol as 2D islands is approximately 0.8 of a monolayer), where If and Ib represent peak currents at forward and backward scans, respectively (a) Pt mass activity for methanol oxidation on commercial PtRu and PtML/Ru electrocatalysts (b) [77] (reproduced with permission from J. Electrochem. Soc. 155, B183 (2008). Copyright 2003, The Electrochemical Society)...

Choosing a proper scanning rate in voltammetry is essential for attaining a steady-state and sufficiently low charging current. Typically, the time required to attain a steady-state is determined by the mass transfer rate inside the pipet. If Op is not very small e.g., > 5°), sigmoidal forward and reverse... 

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