Mass and Charge Selection

The separation of different ions in a QMS is based on the mass to charge ratio (m/z) and is achieved by applying electric fields that vary with respect to time (at 

Solutions of these differentials can be classified into stable and unstable solutions—i.e. ones where an ion passes the mass filter (so called bounded solution where the displacement of the ion along x and y remains finite as t - oo) and others that strike the rods and are filtered out (unbounded solutions, infinite for t oo), respectively. From these equations two parameters with dependence upon the potentials applied can be defined a (R.f. stability parameter) which depends upon U and q (DC stability parameter) which depends upon V. These parameters are defined as follows [25, 26]  

For an ion to pass the mass filter the condition for stability in both x and y direction must be fulfilled and can be plotted as a stability diagram (Fig. 3.2b). Here the bounded, stable trajectories are colored in blue and the mode of operation of the QMS is expressed in a function defined only by the two variables a and q. 

Adjusting the value for the U/V ratio appropriately, the slope of the mass scan line can be set in such way that only ions of one mass fall in the stable region (dark blue area in Fig. 3.2b). The QMS works as a band pass filter with defined boundaries and ideally (here q = 0.706, mass scan line passing through the peak) only one mass is selected [23, 25-27]. 

For unselected clusters (Pin) the R.f. component is zero, thus a = 0 and consequently the mass scan line equals the q axis. As a result all ions in the (dark and light) blue area in Fig. 3.2b would pass the QMS (0 0.908) for small q. However, still a particular value for the R.f. voltage, i.e. q can be set (e.g. q = 0.706, as for the selected case). Now the QMS works as a high-pass mass filter [24, 25, 27] and the stable conditions are 0.706 q 0.905, thus the smallest mass to pass the QMS is 7/9. This ratio can be readily applied to determine the lower 

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The deposition of mass and charge selected ions onto surfaces is underway but is in its infancy. How do the ions survive the collision with a surface This question has a myriad of answers depending on many variables and will have a future in investigative studies. A soft landing is now a possibility (280) and allows the potential spectroscopic investigation of trapped ions. So far no transition metal ions have been examined using this method but it is only a matter of time. Soft landings via inert gas matrices also have potential in the surface deposition of mass selected clusters. 

FIGURE 6.2 (a) The mass spectrum of singly charged Hg ions [9], (b) the charge spectrum of Ca-ions [14], As illustrated the mass and charge selection in SMILETRAP is very good. 

Alternative to DDA, the term information-dependent acquisition (IDA) is used. Both DDA and IDA describe an automatic mode where the MS/MS parameters are collected automatically by calculating the parameter set (e.g., collision energy) based on the mass and charge state of the selected precursor ion. 

An attendant requirement is, of course, that the ionized specie be readily produced. Species with low ionization potentials are therefore required. Of the atomic species, cesium with an atomic mass of 133 AMU and an ionization potential of only 3.89 ev has been the favored propellant. Colloidal species can be either solids or liquids. The criterion of selection of the colloidal material is based on the facility with which colloids of uniform mass and charge can be produced. 

LABEL ATOM worked in conjunction with SET CHARGE and SET MASS to relabel atom symbols All three modes would allow a node to be relabeled with the curently set atom symbol, mass, and charge The general technique would be to select the atom symbol desired (from a sub-menu that appeared after LABEL ATOM had been touched) and then select SET CHARGE and/or SET MASS if necessary Finally, the user would touch the atom(s) to be relabeled ... 

It should be mention here that even when the alcohol membrane selectivity of many of the alternative membranes are higher than that for Nafion, the correlation between this parameter and the DAEC performance using those membranes is very poor. The reason for this could be found in the complex mass and charge transport processes taking place in the three phase region, where ion proton conduction through thin films of the PEM is essential for a good electrochemical efficiency. 

For (p = 0, e" is one for f = rr/2, e is i for f = tt, e is —1 for cp = tt jA, is (1 -f O/V and so on. No matter what value of cp you select, the square modulus e" e is equal to one. The functions p 6), —p(6), iipid), and (i — l)if/(6)/ all have different phase factors, but they have the same square modulus, and their measurable properties, such as where the mass and charge of the particle are located, will be identical. Effectively, phase factors merely shift the coordinate system. A change in the absolute phase cannot affect what the wave-function predicts we will measure in the lab. Multiplying a wavefunction by — 1 is like deciding that you want the z axis to point in the opposite direction when you measure the location of a particle. That decision changes the sign of z in all the... 

Multiple select For a polarized electrode with both mass and charge transfer overpotentials, which parameters will change in value with changes in the bulk solution concentration ... 

Relation (4.11) shows that, in the case of the electrostatic sector, the radius of the ion trajectory is independent of its mass and charge it depends only on the V/E ratio. The value of r is constant because the position of the collector slot is fixed the scanning of E allows the selection of ions according to their kinetic energy. 

In odier words, ions with a particular mass-to-charge ratio, m/z, can be selectively passed tlirough the magnetic sector by appropriate choice of a value of V and B (though nonnally V is held constant and only B is varied). 

Quadrupole analyzer. A mass filter that, creates a quadrupole field with a DC component and an RF (radio frequency) component in such a manner as to allow transmission only of ions having a selected mass-to-charge (m/z) ratio. 

Quadrupole analyzer A mass filter that creates a quadrupole field with dc and rf components so that only ions of a selected mass-to-charge are transmitted to the detector. 

Tandem mass spectrometry (MS/MS) is a method for obtaining sequence and structural information by measurement of the mass-to-charge ratios of ionized molecules before and after dissociation reactions within a mass spectrometer which consists essentially of two mass spectrometers in tandem. In the first step, precursor ions are selected for further fragmentation by energy impact and interaction with a collision gas. The generated product ions can be analyzed by a second scan step. MS/MS measurements of peptides can be performed using electrospray or matrix-assisted laser desorption/ionization in combination with triple quadruple, ion trap, quadrupole-TOF (time-of-flight), TOF-TOF or ion cyclotron resonance MS. Tandem... 

The great advantage of the mass spectrometer is its abihty to use mass, more accurately the mass-to-charge ratio, as a discriminating feature. In contrast to, for example, the UV detector, which gives rise to broad signals with little selectivity, the ions in the mass spectrum of a particular analyte are often characteristic of that analyte. Under these conditions, discrete signals, which may be measured accurately and precisely, may be obtained from each analyte when they are only partially resolved or even completely umesolved from the other compounds present. 

In a different context, a micropipette has been applied to monitor the current through a single-ion channel in a biological membrane. The patch-clamp technique invented by Sackmann and Neher [119] led to their Nobel Prize in medicine. The variations in channel current with voltage, concentration, type of ions, and type of channels have been explored. While the functions of specific channels, in particular their ionic selectivity, have been well known, only a handful of channels have the internal geometry and charge distribution determined. The development of a theory to interpret the mass of channel data and to predict channel action is still lacking. 

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