Mass Analysis of Ions

The ion beam from the source can be separated according to the respective masses (actually mjz, the ratio of the mass to the number of charges, for which the term Thomson is also used) of the ions by a variety of techniques. Magnetic deflection, quadrupole filter, ion trap, time-of-flight, and cyclotron resonance are the separating techniques most commonly used in commercial mass spectrometers. Excellent detailed discussions of these methods are available in the general references. 

The magnetic field acts as the mass analyzer. The mjz value in Thomsons of the ions which can pass through the exit slit depends on the radius (r, cm) of the ion path in the magnetic field, the field strength (B, gauss), and the ion-accelerating potential V, volts) as defined by the fundamental equation (Beynon 1960) 

This can be derived on the basis that the ions of elementary charge e (as distinguished from z, the number of charges on the ion) have all been given the same kinetic energy eV = mv (but different velocities, v) in acceleration, and that the force exerted by the magnet, Bev, must be equal to the centrifugal force, mv /r. Note that an ion of m/z = 100 would require 1 x 10 s to leave the ion source after formation (10-V repeller, 5 mm distance), and 1 x 10 s to travel 1 m after acceleration by 5 kV. 

Changing the magnetic or electric fields that effect separation causes ions of different m/z values to reach the collector. On-line computer systems produce sets of mass and abundance values directly by measuring the field and ion current corresponding to each peak. These systems can record complete mass spectra several times per second, which is especially valuable for GC/MS (see below). However, the scan rate affects the accuracy of ion-abundance measurement, which is important for deducing elemental compositions from isotope ratios check the performance of your instrument with known samples to be sure you have not sacrificed this accuracy unnecessarily. 

For conventional El and Cl, the sample must be vaporized so that the mole-eules will be separated from each other before ionization. Direct introduction into the ion source is preferred for the study of compounds of low volatility samples are inserted with a probe through a vacuum lock into the ion source. 

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Figure 30. Mass analysis of ions extracted from a CF4/O2 discharge using a silicon target extraction electrode. (Reproduced with permission...

Secondary ion mass spectrometry (SIMS) is a highly sensitive surface technique for characterizing materials. The procedure is based on the mass analysis of ions created when an impinging beam strikes the surface of a solid (or liquid, in a few special applications). The impinging ion beam, usually referred to as the primary ion beam, is generally accelerated to energies between 0.2 and 40 keV. Figure 4.1 shows the essential elements of SIMS. 

Bollan, H.R. Stone, J.A. Brokenshire, J.L. Rodriguez, J.E. Eiceman, G.A. Mobility resolution and mass analysis of ions from ammonia and hydrazine complexes with ketones formed in air at ambient pressure, J. Am. Soc. Mass Spectrom. 2007, 18(5), 940-951. 

Eiceman, G.A. Salazar, M.R. Rodriguez, M.R. Limero, T.F. Beck, S.W. Cross, J.H. Young, R. James, J.T., Ion mobility spectrometry of hydrazine, monomethylhydrazine, and ammonia in air with 5-nonanone reagent gas. Anal. Chem. 1993, 65, 1696-1702. Bollan, H.R. Stone, J.A. Brokenshire, J.L. Rodriguez, J.E. Eiceman, G.A., Mobihty resolution and mass analysis of ions from ammonia and hydrazine complexes with ketones formed in air at ambient pressure, J. Am. Soc. Mass Spec. 2007, 18(5), 940-951. 

The TOP mass spectrometer is well suited to the mass analysis of ions produced from a pulsed source. In contrast to scanning mass analysers, the TOP detects all ions sampled from the ion source. Por this reason it is widely used for coincidence PPPICO measurements and various laser ionization experiments. The pulsed nature of synchrotron radiation also lends itself to TOP photoionization studies. Peak shapes for different ions in a TOP mass spectrum can provide valuable information about their kinetic energy distributions. 

Analytical systems based on Py-MS are intended to perform pyrolytic reactions and to analyse the composition of the resultant pyrolysate by mass analysis of ions formed through the ionization of its components molecules. The structural complexity of the spectra recorded is usually considerable, owing to the formation of fragments with the same nominal mass from originally different components and the absence of reliable separation procedures. In addition, the observed complexity originates from secondary fragmentation processes, hence various techniques have been used that minimize these... 

Arnold F and Henschen G 1978 First mass analysis of stratospheric negative ions Nature 257 521-2 Eisele F L 1989 Natural and anthropogenic negative ions in the troposphere J. Geophys. Res. 94 2183-96 Oka T 1997 Water on the sun—molecules everywhere Science 277 328-9... 

The simplest example of this type of instmment is the triple quadmpole ms (27), in which and 2 used for mass analysis of the precursor and product ions, respectively, and 2 is a rf-only quadmpole coUision cell. The maximum possible energy uptake during coUisional activation, ... 

In the process of SNMS analysis, sputtered atoms are ionized while passii through the ionizer and are accelerated into the mass spectrometer for mass analysis. The ion currents of the analyzed ions are measured and recorded as a function of mass while stepping the mass spectrometer through the desired mass or element sequence. If the purpose of the analysis is to develop a depth profile to characterize the surface and subsurface regions of the sample, the selected sequence is repeated a number of times to record the variation in ion current of a selected elemental isotope as the sample surfiice is sputtered away. 

Isotope shifts for most elements are small in comparison with the bandwidth of the pulsed lasers used in resonance ionization experiments, and thus all the isotopes of the analyte will be essentially resonant with the laser. In this case, isotopic analysis is achieved with a mass spectrometer. Time-of flight mass spectrometers are especially well-suited for isotopic analysis of ions produced by pulsed resonance ionization lasers, because all the ions are detected on each pulse. 

The surface is bombarded with a stream of inert gas ions of energy, Eo, and the sputtered target secondary ions of energy, E, are monitored, rather than the backscattered primary beam ions. Mass analysis of the secondary ions is carried out. The intensity and the energy are also determined, Each element has a characteristic value of E/ Eq. This allows the elemental analysis of the surface. 

The FTMS instrument operates in a very different fashion from most other types of mass spectrometers. With FTMS, the principal functions of ionisation, mass analysis and ion detection occur in the same space... 

The ion trap mass analyzer is similar to the quadrupole but with the important distinction that it can isolate and trap ions in an electrical field. Notably, the ion trap differs significantly from quadrupoles in design and operation in that triple quadrupoles perform tandem mass analysis on ions as they pass through an analyzer ion traps are capable of isolating and retaining specific ions for fragmentation upon collision with an inert gas in the same cell. An ion trap is about the size of a tennis ball and consists of a donut-shaped electrode and two perforated disk-like end-cap electrodes. 

M. G. Inghram and R. Gomer. Mass Spectrometric Analysis of Ions from the Field Microscope. J. Chem. Phys., 22(1954) 1279-1280. 

An ultrapure polymer is made of chains of the type G1-AAAAAAA-G2, where A is the repeat unit and G1 and G2 are end-groups. One considers the mass number of one of the MS peaks, subtracts the mass of the cation (e.g., H, Li, Na, Ag), and then repeatedly subtracts the mass of the repeat unit, until one obtains the sum of the masses of G1 + G2. For this purpose, a linear best fit can also be used. Tandem mass spectrometry is particularly useful since, from the analysis of ion fragmentation patterns, one can deduce the mass of G1 and, separately, the mass of G2. 

This basically means that two instruments have been linked together. The first analyser can replace the traditional chromatographic separation step and is used to produce ions of chosen m/z values. Each of the selected ions is then fragmented by collision with a gas, and mass analysis of these product ions effected in the second analyser. The resulting mass spectrum is used for their identification. The potential combinations of the various magnetic sector and quadrupole instruments to form such coupled systems is considerable. Ion traps may also be operated in a tandem MS mode. 

Figure 10.3 Whole-mass analysis of a monoclonal antibody. (A) Direct infusion of the antibody generates an envelope of high m/z ions ranging from 2000 to 3500. Deconvolution of the ion current signal gives the mass of the complete native molecule (147, 100.97 Da) and resolves some heterogeneity linked to the A-glycan structures. The major forms are consistent with molecules carrying biantennary structures capped with 0, 1, or 2 hexose (G = galactose) residues. (Data generated on an ESI-Q-Star instrument, Sciex-Applied Biosystems.)...

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