Mass-analysed kinetic energy

A) MASS-ANALYSED ION KINETIC ENERGY SPECTROMETRY (MIKES)... 

Mass-analysed ion kinetic energy spectrometry (MIKES) A form of MS-MS product-ion scan that may be carried out on a reversed-geometry double-focusing mass spectrometer. 

Magnetic sector mass spectrometers accelerate ions to more than 100 times the kinetic energy of ions analysed in quadrupole and ion trap mass spectrometers. The higher accelerating voltage contributes to the fact that ion source contamination is less likely to result in degraded sensitivity. This is particularly important for analysis that requires stable quantitative accuracy. 

MIKES Mass-analysed ion kinetic energy spectrometry... 

The mass-to-charge ratio, m/n, of those ions that pass through the probe aperture and are analysed in the mass spectrometer is calculated from the equivalence between the potential energy of the atom on the specimen surface at voltage V0, and the kinetic energy that the atom acquires during acceleration to the grounded... 

Instruments are calibrated with substances of known relative molecular mass and very accurate mass measurements can be made with this type of analyser. Sensitivity is very high and there is virtually no upper limit to the working mass range. The performance of TOF instruments can be improved further by the incorporation of an electrostatic reflector (reflectron) which ensures that all ions of identical mass reach the detector simultaneously by correcting for any differences in their kinetic energies. 

A time-of-flight spectrometer can be used as a mass analyzer, an ion kinetic energy analyzer, and an ion reaction time analyzer. We will consider here only what factors affect the resolution of the system in mass analysis.74 The same consideration can easily be extended to find the resolution in other analyses. There are at least two kinds of mass resolution. One refers to the ability of the system to separate two ion species of nearly equal masses in the same mass spectrum. This is related to the sharpness of the mass lines, or the full width at half maximum (FWHM) of the mass lines. The other refers to the ability of the system to distinguish two ion species of nearly identical masses, but not necessarily in the same mass spectrum. This latter mass resolution is related to the sharpness of reference points in the mass lines such as the onset flight times of the ion species, and the overall long-term stability of the system. This latter resolution determined also how accurately the instrument can measure the mass of ion species. Although this latter resolution is more closely related to ion kinetic energy analysis and is as important as the former one, we will consider here only the former kind, or the conventional kind, of mass resolution. 

The principle underlying time of flight (TOF) mass spectrometers is based on the relationship that exists between mass and velocity at a given kinetic energy. The instrument, which uses pulsed ionisation, measures the time taken by each mass to travel the length L of a field-free analyser tube. The basic equation (I6.l l) used in linear TOF analysers is obtained by eliminating the velocity v from equation (16.5) in conjunction with the relationship L = vt ... 

Identification of the different types of ions observed in a mass spectrum through peak-matching and metastable ion analysis allows the determination of molecular structure. Several newer mass spectrometric techniques Mass analysed ion kinetic energy (MIKE) or reversed Nier-Johnson geometry) can also be used in spectral interpretation. These techniques are described in specialised monographs. 

There is one obvious difficulty in determining intramolecular kinetic isotope effects with the present PIPECO techniques for metastable ions (Sect. 3.2.1), which is that peaks for product ions from isotopically labelled molecules will overlap. Indeed, peaks for ions separated by only one mass unit will typically be largely superimposed. The time-of-flight technique is, in effect, measuring velocity and what is required is measurement of mass and translational energy in separate analysers. The problem... 

The most important drawback of the first TOF analysers was their poor mass resolution. Mass resolution is affected by factors that create a distribution in flight times among ions with the same m/z ratio. These factors are the length of the ion formation pulse (time distribution), the size of the volume where the ions are formed (space distribution), the variation of the initial kinetic energy of the ions (kinetic energy distribution), and so on. The electronics and more particularly the digitizers, the stability of power supplies, space charge effects and mechanical precision can also affect the resolution and the precision of the time measurement. 

Schematic description of a continuous extraction mode and a delayed pulsed extraction mode in an linear time-of-flight mass analyser, o = ions of a given mass with correct kinetic energy = ions of the same mass but with a kinetic energy that is too high. Delayed pulsed extraction corrects the energy dispersion of the ions leaving the source with the same mjz ratio.

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