Resolution, mass

According to its definition in Equation 3.35, the mass resolving power is inversely proportional to the resolution. Am. Thus, if Am = 0.01 Da for a peak with a maximum located at 100 Da, then the resolving power would be 10. However, this definition is not universally used and in some definitions the resolving power and the resolution are essentially one and the same quantity. This is highly confusing and so in this book we will stick to the above definitions of resolving power as m/Am and resolution as Am, where Am will be considered to be the FWHM of an isolated peak in the mass spectrum. 

The main function of the mass analyser is to separate (resolve) the ions formed in the ionisation source on the basis of their mass to charge ratios (m/z). The resolving power of a mass spectrometer is a measure of its ability to separate two ions of any defined mass difference. More precisely, the resolution (R) of a mass analyser is defined as its ability to separate the ion envelopes of two peaks of equal intensity, i.e. the ratio of the mass of a peak (M ) to the difference in mass between this peak and an adjacent higher mass peak (Af2), i.e.  

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Figure Bl.7.9. (a) Stability diagram for ions near the central axis of a quadnipole mass filter. Stable trajectories occur only if the and values lie beneath tire curve, (b) Stability diagram (now as a fiinction of U and F) for six ions with different masses. The straight line miming tlirough the apex of each set of curves is the operating line, and conesponds to values of UIVthat will produce mass resolution (reproduced with pennission of Professor R March, Trent University, Peterborough, ON, Canada).

In the simplest fomi, reflects the time of flight of the ions from the ion source to the detector. This time is proportional to the square root of the mass, i.e., as the masses of the ions increase, they become closer together in flight time. This is a limiting parameter when considering the mass resolution of the TOP instrument. 

The reactivity of size-selected transition-metal cluster ions has been studied witli various types of mass spectrometric teclmiques [1 ]. Fourier-transfonn ion cyclotron resonance (FT-ICR) is a particularly powerful teclmique in which a cluster ion can be stored and cooled before experimentation. Thus, multiple reaction steps can be followed in FT-ICR, in addition to its high sensitivity and mass resolution. Many chemical reaction studies of transition-metal clusters witli simple reactants and hydrocarbons have been carried out using FT-ICR [49, 58]. 

Since the microchannel plate collector records the arrival times of all ions, the resolution depends on the resolution of the TOP instrument and on the response time of the microchannel plate. A microchannel plate with a pore size of 10 pm or less has a very fast response time of less than 2 nsec. The TOP instrument with microchannel plate detector is capable of unit mass resolution beyond m/z 3000. 

Assuming that the mass spectrometer has sufficient mass resolution, the computer can prepare accurate ma.ss data on the m/z values from an unknown substance. To prepare that data, the system must acquire the mass spectrum of a known reference substance for which accurate masses for its ions are already known, and the computer must have a stored table of these reference masses. The computer is programmed first to inspect the newly acquired data from the reference compound in comparison with its stored reference spectrum if all is well, the system then acquires data from the unknown substance. By comparison and interpolation techniques using the known reference... 

Although ion transmission guides and ion traps both use the same universal physical laws to achieve control over ion behavior, the ways in which the laws are used are different, as are the objectives. The guides do not retain ions to gain control over their velocities and are used simply to transmit both slow and fast ions over a very wide range of gas pressures. Ion traps retain ions over a relatively long period of time so as to adjust their kinetic energies and thereby improve mass resolution. The so-called bath gas is used at carefully controlled pressures. 

The reflectron increases the spatial separation of the ions of different m/z values by making them travel up and down the flight tube, so the distance traveled is twice what it would be if the ions simply passed once along the tube from one end to the other. The reflectron also narrows the energy spread for individual m/z values, thus improving mass resolution. TOP analyzers are not necessarily equipped with a reflectron. 

Three important parameters for mass spectrometers are mass resolution, mass range, and sensitivity. The resolution, R, required to separate two ions of mass m and (m + Am) is given by equation 1. 

A newer hybrid system available commercially is the magnetic sector—TOF hybrid (38). The precursor ions can be selected with better than unit—mass resolution by msl and the product ion ions detected at high sensitivity by the TOF ms2 (39). 

High mass resolution techniques are used to separate peaks at the same nominal mass by the very small mass differences between them. As an example, a combination of Si and H to form the molecular ion Si H , severely degrades the detection limit of phosphorous ( P) in a silicon sample. The exact mass of phosphorous ( P) is 31.9738 amu while the real masses of the interfering Si H and Si H2 molecules are 31.9816 amu and 31.9921 amu, respectively. Figure 8 shows a mass... 

SiiH2 are completely separated from the peak. Quadrupole instruments are not usually capable of such high mass resolution. 

The choice of mass spectrometer for a particular analysis depends on the namre of the sample and the desired results. For low detection limits, high mass resolution, or stigmatic imaging, a magnetic sector-based instrument should be used. The analysis of dielectric materials (in many cases) or a need for ultrahigh depth resolution requires the use of a quadrupole instrument. 

It is very evident in Figure 3 that the chemical complexity of Hasteloy presents special problems for mass spectrometric analysis using a quadrupole mass spectrometer with low mass resolution. Molecular ions comprised of combinations of matrix and plasma atoms are formed in abundance and will obscure many elements... 

Typical mass resolution values measured on the LIMA 2A range from 250 to 750 at a mass-to-charge ratio M/ Z= 100. The parameter that appears to have the most influence on the measured mass resolving power is the duration of the ionization event, which may be longer than the duration of the laser pulse (5—10 ns), along with probable time broadening effects associated with the l6-ns time resolution of the transient recorder. ... 

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