Resolving power spectrometer

Probably the simplest mass spectrometer is the time-of-fiight (TOP) instrument [36]. Aside from magnetic deflection instruments, these were among the first mass spectrometers developed. The mass range is theoretically infinite, though in practice there are upper limits that are governed by electronics and ion source considerations. In chemical physics and physical chemistry, TOP instniments often are operated at lower resolving power than analytical instniments. Because of their simplicity, they have been used in many spectroscopic apparatus as detectors for electrons and ions. Many of these teclmiques are included as chapters unto themselves in this book, and they will only be briefly described here. 

It would appear that measurement of the integrated absorption coefficient should furnish an ideal method of quantitative analysis. In practice, however, the absolute measurement of the absorption coefficients of atomic spectral lines is extremely difficult. The natural line width of an atomic spectral line is about 10 5 nm, but owing to the influence of Doppler and pressure effects, the line is broadened to about 0.002 nm at flame temperatures of2000-3000 K. To measure the absorption coefficient of a line thus broadened would require a spectrometer with a resolving power of 500000. This difficulty was overcome by Walsh,41 who used a source of sharp emission lines with a much smaller half width than the absorption line, and the radiation frequency of which is centred on the absorption frequency. In this way, the absorption coefficient at the centre of the line, Kmax, may be measured. If the profile of the absorption line is assumed to be due only to Doppler broadening, then there is a relationship between Kmax and N0. Thus the only requirement of the spectrometer is that it shall be capable of isolating the required resonance line from all other lines emitted by the source. 

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. ... 

FTICR-MS is capable of powerful mixture analysis, due to its high mass range and ultrahigh mass resolving power. However, in many cases it is still desirable to couple a chromatographic interface to the mass spectrometer for sample purification, preconcentration, and mixture separation. In the example given above, DTMS under HRMS conditions provides the elementary composition. Apart from DTMS, PyGC-MS can be performed to preseparate the mixture of molecules and to obtain the MS spectrum of a purified unknown. Direct comparison with the pure reference compound remains the best approach to obtain final proof. 

Mass spectrometry is the only universal multielement method which allows the determination of all elements and their isotopes in both solids and liquids. Detection limits for virtually all elements are low. Mass spectrometry can be more easily applied than other spectroscopic techniques as an absolute method, because the analyte atoms produce the analytical signal themselves, and their amount is not deduced from emitted or absorbed radiation the spectra are simple compared to the line-rich spectra often found in optical emission spectrometry. The resolving power of conventional mass spectrometers is sufficient to separate all isotope signals, although expensive instruments and skill are required to eliminate interferences from molecules and polyatomic cluster ions. 

WDSs have excellent resolving power, and the peak-to-background ratio of each line is much higher than can be achieved with a crystal detector. With a suitable crystal of large lattice spacing it is possible to detect and count X-rays as soft as boron K or even beryllium K , and this type of spectrometer is widely used when... 

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