Resolving power instrumental

Today questions such as, Would you rather use deconvolution or build a higher-resolving-power instrument are not uncommon. In fact, today they may actually be justified, considering what we have not learned about de-convolution. These same questions cannot be justified on the basis of a negative reaction to a numerical process that is no more mystical than the operation of any state-of-the-art instrument. 

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. 

Generally, the attainable resolving power of a TOE instrument is limited, particularly at higher mass, for two major reasons one inherent in the technique, the other a practical problem. First, the flight times are proportional to the square root of m/z. The difference in the flight times (t and t ,+i) for two ions separated by unit mass is given by Equation 26.5. 

An added consideration is that the TOF instruments are easily and quickly calibrated. As the mass range increases again (m/z 5,000-50,000), magnetic-sector instruments (with added electric sector) and ion cyclotron resonance instruments are very effective, but their prices tend to match the increases in resolving powers. At the top end of these ranges, masses of several million have been analyzed by using Fourier-transform ion cyclotron resonance (FTICR) instruments, but such measurements tend to be isolated rather than targets that can be achieved in everyday use. 

Finally, instmmental broadening results from resolution limitations of the equipment. Resolution is often expressed as resolving power, v/Av, where Av is the probe linewidth or instmmental bandpass at frequency V. Unless Av is significantly smaller than the spectral width of the transition, the observed line is broadened, and its shape is the convolution of the instrumental line shape (apparatus function) and the tme transition profile. 

The theoretical limit to an instrument s resolving power is determined by the wavelength of light used, and the numerical aperture of the system ... 

The classical polarizing light microscope as developed 150 years ago is still the most versatile, least expensive analytical instrument in the hands of an experienced microscopist. Its limitations in terms of resolving power, depth of field, and contrast have been reduced in the last decade, in which we have witnessed a revolution in its evolution. Video microscopy has increased contrast electronically, and thereby revealed structures never before seen. With computer enhancement, unheard of resolutions are possible. There are daily developments in the X-ray, holographic, acoustic, confocal laser scanning, and scanning tunneling micro-... 

Measurements of surface disorder require a high resolving power (the ability to distinguish two close-lying points in the diffraction pattern). Quantitative measurements of surface disorder are limited in the following manner, the worse the resolving power, the smaller the maximum scale of surface disorder that can be detected. For example, if the maximum resolvable distance of the diffractometer is 100 A, then a surface that has steps spaced more than 100 A apart will look perfect to the instrument. The theoretical analysis of disorder is much simpler than that for atomic positions. 

Other instruments which have been devised for microstructure examination include the X-ray microscope, with greater resolving power than the EM (Ref 41), and the electron microprobe, capable of indicating subtle changes in composition over small specimen areas (Refs 57 62)... 

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. 

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