Signal resolving power

In practice, only a limited number of views are available the scanned sector is typically 180 or 360°, and the angular increment 2°. Moreover the frequency band-width of the employed pulses is very limited, typically one octave. The resolving power of the system is then limited. A typical numerical signal is composed of 1024 samples at a sampling period of 50 nsec. 

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. 

The analytical resolving power can be interpreted as being the maximum number of signals which can find place within a given registration range. Therefore, it is evident that Rz is a measure of the multielement efficiency of analytical methods and influences strongly selectivity. 

It is not true that an increase in the resolving power always simultaneously leads to an increase in the accuracy of mass measurement. As soon as the resolving power allows peaks separation it is useless to increase it more as the signal intensity will decrease with subsequent decrease in accuracy of measurements. 

Therefore, a mass analyzer capable of resolving powers of 26,000 or better is required to reveal the presence of two ions (representing two distinct compounds). If an analyzer with RP< 26,000 is used, the presence of a second compound would be overlooked. Additionally, attempts to measure the exact m/z value of the peak will not provide a true value, but rather some average value (e.g., m/z 937.5072, assuming the compounds produce signals of comparable intensity) that will be of little use in identifying either unknown compound. Ambiguous results such as these only complicate qualitative analysis efforts. 

Good mass accuracy can only be obtained from sufficiently sharp and evenly shaped signals that are well separated from each other. The ability of an instrument to perform such a separation of neighboring peaks is called resolving power. It is obtained from the peak width expressed as a function of mass. 

Note Mass accuracy is highly dependent on many parameters such as resolving power, scan rate, scanning method, signal-to-noise ratio of the peaks, peak shapes, overlap of isotopic peaks at same nominal mass, mass difference between adjacent reference peaks etc. An error of 5 mmu for routine applications is a conservative estimate and thus the experimental accurate mass should lie within this error range independent of the ionization method and the instrument used. [37] There is no reason that the correct (expected) composition has to be the composition with the smallest error. 

Example Regardless of the manufacturer of the hardware, the effect of a time lag on resolution is quite dramatic. The resolving power of linear instruments is improved by a factor of 3-4 and reflector instruments become better by a factor of about 2-3. [36] The advantages are obvious by comparison of the molecular ion signal of Ceo as obtained from a ReTOF instrument with continuous extraction (Fig. 4.7) and from the same instrument after upgrading with PIE (Fig. 4.12), or by examination of MALDI-TOF spectra of substance P, a low mass peptide, as obtained in continuous extraction mode and after PIE upgrade of the same instrument (Eig. 4.11). 

Resolving power R is a measure of specificity and is a primary factor contributing to the probability of false positives. The salient issue is the probability that a background signal will overlap with a target explosives signal. We can qualitatively express this probability using a Poisson distribution function of the form... 

Some of the most prominent spectral interferences can be resolved with a resolution from 4000 up to 10000, depending on the analytical problem. It can be tempting to calculate the resolution necessary to resolve two masses based only on their exact masses and the specified resolving power of the instrument. However, the resolution required will depend on the relative magnitude of the spectral overlap and analyte ion signals. For example, to resolve the overlap of Cl" and H Ar", a resolution of 3900 would be sufficient when considering the exact masses alone. However, as can be seen in Figure 1.10, a resolution of 10000 is needed to provide baseline resolution of the two peaks (because the H Ar ion is much more intense). 

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