Mass-resolving power

High resolution is a desirable figure of merit of a mass spectrometer because it helps to (1) perform accurate mass measurements, (2) resolve isotopically labeled species when the percent incorporation of the label is to be determined, (3) resolve an isotopic cluster when the charge state of high-mass compounds is to be determined, (4) enhance the accuracy of quantification, and (5) unambiguously mass-select precursor ions in MS/MS experiments. 

By definition, the mass resolution of a mass spectrometer is its ability to distinguish between two neighboring ions that differ only slightly in their mass (Am). Mathematically, it is the inverse of resolving power (RP), given as 

Solution First calculate the accurate mass of the ions, using the accurate mass of the most abundant isotope of each element  

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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. ... 

Ultrahigh mass-resolving power (106) and mass accuracy (few ppm)... 

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. 

The analytical resolving power is applied in several analytical fields in form of well-known expressions such as, e.g., spectral resolving power Rx = X/AX or mass resolving power RM = M/AM. 

Let two peaks of equal height in a mass spectrum at masses m and m, Am, be separated by a valley which at its lowest point is just 10% of the height of either peak. For similar peaks at a mass exceeding m, let the height of the valley at its lowest point be more (by any amount) than 10% of either peak. Then the resolution (10% valley definition) is m/Am. The ratio m/Am should be given for a number of values of m [4], Comment. This is a typical example of the confusion regarding the definition of the term resolution. Here resolution is used instead of the more appropriate phrase mass resolving power (which is the inverse of resolution). 

Mass Resolving Power (m/Am) In a mass spectrum, the observed mass divided by the difference between two masses that can be separated, m/Am. The method by which Am was obtained and the mass at which the measurement was made should be reported. 

The molecular ions produced in the MALDI process have relatively high initial velocities, which can cause reduction in mass resolving power and transmission, primarily for TOF analyzers with axial ion extraction (see Section 2.2.1). Hence, the MALDI-MS mass resolving power depends strongly on laser fluence and is highest when the laser fluence is close to the threshold level. 

Finally, the instrument can be operated in the peak-matching mode, which provides optimum mass resolving power and mass accuracy. Here the magnetic field strength is kept constant and the electric sector and acceleration voltages are scanned over a relatively small m/z range. This mode of operation is suitable when two ions that are very close in mass need to be separated or when the elemental composition of a molecule is to be determined at high resolution. 

Performance Parameters. Typical resolving power for the commercial instrument is up to about 130,000 (FWHM) for m/z 400 Th. The mass resolving power is m/z dependent it decreases with fmfz (see the FTICR, described in Section 2.2.6, which decreases linearly with m/z). 

The acquisition speed is, as for the FTICR, resolution dependent. With Thermo Fischer Scientific s orbitrap the desired mass resolving power can be selected. With the lowest setting (7500 FWHM) the acquisition time for one ion injection is 0.3 s and with the highest setting (100,000 FWHM) it is 1.9 s. 

A mass calibration for FTICR analyzers with superconducting magnets is very stable and is valid for many days for normal applications. Mass accuracy < 1 ppm can be obtained over a fairly wide mass range. Unique elemental composition can be determined for masses over 800 Da [262]. Recently, 0.1 ppm mass accuracy, which required a mass resolving power >300,000, has been achieved for several thousand peaks by a 14.5 T instrument [263] and commercial instruments with mass accuracy <0.2 ppm are available. As with the orbitrap (see Section 2.2.5) the frequency is... 

The FTICR analyzer is relatively slow. In a low resolution mode (<25,000 FWHM) scans can be performed in substantially less than a second. A high resolution scan is more time demanding, and more than 1 s is often required for mass resolving powers of 100,000 or more. 

What mass analyzer exhibits the highest achievable mass resolving power (ICR spectrometer, a.k.a. FTMS). 

For example for an ion measured at m/z 552 with a peak width of 0.5 m/z units (FWHM) the mass resolution would be 0.5, while the mass resolving power... 

Figure 5A, B shows the isotopic distribution, of protonated bosentan (C27H30N5O6S, Mr 552.6) with a mass resolution of 0.5 and 0.1 at FWHM, respectively. It is worthwhile to observe the mass shift of the most abundant ion from m/z 552.2006 to m/z 552.1911. This value does not change with a mass resolving power of 15 000 (Fig. 1.5C) or even 500000 (Fig. 1.5D). Accurate mass measurements are essential to obtain the elemental composition of unknown compounds or for confirmatory analysis. An important aspect in the calculation of the exact mass of a charged ion is to count for the loss of the electron for the protonated molecule [M+H]+. The mass of the electron is about 2000 times lower than of the proton and corresponds to 9.10956 x 10 kg. The exact mass of protonated bosentan without counting the electron loss is 552.1917 units, while it is 552.1911 units with counting the loss of the electron. This represents an error of about 1 ppm.

With time of flight instruments, a mass accuracy better than 5 ppm can be achieved, while with Fourier transform ion cyclotron resonance or orbitrap mass spectrometers mass accuracies better than 1 ppm have been reported. It is obvious that, for good mass accuracies, the peaks must be baseline resolved and resolution plays an essential role. For the present example, a mass resolving power of 5000 seems to be quite acceptable. In the case of the [M+H]+ + 1 isotope peak, the situation becomes somewhat more complex for molecules containing nitrogen, sulfur or carbon. Figure 1.5 D illustrates at a mass resolving power of 500000 the contribution of... 

Fig. 1.5 Simulated isotopic distribution of the protonated bosentan (C27H30N5O6S) at mass resolving power (A) R = 1104, with a peak full width at half maximum (FWHM) of 0.5 u. (B) R = 5520, FWHM = 0.1 u. (C) R = 15 000. (D) R = 500000 with isotopic contribution of (peak 1), (peak 2) and (peak 3).

The majority of H/D studies that have been reported employ quadrupole ion trap (QIT) instruments due to their ease of use, excellent sensitivity, ability to perform MS/MS experiments, compact size, and low cost. Other reports discuss the use of instruments with higher mass-resolving power such as the hybrid QqTOF instruments [47]. A few groups have utilized FT-ICR mass spectrometry, which offers ultra-high mass-resolving power and improved mass accuracy [48, 49]. 

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