Mass range

The line width is a measure of the degree to which differentiation can be made between two adjacent lines of the same height. The resolution is normally indicated. It is defined as R = M / AM and is constant for the quadrupole spectrometer across the entire mass range, slightly greater than 1 or AM 1. 

Often an expression such as unit resolution with 15% valley is used. This means that the bottom of the valley between two adjacent peaks of identical height comes to 15 % of the height of the peak or, put another way, at 7.5 % of its peak height the line width DM measured across an individual peak equals 1 amu (atomic mass unit) see in this context the schematic drawing in Fig. 4.10. 

The mass range is characterized by the atomic numbers for the lightest and heaviest ions vi/ith a single charge which are detected with the unit. 

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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. 

Cesium has more isotopes than any element—32—with masses ranging from 114 to 145. 

Thirty isotopes of tellurium are known, with atomic masses ranging from 108 to 137. Natural tellurium consists of eight isotopes. 

When freshly exposed to air, thallium exhibits a metallic luster, but soon develops a bluish-gray tinge, resembling lead in appearance. A heavy oxide builds up on thallium if left in air, and in the presence of water the hydride is formed. The metal is very soft and malleable. It can be cut with a knife. Twenty five isotopic forms of thallium, with atomic masses ranging from 184 to 210 are recognized. Natural thallium is a mixture of two isotopes. A mercury-thallium alloy, which forms a eutectic at 8.5% thallium, is reported to freeze at -60C, some 20 degrees below the freezing point of mercury. 

Twenty five isotopes of polonium are known, with atomic masses ranging from 194 to 218. Polonium-210 is the most readily available. Isotopes of mass 209 (half-life 103 years) and mass 208 (half-life 2.9 years) can be prepared by alpha, proton, or deuteron bombardment of lead or bismuth in a cyclotron, but these are expensive to produce. 

Terbium is reasonably stable in air. It is a silver-gray metal, and is malleable, ductile, and soft enough to be cut with a knife. Two crystal modifications exist, with a transformation temperature of 1289oC. Twenty one isotopes with atomic masses ranging from 145 to 165 are recognized. The oxide is a chocolate or dark maroon color. 

Nitrobenzyl alcohol n-Octyl-3-nitrophenyl ether Wide mass range depending on the glycol used. 

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. 

Accurate mass measurement requires high resolving power. The difference in degrees of difficulty between measuring an m/z of 28 and one of 28.000 is likely to be large. Table 39.3 shows the broad mass ranges achievable with various analyzers. 

Typical Mass Ranges Achievable with Various Analyzers... 

Most ion sources produce singly charged ions, i.e., z = I and the ranges shown here apply to such ions. Matrix assisted methods may produce ions with r > 1. When = 1, m/z. = m, viz., mass can be measured directly. An ES ion source produces ions with z > 1 and this effectively extends the mass ranges that can be examined. For example, with z = 1 and m = 10,000, the m/z value is 10,000 and this would be beyond tbe capabilities of a quadrupole instrument. 

The upper limit of the mass range is about 10,000 mass units (Daltons). 

The ions in a beam that has been dispersed in space according to their various m/z values can be collected simultaneously by a planar assembly of small electron multipliers. All ions within a specified mass range are detected at the same time, giving the array detector an advantage for analysis of very small quantities of any one substance or where ions are produced intermittently during short time intervals. 

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. 

Simultaneous detection of between 4 and 40% of the mass range, gives two orders of magnitude decrease in detection limits (see Photodetectors). 

Very high sensitivity is obtained because almost all the ions formed in the ion source are detected, and the mass range is almost limitless. TOF systems work best when pulsed ion sources are used, and the flight time of the ions is then given by... 

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