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Principles of Instrumental Analysis, 6th ed. Chapter 20
 
 3
 Substituting into Equation 20-9 gives, after rearranging, 
 
2 2 2 19 2
23 2
(0.240 Vs/m ) (0.127 m) 1.60 10 C/ion (Vs) C = = 5.96 = 5.97 V
2 1.246 10 kg/ion m kg
V
−
−
× ×
× ×
 
20-7. After acceleration the velocity v can be calculated with the aid of Equation 20-4. Thus, 
 zeV = ½ mv2
 where m for cyclohexane (M = 84) is given by 
 
+
256 12
23 3
g C H 1 mol 1 kg kg = 84.0 = 1.395 10 
mol 6.02 10 ions 10 g ion
m −× × ×
×
 
 
19 2 2
3
25
2 2 1.60 10 C/ion 5.00 (Vs) C/(m kg) = = = 3.39 10 m/s
1.395 10 kg/ion
zeVv
m
−
−
× × ×
×
×
 
 time = 
2
5
3
15.0 cm 10 m/cm = 4.43 10 s = 44.3 μs
3.39 10 m/s
−
−×
×
×
 
20-8. The presence of a negative dc voltage in the yz plane causes positive ions to move toward 
the rods where they are annihilated. In the presence of an added ac voltage, this 
movement is inhibited during the positive half of the cycle with the lighter ions being 
more affected than the heavier ions. Thus the yz plane acts as a low-pass filter removing 
heavier ions (see Figure 11-????). 
20-9. The resolution of a single focusing mass spectrometer is limited by the initial kinetic 
energy spread of the sample molecules. This spread is minimized in a double focusing 
instrument by accelerating the sample through an electrostatic analyzer, which limits the 
range of kinetic energies of ions being introduced into the magnetic sector analyzer. 
Signicantly narrower peaks are the result.

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