Prévia do material em texto
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.