(a) Describe how ion mobility spectrometry works and state the analogies between this technique and capillary electrophoresis.
(b) As in electrophoresis, the velocity, u, of a gas-phase ion is u = μE, where μ is the mobility of the ion and E is the electric field (E V/L, where V is the voltage difference across distance L). In ion mobility spectrometry, the time to go from the gate to the detector is called drift time, td. Drift time is related to voltage: td = L/μ = L/(μE) L/(μ(V/L)) = L2/μV. Plate number is N 5.55(td/w1/2)2, where w1/2 is the width of the peak at half-height. Ideally, peak width depends only on the width of the gate pulse that admits ions to the drift tube and on diffusive broadening of ions while they migrate:64

where t8 is the time that the ion gate is open, k is Boltzmann's constant, T is temperature, V is the potential difference from the gate to the detector, e is the elementary charge, and z is the charge of the ion. Prepare a graph of N versus V (0 V 20 000) for an ion with μ 8 × 10 -5 m2/(s ˆ™ V), and tg = 0, 0.05, or 0.2 ms at 300 K. Let the length of the drift region be L = 0.2 m. Explain the shapes of the curves. What is the disadvantage of using short tg?
(c) Why does decreasing T increase N?
(d) In a well-optimized ion mobility spectrometer, protonated arginine ion (z = 1) had a drift time of 24.925 ms and w1/2 = 0.154 ms at 300 K. Find N. For V 12 500 V and tg 0.05 ms, what is the theoretical plate number?

Transcribed Image Text:

16kT In 2 Vez Diffusive broadening W1/2 Initial width from gate pulse