Question: The graph shows van Deemter curves for n-nonane at 70°C in the 3.0-m-long microfabricated column in Box 23-2 with a 1- to 2-m-thick stationary phase.

The graph shows van Deemter curves for n-nonane at 70°C in the 3.0-m-long microfabricated column in Box 23-2 with a 1- to 2-m-thick stationary phase.

(a) Why would air be chosen as the carrier gas? What is the danger of using air as carrier gas?
(b) Measure the optimum velocity and plate height for air and for H2 carriers.
(c) How many plates are there in the 3-m-long column for each carrier at optimum flow rate?
(d) How long does unretained gas take to travel through the column at optimum velocity for each gas?
(e) If stationary phase is sufficiently thin with respect to column diameter, which of the two mass transfer terms (22-35a and 22-35b) becomes negligible? Why?
(f) Why is the loss of column efficiency at high flow rates less severe for H2 than for air carrier gas?

0.30 0.25 0.20 0.15 0.10 0.05 Air 50 100 150 Velocity (cm's)

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