A company supervising the cleanup of an explosives manufacturing plant needs to

Question

A company supervising the cleanup of an explosives manufacturing plant needs to determine the amounts of several explosives that are present in the soil at the site. The following data were taken by first extracting these substances from soil samples, followed by liquid chromatography at a flow rate of 1.50 mL/min.

a. Determine the adjusted retention time for each compound.

b. Calculate the retention factors for TNT, RDX, and HMX.

c. How would the adjusted retention times for TNT, RDX, and HMX change if the flow rate used was accidently changed from 1.50 mL/min to 1.00 mL/min? How would the retention factors be changed?

Explanation / Answer

The retention time, t R , is given in seconds by:

a) t R =t S +t M

where t S is the time the analyte spends in the stationary phase and t M is the time spent in the mobile phase. t M is often referred to as the dead, or void time, as all components spend t M in the mobile phase.

So, the adjusted retention time of each compound is tR, which is given.

b) Mathematically, retention factor it is the ratio of the adjusted retention volume (time) and the hold-up volume (time):

k = VR / VM = tR / tM

As stated in a), since tM is null and void, so k = tR

c) Retention factor depend directly on flow rate. If flow rate is decreased, retention time will also decrease.

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