I am taking a LC MS/MS course and we are reviewing a lot of literature about different techniques specially in the pharmaceutical industry. The teacher asked us to come up with an explanation on why most of these papers report retention times almost equal to the void time (sometimes sooner) and very few with retention times longer than the void time. He mentioned that some regulations require a k´ of 2.
Hope you can help me with this.
The retention factor, k*, depends on several parameters: • Flow rate (F) • Gradient time (t) • Column void volume (proportional to column size) (Vm)
Remember: • Any change in flow rate, gradient time or void volume will change k* or retention factor.
If your compound elutes in the void volume on a reverse phase column, it is very hydrophiic.
R. Shah, G. (2010). Determination of pregabalin in human plasma by electrospray ionisation tandem mass spectroscopy. Journal of Advanced Pharmaceutical Technology & Research, 354-357.
Vaidya, V. V. (2007). LC-MS-MS Determination of Pregabalin in Human Plasma. Chromatographi, 925-928.[NL1]
Rosete-Reyes, A. (2016). Bioequivalence Study of Two oral-Capsule formualtions of Pregabalin 300 mg in Healthy Mexican Adult Volunteers. Journal of Bioequivalence Studies.
These are some examples that we discussed where the retention time was shorter than the void time or almost the same.
My question is how can you justify the methodology when the molecule is not being retained in the column. Is it because you are measuring the transition?
FYI i am do not know much about this thats why I am taking the course but this was a problem that is being discussed right now.
Thank you all for your kind replies.
The only time I have personally seen something like what is being described here is when the analyte and stationary phase both carry the same charge state. The charge-charge repulsion renders the analyte unretained and perhaps a little earlier. Since the analyte cannot see the inner pores of the silica gel (repulsion) but the solvent can, it elutes a little earlier than the void volume. I believe I saw this in polar organic mode.
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