Tech Tip 24: The differences between GC and LC Sample Introduction

sample introduction

GC sample introduction is involved, requiring much thought and optimisation.

To get the sample into the analytical column reproducibly and cleanly, from one sample to the next, it helps to use an inlet (also known as an injector). Injections into the inlet may be made manually (less precise) or automated, then transfer to the column requires optimisation to achieve the final goal of a tight representative sample band on the column with no artefacts.

The inlet is pressurised and while it has a large number of potential leak points, a good routine maintenance protocol will eliminate all of them.

Inside the inlet there should be a liner suitable for the injection technique whether it be hot split, hot splitless or something even more advanced. Liner choice, including deactivation, style, capacity and any packing material, is critically important when it comes to good chromatography as are the timings for each part of the injection cycle, from flash volatilisation to transfer and focusing. Cleaning the inlet between injections requires septum purge and split flows with correct empirically derived time intervals.

Advanced inlets allow hot and cold injections, greatly extending the scope of analyses to e.g. large volume injection (LVI) to enable the analysis of compounds compromised by discrimination, thermal lability and very low concentration. At the extreme end of the transfer process, cold trapping and the solvent effect, when harnessed, can also greatly improve early peak shape.

LC sample introduction, whilst simpler than GC, still requires discipline to achieve a tight sample band avoiding peak broadening. The injection solvent is ideally the mobile phase, if it isn’t then it needs to be weaker than the mobile phase for ideal partitioning and better peak shape.

The main challenge with a liquid mobile phase is the large differential between the sample in the vial at atmospheric pressure being injected into 5000 psi of column back pressure. A high pressure switching valve is required which can be operated manually or via an autosampler. The injector is usually a 6 port rheodyne valve, with choice of sample loop size determined by the column loading characteristics and the detector sensitivity. Autosampler injection is preferable to manual injection due to better precision, being able to run large sequences and reduced carryover due to automated wash cycles offering both weak and strong washes.

At the extreme end of this transfer process, injection modes can be either full loop which uses more sample but gives the best precision, partial loop with pressure assist which gives the fastest injection cycle, partial loop with needle overfill which gives the widest injection volume range or injection with flow through needle which gives the lowest carryover of all.

In conclusion, the challenges are different across GC and LC sample introduction but the core principles involved in building a robust method, as always, underpin everything.

To learn more about sample introduction techniques for GC & GC-MS, attend Day 1 of our Complete GC & GC-MS course and Module 2 of our Virtual Classroom Complete GC & GC-MS course.

To learn more about sample introduction techniques for HPLC & LC-MS, attend Day 1 of our Complete HPLC & LC-MS course and Module 3 of our Virtual Classroom Practical Essentials of HPLC & LC-MS course.

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