Chromatograms are like fingerprints. If you can “read” chromatograms well, you often can find a plausible cause. In this series, we will show a series of GC-chromatograms that are obtained from users and discuss some potential causes for the phenomena. Then we can move into some solutions for improvement.
Sometimes chromatogram is obtained as in fig. 1. The separation is very good, but the base line shows “spikes”. Such a spike is not a problem if it happens on the base line, but if a peak elutes, the spike can cause a wrong value and the analysis has to be repeated.
Spikes are electrical signals (pulses), that are initiated by an external event. Sometimes a device in the lab that “switches” electronically, can generate a spike. Usually if it’s a “switch, one can locate it as the spikes will appear systematically.
If spikes appear more randomly, it’s probably not a system in the lab.(although we have to be careful here as sometimes a cryo-valve or even a vacuum cleaner can produce a signal also, that we pick up), and these signals are also not very predictable.
If we look in our GC system, the most probable sources for spikes are electric contacts that have been oxidized. In detection systems, the connections of the electrodes have to be checked as well as the connecting plugs.
If it is not electronics, we also have to suspect the GC setup. Depending on the application, also the separation column can cause spikes. Especially if a PLOT column is used, spiking can be a big problem. PLOT columns are coated with particles and sometimes such particles can elute and “hit” the detector electrodes, thus forming a spike. Spike level depend on stability and operation of the PLOT column.
A solution is to use particle traps which work similar like the analogy you see in fig 2. When you are on the beach amd there is a lot of wind, the sand(particles) blow all over the beach. The moment the sand particles reach the water front, they disappear.
A particle trap works exactly like this. It remains however a solution to work with a relative “unstable” PLOT column. Particles always will accumulate somewhere, see fig.3. I saw recently some developments offering integrated particle traps on PLOT columns. This is nice, as it prevents the coupling with a particle trap, but it does not prevent the particles from accumulating and forming restrictions.
New generation PLOT columns do not need particle traps, because they are based on more “robust” stabilization technology, see: http://www.restek.com/pdfs/B06-Handout-Poster_PLOT.pdf
Transportation can always generate some loose particles in a PLOT column. If you plan to use a PLOT with a valves or a flow switching device, always flush the column at a 50% higher flow, while it’s outlet not connected and run a temperature program.This will allow the loose particles formed during transport, to elute. After that the columns can be used as with new generation PLOT columns, the chance on release of new particles, is minimal.
Tags: "PLOT Columns"