How to Improve Late Eluting Peaks
by Kory Kelly, GC Product Manager and Kristen Parnell, GC Brand Manager
Peaks that elute at the very end of a chromatogram can sometimes be less intense, more broad earlier peaks, and may even have tailing peak shape. Most often, these problems can easily be fixed by adjusting the temperatures of the oven or inlet. Below are some recommendations to help improve late eluting peak response and peak shape.
- Increase the final temperature of the program. Peaks that elute on a very slow or isothermal temperature ramp will be broader. These peaks spend more time transitioning in and out of the phase which gives the peak a slower velocity out of the column. Defined peak width at a slower velocity gives wider peaks. (10cm peak @ 2cm/sec = 5 second wide peak) Increasing the temperature will force the analytes more into the mobile phase and thereby increase their linear velocity (10cm peak @ 5cm/sec = 2 second wide peak). A narrower peak also means taller signal and better sensitivity. Keep in mind that every column has an upper temperature limit. Getting close to or exceeding that temperature limit will result in damage to the phase and cause bleed. This is one great advantage of the XLB-HT and other Inferno columns that offer higher maximum temperatures that may exceed 400°C. Even standard Zebron phases may offer higher temperature limits than other ‘equivalent’ columns like the ZB-5 which offers a maximum temperature higher than competitors.
- Increase the flow. This will also push analytes off of the column faster. The downside is this may effect elution orders of closely eluting peaks. One way to prevent peak switching of earlier eluting peaks is to use flow programming. Keep the normal flow for most of the chromatogram and then increase flow during the last portion of the chromatogram. If using MS, make sure not to provide too much flow that will overload the pumping capacity of the vacuum pumps.
- Decrease the film thickness. A thicker film will offer greater retention for all compounds, especially late eluting compounds. Selecting a thinner film can have drastic differences in retention and cause analytes to elute at much lower temperatures. This will also decrease bleed levels. Verify retention order after making any system changes though as these may result in differences in elution orders, especially for closely eluting peaks.
- If using MS, increase the transfer line temperature. Ensure that the transfer line temperature is higher (at least 5°C higher but 10-15°C is better) than the oven maximum oven program temperature. This may remove any cold spots from the flow path that may cause the analytes to condense or slow down and result broad or tailing peaks.
- Increase the temperature of the inlet. Late eluting peaks are less volatile and require higher temperatures to be forced to the vapor phase. If the temperature of the inlet isn’t high enough, higher boiling point compounds may not transfer well from the inlet onto the column. This trend is known as inlet discrimination and can me minimized by increasing the inlet temperature.