How to Recognize and Eliminate Ghost- and Deformed Peaks in Gas Chromatography

1. How to Recognize and Eliminate Ghost- and Deformed Peaks in Gas Chromatography Jaap De Zeeuw, Restek Corporation, Middelburg, The Netherlands Summary Decomposition of components in an active inlet. If components degrade in a hot inlet, extra peaks will be observed in the chromatogram. The reactivity can be reduced by using highly inert liners (for instance the latest sky-liners). Use lowest possible injection temperature; Often a good injection is possible at a lower liner temperature. Do not use wool; With splitless use a pressure pulse to make transfer faster; Consider to use a different technique that is “milder” for your components: This can be a PTV or the cold-on-column Contamination via Gloves. In our aim to work as clean as possible, we sometimes overlook the obvious. In one of our trace analysis methods, we saw systematically a set of ghost peaks appear. After analysis a small piece of the glove, we found the cause.. Think about it: If you can “smell” something, it’s basically a head space, which just needs concentration to be visible.. The chromatogram is like a fingerprint. If you can read the chromatogram by looking at peak shapes, retention, base line and by comparing with “normal” situation, you have a good chance to solve problems and improve the analysis. A ghost peak is a peak that is showing up, but is not supposed to be there. Sometimes it is referred as a “system” peak. Ghost peaks can be created in many ways. It’s a component that is added/created somewhere in the system, it is injected/trapped/focused onto the column, and will elute. Problems will escalate if a ghost peak interferes with an analyte that has to be quantified. Sources for ghost peaks can be sample vials, gloves, syringes, reagents, carrier gas, tubing, the injection port, operation, memory effects and even the column-phase itself. What can cause a ghost or a deformed peak? Septa is a known source for ghost peaks. Septa is mostly 100% siloxane polymer. When heated it will form bleed products. These products are cyclic siloxanes that show as peaks. Intensity depends on septum type, age and temperature. To reduce: always use septum purge, use low injection temperatures, replace septa regularly. You can also consider to use a septum-less system like a Merlin Seal. “A ghost peak is a peak that appears at a position where we do not expect a peak”. It is a component that shows up in the system and it may show up everywhere in the chromatogram. Sometimes the peak is a sharp peak, sometimes it’s a broad deformed peak or a “hump”, sometime sit is a rising base line.. Some of the key origins for ghost or deformed peaks are: • The purity of the carrier gas • Using non-GC grade carrier gas tubing • Memory effects due to back flash • Contamination of injection port • Injection port reactivity • Septum, O-rings • Analytical column bleed • Column installation • Sample contamination from syringe, vials, vial-septa, gloves Septa particles in the liner can be a big problem. As they are at high temperature, they form siloxanes that are injected on the column. Such particles will also “delay” sample transfer as the siloxane will also retain components. Liner condition upon injection is very important as here the chromatography starts. Use needles with more friendly (tapered/bended) top. Also a septum with a center guide, like the BTO, can be used. Replace liners systematically. If the precision liner is used the septa particles will remain on the top of the wool. Extra peaks showing up as large noise produced by the stationary phase upon fast cooling. High bleed in a column can focus when the oven is cooled down. Because oven will always cool more on one side, there will be a focusing effect for bleed products. In every column winding, bleed will focus. Next run, all these focused bleed-peaks, will elute. We see this especially with high bleed (cyano) phases. Solution is to use temperature controlled cool down. The right signal above was obtained with a -10ºC/min cool program. This effect will depend strongly on the GC type and the temperature gradient in the oven upon cooling. Other sources for Ghost peaks are shown above: Using the MS makes the identification easier. Besides siloxanes, septa also can contain phthalates. (m/z 149,167 and 279) The m/z 277 is indicator for a contaminant that is often released from the O-ring. Carry over of PAH is caused by contamination due to a previous injection, probably to back-flash. Dirty tubing will release residual hydrocarbons, which will be trapped on the column and show up as “ghost” peaks. Use only GC grade copper tubing. Trace 1 in the figure above, shows the materials that elute from a “clean” batch of copper tubing. Specific cleaning procedures eliminate this risk. In column decomposition: Thermo labile components will start to decompose when the temperature reached a certain value. when the decomposition product has less retention, a typical peaks shape is observed as above. Here a brominated diphenylether, BDE-209 is measured. Important is to reduce elution temperature and column activity; this is realized by: 1 Use high flow rate; 2 Slow temperature program; 3 Use thin films; 4 Use short columns; 5 Use inert surface (Rtx-1614); Contamination of Split line can cause broad “blobby” peaks. When sample is splitted, the lower outlet temperature makes sample condense. This can built up a contamination and will be a source for ghost peaks. On the right: A: split line nearly blocked; B: after cleaning Sample contamination in the vial: Just by injecting 2 times through the vial – seal, all the peaks above were introduced. Ghost peaks can be caused by cross-contamination via the wash vial,. Also when the needle punctures through a vial septum, each time the outside will take material of the septum. Select sealing materials that are OK with your solvent. Also clean rinse vials regularly or use multiple rinse vials. Want to learn Chromatography,Tips & Tricks and troubleshooting? Visit ChromaBLOGraphy at www.Restek.com When a system is contaminated, a base line like above can develop. Here hydrocarbon oils were back-flashed in the gas lines. This was confirmed by the MS spectrum. Make sure that you do not inject more then the liner volume. Note that each solvent has a different expansion volume. To prevent, use larger diameter liners, inject less and know the liner and expansion volume of your solvent. Regularly check the split line and clean it. There are also split line filters. Check also flow of split outlet with a flow meter.