Introduction
Chromatographic fronting is a type of asymmetrical peak in which the earlier-eluting side of the peak is wider than the later-eluting, right side of the peak. Visually, this gives us a peak that is less steep on its left side than it is on its right side.
“Peak fronting” describes a shape that can give us essential information about our chromatography.
A peak distorted from its apex towards the earlier part of the chromatogram is an indication that some of the given compound’s molecules are eluting unusually early. Why would that be the case? The good news is that there are a few typical causes of peak fronting:
- Overloading
- Co-elution
- Mismatch between the sample and the chromatographic system
- Column degradation
The Most Common Cause - Overloading
Often times, a case of peak fronting results from overloading the chromatographic system with too much sample. This can mean overloading through excess volume of sample, but it can also mean using a sample with too high a concentration.
Thankfully, these common causes have fairly straight-forward solutions.
- Using less sample (either through reduced injection volume or injecting a lower concentration)
- Changing your column dimensions
- Changing your stationary phase
If circumstances allow, try injecting less sample or introduce less mass on-column through another means. For example, consider diluting the sample or operating in split mode for GC.
If adjusting the injected volume or concentration isn’t a viable option, but you are able to change the column, there are options that can help alleviate column overloading. For GC, using a thicker film column and/or a wider ID column will likely help. A larger ID LC column may also do the trick.
Changing your stationary phase is another possible solution. It might be helpful to re-evaluate your stationary phase if you’re noticing that some peaks are symmetrical while others are fronting. If both compounds are present at roughly the same concentrations, there may be an issue of overall solubility in the stationary phase. Compounds that have less affinity for a given stationary phase are prone to overloading and fronting at concentrations that produce symmetrical peaks for compounds that have a high affinity for the stationary phase. Giving consideration to the compatibility of your stationary phase and your sample may be the answer.
Co-Eluting Peaks: Is it One Fronting Peak, or Multiple Co-Eluting Peaks?
When looking at an unusually shaped peak, one of the first questions we should consider is whether the distorted peak is, in fact, a record of a single compound. The description of a fronting peak as a reflection of com molecules are eluting early only applies if the peak is a record of a single compound’s molecules!
The fixes above (using less sample, changing your column, changing your stationary phase) may be enough to reveal that the apparent fronting is actually a co-elution. However, determining if your peak shape issue is a separation issue is an important part of resolving the problem.
This can be easily done with a mass spectrometer, assuming the compounds don’t share the same mass spectrum. Even without an MS, however, there are signs that additional compounds may be present.
When co-elution is the cause, you may notice that the fronting peak appears in the sample, but does not appear in calibration standards, for instance.
In addition, changing the method conditions to attempt to tease apart the co-elutions may be helpful for diagnosing the problem. If you try this approach, remember that while you may be tempted to lengthen the run by slowing mobile phase flow rate or reducing a temperature of mobile phase composition gradient, sometimes it is actually more successful to reduce the time the compounds spend in the column. This technique can result in narrower peaks and provide you with just enough resolution to discover a sneaky peak.
Match or Mismatch? Assure Compatibility between the Sample and Your Chromatographic System
If only some peaks are exhibiting fronting, this can be a sign that there is a mismatch between your sample and your chromatographic system. This will typically be observed for the earliest-eluting peaks, with the effect getting better with later and later eluting compounds. If you have ruled out co-elutions, then you may be dealing with incompatibility between the sample solvent and either the stationary phase or the initial mobile phase conditions for LC.
The mismatch could be between the polarity of the solvent and the stationary phase in your GC column. For example, poor wetting of a GC column phase can result in peak splitting which may, depending on the severity, have the look of a fronting peak.
In LC, if the sample solvent (diluent) is very different than the initial mobile phase composition, that might also result in peak tailing. If possible, preparing your sample using mobile phase as a solvent can help resolve this issue.
Column Degradation
Exceptions probably exist for just about every type of issue a chromatographer encounters, and it’s true for peak fronting. You may encounter something unusual that turns out to have a different cause.
For instance, if peak fronting is occurring over time, along with an overall loss of retention in your LC analysis, you may be experiencing something called “phase collapse.” Phase collapse can occur in reversed phase HPLC, if highly aqueous mobile phase conditions have caused the hydrophobic stationary phase to “self-associate” and become less available for interactions with your sample.
Another unusual, but not unheard-of possibility is that an LC column’s packed bed may have deteriorated over time, creating “channeling” that might look like a fronting peak.
Of course, in these cases, the significant loss of retention should be another important clue that it isn’t just a matter of column overloading.
PLOT GC Columns – Where Tailing may be “Fronting”
As a final case, it’s worth noting that the most common cause of peak fronting (i.e., overloading) can actually cause peak shapes that are the opposite of fronting in certain circumstances. Overloading a PLOT GC column in particular may frequently appear like a tailing peak, adding to the already myriad reasons why peaks tail.
Nobody ever said chromatography was going to be easy, but knowledge and experience are a chromatographer’s best troubleshooting tools. Identifying the causes of peak fronting is a great first step to finding ways to combat it.
Column Selection
Picking the right column for your sample is the first, best step to avoid problems like peak fronting. You can use our GC and LC Columns Catalogs to find a column that will be just right for your analysis.