Friends;

I am looking for a good description of the causes of "foot shaped" signals in chromatography. This is the type of peak in which you have a slow raising, or plateau shaped, front end in the peak. Otherwise these signals look OK, except that they seem to have tailing quite often as well.

I have heard some discussion about column overloading, silanol activity, metal interaction etc. However, I feel that all these do not adequately explain these peak shapes.

If you know of a good reference that explains these signals and recommends ways to improve them, I would like to hear about them.

pH of the mobile phase or starting phase can also cause this. Your analyte must be either ionized or not ionized in both the mobile phase and the sample solution to give good peak shapes.

Hello Benjamin,
fronting and what you called "foot shaped" signals can come from different problems, from instrument side, from your sample solution, from your mobile phase, and sometimes from a demaged column. Most troubleshooting sections in HPLC books give you some ideas. The way to eliminate such effects is a systematic check. First check injection system. Than check capillaries and all connections before testing your column. Michele is right, pH plays an important role. So if your system is ok, column has shown good performance during test, than have a close look on pH. But this step you should do when developing the method. Also such effects could come from other compounds under your peak. Maybe degradation products, or sample compounds are enriched on your column, and than eluting after some time. When I had such effects in my lab, I went through this systematic check system. Out of my experiance, the only way to eliminate such effects is first a good maintenance of the HPLC system, second to be very careful when doing method development. And one very important point, regarding the column. As long as you will have for every method one specific column, and even for method development you should use also a new column, not a used one, than you be on the good way. Good luck. Gerhard

If a peak "fronts" it means some of the material in the peak was less retained during some part of the separation than the bulk of the material. Often, the fronted material moved out ahead of the main peak at the time of injection because something caused this part of the material to be less retained than normal.

1. Too much material in injection. If there is too much material in the injected sample the capacity of the column to absorb the sample in a narrow band can be exceeded. The fraction not normally absorbed will travel ahaead of the rest of the sample and cause a fronted peak. Dilute the sample 1/10 and if the problem goes away you have identified the source of your problem.

2. Sample solvent strength too strong for sample volumn injected. If the elution strength of the sample solvent is stronger than the elution strength of the mobil phase into which the sample is injected, AND, the sample volumn is too large, then the column may have too little capacity to absorb the injection into a narrow band. The result will be the same as described in #1. Dilute sample in weaker solvent or inject smaller volumn (5-10 uL) and see if problem goes away.
Note, the "solvent strength" of the sample could be a function of pH. So, for example, if an organic acid sample was dissolved in base, and the mobil phase had insufficient acidity to neutralize the base in the injection volumn, then the sample would not absorb as a narrow band at the head of the column and a fronted peak would result. (this example presumes the neutral form of the acid is more retained than the anion)

3. Sample decomposition to a less retained species. If the analyte decomposes in the mobil phase to something less retained, and the decomposition product is also detected by the detector,then a fronting peak will result. As the analyte moves down the column some of it is decomposing to a second less retained species, which will move out ahead of the main peak because it is less retained. A classic example of this phenomenon is in the separation of ahydrides. The acid formed by hydrolysis is always less retained and moves out ahead of the anhydride peak and it is detected as a fronting peak.

4. Column has a channel through the bed. Material traveling the channel takes less time to traverse the column and appears as a front.

Generally, I concur with all of the above. However, I think two points generally cover causes of a peak with a leading edge:

1). The column bed has become damaged. Most of the time the problem is caused by a flow "shortcut" around the outer perimeter of the column bed allowing some of the analyte to bypass the column bed. Less often, this problem can because by plugging of the inlet bed support. At least in a few cases I have corrected a problem of this sort by replacing the bed support.

2). The analyte has a significantly higher affinity for the stationary phase than the eluent species. In this case, the symptom manifests itself as a reduced column loading capacity. If one exceeds this reduced capacity inherent with the eluent system, and then the peak shape will be skewed with a leading edge. As one exceeds the loading capacity with progressively higher injection amounts, the asymmetry will increase. Generally, at some injection concentration you will observe a symmetrical peak although this may not occur at useful concentrations. Generally the best correction for this situation is to use an eluent with stationary phase affinity more comparable to that of the analyte.

In my experience, contrary to the suggestion above by Gerhardt, fluid connections and fittings can not cause this type of peak asymmetry (although they can certainly cause tailing peaks).

I agree with the causes above, as I have seen most of them occur, but would like to add one of my own.
I have seen cases where the extract was not completely clean (e.g. extracting drugs from blood plasma) and the front end of the column was slowly coated with retained material. This essentially lead to fronting of the peaks.

Really depends on if it is a persistant effect or if it slowly builds up over time.

All the above are excellent suggestions of course but might I add that if the analyte is a chelator then I have seen the sort of foot-shaped peak being described. I have seen examples (on my own system) where the chelator pulled trace metals from ??? and the metal complex they formed had a shorter r.t. than the chelator itself. Since the complexes seemed to be formed during elution they had that odd peak shape being described. Just a thought, take it for whatever its worth.

Regards,
Mark

I'm afraid I have to correct my earlier characterization of the "overloading" based cause of "fronting" peaks. I guess I was just thinking backward! Actually, peaks with this characteristic have lower affinity relative to the eluent species. In essence, as the analyte concentration exceeds a critical level, the elution power of the analyte starts to effect its own retention. If the analyte comprises a significantly weaker "eluent" than the eluent species, the elution rate of the analyte will decrease as a concentration of the analyte increases. Once the concentration passes the concentration maximum for the analyte peak, the situation reverses causing a sharp downward drop on the back side of the peak.

By the way, you can find a detailed explanation of this phenomenon described in Hamish Small's book: Ion Chromtography, Plenum Press 1990, pp 20-21