Chromatography Forum

Hello list,

Peak fronting is relatively a rare phenomenon in RPLC as most peaks are tailed. I am trying to understand as to what conditions will "predictably" lead to peak fronting under overloading conditions (e.g. 20 uL injection of 500 mM anion) in ion chromatography. The system is simple: overloading bromide ion on an anion exchanger IC column with conductivity detection. The sample and the eluent are totally aqueous. It turns out that when sodium carbonate is used an an eluent at low conc. i.e. 2.5 mM, peaks front, but when we switch to high carbonate say 25 mM carbonate, peaks tail. What could be fundamentally causing this transition in peak shape? I am curious to see what RPLC users have to say.

Regards.

Fronting Sources
Injection disrupting equilibrium
Column voiding
Guard column degradation / contamination
These are common sources of fronting. In addition so called extra column effects can be involved.

Hope that will help.

Fronting Sources
Injection disrupting equilibrium
Column voiding
Guard column degradation / contamination
These are common sources of fronting. In addition so called extra column effects can be involved.

Hope that will help.

Thanks for your reply. But we are looking at fronting from a more fundamental perspective. Let us assume, column voiding, contamination, injection disruptions are absent. What type interactions of the analyte to the stationary phase can lead to fronting in an overloaded case?

Thanks.

Farooq

Some of the ionic compounds will form a front and some will tail. This can be related to overloading of silanols, problem with pH or reduced buffering capacity of your mobile phase. Usually injecting less will improve tailing/fronting. Running different pH can address the issue too.

viewtopic.php?f=31&t=18426

Fronting is covered about 8.5 minutes in, but you need to watch the first 8 minutes to put it in perspective.

http://www.chromforum.org/viewtopic.php?f=31&t=18426

Fronting is covered about 8.5 minutes in, but you need to watch the first 8 minutes to put it in perspective.

Your lecture is nice and it approaches the fronting problem along the same lines of Langmuirian and anti-Langmuirian peak shapes. I wish I could attach a figure from Guichon's paper here; it shows that by just changing the concentration of one adsorbing additive in a narrow range of 0.1 to 2%, one can manipulate peak shape from tailing, fronting, to partial split, and at least six other forms. The reference is Journal of Chromatography, 461 (1988) 19-34. Figure 6 is the most interesting one. Is there a way to attach to show the figure to you? In general in ion exclusion it is mentioned that using low ionic strength mobile phase leads to fronting peaks. What mechanism could ionic strength be playing in peak fronting on a polymeric charge bearing substrate?
My interest was that in ion-exchange the eluent ions (say carbonate, hydroxide) are definitely very strongly adsorbed by ion exchange phase and so is the analyte.

I'm a practitioner rather than a theoretician, so I couldn't begin to speculate about the detailed mechanism beyond the "boy scout merit-badge" level . At that level, I can imagine that peak shape issues could be driven by mobile-phase effects (e.g., limited solubility as the example I used in the chalk-talk) or stationary-phase effects (the presence of an analyte molecule on/in the stationary phase makes it *more* likely that the next molecule will stick). I would expect more non-linear behavior (in either direction! ) at low buffer concentration simply because the analyte ions represent a larger fraction of the total ionic strength; in effect, there's not enough buffer to "swamp out" the analyte interactions. Beyond that, your guess is better than mine!

By the way, the Forum doesn't allow direct posting, but you can display graphics by linking to them:
viewtopic.php?f=1&t=2617

The only time the fronted peaks under overload conditions when low ionic strength carbonate (2.5 mM) was used an eluent on the anion exchanger. When higher ionic strength was used peak shape shifted to pure tailing (25 mM carbonate) under overload. It does seem that ionic strength is playing a role here, but the rest of the eluents (borate, NaOH) always show tailing under ionic strength. I am trying to understand as to why only carbonate showed this behaviour. Unfortunately explanations don't exist for ion-exchangers. However there is an explanation in one paper for ion-exclusion chromatography. The authors note a similar behaviour:

"However, fronted or ill-defined peak shapes were observed when ionic strength was low. This behaviour may be explained as follows. When an analyte is injected into an eluent of low ionic strength the initial peak shape (at time t0) can be assumed to be Gaussian. The ionic strength of the sample band is much higher than the surrounding eluent. As the sample passes through the column dilution of the sample band in the eluent will occur (t1), with this dilution being more pronounced at the extremities of the band. This results in the ionic strength of the solution being less at the edges of the band than at the centre. Analyte at the leading edge of the band will move progressively further away from the centre of the band because retention is reduced at lower ionic strength, whilst analyte at the trailing edge will “catch up” to the centre of the band by the same mechanism. This effect continues as the analyte band moves through the column so that when the band is eluted from the column (t2) the peak shape is fronted. This mechanism is analogous to that proposed for weak acids in ion-exclusion chromatography using poorly buffered eluents".


Do you agree with the statement (in bold) saying that retention is reduced at lower ionic strength? In fronting, peak times shift towards longer times rather than observing a reduction in retention. What is your view?

Regards,

Generally (and, again, at the boy scout merit-badge level) we do associate peak shape problems with overload, in which case fronting *will* be associated with the bulk of the peak increasing. However, if you visualize an s-shaped isotherm (and I have no idea what mechanism would create it!) like the one in the sketch below, then you would see fronting when you *underload*, and that would indeed result from decreasing retention of the peak edges.

As alluded to above, with low ionic strength carbonate eluent the analyte ions could ovecome the buffer capacity of the eluent and cause a local change in pH and thus a change ionic strength.

I fully agree with Carl, as my experience goes right in this direction: low ionic strenght in buffers in IEC causes "strange" peak behavior.

I fully agree with Carl, as my experience goes right in this direction: low ionic strenght in buffers in IEC causes "strange" peak behavior.

I agree with both of you that low ionic strength can alter pH "local" pH if we have mass overload, but recall that if we are dealing with an strong anion exchanger, it's behaviour should be independent of pH. So still the fundamental reason of low ionic strength eluent showing strange peak shape stands unclear.

Should be, but in IEC at least is not. Slight changes in pH or ionic strength generates major effects in peak shape... the problem as that I have been "theorizing" in its reasons for ages and never reached a logical and "definitive" conclusion - also a reason for being so interested in this subject!

Should be, but in IEC at least is not. Slight changes in pH or ionic strength generates major effects in peak shape... the problem as that I have been "theorizing" in its reasons for ages and never reached a logical and "definitive" conclusion - also a reason for being so interested in this subject!

Good to find someone interested in the topic. We have found some definite trends there in IC...but still in the confirmatory mode as to what condition will lead to fronting and what condition will lead to fronting. Great help came from Guiochon's paper that says that it is all a matter of adsorptive competition in what in the eluent and the analyte. I can discusss more details if you are interested via email.

I would say it's a good theory, as that might be some partition phenomena with the apolar part of molecules with apolar phase and buffers themselves.