October 6, 2022
By Baljit Bains, Marketing Communications Specialist, ACD/Labs
The highly coveted Gaussian peak, a sharp symmetrical shape on a flat baseline, is the ideal peak shape in chromatography. Why is it so highly desired? Naturally, we tend to be drawn to the beauty of symmetry, but more importantly, good peak shape is important for better resolution (Rs) and increased accuracy in quantitation.
The perfect Gaussian peak is not all that common. Peak fronting, peak tailing, and peak splitting are peak abnormalities that frequently occur in chromatography.
What is Peak Fronting in Chromatography?
Peak fronting occurs when an asymmetric peak is broader in the first half and narrower in the second half. Several factors may cause peak fronting including poor sample solubility, column collapse, or saturation/overload of the column.
If the sample has poor solubility, it cannot be evenly dissolved into the mobile phase. This can be resolved by either reducing the injected sample’s volume or the solute’s concentration.
Column collapse is a sudden physical change in the column. This can be due to inappropriate conditions such as the temperature or pH of the column. Column collapse can be avoided by modifying the method so that the column is used within recommended limits, replacing the column with a more robust column, or routinely replacing the column (i.e., every 500 injections).
A column will have a maximum sample capacity. If this is exceeded there may be saturation/overload of the mobile phase. The additional molecules cannot partition between the stationary and mobile phase and will elute faster causing peak fronting. This can be prevented by reducing the amount of sample loaded on the column.
What is Peak Tailing in Chromatography?
Peak tailing is the inverse of peak fronting. The peak is asymmetrical, with a second half that is broader than the front half. Peak tailing can occur for one, a few, or even all the peaks in a chromatograph depending on the causing factor.
What Causes Peak Tailing and How To Avoid it?
Strong interactions between acidic silanol groups on column packing and basic functional groups of the analyte create secondary analyte interactions. This means that not all molecules will travel through the column at the same speed, and this causes the peaks to tail.
There are several ways to minimize these secondary interactions:
- Operate at a lower pH—Performing chromatographic separation at lower pH can ensure silanol groups are protonated and minimize secondary interactions.
- Use a Highly Deactivated Column—Use an “end-capped” column to reduce surface activity. End-capping involves converting residual silanol groups to less polar surface functional groups, reducing the potential secondary interaction they can have with polar analyte molecules.
- Add Buffers to the Mobile Phase of the Chromatography System—Using buffers in mobile phases can control pH, mask residual silanol interactions and reduce peak tailing.
Packing Bed Deformation
Packing bed deformation can occur as the result of the creation of a void at the inlet of the column, the presence of channels in the packing bed, or a collection of particles at the inlet frit.
To resolve this issue, firstly, determine whether there is a column void or blocked inlet frit. This can be done by substituting the column. If there is a suspected void, then reverse the column and wash with a strong solvent to remove any blocking contamination. Regularly replacing solvent filters and using in-line filters and guard columns can help avoid blockage of column frits.
If all peaks tail, then consider the possibility that the column has been mass overloaded.
Column overload can be assessed by diluting the sample and re-assessing the resulting peak shapes. It can be prevented by using a higher capacity stationary phase either with increased % carbon or pore size, using a column with a larger diameter, or decreasing the amount of sample introduced to the column.
Excessive Column Dead Volume
Excessive column dead volume usually affects the peak shape of early eluting peaks.
To avoid excessive dead volume in the column, exercise extra caution when pre-packing the column.
Contaminants in the material can enhance secondary interactions and cause peaks to tail.
Using the purest packing material possible will help to reduce the presence of contaminants, thereby minimizing peak tailing.
Defining Peak Tailing in Chromatography
Since it is very difficult to eliminate all peak tailing, quantifying tailing allows for an acceptable peak tailing limit to be established. There are several ways to measure peak shape, which are included in various chromatography data acquisition software.
There are two main methods for defining peak tailing: The Tailing Factor and the Asymmetry Factor.
*a = the width of the front half of the peak, b = the width of the back half of the peak
Either Tf or AS can be used to measure peak tailing, but not interchangeably.
Good peak shape can be defined by a tailing factor of 1.0, high efficiency, and narrow peak width. Variations indicate poor peak shape.
Why is Peak Tailing a Problem?
- Peaks are harder to accurately integrate
The transition from the baseline to peak or vice versa is much more gradual, making the peak harder to integrate. Peak tailing also causes sloping baselines which makes it difficult to determine peak limits.
- Shorter peaks
Peaks with larger As values tend to have shorter peak heights. Peak height is a limiting factor in determining detection limits, so method limits can be affected.
- Larger time window to be eluted
Peaks with tails can take longer to return to baseline resolution between peaks and therefore the sample will require a longer run time.
- Peak area may be miscalculated due to data analysis systems assigning the end of the peak inaccurately.
What is Peak Splitting in Chromatography?
Peak splitting is when a shoulder or ‘twin’ appears on a Gaussian peak. Split peaks can be caused by column overload, the mismatch between the strength of the mobile phase and injection solvent, a void/channel in the column, or a plugged frit. Peak splitting can indicate that there may be deficiencies in method development that need to be corrected.
How Many Peaks Are Affected and How to Fix Peak Splitting
If only a single peak has split, the problem is likely due to the separation itself. It may be the case that two components are eluting close together. One way to check this is by injecting a smaller sample volume and observing whether this results in two distinct peaks. It may also be that the mobile phase and sample solvent are incompatible. Here, the solvent needs adjusting, and samples need to be injected into the mobile phase. To improve separation resolution, parameters like mobile phase, temperature, flow rate, or column type need to be reconsidered.
If there is peak splitting for all the peaks, a problem occurs before separation and affects every peak similarly. The two common causes of this are a blocked frit and a void in the packing at the head of the column.
A blocked frit is where part of the sample is delayed in entering the column and causes the sample delivery to the column to be spread out thus affecting sample separation and causing all the peaks to split.
Solutions for a Blocked Frit
- Use in-line filters
- Reverse flush the column
- Replace the frit
A void in the packing material can appear as a settled packing bed or a wormhole in the packing, causing some of the sample material to travel faster in the column. The sample is therefore spread out before it enters the column, and all the peaks in the chromatograph are similarly affected and split.
Solutions for a Void in Column Packing
- Use a guard column
- Use a less aggressive mobile phase
- Use a more stable column
- Do a better sample clean-up
- A combination of all the above
Peak shape changes are commonly observed in chromatography, and it is possible to quantitatively track peak shape over time. The causes of peak fronting, peak tailing, and peak splitting are well known and best corrected during method development to ensure system robustness and reproducibility. Minimizing peak abnormalities can help to save time and money and allows for increased chromatograph accuracy.