What do Chromatograms tell us? Peak Shape: Overload using Liquid Phases3 May 2013
Chromatograms are like fingerprints. If you can “read” chromatograms well, you often can find a plausible cause. In this series, we will show a series of GC-chromatograms that are obtained from users and discuss some potential causes for the phenomena. Then we can move into some solutions for improvement.
Fig. 1 a problem we see many times: peaks have a "lead", which is an indicator for overload in gas-liquid separations. Separations become challenging.
A peak shape is observed as shown in figure 1. Peaks start to rise slow, reach their maximum height and sharply fall down. Asymmetry factors can be very big. In gas liquid chromatography, using bonded siloxane or polyethylene glycol-type phases, this peak shape is typically obtained if a component is overloaded.
If the component elutes well-resolved from neighbor peaks, you can work with that as the total area will represent the absolute amount brought on to the column.
Problems develop if another component is eluting close to this peak.
To solve this one can:
Fig. 2 If we allow only 50% of the sample onto the column, the separation improves as peaks will be more symmetrical.
Inject less sample: work in the loadability range of the capillary used. This can be done by dilution of sample, injecting less or using a higher split-ratio; Figure 2 shows the separation if 50% of the sample is injected on to the column.
Use a column with higher loadability: loadability is increased by having more stationary phase in the column. Most easy is to use the same dimension but increase film thickness. Note that increasing the amount of phase or length also will increase retention time. Choosing a higher oven temperature may compensate for that. For every factor 2 increase in film thickness, the oven temperature must be set 15C higher to obtain a comparable retention time. One can also use longer columns and larger diameter to increase loadability, but the effect will be the biggest if film-thickness is increased. Maximum film thickness for non polar phases is about 5-7 um; for polar phases it is about 2 um. If thicker films are used the efficiency of capillaries will rapidly decrease because of the increase of the Cl-term in the Van Deemter equation. (this is the contribution to band-broadening that happens if the component is in the liquid phase)
Fig. 3 Polar compounds (acids) are easily overloaded on a non-polar phase: result: shark-fin peaks.
Fig.4 acids have more solubility in polar phase like Stabilwax-DA. Peak shape remains good even at higher concentrations.
Use a phase with better solubility for the target analytes; sometimes peaks elute like shark-fins because the solubility is very low. Fig.3 shows free fatty acids on a Rtx-1 type phase. One can increase the film thickness, but still experience overload. Using an acid deactivated polar phase, like a Stabilwax-DA, offers highly increased loadability for these highly polar compounds, see fig.4.