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GC Troubleshooting Guide to Broad, Double/Split, Fronting Peaks

The Problem

This is a pretty horrid looking chromatogram that would certainly make most chromatographers despair (Figure 1). 
The peaks are split, broad, and fronting.

The sample is a test mix consisting of 6 analytes, however, from the chromatogram one might be fooled into thinking there could be up to twelve components in the mixture.

Column: 5% Phenyl PDMS.  30 m x 0.25 mm x 0.25 µm
Temperature: 40 °C, 2 min.
40-175 °C, 1.5 °C/min., 20 min.
Inlet: Splitless (purge 2.0 min.), 250 °C
Sample: 1 µL of 2 ppm each (1) 1-octanol, (2) n-undecane,
(3) 2,6-dimethylphenol, (4) 2,6-dimethylanaline,
(5) n-dodecane, (6) n-tridecane in methanol

 

 

This page is part of our series of 6 free Chromatography Troubleshooting Guides.

HPLC

Changes in Selectivity, Retention
and Resolution »

Cycling Baselines and Pressure Fluctuations »

Solving HPLC Peak Splitting Problems »

GC

Broad Peaks and Loss of MS Sensitivity »

Broad, Double/Split, Fronting Peaks »

Peak Area Reproducibility and Reduction
of Sensitivity »

Figure 1: GC chromatogram showing split, broad, and fronting peaks.

 

The CHROMacademy GC Troubleshooter

From our chromatogram we can see that the peaks are fronting, split, and broadened. Using the very unique and intuitive troubleshooter interface, both chromatographic and instrument symptoms can be combined to better specify the problem and derive a more accurate diagnosis of the issue (Figure 2).

There are pictorial representations of each symptom to aid in selecting the correct symptom for your problem. The ability to combine the symptoms pushes the boundaries of regular troubleshooting where traditionally you would have had to deal with each symptom individually.

Figure 2: Input of all three chromatographic symptoms into the CHROMacademy GC Troubleshooter.

 

Hey presto!  In two simple steps the troubleshooter has produced 27 possible causes for the symptoms we have entered (Figure 3).  If we had been using the traditional means of troubleshooting and checked a textbook or wall chart we would doubtlessly have come up with a similar list of possible causes.  However, how would we have decided where to start?  There are always the classic mantras of:

  • Don’t forget the obvious or assume anything.  Check everything.
  • Check the easy things first.
  • Check one thing at a time.

However, the GC Troubleshooter has taken the guess work out of the equation.  Each cause is given a star rating, calculated using our unique and proprietary algorithm, based on the number of symptoms that are matched and the likelihood of this combination of symptoms leading to a certain issue.  The algorithm was developed over several years based on the knowledge and experience of over 70 highly respected chromatographers from across the world. 

Whilst you may still have to work through a couple of the options; however, hopefully the solution will present itself fairly quickly. When you click on one of the causes the solution window displays information on how to remedy the problem and, where applicable, how to avoid it in the future.

 

Figure 3: CHROMacademy GC Troubleshooter results page, including causes of braod, double/split, and fronting peaks,
solutions to each individual cause, and further resources for resolving the problem.

 

The Solution

Using our troubleshooting mantra of try one thing at a time we started at the top of the list with the first cause.

1. Column overload

If you see fronting peaks, think overload, it is a great place to start.  Capillary GC columns have a finite amount of stationary phase which can become saturated with the analyte and sample components causing the sample band to ‘smear’ down the column stationary phase until it encounters an area which is not saturated.  The capacity of the column will depend on the column internal diameter, stationary phase film thickness, and column temperature.  Column overload can be remedied by reducing the sample volume, concentration, or increasing the split ratio (when using split injection). 
In our case the peaks are fronting but this is not the only problem, hence, this is unlikely to be the primary cause of the very poor peak shapes.  Therefore, it is on to the next cause on the list.

2. Sample solvent/stationary phase chemistry mismatch

In splitless injection sample transfer from the inlet to the column is slow which results in broad peaks.  This can be overcome through focusing mechanisms, one of which is solvent focusing.  This involves the low boiling (more highly volatile) components remaining dissolved in the solvent which condenses in the inner walls of the GC column at low initial oven temperatures.  The solvent slowly evaporates to give a narrow concentrated analyte band.  In order for this mechanism to function correctly the polarity of the sample solvent and stationary phase must match, otherwise the deposition if the liquid film onto the inner walls of the column will not be optimal leading to deleterious peak shapes.

You may have noticed in the method conditions outlined earlier that the column being used has a 5% phenyl PDMS stationary phase (this is relatively non-polar) and the injection solvent is methanol (a polar solvent).  Clearly the solvent and stationary phase are not well matched. 
The solvent was changed to dichloromethane to produce an excellent chromatogram which has good peak shapes and the correct number of peaks for the sample
(Figure 4).  Note, the peaks are not fronting, therefore, there is no overload and the original injection volume and concentration are fine for this analysis.

 

Figure 4: Good chromatogram obtained when injection solvent was changed from methanol to dichloromethane.

 

For further on poor GC peak shapes see the following CHROMacademy pages:

Peak Shape Issues section »

Inlet Problems: Chromatographic and Instrument Symptoms and Diagnostic Tests section »

 

Have you ever had a problem with the following symptoms?

Peaks broadening

 

 FID: Unstable baseline

Use the CHROMacademy GC Troubleshooter to see what could be causing the problem and how to resolve it.

GC troubleshooting

 
 

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