Hello I am having variations in my retention times during a run. There isnt a signicant pattern of decreasing or increasing in a specific order, they just seem to be inconsistent.
What are some of the causes of this problem?

It's not clear what you mean. I take it the order of elution is okay, but the actual retention times shift from run to run, and there is no continuous upward or downward trend. Is that it?
Is the column in a thermostated oven? If not, then a change in lab temperature could have such an effect, albeit generally not a very large one.

Otherwise it sounds like a flow rate problem. This could be caused by the pump or a downstream blockage. Try running your effluent into a volumetric flask and timing the flow. Repeat this several times, while running chromatograms so you can correlate the measured flow rates with the changing retention times.

If flow proves to be the problem, the pump or check valves are the likely culprets. However blocked frits, poorly packed columns, leaks, gas in the check valves and other things can cause such problems.

If flow is not the problem, then you may have a more subtle problem. For example, a large injection may "coat" the column with sample, resulting in a change in the column behavior. This can be checked by changing injection volume or sample matrix. This kind of effect should not be observable with repeated injections of the same sample.

Another thing to alter retention times can be in how you make up your samples.
Are they mede up reproducibly?? If not this can also cause a slight shift in retention times.

Variable Retention Times

Common Causes

Leaks
Evaporation of pre-mixed mobil phases
Poorly degased solvents (especially with low pressue mixing)
Running out of He sparge gas
Malfuctioning vacuum degassers
Temperature fluctuations
Column Overload
Sample solvent incompatible with mobil phase
Column Degradation
Poorly performing seals/check valves/proportioning valves
Partially blocked inlet filters
Others that I can't think of right now.

It also should be noted that a small amount of variability is to be expected. The original message did not state the magnitude of the problem. Most (all) manufacturers will provide retention time precision specs for their systems.

A.A.

I would cast my vote for inadequate column (re-)equilibration if a gradient is being used.

The first thing we check when we get undue RT changes is whether the mixing of mobile phase was (is) performed correctly. Also, sample volume obviously can give rise to RT changes, the highly useful trick of injecting sample in a solvent of lower retention power than mobile phase can give enormous shifts without changing peak shape!

Just a comment, since previous writers have covered the topic. I record and review retention time, for my most prevalent analyte, on long runs, even though it serves little purpose. Trends in this parameter can indicate weakness in things like column temp, degassing/ bubbles/ and other pump characteristics.

If the shifts in RT are less than the 0.2% I expect, then all is well. Otherwise some troubleshooting is in order, starting with a repeated injection of that sample, until the chromatogram becomes repeatable.

Hi
We had similar problems with regards to investigating an unknown peak detected in product on stability. The degree of RT shift was as great as 10 minutes. Basically from the investigational studies it was found that the pH of the mobile phase effected the component in question (even slight changes of 0.02 from the theoretical pH) pKa assessment was carried out on the isolated component which conformed that the component was not fully compatable with the mobile phase. I don't know if this is of any help, but may be of interest to someone else.

If the Retention Time shifts are due to pump problems, an obvious change in pressure can usually be observed at the pump's pressure display. This should remain constant through several pump cycles (just a few seconds usually depending on pump speed) if there isn't a problem. If a fluctuation exists, then something as simple as an air bubble in one of the check valves can be the culprit.

If there is no discernable pressure fluctuation, then I would suspect a temperature change in the column's immediate surroundings to be the source of the problem. I have seen room changes of as little as 2 or 3 degrees have enormous influence on retention time drift. It may be as simple to correct as moving the column away from a window or shielding the column body from air conditioning vent drafts.

Lastly, a change in mobile phase of even a fraction of a percent can make a big difference. If the strong solvent (say MeOH on a C18 column)is normally 10% of the mobile phase volume and this changes to 9.5%, this is actually a 5% decrease in strong solvent concentration and will shift the peaks to later elution times.

Inject an unretained compound (such as Uracil for RP columns) and see if its retention is fluctuating or stable. If it is variable then you more than likely have pump problems. If it is stable than you probably have incorrect or changing solvent proportions (due to factors such as temperature changes, evaporation of volatile components etc.) How do you degass? If you are doing helium sparging make sure you turn down the flow of the gas once you have initially degassed the solution.
As mentioned above, more details on your method and problem are needed to pin point what is wrong.

How would the retention times of the components be affected with the ff changes in the chromatographic system?
1.column length doubled
2.Oven temperature lowered
3.Column with smaller internal diameter is used
4.Film thickness in the column is increased
5.Carrier gas changed from nitrogen to hydrogen
6.Head pressure is increased

This is obviously a student homework question. Can anyone point this person to any good on-line GC tutorials?