Chromatography Forum

Hi there!

Is anyone performing a GC-MS method in Ramp Flow mode? I'm currently using constant flow for the majority of the GC-MS methods, normally @ 1.0-1.2ml/min and I would like to have an opinion about using Ramp Flow mode. Does it really work for late eluters and as a fast bake out?
Thanks in advance.

At_om

In my experience, ramping the flow rate definitely helps to improve peak shape compounds with very low volatility and those with low volatility that are degraded by the high temps required to elute them from the column. One example where we have used this is decabromobiphenyl ether. I suppose it could also be used for cleaning the column at the end of each run also, although I don't think this is routinely done in many labs.

Hi

Agree with sassman. Can be useful for troublesome compunds like deca-BDE.

Can also see a general use for it if you have late eluters, long columns and high sample throughput.

It is helpful in shortening your method if your target compund eluted early and the others eluted laterely.

At_om,

One thing I suggest you watch for. It has been my experience that if flows exceed 1 mL/min at high temperatures that you can get roll off of the high boiling components at the upper concentration ranges. Specifically I was doing BNA's at upper concentrations of 120 and 160 ppm and saw significant roll off in response when trying to run constant flow.

Best regards.

If you mean do they come off the column quicker, then yes it works. However, if you need to optimise the separation of closely eluting compounds then you may make it worse by moving away from optimum linear velocity. Swings and roundabouts!
If you have problems with waiting for high boiling point rubbish coming off, try back flushing.

The two simplest ways of getting high boilers off the column more quickly are a thinner stationary phase and/or a shorter column.

Peter

It may sound obvious but make sure that the increased flow does not go above the point where the detector response starts to be suppressed. For our old 5973 the limit is about 2.8-3.0 ml/min.

GCguy

You could use a flow calcualtor to check that your start and end flows will give you optimal linear velocity. there is a calculator on the Agilent website.

I found that increasing the flow rate decrease the sensitivity.
So I use it only for post run column.