I am presently recycling a mobile phase mixture of 22% methanol 78 % water for a process stream analysis. A 0.5 micro litre injection is taken every 15 minutes, 24 hours a day, 7 days a week. By the time the peak ends, the method is started within 2 minutes. In order not to contaminate the reservoir, I've placed a 250mm in length activated carbon column, which I change every week.The method is isocratic and the flow rate is 1 ml/min. This seems to work for a 1 litre bottle for 1 week. Some of the commercial solvent recyclers with switching valves would not be useful due to the frequency of the sampling. Does anyone have any ideas on how to improve the recycling technique without contaminating the mobile phase, and by not using lots of methanol ? I'd be happy to hear any suggestions.

I would not worry about doing anything, unless you have some really strange stuff in your sample. You are doing an injection of of 0.5 microL. Even if this is all organic, you are adding less than half a mL of organic solvent per weak, changing the solvent composition by less than 0.05%. You may get a slight increase in the background due to your sample, but once again, I would not worry about it. The carbon column may not be much more retentive for your sample than the analytical column. I would worry more about contamination from this column.

Let me get this right, you are unhappy using only 220 mL of methanol a week? Why do you feel you need to change this?

Is it that you want the solvent to last longer or just want to reduce the amount of methanol used per week.

Changing your column diameter and reducing the flow rate (ie going from a 3.9 mm id to a 3.0 mm id and reducing from 1.0 to 0.60 mL/min) could potentially provide a solution.

I also agree with the previous message, I doubt the carbon column is doing much good and I would be prone to getting it out of the system.

Commercial solvent recyclers divert the mobile phase to waste while your main analyte is eluting. Some can use the detector output to detect a peak and some just use time. I don't understand why you think they would not be useful because of the frequency of sampling. The activated carbon column may or may not be retaining your analyte and preventing the background absorbance from increasing.

I don't know how to help you. Maybe a solvent recycler would allow you to run for two weeks and cut your methanol consumption to 110mL/week. But then again you could probably buy a 20 year supply of methanol for the cost of a solvent recycler (220mL/wk)(50wk/yr)(20yr)($45/4liters)= $2475 .