Chromatography Forum

Dear peter , bruce, Rod and all

There seems to be small problem once more. Please give your expert advice

Base line gives valleys during blank run and i do it for three of four times and it happens always.

Is there any change in presure? The column flow is 5ml/min and linear velocity varies from 32cm/sec to 38 cm/ sec during the complete run.

The detail method is also posted some where in the forum.

I am also posting the image.

http://i14.tinypic.com/498mxqt.jpg

[IMG]http://i14.tinypic.com/498mxqt.jpg[/

This looks like column bleed maybe with some septum bleed as well. First bit of advice; change your carrier gas purifier.

Second bit of advice - do not expect people to go hunting "some where in the forum." for the details of your method , take the trouble to copy and paste them yourself.

Good luck Peter

rajdeeproy wrote originally,

I am anaysisng some seawater samples for dissolved halocarbons by liquid extraction in pentane, but my base line in chromatogram is wandering and not stable. I am using a megabore comumn DB624(60m )coupled with HP 5 (5 m length) I am using varian GC with ECD but no autosampler. So i do manual injection
The Temp ramp program are as follwos.
40degree for 1 min
raised to 160 degree @4 degree per min
detector (ECD 220)
Column flow He 4ml/min
make up N2 (30 ml/min)

please suggest something

Dear rajdeeproy,

I would suggest that you 'bake off' your column between samples. You have noted that you see these peaks only in samples not in standards. I don't think you are seeing negative peaks but rather valleys between the material eluting from the column.

This looks like garbage extracted from your original matrix (sea water) that is eluting late in the run. Garbage in..........Garbage out.

I see little that you can do. This garbage may be backflashed in your GC neumatics and keep coming out for a long time.

If it is coming out from your fresh samples then you have to perform a bake out after each sample.

Perhaps some bare retention gap might help or a small micropacked silica column as a 'trap' column to retain this material might be possible and you can still analyze for your halogenated volatiles.

best wishes,

Rod

Dear peter and rod

Thanks for those valuable messege. I will start working on those advice.

To be frank the forum have been a great help to me and especially experts like you all giving such a good advice.

Thankyou once again