strange negative peaks

[jadallah]

Hi all,

We are having a problem and request your help & suggestions to find solution to this problem.

we are using DRI detector to see the concentration profile of acetone water system along thin tube. we use this concentration profile to measure the diffusion coefficient.

the sample injected is 10g/l acetone, and the solvent is pure water

*for the 1st sample injection we get nice +ve normal distribusion peak
*for the 2nd we get +ve normal distrubusion followed with -ve peak almost with the same hight of the +ve one. it dosent affect the +ve peake and gives good results(the same as 1st injection).
*for the 3rd we get +ve peak affected by a -ve peak. its like the negative peak is shiffting towards the positive one.

in order to test if the problem from the RI or not we tried to use both sample and solvent as pure water and the surprise that we didnt get any nigative or positive values???? im still not confinced that IR can make such an error and its not always teh same

note: all liquids are degased

Await for your replay

best regards,
Mohammed

[Uwe Neue]

Have you connected the reference cell into the flow path?

[jadallah]

yes i did before the 1st run then i separated the refrance cell from the flow path and i did the three pulses with the same sample 10g/l acetone but i got negative beaks after the 2nd and 3ed one ???

[jadallah]

thanks all,

i think i find what is the problem. there is a leak in the valve disconnecting the referance cell and the flow cell. so a sample of what i am pulsing is passed to the referance cell and give the negative peak