dead time

[tatianak]

What does it mean when dead time does not appear in my base line.

THANKS
TK

[coquim]

HI TK!

Have you checked the scale magnification you are watching? may be the the signal is very low.
Have you got a peak response at a very short time? may be this peak overlap the solvent front...

Bye!

[mbicking]

Peaks that appear at the void time are sometimes called "vacancy" peaks. They appear when your injection solvent and mobile phase are different. The greater the difference, the greater the effect. The "peaks" arise from the band of solvent going through the column that has a slightly different composition - it is temporarily disturbing the equilibrium between mobile and stationary phase.

For example, if your sample is in 100% acetonitrile, and your mobile phase is 80% acetonitrile, you probably wouldn't see much of a response. But if your sample was in methanol, and mobile phase was acetonitrile/water, you might see one or more peaks. Peaks can be positive or negative, and sometimes there is more than one, depending on the differences between the solutions. Of course, anything in your sample that is not retained may also appear as peak, but this is a different issue.

Of course, you have to be able to "see" these disturbances with the detector. I find that vacancy peaks are much more common at shorter wavelengths, as would be expected, since acetonitrile and methanol absorb somewhat at those wavelengths. But I also see them at longer wavelengths, and perhaps this is a refractive index effect.

To summarize, if you are dissolving sample in mobile phase, you will not see any peaks, unless something in your sample is not retained. When the compositions don't match, then expect to see something. If the peaks change or disappear, then maybe something has happened that you need to investigate.

[coquim]

To summarize, if you are dissolving sample in mobile phase, you will not see any peaks, unless something in your sample is not retained.

Hi !!!
a simple cuestion:
As far as i know, the solvent front (of an inyection) ALWAYS give us a response (because the flow turbulence), no matter the mobile phase is the same than the eluent...
is´n it??