I am running a DSC experiment of protein sample. How to interpret peaks at the beginning of the experiment? I've read about start-up hooks, but this seems not to be the case as these peaks are not reproducible in different measurements.
Please look at the pictures attached.
I've analyzed one sample thrice (each time I prepared a new sample and reference pans)
Sample: protein solution 50 µl
Reference: solvent 50 µl
Here is my procedure:
Initial heat flow: 20 mV
1) Hold for 1.0 min at 30.00°C
2) Heat from 30.00°C to 95.00°C at 1.00°C/min
3) Hold for 1.0 min at 95.00°C
I used baseline correction for all measurements.
It really looks strange for a initial signal transient (isotherm -> heating) as the equlibration time is quite long (about 3 min). I'm not sure what type of DSC did you use (a PerkinElmer one, apparently). For power-compensated DSCs (like PE diamond, DSC8000/8500) the initial transients are short (under 1 min). For a heat-flux DSC (DSC4000/6000 ) they are longer (1-3 min).
Nonetheless, my first idea would be that this is caused by a slightly different amounts of the solvent in sample and reference pans. The amounts you use are quite large and if the solvent is water (high cp) then even a small difference would be observed as a large (and perhaps wide) signal transient at the beginning of the heating ramp. That would explain why the endo/exo direction changes from run to run.
What is your sample? Is it starch free? If yes, then you are getting protein denaturation temperature (Td) around 75C irrespective of the isotherms.
I believe that is the start up hook due to slightly different weight between sample and reference, as you mentioned the same volume were used instead of weight.
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