I know that V=IR where V is voltage, R is resistance and I is the current in mA; so my problem is that the electrophoresis units available to me allow fellow researchers to change these parameters at random. Some ramp up the current for speed, leaving others, if we're not careful to observe this, with our bands quickly shooting down the gel with poor resolution.
I know that the resistance of a gel is based on a number of parameters including pore thickness (for example 2% gel) and thickness of the gel slab itself (mine are generally thin/ 5mm) Also, I am using TBE as my buffer. I know the salt concentration of the TBE will also be a factor.
What I don`t know is how to take this information and use it to calculate resistance, so that I can then pump it into the equation (knowing my voltage is 110 V) which would allow me to adjust the ampage appropriately for the gel run.
I know these two factors should be interdependent but it seems that researchers can adjust settings adhoc and I will show up on different days to find that while voltage is always set at 100 the ampage can be anywhere from small to 100mA; and I don't know at what ampage to reset it correctly.
Hi Chinmaya, Well the problem in the last run was that one researcher had set the current all the way to 100 mA (I did not check for this, my mistake) and this made the bands rush through the gel at great speed so that even the hyperladder was a big 'swoosh' with the occasional band but none of the clarity of slower speeds of passage. At other times it has been set at lower mA and this has not happened. Given that I will likely need to correct and reset the machine for my own use each time (due to sharing the lab) I would like to calculate the appropriate mA for my 110 volt which I can only do if I could calculate the electrical resistance of my TBE solution and gel. I do get bands when this happens but they can be smeared and not as clear cut as would be optimal, even the hyperladder is sub-optimal in these cases.
All the best Andrew.
Don't know why anyone still uses TBE. Sodium borate buffer is less costly and allows faster run times.
Basic recipe is:
8g NaOH
45g Boric Acid
adjust to 1l H2O
(makes a 20x solution)
pH should be around 8.3
See pdfs attached to this and the next post
By the way, if you want to know the resistance of your gel just run it at some known voltage and read the current (make sure that the power supply is not set to a "current limiting" setting.) Then just use your formula I=V/R (R=V/I) to calculate the resistance. The power supply must be set to a "voltage limiting" setting for the current reading to reflect the gel's resistance--in other words, the power supply must actually deliver the set voltage.
Wow; $200 is expensive for such a simple device! I would like to know what it has got that makes it worth such a price (a fancy digital display not being one of them...)
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