Electrophoresis safety question

Jennifer A. Griffin losteye at ix.netcom.com
Thu Jun 18 19:27:32 EST 1998


jgriffin at incyte.com wrote in message <6m6eir$1e5$1 at nnrp1.dejanews.com>...
>We run alot of horizontal agarose gels in our lab using standard power
packs
>at 100V (DC -- up to 3 amps) with 1X TAE running buffer.
>
>A number of people run their gels without the lid on, meaning there are
open
>gel rigs running on the bench. My question is:  Is this dangerous, and, if
>so, how dangerous?  I know that if I actually touched the wires in the box
I
>would be badly (fatally?) shocked.  Would touching the buffer with one
>(unprotected) hand do anything to me?  With both hands?  With nitrile
gloves
>on?
>
>I don't know enough about electricity to know how dangerous this is.
>Intuitively, it seems bad, but in reality perhaps it isn't.  I always wear
>gloves when working around the boxes, because of EtBr, but I am always a
>little worried about taking a gel out of a rig only to find it on for some
>reason. It's not the sort of thing I want to experiment with on my own, and
>was hoping someone with more lab experience would have some insight into
>this.





Jen,

I'm not a molecular biologist, so I don't know what types of safety hazards
are considered severe in your lab, but I noticed a few misunderstandings
about electrical shocks in some of the replies you've received, so I thought
I should point them out.


1) "It's current that kills, not voltage."

While true, this statement is often misunderstood.  The point to the
statement isn't that high voltages are safe, it's that even tiny voltages
(such as 5V) can be deadly if the conditions are right.  The amount of
current which will flows through your body is determined by two factors:
the voltage applied and your body's electrical resistance.  Therefore, you
can increase the current that will flow through your body by either raising
the voltage or decreasing your body's resistance.  The power supplies you
mentioned are rated at a maximum of 3A -- this means that if the voltage is
set high enough and/or your body's electrical resistance is low enough, the
power supply could deliver up to 3A through your body.  This is much more
than enough to kill human beings.


2) "You are safe because most of the current will flow through the medium
since it conducts better."

The amount of current is not constant.  The types of power supplies you are
using deliver constant voltage -- but the current can vary anywhere up to
3A.  When you hook two gels up in parallel, the amount of current going
through each one is the same as that through one if you were running it by
itself.  Likewise, if you put your hands into the gel, additional current
would start flowing through your hands without affecting the current in the
gel at all.  In fact, as long as you don't max out the power supply's
current rating, the exact same amount of current will flow through you with
or without the gel in place.


3) "Since it is unlikely that you could touch both electrodes, you are
safe."

When the gel is running, there is a uniform voltage gradient across the
entire gel.  One electrode is at 0V and the other is at 100V.  The futher
apart your hands get (in line with the electrodes), the more voltage you are
exposed to and the more current will flow through your body.  While it may
be difficult to be exposed to 100V, it would be quite easy to be exposed to
25-75V.  (If you are evaluating the setup as a safety hazard, you should
also consider the maximum voltage that the power supplies can be set to.)


Another important point to keep in mind is that the electrolytic solution in
the gels would greatly reduces the resistivity of your skin.  This makes the
risk of harmful/fatal shock much higher than if you touched the electrodes
with dry hands.


I just did a quick web search which turned up the following site:

http://pchem.scs.uiuc.edu/pchemlab/electric.htm

It has good concise information on shock hazards.


The page states that the most deadly currents are in the 0.1 - 0.2 ampere
range.  (Above 0.2 amps, the heart clamps and ventricular fibrillation
cannot begin...  so people have a better chance of surviving it.)  When your
skin is wet, your resistance can drop as low as 1000 ohms, which means that
the *deadliest* voltages begin around 0.1A * 1000 ohms = 100V DC.

Presumably, one could be killed by voltages far less - in fact, the author
mentions that people have been killed by as little as 42V DC.  (According to
the chart, severe shock and the inability to breathe can occur at .05A *
1000 ohms = 50V.)


Given this information, I would say that running open gels with 100V @ 3A
power supplies consitutes an extremely dangerous hazard.


---Gary Snethen







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