eniko - 28-4-2016 at 08:18
Hey. I am working on a Metrohm instrument, 797 VA Computrace 1.3, using HMDE as working electrode (Polarography). I am measuring between -1.2 V and
+0.2 V. Recently the measured current keeps falling to 0 A at about -0.6 V or I am not measuring any current at all, although my mercury drop hasn't
fallen off. Can anyone help me with this problem? I can't figure out what is the problem.
aga - 28-4-2016 at 11:18
It would help to explain more accurately what you are actually doing, maybe with a photo or two.
Artemus Gordon - 28-4-2016 at 15:04
Why don't you contact Metrohm for suggestions? They would be more likely to understand your problems than any of us because they built the equipment.
I'm sure you don't have a service contract with them, but my experience with application engineers is that they are happy to help, as long as you make
an honest effort to not waste their time. I would suggest you try to come up with 2 or 3 possible reasons for your problem and ask them which seems
most likely. Even if all of your ideas are way off the mark, you would have demonstrated that you are engaged in finding the solution.
One suggestion: I don't really understand your equipment, but it is interesting that your current falls off at around 0.7 or 0.6v, because that is the
typical forward threshold voltage of a silicon diode in the conducting mode. If you do have a diode in your current path, once you drop below its
threshold voltage, its resistance becomes very high and very little current can pass through.
[Edited on 28-4-2016 by Artemus Gordon]
eniko - 28-4-2016 at 21:35
First of all, thank you for answering. My answer for Gordon is that, this phenomenom being something new, we (I, my teather at the university and
other teachers that are using this instrument) are trying to figure outt what is the problem on our own first, beacuse if we contact those from the
instrument, we'll probably have to pay for their service, and we are trying to avoid that, as long as possible 
. Besides, one of the teachers knows electronics, but can't figure out where could the problem be.
And for aga's request a few more details on what I'm actually doing. We are measuring heavy metals, more exactly Zn, Cd, Pb and Cu from digested
mushrooms, digested with nitric acid and perchloric acid. We are measuring, as I said earlier, between -1.2 V and 0.2 V using HMDE as the working
electrode. The earlier attached image shows a voltammogram of a solution composed of 9 mL deionized water, 1 mL sodium acetate buffer solution and 0.3
mL of digested sample. On a few occasions it also happened, that we didn't measure any current at all, from the beginning of the potential range to
the end. So,in the beginning, we suspected that the problem lies in a phenomenom caused by the digested sample, but it isn't, since recently the same
problem occured using only water and buffer solution, or water, buffer solution and standard solution. More interesting is the fact that, if we switch
to SMDE mode, everything works fine, but since the metal contents in these mushrooms is very small, especially Cd and Pb, we have to work in HMDE,
since it's a more sensitive method, having lower detection limits.
Thousands of thanks for anyone who can help.
aga - 29-4-2016 at 00:02
Unless you've accidentally discovered a new semiconductor material, i'd supsect it's air or dust getting into the capillary, possibly due to
insufficient Hg in the resevoir, possibly because there is some dust in the mercury.
What feed mechanism is it : auto or manual ?
[Edited on 29-4-2016 by aga]
eniko - 29-4-2016 at 00:42
The mercury feed is auto; there is enough mercury in the reservoir, it has been recently filled, and cleaned by the use of a filter paper placed in a
funnel, in which we made a small hole on the bottom.
I think that air or dust can't go into the capillary since the inner diameter is very small, besides we got this faulty voltammogram 10-15 times in a
row.
aga - 29-4-2016 at 00:44
is the Timing of the fault the same for each experiment ?
eniko - 29-4-2016 at 00:52
if by timing you understand the potential at which the current falls, than no, it changes with each experiment, and as I said in my previous post,
sometimes i'm not measuring any current from the beginning to the end of the potential range.
aga - 29-4-2016 at 00:56
OK, so it's basically open circuit somewhere, so work through all the connections, of which the drop of mercury/mercury connection path sounds most
likely.
The spec sheet says:
"HMDE or SMDE; requires mercury 99.9999% and nitrogen 99.995%, whose
pressure must be reduced with a reduction valve to 1.0 ± 0.2 bar"
Just a guess: perhaps that's required to keep the drop geometry stable.
eniko - 29-4-2016 at 01:06
if it would be an open circuit somewhere, than wouldn't it affect the SMDE also? because we tried it in that mode to, and it works fine that way. Also
we have done a Dummy cell test too, which tests the electronics of the instrument, as the instruments manual instructed, and it came out fine
aga - 29-4-2016 at 05:28
If the machine electronics check out ok, and a static drop works ok, you're left with the dynamic blob mechanism.
As i understand it, you've got a resevoir, capillary tube and some kind of pump.
I'll bet it's something like air/gas in the capillary being introduced by the pump somehow (faulty seal etc). Possibly some sticky liquid crap got
into it earlier and is still in there.
If you can get it to fail, freeze the setup and visually inspect the capillary for a break in the Hg column with a magnifying glass (if possible) or
simply test for conductivity from the blob to the resevoir.