Sample jar loses mass?

[Walt Donovan]

Please excuse my ignorance as I am not a chemist but....

I prepare metallographic samples for ICP AES spectrography by digesting
them in aqua regia then diluting after microwave cycles.

In the dilution process I place an empty, covered polypropylene specimen
jar in an analytical balance. The jars are the type used for urine
specimen collection. They come sterilized and wrapped in a plastic bag.
They have a paper label. The top is an unknown plastic, possibly not the
same as the jar. The balance should tare in a few seconds (Mettler
AE240). It slowly goes negative as if the jar is losing mass. It will
drop by .004 to .007g. I thought that I had handled them with damp
hands, and the moisture was slowly drying from the jar or label,
reducing it's weight. Eliminated this by care in handling. Then I
thought that the containers had a gas inside less dense than air,
possibly from the polymer outgassing (I remove the original top and
replace it with one having a hole in it). I purged the jar with
compressed air. No difference. Now I suspect that a static charge plays
a role in this mystery but I'm not sure how that is possible. The mass
takes 5 or 10 minutes to stabilize. Any ideas?

[David Stone]

Post by Walt Donovan
Please excuse my ignorance as I am not a chemist but....
I prepare metallographic samples for ICP AES spectrography by digesting
them in aqua regia then diluting after microwave cycles.
In the dilution process I place an empty, covered polypropylene specimen
jar in an analytical balance. The jars are the type used for urine
specimen collection. They come sterilized and wrapped in a plastic bag.
They have a paper label. The top is an unknown plastic, possibly not the
same as the jar. The balance should tare in a few seconds (Mettler
AE240). It slowly goes negative as if the jar is losing mass. It will
drop by .004 to .007g. I thought that I had handled them with damp
hands, and the moisture was slowly drying from the jar or label,
reducing it's weight. Eliminated this by care in handling. Then I
thought that the containers had a gas inside less dense than air,
possibly from the polymer outgassing (I remove the original top and
replace it with one having a hole in it). I purged the jar with
compressed air. No difference. Now I suspect that a static charge plays
a role in this mystery but I'm not sure how that is possible. The mass
takes 5 or 10 minutes to stabilize. Any ideas?

Electronic balance drift due to static charge on the weighing
vessel is exceedingly common, especially with analytical balances.

The static bleeds through the balance pan; since the actual
"balance" is achieved via some electromagnetic or piezoelectric
means, the reading is affected by the static charge.

I've seen this happen with both glass weighing bottles and plastic
weighing boats many times, and tell my students about it repeatedly.
They always seem surprised when it happens to them, though!

The only thing you can do is reduce handling as much as possible,
and use e.g. crucible tongs to move the vessel on and off the
balance. If a vessel is already charged, you can try placing it
on a grounded metal plate to achieve discharge.

Oh, and gloves don't help - actually, they can make things worse!

[Walt Donovan]

Ahh... I forgot about the electrically supersensitive piezo or strain
gage that's responsible for "reporting" the weight in the first place.
It's all clear now, knowing what a few microvolts can do to a load cell.
Unfortunately, the handling that is part of the method (and thus
absolutely necessary) is: 1) tearing open a plastic bag (maybe most
likely culprit), 2) unscrewing and screwing caps to change to one with
a pouring hole. The tops seem to be a diferent polymer which I would
think that friction between the unlike materials could be a contender. I
will try grounding the jars. I will also try removing them from the
sterile bag ahead of time. I wonder...Could there be merit in covering
the metal weighing pan with a teflon watch glass to insulate the sample
jar from the scale?

Post by David Stone

Post by Walt Donovan
Please excuse my ignorance as I am not a chemist but....
I prepare metallographic samples for ICP AES spectrography by digesting
them in aqua regia then diluting after microwave cycles.
In the dilution process I place an empty, covered polypropylene specimen
jar in an analytical balance. The jars are the type used for urine
specimen collection. They come sterilized and wrapped in a plastic bag.
They have a paper label. The top is an unknown plastic, possibly not the
same as the jar. The balance should tare in a few seconds (Mettler
AE240). It slowly goes negative as if the jar is losing mass. It will
drop by .004 to .007g. I thought that I had handled them with damp
hands, and the moisture was slowly drying from the jar or label,
reducing it's weight. Eliminated this by care in handling. Then I
thought that the containers had a gas inside less dense than air,
possibly from the polymer outgassing (I remove the original top and
replace it with one having a hole in it). I purged the jar with
compressed air. No difference. Now I suspect that a static charge plays
a role in this mystery but I'm not sure how that is possible. The mass
takes 5 or 10 minutes to stabilize. Any ideas?

Electronic balance drift due to static charge on the weighing
vessel is exceedingly common, especially with analytical balances.
The static bleeds through the balance pan; since the actual
"balance" is achieved via some electromagnetic or piezoelectric
means, the reading is affected by the static charge.
I've seen this happen with both glass weighing bottles and plastic
weighing boats many times, and tell my students about it repeatedly.
They always seem surprised when it happens to them, though!
The only thing you can do is reduce handling as much as possible,
and use e.g. crucible tongs to move the vessel on and off the
balance. If a vessel is already charged, you can try placing it
on a grounded metal plate to achieve discharge.
Oh, and gloves don't help - actually, they can make things worse!

[Marvin]

Post by David Stone

Post by Walt Donovan
Please excuse my ignorance as I am not a chemist but....
I prepare metallographic samples for ICP AES spectrography by digesting
them in aqua regia then diluting after microwave cycles.
In the dilution process I place an empty, covered polypropylene specimen
jar in an analytical balance. The jars are the type used for urine
specimen collection. They come sterilized and wrapped in a plastic bag.
They have a paper label. The top is an unknown plastic, possibly not the
same as the jar. The balance should tare in a few seconds (Mettler
AE240). It slowly goes negative as if the jar is losing mass. It will
drop by .004 to .007g. I thought that I had handled them with damp
hands, and the moisture was slowly drying from the jar or label,
reducing it's weight. Eliminated this by care in handling. Then I
thought that the containers had a gas inside less dense than air,
possibly from the polymer outgassing (I remove the original top and
replace it with one having a hole in it). I purged the jar with
compressed air. No difference. Now I suspect that a static charge plays
a role in this mystery but I'm not sure how that is possible. The mass
takes 5 or 10 minutes to stabilize. Any ideas?

Electronic balance drift due to static charge on the weighing
vessel is exceedingly common, especially with analytical balances.
The static bleeds through the balance pan; since the actual
"balance" is achieved via some electromagnetic or piezoelectric
means, the reading is affected by the static charge.
I've seen this happen with both glass weighing bottles and plastic
weighing boats many times, and tell my students about it repeatedly.
They always seem surprised when it happens to them, though!
The only thing you can do is reduce handling as much as possible,
and use e.g. crucible tongs to move the vessel on and off the
balance. If a vessel is already charged, you can try placing it
on a grounded metal plate to achieve discharge.
Oh, and gloves don't help - actually, they can make things worse!

You can use a static eliminator to help bleed off the charge
into the air. They have a radioactive material that emits
low energy alpha particles. Lab supply houses sell them.
They aren't dangerous to use unless one breaks open. The
alpha particles can't even penetrate our skin.

[John Dulak]

You can use a static eliminator to help bleed off the charge into the
air. They have a radioactive material that emits low energy alpha
particles. Lab supply houses sell them. They aren't dangerous to use
unless one breaks open. The alpha particles can't even penetrate our skin.

Marvin:

You can sometimes find such dwvices at large photography supply
houses. They were used to remove dust from negatives before most
photography became digital.

http://www.amstat.com/solutions/staticmaster.html

HTH & GL

John

[Shankar Bhattacharyya]

Post by Marvin

[...] The balance should

tare in a few seconds (Mettler AE240). It slowly goes negative as
if the jar is losing mass. It will drop by .004 to .007g. [...]
Now I suspect that a static charge plays
a role in this mystery but I'm not sure how that is possible. The
mass takes 5 or 10 minutes to stabilize. Any ideas?

Electronic balance drift due to static charge on the weighing
vessel is exceedingly common, especially with analytical balances.
[...]
I've seen this happen with both glass weighing bottles and plastic
weighing boats many times, and tell my students about it
repeatedly. They always seem surprised when it happens to them,
though!

So true. We have the same training issue in an analytical laboratory
with some thirty balances distributed around the lab.

We used to dry-ash food samples in crucibles and borosilicate glass
beakers. Those showed some of this static-related drift but it was not
too bad. Recently, in the metals operation in my department, we moved
to microwave digestion in PTFE vessels which are quite tall, so that
we have to leave the top of the balance enclosure open while weighing
samples in the digestion vessel. Balance stability went all to hell.
We had some difficulty in getting younger staff to believe that we
should address static electricity issues before we started encouraging
building maintenance to start any major re-engineering of air
circulation.

Post by Marvin
You can use a static eliminator to help bleed off the charge
into the air. They have a radioactive material that emits
low energy alpha particles. Lab supply houses sell them.
They aren't dangerous to use unless one breaks open. The
alpha particles can't even penetrate our skin.

I have seen two types. You can buy little polonium sources to put in
the balance enclosure. We have these things taped to the back wall of
most of our analytical balances. They cost only a few dollars. That
helps when the problem is not too bad. There are also anti-static
brushes which have such a source built into them. In some of our more
economy-minded labs that brush gets left in the enclosure, as a sort
of dual-use device.

There is also at least one device which costs many, many times more,
which provides a more serious source of ionized air.

We are addressing our problem in the short term by weighing samples by
difference but that is a pain in a production lab. Any additional step
is a pain in a production lab.

We are working with a CEM MARS microwave digestor. If anyone has
addressed this issue with the tall vessels used by this or other
similar digestors, I would appreciate the benefit of your experience.

Incidentally, this business with static charges gets much more serious
in northern winters, when inside-the-building humidity drops and
people are walking around in sweaters which carry a few thousand
volts. We encourage people to ground themselves before they use the
balance. We have been considering antistatic wrist straps.

If you put the balance in an isolated room and humidify the lab air
without putting a mist in the air, most of the problem will go away.
We don't have quite that luxury in terms os available space.

- Shankar

[beav]

Post by Walt Donovan
Please excuse my ignorance as I am not a chemist but....
I prepare metallographic samples for ICP AES spectrography by digesting
them in aqua regia then diluting after microwave cycles.
In the dilution process I place an empty, covered polypropylene specimen
jar in an analytical balance. The jars are the type used for urine
specimen collection. They come sterilized and wrapped in a plastic bag.
They have a paper label. The top is an unknown plastic, possibly not the
same as the jar. The balance should tare in a few seconds (Mettler
AE240). It slowly goes negative as if the jar is losing mass. It will
drop by .004 to .007g. I thought that I had handled them with damp
hands, and the moisture was slowly drying from the jar or label,
reducing it's weight. Eliminated this by care in handling. Then I
thought that the containers had a gas inside less dense than air,
possibly from the polymer outgassing (I remove the original top and
replace it with one having a hole in it). I purged the jar with
compressed air. No difference. Now I suspect that a static charge plays
a role in this mystery but I'm not sure how that is possible. The mass
takes 5 or 10 minutes to stabilize. Any ideas?

it couldn't be as simple as moisture inherent in the packaging?

can you dry it to a constant weight in an oven?

if it is outgassing (which I doubt anyway), it should drop to a
constant weight.....