Chromatography Forum

Does anyone know how to weigh an cold object accurately using an analytical balance with readability down to 0.1mg? I find it difficult to obtain a stable reading as it increases gradually, probably due to condensation of water vapor on object's surface and convection current inside the balance due to temperature gradient.
Please advise.

A cooled room, operating in the range specified in the instrument manual? How cold are you working at? Can you run a dehumidifier in the room?

Drop it into a tared weighing bottle, cover?

Thanks for your reply.
Regarding the temperature of the cold object, what I need is to get the weight straightaway from the object taken out immediately from the freezer.
I did drop the cold object into a tared and capped weighing bottle but the reading of the analytical balance increases gradually; probably beause:-
1) water condensation on the cold surface of the weighing bottle conducted from the cold object (P.S. I did put a lot of silica gel surrounding the balance's chamber)
2) convection current due to temperature gradient created from the cold object and the balance's surrounding, causing a downthrust onto the pan of the balance.

I will insert and hang the cold object inside a relatively big capped bottle containing silica gel, trying to avoid the above problems. Makes any sense?

Hi Hubert

I have seen a reference to putting cold objects into styrofoam holders. The outside surface stays at room temp due to the insulation. You might get some minor problems with static. I have to admit that I have never tried this myself, but it sounds as if it might work.

Rainer Schuhmacher · Manuela Führer ·Wolfgang Kandler · Caroline Stadlmann · Rudolf Krska. Interlaboratory comparison study
for the determination of methyl tert-butyl ether in water. Anal Bioanal Chem (2003) 377 : 1140–1147.
DOI 10.1007/s00216-003-2215-4

They weighed to 1 mg:

" For each solution approximately 95 g methanol
(Baker, HPLC analysed) was exactly weighed into a 100-mL
Erlenmeyer flask. The flask was sealed with a screw cap containing
a Teflon-lined septum and cooled to –20 °C for 1 h. Thereafter,
the flask was put into a polystyrene beaker, wrapped with laboratory
Film (Parafilm “Mâ€

Hi Peter,

Thanks for your suggestion. I will try to look up the article you mention. Besides, I have tried the method that I previously mention in my last post and it seems okay.
Briefly, I get a 200-mL plastic bottle with gasket inside the cap. Then I attach a 50-mL centrifuge tube under the bottom of the cap by "blue-tac" and screw the cap tightly. The tube is settled in such a way that it won't touch the inner surface of the bottle and the distance between them is at least 1 inch. The whole setup is then placed on an analytical balance until the reading is steady. Then I take out the tube and place the cold object immediately into it. Reassemble the setup rapidly and place on the analytical balance again. The reading becomes steady within 30 sec. The weight of the object is measured by difference. I will modify the setup 'cos it takes some time to re-attach the tube after placing the cold object using the "blue-tac". Any other idea?

Hubert

Hi Hubert

I would be tempted to get a 50 ml screw=capped tube for the inner tube, and glue the cap (probably with silicone to stick to the plastic) to the underside of the lid of the 200 ml jar. Then you can attach and remove the 50 ml tube just by scrweing it into and out of its cap.

Alternatively (and maybe easier), cut a piece of foam plastic to fit inside the 200 ml bottle, with a hole in the middle for your sample tube.

The extract form the paper that I pasted into my post is all the detail that they give on their cold weighings, and I have not managed to track down anyting on accepted best practise. If you find something please let me know.

Peter

set up balance in freezer, allow sufficient equilibration time then weigh.

Hi Neaman

Have you ever actually done this ?

Since the temperature range that balances are certified for does not go down below zero I would anticipate all sorts of problems with validation, and some really interesting issues with traceability to the Kilogram.

Peter

Hi Neaman

Have you ever actually done this ?

Since the temperature range that balances are certified for does not go down below zero I would anticipate all sorts of problems with validation, and some really interesting issues with traceability to the Kilogram.

Peter

not to mention the vibration, lighting and surface flexibility issues... probably not a viable solution.

Field-Lab/Poor-man's method:
Tare-out a styrofoam cup and watch-glass or equivalent, after shooting them with an anti-static gun. Place the specimen in the cup and cover.

just off the top of my head