micro balance

[bartjoosen]

As I remember good, the requirements for the microbalance are for assays (I don't have a USP at hand).
The precision for an assay is also much better as a loss on drying.
(And what about a loss of drying of 0%? What about the accuracy of your microbalance in this case?).

The whole thing with %RSD and SD was a way of explanation why the differential weighing procedure isn't good for an assay.

But remember: This is only my point of view and not neccesairy the correct one....

Bart

[SSGG]

Very fascinating discussion.

From what I understand:

JM can't weigh 2mg, because if he weighs 2mg (by difference OR by absolute weighing), the size of the measurement uncertainty approaches the size of the signal

I thought weighing guidelines were quite clear (until I read this thread )...

BTW, what do most of you guys use for weighing on a microbalance and quantitative transfer? Al foil squares? Weighing boats? weighing funnels?

[rnelson]

Allow me to confuse the issue further.

Many times I've seen the minimum weight for a balance calculated in reverse. In other words, take the balance specs for repeatability (for example 0.001mg) multiply by 3 (for the 3SD) then multiply by 1000 (inverse of 0.001). For my example, the lowest weight I can weigh on this balance would be 0.001x3x1000 = 3mg. Of course, these are ideal conditions. Each balance should have their own minimum weight determined since each may have a slightly different environment.

The other question this brings up for me is how do people perform a daily weight check on a micro balance? If I'd like to weigh down to 3mg and want to use a 3mg traceable weight during my daily check, what is the standard practice when USP allows using Class 1 weights? A Class 1 3mg weight is only accurate to 0.010mg, well outside the 0.1% requirements. Do other companies use the certified value of the weight to determine the limits? (Example: My 3mg weight actually weighs 3.005mg, my limits would then be 3.005mg +/- 0.003mg).

Here's where my true confusion lies: If I weigh 3mg ten different times, and every time I get a number that meets the 0.1% requirements, how critical is the uncertainty calculation? Assuming worst case, I could get numbers ranging from 2.997 to 3.003 and calculating uncertainty as per USP I get nearly 0.002. (In this example, uncertainty actually equals sd, since we're taking sd multiplying my 3, then dividing by 3(mg)). What are we truly measuring with uncertainty?

[unmgvar]

rnelson

i will also add to the confusion.
in the company i used to work before we would go according to the accuracy test in ordere to set the minimum weight to be used with a balance. for example for an analitical balance of a resolution of 0.01 mg the smallest weight that you can accurately weight is 10 mg and still have 0.1 % accuracy. if 9 out of 10 the measurement was the same and only once it was something else then you have passed the second test.

the important thing to remember looking at those two tests is:
the first test is an accuracy test. no matter how many times you measure each individual measurement has to be within +-0.1%
the second test is a reproducability test. this is a statistical test whose population size is 10 (that is a small statistical population which is set by a set of rules in accordance, in opposition to a large population which is generally the case in statistics, more then 100 or so)

each test is individual but they are complementary in order to help us decide if a balance is working properly, because you can have a balance which is accurate but non reproducable and you can have a balance which is inaccurate but very reproducable in it's offset. like you yourself saw even if accurate a balance ca still fail the reproducability test.

we use to perform the accuracy test once a month, and the reproducability test every 6 month. for the daily test we would go with the internal calibration test of our mettler balance.

[HW Mueller]

No need for confusion if you stay far away from the limits of your scale.

[bartjoosen]

More confusion:....

If you can prove your reproducability, why do you test your accuracy 10 times?
And if you test your accuracy 10 times, you can use this results for reproducability also?

[DR]

[JM]

DR !! nice one!!

The issue is not dead yet, recently i was looking at the recent EU pharmacopoeia supplement 5.6 ( page 4389)and i noticed one change in the sulphated ash method, the following has been added,

" The amount of substance used for the test ( usually 1-2 gm) is chosen so that the prescribed limit the mass of the residue( usually about 1 mg) can be measured with sufficient accuracy"

What does that sufficient accuracy means? accuracy of 1 mg residue we weigh? on normal analytical balance? 1 mg is out of limits of normal analytical balance? We can not use microbalance in this case as the crucible weight will be far more than the upper limit of any microbalance.

in otherwords, the weight of empty and with residue crucible is with in the accuracy limits of analytical balance but the difference ( 1 mg ) will not ( not accurate) i.e with out sufficient accuracy?

HW/DR pls comment.

I love this forum!!

JM

[HW Mueller]

Forget that garbled sentence and please see to it that the pills we swallow contain nearly the correct amount of poison.

[DR]

Please see to it that the pills we swallow contain nearly the correct amount of poison.

With your permission, I think I'll submit that as a candidate for my department's new mission statement.

"Sufficient Accuracy" - weasel words - they know that there are no semi-micro balances that can tare out a crucible. I guess you could weigh a 2g sample, ash it and quantatively scrape the residue into a 4mm weigh boat (don't spill any)

[HW Mueller]

Dr, certainly, it should be interesting to see whether a company exists which would foster that:

[Bruce Hamilton]

I'd come riding to the rescue of the EP, but somebody above is flogging my recently-deceased environmentally-friendly transport .

Section 1.2 of EP 5.0 notes " Quantities are weighed or measured with an accuracy commensurate with the indicated degree of precision. For weighings, the precision corresponds to plus or minus 5 units after the last figure stated ( e.g. 0.25 g is to be interpreted as 0.245 g to 0.255 g ). ...

It is recognised, however, in certain cases the precision with which quantities are stated does not correspond to to the number of significant figures stated in a specified numerical limit. The weighings and measurements are then carried out with a sufficiently improved accuracy ".

How much more clarity is needed, "sufficient accuracy" could be trumped by "sufficiently improved accuracy" .

Please keep having fun. Given Pharmacopoeia language, this thread may never die...

Bruce Hamilton

[JM]

DR,
Scraping out 1 mg of residue?? NO way!! Lets be practicle , it is impossible and never done in any lab. it would be even more inaccurate. by the way the this test is to control the Poison ( Heavy metals).

Bruce, The whole argument is based on Pharmacopoeia language and their requirment, and we all are following them without a second thought.

JM

[DR]

The whole argument is based on Pharmacopoeia language and their requirment, and we all are following them without a second thought.

The whole argument is based on Pharmacopoeia language and their requirment, and we all are following our own interpretations of them without a second thought.


Fixed it.

[mbreslav]

Just a note. I found it very convenient to record results directly to the excel spreadsheet automatically using Mettler Toledo BalanceLink software.