2. Content
• Significance of balances in the QC Laboratories
• Types of Balances
• Definition
• Minimum weight
• Location for installation Balances
• USP requirement
• Performances test
• Calibration
– Repeatability
– Linearity
– Eccentricity
– Sensitivity
• Factor influence the accuracy of weight
• Types of samples and handling
• Precaution to be taken while weighing
3. Impact of Weighing in Analysis
• Weighing is a one of key activities in all the QC laboratories
• Most of the time, our understanding is not at sufficient level
• Its importance or complexity is underestimated.
• Quality of weighing determines the Quality & Accuracy of final
test result.
• The USP specifically requires highly accurate results when
weighing analytes for quantitative measures
• Right choice of balances (Analytical/semi-micro/micro ) with
desired resolution, accuracy & repeatability is essential to
reduce the error and meet the compliance
4. Type of balances
Balance name Resolution Quantity of
decimal digits (gm)
Ultra-microbalances 0.1 µg 0.0000001
Microbalances 1 µg 0.000001
Semi-microbalances 0.01mg 0.00001
Analytical balances 0.1mg 0.0001
Precision balances 1g ÷ 1mg 1g ÷ 1mg
7. What is minimum weight ?
• Minimum weight is the minimum sample quantity required to
perform an accurate quantitative analysis is based on the
measurement error of the balance used
• In order to satisfy the required weighing tolerance, when
samples are weighed the amount of sample mass (i.e., the net
weight) must be equal to or larger than the minimum weight.
• The minimum weight applies to the sample weight, not to the
tare or gross weight.
• If the sample quantity is too small, the measurement error will be
huge and result of the analysis will be unreliable.
8. Calculation of Minimum Mass
Minimal mass value of balance can be established based on
repeatability
- analysis required precision 0,1%,
- standard deviation (Balance specification)
Eg Max 220g; d=0.01mg/0.1mg; s = 0,025mg; k = 2; A = 0.1%
• Minimal weighing for the balance is 50mg, if analysis is supposed to
be performed with precision 0.1%.
9. Definition:
Accuracy
• Closeness of agreement between a measured quantity value and a
true quantity value of a measurand. VIM *
• Difference between measurements average value and the real value
according to USP
Precision
• Closeness of agreement between indications or measured quantity
values obtained by replicate measurements on the same or similar
objects under specified conditions. VIM
Trueness
closeness of agreement between the average of an infinite number
of replicate measured quantity values and a reference quantity value
(*) VIM – International Vocabulary of Terms in Legal Metrology
(**) USP – United States Pharmacopeia
11. USP General Chapters
• Measurement is stated to be 'accurately measured' or
'accurately weighed',
– (41) Balances
– (1251) Weighing On An Analytical Balance
12. Understand the USP Requirement
USP General Chapter <1251>:
"In order to satisfy the required weighing tolerance, when
samples are weighed, the amount of sample mass (i.e.,
the net weight) must be equal or larger than the minimum
weight.
The minimum weight applies to the sample weight, not to
the tare or gross weight.
13. Requirement of Balances
Select the appropriate balance based on the accuracy,
repeatability, stability, access control, printout or connect to
other instrument or LIMS etc.
URS
Select Correct accuracy and repeatability
Qualification
Installation at right place / location
Operation qualification
Performance qualification / Calibration
Password protection, Access control , printer, etc
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