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Good Weighing Practices At QC Laboratory_Dr.A.Amsavel

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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 – PowerPoint PPT presentation

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Title: Good Weighing Practices At QC Laboratory_Dr.A.Amsavel


1
Good Weighing Practices At QC Laboratory
  • Dr. A. Amsavel M.Sc., B.Ed., Ph.D.

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
5
Balance Selection for Minimum Weight
Resolution Repeatability ( S.D.) Minimum weight (typical)3
0.001 mg 2.5 µg 3.0 mg
0.01 mg   0.03 mg 20 mg
0.01 mg   0.02 mg 30 mg
0.1 mg 0.2 mg 120 mg
6
Requirement of Balances
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 d0.01mg/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

10
Accuracy Precision
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 lt1251gt "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

14
Balances Requirement - USP
REPEATABILITY  USP General Chapter lt41gt
Repeatability tolerance 0.10
Expansion factor, K 2
Acceptance criteria 2 x SD/N 0.10
Smallest possible sample weight/lowest starting point 820 d (in optimal conditions)
Operating Range (2 x SD x 1000) to max capacity or (2 x 0.41d x 1000) to max capacity (if SD lt 0.41d)
ACCURACY  USP General Chapter lt41gt
Tolerance Satisfactory if its weighing value is within 0.10 of the test weight value
Test weight Between 5 and 100 of the balance's capacity
Calibration uncertainty of the weight Must be 1/3 of 0.10
15
Location Installation of Balances
  • Install the balance on anti- vibration table
  • non-magnetic surface and grounded to prevent
    static electricity
  • Room should be temperature and humidity
    controlled
  • vibration, air currents,
  • Should be free of drafts and away from air
    conditioner or fans or windows to avoid strong
    air current or direct sunlight
  • Away from magnetic fields (magnetic stirrer),
    electromagnetic radiation eg RF
    generators/communication devices and electric
    motors,
  • Away from Corrosive materials are used nearby.

16
Operational Qualification
  • OQ has to cover following, but not limited to
  • Control of stable indication
  • Mechanical mobility of all moveable parts
  • Manually triggered or automatic adjustment by
    means of built-in weights.
  • Automatic adjustment reduce the drift of the
    balance.
  • Operation of ancillary equipment
  • Tare function
  • Calibration part of OQ

17
Performance Test
Prop- erty Definition Acceptance Criteria Examples
Sensitivity Change in weighing value divided by the change in load, usually measured between zero and the capacity of the balance. NMT 0.05 deviation where lt41gt is applicable. For other uses, respective tolerance requirement divided by 2. The test load at or sufficiently close to the capacity of the balance.
Repeat-ability Ability of a weighing instrument to display identical measurement values for repeated weighings of the same objects under the same conditions, e.g., the same measurement procedure, operator, measuring system, operating conditions, and same location over a short period of time. Repeatability usually is expressed as the standard deviation of multiple weighings. Requirement from lt41gt where applicable. For other uses, user specified requirements will apply. 10 replicate weighings (test weight -few percent of the nominal capacity of the balance).
18
Performance Tests
Pro perty Definition Acceptance Criteria Examples
Linearity Ability of a balance to follow the linear relationship between a load and the indicated weighing value. Nonlinearity usually is expressed as the largest magnitude of any linearity deviation within the test interval. NMT 0.05 deviation where lt41gt is applicable. For other uses, respective tolerance requirement divided by 2. From 3 to 6 points over the range of the balance.
Eccentricity Deviation in the measurement value caused by eccentric loadingin other words, the asymmetrical placement of the center of gravity of the load relative to the load receiver. Eccentricity usually is expressed as the largest magnitude of any of the deviations between an off-center reading and the center reading for a given test load. NMT 0.05 deviation where lt41gt is applicable. For other uses, respective tolerance requirement divided by 2. Performed in the center of gravity 4 quadrants ( Test load usually should be 30 of the capacity of the balance or higher
19
Accuracy lt41gt
  • Accuracy
  • The accuracy is satisfactory if its weighing
    value, is within 0.10 of the test weight value.
  • A test weight is suitable if it has a mass
    between 5 and 100 of the balance's capacity.
  • maximum permissible error (mpe) or uncertinity
    shall 1/3 of the applied test ie 0.03. (ASTM
    E617)
  • Note A readability of 0.1 mg of balance
    is believed as accurate to 0.1 mg as
    misconception.

20
Balance Calibration
  • According to USP General Chapter lt41gt
    Balances, for substances to be accurately
    weighed, the balance used must be calibrated over
    the operating range
  • The most important are
  • Repeatability (RP),
  • Eccentricity (EC),
  • Linearity (L) and
  • Sensitivity (SE),

21
Repeatability
  • Why Repeatability is so important?
  • It will have significant impact on all the
    quantitative analysis
  • Ability of a weighing instrument to display
    identical measurement values for repeated
    weighings of the same objects under the same
    conditions, e.g., the same measurement procedure,
    same operator, same measuring system, same
    operating conditions, and same location over a
    short period of time.
  • Systematic deviations normally can be prevented
    if Repeatability is performed.

22
Repeatability
Repeatability Individual measurement deviation
from average value does not exceed standard
deviation, that is 0,0003g with probability
68,5. Individual measurement deviation does not
exceed three standard deviations, that is
0,0009g with probability higher than 99,7, so
very close to certainty.
23
Repeatability test ltUSP 41gt
  • Perform 10 measurements with the same reference
    weight
  • Calculate the Standard Deviation
    SD Repeatability is satisfactory if
  • 2 x SD/Nominal reference weight used is 0.10
  • If the standard deviation obtained is less than
    0.41d, (d-is the scale interval), replace this
    standard deviation with 0.41d.
  • 2x SD x 1000 If SD lt 0.41d,
  • In this case, repeatability is satisfactory if
    two times 0.41d, divided by the nominal value of
    the weight used, does not exceed 0.10.
  • 2x SD (0.41d) x 1000 820d
  • analytical balance with a readability of 0.1 mg,
    this means the starting point yielded is 82 mg.

24
Linearity test
  • Linearity To ensure that balance is accurate at
    the desired level in the operating range
  • Ability of a balance to follow the linear
    relationship between a load and the indicated
    weighing value.
  • Non-linearity usually is expressed as the largest
    magnitude of any linearity deviation within the
    test interval.
  • Perform 3 to 6 points over the range of the
    balance.
  • Limit NMT 0.05 deviation where lt41gt is
    applicable. For other uses, respective tolerance
    requirement divided by 2.

It is a deviation of balance real curve from
straight line joining two points AB ideal
weight.
25
Eccentricity Test
  • Deviation in the measurement value caused by
    eccentric loading in other words, the
    asymmetrical placement of the center of gravity
    of the load
  • Eccentricity usually is expressed as the largest
    magnitude of any of the deviations between an
    off-center reading and the center reading for a
    given test load.
  • In practice, a difference is defined between
    indication when mass standard is put at central
    point of weighing pan and indication when the
    same mass standard is located at another place on
    the weighing pan.
  • Performed in the center of gravity and the four
    quadrants
  • Test load usually should be 30 of the capacity
    of the balance or higher. Limit NMT 0.05
  • Deviation where (41) is applicable.

26
Sensitivity Test
  • Change in weighing value divided by the change in
    load, usually measured between zero and the
    capacity of the balance.
  • Use certified weights with an appropriate weight
    class
  • Perform as performed for repeatability test
  • Limit NMT 0.05 deviation where lt41gt is
    applicable. For other uses, respective tolerance
    requirement divided by 2.

27
Testing of Balance parameters
  • How often balance parameters should be
    tested? Intervals defining balance
    calibration/testing shall be based on range of
    operation, their intensity, balance stability in
    time and expected weighing process measurement
    precision.
  • Assuming that external conditions are stable,
    following balance parameters control periods can
    be fixed
  • Calibration annually
  • Repeatability centricity monthly
  • Sensitivity change weekly
  • Verification/ adjustment daily

28
Factors affects the weighing Accuracy
  • There are several external factors influence the
    accuracy of weighing
  • Ambient area and people influence result of
    weight measurement
  • Major balance external factors are
  • Oscillations, vibrations
  • Breeze of air
  • Temperature drifts
  • Electrostatics
  • Evaporation and absorption phenomena
    (hygroscopicity)
  • Magnetism
  • Other factors is nature of sample ,
  • Both balance and the sample will influence the
    accuracy of results
  • To prevent any such issue , need to identify the
    reason properly
  • Is it due to Balance external factor or
    sample external factor?

29
Impact of Vibration Strong Air
  • Oscillations vibrations are transmitted by the
    ground and walls that are generates and affects
    weighing balance
  • Effect is longer measurement time and higher
    indication dispersion.
  • Prevent vibrations by keep away from vibration
    area
  • Keep on anti-vibration table. Anti vibration
    double rubber console, to suppress vibrations
  • Breeze of air influence to instability and long
    weighing time.
  • Balance workstation should not be located close
    to doors or windows.
  • Closeness to devices such as air-conditioning,
    fans, should be avoided.

30
Impact due to Temperature
  • How Temperature affect the weighing
  • Weighing room temperature should be maintained at
    constant level. Eg variation must not more than
    0.5C/hour.
  • Equilibrate the sample to room temperature before
    weighing , will give error due to heat
    convection ( hot sample will be less than true
    value)
  • Precaution/ aspects for weighing process
  • pick up the samples with use of tweezers or other
    holders
  • should not put their hands into weighing chamber
  • Touched with hands, samples may change their
    temperature

31
Hygroscopic samples
  • Sample nature influence the weighing accuracy
  • Hygroscopic samples absorbs the moisture from
    ambient air and steadily gain the weight
  • Measured weight will have higher mass than actual
  • Weighed promptly
  • Use the hermetic vessels or a gas-tight
    enclosure.
  • Weighing vessel should be clean dry and easily
    transferable
  • Add the desired amount of sample, and replace the
    enclosure

32
Volatile or hygroscopic samples
  • Sample nature influence the weighing accuracy
  • Volatile liquids (low boiling solvents or solid
    with volatile solvents ) can undergo evaporation
    during weighing
  • Balance indicates fluctuation / drift ie
    weighing continuously decrease while measurement
  • Use appropriate weighing vessels, like bulbs with
    narrow necks or vessels with top cover.
  • Weighing of Volatile samples
  • Weighing of low boiling liquid point in a vessel
    with a gas-tight enclosure of small diameter.
  • Close immediately after transfer of material
  • After the balance display stabilizes, the analyst
    records the specimen weight

33
Corrosive and Bio-hazardous Samples
  • Aseptic or Biohazardous Samples
  • Weighing the samples in the confines area /
    bio-safety cabinet /isolator, or similar
    containment device.
  • Care should be taken if airflow in the hood may
    cause balance instability
  • Corrosive Materials
  • Extra care is essential when materials of this
    nature are weighed.
  • Use sealed containers such as weighing bottles or
    syringes

34
Samples with Electrostatics
  • Effect of electrostatic presence.
  • Slow drift of weighing result,
  • Large dispersion of weighing results in a series
    of measurements, and
  • No return to zero if a load is taken off the
    weighing pan.
  • Possible source
  • Dry, finely divided powders may be charged with
    static electricity
  • The static charge may develop due to low relative
    humidity, clothing worn and gloves used
  • How to prevent
  • Antistatic weigh boats, antistatic guns, and
    antistatic screens
  • Placing the container in a metal holder
  • Balances with a built-in antistatic device is
    available (piezoelectric components or low amount
    of a radioactive polonium) to generate a stream
    of ions that dissipate

35
Magnetism as interfering factor
  • If magnetic load is measured, electromagnetic
    field of a balance is disturbed or weighed sample
    is influenced by magnet installed in a balance.
  • It will lead to incorrect mass reading of a
    weighed sample.
  • High resolution balances are constructed on basis
    of electromagnetic sets which include a
    force-motor and magnet.
  • How to avoid Increasing a distance between a
    sample and balance mechanism. Use under-hook
    weighing with application of special racks or
    hooks made of aluminum.

36
Precaution while weighing
  • All receivers must be clean, dry, and inert.
  • weighing uncertainty for small samples, i.e., net
    weights with a mass determined around
    repeatability.
  • Receivers should be nonmagnetic used at ambient
    temperature
  • Weighing dishes should be polymer or aluminum.
    Antistatic and compatible with the liquid
    sample.
  • Safety measures
  • Use proper PPEs gloves , mask, goggles etc
    during a weighing to avoid exposure
  • Hazardous materials should be handled in an
    enclosure that has appropriate air
    filtration. Many toxic and possibly allergenic
    substances are present as liquids or finely
    divided particles.

37
Reference
  • US Pharmacopeia- 42
  • General Chapter lt41gt Balances,
  • General Chapter lt1251gt Weighing on an Analytical
    Balance,
  • Reichmuth, A., Weighing Small Samples on
    Laboratory Balances, 13th International Metrology
    Congress, Lille (F), 2007

38
  • Thank you
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