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MASS MEASUREMENT

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					MASS MEASUREMENT




by Mr.Vikram Joshi
Vikram.joshi@ranbaxy.com
MASS MEASUREMENT

    INTRODUCTION: -

   Measurement of mass one of the mist frequently carried out
    operation

   Accurate mass measurement required for such purpose as
-   Obtain a known quantity of Sample for analysis
-   Preparation of analytical reagents
-   Preparation of calibration standards
MASS MEASUREMENT

   Units of mass measurement

   in the international system (SI) of measurement unit base unit
    of mass is the kilogram (Kg)

   Kilogram is one of the seven base measurement units

   In routine analytical work, it is more convenient to work in
    terms of grams (g) where 1000 g = 1 Kg or 1 g = 10-3 Kg
MASS MEASUREMENT

   Other sub-division and multiples of the gram are
    Unit             Symbol   Relationship to the gram

    Fematogram       fg       1fg = 10-15 g

    Picogram         pg       1pg = 10-12 g
    Nanogram         ng       1ng = 10-9 g
    Microgram        Mg       1Mg = 10-6 g
    Milligram        mg       1mg = 10-3 g
    Gram             g        1g = 1 g
    Kilogram         kg       1Kg = 103 g
    (metric) tonne   t        1t = 106 g
MASS MEASUREMENT

    TYPES OF BALANCE USED IN ANALYTICAL
    LABORATORIES
   Balances are distinguished by specific features
     Typical minimum     Typical
                                       Type of balance
     capacity            readability
     10 g                0.001 mg      micro-balance 6-figure
                                       balance
     150g                0.01 mg or    Analytical balance 3,4 or 5-
                         0.1 mg        figure balance
     1000g               0.01g         general purpose or “top-
     5000g               0.1g          pass” balance
     3000g               0.1g or 1g
MASS MEASUREMENT

    TYPES OF BALANCE USED IN ANALYTICAL
    LABORATORIES

   Balance capable of weighing to 0.001 mg are commonly
    referred to as micro balances
   Normally used for weighing quantities of < 0.1 g.
   Balance weighing to 0.01 mg, 0.1 mg or 1 mg are
    commonly referred to as analytical balance
   They are normally used weighing quantities of about 0.1
    to 100 g
MASS MEASUREMENT

    MASS: - Mass is the amount of material in an object and
    does not change with environment in which the object is
    located

    WEIGHT : - Weight is a force arising from the interaction of
    mass with earth’s gravitational field which varies with the
    location.
   Sophisticated balance provide “error-free” measurements

   All the measurements have error that trouble the accuracy
    and create uncertainty about the quality of the measured
    mass.
MASS MEASUREMENT

   Weighing process is distorted by several influences that
    introduce both systematic and random error

   Bias is due to buoyancy effort or to the deviation of
    reading from the reference mass

   Common random contribution involves readability,
    repeatability

   The measure of the weighing process is the mass “m”
    weighing object which is evaluated from the reading R of
    a calibrated and verified electronic balance
MASS MEASUREMENT

    LOCATION OF THE BALANCE

   DRAUGHTS: - Chosen to minimize draughts from doors,
    windows, passer-by

   VIBRATION: - Vibration had affect on balance for
    measurement 0.01 g or less, locate the balance on a
    vibration free surface.

   LEVEL SURFACE: - The surface on which the balance is
    mounted should be level and balance feet should be
    adjusted, using the spirit-level device to show when the
    balance is level
MASS MEASUREMENT

   TEMPERATURE: -Ideally the ambient temperature should be
    stable to within ± 3°C. temperature fluctuations can cause
    gradients in the balance mechanism

   HUMIDITY: - Humidity is relatively un-important provided
    condensation does not occur

   MAGNETIC FIELDS: - There to be avoided as they may cause
    permanent changes in the response of the balance

   ELECTRICAL INTERFERENCE: - Subject to electrical
    interference and left on all the times
MASS MEASUREMENT

    OPERATIONAL PRINCIPLES

   Typical balance consists of a single pan (Usually SS)

   For analytical balance is enclosed by a movable transparent
    shield to protect pan from draughts

   EMF COMPENSATION BALANCE: -
-   A coil is rigidly attached to the balance
-   Pan assemble is placed in the field of magnet
-   Object placed on pan lowers it, causing and increase in the
    current in the coil.
-   Magnetic counter-force generated which returns the pan to
    its original position
MASS MEASUREMENT

-   The increase in current in the coil is measured as a voltage
    on a digital voltmeter
-   Mass of the object is directly proportional to the measured
    voltage.
-   A digital read-out of the mass of the object readily obtained
-   The balances are called electronic balance
MASS MEASUREMENT

    PERFORMANCE VERIFICATION: -

   The need for regular and appropriate assessment of
    balances vital both in providing traceable and accurate
    results.

   Achieved by means of calibration and verification

   CALIBRATION: - set of operation that establish under
    specified conditions, the relationship between values of
    quantities indicated by a measuring instrument or measuring
    system or values represented by a material measure or a
    reference material and the corresponding values realized by
    the standards
MASS MEASUREMENT

   Calibration shows how the nominal values of a material or
    the indication of an instrument relates to the convention
    values of the measured.

   The conventional value is realized by a traceable reference
    standard

   Calibration not sufficient to ensure the suitable performance
    of the balance and comparability of its measurements

   Number of specifications to accomplish

   Examination of conformity of results with the specifications
    (manufacturers) generally expressed as tolerances
MASS MEASUREMENT

   Daily or before-use checks should be made on balances
    results recorded.
   If the balance has the adjustment facility that allows the
    output to be adjusted between zero and an internally or
    externally applied weight, this facility to be operated
    regularly before the balance is used.
   The checks to include adjusting the zero of the balance
    (performed with adjusting facility) followed by the placement
    of a single weight (Calibrated or control weight) on the pan.
   This procedure for daily check should define on action limit
    or maximum permissible error, and if it is exceeded, a full
    calibration should be carried out.
   Other regular checks called intermediate checks required
    between full calibrations
MASS MEASUREMENT
     E CENTERING LOADING: -

    This testing verifies the instrument delivers the same weight
     reading regardless of where on the weighing pan the object
     is placed.
    Procedures
a.   Select a test weight close to the weighing capacity of the
     instrument
b.   Place the weight in the center of the pan and re-zero the
     display
c.   Move the weight on half way from the center to the front
     edge of the pan
d.   Repeat step (c) at the half way locations for right, rear and
     left edges, recording the reading as appropriate
e.   Compare there readings with the corresponding corner load
     tolerances
MASS MEASUREMENT

            Corner load   Tolerance           Lab
             resolution   (digits) for   environments

 Capacity      1mg          0.1mg          0.01 mg

  30 g           -             2              5

  100g          2              4             10

  300g          4             10              -

  1000g         10             -              -
MASS MEASUREMENT
    REPRODUCIBILITY TESTING

   Repeatability/Reproducibility refers to instrument’s ability to
    repeatedly deliver the same weight reading for a given
    object.
   Expressed as standard deviation
   Select a test mass equal to, or nearly equal to the weighing
    capacity of the instrument
   Procedure
-   Tare the balance to read all zero
-   Place the test mass on the pan. Record the reading under
    heading “full scale”
-   Remove the weight (do not re-zero) and record the reading
    under zero
-   Repeat the above two steps 10 to 11 times
MASS MEASUREMENT

          Line                 Zero               Full scale


    Standard deviation
          (SD)


-     Calculated the standard deviation for the measurements
      set of “Zero” and “full scale”
-     Calculated standard deviation larger than allowed in the
      instrument specification indicated instrument is either
      operating in an unstable environment (air draft, static,
      warm-up, vibration, etc.) or the instrument is need of
      repair.
MASS MEASUREMENT

    LINEARITY TESTING

   Linearity: - A balance to exhibit perfect linearity if two
    masses that differed by a particular factor give read-out that
    differed by the same factor
   Perfect linearity is not capable of being achieved in practice
    or of being demonstrated experimentally
   Linearity checks verifies the accuracy of the instrument at
    intermediate values of weight
   Specified by the manufacturer as a tolerance which
    represents the maximum deviation of the balance indication
    from the value that would be obtained by linear
MASS MEASUREMENT

   Interpolation between the adjusting points (zero and internal/
    external weight loading)
   The linearity error typically takes the form of an S-shaped
    curve
   Routine QC ensures that linearity remains within acceptable
    limits
   A convential procedure
-   Use two weights, each of approximately one-half the
    weighing capacity of the instrument
-   Imperative that these two weights not be interchanged within
    this procedure
MASS MEASUREMENT

-   Refer to the individual weights as “A” and “B”
-   Re-zero the display place “A” on the pan (at the center)
    record the reading (x)
-   Remove “A” and place “B”. Re-zero the display
-   Again place “A” on the pan. Record the reading (y)
-   Calculate the difference between the two readings.
-   The difference should be less than the advertised tolerance
    and for linearity and accuracy.
MASS MEASUREMENT
    SPECIAL NOTE: -

   A common error in linearity (accuracy) testing is simply
    place test weights on the pan and observes the difference
    between the indicated weight and nominal value of the test
    weight.
   This process fails to account for the fact that test weight are
    imperfect and that difference between nominal value and
    weight might be significant
   True with analytical balances, where the balance may be
    more accurate then any standard test weight.
   Nullifies the problem by comparing the weight readings of
    the same object, both with and without a preload.
   The accuracy of the test weight is thus immaterial
MASS MEASUREMENT
    THE CALIBRATION PROCESS

   It refers to the difference between the weight reading of a
    given mass standard and the actual value of that standard
   Calibrated weight: - A weight with a mass value and an
    associated uncertainty that has been formally established by
    a calibration laboratory that ensures that traceability of the
    mass value the national standard of mass.
   Calibrated weights have and uncertainty that is 1/3 to 1/5 of
    the OMIL tolerances
MASS MEASUREMENT
   TRACEABILITY: - A measurement is said to exhibit
    traceability if it can be related to a recognized reference of
    appropriate quality through an unbroken chain of
    comparisons, each comparison having a known uncertainty.
    Ideal, all measurements should have such traceability.

   CLASS OF WEIGHT: - Classified by International Organization
    of Legal Metrology (OIML) according to their material of
    construction and maximum permissible error (tolerance)
MASS MEASUREMENT

     CLASSIFICATION OF WEIGHTS ACCORDING TO OIML

OIML             Material of construction           Nominal      Permitted
classification                                      mass value   tolerance
E1               Integral stainless steel without   50g          ± 0.00003 g
                 marking or adjustment chamber
E2               Integral stainless steel without   50g          ± 0.0001 g
                 marking or adjustment chamber
F1               Stainless steel with a screw       50g          ± 0.00003 g
                 know
F2               Plated brass                       50g          ± 0.001 g

M1               Brass or cast iron with painted    50g          ± 0.003 g
                 finish
MASS MEASUREMENT

   To perform calibration suitably, the balance should be clean
    and located in a position free from vibration and thermal
    sources.
   Standard weights used for calibration should be cleaned
   Calibration to cover at least ten points evenly spread over
    the loading range.
   A series of repeated measurements are carried out (six to
    ten) for each standard weight
   From there data, at each calibration point, the average of
    readings of standard weight is evaluated.
   All measurements are subject to unavoidable measurement
    uncertainty
MASS MEASUREMENT

   Balances are highly engineered, sophisticated and reliable
    items of equipment
   Source of uncertainty
-   The reproducibility of balance readings
-   The linearity of balance readings
-   The uncertainty of calibrated weights used to calibrate the
    balance response.
-   The uncertainty of calibration procedure
   The repeatability provides decisive contribution to the
    uncertainty of measurement
MASS MEASUREMENT

    DETERMINATION OF THE MINIMUM SAMPLE WEIGHT: -

   On site determination the minimum sample weight
   To ensure that influence of the person performing the
    weighing and environments are included.
   The lowest permissible sample weight is defined by
-   Repeatability of a balance at its location
-   Readability
-   Require relative tolerance limit of the maximum error of
    measurement
-   A probability of occurrence
MASS MEASUREMENT

   USP-30 section 41, weighing is to be performed with a
    weighing device whose measurement uncertainty (random
    plus systematic error) does not exceed 0.1 % of the reading
   The minimum weight for a balance is that which will have
    loss than 0.1 % of that weight
   Associated with expanded uncertainty with a coverage
    factor of 3 which takes into account the defined probability of
    occurrence of 99.73 %
   The minimum weight that can be weighed on an analytical
    balance is 3000 SRP or 1200d which is ever is larger
    SRP -> Standard deviation for n replicate weighing
    d-> Readability of the balance
MASS MEASUREMENT

    “Reference standards are weighed on and analytical
    balance and weights are recorded to 4 decimal places (e.g.
    0.0100). Typically 10 mg quantities are weighed out,
    however in some instances smaller quantities are weighed.”


    A FDA 483 OBSERVATION
   Ensure to use and appropriate analytical balance and
    sufficient material is weighed to reduce the error
   The greater the number of decimal point could reduce the
    amount we weigh, whether it is fit for its purpose?
   Large number of analytical results that are out of its
    expected range, one of the factor may be weighing
MASS MEASUREMENT
    CHOICE OF BALANCE
   The choice of balance depends on the quantity to be
    weighed and nominal accuracy (number of decimals)
    required in the weighing
   Balance selection for a particular weighing operation
    Nominal Quantity to   Recommended             Nominal Accuracy
    be weighed            Balance                 obtained

    0.01 g                Micro (6-figure)        0.000001 g (0.01 %)

    0.1 g                 Analytical (5-figure)   0.00001 g (0.01 %)

    1g                    Analytical (4-figure)   0.0001 g (0.01 %)

    100 g                 Top-pan (2-figure)      0.01 g (0.01 %)
MASS MEASUREMENT
    CHOICE OF BALANCE
   Mass values with a nominal relative accuracy of 0.01 % or
    better which is more than accurate for all routine analytical
    work

    CONTAINERS AND TYPE OF SUBSTANCE TO BE WEIGHED
   Substances are nearly always weighed in containers of
    same sort
   Empty container should be clean, dry, free from dust and
    be of a design that facilitates easy transfer of the weighed
    substance in to the vessel or apparatus subsequently
    required for analysis
   Mass of the empty container should not be larger than
    necessary compared to the mass of the substance being
    weighed
MASS MEASUREMENT
    CONTAINERS AND TYPE OF SUBSTANCE TO BE WEIGHED
   For volatile liquids being weighed, a container with a well-
    fitting stopper to minimize losses through evaporation
   For hygroscopic substances, a stopper container helps to
    minimize pick-up of the moisture from the ambient air.

    SEQUENCE OF OPERATION INVOLVED IN WEIGHING A
    SUBSTANCE
   Setting up the balance
   Switch on the power supply and at least 20 minutes
    allowed for balance to “warm up”
   Recommended that balance are left permanently switched
    on and left in the stand-by mode
MASS MEASUREMENT
    SEQUENCE OF OPERATION INVOLVED IN WEIGHING A
    SUBSTANCE
   Check that balance is level and adjust if necessary, using
    the leveling feet to centre the bubble in the spirit level
    device
   Gently clean the balance pan with a brush to remove any
    dust or loose particulate matter
MASS MEASUREMENT
    CHECKING THE ACCURACY OF THE BALANCE
   Set the balance to read zero and check that zero is
    displayed
    Note: - Depending on the balance, its location and the
    substance being weighed, it may be un-realistic to expect
    the stability in the last decimal place of the balance reading
   Some judgment will have to be exercised as to what digit is
    taken for the final decimal place of the reading
   An additional uncertainty in measured mass value is
    introduced by such instability
   Analyst should satisfy them selves that uncertainty is
    acceptable in terms of fitness-for-purpose
   The calibrated weights should be handled with forceps and
    not with hands
MASS MEASUREMENT
    CHECKING THE ACCURACY OF THE BALANCE
   The forceps should be made of plastic or non-metallic
    material
   If made of metal they should be tipped with material such
    chamois lather
   Place the calibrated weight on the center of the pan close
    the balance door and weight for mass reading to stabilize,
    record the reading
   Record the result of the accuracy check
   The difference between the measured mass and the
    expected value should be within the stated limits
   This check only needs to be carried out at the start if series
    of weighing, not before every individual weighing
MASS MEASUREMENT
    CHECKING THE ACCURACY OF THE BALANCE
   Remove the calibrated weight and check that zero is
    displayed on the balance
   If zero is not displayed, check the balance pan is clean, the
    doors have been closed
   For small departure from zero (i.e., in the final decimal
    place of the reading, the balance may be zeroed
   Larger departure from zero should be investigated and if
    necessary the balance should be serviced
MASS MEASUREMENT
    WEIGHING BY DIFFERENCE
   Place the empty container on the centre of the pan, close
    the balance doors, wait until the reading is stable (W0
    grams)
   Remove the empty container from the pan, transfer the
    substance to be weighed to the container and place the
    container on the center of the pan. Close the balance
    doors, wait for the reading to stabilize and record the
    reading (W1 grams)
   Remove the container from the balance pan
   If the substance is transferred quantitatively (by washing
    with water or solvent) to the required vessel, the mass of
    the substance is obtained by subtraction of the mass of the
    empty container from the mass of container plus substance
    (W1-W0)
MASS MEASUREMENT
    WEIGHING BY DIFFERENCE
   If the substance is transferred ‘dry’ to another vessel, once
    the transfer has been carried out replace the container on
    the center of the balance pan, close the doors, wait for the
    reading to stabilize and record the reading (W2 grams).
    The mass of the substance transferred is then obtained by
    subtraction (W1-W2)
MASS MEASUREMENT
    USE OF TARE FACILITY
   If a substance is to be weighed directly on to the vessel
    required in the subsequent analysis, the tare facility is used
   Place the empty container on the center of the balance
    pan, close the doors and press the tare button. Wait for
    reading to stabilize at zero
   Remove the container from the balance, add the
    substance to the container, replace the container on the
    balance close the doors, wait for the reading to stabilize
    and record the reading. The reading gives the mass of the
    substance in the container
MASS MEASUREMENT
    COMPLETION AND TIDY UP
   On completion of the weighing activities, gently clean the
    balance pan with a brush, collect any debris and discard it
    appropriately
   Close the balance door and leave the balance in the stand-
    by mode
   Ensure any calibrated weights used are returned to their
    storage box
   Ensure that an appropriate and permanent record of the
    weighing is made
MASS MEASUREMENT

    EFFECT OF BUOYANCY ON WEIGHING

   Buoyancy is uplifting force on a weighed object due to the
    fluid in which it immersed.
   Magnitude of the uplifting force depends upon the density of
    the air
   True mass = Ma {1+ PA (1/Ps – 1/Pcw)}

    Ma = Mass value obtained by weighing in air
    PA = density of air (kg per meter cube)
    Ps = density of object weighed)
    Pcw = density of the calibrated weight used to calibrated the
          balance
MASS MEASUREMENT

    OBSERVATION

    SIGN THAT INDICATE ALL IS NOT WELL
   Messy area where the analyst has been working
   Spilt solid on or under the balance (and the pan)
   Chemical left out on the open bench rather than being
    returned to the place where it is stored
   The door(s) on the balance being left open when weighing
   Moving a balance but not checking to see if it is level, before
    use
   Weighing results not recorded, written down on a scrap of
    paper, or inappropriately recorded, e.g., rounding a weighing
    such as 20.3974 g and recording as 20.4000 g
MASS MEASUREMENT

   Incomplete recording of information, e.g., not including what
    has been weighed, when and by whom
   Re-calibrating the balance for no apparent reason
   Modern balances are reliable instruments and some
    automatically re-calibrate whilst others rarely need to be re-
    calibrated
   If there is a problem with a balance then the problem should
    be identified and rectified before re-calibration of the
    balance
   Using the tare facility in a haphazard way - this is particularly
    important to get right in a busy laboratory where several
    analysts may be using the same balance
   Balance not recording zero when there is nothing on the
    balance pan
MASS MEASUREMENT

   Quality control checks not performed at the correct time to
    determine that the balance is in calibration
   Quality control check performed but the analyst does not
    know the defined limits for acceptable performance, e.g.,
    analyst records the measurement of a check weight but
    does not check to see if it passes or fails
   The manufacturers' calibration certificate
   When to calibrate a balance and the appropriate calibration
    interval(s)
   As part of a routine calibration procedure
   After a balance has been moved
   The calibration expiry date which should be clearly visible to
    analysts, e.g., displayed on the balance
MASS MEASUREMENT

   The use of standard certified weights for balance
    calibrations
   The use of check weights as a quick way to determine
    whether or the not there is a problem
   The need to monitor run to run variations by calculating the
    standard deviation of successive weighing
   How to monitor daily drift by, e.g., constructing and using a
    control chart?
   How to determine linearity using standard certified weights?
MASS MEASUREMENT




       Thank you

				
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