Transcript MASS MEASUREMENT - Pharma Knowledge Park

MASS MEASUREMENT
by Mr.Vikram Joshi
[email protected]
MASS MEASUREMENT
INTRODUCTION: 
Measurement of mass one of the mist frequently carried out
operation
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Accurate mass measurement required for such purpose as
Obtain a known quantity of Sample for analysis
Preparation of analytical reagents
Preparation of calibration standards
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MASS MEASUREMENT
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Units of mass measurement
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in the international system (SI) of measurement unit base unit
of mass is the kilogram (Kg)
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Kilogram is one of the seven base measurement units
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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
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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
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Balances are distinguished by specific features
Typical minimum
Typical
capacity
readability
10 g
0.001 mg
micro-balance 6-figure
balance
150g
0.01 mg or
0.1 mg
Analytical balance 3,4 or 5figure balance
1000g
0.01g
general purpose or “top-
5000g
0.1g
pass” balance
3000g
0.1g or 1g
Type of balance
MASS MEASUREMENT
TYPES OF BALANCE USED IN ANALYTICAL
LABORATORIES
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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
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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
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Weighing process is distorted by several influences that
introduce both systematic and random error
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Bias is due to buoyancy effort or to the deviation of
reading from the reference mass
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Common random contribution involves readability,
repeatability
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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
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DRAUGHTS: - Chosen to minimize draughts from doors,
windows, passer-by
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VIBRATION: - Vibration had affect on balance for
measurement 0.01 g or less, locate the balance on a
vibration free surface.
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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
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TEMPERATURE: -Ideally the ambient temperature should be
stable to within ± 3°C. temperature fluctuations can cause
gradients in the balance mechanism
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HUMIDITY: - Humidity is relatively un-important provided
condensation does not occur
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MAGNETIC FIELDS: - There to be avoided as they may cause
permanent changes in the response of the balance
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ELECTRICAL INTERFERENCE: - Subject to electrical
interference and left on all the times
MASS MEASUREMENT
OPERATIONAL PRINCIPLES
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Typical balance consists of a single pan (Usually SS)
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For analytical balance is enclosed by a movable transparent
shield to protect pan from draughts
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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
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MASS MEASUREMENT
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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.
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Achieved by means of calibration and verification
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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
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Calibration shows how the nominal values of a material or
the indication of an instrument relates to the convention
values of the measured.
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The conventional value is realized by a traceable reference
standard
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Calibration not sufficient to ensure the suitable performance
of the balance and comparability of its measurements
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Number of specifications to accomplish
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Examination of conformity of results with the specifications
(manufacturers) generally expressed as tolerances
MASS MEASUREMENT
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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: 
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a.
b.
c.
d.
e.
This testing verifies the instrument delivers the same weight
reading regardless of where on the weighing pan the object
is placed.
Procedures
Select a test weight close to the weighing capacity of the
instrument
Place the weight in the center of the pan and re-zero the
display
Move the weight on half way from the center to the front
edge of the pan
Repeat step (c) at the half way locations for right, rear and
left edges, recording the reading as appropriate
Compare there readings with the corresponding corner load
tolerances
MASS MEASUREMENT
Corner load
resolution
Tolerance
(digits) for
Lab
environments
Capacity
1mg
0.1mg
0.01 mg
30 g
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2
5
100g
2
4
10
300g
4
10
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1000g
10
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MASS MEASUREMENT
REPRODUCIBILITY TESTING
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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)
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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
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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
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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
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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: 
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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
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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
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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.
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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
classification
Material of construction
Nominal
mass value
Permitted
tolerance
E1
Integral stainless steel without
marking or adjustment chamber
50g
± 0.00003 g
E2
Integral stainless steel without
marking or adjustment chamber
50g
± 0.0001 g
F1
Stainless steel with a screw
know
50g
± 0.00003 g
F2
Plated brass
50g
± 0.001 g
M1
Brass or cast iron with painted
finish
50g
± 0.003 g
MASS MEASUREMENT
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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
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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: 
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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
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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.”
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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
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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
be weighed
Recommended
Balance
Nominal Accuracy
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
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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
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For volatile liquids being weighed, a container with a wellfitting 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
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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
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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
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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
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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
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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
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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
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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
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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
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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 standby 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
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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
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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
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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 recalibrated
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
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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
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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