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Study Notes: Reference Standards

Reference standards or reference materials may be defined as a material or substance with one or more property values that are sufficiently homogeneous, stable and well established to be used for the calibration of an apparatus, the assessment of a measurement method or for assigning values to materials. Without reference materials or substances there could be no calibration. What would we calibrate the apparatus or method against?

Think of a top loading balance. How is this balance calibrated so that it gives accurate and reproducible (precise) measurements in grams? At some stage in its manufacture and subsequent re-calibration a standard mass must be used to calibrate the balance. This standard mass must be calibrated against another standard mass and so on until finally the standard mass is traceable to the standard kilogram housed under special and secure conditions at an international site.

There are two kinds of chemical reference materials used in chemical and biological laboratories:

  1. Pure substances, e.g. 1.00M NaCl, or
  2. Matrix Materials, e.g. 3.00mM glucose in human serum.

The benefit of using matrix reference materials is that it gives a better representation of the accuracy and precision of measuring this analyte in this matrix.

There are three main types of reference materials.

  1. Standard Reference Materials (SRM): a Certified Reference Material (CRM) issued by the National Institute of Standards and Technology (NIST)
  2. Certified Reference Material (CRM): a reference material issued and certified by an organisation generally accepted to be technically competent, for example the National Analytical Reference Laboratory (NARL), and
  3. In-house reference material: a material developed by the laboratory for its own internal use.

Reference materials have five main uses in the laboratory:

  1. to develop and validate accurate methods of analysis, ensuring traceable measurement results at a working level
  2. to verify that test methods are performing according to validated performance levels
  3. to calibrate measurement systems
  4. to assure the long term integrity of measurement quality assurance programs
  5. to use as test materials for inter-laboratory comparisons and proficiency programs.

Reference materials come under the guidelines of the National Measurement Act (NMA), 1999 and the purpose of the NMA can be summarised as:

“That we must ensure that measurements are what they purport to be.”

For instance, when you purchase 25L of petrol you want and expect that you will receive 25L +/- a small acceptable variation. You would be very upset if you found that you only received 23L!

Of course all chemical and physical standards must be eventually traceable to international standards based on the SI units and such standards shall be calibrated by a competent body that can provide traceability to a national or international standard of measurement.

The SI base units are:

Physical Quantity
Unit
Symbol
Length
metre
m
Mass
kilogram
kg
Time
second
s
Electric current
ampere
A
Thermodynamic temperature
kelvin
K
Luminous intensity
candela
cg
Amount of substance
mole
mol

Other SI units are derived from these seven base units, for instance, joule the unit of energy is kg m2 s-2. The other derived units are:

Physical Quantity
Unit
Symbol
Frequency
hertz
Hz
Energy
joule
J
Force
newton
N
Pressure
pascal
Pa
Power
watt
W
Electric charge
coulomb
C
Electric potential difference
volt
V
Electric resistance
ohm
omega
Electric conductance
siemens
S
Electric capacitance
farad
F
Magnetic flux
weber
Wb
Inductance
henry
H
Magnetic flux density (magnetic induction)
tesla
T

All reference standards in Australia have to be traceable back to SI base unit standard though State, National and ultimately international organisations.

Study Notes: Practical Use of Standards provides more information on reference standards.

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