Performance Verification of Analytical Balances according to ISO/IEC 17025 | Chemistry Net

Performance Verification of Analytical Balances according to ISO/IEC 17025

Verification of Analytical Balances according to ISO/IEC 17025 Standard

 

Verification of Analytical Balances according to ISO/IEC 17025 Standard

 

Accurate analytical measurements are fundamental to the functioning of modern society. Inaccurate results can have an enormous social and economic impact (Prichard et al. “Quality Assurance in Analytical Measurement”, 2007) such as:

  • “In trade, it could lead to the supply of sub-standard goods and the high cost of replacement with subsequent loss of customers”
  • “In environmental monitoring, mistakes could lead to hazards being undetected or to the identification of unreal hazards”
  • “In supply of drinking water, it could lead to harmful contaminants being undetected”
  • “In healthcare, the incorrect medication or the incorrect content of an active ingredient in a tablet can be catastrophic for the patient”.

 

Calibration and verification are the most important actions to ensure the correct indication of measuring instruments (K. Sommer et al. “Calibration and Verification: Two Procedures

Having Comparable Objectives and Results”; OIML BULLETIN; vol. XLII, number 1, 2001).

They are considered major requirements for the competence of testing and calibration laboratories according to the International Standard  ISO/IEC 17025:

“Equipment used for testing, calibration and sampling shall comply with specifictions…” (5.5.1)  and

“Before being placed into service, equipment (including that used for sampling) shall be calibrated or checked to establish that it meets the laboratory’s specification requirements and complies with relevant standard specifications. It shall be checked and/or calibrated before use”. (5.5.2)

ISO/IEC Guide 2 defines verification as: “Confirmation by examination and provision of evidence that specified requirements have been met”.

Verification is the confirmation, based on evidence (test results) that a certain number of specified requirements have been fulfilled. For example, the verification of an analytical balance  will prove that the performance of the balance is still in agreement with the calibration certificate.

The acceptance / rejection criteria used for the verification of analytical balances are:

i) Trueness (% relative error), comparing to the weights of certified standard weights. The average weight xaverage of ten consecutive measurements of a certified standard weight is measured using the analytical balance. The absolute error is calculated and then the % relative error according to:

 

Absolute error  = xaverage – xt

 

% relative error  = [ (xaverage – xt )/ xt]  x 100

Where xt  the true value of  the certified weight and xaverage  the average weight of the certified weight determined experimentally ( ten consecutive measurements of the certified weight using the analytical balance of interest).

ii) Absolute and % relative error are a measure of the accuracy (trueness) of the analytical balance.

The definition of trueness according to VIM 2010 (International Vocabulary of Terms in Legal Metrology)  is:  closeness of agreement between the average of an infinite number of replicate measured quantity values and a reference quantity value.

 

iii) Repeatability  (% coefficient of variance, %RSD), comparing to the weighs of certified standard weights.

The standard deviation from the mean value of ten consecutive measurements of a certified weight is determined. Then the % relative standard deviation (%RSD) is calculated.

Standard deviation = s = [Σ(xi-xaverage)2 / (n – 1)]1/2

Where xaverage the average value of the ten consecutive measurements and x each individual measurement and n the number of measurements

% relative standard deviation (%RSD) =  s / xaverage* 100

Repeatability  (%RSD) is a measure of the precision of the analytical balance.

The definition of precision according to VIM 2010 (International Vocabulary of Terms in Legal Metrology)  is: closeness of agreement between indications or measured quantity values obtained by replicate measurements on the same or similar objects under specified conditions.

 

iv) Drift, refers to weight readings that do not stabilize, or unstable readings with no weight applied. All analytical balances show some uncertainty. The clearest indication of a malfunction is a change in the amount of instability.

 

The acceptance / rejection criteria (tolerance) for analytical balances are given in the table below:

 

Acceptance / Rejection Criteria for Analytical Balances

Trueness, % Relative Error

0.01

Repeatability, % RSD

0.0001

Drift, g

0.001

The above procedure was followed for the verification of a KERN 770-13 precision balance. The results are given in the table below:

 

Table 1: Balance Verification Table – Trueness and Repeatability Assays

Balance I.D.
Standard Weight I.D.
Brand
KERN
Nominal Value
99,9996
Model
770-13
 
 
 
 
 
 
Measurement #
Value (g)
Drift (g)
1
99.9998
t = 0     1.0000
2
99.9997
             t = 5     1.0001
3
99.9996
             t = 10   1.0001
4
99.9997
 
5
99.9998
 
6
99.9998
 
7
99.9997
 
8
99.9997
 
9
99.9997

 

10
99.9997

 

Average (g)
99.9997

 

Absolute Error (g)
0.0001

 

Relative Error (%)
0.0001

 

Accepted /not Accepted  Trueness
Accepted

 

Std. Deviation (g)
0.000063

 

%RSD
0.000063

 

Accepted /not Accepted  Repeatability
Accepted

 

Drift (g)
0.0001

 

Accepted /not Accepted  Drift
Accepted
 
 
 
 

Relevant Posts

Calibration and Outliers - Statistical Analysis

 


References

  1. K. Danzer, L.A. Currie, Pure & Appl. Chem., Vol. 70, 4, 993-1014 (1998)
  2. D. Harvey,  “Modern Analytical Chemistry”, McGraw-Hill Companies Inc., 2000
  3. D.A. Skoog, F.J.  Holler, T.A.  Nieman,  “Principles of Instrumental Analysis”. Saunders College Publishing: Philadelphia, 1998.
  4. Prichard et al. “Quality Assurance in Analytical Measurement”, 2007
  5. K. Sommer et al. “Calibration and Verification: Two Procedures Having Comparable Objectives and Results”; OIML BULLETIN; Vol. XLII, 1, (2001)

Key Terms

calibration, verification, analytical balances, measuring instruments, trueness, %relative error, repeatability, drift,

 


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