T309 Traceability of spectrophotometric …...• In addition, measurements also required to...
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© NMISA 2013 TRACEABILITY OF SPECTROPHOTOMETRIC MEASUREMENTS TO THE HIGHEST STANDARD OF CALIBRATION Refuoe Pepenene Test and Measurement conference (09 October 2013)
Transcript of T309 Traceability of spectrophotometric …...• In addition, measurements also required to...
© NMISA 2013
TRACEABILITY OF SPECTROPHOTOMETRIC
MEASUREMENTS TO THE HIGHEST STANDARD
OF CALIBRATION
Refuoe PepeneneTest and Measurement conference (09 October 2013)
© NMISA 2013
Layout
• Introduction• Traceability• NMISA Mission• Reference Transmittance Spectrophotometer (RTS)• Measurements technique and the results• Conclusion
© NMISA 2013
Introduction
Regular transmittance - form basis of number of quantities used in spectrophotometer
Such as:Optical density (absorbance)Molar Absorptivity
Industries:PharmaceuticalsAnalytical ChemistrySugar MillingManufactures of: (glass, optical materials and coating)Research Institute
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Introduction cont
• Regular Transmittance is measured
• It is customary to express transmittance as a percentage:
• Absorbance can be obtained using Lambert law :
•
II
T0
=
1000
×=II
T
=
IIA 0log
=
TA1
log
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• For analytical purposes, two main propositions define the laws of light absorbance.
• Lambert's law:
• Beer’s law :
• Combination of the two laws - Beer-Lambert Law ,- Which defines the relationship between absorbance (A)and transmittance (T).
=
TA1
log
clA ε=
clTI
IA ε=
=
= 1
loglog 0
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• Wavelength accuracy and photometric / absorbanceaccuracy is important for the spectrophotometry.
• Spectrophotometer needs to show compliance with test procedures and, have documented evidence for data integrity.
• In addition, measurements also required to demonstrate traceability to the national standards.
• To ensure equivalent measurements regardless of theinstrument design or region of the world wheremeasurements are made.
Introduction Cont.
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Traceability
• NMISA obtain traceability via NPL spectral transmittance scale,
• Participated in the inter-comparison.• For end users to establish traceability, reference
standard filters traceable to National Metrology Institute are use to validate the measurement technique employed.
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• Establishes - national scale for regular spectraltransmittance measurements.
• Linked to the primary transmittance scale by means ofoptical filter reference standards,
• Manufactured and calibrated at the National PhysicalLaboratory (NPL) in United Kingdom.
Reference Transmittance Spectrophotometer
(RTS)
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Diagram Reference Transmittance
Spectrophotometer
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1) To maintain the national transmittance/absorbancescale and,
2) Disseminate it for the benefit of local laboratories,industries, and others who need the highest accuracymeasurements.
NMISA Mission:
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• Sending the spectrophotometer to the calibrationlaboratory - be costly and cumbersome.
• Limitations due to sample compartment size.• Ideal approach – use of reference standards.• Standards need to be certified by either a National
Metrology Institute or an accredited calibrationlaboratory.
• Wavelength accuracy - Holmium oxide solution,Holmium oxide glass filters or Didymium oxide glassfilters.
• Photometric accuracy (absorbance accuracy) - neutraldensity filters / solutions.
Verification of spectrophotometer (Reference Transmittance Spectrophotometer & End user
Spectrophotometer)
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Wavelength accuracyMaintenance (RTS)
Photometric scale accuracyMaintenance (RTS)
Linearity of photo-detectorMaintenance (RTS)
End user (Neutral Density & Holmium filter) Dissemination
End User’s Spectrophotometer
Maintenance & Dissemination Transmittance /
Absorbance Scale
© NMISA 2013
Measurement Technique and results
• Operational performance and data quality optimised toinclude wide range of spectral regions.
• For the purpose of this presentation, UV/Vis data isdiscussed.
• Wavelength Accuracy ( Example)- Range (200 nm – 800 nm)- Bandwidth setting: 2 nm- Selected Absorbance peaks measured and compared
with NPL valuesSELECTED ABSORBANCE MAXIMA
0.0
0.5
1.0
1.5
2.0
2.5
3.0
200 300 400 500 600 700 800
Wavelength (nm)
Abs
orba
nce
(Abs
)
12
3
4
5
67
810
9
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(RTS) Wavelength Accuracy
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• Absorbance peak values – are reproducible and withinNPL uncertainties.
• Performance of the moving mechanical components(responsible for positioning the monochromatorcomponents) stable.
(RTS) Wavelength Accuracy Cont.
588
588.2
588.4
588.6
588.8
589
589.2
589.4
2001 2004 2007 2010 2013
Wav
elen
gth
(nm
)
Calibration Interval
e) Wavelength of absorption maxima (588,57nm)
NMISA
NPL
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Photometric Scale
• Transmittance filters having different nominaltransmittance levels (low, medium and high) selected
• Range (200 nm – 800 nm)• Bandwidth setting: 2nm• Average transmittance values over a period of nine
years presented• NMISA values - compared with NPL valuesLinearity
• Filter set used, Range (200 nm – 800 nm)• NMISA values plotted against NPL values
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RTS Photometric Scale
• Average measurement data reproducible• Stability of photometric scale adequate• Variation that might occur during the calibration will not
limit the precision and accuracy of the measured data
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• The photo-detector linearity is appropriate over thetransmittance / absorbance range of calibration.
• Output signal of the photo-detector system is proportional tothe input signal.
• Transmittance values vary from 0,08% to 80%,• This correspond to absorbance values 3 to 0,09.
RTS Linearity check
© NMISA 2013
Calibration of End user’s Reference Standards
Neutral density filter• Absorbance measured at wavelength 420 nm, 480 nm,
546 nm, 600 nm and 700 nm.• Bandwidth setting 2nm,
© NMISA 2013
Calibration of End user’s Reference Standards
Holmium oxide filter• Spectral measurement range (200 nm – 800 nm),
Bandwidth setting 2nm.• Nominal wavelength of selected absorption peaks
measured.• Expanded uncertainty (k=2) of the wavelength values
was ± 0,4 nm .
© NMISA 2013
End user (Neutral Density Filters)
2.49
2.5
2.51
2.52
2.53
2.54
2.55
2.56
2.57
2.58
Abs
orba
nce(Abs)
Wavelength (420 nm) 2004
2005
2006
2007
2008
2009
2010
2012
2.28
2.29
2.3
2.31
2.32
2.33
2.34
2.35
2.36
Abs
orba
nce
(Abs
)
Wavelength (480 nm) 2004
2005
2006
2007
2008
2009
2010
2012
2013
2.29
2.3
2.31
2.32
2.33
2.34
2.35
2.36
2.37
2.38
Abs
orba
nce
(Abs
)
Wavelength (546 nm) 2004
2005
2006
2007
2008
2009
2010
2012
2.32
2.33
2.34
2.35
2.36
2.37
2.38
2.39
2.4
Abs
orba
nce
(Abs
)
Wavelength (600 nm) 2004
2005
2006
2007
2008
2009
2010
2012
© NMISA 2013
End user (Neutral Density Filters) Cont.
• Measured absorbance within the claimed uncertainty.• Spectrophotometer is capable of reproducing the values
through the years.
1.795
1.8
1.805
1.81
1.815
1.82
1.825
1.83
1.835
1.84
1.845
Abs
orba
nce
(Abs
)
Wavelength (700 nm) 2004
2005
2006
2007
2008
2009
2010
2012
© NMISA 2013
End-user (Holmium oxide filter)
286.5
287
287.5
288
288.5
2002 2004 2006 2008 2010 2012 2014
Wav
elen
gth
(nm
)
Calibration Year
Peak wavelength (287,5 nm)
360.2360.4360.6360.8
361361.2361.4361.6
2002 2004 2006 2008 2010 2012 2014
Wav
elen
gth
(nm
)
Calibration Year
Peak wavelength (361,0 nm)
418
418.5
419
419.5
2002 2004 2006 2008 2010 2012 2014
Wav
elen
gth
(nm
)
Calibration Year
Peak wavelength (419,0 nm)
278
278.5
279
279.5
280
2002 2004 2006 2008 2010 2012 2014
Wav
elen
gth
(nm
)
Calibration Year
Peak wavelength (278,8 nm)
© NMISA 2013
End-user (Holmium oxide filter) Cont.
445.6445.8
446446.2446.4446.6446.8
2002 2004 2006 2008 2010 2012 2014
Wav
elen
gth
(nm
)
Calibration Year
Peak wavelength (446,2 nm)
459
459.5
460
460.5
461
2002 2004 2006 2008 2010 2012 2014
Wav
elen
gth
(nm
)
Calibration Year
Peak wavelength (460,3 nm)
536536.2536.4536.6536.8
537537.2537.4
2002 2004 2006 2008 2010 2012 2014
Wav
elen
gth
(nm
)
Calibration Year
Peak wavelength (536,7 nm)
637
637.5
638
638.5
639
2002 2004 2006 2008 2010 2012 2014
Wav
elen
gth
(nm
)
Calibration Year
Peak wavelength (637,4 nm)
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• Absorbance peaks within the claimed uncertainty.• Spectrophotometer is capable of reproducing the values
through the years.
End-user (Holmium oxide filter) Cont.
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Conclusion
• Verification tests facilitate the acceptance and use ofthe reference transmittance spectrophotometer todisseminate the transmittance/absorbance scale withgreat confidence.
• Calibration of the End user filters - calibration valuesare within NMISA claimed uncertainty and traceable toprimary reference standards.
• Through traceability the laboratory’s qualityconsciousness and reputation will be enhanced, whichwill further support its commitment to analyticalexcellence in the competitive global marketplace.
© NMISA 2013
Acknowledgements:
Dr. Angelique Botha Dr. Clive OliphantElsie Coetzee Margaret BudzinskiMariesa Nel
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