Comparison of FTIR with Wet Chemical Reference Methods

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Comparison of FT-IR with Wet Chemical Reference
Methods

• Dr. Marc Coleman
• National Physical Laboratory

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Contents

• Description of measurements performed at a
  fibreglass manufacturing plant
• Comparison of HCl readings made by FT-IR and wet
  reference methods
• Validation of HCl FT-IR reading via. modelling
• Benefits of on-line FT-IR monitoring to
  fibreglass manufacturers and related industries

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Fibreglass Case Study Aims

• VAM 1.6 funded by NMS
• To promote and validate the use of spectrometric
  techniques for multi-component on-line analysis
• Currently, a contractor performs measurements of
  HCl and HF
• Total fluorides and chlorides by a method based
  on USEPA 26a
• Two-fold aim of case study
• Compare FT-IR to a recognised reference method
• Validate FT-IR reading via. modelling

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Detected Emission Species
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Experimental Design
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Experimental Theory

• Contractors in-house monitoring method
• 5 impingers of H2O2 with silica gel in last
  impinger
• Will dissolve total chlorides and fluorides
• FT-IR cannot measure Cl2 due to spectroscopic
  selection rule
• USEPA method 26
• 1 empty impinger followed by 2 impingers of H2SO4
  , 2 of NaOH and one of silica gel
• Can discriminate between HCl and Cl2
• Enabling direct comparison to FT-IR HCl readings
• Four 1h runs performed of each method
• Contractor sent all samples to a UKAS accredited
  lab for analysis
• Samples from last run of each technique sent to a
  2nd UKAS accredited lab for comparison

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FT-IR vs. USEPA for HCl
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Cl2 Determined from Chloride Found in NaOH
Impingers
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FT-IR vs. Contractors In-House Method for HCl
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Remarks

• FT-IR and USEPA values agree well
• Difference between FT-IR and contractors in-house
  method greater
• USEPA method indicates small quantities of Cl2
  present
• May rationalise difference in FT-IR and
  contractors in-house method readings
• HCl and Cl2 can be inferred as the only Cl based
  species present. Otherwise wet methods would
  report higher chloride readings than the FT-IR
  HCl reading
• Only holds for chlorides soluble in either H2SO4
  or NaOH

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Modelling

• Raw IR spectra analysed and concentrations
  reported in real time by commercial software
• How valid is analysis of commercial software?
• Can answer by fitting a model calculated from an
  internationally accepted database
• HITRAN
• Modelling methodology complex issue but briefly
• Remove any interferents
• Fit model by matching ILS
• Compare HCl concentration obtained from modelling
  to that reported by commercial software to
  determine degree of validity

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Species Model
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Region for HCl Analysis
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Fitting HClH2O Model to Measured Spectrum
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HCl Fitted Concentration

• Fitted model yields an HCl concentration of 6.4
  ppm
• Analysing same spectrum commercial software
  determines a concentration of 5.8 ppm
• Within error limits of commercial software the
  fit matches
• Conclusions drawn in regard of comparison to wet
  chemical reference methods valid

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Advantages of FT-IR
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Benefits to Fibreglass Industry

• Economic
• 35k on contractor monitoring
• 6.5k on internal monitoring
• Installing an FT-IR (50k) operated in-house
  could save 27k per annum. 2 year payback!
• Technique applicable to many other industries. In
  particular, HF monitoring important to aluminium
  smelting and brick work industries.