Uncertainties in Optical Charring Correction Schemes

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Uncertainties in Optical Charring Correction
Schemes

• Jian Zhen Yu and Hong Yang
• Department of Chemistry
• Hong Kong University of Science Technology

EC/OC workshop, Durango, CO March 2003
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Two assumptions underlying charring correction
scheme

1. PEC evolves before native EC evolves in the
   analysis, or
2. PEC and native EC have the same absorption
   coefficients at the monitoring light wavelength.

The EC/OC split is set at where filter
transmittance returns to its initial value.
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Definition of terms

• PEC pyrolytically-generated EC
• NEC native EC
• AEC apparent EC, as determined by the instrument
  using optical charring correction scheme
• s filter light absorption coefficient at the
  monitoring wavelength
• A filter light absorbance at the monitoring
  wavelength

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Hypothetic thermograms of pure PEC, pure NEC, and
a mixture of the two in a He/O2 atmosphere
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How much does AEC deviate from NEC?
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Unburned EC on the filter in the He/O2 stage
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Implication of an invalid assumption 2
If ?NEC lt ?PEC2 , AEC underestimates NEC If ?NEC
gt ?PEC2 , AEC overestimates NEC
The magnitude of deviation of AEC from NEC is
proportional to (1) the amount of NEC that is
oxidized before the split line (2) the ratio
between the ? values of NEC and PEC2.
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AEC overestimates NEC
Analyzed with the same temperature program.
Filter alone AEC 1.87/-0.03
mgC/cm2 FilterWSOC AEC2.25/-0.26 mgC/cm2
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Conclusions

• The two assumptions underlying the charring
  correction scheme by optical methods are proved
  not to hold.
• Pyrolytically generated EC is indistinguishable
  from native EC.
• The two types of EC have different light
  absorption coefficients (s) at the monitoring
  light wavelength.
• The unequal s values of PEC and NEC could lead to
  either a negative or a positive bias in the EC
  measurement using the charring correction scheme.
  
• The deviation from the underlying assumptions
  highlights the importance of minimizing PEC
  formation in order to minimize uncertainties in
  OC and EC determination.