Reflectance spectra were collected
using a halogen light source (Ocean Optics, HL-2000) and a spectrometer
(Ocean Optics, HR2000+) equipped with a bifurcated optical probe (Ocean
Optics, QR400–7-UV/Vis). The optical probe was normal to the
rugate filter surface, generating an illuminated area of about 2 mm
in diameter. Vapor concentrations were varied by bubbling the nitrogen
carrier gas at a particular flow rate into the analyte liquid so that
the partial pressure of the analyte vapor was between 0.02P0 and 0.30P0, where P0 is the saturated vapor pressure at 20 °C.
The total gas flow rate for all measurements was held constant at
400 mL min–1 by mass flow controllers (Alicat Scientific),
and the temperature was held constant at 20 °C. The reflectance
measurement cell had a volume of 500 mL, while the gas tubing had
a total volume of 270 mL. Each spectrum was collected after 10 min
of exposure to the vapor. Mild heating of the pSi rugate filter to
80 °C facilitated the rapid return of the spectra to the nitrogen
baseline between analyte tests.28 (link) For both
pSi rugate filters, three separate runs of each analyte concentration
were collected and averaged. Rather than a reflectance standard, differential
reflectance spectra were collected and analyzed.4 (link),11 (link),38 (link) PCA was performed on the averaged differential
spectra relative to nitrogen after mean-centering the data.
using a halogen light source (Ocean Optics, HL-2000) and a spectrometer
(Ocean Optics, HR2000+) equipped with a bifurcated optical probe (Ocean
Optics, QR400–7-UV/Vis). The optical probe was normal to the
rugate filter surface, generating an illuminated area of about 2 mm
in diameter. Vapor concentrations were varied by bubbling the nitrogen
carrier gas at a particular flow rate into the analyte liquid so that
the partial pressure of the analyte vapor was between 0.02P0 and 0.30P0, where P0 is the saturated vapor pressure at 20 °C.
The total gas flow rate for all measurements was held constant at
400 mL min–1 by mass flow controllers (Alicat Scientific),
and the temperature was held constant at 20 °C. The reflectance
measurement cell had a volume of 500 mL, while the gas tubing had
a total volume of 270 mL. Each spectrum was collected after 10 min
of exposure to the vapor. Mild heating of the pSi rugate filter to
80 °C facilitated the rapid return of the spectra to the nitrogen
baseline between analyte tests.28 (link) For both
pSi rugate filters, three separate runs of each analyte concentration
were collected and averaged. Rather than a reflectance standard, differential
reflectance spectra were collected and analyzed.4 (link),11 (link),38 (link) PCA was performed on the averaged differential
spectra relative to nitrogen after mean-centering the data.
