Gas phase infrared spectroscopy

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Introduction to FTIR
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Gas phase infrared spectroscopy

• Molecules in gas phase vibrate and rotate at
  frequencies characteristic to each molecule. Each
  frequency is associated with an energy state of a
  molecule
• Infrared radiation moves the molecules to higher
  energy states characteristic frequencies are
  absorbed by the molecule in the process
• Each molecule absorbs infrared radiation at
  several characteristic frequencies (wavelengths)
• The result is an IR absorption spectrum a
  fingerprint unique to each molecule

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Gas phase infrared spectroscopy

• All molecules can be identified on the basis of
  their characteristic absorption spectrum (except
  diatomic elements such as O2 and noble gases)
• Each molecule absorbs infrared radiation at its
  characteristic frequencies
• IR absorption spectrum is a fingerprint unique to
  each molecule
• Beers law Absorption strength i.e absorbance is
  directly proportional to concentration

HCl molecule stretching vibration at 2880 cm-1

IR spectrum of HCl
Absorbance
Wavenumbers
All gases except O2, N2, H2, Cl2, F2, H2S, and
noble gases can be measured
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IR technologies

• Gas Filter Correlation IR (GFC)
• Measures only separate wavelength bands with
  gas-filled filters
• Only one component can be measured with each
  filter
• Multiple gases can be measured and spectral
  interference resolved only with additional
  filters (typically maximum 6 gases)
• Multiple gas filled filters means multiple
  calibration checks
• Fourier Transform Infrared (FTIR)
• Spectrometer measures all the IR wavelengths
  simultaneously and produces a full spectrum.
• Any number of components (up to 50) can be
  analysed from single measurement and
  interferences are automatically resolved
• Same optical elements used for each measurement,
  multiple calibration checks are not necessary

Optical filters
Broad band light source
Sample cell
B
A
C
B
Interferometer
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FTIR spectroscopy
Interferometer

• Based on the use of an optical modulator
  interferometer
• Interferometer modulates radiation emitted by an
  IR-source, producing an interferogram that has
  all infrared frequencies encoded into it
• Interferometer performs an optical Fourier
  Transform on the IR radiation emitted by the IR
  source
• The whole infrared spectrum is measured at high
  speed
• Spectral range is continuously calibrated with
  HeNe laser
• Fast, extremely accurate measurements

Modulated IR Beam
Interferogram
Fourier Transformation
IR Spectrum
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Michelson interferometer

• Simplest interferometer design
• Beamsplitter for dividing the incoming IR beam
  into two parts
• Two plane mirrors for reflecting the two beams
  back to the beamsplitter where they interfere
  either constructively or destructively depending
  on the position of the moving mirror
• Position of moving mirror is expressed as Optical
  Path Difference (OPD)

OPD Distance travelled by red beam minus
distance travelled by yellow beam
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Interference

• Electromagnetic (EM) radiation can be described
  as sine waves having definite amplitude,
  frequency and phase
• When EM-waves interact, interference is observed
• Depending on the relative phase of the waves,
  interference is either destructive or constructive

constructive interference
destructive interference
A
A
A
A
Interference signal
Interference signal
EM waves with same amplitude and frequency, out
of phase
EM waves with same amplitude and frequency, in
phase (OPD 0)
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Mirror movement and interference of single
wavelength beam
When moving mirror is in the original position,
the two paths are identical and interference is
constructive When the moving mirror moves ¼ of
wavelength, the path difference is ½ wavelength
and interference is destructive Mirror moves
back and forth at constant velocity the
intensity of the interference signal varies as a
sine wave
OPD Distance travelled by red beam minus
distance travelled by yellow beam
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Fourier transformation

• The interferogram signal is recorded as a
  function of optical path difference
• The interferogram is comparable to a time domain
  signal (eg. a recorded sound) and the spectrum
  represents the same information in frequency
  domain (eg. the frequency of the same sound)
• Fourier transformation is the mathematical
  relation between the interferogram and the
  spectrum (in general, between time domain signal
  and frequency signal)
• A pure cosine wave in the interferogram
  transforms to a perfectly sharp narrow spike in
  the spectrum

Intensity
Intensity
Fourier transformation pair
OPD / cm
Wave number / cm-1
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Interferogram and spectrum
Spectrum consisting of three discrete frequencies
E(? )
Spectrometer IR source Continuous emission
?
FT
Each frequency contributes a cosine wave to the
interferogram
Observed interferogram of wide band of frequencies
FT
OPD
0
Observed interferogram with centerburst
OPD
OPD
0
Fourier transform analysis converts the recorded
interferogram back into a frequency spectrum by
reversing the process shown at left
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IR and laser interferograms

• IR interferogram is recorded after the IR beam
  passes through the interferometer and sample cell
• IR interferogram contains the absorption of
  sample gas
• Laser interferogram is produced by a helium-neon
  laser beam travelling through the interferometer
  into a special detector
• Laser interferogram is a nearly ideal cosine wave
• Laser interferogram tells the position of moving
  mirror with excellent accuracy

A
IR-interferogram
Laser-interferogram
OPD
?x 632.8 nm
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Recording an interferogram

• Laser interferogram signal is used to digitize
  the IR interferogram
• Single mode HeNe-laser provides a constant
  wavelength output at 632.8 nm
• Accurate and precise digitization interval
  provides high wavelength accuracy in the spectrum
• The data points for IR interferogram are recorded
  every time the mirror has moved forward by one
  HeNe laser wavelength

Infrared source
Helium-neon laser
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Recording an interferogram

• The digitized IR interferogram (an XY table) is
  transmitted to computer where the Fast Fourier
  Transform (FFT) algorithm computes the spectrum

Infrared source
Infrared source
Helium-Neon laser
Helium-neon laser
0
-L
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Measurement sequence

• Transmittance spectrum is a single beam sample
  divided by background
• Absorbance spectrum negative logarithm of
  transmittance
• Calcmet automatically converts and displays
  spectra as absorbance spectra

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Background and absorbance spectra

• Absorbance spectrum is calculated from the
  background and a single beam sample spectrum

• The absorbance peak height depends also on the
  concentration c of the sample, absorptivity
  epsilon (this is a physical constant specific to
  each gas and wavelength) and cell lenght l

Zero absorbance means that the amount of light
arriving at the detector is the same in both
sample and background. This is why the background
measurement is often called a zero calibration.
High absorbance means less light arriving at the
detector (-1 in the formula). If the baseline
(region of spectrum without peaks) is above zero,
transmission of light is less than in the
background.
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Spectral resolution and signal-to-noise ratio
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Resolution and interferogram
Absorption line shape due to truncation
Truncation points L, L (cm)
Resolution is limited by interferogram truncation
i.e. length of mirror movement
Resolution is also limited by aperture size

• High resolution comes at a cost
• long mirror movement (slow)
• small aperture (little signal)
• Gasmet design matches interferogram truncation
  with aperture size optimized for high signal to
  noise ratio

Source
Absorption line shape due to finite aperture
2? angle at which the source is seen from the
collimating lens
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Signal-to-noise ratio
The most important property of the spectrum in
quantitative analysis
spectra are on same absorbance scale
1 cm-1

• Due to low (8 cm-1) spectral resolution,
  Gasmet has an excellent signal-to-noise ratio
  (SNR).
• SNR affects the uncertainty (error limits) of
  the analysis
• - Precise and accurate
• measurements
• - Low detection limits
• and reliable analysis

Absorbance (a.u.)
8 cm-1
3000
2800
2600
2400
2200
2000
1800
1600
1400
1200
1000
800
Wave number (cm-1)
Figure adapted from Instrumental Resolution
Considerations for Fourier Transform Gas-Phase
Spectroscopy. Applied Spectroscopy. Volume 51,
Number 8, 1997.
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Resolution and dynamic Range
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Spectral resolution

• In high resolution spectrum, the band
  intensities are high and can get
• saturated at relatively low concentrations
  (concentration ? path length)
• Quantitative analysis precision deteriorates
  when band absorbance is
• higher than approximately 0.434 A.U.
• In low resolution spectrum, band intensities are
  low and get saturated
• only at very high concentrations

• Low resolution
• High signal-to-noise ratio (SNR)
• Wide dynamic range
• Non-linear calibration
• Strong spectral overlap
• Short measurement times

• High resolution
• Low signal-to-noise ratio (SNR)
• Low dynamic range
• Linear calibration
• Weak spectral overlap
• Long measurement times

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Interferogram and resolution

• Only an envelope without any fine structure of a
  vibrational absorption band can be detected when
  only a short portion of the interferogram is
  recorded (low resolution)
• When a longer interferogram is recorded
  containing the centerburst and the signatures,
  the rotational fine structure beneath the
  envelope becomes detectable (high resolution).
• No information of the molecule is contained in
  the interferogram data points between the
  centerburst and the signatures

noise, no information
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Advantages of FTIR spectroscopy

• Speed (Felgett advantage) All the frequencies
  are recorded simultaneously a complete spectrum
  is measured in less than a second.
• Sensitivity (Jacquinot or Throughput advantage)
  In the interferometer, the radiation power
  transmitted on to the detector is very high which
  results in high sensitivity.
• Internally Calibrated (Connes advantage) FTIR
  spectrometers employ a HeNe laser as an internal
  wavelength calibration standard, no need to be
  calibrated by the user.
• Multicomponent capability Since the whole
  infrared spectrum is measured continuously, all
  infrared active components can be identified and
  their concentrations determined.

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Analysis of FTIR spectra
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Analysis of FTIR spectra

• Modified Classical Least Squares (CLS)
• Use of single component library spectra
• Use of both line shape and line intensity
• Cross-correlation effects compensated
• Residual spectrum and confidence intervals for
  QA/QC
• Identification of unknowns

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Cross-Interference compensation

• Spectral analysis by a line-shape fitting
  modified CLS routine
• The source of cross-interference is spectral
  overlap
• Spectra of the interfering (overlapping) species
  used in the CLS routine as interfering
  components

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Calcmet analysis
Sample spectrum
Reference Spectra (not to same scale)
Calcmet analysis 0.881 Water 10 vol-1.112
CO2 10 vol- 0.995 CO 1000 mg/Nm3 0.910 NO
300 mg/Nm3 0.810 SO2 300 mg/Nm3 0.660 NH3
100 mg/Nm3 0.082 HCl 50 mg/Nm3 0.210 Methane
50 mg/Nm3
Calculated spectrum
Concentrations Water 8.81 vol-
CO2 11.12 vol-
CO 955 mg/Nm3
NO 274 mg/Nm3
SO2 243 mg/Nm3 NH3
66.0 mg/Nm3 HCl 4.1 mg/Nm3 Methane
10.5 mg/Nm3
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CLS analysis
Example mixture of 80 ppm propane and 150 ppm
ethane
Spectra are completely overlapping.
How to analyse?
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CLS analysis

• CLS analysis is an iterative process
• At each step every individual reference spectrum
  is given a coefficient (k)
• Model spectrum is calculated as a sum of
  reference spectra weighted by coefficient (k)
• The difference between measured spectrum and
  model spectrum is called residual spectrum
  (residual)
• The residual is calculated in every data point of
  the selected analysis area
• The CLS algorithm searches for smallest possible
  residual by changing the k values
• When the minimum residual is found, the
  concentrations in the sample spectrum are k times
  concentration of the reference spectra

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Initial quess Propane 100 ppm, ethane 100 ppm
Residual Sample spectrum Calculated spectrum
Residual not in minimum -gt optimisation continued
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• The optimisation stops when
• k for ethane is 1.5
• k for propane is 0.8

Residual is only noise Succesful analysis!
Concentrations Ethane 1.5 X 100 ppm 150
ppm Propane 0.8 X 100 ppm 80 ppm
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Cross interference correction
Methane not included
Successful analysis

• Cross interference occurs when one or more gases
  are missing from the library
• Incomplete library leads to large difference
  between measured and calculated spectrum ?
  analysis error
• Cross interference may be avoided by selecting
  suitable analysis areas avoiding the interfering
  absorption if the library cannot be expanded.

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Analysis Areas and typical sample spectrum
CO2, NH3, C2H4
CO, N2O
NO2 CH4 C3H8
H2O
HF
NO
HCl HCHO
12 September 2006
Gasmet Technologies Oy 2006
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Extended CEM settings
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Gasmet spectrum file (.spe)
Analyzer parameters
Analysis results

• Stored ample spectrum is an absorbance spectrum,
  no need to ratio it againts background again
  (different from lab FTIRs)
• Sample spectrum includes analyser hardware status
  information
• Without sample spectrum, verification of results
  and re-analysis is impossible

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Quality assurance and control
Toluene added into analysis. Positive reading and
no errors.
Residual error for methane. Invalid results.
Pure noise
Residual spectrum indicating missing gas
(toluene)
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Gasmet FTIR structure
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Gasmet structure and outline
FTIR spectrometer
IR source
SAMPLE CELL
Broad band infrared radiation
IR SOURCE
Detector
Signal and data processing
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Corrosion resistant sample cells

• Nickel-rhodium-gold plated
• Fixed mirrors
• Absorption lengths vary from 1 cm to 9.8 m
  according to application
• Single pass and multipass (White cell)
• Can be heated up to 180 oC

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Sample cells and optical path length
Different path lengths for different measurement
ranges
High Sensitivity (Multipass) Sample Cell V
0.4 l L 60 980 cm T90lt 10 sec (4 lpm)
L 9.8 meter c 10 ppm A 0.0047 a.u
L 2.5 meter c 39 ppm A 0.0047 a.u
L 10 centimeters c 980 ppm A 0.0047 a.u
Single pass cellV 0.013 0.031 l L 1, 4, or
10 cm T90 lt 1 sec (4 lpm)
L 4 centimeters c 2450 ppm A 0.0047 a.u
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Hot extractive sampling
Sample gas in
Hot 3-way solenoid valve
Flow restricted to 0.5 lpm
Pump
Zero / test gas in
to O2 analyser
Particle filters (two-stage filtration)
to FTIR
Hot zone maintained at 180 oC

• Wet and hot sample gas is transferred through
  heated lines and pump to the analyzer
• Sample gas must always be free of particles and
  in gaseous form
• Sample gas should always be measured against
  ambient air

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