1. © Gasmet Technologies 02 January 2006Introduction to FTIR

2. © Gasmet Technologies 02 January 2006Introduction to FTIR

3. © Gasmet Technologies 02 January 2006Introduction to FTIR 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

4. © Gasmet Technologies 02 January 2006Introduction to FTIR All molecules can be identified on the basis of their characteristic absorption spectrum (except diatomic elements such as O 2 and noble gases) Each molecule absorbs infrared radiation at its characteristic frequencies IR absorption spectrum is a fingerprint unique to each molecule Beer’s law: Absorption strength i.e absorbance is directly proportional to concentration Gas phase infrared spectroscopy IR spectrum of HCl Wavenumbers Absorbance All gases except O 2, N 2, H 2, Cl 2, F 2, H 2 S, and noble gases can be measured HCl molecule stretching vibration at 2880 cm -1

5. © Gasmet Technologies 02 January 2006Introduction to FTIR 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 AC B B Optical filters Broad band light source Sample cell Interferometer

6. © Gasmet Technologies 02 January 2006Introduction to FTIR FTIR spectroscopy 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 Interferogram IR Spectrum Fourier Transformation Interferometer Modulated IR Beam

7. © Gasmet Technologies 02 January 2006Introduction to FTIR 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) Michelson interferometer IR Source Moving mirror Stationary mirror Beamsplitter OPD = Distance travelled by red beam minus distance travelled by yellow beam

8. © Gasmet Technologies 02 January 2006Introduction to FTIR 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 Interference destructive interference constructive interference Interference signal EM waves with same amplitude and frequency, out of phase EM waves with same amplitude and frequency, in phase (OPD = 0) AA A A

9. © Gasmet Technologies 02 January 2006Introduction to FTIR 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 OPD = 0 at the white line

10. © Gasmet Technologies 02 January 2006Introduction to FTIR Fourier transformation Fourier transformation pair 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 OPD / cm Intensity Wave number / cm -1

11. © Gasmet Technologies 02 January 2006Introduction to FTIR Interferogram and spectrum Fourier transform analysis converts the recorded interferogram back into a frequency spectrum by reversing the process shown at left FT OPD Spectrometer IR source Continuous emission OPD Observed interferogram of wide band of frequencies Observed interferogram with centerburst Each frequency contributes a cosine wave to the interferogram Spectrum consisting of three discrete frequencies E( ) OPD FT 0 0

12. © Gasmet Technologies 02 January 2006Introduction to FTIR 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 IR and laser interferograms OPD A IR-interferogram Laser-interferogram  x =632.8 nm

13. © Gasmet Technologies 02 January 2006Introduction to FTIR 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

14. © Gasmet Technologies 02 January 2006Introduction to FTIR 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 Helium-Neon laser 0 -L Infrared source Helium-neon laser Optical path difference

15. © Gasmet Technologies 02 January 2006Introduction to FTIR Measurement sequence Interferogram with N 2 Interferogram with sample Background Single beam sample spectrum Transmittance spectrum Absorbance spectrum 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

16. © Gasmet Technologies 02 January 2006Introduction to FTIR 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.

17. © Gasmet Technologies 02 January 2006Introduction to FTIR Spectral resolution and signal-to-noise ratio

18. © Gasmet Technologies 02 January 2006Introduction to FTIR Resolution is limited by interferogram truncation i.e. length of mirror movement Absorption line shape due to finite aperture Absorption line shape due to truncation Truncation points – L, L (cm) Source 2  = angle at which the source is seen from the collimating lens Resolution and interferogram 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

19. © Gasmet Technologies 02 January 2006Introduction to FTIR Signal-to-noise ratio Figure adapted from: Instrumental Resolution Considerations for Fourier Transform Gas-Phase Spectroscopy. Applied Spectroscopy. Volume 51, Number 8, 1997. 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 Wave number (cm -1 ) mixture: 1 cm -1 residual: 1 cm -1 mixture: 8 cm -1 residual: 8 cm -1 1 cm -1 8 cm -1 The most important property of the spectrum in quantitative analysis spectra are on same absorbance scale 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.)

20. © Gasmet Technologies 02 January 2006Introduction to FTIR Resolution and dynamic Range 0 0,1 0,2 0,3 0,4 0,5 020004000600080001000012000 Concentration (ppm) Absorbance High resolution 8 cm -1

21. © Gasmet Technologies 02 January 2006Introduction to FTIR Spectral resolution 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 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

22. © Gasmet Technologies 02 January 2006Introduction to FTIR 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

23. © Gasmet Technologies 02 January 2006Introduction to FTIR 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.

24. © Gasmet Technologies 02 January 2006Introduction to FTIR Analysis of FTIR spectra

25. © Gasmet Technologies 02 January 2006Introduction to FTIR 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

26. © Gasmet Technologies 02 January 2006Introduction to FTIR 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

27. © Gasmet Technologies 02 January 2006Introduction to FTIR Calcmet analysis: 0.881 * Water 10 vol-% 1.112 * CO 2 10 vol-% 0.995 * CO 1000 mg/Nm 3 0.910 * NO 300 mg/Nm3 0.810 * SO 2 300 mg/Nm3 0.660 * NH 3 100 mg/Nm 3 0.082 * HCl 50 mg/Nm 3 0.210 * Methane 50 mg/Nm 3 Concentrations: Water 8.81 vol-% CO 2 11.12 vol-% CO 955 mg/Nm 3 NO 274 mg/Nm 3 Calcmet analysis: Reference Spectra (not to same scale): Water 10 vol-% Methane 50 mg/Nm 3 SO 2 300 mg/Nm 3 CO 2 10 vol-% CO 1000 mg/Nm 3 NO 300 mg/Nm 3 NH 3 100 mg/Nm 3 HCl 50 mg/Nm 3 SO 2 243 mg/Nm 3 NH 3 66.0 mg/Nm 3 HCl 4.1 mg/Nm 3 Methane 10.5 mg/Nm 3 Sample spectrum Calculated spectrum

28. © Gasmet Technologies 02 January 2006Introduction to FTIR CLS analysis Example: mixture of 80 ppm propane and 150 ppm ethane Spectra are completely overlapping. How to analyse?

29. © Gasmet Technologies 02 January 2006Introduction to FTIR 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

30. © Gasmet Technologies 02 January 2006Introduction to FTIR Initial quess: Propane = 100 ppm, ethane = 100 ppm Residual = Sample spectrum – Calculated spectrum Residual not in minimum -> optimisation continued

31. © Gasmet Technologies 02 January 2006Introduction to FTIR 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

32. © Gasmet Technologies 02 January 2006Introduction to FTIR Cross interference correction 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. Methane not includedSuccessful analysis

33. © Gasmet Technologies 02 January 2006Introduction to FTIR 12 September 2006Gasmet Technologies Oy 2006 Analysis Areas and typical sample spectrum CO 2, NH 3, C 2 H 4 H2OH2O CO, N 2 O NO 2 CH 4 C 3 H 8 HCl HCHO NO HF

34. © Gasmet Technologies 02 January 2006Introduction to FTIR Extended CEM settings ComponentGasStandard analysis areaInterferents Water vaporH2OH2O3200 – 3401NH 3 Carbon dioxideCO 2 926 – 1150H 2 O, N 2 O, SO 2, NH 3, C 2 H 4 Carbon monoxideCO2000 – 2200, 2540 – 2590H 2 O, CO 2, N 2 O Nitrous oxideN2ON2O2000 – 2222, 2540 – 2590H 2 O, CO 2, CO Nitrogen monoxideNO1875 – 2138H 2 O, CO 2, CO, N 2 O Nitrogen dioxideNO 2 2700 – 2950H 2 O, HCl, CH 4, C 2 H 4, C 3 H 8, HCHO, N 2 O Sulfur dioxideSO 2 1200 – 1366H 2 O, N 2 O, NH 3, CH 4, C 3 H 8 AmmoniaNH 3 910 – 1150H 2 O, CO 2, N 2 O, SO 2, C 2 H 4 Hydrogen chlorideHCl2617 – 2880H 2 O, NO 2, CH 4, C 2 H 4, C 3 H 8, HCHO, N 2 O MethaneCH 4 2700 – 3200H 2 O, HCl, NO 2, C 2 H 4, C 3 H 8, HCHO, N 2 O Hydrogen fluorideHF3200 – 3400, 4020 – 4200H 2 O, NH 3 PropaneC3H8C3H8 2600 – 3200H 2 O, HCl, NO 2, CH 4, C 2 H 4, HCHO, N 2 O EthyleneC2H4C2H4 910 – 1150H 2 O, CO 2, N 2 O, SO 2, NH 3 FormaldehydeHCHO2550 – 2850H 2 O, CO 2, NO 2, HCl, C 2 H 4, CH 4, C 3 H 8

35. © Gasmet Technologies 02 January 2006Introduction to FTIR Gasmet spectrum file (.spe) 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 Analysis results Analyzer pa r ameters

36. © Gasmet Technologies 02 January 2006Introduction to FTIR Quality assurance and control Residual error for methane. Invalid results. Toluene added into analysis. Positive reading and no errors. Residual spectrum indicating missing gas (toluene) Pure noise

37. © Gasmet Technologies 02 January 2006Introduction to FTIR Gasmet FTIR structure

38. © Gasmet Technologies 02 January 2006Introduction to FTIR Gasmet structure and outline Broad band infrared radiation FTIR spectrometer IR source Transmitted infrared radiation Sample cell Modulated infrared radiation Interferometer Measured signal Detector Signal and data processing SAMPLE CELL IR SOURCE Detector

39. © Gasmet Technologies 02 January 2006Introduction to FTIR 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 o C Corrosion resistant sample cells

40. © Gasmet Technologies 02 January 2006Introduction to FTIR Sample cells and optical path length Single pass cell V = 0.013 … 0.031 l L = 1, 4, or 10 cm T 90 < 1 sec (4 lpm) High Sensitivity (Multipass) Sample Cell V = 0.4 l L = 60 … 980 cm T 90 < 10 sec (4 lpm) Different path lengths for different measurement ranges 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 L = 4 centimeters c = 2450 ppm A = 0.0047 a.u

41. © Gasmet Technologies 02 January 2006Introduction to FTIR Hot extractive sampling 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 Sample gas in to FTIR to O 2 analyser Zero / test gas in Particle filters (two-stage filtration) Hot 3-way solenoid valve Pump Flow restricted to 0.5 lpm Hot zone maintained at 180 o C

42. © Gasmet Technologies 02 January 2006Introduction to FTIR