Mid-IR Methods. Matti Hotokka

Transcription

1 Mid-IR Methods Matti Hotokka

2 Traditional methods KBr disk Liquid cell Gas cell Films, fibres, latexes Oil suspensions GC-IR PAS

3 KBr Disk 1 mg sample, 200 mg KBr May be used for quantitative analysis Mix and grind Fill mold Press the disk Disk

4 KBr disk Insufficient grinding gives asymmetric bands Standard mg L Semimicro mg 1 4 L Micro 50 g L Ultramicro 50 ng 1 g

5 Liquid cell

6 Liquid cell Technical The thickness of sample layer is adjusted by using a suitable spacer The window material must be resistant to sample Most often gives interferences Solvents absorb, use complementary solvents Cannot be calibrated, no quantitative measurements

7 Liquid cell Interference

8 Liquid cell Example of complementary solvents

9 Simple Gas Cell

10 Gas Cell Pressure inside the cell a few torr Simple cell Path length typically 10 cm Single pass White cell For professional use Multiple pass cell Pathlength several meters

11 Gas cell, White cell

12 Gasmet gas analyzer Gasmet DX

13 Films Single component Press thin Mount on suitable holder Multicomponent films Cast in resin Cut using microtome Use microscope Interference patterns will appear

14 Fibres Single fibre Press this, use microscope or diamond ATR Bundles Make a mat Dip in liquid to fill the gaps (less scattering) Mount in the beam, e.g., using tablet holder

15 Latex A drop of latex or similar stuff may be deposited on an ir-window and dried Wash off the surface treatment chemicals Resins may be precipitated on a window or dried and used in a KBr disk Oil suspension (Nujol mull) may be used instead of KBr disk. Oil absorptions appear at 2900, 1460 and 1380 cm -1.

16 GC-IR

17 PAS Photoacoustic spectrometry Microphone Incident modulated IR light IR window He Pressure waves = sound Sample

18 PAS Suitable for samples that cannot be measured with other methods Traditionally sensitive to environment Modern PAS is quite useful Today, e.g., capasitive cantilever detector is used, for instruments, see gasera.fi

19 PAS Surface sensitive method Depth of penetration depends on Modulating frequency Thermal diffusivity of the sample Absorption coefficient of radiation Surface reflectance of the sample Homogeneouity of the sample Quantitative analysis is possible, if...

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21 Refraction Snell s law sin sin' n' n n n

22 Total reflection Critical angle: refraction angle is n c n sin c n n'

23 ATR Internal reflection Attenuated total reflection Sample placed on a crystal with high refractive index, shaped to give total reflection The totally reflected beam tests the sample Sample must be pressed against crystal Sample

24 ATR Multiple reflection ATR Evanescent wave, 1 m Sample: powder, liquid, fiber, film IR beam Crystal, e.g., Ge or ZnS Total reflection

25 ATR Evanescent wave Classically no penetration into the sample The parallel component induces a field beyond the interface Penetration depth: Harrick s equation d p 2n n sin ( n2 / 1)

26 ATR Penetration depth 1723 cm -1, n 2 = 1.5 Crystal n 2 /n 1 () d p (m) Ge KRS KRS ZnSe Diamond

27 ATR Some common crystal materials Crystal n 1 Transparent (cm -1 ) Ge Si , AMTIR ZnSe ZnS KRS Sapphire Diamond

28 ATR Higher penetration at low wavenumbers Intensity of peaks high at low wavenumbers Diamond 600/3000 cm -1 : d P = 3.4/0.7 m Refractive index changes at absorptions Asymmetrical spectral bands Critical angle varies along spectrum You may loose total reflection Wavenumber changes usually neglected

29 ATR ATR spectrum of polyethylene Red trace: FTIR spectrum, Copyright NICODOM Ltd.,

30 Diffuse Reflectance A.k.a. DRIFT For powder samples Light must penetrate into the sample Reflections from particles surface => Surface analysis Kubelka-Munk transformation required

31 Diffuse Reflectance Spherical reflectors Schematically Shield Sample cup Flat mirrors

32 Diffuse Reflectance

33 Diffuse Reflectance Sample Powder mixed with KBr; bottom of sample cup filled with pure KBr (the surface layer gives infinite thickness required by theory) Kubelka-Munk Transformation from reflection to absorbance 2.303ac S (1 R 2R ) 2

34 Diffuse Reflectance Original spectrum, phenacetin

35 Diffuse Reflectance Kubelka-Munk transformed spectrum

36 Diffuse Reflectance Phenacetin standard spectrum

37 Highlights Infinitesimal change in intensity for light passing through the layer dx di ( ) uidx vjdx dj ( ) vjdx uidx = scattering coefficient = absorption coefficient i, j = total intensities u, v = mean free paths Totally diffuse material: u=v=2 Define K 2 u v S 2 u v The in-coming intensity diminishes as radiation is absorbed (K) or scattered (back) (S). It increases from the out-going intensity scattered back into sample. I 0 di dj J d dx 0

38 Highlights Both Kubelka-Munk absorption and scattering are assumed to be linear di KIdx dj SJdx Let I t be the intensity at the bottom of the sample, I 0 the incident intensity and J r the flux up at the surface of the sample. Then transmittance and reflectance are I J t r T R I0 I 0 The general solution of the system of diff-equations is (R g is reflectance of substrate) 1 Rg a bcoth( bsd) b K S R T a b a 2 1 a Rg coth( bsd) asinh( bsd) bcosh( bsd) S Let R be the reflectance of an infinitely sample that completely hides the substrate. K S 1 R 2R 2 R 1 K S K S V. Džimberg-Malčić et al., Technical Gazette 18, 1(2011), K S

39 Highlights Let the layer thickness be dx. A perfectly diffuse light falls on the layer. The incidence angle can be 0 to 90. The mean optical path length is then / 2 i d di dx udx 0 i cos d j dx / 2 For perfectly diffuse material 0 j d vdx j cos i i sin(2 ) 2i sin cos j j sin(2 ) 2 j sin cos For = 60 one does not need to calculate the mean path because dx dx di 2dx cos dx dx d j 2dx cos60 0.5

40 Microscopy Most important method in industry Various objectives, including ATR, reflection, microdisk, fiber holder etc Tomographic samples Mapping, imaging, data cube

41 Microscopy Various methods in micro scale KBr microdisk Microtome cuts of resin-embedded samples Micro ATR, Diamond ATR accessories Diamond pressure cell Grazing angle reflection spectroscopy Confocal microscopy

42 Microscopy Micro KBr disks Down to 100 ng

43 Microscopy Thin layers paint chips, multilayer films etc. Cast in resin, cut with microtome

44 Microscopy Diamond pressure cell

45 Microscopy Wide field microscope Scanning microscope Illuminated area CCD grid

46 Microscopy C=O band Amide I band

47 IR Dichroism Every molecule belongs to a point group The vibrations follow the symmetry of that molecule In solid samples the symmetry axes are fixed Then polarized light will only excite certain modes Useful for, e.g., polymers and proteins Circular dichroism (VCD) for optical activity

48 Scans Scans Step-scan spectrometer Ordinary rapid scan FTIR Step-scan

49 Step-scan spectrometer The mirror must be held at measuring point for extended time Fine-adjustments with piezo elements Mirror movement in rapid scan is slow Detector electronics is fast Process must be repeatable Time resolution less than 50 ns

50 Step-scan spectrometer Ignition of a GaAs diode. Spectral resolution 0.4 cm-1. Time resolution 50 ns. T. J. Johnson et al., Bruker, 1993