CH 502 Experimental & Analytical Methods

1. CH 502 Experimental & Analytical Methods Supplement - 1

2. Glass Membrane Electrodes • Ion Selective Field Effect Transistor (ISFET) electrode • pH electrode csrg.ch.pw.edu.pl/tutorials/isfet/ www.ph-meter.info

3. Spectroscopic Methods • Based on the interaction between electromagnetic radiation (EMR) and matter • EM spectrum ranges from Gamma rays to Radio waves • The energy associated with the rays is related to the wavelength by the Planck’s law equation • E = hc / λ • h – Plancks Constant c – velocity of light λ - wavelength • Gamma rays have the lowest wavelength and hence the highest energy

4. Gamma Rays: < 0.01 nm Hard X-rays: 0.01 – 0.1 nm Soft X-rays: 0.1 – 10 nm Vac. UV: 10 – 180 nm Visible: 380 – 700 nm Near UV: 180 – 380 nm Near IR: 700 – 1400 nm Mid IR: 1400 – 15000 nm Far IR: 15 – 1000 μm Microwave: 1 – 10 mm Radio waves: > 10 mm Electromagnetic Spectrum http://nationalatlas.gov/articles/mapping/IMAGES/

5. Types of Interaction between EMR and Matter • NMR (Radio frequency): • Change of Spin – absorption on RF energy • Microwave • Change of Orientation • Infrared • Change of Configuration – rotational/vibrational • UV/Vis • Change in electronic structure • X-rays • Change in electronic structure • Gamma Rays • Change in nuclear configuration/mutation/isotopes

6. Instrumentation for Spectroscopy Absorption Spectroscopy Source Sample Wavelength Selector Detector Data Readout Emission Spectroscopy Sample EMISSION Wavelength Selector EXCITATION Wavelength Selector Detector Data Readout Source

7. Optical Materials • Transmittance is the main property for selecting material for optical elements • LiF – transmittance between 150 – 7000 nm • Borosilicate (normal glass) absorbs below ~380 nm • Quartz or Fused Silica is ideal for UV applications and have very high transmittance • IR optical elements – halide salts (AgCl, KBr, NaCl), polymers have high transmittance

8. Sources • Continuous Sources • Xenon Arc Lamp – 250 – 600 nm • Molecular Fluorescence • Hydrogen or Deuterium Lamp – 180 – 380 nm • UV Molecular Absorbance • Tungsten/Halogen – 240 – 2500 nm • UV/Vis/NIR – Molecular Absorbance • Tungsten – 350 – 2200 nm • Vis/NIR • Nernst Glower – 400-20,000 nm • IR – molecular absorbance • Nichrome – 750 – 20,000 nm • Globar – 1200 – 40,000 nm • Tunable Dye Lasers

9. Sources • Continuous Source Lamp Spectra & Irradiance From “Oriel Corporation”: (www.lot-oriel.com)

10. Sources • Line Sources • Narrow Discrete Spectra From “Oriel Corporation”: (www.lot-oriel.com)

11. Sources • Pulsed Sources • Lasers (Light Amplification by Stimulated Emission of Radiation) • Permits light of low divergence • Well defined wavelength • Low Energy and High Energy • Nd: YAG Lasers

12. Wavelength Selectors • Radiation is measured in ‘bands’ • Selectivity – narrow bands • Sensitivity – broader bands – allow more energy • Devices to select wavelength • Absorption/Emission Wavelengths • Monochromator • Band Filters • Interference Filters

13. Wavelength Selectors • Monochromator • Czerny-Turner • A – incident beam • B – Entrance Slit • C – Mirror 1 • D – Diffraction Grating (Rotating) • Blazed Grating • Prism • Concave • Holographic • E – Mirror 2 • F- Exit Slit Image: http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/spectro.htm#contnt

14. Wavelength Selectors • Blazed Diffraction Grating From www.edmundoptics.com

15. UV Absorption Spectroscopy • Transmittance, T = I / I0 • Absorbance, A = -log T • Beer’s Law • A = ε.L.C • ε = absorption coefficient • L = path length • C = concentration of analyte • Example Absorption Spectra shown here Image: http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/spectro.htm#contnt

16. UV Absorption Spectroscopy • Double Beam Spectrophotometer (figure above) • Analysis of Reference and Sample Simultaneously • Single Beam • Only one detector – reference and sample cuvettes move to come in line to the path of light.

17. Infra-red Spectroscopy • Structural Features: • Stretching or Bending Vibrations • Organic or Inorganic Bonds http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/spectro.htm#contnt

18. IR Vibrational Spectroscopy Formaldehyde – Gas Phase IR spectra Some Common Structural Features http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/spectro.htm#contnt

19. IR Vibrational Spectroscopy • Examples of Functional classes that exhibit IR absorbance • S compounds • S-H, S-OR, S-S, C=S, S=O • P compounds • P-H, (O=)PO-H, P-OR, P=O • Si Compounds • Si-H, Si-OR, Si-CH3 • Oxygen-Nitrogen • NOH, N-O, N=O http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/spectro.htm#contnt

20. Michelson Interferometer http://www.biophysik.uni-freiburg.de/Spectroscopy/Time-Resolved/spectroscopy.html

21. IR Spectrophotometer www.thermonicolet.com

22. Fourier Transform FTIR • The interferogram is the signal that is received by the detector (which is a thermal detector). • This interferogram is processed using a fourier transform to obtain spectral information (or an absorption spectrum) shown on the right side. • The transmission is the usual representation of IT spectra – absorption can be converted to transmission easily. Processor Absorption Spectra Interferogram http://www.biophysik.uni-freiburg.de/Spectroscopy/Time-Resolved/spectroscopy.html

23. NMR Spectroscopy • Nuclei of elements have a characteristic spin – I • Integral: e.g. 1,2,3… (# protons and # nuetron are odd) • Half integral: e.g. ½, 3/2, 5/2…(#protons + # nuetrons are odd) • Zero (#protons and #nuetrons are even) • Spinning charge generates a small magnetic field resulting in a magnetic moment • In the presence of an external magnetic field, B0 , two spin states exist, +1/2 and -1/2. • The magnetic moment of the lower (+1/2) energy spin state is aligned with the external magnetic field and that of the higher (-1/2) is aligned in the opposite direction to B0.

24. Spin states (two energy) in external magnetic field No Magnetic Field B0 – Magnetic Field Strength http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/nmr/nmr1.htm

25. Magnetic Fields • Earth’s Magnetic field – 10-4 T (Tesla) • Modern NMR instruments use magnetic fields 1 – 20 T. • Even at high B0, the energy difference observed between the two spin states is less than 0.1 cal/mole • IR (1-10 cal/mole), Electronic transitions are 100 times higher • Energy difference is expressed in terms of frequency in MHz • Ranging from 20 – 900 MHz depending on the field and the molecule • Irradiation of the sample with Radio Frequency energy corresponding to this energy difference between the two spin states causes absorption and excitation from the +1/2 to the -1/2 state. • For ½ spin nuclei, the energy difference is proportional to their magnetic moments at a given B • Common nuclei used in NMR spectroscopy are • 1H, 13C, 19F, 31P, • Most common is the 1H NMR also referred to as Proton NMR.

26. Proton NMR Spectroscopy http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/spectro.htm#contnt

27. Proton NMR Spectroscopy • Consider a sample of water • External magnetic field B = 2.3487 T • 100 MHz - radiation • If B is increased slowly to 2.3488 T • H nuclei will absorb RF energy and a resonance signal will appear http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/spectro.htm#contnt

28. Chemical Shift • Different H nuclei have different signals at different frequencies • Electron density around a nucleus produce a magnetic field that opposes the much stronger external field. • This secondary field shields the nucleus from the external field • To compensate for this shielding B must be increased to achieve resonance • Scanning of B at a RF frequency produces a SHIFT • Compounds that have higher electron density have the resonance at higher energy frequencies

29. Chemical Shift • The characteristic frequencies are dependent on the magnitude of B and the radio frequency • No two magnets have the same field and resonance frequencies vary accordingly • Reference proton: TMS (Trimethylsilane) • Shift is measured as shown below in terms of ppm • Solvent – CDCl3

30. Chemical Shift

31. NMR Spectroscopy – examples Red peaks indicate different characteristic groups of the chemical, which exhibit a certain chemical shift with respect to a reference such as TMS.

32. Chemical Shift • Signal Strength • Number of Hydrogen atoms • Concentration • OH proton signal • Aliphatic or aromatic • Π-electrons • Solvent effects • Spin-Spin Interactions • J Coupling

33. J-Coupling • Signal Splitting • Doublet • Triplet • Quartet • Quintet • Concentration

34. 13C NMR Spectroscopy • For Molecules that lack significant C-H bonds – one cannot use proton (or 1HNMR spectroscopy) • Polychlorinated compounds • Triple bond compounds • Polycarbonyl ass • 13C isotope is a viable alternative • Less abundant  less sensitive signal splitting due to interaction with H • Solid State NMR • anisotropy

35. Raman Spectroscopy • Used in chemistry, • Vibrational information is very specific for the chemical bonds in molecules. • Provides a fingerprint by which the molecule can be identified. The fingerprint region of organic molecules is in the range 500-2000 cm-1. • Study changes in chemical bonding, e.g. when a substrate is added to an enzyme. • Raman gas analyzers have many practical applications, for instance they are used in medicine for real-time monitoring of anaesthetic and respiratory gas mixtures during surgery. • Characterize materials – measure T • Crystallographic Orientation of a sample • Inelastic Scattering or Raman scattering of monochromatic light usually from a laser in the visible, NIR or near UV range. • Holographic diffraction gratings and multiple dispersion stages to separate the weak inelastic Raman scattering and the strong Rayleigh scattering • Spontaneous and stimulated RS

36. Atomic Spectroscopy • Qualitative/Quantitative determination of elements • Analysis of metals in water, soils, solids, tissue • Ionization/Atomization of sample • Flame Atomization • Plasma • Analysis • Atomic Optical Spectrometry • Atomic Mass Spectrometry • Generic Instruments • Atomic Absorption (AA) - Flame • ICP-AES/OES - Plasma • ICP-MS - Plasma SAMPLE Sample Processing Spray /Vapor (Nebulizer or Vaporizer Flame or Plasma Analyzer

37. Atomic Absorption Spectroscopy - AA • Source – Hollow Cathode Lamp • Tungsten anode/Cylindrical Cathode in quartz with inert gas • Cathode is made of the analyte metal or has a coating of the metal • Line spectra (narrow) • Individual lamps for different metals – about 70 elements • Sample is aspirated into the atomizing zone as a spray – • Nebulizer • Ionization/Atomization of sample • Flame – Burner – 1700 – 3500 C • Fuel + air (natural gas, Hydrogen, acetylene) • Flow rate is high – efficiency of atomization is not good • Higher T increases sensitivity • Graphite Furnace • Graphite tube with electric current • Thermal energy causes electronic transition • Light Absorption • Monochromator • Detector • Photometers • Spectrophotometer – UV/Vis monochromator

38. Atomic Absorption Spectroscopy - AA • Source of Error • Flame Emission • Modulation – Chopper/Reflected/Sample • Alternating signal from the lamp and continuous signal from the flame • Correction is made at the data analysis. • Background Correction • Hollow cathode lines are narrow so very little chance of interferences • Molecular species in the range of the wavelength absorbed can cause interference • AT = AA + AB • Continuous Source Background Correction • Part of the flame used for quantitative measurement • Primary combustion zone (very close to nozzle) - hottest • Interzonal region • Secondary combustion zone (outer ring) – oxidation may occur • Flame position must be adjusted since the response of each metal varies with it. • Beer’s Law • Internal Standards • Hg – Cold Vapor AAS • Detection limits • 0.2 – 30 ppb (mg/L) (for different elements)

39. Plasma For Atomization • Plasma tube • 3 concentric quartz tubes • Argon Gas – as ionizing gas and for cooling • RF Coils – 2KW energy at 27/40 MHz • Temperature – 6000 – 8000 K • Ionization efficiency is high • Tangential Gas flow allows for thermal separation of plasma zone from the region before it

40. ICP-AES

41. ICP-AES • Interference • Blank interference • MilliQ water – relative • Blank – MDL and IDL • Spectral interference • Analyte interference • Physical/chemical effects • Operating conditions • Detection limits

42. ICP-AES • Wavelength selection • Monochromator • Sequential analysis/scan of wavelengths and a PMT • Spectrograph • Large aperture for a band and detection using a CCD or array detector • Polychromator • Grating + Simultaneous detection using a CCD or array detector

43. ICP-MS icpms.ucdavis.edu/InstrumentationQuad.html

44. ICP-MS • Operates under Vacuum – 10-6 torr • After ionization sample is introduced to the analyzer zone through two orifices • Skimmer cone • Sampler cone • Ion Optics • Mass Analyzer • Selection on the basis of Isotope (or Mass/Charge ratio) • Capable of isolating “isotopes” • Very low detection limits.

45. ICP-MS – Mass Analyzer • Ion Optics • Focus the ions in a barrow beam towards the Mass Analyzer • Mass Analyzer • Quadrupole • Allows only a certain M/Z set to pass through to the detector • Detector • PMT/Dynode • Interferences • Ar+, ArO+ , ArH+, H2O+, Ar2+ or combination • Same isotope • Matrix effects • Other Types of analysers • Ion Trap • Time of Flight(TOF) MS

46. ICP-MS • During analysis, the use of blank is important since it accounts for contamination • in the instrument (previous sample memory effects) • the surroundings (cleanliness of the lab). • For very sensitive (or low concentration) work, a clean room (with very low particulate contamination) may be required. • Internal standard is recommended in all samples (same amount in all samples, standards and blanks) • The internal standard should be an element that is not present in the sample • Selection of internal standard depends on the nature of the analysis matrix