Raman Spectroscopy A practical approach JASCO NRS3100 Raman Spectroscopy
A Historical Perspective What is Raman Spectroscopy? Theoretical Principles Analytical Applications Advantages and Disadvantages Why use Raman spectroscopy? Raman Spectroscopy
Sir Chandrasekhara Venkata Raman 1930 Nobel Physics Laureate
Investigations of the Raman Effect began in many countries in 1930’s
In the first twelve years after its discovery, about 1,800 research papers were published and about 2,500 chemical compounds were studied History of Raman Spectroscopy 1928 - First experiments with light scattering by C. V. Raman using sunlight as source 1930's - mercury arc lamps and photographic film used to capture spectra. Raman is the spectroscopic technique of choice 1950’s - Cheaper IR technique overtakes Raman 1960's - gas lasers and photo-multiplier (PMT) detectors available for collection of Raman spectra 1980's - diode array detectors are used 1990's - FT-Raman instruments commercially available Present day: solid state lasers, Peltier cooled CCD detectors…. Vibrational Spectroscopy
Absorption
Scattering (“emission”)
A molecule can be characterised (or identified) based on the wavelength and intensity of the spectral peaks. Simple Raman Spectrograph
Sample Spectrograph Laser
Detector
Laser / Rayleigh Rejection notch filter (10-4 ~ 10-6) The Raman Spectrum
Intensity
Excitation n0
Stokes Anti-Stokes (n0 -n1) (n0 +n1)
0 Raman shift in cm-1 (“Kaisers”) The Raman Spectrum
CCl4 Stokes anti-Stokes
Decreasing absolute wavenumber
Red Blue
The 'complete' Raman spectrum contains both the red-shifted Stokes and blue-shifted anti-Stokes scattering. Normally we only collect the Stokes peaks as they are more intense. Note the strong attenuation of the laser / Rayleigh scattering. Comparison of Raman and Infrared Infrared is more sensitive to functional groups with very oriented dipole moments. Raman is more sensitive to bonds that are equally shared. Generally, what is strong in the Raman is weak in the infrared and vice versa.
In simple, highly symmetric molecules such as benzene and CCl4, the Raman and infrared spectra can be mutually exclusive; some bands only present in the Raman and some only in the infrared. Usually, however, either spectrum fully describes the molecular vibrational activity. Inorganics, with polar bonds, have few Raman active vibrations. In these materials, phonons (lattice vibrations) can dominate the Raman spectrum. Think how the electrons are distributed in the
bonds in C6H6 benzene and NaCl, and how strongly the electrons are localised in the latter. We can study the solid-state structure with Raman spectroscopy. Some materials (silicon, diamond) characterised by a single sharp phonon. Comparison of Raman and Infrared Sample preparation for infrared spectroscopy is critical and often tedious. There is generally no sample preparation for micro-Raman samples. Infrared samples require fragile, water-sensitive sample cells or complex, expensive sampling accessories with limited spectral ranges. Raman samples are placed in the path of the laser. Water is a nuisance for most infrared analyses. Water is an ideal solvent for Raman sampling, with a very weak interference. Infrared quantitation can approach ppm levels for some samples and even ppb levels for gases, but most samples are limited to detection levels in the 0.01 to 0.001% range. Raman spectra can be used for quantitation with a detection limit approaching 0.01% in some cases. But only TINY amounts of sample needed! Raman Sampling Advantages
No sample preparation Depth profiling and microscopic mapping of samples with spatial resolution approaching 1 micron Quartz optical fibres used for in-situ analysis, remote probes No combinations or overtones, less 'clutter' in Raman spectra No water sensitive or range limiting optics, cells or accessories A complete spectrum is collected, from >4,000 to ~50 cm-1 or less Opaque or cloudy samples OK Sample containers can be glass or plastic and sample in-situ Sampling Configurations
360 degree transmission Macro Micro Macro
45 degree pseudo-backscattering
Macro
Micro
90 degree backscattering 180 degree backscattering Raman - the Old Way
Gas lasers used for Raman spectroscopy were large, expensive and difficult to maintain. Raman instruments required pre-monochromator systems to eliminate plasma lines near the laser wavelengths. To reject scattered light from the laser and provide resolution of the Raman bands, Raman instruments used (expensive) double and triple monochromators - identical monochromators in sequence. The photo-multiplier (PMT) detectors used could only "read" one wavelength at a time. As a result, spectral collection took a long time, especially high resolution scans of weakly scattering molecules. Raman sampling limited to samples that were contained within a quartz cuvette or otherwise held in the path of the laser beam. Choices of excitation wavelength and compatible PMT detectors were limited. Old vs New: Spectrographs
PMT slit slit scanned grating
scanned grating (1 or 2 required) slit
The old way: 2 or 3 gratings and PMT detector Old vs New: Spectrographs
slit slit CCD notch filter Grating is set to 1 or more positions
Each grating position generates a broad spectrum
The new way: notch filter, 1 grating and CCD detector Raman - the 21st Century
Compact, solid-state, diode lasers with powers of 10mW ~ 100mW provide stable, easy to maintain laser platforms while costing less than $10k. Notch filters eliminate the requirement for additional laser optics and reduce noise due to Rayleigh scattering. Sensitive CCD detectors with extended wavelength ranges and excellent spectral response allow simultaneous collection of multiple wavelengths - often, the complete spectrum can be collected at moderate resolution, ~6-8 cm-1 with a single 'exposure' of the CCD. A single monochromator can be used to disperse Raman scattering onto the CCD detector element, interchangeable gratings can be used to change resolution and/or spectral range. Sampling options now include micro-, macro-, mapping and remote sampling of liquids, solids or any sample in between. Why Collect Raman Spectra? Chemical and Structural Information Qualitative and Quantitative Rapid and Sensitive Microgram or Bulk Samples Non-contact and Non-destructive QC or R & D Simple Sampling Conditions Complementary to FT-IR Data Often succeeds where IR cannot Raman and photoluminescence data collected simultaneously Single or dual grating?
With one laser (532nm) and one grating (1,800gr/mm), 90% of samples can be analysed at good resolution
Different lasers will probably require different gratings to get the same resolution (and so, coverage of the CCD)
So 2 lasers really calls for 2 gratings, which then act as high and low resolution
Having 2 gratings also allows for wider range of working styles and future system changes Dual Grating Benefits
(A) 1800 gr / mm Changing the grating allows increased resolution A.U.
18001500 1000 500 250
(B) 2400 gr / mm To fully resolve the A.U. shoulder peak
1000800 600 400 Raman Shift (cm-1) Laser selection
90% of samples run well using 30-60mW 532nm standard laser Sometimes, 785nm offers lower fluorescence level but should not be considered as “standard” laser due to inefficiency (0.2x standard laser!) and lower beam quality (=larger focussed spot) inherent in diode lasers Generally, gas lasers (Ar, Kr) can be used without problems; will usually be external and require fore-monochromator option if >100mW Gas lasers offer multiple lines, excellent beam and plenty of power but $$$! Only very few applications require HeNe 633nm; mostly historical. HeNe 633nm efficiency 0.5 of 532nm! And fluorescence is still strong. UV lasers require different and specialised optics, will not work in a standard Raman system External fiber optics
JASCO RMP-100 microscope type probe with CCD-TV camera built in
Immersion type (Inphotonics) macro probe for various environments Block diagram of the laser Raman spectrophotometer system
External laser (three types available) Externalaser Laser (accepts(1-4) 3 tyles)s Internal laser (two types available) UV Laser UV Laser InternalInternal Laser laser (2(up typed) to 2) Laser Raman Spectrophotometer
Imaging NRS-3200(NRS-3200) Auto alignment
Main CCD Power monitor monochromatorPolychromator detector
Objective lens PC Sample Laser Raman spectrometer system (front view)
Observing analyzer mounting section Observing1/2 wavelength analyzer plate holder holder Trinocular mounting section Press the lid to open and install the When the analyzer is not used, (factory option) analyzer on the holder inside. a perforated plate is installed.
Micrometer for focusing laser beam(normally, no adjustment required) Interlock warning lamp Laser lamp WARNING LASER Measurement lamp MEASURE (Lit during measurement and blinks DOOR when the instrument is running) Sample compartment door
POWER Power switch
Sample compartment Laser Raman spectrometer system (right side view)
Optical fiber mounting position (inlet)
Optical fiber mounting position (outlet) Key Hardware When selecting a Raman spectrometer, there are many different combinations of hardware which require consideration. The four main criteria are: 1. Excitation laser: wavelength, power, cost, maintenance 2. Spectrograph: resolution, wavelength range 3. CCD detector: range, spectral response 4. Sampling hardware: micro, macro, mapping, remote
(If the application calls for FT-Raman, usually only a NIR laser of 1064 nm is used and the spectrograph is based on an FT-IR system) Block diagram of the laser beam input
Auto alignment mechanism
1/2 wavelength plate Attenuator (option) Power monitor Inter Interlocklock Inter lockInterlock Lens Laser Beam splitter
Objective lens
Sample Simple Raman Spectrograph
Sample Spectrograph Laser
Detector
Laser / Rayleigh Rejection notch filter (10-4 ~ 10-6) Laser Sources NRS-3100 Standard model Visible - NIR NRS-3200 Multi-configuration model UV - NIR NRS-3300 Advanced applications model UV - NIR
Applicable Laser Sources UV NIR
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244 nm 488 nm 514.5 nm 785 nm 257 nm Blue Ar Deep Red Ar Ar Laser 632.8 nm Diode Laser Laser Laser Red 532 nm He-Ne 325 nm Green Laser He-Cd Diode Laser Laser Why use different lasers?
Raman intensity depends on excitation wavelength but Raman shift stays the same
Intensity
Excitation n0
Red changed to green laser: Scattering intensity increased but position unchanged
0 Raman shift in cm-1 (“Kaisers”) Multi Laser Benefits
532 nm excitation
785 nm excitation
18001500 1000 500 300 Raman shift cm-1
532 nm laser excitation generates a high fluorescence background for the sample of white wine which is greatly reduced by switching to 785 nm excitation Sample compartment
Lens holder SetSets the objective lens in thelens opticalby turning path the byholder. A turningmaximum the of 6holder. lenses can Maximum be mounted. six lenses can be mpunted. Lens are identified by No.1 numbers 1 through 6. No.3 X5 No.2 Lens X20 X100 Sample clip Stage SampleSample holding plate plate
XY adjustment knob Back and front movement knob Moves the stage to back and front (moves the stage back and forth) Side-to-side movement knob (movesMoves the the stage sample from sideholding to side plate side to side) Z axis control knob (moves the stage up and down) Fine control knob Coarse control knob Block diagram of the microscope and spectrograph
CCD detector
Mirror Optical path Neon lamp change mirror (for wavelength correction) Ne MonochromatorPolychromator
Optical path change mirror Grating Notch filter Entrance slit Depolarization plate(Option) Polarizer(option)
Beam splitter CCD camera (for observation) Optical path change mirror Laser beam Beam splitter
LED Objective lens (for reflection illumination)
Sample
W Stage Tungsten lamp (forSample tramsmission Illumination illumination) Filtri notch Reticoli di Diffrazione
Reticoli di diffrazione
• Legge di Bragg • 2dsenθ=nλ D rappresenta la proprietà del reticolo e λ la componente della radiazione policromatica da disperdere • Risoluzione cresce con 1/d, quantificato dal numero di linee per mm (grooves/mm) è direttamente proporzionale alla risoluzione (1/d) Multi Grating Benefits
(A) 1800 gr / mm Changing the grating allows increased A.U. resolution
18001500 1000 500 250
(B) 2400 gr / mm To fully resolve the A.U. shoulder peak
1000800 600 400 Raman Shift (cm-1) CCD detectors
NSR-3000 uses a TE cooled charge-coupled device (CCD) detector or “Camera” that allows simultaneous collection of a wide spectral wavelength range.
A water cooling option allows -90º C operation.
The ANDOR thermoelectrically (TE) cooled CCD. CCD Detector Options
Different types of CCD elements are available from ANDOR to fine-tune the spectral range, e.g. UV or NIR excitation. The plot compares the various CCD detectors (FI is the standard CCD for NRS-3000). Why Micro-Raman?
• Raman is inherently a micro-technique; wherever the laser is focused, sample scattering will occur • Raman microscopes are optically simple, microscope optics define the sampling area and spatial resolution • Microscope optics are readily available, reducing manufacturing costs • Instrument can use smaller optics, thus a more compact design • Laser micro-beam = lower power and smaller lasers • Requires less sample and generates much less fluorescence • A confocal microscope can provide depth profiling capability Confocal Microscopy
Sample lens aperture
Sample surface Confocal pinhole aperture allows z (depth) resolution because only the rays from the in-focus (green) region enter the spectrometer. To change the sampling depth, the focus is adjusted slightly.