Infrared Spectroscopy Dispersive IR Fourier-Transform IR (FTIR) PDF

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Summary

Why is IR used in Forensics?
Things you could look from a Crime scene and analyse using FTIR.
Detection of Explosive Particles in Latent Fingerprints.
document examination, distinguishing between different fibres, detecting impurities, structural elucidation.
Electromagnet...

Description

Infrared Spectroscopy Dispersive IR Fourier-Transform IR (FTIR)

Universal technique able to identify compounds present within many different sample types. It is using the infrared region of the electromagnetic spectrum. Can give an indication of what compounds are present. From spectroscopy, it is the study of how materials respond to electromagnetic radiation. Why is IR used in Forensics? ● Fast - Sample preparation - Analysis time ● Simple sample preparation ● No solvents required - Compare methods to each other ● Non-destructive - When you put your sample into the FTIR, you can still use it for other experiments. It may become a little squashed but still usable. ● Wide range of applications ● Qualitative (and quantitative) - Compounds with higher/hower m/z ratios - Non-soluble compounds - Qualitative: is something present or absent - Quantitative: quantity of sample present ● Absolute identification of compounds - Because it's looking at the bonds between diff atoms, you can work out the structure of your compound which then tells you more definitely what compound is present. However, not fallible ● Not as sensitive as chromatographic and mass spectrometric techniques - when you do Chromatographic techniques you can suppress matrix effects, however when using FTIR, you analyse everything so can get different peaks which then takes a trained eye to pull out the relevant ones from the background ● Adversely impacted by water- containing samples - You need to ensure samples are dry otherwise when you look at your spectrum at the end you get a tongue shape that covers ⅓ of your spectrum which will hide any interesting peaks. ● Complex mixtures can be difficult to interpret ● Have to ensure sample is homogeneous Things you could look from a Crime scene and analyse using FTIR: ● Paints - can compare paint from a hit and run scene to paint found on a car ● Polymers ● Drugs ● Inks ● Fuels ● Biological samples ● Rubbers ● Plasticisers ● Air pollutants

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Explosive residues Food additives

Detection of Explosive Particles in Latent Fingerprints - Ammonium nitrate, a) fingerprint; b) library Ammonium nitrate- common fertilizer and when you combine it with the hydrocarbons e.g. diesel, it can create an explosive mixture. Document examination - Differentiating inks - Sequence of events - important in cases of altered wills or contracts. Can use it for different pens - anything with ink! Distinguishing between different fibres - Manufacturers put on different materials onto fibres e.g. waterproofing materials which you can use to compare. Detecting impurities E.g. an unknown white powder found at a crime scene, FTIR will tell you if a drug is present, and ratios of any contaminants or cutting agents present. Can be used to then link back to a dealer. Structural Elucidation - Diastereoisomers - Positional isomer of aromatic substances - Other stereoisomers - Isomeric functional groups

How does FTIR work? Using a portion of.. The Electromagnetic Spectrum ● Made up of Electromagnetic radiation. Form of energy that is all around us and consists of an: - Electric field - Magnetic field - They vibrate together in space and time and synchronous. When one of them is affected then you will have an equal effect in the other field. They travel through vacuums and space and travel in a wave formation. Output from the FTIR: spectrum X axis: wavelength or wave number Most common to appear on spectrums.. ● Wavelength (Units: nm, cm, m) - Distance between two identical points on two adjacent waves e.g. Measuring crest to crest or trough to trough. ● Wave number (nm-1/ cm-1/ m-1) - 1 divided by the wavelength. Both tell you the amount of energy that radiation has.

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Frequency ()

- Number of wavelengths that pass a point in one second. Electromagnetic Spectrum Different types of electromagnetic radiation. Majority we are exposed to everyday. We can differentiate these by their wavelengths and frequencies. The diagram shows different radiations: the wavelengths, wavenumber and frequency. Radio waves: long wavelengths, and low frequency meaning low energy. Gamma rays: short wavelengths, high frequency meaning high energy so able to break things up. If gamma rays are exposed to an uncontrolled environment, they can be dangerous as they have the capacity to knock our atoms apart and break them. We used Gamma Rays medicinally e.g. to kill cancer cells. Infrared region - low energy than Gamma Rays so not able to break our drugs into little pieces but will cause slight vibration. FTIR is measuring the little vibrations that happen within our drugs of interest. Infrared - Can be further split into 5 zones with different wavenumbers. - It doesn't have the capacity to split atoms apart, just cause vibrations

Different type of vibrations: Molecular Vibrations Bending: Diagram: red is carbon and the blue are two hydrogens with bonds in between them. Scissoring: hydrogens are coming together and apart (like scissors) Rocking: hydrogens are getting any closer together, they are moving side to side. Wagging: coming towards you Twisting: the hydrogen atoms are twisting around the carbon. Bending requires a small amount of energy. Stretching:

To make your atoms stretch away from each other, you are looking at the higher end of the infrared spectrum. Two different types: Symmetric: They pull apart equally at the same time Asymmetric: one goes in and one goes out These require a lot more energy than bending.

How does this happen? IR Source- releases infrared spectrum and that energy is directed towards your molecules of interest. These bonds are resonating ever so slightly already because of the frequency (always slightly resonate). As energy is being emitted it causes the exciting radiation because it has the chance to excite this and cause a stretch. Only certain wavelengths are going to cause that stretch to happen. Can have your Y-axis as absorbance, which would show how much of the wave number has been absorbed by the sample. (Opposite of Y-axis transmission) Transmission as Y-axis- how much reaches the detector. All your different wavelengths will be reaching your detector.. If your wavelength reaches the detector that means your molecule didn't absorb anything. As you go through different wavenumbers, and some of a particular wavenumber did not reach the detector it shows the sample absorbed the wavenumber. Peaks on a graph relate to which wavelength was absorbed which is attributable to a type of bond which is vibrating in a particular manner.

Beer-Lambert Law A= a x b x c A = absorbance a = absorptivity b = path length c = concentration Obtain a signal when: applied infrared frequency = Natural frequency of vibration. Dipole occurs during vibration Degrees of Freedom - Degrees of freedom = 3n - 6 n = number of atoms (Number of atoms x3, -6) Molecule: we can make predictions on how its spectrum would look because we can work out the degrees of freedom which will tell us the number of ways it will respond and anticipate how many peaks and troughs we are going to obtain. Example: Formaldehyde: give us 6 different peaks. Different peaks and troughs are associated with a particular bond occurring between atoms and vibrating in some way or another.

Types of Infrared Spectroscopy Classic - Dispersive Infrared -

Modern - Fourier Transform Infrared Dispersive Infrared IR source gets separated into two beams and the beam from the Reference Cell and beam from the Sample Cell will then alternate and you can get a comparison.

Inside of a dispersive IR

Monochromators Filtration of desired frequency of radiation Different ways of splitting light: ● Prismatic monochromator

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(prism) - Glass/quartz coated with alkyl halide (NaCl) Grating monochromator ○ Grooves made up of aluminium (Al) ○ Change the at which your beam of light is hitting and then separates into different wavelengths.

Dispersive IR-Spectrometer Negatives: ● Sensitivity - Not all of the light reaches the sample, some is lost in the narrowness of the focussing slits. Hitting so many mirrors and gets deflected - few % reaches the detector ● Speed - It takes minutes to record the spectrum, individually measures each infrared frequency ● Wavelength accuracy - Scan over the wavelength but the mechanical movements of the gratings and the rotation of the mirror cause inaccuracies FTIR: -

(compared to Infrared Spectroscopy) Less things that can go wrong.

Michelson interferometer ● Made up of a beam splitter, a movable mirror and a fixed mirror.

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● You have your light source which hits the toroidal mirror which focuses the light which then hits the beam splitter and half goes to the movable mirror and half goes to the fixed mirror and they both recombine to reach the detector simultaneously. Moveable mirror moves at a very small amount

Your beams are recombined but as they have travelled different lengths (moving mirror means some of the beam will take longer to reach the detector) the signal can either be enhanced or reduced. So when both mirrors are an equal distance from the beam splitter this is called Constructive interference. Whereas when some move at an unequal distance so when your mirror is further away it means that beam light is going to take longer to travel. When they recombine, it can cause Destructive interference and cause a flat line.

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Helium and Neon used as your calibrater. Known wavelength of 633, so to make sure everything is working properly, using the calibrator gas to work out whether it's all operating properly.

When the Constructive or Destructive hits the detector it introduces the interferogram. This is spectral coding - a unique type of signal that has all the infrared frequencies encoded within it. It then takes a mathematical formula called Fourier Transformation to then convert that ferofram into your spectrum.

Fourier Transform Infrared Ways of looking at your sample: ● Transmission - direct your IR source through the sample. Looking straight through it will give you an idea of the entirety (all layers) of the sample ● Reflectance - only looking at the outer coating e.g. what's present in a capsule, or first layer of paint

Sample Preparation for Dispersive IR- Liquids Get your liquid onto a pipette and drop it onto salt plates. There are a variety of different salt plates and they are looking at different parts of the spectrum e.g. lower end of the infrared and the higher end. Salt plates (compressed salts) ● Sodium Chloride (NaCl) ● Potassium bromide (KBr) ● Calcium Fluoride (CaF2 ● Barium Fluoride (BAF2) ● Zinc Selenide (ZnSe) Sample Preparation for Dispersive IR- Solids (as Nujol mulls) 1) Ground into a fine powder with a pestle and mortar 2) Mixed with liquid paraffin (Nujol) to form a paste/liquid 3) Spread onto a salt plate 4) Press plates together Sample Preparation for Dispersive IR- Solids (in solution) 1) Dissolve in a suitable solvent - NOT water e.g. dichloromethane 1) Directly onto salt plates 2) In a test-tube

2) Press plates together Sample Preparation for Dispersive IR - Solids (KBr pellet/disk) 1) Make a 1-2% mixture of sample and potassium bromide (KBr) 2) Grind into a fine powder 3) Press into a disk 1) Hydraulic press 2) Bolt press -

Have to work quickly as potassium bromide absorbs humidity Ensure the risk is not cloudy

Sample Preparation - Gases - Need sufficient sample to flush out any ambient air

FTIR: put into machine, close lid - much simpler!!

FTIR: Attenuated Total Reflectance (ATR) ● Using the reflective approach to analyse samples. This can be done via: - Single sample reflection - Multi sample reflection Single Reflection - Once your sample is on top of diamond/Ge/ZnSe/Si (depends on the wavenumber you're interested in), you would hit it with the IR source and any absorbance would happen. You need to make sure your sample is homogeneous because you won't get true reflection of what you are looking for. Mixed everything thoroughly to get an idea of what's present! Multi Reflection - Can change the instance of your peak to make it hit your sample at separate times. Depending on the angle, you can then change how many parts you want to analyze. If you are worried that your sample is not completely homogeneous then you're going to get a better idea. - If you think you have a low concentration of drug within your sample/chemical of interest you can take repeated exposures which will give you greater peaks being present. - Get increased peak or trough sizes. - Trough is related to transmission! - Peak is related to absorptions!

Specular Reflection and Diffuse Reflection (DRIFTs)

Specular Reflection- good for measuring coatings or thin layers

Spectra Interpretation - Properties of Peaks With FTIR, you produce a spectrum and you can talk about the peaks/troughs in terms of intensity. Peaks in terms of absorption and troughs in terms of transmission ● Intensity - Weak - Medium - Strong ● Shape - Broad - Sharp ● Position in the spectrum First Trough is strong and broad, as it's gone down to 0% and not particularly narrow so I know it is an O-H or N-H or C-H. Second Peak is a strong one, sharp peak. Region where you expect to see triple bonded carbons to hydrogens or triple bonded carbons to nitrogens....