Let's start the discussion of spectroscopy by understanding the PHOTOELECTRIC EFFECT.
The Photoelectric Effect
The photo-electric effect explained
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It's been determined experimentally that when light shines on a metal surface, the surface emits electrons. This is called the photoelectric effect.
Characteristics of Photoelectric effect
- The electrons were emitted immediately.
- Increasing the intensity of the light increased the number of photoelectrons, but not their maximum kinetic energy.
- When Na is the metal red light will not cause the ejection of electrons, no matter what the intensity.
- Weak violet light will eject only a few electrons, but their maximum kinetic energies are greater than those for intense light of longer wavelengths.
In order for the photoelectric effect to work, light must have particle like characteristics. Waves vs Particles - If light only has wave properties (i.e. wavelengths) then only the intensity of the light and not its frequency should create the photoelectric effect. So light must also have particle characteristics.
The term intensity has a particular meaning here: it is the number of waves or photons of light reaching your detector; a brighter object is more intense but not necessarily more energetic. Remember that a photon's energy depends on the wavelength (or frequency) only, not the intensity. The photons in a dim beam of X-ray light are much more energetic than the photons in an intense beam of infrared light because their frequency is much higher.
Based on Planck's work, Einstein proposed that light also delivers its energy in chunks; light would then consist of little particles, or quanta, called photons, each with an energy of Planck's constant times its frequency.
$$ E = hc / \lambda $$
Light falling on the metal, consisting of photons having the appropriate amount of energy, knocks the electrons out. The electrons of metal just absorb the photons and take all their energy. So when light intensity increases, the number of photons having proper energy increases too. They knock out a greater number of electrons giving each of them the same energy by the smaller intensity of light, but when light frequency increases, the energy of photons increases too.
So an increase in intensity just increases the number of photons created but an increase in frequency increases the energy of the photons created.
Using the Photoelectric Effect
As indicated previously, the wavelengths of light absorbed and emitted by each element is unique. This means that as elements are formed into compounds, the compound will have specific wavelengths at which it will best absorb the light. We can use this information to specifically set a wavelength of light that will be absorbed by a compound of interest in a sample. We can use the light that passes through the sample and is not absorbed to not only confirm the presence of our molecule in a sample but also quantify how much of the compound is present based on the response.
UV/Vis Spectroscopy
The Visible and Ultraviolet range of light is given in nanometers below:
- Visible - (380-780 nanometers)
- Ultraviolet (UV) - (10-380 nanometers)
UV/VIS spectrometers excite and measure response in samples in this range of the electromagnetic spectrum. UV/Vis spectra can be used to some extent for compound identification; however, many compounds have similar spectra. Solvents can also cause a shift in the absorbed wavelengths. Note, the same solvent must be used when comparing absorbance spectra for identification purposes. UV/VIS spectroscopy is therefore often used in combination with other spectroscopies to build a case but not necessarily be used as the solitary analysis technique.
How the Spectrometer Works
Inside the spectrometer, light is passed through the sample cell at the appropriate wavelength selected by a diffraction grating. The light is then focused through an aperture to pass through the sample cell. The light at that point can scatter, or be absorbed by the molecule of interest and re-emitted in any direction or pass clean through the sample cell without interacting with the sample at all. Thus the amount of light absorbed and emitted is concentration dependent as well as wavelength dependent. The amount of light that passes through the sample is quantified by the detector.
How does a spectrophotometer work?
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Spectrometers like the one above can be set to read in two different units, either absorbance (A) or percent transmission (%T). The Beer-Lambert Law expresses the correlation between the absorbance of a sample, its concentration, and its thickness. The law can be written as:
$$ A = \varepsilon bc $$
where $ \varepsilon $ is the molar absorptivity (this is a constant which depends on the nature of the absorbing system and the wavelength passing through), b is the path length (the width of the sample cell or cuvette, usually 1 cm), c is the concentration of the sample and A is the absorbance of the sample. If the molar absorptivity and path length are known then a measurement of the absorbance of a substance can yield its concentration. In forensic investigations, the most common use of UV/VIS spectroscopy is the determination of exact colors. For instance if you have a chip of paint from an accident scene that you need to match to a suspect's car, you can use UV/VIS spectroscopy to determine the wavelength of maximum absorbance and that will give you the specific color of the paint chip for comparison. UV/VIS spectroscopy is also used for determining the concentration of illegal substances in mixtures. For example, if you suspect an individual is hiding heroin in sugar, you can use UV/VIS spectroscopy to not only identify the heroin but also quantify it since the sugar does not absorb light at the same wavelength as the heroin. Other spectroscopic techniques, like Mass Spectrometry or IR spectroscopy, are used to confirm the identification of the illegal substance.
Infrared Spectroscopy
Infrared spectroscopy is similar to UV/VIS spectroscopy in that it uses light to determine the structural components in a substance, but unlike UV/VIS spectroscopy, the wavelength of Infrared light is not sufficiently energetic to cause an emission. Rather, IR wavelengths of light, ~10-5m, cause the bonds in molecules to vibrate and rotate which is what we detect and use to define the identity of the compound.
IR spectra is quite complicated. The bands that are shown in a spectrum are defined by their locations:
| wavenumber, cm-1 | bond | functional group |
| 3640-3610 (s, sh) | O-H stretch, free hydroxyl | alcohols, phenols |
| 3500-3200 (s,b) | O-H stretch, H-bonded | alcohols, phenols |
| 3400-3250 (m) | N-H stretch | primary, secondary amines, amides |
| 3300-2500 (m) | O-H stretch | carboxylic acids |
| 3330-3270 (n, s) | -C(triple bond)C-H: C-H stretch | alkynes (terminal) |
| 3100-3000 (s) | C-H stretch | aromatics |
| 3100-3000 (m) | =C-H stretch | alkenes |
| 3000-2850 (m) | C-H stretch | alkanes |
| 2830-2695 (m) | H-C=O: C-H stretch | aldehydes |
| 2260-2210 (v) | C(triple bond)N stretch | nitriles |
| 2260-2100 (w) | -C(triple bond)C- stretch | alkynes |
| 1760-1665 (s) | C=O stretch | carbonyls (general) |
| 1760-1690 (s) | C=O stretch | carboxylic acids |
| 1750-1735 (s) | C=O stretch | esters, saturated aliphatic |
| 1740-1720 (s) | C=O stretch | aldehydes, saturated aliphatic |
| 1730-1715 (s) | C=O stretch | alpha,beta-unsaturated esters |
| 1715 (s) | C=O stretch | ketones, saturated aliphatic |
| 1710-1665 (s) | C=O stretch | alpha,beta-unsaturated aldehydes, ketones |
| 1680-1640 (m) | -C=C- stretch | alkenes |
| 1650-1580 (m) | N-H bend | primary amines |
| 1600-1585 (m) | C-C stretch (in-ring) | aromatics |
| 1550-1475 (s) | N-O asymmetric stretch | nitro compounds |
| 1500-1400 (m) | C-C stretch (in-ring) | aromatics |
| 1470-1450 (m) | C-H bend | alkanes |
| 1370-1350 (m) | C-H rock | alkanes |
| 1360-1290 (m) | N-O symmetric stretch | nitro compounds |
| 1335-1250 (s) | C-N stretch | aromatic amines |
| 1320-1000 (s) | C-O stretch | alcohols, carboxylic acids, esters, ethers |
| 1300-1150 (m) | C-H wag (-CH2X) | alkyl halides |
| 1300-1150 (m) | C-H wag (-CH2X) | alkyl halides |
| 1250-1020 (m) | C-N stretch | aliphatic amines |
| 1000-650 (s) | =C-H bend | alkenes |
| 950-910 (m) | O-H bend | carboxylic acids |
| 910-665 (s, b) | N-H wag | primary, secondary amines |
| 900-675 (s) | C-H "oop" | aromatics |
| 850-550 (m) | C-Cl stretch | alkyl halides |
| 725-720 (m) | C-H rock | alkanes |
| 700-610 (b, s) | -C(triple bond)C–H: C-H bend | alkynes |
| 690-515 (m) | C-Br stretch | alkyl halides |
| m=medium, w=weak, s=strong, n=narrow, b=broad, sh=sharp | ||
The bands once they are identified as above can then be used in combination to identify the molecule(s) that are present in a sample.
Characteristic spectra are catalogued and placed into databases for comparison. This means that when a forensic analyst generates an IR spectrum, he or she can have a computer run a comparison check to see if the spectrum matches a known chemical. If you were to take, organic chemistry or analytical chemistry, learning to interpret these spectrums would be part of your assignment. But since this just an introductory course, we will assume that any spectra we produce can be run through a database and identified that way.
Running an Infrared Spectrum
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To run an IR spectrum you must prepare your sample. Samples can be pastes or liquids. Pastes are loaded into the IR spectrometer using KBr disks and a disk holder. Solutions or liquids are added to a special u-shaped holder and then placed into the spectrometer.
The FTIR or Fourier Transform Infrared Spectrometer
The recombined beam passes through the sample. The sample absorbs all the different wavelengths characteristic of its spectrum, and this subtracts specific wavelengths from the interferogram above. The detector now reports variation in energy versus time for all wavelengths simultaneously. Fourier Transform is then used to convert the Intensity versus time pattern to one that shows frequency versus time (remember that frequency is a per time event so we can use that relationship to make the conversion). The result is an FTIR spectrum that can then be analyzed.
Mass Spectrometry
Simple explanation of the Mass Spectrometer
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The mass spectrometer uses a different method of detection and excitement than UV/VIS or Infrared spectroscopy. Mass spectrometers use a stream of electrons to convert molecules or atoms into positive ions so that they can be attracted using a magnet.
The steps of Mass Spectrometry:
- A small sample (gas or liquid) must be collected and injected into the system
- The sample if liquid is vaporized
- The sample is then passed through an electron stream to produce positive ions out of its molecules
- An electrical field is used to push the ions towards the magnet at the end of the spectrometer
- Charged plates with narrow slits are placed in the path of the ion beam to focus the stream
- Upon entering the chamber with the magnet, the ions with the largest charge to mass ratio are the most affected and those with the lowest are the least affected. This means the largest affected molecules will curve into the wall away from the magnet as the tube bends; those with the lowest m/z ratio will continue straight running into the wall at the other side of the curve. Those with m/z ratios in between the two extremes will pass through the curve at different points and run into the detector
- The more ions of the same mass that strike the detector, the higher the peak that is shown in the mass spectrogram and this means the molecule is of higher concentration
Mass spectrometry, often in conjunction with separation techniques like gas chromatography, is used as a confirmation test for the presence of drugs or flammable liquids in forensic analysis. Because the mass of a substance can be determined down to the isotopic level (differing in mass only by the mass of one neutron in some cases) and can also be used with very small sample amounts (picogram or 10-12 g amounts), it is a perfect technique for working with dirty mixtures of samples which are often the only kind available at a crime scene.