Chemistry for Liberal Studies - Forensic Academy / Dr. Stephanie R. Dillon

The Spectrometer & Spectroscopy

Let's start the discussion of spectroscopy by understanding the PHOTOELECTRIC EFFECT.

The Photoelectric Effect

The photo-electric effect explained
LangtonStarCentre (YouTube)

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

  1. The electrons were emitted immediately.
  2. Increasing the intensity of the light increased the number of photoelectrons, but not their maximum kinetic energy.
  3. When Na is the metal red light will not cause the ejection of electrons, no matter what the intensity.
  4. 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:

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?
BioNetwork (YouTube)

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
RSCteacherfellows (YouTube)

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 spectrometer works by sending source energy through an interferometer and through the sample. In every scan, all source radiation goes through the sample. The interferometer is a fundamentally different piece of equipment than the monochromator found in UV/Vis spectrometers. In the interferometer, the light passes through a beam splitter, which sends the light in two directions at right angles. One beam goes to a stationary mirror then back to the beam splitter. The other goes to a moving mirror. The motion of the mirror makes the total path length variable versus that taken by the stationary-mirror beam. When the two meet up again at the beam splitter, they recombine, but the difference in path lengths creates constructive and destructive interference producing a complete interferogram:

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
FranklyChemistry (YouTube)

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:

  1. A small sample (gas or liquid) must be collected and injected into the system
  2. The sample if liquid is vaporized
  3. The sample is then passed through an electron stream to produce positive ions out of its molecules
  4. An electrical field is used to push the ions towards the magnet at the end of the spectrometer
  5. Charged plates with narrow slits are placed in the path of the ion beam to focus the stream
  6. 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
  7. 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
  8. 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.