Infrared (IR) Spectroscopy Flashcards
What does IR spec trigger?
Molecular vibrations through irradiating the sample with infrered light
What else happens to the atoms/ molecules during IR spec?
- Specific functional groups will stretch and bend depending on the bonds and atoms in the specific functional group
- Observed IR peaks (stretches) are characteristic to the corresponding functional group, and as a result are diagnostic and can be exploited to determine what functional groups are present in a sample, as well as spectroscopically determine which functional groups are absent from the molecule to determine its’ identity
o Useful tool in chemist’s toolbox to quantify functional groups present
- **Different functional groups stretch and bend differently under applied infrared energy, and those differences can be quantified for analysis of an unknown compound
How can IR spec be used in forensics?
- Drugs
- toxicology
- Fingerprints
- Paint
- Documents
- Dyes
- Polymers
- Explosives
What does IR spec observe?
- IR spectroscopy looks at the interaction between a molecule and applied infrared radiation
What can the applied radiation in IR spec cause? Why is this important?
- This applied radiation causes the excitation of the vibrations of covalent bonds within that molecule that is being analysed
- How the bonds within the molecule **bend and stretch under applied radiation is dependent on the functional groups **present (different atoms and bonds will behave differently under the applied radiation)
o Of utmost importance to not only readily identify functional groups, but also understand and readily recognize what “diagnostic” peaks you’re looking for or when seeing a peak, what that could be telling you about the molecule
https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/spectrpy/infrared/infrared.htm
Use this link when looking up and assigning IR stretches
How is the IR specta often presented?
- The IR spectrum is split into two approximate regions:4000-1000 Cm-1 known as the functional group region and <1000cm-1 known as the fingerprint region
What are some important things to consider when looking at analysing with the IR spectrum?
The types of bonds that are “IR active”
What bonds are “IR active”?
Only polar bonds i.e., those that have a dipole moment
Ketone
Alcohol
Cocaine
LSD
Example) Police have seized an unknown white powder, which through analytical tests you have narrowed down the identity of the molecule to one of the following structures. Of the following molecules, identify the functional groups present in each structure and determine which structure matches the corresponding IR spectrum.
- represents the CA due to broad stretch, peak at roughly 3000 and 1700
Forensic Chemistry Applied: The Discrimination Potential of FT-IR (and other techniques) for the Forensic Analysis of Black and Blue Ballpoint Inks
- Ink analysis crucial for cases involving questioned documents. Recall that inks are made up of a variety of components, are often complex mixtures, and as such it is imperative an accurate protocol is in place for the identification and discrimination of ink samples in a forensic laboratory
- Recall, many different types of inks (organic and inorganic pigments), as well as those found in gel pens. Inks are sought after in writing utensils for their ability to not dry out, readily affix to the substrate and dry quickly once on the substrate, must be fluid, not too thin, etc. As a result, there are many components and variables to quantify when undergoing the analysis of questioned documents
- In this experiment, 33 ballpoint pens (21 black and 12 blue) from different manufacturers and different models were chosen randomly from several stationary shops in the Italian city of Messina to undergo this analysis, testing the accuracy of FT-IR, as well as UV-VIS, and TLC in the correct identification of the 33 pen sources.
- Fourier Transform Infrared Spectroscopy, better known as FT-IR, is a technique of interest for this study, not only for its ability to confirm what functional groups are present in each ink, but also was employed to analyse the non-colourant components found in each of the writing inks. Through UV-VIS spectroscopy it was confirmed the typical absorption bands expected in each sample, so from there any inks that had deviations from the typical expected analysis were tested further using FT-IR.
- Diagnostic bands are indicated on the above spectra in Figure 5, where a = acid or ester, d = dye, e = epoxy resin, g = ethylene glycol, s = solvent (styrene and benzyl alcohol). A gparticularly diagnostic peak is denoted by ‘a,’ representative of the presence of an acid or an ester, appearing at ~1700 cm-1. Doublet peaks appearing around 700 and 1475 cm-1 were indicative of a solvent present in the ink, such as styrene and benzyl alcohol.
- Since some of the ink samples would produce very complex spectra, individual peak assignment wasn’t always possible, so having this baseline available in terms of ‘diagnostic’ bands to look out for was crucial in this (and all) analysis.
- It is well known in the literature that writing inks undergo an aging process over time once they are deposited on a substrate, which primarily consists of the evaporation of volatile organic components, some oxidation of other components, and general drying of resins. This gradual change in the ink over time (as it dries) can be quantified through FT-IR
- It was discovered that the aging process affected the intensity of the OH band at 3300 cm-1, of the CH band at 2950 cm-1, and of the C=O band at 1700 cm-1. Across a variety of studies, a general decrease in the intensities of all bands was observed over time, with one band at 1245 cm-1 disappearing almost completely over time.
- Differences between samples 14 (a) and 18 (b) were indistinguishable with UV-VIS, but here with FT-IR can readily quantify the change occurring over time as the ink ages.
Position of IR on the EMS
780 nm - 1mm
