In order for IR absorbance to occur two conditions must be met: 1. What is the major requirement for IR absorption? IR spectroscopy is simply absorption measurement of different IR frequencies of a sample positioned in the path of an IR beam. Different functional groups absorb characteristic frequencies of IR radiation. IR spectroscopic analysis is used to determine the various chemical functional groups present in the sample. What does IR spectroscopy allow us to determine? More molecules means less light gets through the sample, thus the size of the dip in the IR light intensity at the frequencies of your molecule’s absorption feature is proportional to the concentration. How does concentration affect IR spectrum? 5.2 Resonance and Inductive Electronic Effect.Īnalyses of IR spectra showed that the apparent positional shifts of peak maxima in these systems are actually due to relative contribution changes of two overlapped bands, instead of the gradual frequency shift of a single band induced by the change in the strength of molecular interactions.Factors Influencing Vibrational Frequencies.What are the factors affecting IR spectroscopy? The impurities can melt at lower temperatures and alter the purity of the extracted caffeine.
Why is the melting point of your extracted caffeine lower or higher than the literature melting point?Īnswer and Explanation: The melting point of your isolated caffeine is likely lower than the actual literature value because it contains impurities. The protons in different fields are shown as follows: Thus, the caffeine showed four different signals in proton NMR. The IR radiation absorbed allows the bond to vibrate a bit more, that is, increase the amplitude of vibration, but the vibrational frequency will remain the same. If a molecule is exposed to IR radiation, it will absorb the radiation that matches the frequency of the vibration of one of its bonds. Therefore, we could use IR spectroscopy and Beer’s Law to find the concentration of substance or the components of mixture. The intensity also depends on the number of bonds responsible for the absorption, and an absorption band with more bonds involved has a higher intensity.Ĭan IR be used to determine concentration?įrom the Beer’s Law, we could figure out the relation between the absorbance and the concentration of the sample since the analytes have a particular molar absorptivity at a particular wavelength. The intensity of an absorption band depends on the polarity of the bond, and a bond with higher polarity will show a more intense absorption band.
What does IR spectroscopy allow us to determine?.How does concentration affect IR spectrum?.Why is the melting point of your extracted caffeine lower or higher than the literature melting point?.What peaks should be observed in the IR spectrum of caffeine?.(And so you don't have to go hunting, Caffeine structure: And Paracetamol Structure: ). Strip away the confusion and my question is essentially, why is Paracetamol more polar than caffeine? What's the obvious thing I'm missing here. Just looking at the amount of N's and O's in each of them would suggest that Paracetamol would be less polar, and therefore have the longer retention time in the column: but practically that's exactly what didn't happen. After drawing out the structures and trying to get my head around the polarity of both of them, I can only come to the conclusion that I think that's backwards. Under the conditions we used (Octadecylsilyl column, methanol/water mobile phase) Caffeine had the much larger retention time (almost double that of paracetamol). Experiment went well, Worked out the concentration and such: now I'm just working on the theory that goes with it, and my problem is this. In the lab recently, we ran a Reversed Phase HPLC to find the concentration of an unknown solution containing Caffeine and paracetamol. I have what I think should be one of those Simple questions that's making me tear my hair out because I'm not seeing something obvious.
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