Week #5 Progress

This week I used another method to acquire more chemical information of our samples. The method is called Infrared Spectroscopy (IR). It records the vibration modes of chemical bonds in the examined chemicals. By interpreting the IR spectrum for the sample, we are able to determine the kinds of functional groups exist in our sample. Moreover, by tracking the changes in IR spectra of one sample over a certain timescale, chemists are able to monitor the changes in chemical composition of the sample and speculate about the probable chemical reactions happened during the span of time.


The following picture serves as an example. It is the IR spectra taken for a new biomass burning aerosol filter sample. We are trying to understand to chemical effect of photolysis (irradiation of sunlight) on the sample by cutting the filter into 6 pieces, leaving them under simulated sunlight for different numbers of days, and taking their respective IR spectra. The numbers of days irradiated are showed along with the legend. The x-axis of the spectra represents the frequency of the vibration signals (in cm-1), and y-axis indicates the intensity of the signal.

IR example

In an IR spectrum, each peak represents the vibration mode of a certain chemical bond, and therefore, may indicate the existence of a certain functional groups. For example, the broad peak around 3200 – 3500 cm-1 are generally assigned as the vibrational mode of O-H bond, which suggests the present of a -OH, or alcohol group. However, one thing to keep in mind while interpreting an IR spectrum is: sometimes different chemical bonds may have a similar vibrational frequency and can contribute to the same peak in the spectrum. Notice the small spike around 1000 cm-1. A peak in this frequency region may indicate the existence of some polysaccharide-like substances, but can also be the vibrational modes of aromatic C-H bonds. The interpretation of an IR spectrum is complicated in such cases, especially for our sample being a complex mixture of chemicals.