in the last lecture we left off talking about the qualitative analysis aspect of ir spectroscopy we can also use ir for um quantitative but let's look at a few more qualitative analysis aspects first um first is you must have a spectrum of a pure compound so you have to make sure that when you're doing it for qualitative purposes that you um have a pure compound so that you don't accidentally identify something else you're going to use the group frequencies as a guide um so understanding the different frequency shifts for the stretches and bends that we see for the different functional groups use a correlation chart for more compound specific functional group analysis we can use computerized spectral search engines and we can use ir assignments in conjunction with other information such as chemical physical or spectroscopic properties when you are using ir for qualitative analysis the match of two spectra in the fingerprint region so again your fingerprint region has 650 to 1200 wave numbers magic2 spectra in that region constitutes almost certain evidence the two compounds are identical so we can use this idea to help us identify an unknown if we have an idea of a general idea of what it is as i mentioned we can also use this for quantitative analysis this is limited use but with care you can get reasonable results you can get trace detection limits so parts per million detection limits with gases with pretty excellent selectivity the good side of this is that almost all compounds absorb in the ir region and solids liquids and gases are all accessible so you can test all of your samples the problem with this is that you have poor sensitivity and short cell path lengths mandate relatively high solution concentrations so you need one to two percent or more weight weight percent for your concentrations so you need relatively high concentrations but remembering high concentrations results in beer's law deviations so you kind of have that trade-off of what concentration do you need to see results without too much deviation from beer's law narrow absorption bands and complex spectra with overlapping bands can cause polychromatic errors so you have to be careful of the overlapping spectra causing errors with that so i want to show you briefly of how you would use an ir spectrum for qualitative analysis so let's say this is your spectra so on this side we have percent t on this side you have your wavelength or wave number whatever you're looking at and so let's say 50 is here and you get something that kind of looks like this whatever your spectra looks like so when we're measuring on here the absorption is going to be equal to the log of t naught over t s well on the spectra the distance from the bottom of the peak to the bottom right here that represents ts and then if you're looking at the peak you're going to the top of the peak here and that distance from there down to there that represents t nut and this all is equal to log of p naught over p so again t naught is going to be your full amount here versus the amount here so it's kind of like intensity and intensity what kind of idea