Infrared spectrophotometer

[carbocation]

Is it possible to use an infrared spectrophotometer to determine the composition of a sample or the structure of a substance. Can you also use it to detect seperate substances remaining in the final product from previous processes. I have seen some models (mainly unicam) on ebay for between £30 and £200. I have used the FSE but i cant find exactly what Im looking for, i've also used google fruitlessly. can anyone help?

[politoxicomania]

u can see specific groups in infrared spectras, but cant determine the structure of a compound by infrared spektometrie. If u have found a spektrum from the pure substance u want u can compare it with your spektrum.
To determine a stucture of a substance always use different methods like CHNS-Analyse, NMR, powder- or cristall-x-ray-analysis.
What substances do u want to analyse ?

[KidCurry]

Short question: Is there any  on-line service that gives access to a database of IR-spectrums for different compounds?

[Rhodium]

TFSE gave the following hits for "IR spectrum database", among others.

Post 181182

(LaBTop: "Professional Chemistry Links", Methods Discourse)

Post 497867 (missing)

(7is: "Drugbase - A Structural Database of Drugs", The Server Room)

[embezzler]

i find all i need of the spectra in a google images search...

for carbocation:

ir is most useful for testing the purity of a compound,that is confirmation of a suspected cmpd, the fingerprint region of the ir is specific for pretty much every compound.

 it is often used in conjuntion with some other form of analytical examination such as hplc to verify the scan since the combination of bonds can yield different compounds. it is not the best tool for id an unknown mix since it will get real hard to interpret real fast.

it will tell you if there is anything there that isnt expected, by  providing an extra peak or two if you are lucky but the best would be some form of chromatography(imho)

[Chimitant]

To my knowledge it will be very difficult to do a purity check when you have impurities below 10 % level in your sample. It will depend on the type of material you want to investigate and whether the impurities are modifications of your target compound or completely different chemically spoken.

[embezzler]

do you have a ref for that figure?i have never heard thet quoted before.

swim knows that sensitivity is really dependant on the machine you are using there is a great variation in sensitivity across the board depending on the type of IR instrument used. unless the threshold of detection has not been reached i see no reason why a chemical impurity would not show up.

however if the impurity is a modification of the original molecule then to validate the results , even IF nothing shows up , hplc is usually run with ir to ensure accurate results.if properly implemented the hplc should detect the kind of structural changes ir would miss.it all depends on the type of modification to the original molecule.

depending on the level of confidence one requires  in their results ir can be ideal, fast an accurate if the sample spectrum can be compared to a standard then impurities are readily visible if not easily identifiable

[Chimitant]

Embezz: I obtained this figure from personal experience with IR analyses. In combination with GC/MS I was able to identify impurities within my samples that did not show up with IR (or at least not whithout the knowledge what functional groups are in there).

You make a good point about the reference material. If you have pure materials at your disposal you could always substract its spectrum from the spectrum of your unknown material. This will give a mixed spectrum of the impurities.

But you´re right about IR being a very fast, simple and cheap technique for screening if you have relatively pure materials. Mixtures will be difficult to deal with.