Question

1.) How can you use IR spectroscopy to confirm that the collected product is correct and pure?

2.) How can you use gas chromatography to confirm that the collected product is pure?

Answer 1

1,# When a sample is run in IR spectroscopy, IR light is made to pass through the sample and it absorbs certain amount and wavelength of the light.This is measured and used to analyse the unknown sample.The IR spectroscopy uses fingerprint method (1500-500cm-1),where all the complex characteristics of a molecule or atom can be found and analysed.The obtained sample can be checked if it is correct or no by running the sample in IR and correlating the characteristic peaks.

Characteristics of a sample like single bond, double bond, bond stretch ,bond wagging can all be found out using IR hence making it easy to give a structure to the sample.

#To find out the purity of the sample,3 methods can be used.

*Use of the reference sample. A reference sample is a known one with highest purity and is used to comparision.2 methods are used for this.

a, Comparing with the standard sample:

By running the sample in IR and comparing the obtained result to that of the standard sample, one can simply subtract the spectrum and the difference between the 2 spectrums will give you the amount of impurity present in the sample. However this method is not really effective.

b, Dynamic difference sprectroscopy :

There is a difference factor between the spectrums of standard and sample. This method will give importance to this factor and using this during comparision gives better results.

The optimal compensation is now calculated my firstly multiplying the standard spectrum with the optimal difference factor and then subtracting it with the sample.

Another method used for checking purity of the sample is by transforming the standard sample to a less developed state and calculating the correlation coefficient between the sample and the standard. But this method is effective only when the impurity present in the sample is very less.

2, In a GC, the sample is injected and the compound is made to elute and the retention time of it is observed. There are formation of peaks obtained to help us identify the nature of the compound.However, only the impurities of sample that is stable and volatile for a GC system can be analysed. Also the sample amount should be more in order to find purity present in very small quantities.The unknown compound can be found as the GC has characteristic retention time.

To find the purity of sample, the standard and the sample are run in GC under identical experimental conditions. The obtained 2 chromotograms are compared. The standard sample has the highest purity and the impurities level are known. The peak height, peak area are compared with that of the sample.

Percent volume of impurities(The impurities present in sample)

= (Peak height of sample / peak height of standard ) *Percent volume of impurity in standard

Using the above formula, the impurities present in any sample can be found.