why is an internal standard used and in what ways does it improve the quantitative methodology?

Question

and why has benzene used in this case?

Answer 1

An internal standard is used in those methods that have some inherent problem with reproducibility. This means that each time the method is run, you would expect a high degree of variation in the answer, even if you used exactly the same sample amount each time. To compensate for, and reduce the variation, an internal standard is used. It is best if the internal standard is structurally very close to the sample, so that the standard and sample experience a similar test. Benzene would be most useful for similar chemicals like methyl benzene.

Answer 2

An internal standard is used to provide a known amount of the material being measured. This known amount of the internal standard is compared to the unknown amount in the sample that you are measuring.

A known amount of Benzene can be compared to the unknown amount of Benzene that you need to measure.