The primary goal of instrument analysis is to measure an amount of substance. This is done by comparing the relative concentrations among the atomic masses in the generated mass spectrum. Two kinds of analysis are possible, comparative and original. Comparative analysis essentially compares the given spectrum to a spectrum library to see if its characteristics are present for some known sample in the library. This is best performed by a computer because there are many visual distortions that can take place due to variations in scale. Computers can also correlate more data (such as the retention times identified by the GC), to more accurately relate certain data.