The determination of substance in the unknown sample in Figure 6.8 can be done. First step, the unknown substance should be identified. From the retention time (1.17 min) of the peak appeared in Figure 6.8 (d), the substance in the sample is methylbenzoate because the retention time of methylbenzoate is ~1.2 min. To find the concentration of methylbenzoate in the sample, the steps shown in Figure 6.10 should be followed.