nicotine. If a 500 mL sample of urine is used and all of the nicotine present is concentrated during sample preparation into a final solution volume of 10.00 mL, what is the minimum concentration of nicotine (in ?g?L-1) that can be detected in urine by the assay?

注：nicotine——尼古丁，urine——尿。

27. A spectrophotometric assay was to be carried out on a compound with κ = 5 000 L?mol -1?cm-1.The stock solution of standard had been made with a concentration of 0.020 3 mol?L-1. Given a path length of 1.000 cm and the fact that the final absorbance should be around 0.40 to 0.5, by what factor should the stock solution be diluted?

注：stock solution——储备溶液

28. 画出单光束原子吸收分光光度计结构方框示意图，并简要说明各部分作用。