以盐酸酸化的二甲亚砜为溶剂，铁盐催化空气氧化安息香制备苯偶酰，再由苯偶酰与尿素缩合制备苯妥英。在此合成过程中需对反应液中安息香的浓度、中间产物苯偶酰的含量以及最终产物苯妥英的浓度进行检测。试验提出了采用反相高效液相色谱法结合计时波长切换紫外检测法对上述各化合物进行测定。将所需测定的样品溶于乙腈中，所得溶液经0.45 μm滤膜过滤。取滤液（进样量为20 μL）通过kromasil 100-5C₁₈色谱柱（200 mm×4.6 mm，5 μm），以体积比为80：20的乙腈-水溶液作为流动相进行等度洗脱，洗脱液按预设程序进行计时紫外检测。在0~3.0 min和波长218 nm处检测苯妥英；在3.0~4.0 min和波长247 nm处检测安息香；在4.0~6.0 min和波长260 nm处检测苯偶酰。3种化合物标准曲线的线性范围均为0.1~100 mg·L^-1，并测得其检出限（3S/N）依次为0.010，0.003 0，0.007 0 mg·L^-1。在实际样品基础上加入3个浓度水平的混合标准溶液进行回收试验，测得3种化合物的回收率为95.4%~105%；测定值的相对标准偏差（n=6）为1.8%~4.2%。

Benzoin was oxidized by air in the presence of iron salt as catalyst and using dimethyl sulfoxide (acidified with HCl) as solvent to give benzil, which was in turn condensed with urea to give the object product phenytoin. In the course of this synthetic process, it was required to determine phenytoin, the intermediate benzil, and also the starting material benzoin. A method for determination of the 3 compounds mentioned above by RP-HPLC in combination with the UV-detection by timing wavelength switching was tested and proposed. The sample containing the analytes was dissolved with acetonitrile and solution obtained was filtered through 0.45 μm filtering membrane. The filtrate was used for LC analysis. The kromasil 100-5C₁₈ chromatographic column was used as stationary phase, and acetonitrile-H₂O solution with the volume ratio of 80:20 was used as mobile phase in isogradient elution. Absorbances (expressed as their peak areas) of the eluates were measured by the programme of timing wavelength switchings as follows:at 0-3.0 min and wavelength of 218 nm (for phenytoin); at 3.0-4.0 min and wavelength of 247 nm (for benzoin); and at 4.0-6.0 min and wavelength of 260 nm (for benzil). Linearity ranges of the standard curves of the 3 compounds were in the same range of 0.1-100 mg·L^-1. Detection limits (3S/N) found were 0.010 mg·L^-1, 0.003 0 mg·L^-1 and 0.007 0 mg·L^-1 respectively. Test for recovery was made by addition of mixed standard solutions at 3 concentration levels to matrixes of substantial sample, results of recovery found for the 3 analytes were in the range of 95.4%-105%. Values of RSDs (n=6) were found in the range of 1.8%-4.2%.