Measuring the internal dose of toxicants in blood has several advantages over measuring it in urine. Generally, the parent compound, instead of a metabolite, can be directly monitored in blood products such as whole blood, plasma, or serum; therefore, the development of a blood measurement technique usually does not require detailed information on the metabolism. The major disadvantages of blood measurements are the venipuncture required to obtain the sample and the low concentration levels. In addition, when samples can be obtained, the amount of blood available to perform the analysis is often limited. These facts stress the need for developing especially sensitive analytical procedures.