Kinetics of bilirubin photodegradation with application to amniotic fluid

By spectrophotometric methods we have studied the photodegradation of bilirubin in simple solution and in amniotic fluid and have found that: (1) the photodegradation process can be described by kinetics which are first-order in concentration and second-order in time; (2) the photodegradation products of bilirubin do not prevent the determination of the absorbance of the parent compound; (3) although the rate of the photodegradation process depends upon both the oxygen partial pressure and the irradiation intensity, the determination of the original bilirubin absorbance is independent of both of these parameters so long as they are constant over the period of photodegradation; (4) other chromagens in amniotic fluid (including hemoglobin and methemoglobin) experience little photodegradation in the time required to degrade bilirubin; (5) several absorbance measurements over a short (approximately five-minute) period of photodegradation are sufficient for the determination of the original bilirubin absorbance for the complete range of interest in amniotic fluid with an accuracy of about five percent.