Degradation of Nitrofurantoin
Nitrofurantoin 2.00 mg/mL was incubated overnight at 40°C, exposed to daylight at room temperature for 7 days, and then boiled for 1 min. The sample was then cooled to room temperature. A 0.5-mL aliquot was mixed with 0.5 mL furazolidone 2.00 mg/mL, the mixture was filtered, and a 10-pL sample was injected onto the column. The chromatogram obtained for the degraded preparation was compared with a chromatogram obtained from a standard (1.00 mg/mL) to determine any changes in concentration, retention time, and peak shape.
Preparation of Samples
Nitrofurantoin (10 mg/mL) study samples were thawed, and a 0.2-mL aliquot was diluted with 0.8 mL HPLC-grade water and then centrifuged for 1 min at 10 000 rpm. A 0.5-mL aliquot of nitrofurantoin was added to a 0.5-mL aliquot of internal standard in HPLC- grade acetonitrile. The final theoretical nitrofurantoin concentration was 1.00 mg/mL. Each sample was passed through a 0.45-pm microfilter before a 10-pL sample was withdrawn and injected onto the column.
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The means, standard deviations, and coefficients of variation were calculated for samples analyzed in triplicate and quadruplicate. For each study day, the percentage of initial nitrofurantoin concentration remaining was calculated for each sample. The percentage of nitrofurantoin remaining on day 91 was calculated from the concentration on day 91 as determined by linear regression and the concentration observed on day 0, according to the following formula: concentration on day 91/concentration on day 0 x 100%. The 95% confidence interval (CI) of the amount remaining on the last study day was calculated from the lower limit of the 95% CI of the slope of the curve relating concentration to time, determined by linear regression, according to the following formula: lower limit of the 95% CI of the concentration on day 91/concentration on day 0 x 100%. Stability was defined as maintenance of at least 90% of the initial nitrofurantoin concentration.