A background subtraction approach for determination of endogenous cortisol and 6β-hydroxycortisol in urine by UPLC-MS/MS with application in a within-day variability study in HIV-infected pregnant women - Benzi, Jhohann Richard de Lima; Moreira, Fernanda de Lima; Marques, Maria Paula; Duarte, Geraldo; Suarez-Kurtz, Guilherme; Lanchote, Vera Lucia (Journal of Chromatography B, 2020)