title: A sensitive method for digoxin determination using formate-adduct ion based on the effect of ionization enhancement in liquid chromatograph-mass spectrometer.
abstract: A sensitive and rapid method based on formate-adduct ion detection was developed and fully validated for digoxin determination in rat plasma. For LC/MS/MS detection with formate-adducts as precursor ions, transitions of m/z 825.5→779.9 for digoxin and m/z 809.5→763.4 for the internal standard (digitoxin) were monitored in negative mode. To investigate the impact of formic acid on the mass response and method sensitivity, a formic acid concentration range of 0-0.1% (0, 0.0005%, 0.002%, 0.01%, 0.1%, v/v) was evaluated. A concentration of 0.002% gave the highest sensitivity, which was 16- to 18-fold higher than deprotonated ions, and was designated as the contribution giving the strongest ionization enhancement and adduction. A number of parameters were then varied in order to optimize the method, and a limit of quantitation (LOQ) at 0.2 ng/mL was reached with an injection volume of 5 μL, a total run time of 3 min, and 0.1 mL of rat plasma. A calibration curve was plotted over the range 0.2-50 ng/mL (R(2)=0.9998), and the method was successfully applied to study pharmacokinetics in rat following a single oral administration of digoxin (0.05 mg/kg). Four additional steroid saponins (digitoxin, deslanoside, ginsenoside Rg1 and Rb1) were investigated to assess the impact of formic acid on the mass response of steroid saponins. Compounds with a conjugated lactonic ring in their structures such as digoxin, digitoxin and deslanoside tended to form stable formate-adduct ions more easily. The LC/MS/MS method developed here is therefore well suited for the quantification of steroid saponins that are difficult to deprotonate using other MS approaches.
authors.affiliation: School of Pharmacy, Fudan University, Shanghai 201203, PR China.
authors.forename: Xia
authors.initials: X
authors.lastname: Li
authors.affiliation: School of Pharmacy, Fudan University, Shanghai 201203, PR China.
authors.forename: Yu
authors.initials: Y
authors.lastname: Wang
authors.affiliation: School of Pharmacy, Fudan University, Shanghai 201203, PR China.
authors.forename: Qingyuan
authors.initials: Q
authors.lastname: Zhou
authors.affiliation: School of Pharmacy, Fudan University, Shanghai 201203, PR China. Electronic address: yqyu@shmu.edu.cn.
authors.forename: Yunqiu
authors.initials: Y
authors.lastname: Yu
authors.affiliation: Department of Dermatology, Ruijin Hospital, School of Medicine, Shanghai Jiaotong University, Shanghai 200025, PR China.
authors.forename: Lihong
authors.initials: L
authors.lastname: Chen
authors.affiliation: Department of Dermatology, Ruijin Hospital, School of Medicine, Shanghai Jiaotong University, Shanghai 200025, PR China.
authors.forename: Jie
authors.initials: J
authors.lastname: Zheng
mesh_terms: D000818:Animals; D002853:Chromatography, Liquid / Q000379:methods*; D004077:Digoxin / Q000097:blood* / Q000737:chemistry / Q000493:pharmacokinetics; D004355:Drug Stability; D005561:Formates / Q000737:chemistry*; D006801:Humans; D016014:Linear Models; D051381:Rats; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D053719:Tandem Mass Spectrometry / Q000379:methods*
chemical_list: D005561:Formates; C030544:formic acid; D004077:Digoxin
keywords: Digoxin; Formate-adduct; LC/MS/MS; Rat plasma; Sensitivity