The Potential of Restricted Access Media Columns As Applied in Coupled-Column LC/LC−TSP/MS/MS for the High-Speed Determination of Target Compounds in Serum. Application to the Direct Trace Analysis of Salbutamol and Clenbuterol

Abstract

This study investigated the potential of restricted access media (RAM) columns used as a first column in coupled-column LC hyphenated to thermospray tandem mass spectrometry (LC/LC−TSP/MS/MS) for the fast, selective, and sensitive determination of target drugs in serum samples. Because of their wide range in polarity, salbutamol and clenbuterol were chosen as model compounds and representatives of the class of β₂-agonists. Three types of RAM columns were tested: (i) Pinkerton ISRP (internal surface reversed phase, 5 μm), (ii) SPS (semipermeable surface, 5 μm C18), and (iii) RP-18 ADS (alkyl-diol silica, 25 μm). A 3-μm C18 column (50 mm × 4.6 mm i.d.) was chosen as the second column. Tandem mass spectrometric detection was carried out in the selected reaction monitoring (one parent→one daughter) mode. With regard to retention and, moreover, the peak elution volume of the analytes, the ISRP material was found to perform best: a 50-mm × 4.6-mm i.d. ISRP column in combination with a 100% aqueous buffer (pH of 7.0 ± 0.2) allowed the injection of large volumes (up to 200 μL) of sample without additional band broadening of the analytes and provided sufficient preseparation between analytes and large-molecule serum constituents. Under the selected conditions, both analytes could be determined in serum samples up to a limit of quantification (LOQ) of 0.5 ng/mL, with a sample throughput of 7 and 5 h^-1 for salbutamol and clenbuterol, respectively. Method validation was carried out by analyzing, in the course of several days, calf and human serum samples spiked with the analytes. In the case of salbutamol, the overall recovery from serum samples spiked at levels between 0.5 and 50 ppb (n = 33) was 103.4%, with a repeatability of 12.7% and reproducibility of 14.3%. The overall recovery for clenbuterol was 99.6% (n = 15, spiked level 0.5−5 ppb), with a repeatability of 15.2% and reproducibility of 16.4%. The adopted LC/LC−TSP/MS/MS analyzer appeared to be very robust under the selected conditions, and, after the period of analysis involving the processing of more than 100 mL of serum, neither loss of chromatographic performance nor pressure increase of columns or of the interface was observed.