Silica coated paper substrate for paper-spray analysis of therapeutic drugs in dried blood spots

Abstract

Paper spray is a newly developed ambient ionization method that has been applied for direct qualitative and quantitative analysis of biological samples. The properties of the paper substrate and spray solution have a significant impact on the release of chemical compounds from complex sample matrices, the diffusion of the analytes through the substrate, and the formation of ions for mass spectrometry analysis. In this study, a commercially available silica-coated paper was explored in an attempt to improve the analysis of therapeutic drugs in dried blood spots (DBS). The dichloromethane/isopropanol solvent has been identified as an optimal spray solvent for the analysis. The comparison was made with paper spray using chromatography paper as substrate with methanol/water as solvent for the analysis of verapamil, citalopram, amitriptyline, lidocaine, and sunitinib in dried blood spots. It has been demonstrated that the efficiency of recovery of the analytes was notably improved with the silica coated paper and the limit of quantitation (LOQ) for the drug analysis was 0.1 ng mL(-1) using a commercial triple quadrupole mass spectrometer. The use of silica paper substrate also resulted in a sensitivity improvement of 5-50-fold in comparison with chromatography papers, including the Whatman ET31 paper used for blood cards. Analysis using a hand-held miniature mass spectrometer Mini 11 gave LOQs of 10-20 ng mL(-1) for the tested drugs, which is sufficient to cover the therapeutic ranges of these drugs.

Figure 1

SEM and photographic images of Grade 4 chromatography and silica coated papers without and with dried blood spots: SEM images of (a) chromatography and (b) silica coated paper, and (c) close-up image of the selected area in (b) without dried blood spots; top view of (d) chromatography and (e) silica coated paper with dried blood spots; photograph images of the (f) top and (g) back sides of chromatography paper, and (h) top and (i) back sides of silica coated paper with blood spots.

Figure 2

(a) Effect of spray solvent on the analysis of verapamil [(M+H)⁺, m/z 455, product ion, m/z 303] and (b) effect of isopropanol percentage in dichloromethane on the signal of verapamil with a triple quadrupole. Silica-coated paper substrate used. The peak intensity is an average of total ion chronogram values. The concentration of verapamil in the blood sample was 500 ng mL⁻¹.

Figure 3

Comparison of the elution behavior of verapamil with silica coated paper and chromatography papers: (i) verapamil in pure water (5 μL, 500 ng mL⁻¹) deposited onto surface of (a) silica coated paper, (b) Grade 4 chromatography paper and (c) ET31 chromatography paper; (ii) verapamil in blood (5 μL, 500 ng mL⁻¹) deposited onto surface of (d) silica coated paper and (e) Grade 4 chromatography paper and (f) Grade ET31 chromatography paper. Note: the experiments were performed after the paper had dried. Solvent for silica coated paper was 9:1 dichloromethane/isopropanol, chromatography paper 9:1 methanol/water. a.u. : arbitrary units.

Figure 4

Comparison of the LOQ and linear dynamic range of verapamil with (a) silica coated paper (0.27 mm thick; 9:1 dichloromethane/isopropanol), (b) Grade 4 chromatography paper (0.21 mm thick; 9:1 methanol/water), and (c) Grade ET31 chromatography paper (0.50 mm thick; 9:1 methanol/water) . Note: 5 μl of blood sample was used, product ion m/z 303 of verapamil was monitored.

Figure 5

Linear dynamic range for (a) lidocaine and (b) verapamil, and typical spectra of lidocaine (c) and verapamil (d) with concentrations in blood of 20 ng mL⁻¹ and 10 ng mL⁻¹, respectively, obtained with Mini 11. Silica-coated paper (0.27 mm thick) with 9:1 dichloromethane/isopropanol.