Investigations of Some Liquid Matrixes for Analyte Quantification by MALDI

Abstract

Abstract. Sample inhomogeneity is one of the obstacles preventing the generation of reproducible mass spectra by MALDI and to their use for the purpose of analyte quantification. As a potential solution to this problem, we investigated MALDI with some liquid matrixes prepared by nonstoichiometric mixing of acids and bases. Out of 27 combinations of acids and bases, liquid matrixes could be produced from seven. When the overall spectral features were considered, two liquid matrixes using α-cyano-4-hydroxycinnamic acid as the acid and 3-aminoquinoline and N,N-diethylaniline as bases were the best choices. In our previous study of MALDI with solid matrixes, we found that three requirements had to be met for the generation of reproducible spectra and for analyte quantification: (1) controlling the temperature by fixing the total ion count, (2) plotting the analyte-to-matrix ion ratio versus the analyte concentration as the calibration curve, and (3) keeping the matrix suppression below a critical value. We found that the same requirements had to be met in MALDI with liquid matrixes as well. In particular, although the liquid matrixes tested here were homogeneous, they failed to display spot-to-spot spectral reproducibility unless the first requirement above was met. We also found that analytederived ions could not be produced efficiently by MALDI with the above liquid matrixes unless the analyte was sufficiently basic. In this sense, MALDI processes with solid and liquid matrixes should be regarded as complementary techniques rather than as competing ones.

Quantitative transfer of polar analytes on a solid surface to a liquid matrix in MALDI profiling

“…It has been our experience that the calibration curve determined by our method for a given combination of analyte, matrix and instrument remains valid even a few years after its construction . The calibration curve shown above is essentially the same as the one reported previously . The calibration curve for imipramine (structure shown in the Fig.…”

“…Fournier et al demonstrated the imaging, although not quantitative, of lipids in cells by MALDI with ionic liquid matrixes . We reported that a genuinely fluidic liquid could be produced by the nonstoichiometric mixing of the above acids and bases and air‐drying of the mixture to eliminate the solvent—in profiling, a solution of liquid matrix in a solvent is loaded and dried . Such a liquid, which is not entirely ionic, remained fluidic, and hence homogeneous, even after prolonged storage inside a vacuum and after extensive bombardment—10 000 shots or more—by laser pulses .…”

“…TIC control is useful for the acquisition of reproducible spectra from a solid sample. Previously, we found that MALDI spectra acquired with a liquid matrix were reproducible even without the TIC control when the top center of a droplet was irradiated by the laser . However, using the TIC control made the experiment easier.…”

“…We reported that a genuinely fluidic liquid could be produced by the nonstoichiometric mixing of the above acids and bases and air‐drying of the mixture to eliminate the solvent—in profiling, a solution of liquid matrix in a solvent is loaded and dried . Such a liquid, which is not entirely ionic, remained fluidic, and hence homogeneous, even after prolonged storage inside a vacuum and after extensive bombardment—10 000 shots or more—by laser pulses . For MALDI with a solid matrix, the ion signals from a spot changed as the laser irradiation continued .…”

“…Hence, a method was needed to make MALDI reproducible. In contrast, we could generate reproducible ion signals without much effort in liquid MALDI , an advantage in quantitative profiling. More importantly, the fact that the liquid matrix remains fluidic over an extended period of time can be an advantage for the efficient extraction of analytes from a solid sample surface.…”