ABSTRACT
Here, we report the utility of induction-based fluidics (IBF) for the introduction of oligonucleotides to a mass spectrometer via charged droplets. The device produces nanoliter-sized droplets, which are field transported with minimal concerns related to source plugging or sampling loss. The IBF source enabled detection of oligonucleotides at the nanomolar concentration level. Importantly, analysis of individual droplets revealed that oligonucleotide mixtures could be detected with ion abundance ratios that closely match the initial concentration ratios within the sample.
ABSTRACT
Electrospray ionization mass spectra of lanthanide coordination complexes were measured by launching nanoliter-sized droplets directly into the aperture of an electrospray ionization mass spectrometer. Droplets ranged in size from 102 nL to 17 nL, while metal concentrations were 293 µM. The sample solution was delivered to a source capillary by a nanoliter dispenser at a rate of 21 nL/s, and droplets were ejected from the capillary by pulsing a potential onto the capillary. The end of the capillary was situated in front of the mass spectrometer and aimed directly at the aperture. The period and power of the electrical pulse was controlled by a digital energy source. The intensity of the extracted ion time profiles from the experiment showed reproducible production of lanthanide nitrato-anion complexes (Ce, Tb, and Lu). The integrated ion intensities of the complexes were reproducible, having relative standard deviations on the order 10% for anions, and 10-30% for cations. The integrated ion intensities were proportional to the droplet size, and the response was linear from about 100 to 650 pmol. However, the intercept is not zero, indicating a nonlinear response at lower analyte quantities or droplet sizes. Cation complexes were generated in separate experiments that corresponded to lanthanide nitrate ion pairs coordinated with the separations ligand octyl,phenyl,(N,N-diisobutylcarbamoyl)methylphosphine oxide (CMPO). Experiments showed a preference for formation of CMPO complexes with Ln(3+) having larger ionic radii. The relative standard deviation values of the cation abundance measurements were somewhat higher for the more highly coordinated complexes, which are also less stable. The mass spectral quality was high enough to measure the ratios of the minor isotopic ions to a high degree of accuracy. The approach suggests that the methodology has utility for analysis of solutions where the sample quantity is limited, or where the sampling efficiency of a normal ESI source is limiting on account of hazards derived from the sample solution.
ABSTRACT
This study preliminarily investigates whether nanoliter volumes of concentrated polar liquids and organic monomers launched to targets using induction based fluidics (IBF) can be verified through the real time charge measurements. We show that using a nanoliter IBF dispensing device and nanocoulomb meter, charge measurements made on nanoliter drops in real time are correlated with surface area following Gauss's Law. We infer the "induction only" formation of the double layer showing the ability to determine nanoliter volumes, nearly instantaneously, in real time. We discuss the implications that these observations may have for on improving/monitoring MALDI quantitation and its quality control.
ABSTRACT
A new contact-free, small droplet deposition method using an induction-based fluidics (IBF) technique to dispense nanoliter drops is described and evaluated for sample preparation in matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). The signal intensities available when using nanoliter spots are greater than those obtained with normal, microliter spots when the same amount of analyte is used. When using an ionic-liquid matrix, the improvement in sensitivity is equal to the concentration enhancement that was achieved by using smaller quantities of matrix. When using a conventional solid matrix, however, the increase in signal intensity shows a more complicated relationship to concentration. The approach of nanoliter deposition also supports multiple spotting to increase sample concentration and, thus, sample signal intensity. Nanoliter spotting not only improves the signal intensity and sensitivity achieved by MALDI-MS but also allows a major fraction of trace samples to be saved for other experiments, thus expanding the application of MALDI-MS to biological studies where sample quantity is limited.
