Improving the signal intensity and sensitivity of MALDI mass spectrometry by using nanoliter spots deposited by induction-based fluidics

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. A lthough matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has wide applicability to biological and synthetic polymers, obtaining high-quality MS signals when the quantity of sample is small is difficult-a challenge that is endemic in biological studies. A limited sample amount gives rise to poor signal intensity, hampering spectra interpretation, unknown identification, and quantification, thus limiting the application in biological studies. One way to increase the signal intensity without increasing the sample amount is to make MALDI spots smaller and more concentrated.Small-scale sample preparation on a MALDI target has been achieved in several ways. One approach minimizes dispersion of sample materials by etching or drilling small channels on the target [1, 2] or using hydrophobic materials as MALDI plate surfaces [3][4][5][6]. A second approach uses microdispensing, including use of capillary tubes [7][8][9][10], piezoelectric techniques [11][12][13][14], electrodynamic methods [15], and acoustic ejection [16]. This general approach improves MALDI for imaging [14,16], on-line analysis [13,17], and concentration enhancement by microextraction [10]. Common in both approaches [1-12, 15, 17] is the concept that the sensitivity of MALDI-MS increases when small sample spots are used and that the search for "hot spots" within the sample area becomes more productive when solid matrices are used. Our approach of using small volumes of concentrated analytes is motivated by the concept of purposefully creating a "hot spot."Herein, we describe an approach by which a nanoliter syringe and induction-based fluidics (IBF) deposit small-volume droplets (in the nanoliter range) in a noncontact, flight-controlled way. Using this technique, we first investigated the relationship between the spot size and signal intensity with an ionic-liquid matrix (ILM), taking advantage of the homo...

The measurement of charge for induction-based fluidic MALDI dispense event and nanoliter volume verification in real time

“…Yergey has also observed up to a 100-fold increase in analysis sensitivity [6] for a major class of proteins, tubulins. These and similar observations on the analytes in the Applied Biosystems (ABI) 4700 Standard [4] show significant improvement (increases of around 5-, 10-, to 100-fold) in sensitivity and a 3 to 20% increase in reproducibility using nanoliter IBF depositions for proteins, peptides, and synthetic polymers. These enhancements in a wide range of molecules and mass ranges in both positive-ion reflectron mode and negative-ion linear mode indicate that nL-IBF deposition improves sensitivity across many MALDI applications.…”

“…In IBF there are no Faradaic processes, only capacitance based ones, unlike ESI. The physics behind IBF reveals [2,3] that, unlike piezoelectric, sound, or any other technologies that are applied to transport liquids at low volumes, IBF kinetically launches drops to targets, as it dynamically directs the liquids to targets, and-as we show here-measures them on arrival, in real time.One major IBF application, nanoliter (nL) depositions for the production of matrix-assisted laser desorption/ ionization mass spectrometry (MALDI-MS) plates, has been shown to increase 10-fold in signal-to-noise ratios for both bradykinin measurements and analysis precision [4]. Nanoliter-IBF depositions also produce a major increase in MALDI sensitivity and reproducibility for synthetic polymers [5] (PMMA, PEG, and PS), even with polymers with Mn values greater than 90,000 units.…”

“…One major IBF application, nanoliter (nL) depositions for the production of matrix-assisted laser desorption/ ionization mass spectrometry (MALDI-MS) plates, has been shown to increase 10-fold in signal-to-noise ratios for both bradykinin measurements and analysis precision [4]. Nanoliter-IBF depositions also produce a major increase in MALDI sensitivity and reproducibility for synthetic polymers [5] (PMMA, PEG, and PS), even with polymers with Mn values greater than 90,000 units.…”

“…The ability to accurately measure and to verify the volume deposited per event by IBF in real time could further aid the ability to QC and to improve MALDI quantitation. The ability to easily manipulate small-volume solvents or solutes to targets also has applications in the areas of green chemistry and in biological MALDI with limited sample sizes [4]. Applications of nL-IBF exist in nontouch dispensing (micro-, nano-, and picoliter samples), in parallel LC/MALDI, in defense homeland security [7], chemistry, for desorption electrospray ionization/ direct analysis in real time (DESI/DART) standardization and applications [8], DNA/RNA sample preparation [9], and in thin-layer chromatography (TLC) applications [10].…”

Quantitative matrix-assisted laser desorption/ionization mass spectrometry

“…This allowed the investigators to dispense nanolitre drops and they report that this improves the signal intensity and sensitivity achieved in MALDI-MS [27].…”