High Sensitivity Analysis of Nanoliter Volumes of Volatile and Nonvolatile Compounds using Matrix Assisted Ionization (MAI) Mass Spectrometry

Abstract

Abstract. First results are reported using a simple, fast, and reproducible matrixassisted ionization (MAI) sample introduction method that provides substantial improvements relative to previously published MAI methods. The sensitivity of the new MAI methods, which requires no laser, high voltage, or nebulizing gas, is comparable to those reported for MALDI-TOF and n-ESI. High resolution full acquisition mass spectra having low chemical background are acquired from low nanoliters of solution using only a few femtomoles of analyte. The limit-of-detection for angiotensin II is less than 50 amol on an Orbitrap Exactive mass spectrometer. Analysis of peptides, including a bovine serum albumin digest, and drugs, including drugs in urine without a purification step, are reported using a 1 μL zero dead volume syringe in which only the analyte solution wetting the walls of the syringe needle is used in the analysis.

Novel ionization processes for use in mass spectrometry: ‘Squeezing’ nonvolatile analyte ions from crystals and droplets

“…Consequently, using mass spectrometers optimized for ESI‐ and MALDI‐MS, the ease of adaptation of the methods based on the new ionization processes varies. It may only require turning off the high voltages and/or laser, or it may necessitate more significant physical modification(s) to obtain analytical useful results . As an example, the currently best performing MAI matrix, 3‐NBN, on Waters SYNAPT G2 and G2S instruments works exceptionally well for analyte ionization on the intermediate pressure MALDI sources without the use of the laser, but performs best at room temperature on the ESI Z‐Spray source blocks, and above the melting point of the matrix, 114–117 °C, the analyte ion abundance essentially ceases on this instrument .…”

“…A number of different mass spectrometers, in terms of analyzer types and vendors, have been used with the novel ionization processes demonstrating the importance of pressure, temperature, and collisions in optimizing ion formation . An example is shown in Figure for bovine serum albumin (BSA), 66 kDa, using mass spectrometers to ionize this 66 kDa protein intact and multiply charged using the new ionization processes.…”

“…However, it should be noted that introduction of a wet sample into an inlet tube increases the potential for carryover in subsequent acquisitions. A variety of different commercial MALDI and ESI sources have been employed including those from Thermo, Waters, Sciex, Bruker, and Advion mass spectrometers using MAI conditions …”

“…In order to achieve ionization under these conditions, matrix compounds such as 3‐nitrobenzonitrile (3‐NBN), amongst others, are used . This matrix‐assisted ionization (MAI) method is ultimately simple and has excellent sensitivity with a limit of detection reported to be <50 attomoles and produces clean full‐scan mass spectra requiring only a few femtomoles of, for example, drugs and peptides . Mass spectra of complex mixtures can be acquired in seconds directly from biological fluids, tissue, or buffered solution, which is especially effective in conjunction with ultra‐high resolution .…”

“…By placing matrix only on the specific area(s) of interest on a surface and exposing the surface to the vacuum of the mass spectrometer, ions are observed from compounds exposed to the matrix solution, allowing rapid interrogation of ‘features of interest’ . The discoveries of these processes and development of suitable technologies were accomplished through fundamental research and using commercially available ion source inlets ’as is’ or with simple physical source modifications . Recently, we have begun to design and build dedicated inlets with the promise of a better understanding of the ionization processes and making improvements in applications .…”