Droplet Assisted Inlet Ionization for Online Analysis of Airborne Nanoparticles

Abstract

Airborne nanoparticles play a key role in climate effects as well as impacting human health. Their small mass and complex chemical composition represent significant challenges for analysis. This work introduces a new ionization method, droplet assisted inlet ionization (DAII), where aqueous droplets are produced from airborne nanoparticles. When these droplets enter the mass spectrometer through a heated inlet, rapid vaporization leads to the formation of molecular ions. The method is demonstrated with test aerosols consisting of polypropylene glycol (PPG), angiotensin II, bovine serum albumin, and the "thermometer" compound p-methoxybenzylpyridinium chloride. High-quality spectra were obtained from PPG particles down to 13 nm in diameter and sampled masses in the low pictogram range. These correspond to aerosol number and mass concentrations smaller than 1000 particles/cm and 100 ng/m, respectively, and a time resolution on the order of seconds. Fragmentation of the thermometer ion using DAII was inlet temperature dependent and similar in magnitude to that observed with a conventional ESI source on the same instrument. DAII should be applicable to other types of aerosols including workplace aerosols and those produced for drug delivery by inhalation.

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

“…Inlet ionization is soft even when high temperatures are applied to the inlet . Similar observations have been reported by Johnston's group using inlet tube temperatures as high as 850 °C . With volatile solvents the inlet conditions can be at or below room temperature, but increasing heat typically enhances ionization, including chemical background and metal cation (e.g., sodium) adduction, as is the case with some solid matrices .…”

“…Using a temperature range from 70 to 400 °C on an Orbitrap Q‐Exactive Focus mass spectrometer produces for 3‐NBN matrix exceptionally high analyte ion abundance without the need for high voltages or a laser , while other matrices, especially typical MALDI matrices, produce less abundant analyte ions, multiply and singly charged, and only at high temperatures. The temperature can be applied directly or indirectly to the tube, typically used as an ESI inlet . Charge separation occurs after the matrix is introduced to sub‐atmospheric pressure.…”

“…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.…”

“…Other means for solution introduction for voltage‐free analysis have been reported. These include inoculation loops, zero‐volt paper spray, and introduction of solvent mist droplets . For example, Johnston's group heated a homebuilt inlet tube to >800 °C on a SYNAPT G2S and analyzed aerosol particles condensed with solvent vapor .…”

“…peptide and protein analyses . With higher heat (energy imparted), more matrices become useful and more analyte types can be ionized (similar to MALDI and increased laser fluence), but this also increases the background . Increased voltage applied to accelerate ions through an intermediate pressure region (as with in‐source fragmentation), with the new ionization processes, typically increases background, and for small molecules fragmentation, but for larger molecules provides increased abundance possibly due to desolvation .…”