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Supercritical fluids are typically electrosprayed using an organic solvent makeup flow to facilitate continuous electrical connection and enhancement of electrospray stability. This results in sample dilution, loss in sensitivity, and potential phase separation. Premixing the supercritical fluid with organic solvent has shown substantial benefits to electrospray efficiency and increased analyte charge state. Presented here is a nanospray mass spectrometry system for supercritical fluids (nSF-MS). This split flow system used small i.d. capillaries, heated interface, inline frit, and submicron emitter tips to electrospray quaternary alkyl amines solvated in supercritical CO2 with a 10% methanol modifier. Analyte signal response was evaluated as a function of total system flow rate (0.5–1.5 mL/min) that is split to nanospray a supercritical fluid with linear flow rates between 0.07 and 0.42 cm/sec and pressure ranges (15–25 MPa). The nSF system showed mass-sensitive detection based on increased signal intensity for increasing capillary i.d. and analyte injection volume. These effects indicate efficient solvent evaporation for the analysis of quaternary amines. Carrier additives generally decreased signal intensity. Comparison of the nSF-MS system to the conventional SF makeup flow ESI showed 10-fold signal intensity enhancement across all the capillary i.d.s. The nSF-MS system likely achieves rapid solvent evaporation of the SF at the emitter point. The developed system combined the benefits of the nanoemitters, sCO2, and the low modifier percentage which gave rise to enhancement in MS detection sensitivity.
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Ion formation in droplet‐assisted ionization
RationaleIn droplet‐assisted ionization (DAI), intact molecular ions are generated from molecules in aerosol droplets by passing the droplets through a temperature‐controlled capillary inlet. Ion formation is explored through the effects of analyte mass flow, droplet solvent composition, and capillary temperature on ion signal intensity. MethodsA Waters SYNAPT G2‐S is adapted for DAI by reconfiguring the inlet with a temperature‐controlled capillary. Droplets are generated by atomization of a solution containing analyte and then sampled through the inlet. If desired, solvent can be removed from the droplets prior to analysis by sending the aerosol through a series of diffusion dryers. Size distributions of the dried aerosols allow the mass flow of analyte into the inlet to be determined. ResultsAnalyte signal intensities are orders of magnitude higher from droplets containing a protic solvent (water) than an aprotic solvent (acetonitrile). The highest signal intensities for DAI are obtained with inlet temperatures above 500°C, though the optimum temperature is analyte dependent. At elevated temperatures, droplets are thought to undergo rapid solvent evaporation and bursting to produce ions. The lowest signal intensities are generally obtained in the 100–350°C range, where slow solvent evaporation is thought to inhibit ion formation. As the temperature decreases from 100°C down to 25°C, the signal intensity increases significantly. When 3‐nitrobenzonitrile, a common matrix for solid‐state matrix‐assisted ionization (MAI), is added to droplets consisting of 50/50 v/v water and acetonitrile, the matrix enhances ion formation to produce a signal intensity comparable to DAI in 100% water. ConclusionsThe results are consistent with other inlet ionization techniques, suggesting that similar ion formation mechanisms are operative. Optimized ion yields (the combined effects of ionization probability and ion transmission) for DAI are currently in the 10−5to 10−6range, which is sufficient for many aerosol applications.
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- Award ID(s):
- 1649719
- PAR ID:
- 10453802
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Rapid Communications in Mass Spectrometry
- Volume:
- 35
- Issue:
- S1
- ISSN:
- 0951-4198
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Supercritical fluids are typically electrosprayed using an organic solvent makeup flow to facilitate continuous electrical connection and enhancement of electrospray stability. This results in sample dilution, loss in sensitivity, and potential phase separation. Premixing the supercritical fluid with organic solvent has shown substantial benefits to electrospray efficiency and increased analyte charge state. Presented here is a nanospray mass spectrometry system for supercritical fluids (nSF-MS). This split flow system used small i.d. capillaries, heated interface, inline frit, and submicron emitter tips to electrospray quaternary alkyl amines solvated in supercritical CO2 with a 10% methanol modifier. Analyte signal response was evaluated as a function of total system flow rate (0.5–1.5 mL/min) that is split to nanospray a supercritical fluid with linear flow rates between 0.07 and 0.42 cm/sec and pressure ranges (15–25 MPa). The nSF system showed mass-sensitive detection based on increased signal intensity for increasing capillary i.d. and analyte injection volume. These effects indicate efficient solvent evaporation for the analysis of quaternary amines. Carrier additives generally decreased signal intensity. Comparison of the nSF-MS system to the conventional SF makeup flow ESI showed 10-fold signal intensity enhancement across all the capillary i.d.s. The nSF-MS system likely achieves rapid solvent evaporation of the SF at the emitter point. The developed system combined the benefits of the nanoemitters, sCO2, and the low modifier percentage which gave rise to enhancement in MS detection sensitivity.more » « less
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