title: Electrospray ionization on porous spraying tips for direct sample analysis by mass spectrometry: enhanced detection sensitivity and selectivity using hydrophobic/hydrophilic materials as spraying tips.
abstract: BACKGROUND
  Despite various porous materials having been widely adopted as spraying tips for direct sample analysis using electrospray ionization mass spectrometry (ESI-MS), the effect of surface property and porosity of spraying tip materials on their analytical performances is not clear. Investigation of their relationships could provide insight into the proper choice and/or design of spraying tip materials for direct sample analysis.
  
  
  METHODS
  The effect of spraying tip materials with different polarities, including polyester and polyethylene (hydrophobic) and wood (hydrophilic), on the detection sensitivity for a variety of compounds, and on the ESI onset voltage, were studied using ESI-MS. The porosity of each type of spraying tip was characterized by scanning electron microscopy (SEM). Factors governing the detection sensitivity were determined based on the correlation of the detection sensitivity to the ESI onset voltage, the polarity, and the porosity of the spraying tip materials.
  
  
  RESULTS
  Hydrophobic tips (i.e., polyester and polyethylene) show better detection sensitivity for polar compounds but not for non-polar compounds, while hydrophilic tips (wooden tips) show the opposite effect. This phenomenon could be due to the difference in interaction between the analytes and the tips, causing the analytes to adsorb on the tip to different extents. In addition, the micro-porous nature of the tips could facilitate solvent diffusion for transporting analytes to the tip and maintain a stable spray for recording MS data. With the proper choice of spraying tip materials, trace amount of analytes at the picomole level can be detected with minimal sample pretreatment.
  
  
  CONCLUSIONS
  Both the polarity and the porosity of the spraying tip materials could significantly affect detection sensitivity for a wide variety of analytes. With proper choice of spraying tip material, ESI on a porous spraying tip could be a sensitive method for the direct analysis of daily life samples.
authors: Department of Chemistry, Open Laboratory of Chemical Biology of the Institute of Molecular Technology for Drug Discovery and Synthesis, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, PR China.
authors: Melody Yee-Man
authors: MY
authors: Wong
authors: Ho-Wai
authors: HW
authors: Tang
authors: Sin-Heng
authors: SH
authors: Man
authors: Ching-Wan
authors: CW
authors: Lam
authors: Chi-Ming
authors: CM
authors: Che
authors: Kwan-Ming
authors: KM
authors: Ng
mesh_terms: D004058:Diffusion; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D057230:Limit of Detection; D011091:Polyesters / Q000737:chemistry; D020959:Polyethylene / Q000737:chemistry; D016062:Porosity; D046351:Protoporphyria, Erythropoietic / Q000097:blood; D011524:Protoporphyrins / Q000097:blood; D021241:Spectrometry, Mass, Electrospray Ionization / Q000295:instrumentation* / Q000379:methods*; D014934:Wood / Q000737:chemistry
chemical_list: D011091:Polyesters; D011524:Protoporphyrins; D020959:Polyethylene; C028025:protoporphyrin IX