title: Sensitive ionization of non-volatile analytes using protein solutions as spray liquid in desorption electrospray ionization mass spectrometry.
abstract: BACKGROUND
  Desorption electrospray ionization (DESI) is the most popular ambient ionization technique for direct analysis of complex samples without sample pretreatment. However, for many applications, especially for trace analysis, it is of interest to improve the sensitivity of DESI-mass spectrometry (MS).
  
  
  METHODS
  In traditional DESI-MS, a mixture of methanol/water/acetic acid is usually used to generate the primary ions. In this article, dilute protein solutions were electrosprayed in the DESI method to create multiply charged primary ions for the desorption ionization of trace analytes on various surfaces (e.g., filter paper, glass, Al-foil) without any sample pretreatment. The analyte ions were then detected and structurally characterized using a LTQ XL mass spectrometer.
  
  
  RESULTS
  Compared with the methanol/water/acetic acid (49:49:2, v/v/v) solution, protein solutions significantly increased the signal levels of non-volatile compounds such as benzoic acid, TNT, o-toluidine, peptide and insulin in either positive or negative ion detection mode. For all the analytes tested, the limits of detection (LODs) were reduced to about half of the original values which were obtained using traditional DESI. The results showed that the signal enhancement is highly correlated with the molecular weight of the proteins and the selected solid surfaces.
  
  
  CONCLUSIONS
  The proposed DESI method is a universal strategy for rapid and sensitive detection of trace amounts of strongly bound and/or non-volatile analytes, including explosives, peptides, and proteins. The results indicate that the sensitivity of DESI can be further improved by selecting larger proteins and appropriate solid surfaces.
authors.affiliation: Jiangxi Key Laboratory for Mass Spectrometry and Instrumentation, East China Institute of Technology, Nanchang, Jiangxi Province, 330013, PR China.
authors.forename: Zhiqiang
authors.initials: Z
authors.lastname: Zhu
authors.forename: Jing
authors.initials: J
authors.lastname: Han
authors.forename: Yan
authors.initials: Y
authors.lastname: Zhang
authors.forename: Yafei
authors.initials: Y
authors.lastname: Zhou
authors.forename: Ning
authors.initials: N
authors.lastname: Xu
authors.forename: Bo
authors.initials: B
authors.lastname: Zhang
authors.forename: Haiwei
authors.initials: H
authors.lastname: Gu
authors.forename: Huanwen
authors.initials: H
authors.lastname: Chen
mesh_terms: D000818:Animals; D019817:Benzoic Acid / Q000032:analysis; D053834:Explosive Agents / Q000032:analysis; D006801:Humans; D007328:Insulin / Q000032:analysis; D007477:Ions / Q000737:chemistry; D057230:Limit of Detection; D010455:Peptides / Q000032:analysis; D011506:Proteins / Q000737:chemistry; D012996:Solutions / Q000737:chemistry; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D014052:Toluidines / Q000032:analysis; D014303:Trinitrotoluene / Q000032:analysis
chemical_list: D053834:Explosive Agents; D007328:Insulin; D007477:Ions; D010455:Peptides; D011506:Proteins; D012996:Solutions; D014052:Toluidines; D014303:Trinitrotoluene; D019817:Benzoic Acid; C023622:2-toluidine