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Direct analysis: Current progress and future potential

Direct analysis: Current progress and future potential. Jim Scrivens Centre for Biomedical Mass Spectrometry and Proteomics University of Warwwick. Ambient ionisation mass spectrometry. Advantages of direct desorption mass spectrometry approaches. Ambient mass spectrometry Rapid analysis

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Direct analysis: Current progress and future potential

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  1. Direct analysis: Current progress and future potential Jim Scrivens Centre for Biomedical Mass Spectrometry and Proteomics University of Warwwick

  2. Ambient ionisation mass spectrometry

  3. Advantages of direct desorption mass spectrometry approaches • Ambient mass spectrometry • Rapid analysis • Little or no sample preparation • High sensitivity • Very wide application range

  4. EESI [Extractive electrospray] DAPPI [Desorption atmospheric pressure photoionisation] LAESI [Laser ablation electrospray ionisation] DeSSI [Desorption sonic spray ionisation] PADI [Plasma assisted desorption/ionisation] Paper Spray DESI [Desorption electrospray ionisation ] Reactive DESI DAPCI [Desorption atmospheric pressure chemical ionisation] ASAP [Atmospheric solids analysis probe] DART [Direct analysis in real time] ELDI [Electrospray assisted laser desorption/ionisation] LTP Low temperature plasma Direct ionisationmethods

  5. Ambient ionisation methods: - Issues • Sensitivity • Range of polarities • Compound classes that can be studied • Compatibility with instrumentation • Sample damage • Mass range • Sampling/ionisation coupled or decoupled • Spatial resolution

  6. Need for additional experimental data • Protonated, or deprotonated, molecule ion often formed • Samples are often present in a complex mixture • Additional structural information is required • A range of polarities may be present

  7. Direct desorption Cooks et al.: Science 311 (2006) 1566-1570

  8. DESI • A pneumatically assisted electrospray is directed onto an analyte surface, coupled with collection of the desorbed ions by the mass spectrometry inlet. • Charged solvent droplets collide with the analytes on the surface which are desorbed and ionisation is thought to occur by a droplet ‘pick-up’ mechanism • Charge transfer between gaseous ions generated by ESI and the analyte surface – desorption of ions by a chemical sputtering method

  9. DAPCI • In this approach gaseous reagent ions impact the surface. • Volatilisation followed by gas-phase ionisation via ion-molecule reactions using a corona discharge ion source.

  10. Solventless DAPCI Thermal vaporisation of condensed-phase samples followed by ionisation by a corona discharge

  11. Atmospheric Solids Analysis Probe (ASAP) In the ASAP method, vaporization of the sample occurs when it is exposed to a hot nitrogen desolvation gas from an ESI or APCI probe Useful for the analysis of volatiles and semivolatiles in tissue and other solid material samples

  12. DESI Schematic

  13. DART This non-contact ion source consists of a remote non-thermal plasma from which charged species are rejected with a grid system and metastable species are directed toward the surface of the analyte. Cody R.B., Laramee J.A., Durst H.D.Versatile New Ion Source for the Analysis of Materials in Open Air under Ambient ConditionsAnal. Chem., 2005, 77(8), 2297

  14. Schematic of DART source

  15. Mechanism of DART ionisation • Transfer of energy to the surface by metastable atoms and molecules leads to sample desorption and ionization. • When helium is used the mechanism of ionization can involves the formation of ionized water clusters followed by proton transfer reactions. • Dopants such as ammonium (from ammonium hydroxide headspace vapour) or chloride (from methylene chloride) ions can modify the chemistry of ion formation.

  16. Schematic of DESI and DART sources

  17. Recently introduced DART SVP source DART (Simplified voltage and pressure [SVP] source) Versatile sample placement More rugged design Easier to operate, conserves gas and uses less energy Enables the use of low cost nitrogen gas. Uses iPod for setup and control

  18. Presentation of sample using DESI in our laboratory Sampled for 1-5s, solvent and gas immediately switched off [OR] Sample mounted onto surface within closed source

  19. Anadin Extra 45mg caffeine 200mg paracetamol 300mg aspirin [M+H]+ [M+H]+ (A) (B) 195.07 195.09 100 100 [M+H]+ 163.04 198.07 152.07 121.03 198.08 163.04 121.02 [M+H]+ % % [M+H]+ 152.07 203.03 [M+H]+ 181.04 181.04 0 m/z 0 m/z 60 80 100 120 140 160 180 200 60 80 100 120 140 160 180 200 (C) [M+H]+ (D) 138.0675 100 [M+H]+ % [M+H]+ 110.0729 195.0882 139.0703 0 m/z 50 75 100 125 150 175 200 225 195 138 DESI DAPCI DART DAPCI MS/MS

  20. Reactive DESI

  21. Reactive DESI MS/MS

  22. DESI: Ibuprofen gel detected off skin after 30 minutes [M-H]- 205.15 100 % 206.15 0 m/z 50 100 150 200 250 300 350 161.1321 205 100 % [M-H]- 205.1229 161 0 m/z 100 125 150 175 200 225 250 5% w/w ibuprofen

  23. DESI spectrum α-methyl styrene [4 + Ag]+ n = 3 [4 + Ag]+ n = 4 100 [4 + Ag]+ n = 5 [4 + Ag]+ n = 6 [4 + Ag]+ n = 7 % [4 + Ag]+ n = 8 [4 + Ag]+ n = 2 [4 + Ag]+ n = 9 [4 + Ag]+ n = 10 0 200 400 600 800 1000 1200 1400 1600 1800 m/z

  24. DESI MS/MS spectrum α-methyl styrene A2 100 B1* % [M + Ag]+ G1 A1 A1* G2 B1 B2 0 100 200 300 400 500 600 700 800 900 1000 1100 m/z

  25. Polarity switching DESI Anadin extra [M+H]+ [M+H]+ [M-H-CH2=CO]- [M-H]- [M-H]- 45mg caffeine 200mg paracetamol 300mg aspirin Paracetamol Caffeine Aspirin

  26. Electrospray assisted laser desorption/ionisation (ELDI) • Utilizes a nitrogen laser pulse to desorb intact molecules from matrix-containing sample solution droplets. • This is followed by electrospray ionization (ESI) post-ionization. • The ELDI source has been coupled to a quadrupole ion trap mass spectrometer and allows sampling under ambient conditions. • Preliminary data showed that ELDI produces ESI-like multiply charged peptides and proteins up to 29 kDa carbonic anhydrase and 66 kDa bovine albumin from single-protein solutions, as well as from complex digest mixtures. • Spatial resolution should be higher than DESI. • Separates desorption and ionisation steps. Ivory X. Peng, JentaieShiea, Rachel R. Ogorzalek Loo and Joseph A. Loo Rapid Commun. Mass Spectrom. 2007; 21: 2541–2546

  27. Schematic of ELDI experiment

  28. ELDI experimental results

  29. Sonic spray ionization (SSI) • In DESI, high voltages (typically 4 kV) are applied to the capillary tip • As with conventional ESI, sonic spray ionization (SSI) uses polar (typically methanol/water) solutions of the analyte that are sprayed from a fused-silica capillary with a supersonic nebulizing gas flow coaxial to the capillary. • In SSI however neither heating nor voltage is used for ion formation. • Charged droplets and consequently gaseous ions are produced at atmospheric pressure due to statistical (unbalanced) charge distribution during droplet formation in the supersonic pneumatic spray. • SSI is therefore gentler than ESI, producing ions of lower internal energy and lower charge states. • Electrosonic spray ionization (ESSI) is a variant of SSI, and its main feature is the variable electrostatic potential that can be tuned for most efficient ionization.

  30. Schematic of desorption sonic spray ionization (DeSSI) Renato Haddad, Regina Sparrapan and Marcos N. EberlinRapid Commun. Mass Spectrom. 2006; 20: 2901–2905

  31. Sonic spray ionisation resultsa) aspirin tablet (-ve ion) b) MS/MS of protonated amlopidine

  32. Laser ablation electrospray ionisation (LAESI) • Uses mid-infrared laser ablation coupled with electrospray ionisation. • Used for ambient ionisation of complex biological fluids and tissue samples with water content. • Does not need an external matrix. • Can provide profiling information. • Depends on water content of the sample.

  33. Schematic of laser ablation electrospray ionisation (LAESI) experiment Peter Nemes and AkosVertes Anal. Chem. 2007

  34. LAESI-MS analysis of whole blood and serum. • Phosphocholine and glycerophosphocholines (GPC) can be identified. • The mass spectrum is dominated by the heme group of human hemoglobin. • Deconvolution of the spectrum of multiply charged ions (inset) in the higher m/z region identified the α- and β-chains of human hemoglobin with neutral masses of 15,127 Da and 15,868 Da, respectively.

  35. Desorption Atmospheric Pressure Photoionization [DAPPI] • In the DAPPI technique, a confined heated vapour jet from a heated nebulizer microchip and photons emitted by a krypton discharge lamp are directed toward the sample. The vapour jet and photons desorb and ionize analytes from the surface, and the ions are collected into a mass spectrometer. Markus Haapalaet al Anal. Chem. 2007, 79, 7867-7872

  36. Desorption Atmospheric Pressure Photoionization [DAPPI]

  37. Mechanism of DAPPI • The desorption/ionization mechanism of DAPPI is a combination of thermal and chemical processes. • Since the intensity increases with the temperature of the vapour jet, the desorption is probably largely thermal, although the dissolving properties of the solvent may enhance the desorption. • The effect of the nebulizer gas velocity in the desorption process differs in DAPPI and DESI, since the gas velocity in DAPPI is only a fraction of the velocity of solvent droplets in DESI.

  38. Mechanism of DAPPI • In DAPPI, with a gas flow rate of 180 mL/min, the average linear velocity at the chip nozzle is 30 m/s and lower further in the jet, while in DESI, the mean velocity of the solvent droplets is typically 120 m/s. In addition, in DAPPI the high temperature of the chip vaporizes the solvent efficiently and it is not probable that actual droplets exist in the heated vapour jet. • The ionization in DAPPI seems to be similar to that in APPI, where photons emitted by the UV lamp initiate the ionization process.

  39. Mechanism of DAPPI • The presence of dopant-like solvent (toluene or acetone) is necessary for the ionization of the analytes, which suggests that the ionization in DAPPI is initiated by the photoionization of the dopant, just as in APPI. • Similarly to APPI, the selectivity of ionization in DAPPI can be controlled by choosing solvents that promote either charge exchange or proton transfer.

  40. Mechanism of DAPPI

  41. DAPPI results (A)DAPPI-MS toluene as solvent (B)DAPPI-MS acetone as solvent • DESI-MS water/methanol/formic acid (50/50/+0.1%) as solvent. • The amounts of anthracene, testosterone, MDMA, and verapamil were 10, 1, 10, and 10 pmol, respectively, with DAPPI and 10 pmol with DESI.

  42. Plasma assisted desorption/ionisation (PADI)

  43. Plasma assisted desorption/ionisation (PADI) • PADI consists of generating a non-thermal radio frequency-driven atmospheric pressure plasma and directing it onto the surface of the analyte without charged particle extraction. • The PADI source is a non-thermal atmospheric glow discharge which is characterized by a lower operating voltage and higher current characteristics than corona discharges. • This results in a truly non-thermal, or cold, plasma with an operating temperature close to that of the ambient surroundings.

  44. Plasma assisted desorption/ionisation (PADI) • The non-thermal plasma of PADI is cold to the touch and does not heat the sample. • The surface of the analyte is in direct contact with the active part of the plasma. • This direct interaction of the plasma with the sample results in surface interactions not only with metastable helium atoms, thought to be the dominant desorption/ionization mechanism in DART but also with energetic ions and radicals. • The ionisation mechanism is suggested to proceed via a combination of ionized water cluster formation and proton-transfer reactions

  45. Plasma assisted desorption/ionisation (PADI)

  46. Low temperature plasma (LTP) A dielectric barrier discharge is induced The plasma is generated in a probe configuration, allowing analysis of any type of object regardless of size, in the earlier Dielectric Barrier Discharge Ionisation (DBDI) method, the sample was placed between two counter electrodes. Sample is not exposed to the discharge No solvent is needed, the method operates in ambient air rather than in helium or nitrogen Wide application range – Explosives, drugs of abuse, pharmaceuticals, agrochemicals including contamination of food products Anal.Chem., 2008, 80(23), pp 9097-9104

  47. LTP

  48. Paper Spray Analyte ions generated by applying high voltage to paper triangles that have had a small volume of wetting solution loaded. Samples may be pre-loaded in the wetting solution or transferred from surfaces by using the paper as a wipe. It is believed that a high electric potential between the paper and the inlet of the mass spectrometer induces charges that accumulate at the paper apex adjacent to the inlet. Coulombic forces then produce charged droplets which desolvate to form ions, as per ESI Angew.Chem.Int.Ed. 2010, 49, 877-880; Anal. Chem., 2010, 82 (6), pp 2463-2471

  49. Paper Spray

  50. Ambient ionisation methods: - Issues • Sensitivity • Spatial resolution • Range of polarities • Compound classes that can be studied • Compatibility with instrumentation • Sample damage • Mass range • Sampling/ionisation coupled or decoupled

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