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Mass Spectrometry Ionisation Techniques

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Mass Spectrometry Ionisation Techniques Title Text Mass Spectrometry Ionisation Techniques How do we handle different types of sample ? Types of ionisation techniques ◗ Volatile samples – Electron Ionisation – Chemical Ionisation – GC (and LC) inlets ◗ Non-volatile samples – Fast Atom Bombardment –Thermospray – Matrix Assisted Laser Desorption Ionisation – Electrospray Ionisation – Atmospheric Pressure Chemical Ionisation – LC (and GC) inlets Comparison of Ionisation Techniques 200,000 ESI 15,000 1,000 APCI Molec. TSP FAB Weight EI Non Highly Polar Polar Electron Ionisation ◗ Widely used technique when coupled to GC ◗ Suitable for volatile organic compounds – eg hydrocarbons, oils, flavours, fragrances ◗ Not really coupled to LC today ◗ Also called electron impact Electron Ionisation ◗ Produces M+. radical cation giving molecular weight ◗ Produces abundant fragment ions ◗ Library searchable spectra ◗ Energetic process. A heated filament emits electrons which are accelerated by a potential difference of usually 70eV into the sample chamber. Ionisation of the sample occurs by removal of an electron from the molecule thus generating a positively charged ion with one unpaired electron. Electron Ionisation Filament Extraction lenses Sample Inlet + + + + + + + + + + + + + + + + Collector Source magnets Electron Ionisation M + e- M+. + 2e- Fragmentation M+. A+ B+ A+ M+. B+ Chemical Ionisation ◗ Development from EI ◗ Same compound classes as EI ◗ Gives molecular weight ◗ Softer ionisation technique ◗ Produces M+H+ ions or M - H- ions ◗ Used to produce more abundant molecular ions when the molecule under investigation fragments using EI Chemical Ionisation ◗ Similar ionisation technique to EI except that a reagent gas is introduced into the chamber in excess of the sample ◗ Positive CI uses methane, isobutane or ammonia as reagent gases ◗ Negative CI uses methane reagent gas in electron capture mode ◗ Ionised reagent gas protonate the sample molecules leaving a neutral reagent gas species ◗ Not reproducible from lab to lab, hence no CI libraries. Chemical Ionisation Extraction Filament lenses + + Sample Inlet + + + + + + + + + + + + + + + + + + ++ + + + + + Collector + + + Source magnets Chemical Ionisation Primary ions - +. - CH4 + e CH4 + 2e +. + . CH4 CH3 +H Secondary ions +. + . CH4 + CH4 CH5 + CH3 + + CH3 + CH4 C2H5 +H2 Proton donation + + CH5 + M CH4 + MH Fast Atom Bombardment ◗ Used for large compounds with low volatility (eg peptides, proteins, carbohydrates) ◗ Solid or liquid sample is mixed with a non-volatile matrix (eg glycerol, crown ethers, nitrobenzyl alcohol) ◗ Immobilised matrix is bombarded with a fast beam of Argon or Xenon atoms. Charged sample ions are ejected from the matrix and extracted into the mass analysers ◗ Gives M+H+ or M+Na+ ions ◗ Choosing correct matrix is difficult FAB Source Atom / Ion Gun delivering eg Argon atoms + + + + + + + ++ + + Target + + + Sample + Matrix Lenses Matrix Assisted Laser Desorption Ionisation ◗ Similar process to FAB ◗ Sample is dissolved in matrix which absorbs light from a short pulse of laser of a specific wavelength. The sample becomes ionised and extracted towards the mass analysers ◗ Coupled to Time of Flight MS ◗ Not coupled to LC ◗ High mass range achievable ◗ Calibrants may be external or included in sample ◗ Reproducibility issues Thermospray ◗ First widely used LC/MS interface ◗ Flow rates 0.5 - 1.5 ml/min ◗ Good for polar compounds ◗ LC eluent containing sample and ammonium acetate is pumped through a heated vaporiser. The jet of vapour contains small charged droplets which evaporate under the heat and vacuum expelling charged ions from the surface ◗ Produces M+H+ or M - H- ions ◗ Not commercially available today Thermospray Process Thermospray nozzle Solvent evaporation due to heat and reduced pressure Atmospheric Pressure Ionisation ◗ Most important and widely used LC / MS technique ◗ API two types – Electrospray – Atmospheric Pressure Chemical Ionisation ◗ > 99% new LC/MS use API source ◗ Ionisation takes place outside vacuum region Atmospheric Pressure Ionisation ◗ API coupled to LC or CE or Nanospray ◗ Handle wide range of flow rates ◗ Produce Intense M+H+ ions ◗ Very little fragmentation – Need MS/MS for structural information ◗ Applicable to wide range of compounds ◗ Sample must be in solution Electrospray ◗ Electrospray also known as : –Ionspray – Nanospray –Sonic Spray –“Pure” Electrospray –ESI, ES, IS Electrospray ◗ Softest ionisation technique ◗ Best for polar non-volatile compounds (proteins, peptides, nucleic acids, Pharmaceuticals, natural products) ◗ Coupled to LC at a flow range of 2-1000 ul/min, nanospray (10 nL/min – 2 uL/min) ◗ Ions are ejected from charged vapour droplets to gas phase producing M+H+ or M - H- ions ◗ Can produce multiply charged ions allowing determination of high molecular weight proteins ◗ Not very tolerant of non-volatile salts ESI Probe Electrospray Process Solvent evaporation due Atmospheric Pressure Region to high electric field Electrospray Heated Probe Capillary (+3 to +4 kV) (0 to +196 kV) m m m m m m m m Original Solvent Positive Droplet Evaporation Ions High potential capillary creates electrostatic spray of multiply charged droplets APCI ◗ Atmospheric Pressure Chemical Ionisation, also known as : –APCI – Heated nebuliser – APcI APCI ◗ Used for wide range polarity of compounds ◗ HPLC eluent (up to 2ml/min flow rate) is vaporised at up to 600 oC ◗ The Corona discharge needle ionises solvent molecules. A combination of collisions and charge transfer reactions between the solvent and the analyte results in the transfer of a proton to form either M+H+ or M-H- ions ◗ Compounds can thermally degrade ◗ Multiply charged ions rare ◗ More tolerant to salts APCI Probe APCI Process LC eluent evaporated from heated vaporiser Corona discharge needle ionises solvent to generate a chemical ionisation reagent gas plasma API Publications 1000 900 800 700 600 500 400 300 200 100 0 1991 1992 1993 1994 1995 1996 1997 Halket JM and Down S, LC/MS Update, HD Science, Nottingham Solvent suitability ◗ HPLC buffers – Reversed phase most often used –MeOH, ACN, H2O, – TFA, formic acid, acetic acid, Ammonium formate, ammonium acetate – Normal phase can be used ◗ Non-volatile buffers – OSA, aQa self cleaning source, off-axis probe Orthogonal Sampling Adaptor Orthogonal Sampling Adaptor Blocking disk Heated capillary ESI Probe Cap attachment Focusing ring Liquid drains OSA fitted – 10mM Phosphate solution Problems How would you analyse this compound ? Naphthalene What sample introduction technique could you use ? Which ionisation technique ? A: EI, GC/MS Problems How would you analyse this compound ? Phenacetin What sample introduction technique could you use ? Which ionisation technique ? A: API (either APCI or ESI), LC/MS Problems How would you analyse myoglobin ? Myoglobin is a protein with a molecular weight of 16,951. If the Mass Spectrometer has a mass range of up to 4,000, how can you analyse high molecular weight proteins ? Multiply charged myoglobin ions from ESI (M2-1.008) /M1-M2 = Z1 1060.5 100 M1 (Z1 * M1)-(Z*1.008) = Mwt 90 1131.1 998.2 1211.9 M2 942.9 80 1305.0 70 893.3 60 848.6 1413.5 50 40 808.2 1541.9 30 771.5 20 1696.0 616.2 738.1 10 707.3 1310.9 1884.2 1428.7 1563.0 1820.8 1888.9 0 600 800 1000 1200 1400 1600 1800 2000 m/z Deconvoluted myoglobin spectrum 16951.0 100 90 80 70 60 50 40 30 20 10 15931.0 16104.0 16392.0 16582.0 16784.0 17088.0 17280.0 17562.0 17830.0 17995.0 0 16000 16200 16400 16600 16800 17000 17200 17400 17600 17800 18000 mass.
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