1Electrospray Ionization Mass Spectrometry
Jessica GilmanCourtney Mashburn
17 September 2002Chemistry 5181
“Many users tend to view ESI as a ‘Black Box,’ because sources of instability, background, interference, competition, and suppression are not always understood.”2
OutlinelIntroductionlIonization ProcesslIntroduction of Ions into MSlOperational Conditions and ParameterslSolvent and Analyte CharacteristicslSensitivity and Detection LimitslTandem TechniqueslSummary
IntroductionlESI allows for large, non-volatile molecules to be analyzed directly from the liquid phase
lUsed for:lMass determination of biomoleculeslAnalysis and sequencing of proteins and oligonucleotides
lAnalyzing drugs, pesticides, and carbohydrateslLong chain fatty acids3
ElectrosprayIonization ChamberAPI-Electrosprayoccurs at 760 torrProducts = neutrals, ions, and clusters of ions
Ionization MechanismslCoulomb Fission : lAssumes that the increased charge density, due to solvent evaporation, causes large droplets to divide into smaller droplets eventually leading to single ions.
lIon Evaporation: lAssumes the increased charge density that results from solvent evaporation causes Coulombicrepulsion to overcome the liquid’s surface tension, resulting in a release of ions from droplet surfaces4
Charging the AnalytelCharge SeparationlGas-phase ions are formed when the droplets from the Taylor Cone evaporate and the ions carrying excess charge are released into the gas-phase.lAdduct FormationlPolar molecules that do not have acidic or basic groups can be charged through formation of adducts with various ions.
lNegative ion ESI: form adducts with Cl-ions
lPositive ion ESI: form adducts with Na+, Li+, NH4+, or other cationic species
lProblem: High [salt] causes background interference.
Charging the Analyte, Cont’dlGas-Phase Reactions:lIonized at atmospheric pressure.lGenerally through gas-phase proton transfer.lProton goes to species with higher gas-phase proton affinity lAnalyte must have higher proton affinity than the solvent.lElectrochemRedoxRxns: l“The continuous flow of charge from the metallic contact to the sample solution must occur via an electrochemical reaction at that contact.”
lPositive ion ESI: oxidation
lNegative ion ESI: reduction5
Pneumatically-Assisted ESIlMust separate ions from neutrals and establish complete desolvation.
lUse a neutral sheath gaslAids in droplet formation and desolvation.lOff-Axis Positioning
lMax amount of desolvatedanalyte
lSelects against un-evaporated droplets
Introducing Ions into the MSlUse a dry N2“curtain” gas:
lCharged species penetrate the curtain because they are electrostaticallyattracted toward the orifice by an electric field gradient.
lUse a heated metal capillary interfacelAids in desolvationand declusteringof ions from neutrals.6
ESI Operating ConditionslESI operation depends on the ability to balance many variables simultaneously
lSelf-adjusted flow rate of samplelSelf-adjusted voltage between power supply and contact with solution
lCapillary tube parameters:
lNon-conductive: fused silica tubinglConductive: metalizedglass capillarieslInner diameter = flow ratelOuter diameter = Taylor cone
Instrumental ParameterslStable and effective ESI spray conditions:lTaylor cone has constant shapelConstant stream of droplets from Taylor cone
lNebulizinggas and solution flow rates
lApplied voltage, viscosity, and dielectric constantlDistance between spray capillary and counter electrode
lMethanol or pneumatic assistance is required
lHigh water content means high surface tensionlHigher voltages must be applied7
Analyte CharacteristicslSurface-active analyteshave a higher responselThey follow charge during fissioningprocesslESI response is directly related to:
lNonpolarsurface areal∆G transfer from NP to P solutionslReverse-phase HPLC retention time
lDifference between analyte pH and solvent pH
lIf low response, change the molecular structure
lDerivatitationcan make the analyte more easily charged or can increase the surface activity
Surface ActivitylEquimolaramounts and all other variables equal
lSurface–inactive = Cs+lSurface–active = DTMA+
lFor quantitative work, calibration curves, etc. must be employed
lIntensity of peak is not solely related to conc.
