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Mass Spectrometry A.) Introduction : Mass Spectrometry (MS) measures the atomic or molecular weight of a ion from the separation based on its mass to charge.

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Presentation on theme: "Mass Spectrometry A.) Introduction : Mass Spectrometry (MS) measures the atomic or molecular weight of a ion from the separation based on its mass to charge."— Presentation transcript:

1 Mass Spectrometry A.) Introduction : Mass Spectrometry (MS) measures the atomic or molecular weight of a ion from the separation based on its mass to charge ratio (m/z) - elemental composition of matter - structures of inorganic, organic and biological molecules - qualitative and quantitative composition of complex mixtures - isotopic ratios of atoms in the sample One of the MOST Routinely used Analytical Techniques

2 Mass Spectrometry Nobel Laureates: Joseph John Thomson Physics 1906 first mass spectrometer Francis William Aston Chemistry 1922 mass spectrometry of isotopes Wolfgang Paul Physics 1989 quadrupole and quadrupole ion trap MS John B. Fenn Chemistry 2002 electrospray ionization of biomolecules Koichi Tanaka Chemistry 2002 Matrix-assisted laser Desoprtion/ionization (MALDI) A Long and Continuing History of Achievements

3 Mass Spectrometry Abundance Qualitative Information Quantitative Information Mass Spectrometry Data Qualitative Analysis - Molecular Weight Determination - Structure Determination Quantitative Analysis - Biotechnology  analysis of proteins & peptides  analysis of oligonucleotides - Pharmaceutical  drug discovery, combinatorial chemistry  pharmokinetics, drug metabolism - Clinical  neonatal screening, hemoglobin analysis  drug testing - Environmental  water, food, air quality (PCDs etcs) - Geological  oil composition - Toxicology - Forensics

4 Mass Spectrometry Advantages Over Atomic Optical Spectrometric Detection limits three orders of magnitude betterDetection limits three orders of magnitude better Remarkably simple spectra that are unique and easily interpretedRemarkably simple spectra that are unique and easily interpreted Ability to measure isotopic ratiosAbility to measure isotopic ratiosDisadvantages Instrument costs are two to three times higherInstrument costs are two to three times higher Instrument drift that can be as high as 5- 10% per hourInstrument drift that can be as high as 5- 10% per hour Interference effectsInterference effects

5 Atomic Weights in MS Discriminates among the masses of isotopesDiscriminates among the masses of isotopes - differs from other analytical techniques Atomic mass units (amu) or daltons (Da)Atomic mass units (amu) or daltons (Da) - Relative scale:  exactly 12 amu - amu or Da equals 1/12 mass of  x g/atom All measured masses relative toAll measured masses relative to - has a mass times Da x = Da  atomic weight = g/mol Exact Mass (m)Exact Mass (m) - Sum of specific set of isotopes within compounds  12 C 1 H 4 : m = x x 4 = Da  13 C 1 H 4 : m = x x 4 = Da  12 C 1 H 3 2 H 1 : m = x x x 1 = Da  3-4 significant figures to right of decimal Nominal Mass (m)Nominal Mass (m) - Whole number precision in mass measurement  12 C 1 H 4 : m = 12 x x 4 = 16 Da  13 C 1 H 4 : m = 13 x x 4 = 17 Da  12 C 1 H 3 2 H 1 : m = 12 x x x 1 = 17 Da

6 Atomic Weights in MS Chemical atomic weight or average atomic weightChemical atomic weight or average atomic weight  A 1, A 2, A n :atomic masses in Da  p 1, p 2, p n :fractional abundance of each isotope  n :number of isotopes Weight of interest for most purposesWeight of interest for most purposes Sum of chemical atomic weights for the atoms in the compound formulaSum of chemical atomic weights for the atoms in the compound formula - CH 4 (m) = x = Da - typical atomic masses in periodic table Boron: 10 B 23% 11 B 100% B (m) = (23x x11)/123 = Zirconium: 90 Zr 51.5% 91 Zr 11.2% 92 Zr 17.1 % 94 Zr 17.4% 96 Zr 2.8% Zr (m) = (51.5x x x x96)/100 = 91.22

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8 Molecular Formulas from Exact Molecular Weights Requires identification of molecular ion peakRequires identification of molecular ion peak Exact mass needs to be determinedExact mass needs to be determined  High resolution instruments  detect mass differences of a few thousands of amu - Purine, C 5 H 4 N 4 (m = ) - Benzamidine, C 7 H 8 N 2 (m = ) - Ethyltolune, C 9 H 12 (m = ) - Acetophenone, C 8 H 8 O (m = )  Molecular ion peak is ±  only C 7 H 8 N 2 is possible formula  Precision of a few parts per million is routinely possible

9 Molecular Formulas from Isotopic Ratios Low-resolution instrument  only differentiate whole number massesLow-resolution instrument  only differentiate whole number masses Requires sufficiently intense molecular ion peakRequires sufficiently intense molecular ion peak Requires accurate heights for (M+1) + and (M+2) +Requires accurate heights for (M+1) + and (M+2) + C6H4N2O4C6H4N2O4C6H4N2O4C6H4N2O4 13 C 6 x 1.08 = H2H2H2H 4 x = 0.060% 15 N 2 x 0.37 = 0.74% 17 O 4 x 0.04 = 0.16% (M+1) + /M + = 7.44% (M+1) + /M + = 7.44% C 12 H C 12 x 1.08 = 12.96% 2H2H2H2H 24 x = 0.36% (M+1) + /M + = 13.32% (M+1) + /M + = 13.32%

10 Resolution in MS (R) differentiate between massesdifferentiate between masses R = m/  m or or Higher the number the better the resolutionHigher the number the better the resolution  500,000 is better than 500  Resolution of 4000 would resolve peaks occurring at m/z values of:  and  and 40.01

11 Example 22: Calculate the resolution to differentiate (a) C 2 H 4 + (m = ) and CH 2 N + (m = ) and (b) N 2 + (m = ) and CO + (m = ).

12 Identification of odd electron ions – The Nitrogen Rule Based on an anomaly in the relationship between the atomic weights and valences of the common elementsBased on an anomaly in the relationship between the atomic weights and valences of the common elements An organic molecule containing the elements C, H, O, S, P or halogenAn organic molecule containing the elements C, H, O, S, P or halogen - odd nominal mass if it contains an odd number of nitrogen atoms - even nominal mass if it contains an even number of nitrogen atoms (including 0) ElementAt WtValence H11 C124 N143 O162 F191 S322 Cl35/371 CompoundMW CO 2 44 CO28 CH 4 16 HCO 2 H46 HCl36/38 HCN27 N2N2 28 NH 3 17 Even nominal mass  even valence Odd nominal mass  odd valence Nitrogen is the exception

13 Double Bond Equivalent (D) number of rings or double bonds that an ion containsnumber of rings or double bonds that an ion contains Calculated from the elemental formula as follows:Calculated from the elemental formula as follows:where:  D is unsaturation  i max is the total number of different elements  N i the number of atoms of element i and  V i the valence of atom i valence of 1 (H, F, Cl), valence of 2 (O, S).valence of 1 (H, F, Cl), valence of 2 (O, S). valence of 3 (N, P), valence of 4 (C, Si).valence of 3 (N, P), valence of 4 (C, Si). Hexane: C 6 H 14 Mass of molecular ion: 86 D = 1+1/2((6x(4-2) + 14x(1-2)) = 0 Hexene: C 6 H 12 Mass of molecular ion: 84 D = 1+1/2((6x(4-2) + 12x(1-2)) = 1

14 Basic MS Instrument Design Atomic Mass Spectrometry involves the following steps :Atomic Mass Spectrometry involves the following steps : - atomization - conversion of atoms to ions  most ions are single charge - separation of ions based on mass-to-charge ratio (m/z) - Counting the number of ions of each type or measuring the ion current Principal ComponentsPrincipal Components - Vacuum system  maintain low pressure(10 -5 to torr) - Inlet: introduce  -amount of sample into ion source - Ion source: sample converted into gaseous ion by bombardment with:  Electrons  Photons  Ions  Molecules  Thermal/electric energy - Positive/negative ions accelerated into mass analyzer into mass analyzer - Mass analyzer: disperse ions (m/z) - Transducer: convert beam of ions to electrical signal electrical signal

15 Types of Atomic and Molecular MS Thermal ionization & Spark source  first MSThermal ionization & Spark source  first MS Inductively coupled plasma (ICP)  current common approachInductively coupled plasma (ICP)  current common approach - Differ by types ion sources and mass analyzer

16 MS Theory: 1.) Mass analyzers use electric and magnetic fields to apply a force on charged particles F = ma (Newton's second law) F = e(E+ v x B) (Lorentz force law) where: F - force applied to the ionm - mass of the ion a - acceleratione - ionic charge v x B - vector cross productE - electric field v x B - vector cross product E - electric field of the ion velocity and the applied magnetic field 2.) Force is therefore dependent on both mass and charge - spectrometers separate ions according to their mass-to-charge ratio (m/z) - not by mass alone

17 Inlet Systems: Introduce sample into ion source with minimum loss of vacuumIntroduce sample into ion source with minimum loss of vacuum Spectrometer equipped with multiple inlets for different sample typesSpectrometer equipped with multiple inlets for different sample types  Batch Inlet  Direct Probe Inlet  Gas Chromatography  Liquid Chromatography

18 External (Batch) Inlet Systems: Simplest Simplest Gas & liquid samples (bp < 500 o C) Gas & liquid samples (bp < 500 o C) Sample heated (<400 °C) in small external oven Sample heated (<400 °C) in small external oven Sample pressure to torr Sample pressure to torr Vapor admitted to ionizer through valve Vapor admitted to ionizer through valve Gas stream added to analyte Gas stream added to analyte Inlet Systems:

19 Direct probe inlet system for solids Direct Probe Solids and non-volatile samples Solids and non-volatile samples - Less sample is required & wasted (few ng) - sample held on the surface of glass or aluminum capillary tube, fine wire or small cup Sample vial inserted through air-lock into ionizer chamber Sample vial inserted through air-lock into ionizer chamber - Lock system minimizes amount of air that must be pumped from system Vial heated to vaporize sample Vial heated to vaporize sample Vial can be reduced to capillary or surface plate for small quantities Vial can be reduced to capillary or surface plate for small quantities The direct probe is only ¼" in diameter. Probe moves through various lock system stages permits for a step-wise increase in the vacuum Inlet Systems:

20 LC & GC coupled to mass spectrometerLC & GC coupled to mass spectrometer Permits separation and determination of components for complex mixturesPermits separation and determination of components for complex mixtures  Requires specialized inlet systems  Major interface problem – carrier gas dilution  Jet separator (separates analyte from carrier gas) - Lighter carrier gas deflected by volume - Heavier sample travels in straight line

21 Ion Sources: Formation of gaseous analyte ionsFormation of gaseous analyte ions Mass spectrometric methods are dictated by ionization techniquesMass spectrometric methods are dictated by ionization techniques Appearance of spectrum highly dependant on ionization techniqueAppearance of spectrum highly dependant on ionization technique Gas-phaseGas-phase  Sample first vaporized then ionized  Thermally stable compounds boiling points < 500 o C  MW < 100 amu DesorptionDesorption  Solid or liquid directly converted to gaseous ion  MW as large as 10 5 daltons TypeName and AcronymIonizing Process Gas PhaseElectron Impact (EI)Exposure to electron stream Chemical Ionization (CI)Reagent gaseous ions Field Ionization (FI)High potential electrode DesorptionField Desorption (FD)High potential electrode Electrospray Ionization (ESI)High electric field Matrix-assisted desorption ionization (MALDI)Laser beam Plasma Desorption (PD)Fission fragments from 252 Cf Fast Atom Bombardment (FAB)Energetic atomic beam Secondary Ion Mass Spectrometry (SIMS)Energetic beam of ions Thermospray Ionization (TS)

22 Ion Sources: Hard sourcesHard sources  Sufficient energy so analyte are in highly excited energy state  Relaxation involves rupture of bonds - Produces fragment ions with m/z < molecular ion - Kinds of functional groups  structural information Hard Ionization

23 Ion Sources: Soft sourcesSoft sources  Cause little fragmentation  Mass spectrum consists of molecular ion and only few, if any, other peaks  Accurate mass Soft Ionization

24 Ion Sources: Electron-Impact Source (EI)Electron-Impact Source (EI)  Sample heated to produce molecular vapor  Bombard with a beam of electrons - Electrons emitted from heated tungsten or rhenium filament - Electrons accelerated by a potential of 70V - Path of electrons and molecular ion at right angles  Form positive ions  electron beam expels electron due to electrostatic repulsion M + e -  M ● + + 2e -  Not very efficient  one molecule in a million ionized  Positive ions attracted to first slit by small potential 5V  High potential applied at accelerator plates 10 3 to 10 4 V - Generates molecular ion velocity

25 Ion Sources: Electron-Impact Source (EI)Electron-Impact Source (EI)  Hard source 50V higher energy than chemical bond  Highly excited vibrational and rotational state - Electron beam does not increase translational energy  Relaxation results in extensive fragmentation - Large number of positive ions of various masses - Typically less mass than molecular ion - Lower mass ions called daughter ions - Sometimes molecular ion not present

26 Ion Sources: Electron-Impact Source (EI)Electron-Impact Source (EI)  Base peak  most intense peak - Usually a daughter ion or fragment ion  Peaks at MW greater than molecular ion - Same chemical formula but different isotope composition - Size of peak depends on relative natural abundance of isotopes  Collision Product peak (M+1) + - Collision transfers a hydrogen atom to the ion to generate a protonated molecule - Second order reaction  depends on concentration - Increases with increase in pressure

27 Ion Sources: Electron-Impact Source (EI)Electron-Impact Source (EI)  Advantages - Good sensitivity - Fragmentation  unambiguous identification of analytes  Disadvantages - Need to volatize sample  thermal decomposition before ionization - Fragmentation  disappearance of molecular ion peak - MW not determined

28 Ion Sources: Chemical Ionization Source (CI)Chemical Ionization Source (CI)  Electron Impact and Chemical Ionization are Interchangeable in a Spectrometer  Chemical Ionization is the second most common procedure for generating ions  Gaseous atoms from the sample are: - Heated from a probe - Collide with ions produced reagent gas bombarded by electrons - Usually positive ions are used  Need to modify electron beam - Add vacuum pump capacity - Reduce width of slit for mass analyzer - Allow a reagent pressure of 1 torr in ionization area - Keep pressure below torr in analyzer  Concentration ratio of reagent to sample is 10 3 to Electron beam preferentially interacts with reagent instead of sample

29 Ion Sources: Chemical Ionization Source (CI)Chemical Ionization Source (CI)  Soft Source  Methane is common reagent - Also use propane, isobutane and ammonia - Reacts with high-energy electron beam to generate several ions - CH 4 +, CH 3 + (~90% of product) and CH React with other methane molecules CH CH 4  CH CH 3 CH CH 4  C 2 H H 2  Collisions Between sample molecule and CH 5 + & C 2 H 5 + are highly reactive - Involve proton or hydride transfer CH MH  MH CH 4 proton transfer C 2 H MH  MH C 2 H 4 proton transfer C 2 H MH  M + +C 2 H 6 hydride transfer - Proton transfer  (M + 1) + - Hydride transfer  (M – 1) + - C 2 H 5 + transfer (M + 29) + EI Hard Ionization CI Soft Ionization

30 Ion Sources: Matrix-Assisted Laser Desorption/Ionization (MALDI)Matrix-Assisted Laser Desorption/Ionization (MALDI)  Accurate MW for polar biopolymers - DNA, RNA, Proteins - Few thousands to several hundred thousand Da  Sample is mixed with large excess of radiation-absorbing matrix material  Solution is evaporated onto solid surface  Sample exposed to pulsed laser beam - Sublimation of analyte ions - MS spectra recorded between laser beam pulses

31 Ion Sources: Matrix-Assisted Laser Desorption/Ionization (MALDI)Matrix-Assisted Laser Desorption/Ionization (MALDI)  Low background noise - At high MW, matrix causes significant background at low MW  Absence of fragmentation  Multiple charged ions (+2, +3)  Observe dimers trimers  Mechanism is not completely understood - Matrix compound must absorb the laser radiation radiation - Soluble enough in sample solvent to be present in large excess present in large excess - Analyte should not absorb laser radiation  Fragmentation will occur Simulation of MALDI

32 Ion Sources: Matrix-Assisted Laser Desorption/Ionization (MALDI)Matrix-Assisted Laser Desorption/Ionization (MALDI)  MALDI spectra are greatly influenced by type of matrix, solvent and additive - At high MW, matrix causes significant background at low MW Dariusz Janecki et al. (2002) ASMS

33 Ion Sources: Electrospray Ionization (ESI)Electrospray Ionization (ESI)  One of the most important techniques for analyzing biomolecules - Polypeptides, proteins and oligonucleotides - Inorganic species synthetic polymers - MW >100,000 Da  Uses atmospheric pressure and temperature  Sample pumped through a stainless steel capillary - Rate of a few  ls per minute - Needle at several KVs potential  Creates charged spray of fine droplets

34 Ion Sources: Electrospray Ionization (ESI)Electrospray Ionization (ESI)  Passes through desolvating capillary - Evaporation of solvent - Attachment of charge to analyte molecule - Molecules become smaller  charge density becomes greater  desorption of ions into ambient gas

35 Ion Sources: Electrospray Ionization (ESI)Electrospray Ionization (ESI)  Little fragmentation of thermally fragile biomolecules  Ions are multiply charged - m/z values are small - Detectable with quadrupole with mass range of 1500 or less  Average charge state increases ~linearly with MW - MW determined from peak distribution

36 Ion Sources: Fast Atom Bombardment Sources (FAB)Fast Atom Bombardment Sources (FAB)  Major role for MS studies of polar high molecular-weight species  Soft Ionization technique - MW > 10,000 - Structural information for MW ~3,000  Samples are in a condensed state - Glycerol solution matrix - Liquid matrix helps reduce lattice energy  Ionized by bombardment with energetic (several keV) xenon or argon atoms - Very rapid sample heating - Reduces sample fragmentation  Positive & negative analyte ions are sputtered from the surface - Desorption process - Must overcome lattice energy to desorb an ion and condense a phase - “healing” the damage induced by bombardment

37 Ion Sources: Fast Atom Bombardment Sources (FAB)Fast Atom Bombardment Sources (FAB)  Beam of fast energetic atoms are generated by: - Passing accelerated argon or xenon ions from an ion source through a chamber - Chamber contains argon or xenon atoms at torr - High-velocity ions undergo a resonant electron-exchange reaction without substantial loss of translational energy Ar + * + Ar > Ar + + Ar 0 * Production of “fast atoms” Charge transfer Accelerated argon ion from “ion gun” Ground state argon atom “slow ion” “fast atom” Focusing Extraction plate Analyte ion beam (secondary ions) Probe tip Analyte metrix Atom beam

38 Example 23: Identify the ions responsible for the peaks in the following mass spectrum for 1,1,1,2-tetrachloroethane (C 2 H 2 Cl 4 ): MW =

39 Mass analyzers: Double-Focusing analyzerDouble-Focusing analyzer  Two devices for focusing an ion beam - Electrostatic analyzer - Magnetic sector analyzer  Ions accelerated through slit into curved electrostatic field - Focus beam of ions with narrow band of kinetic energies into slit  Ions enter curved magnetic field - Lighter ions deflected more than heavier ions

40 Mass analyzers: Double-Focusing analyzerDouble-Focusing analyzer  Curved magnetic field of 180, 90 or 60 degrees - Vary magnetic field strength or accelerating potential to select for ions of different mass  Kinetic energy of accelerated ion (before magnetic field) where: V = voltagev = ion velocity e = electronic charge (1.60x C) z = ion charge  Path through magnet depends on the balance of two forces: - Magnetic force (F M ) and centripetal force (F c ) where: B = magnetic field strengthr = radius of curvature of magnetic sector  Substitute velocity into kinetic energy equation  Select masses by varying B by changing current in magnet

41 Mass analyzers: Double-Focusing analyzerDouble-Focusing analyzer

42 Mass analyzers: Quadrupole mass analyzerQuadrupole mass analyzer  More compact, less expensive, rugged  High scan rate  spectrum in < 100ms Four parallel cylindrical rods serve as electrodesFour parallel cylindrical rods serve as electrodes  Opposite rods are connected electrically - One pair attached to positive side of variable dc source - One pair attached to negative side of variable dc source  Variable radio-frequency ac potential (180 o out of phase) applied to each pair of rods Ions accelerated through space between rodsIons accelerated through space between rods  Potential of 5 to 10 V  ac and dc voltages increased simultaneously with ratio being constant  All ions without specific m/z strike rods and become neutral - only ions having a limited range of m/z reach transducer (detector)

43 Mass analyzers: Quadrupole mass analyzerQuadrupole mass analyzer Positive rodsPositive rods  Alternating ac causes ions to converge during positive arc and diverge during negative arc - If ions strike rod during negative arc  neutralized and removed by vacuum  Striking a rod depends on: - rate of movement through rod - Mass to charge ratio - Frequency and magnitude of the ac signal  dc current effects momentum of ions - Momentum directly related to square-root of mass - More difficult to deflect heavy ions than lighter ions - Prevents heavier atoms from striking rods  High-pass mass filter

44 Mass analyzers: Quadrupole mass analyzerQuadrupole mass analyzer Negative rodsNegative rods  In the absence of ac, all positive ions drawn to rods  annihilated  Offset for lighter ions by ac  Low-pass mass filter For ions to pass through the rods (band of ions):For ions to pass through the rods (band of ions):  Significantly heavy to pass positive rods  Significantly light to pass negative rods - One pair attached to positive side of variable dc source - One pair attached to negative side of variable dc source Adjusting ac & dc moves the center of band of ions which pass the rodsAdjusting ac & dc moves the center of band of ions which pass the rods

45 Mass analyzers: Time of Flight (TOF) Mass AnalyzersTime of Flight (TOF) Mass Analyzers Ions generated by bombardment of the sample with a brief pulse of:Ions generated by bombardment of the sample with a brief pulse of:  Electrons, secondary ions, laser-generated photons  Frequency of pulse 10 to 50 kHz, duration of pulse 0.25  s Ions accelerated by electric field pulse 10 3 to 10 4 VIons accelerated by electric field pulse 10 3 to 10 4 V  Same frequency of ionization pulse, but lags behind Accelerated particle enter field-free drift tubeAccelerated particle enter field-free drift tube  Ions enter tube with same kinetic energy  Ion velocity vary inversely with mass - Lighter particles arrive at detector before heavier particles - Flight times are 1 to 30  s - Requires fast electronics  Peak broadening due to variability in ion energies and initial position - Limits resolution compared to magnets and quadrupole - Less widely used than quadrupole  Advantages: unlimited mass range, rapid data acquisition, simplicity, ruggedness, ease of access to ion source

46 Transducers (detectors) for Mass Spectrometry: Electron MultipliersElectron Multipliers  Detects positive ions  Similar to photomultiplier used in UV/vis - Each dynode held at successfully higher voltage  Dynodes have Cu/Be surfaces - Burst of electrons emitted when struck with energetic ions or electrons  Electron multiplies contain upwards of 20 dynodes - Typical current gain of 10 7  Continuous-Dynode electric multiplier - Glass that is heavily doped with lead - Potential of 1.8 to 2 V across length of transducer - Trumpet shaped - Ions strike surface near entrance that ejects electrons - Electrons then skip along surface ejecting more electrons with each impact - Typical current gain of 10 5 to 10 8  Rugged, reliable with high current gains  Positioned directly at the exit slit of magnet - Ions have enough energy to eject electrons - Requires accelerator with quadrupole MS

47 Transducers (detectors) for Mass Spectrometry: Faraday CupFaraday Cup  hollow collector, open at one end and closed at the other, used to collect beams of ions - incident ion strikes the dynode surface which emits electrons - induces a current which is amplified and recorded  Surrounded by cage - Prevents the escape of reflected ions and ejected secondary electrons  Inclined with respect to ion beam - Particles striking or leaving the electrode are reflected away from entrance  Connected to ground potential through a large resistor - Ions striking plate are neutralized by flow of electrons from resistor - Causes a potential drop across resistor  Independent of the energy, mass or chemical nature of ion  Inexpensive and simple mechanical and electronic device  Disadvantages: - Need for a high-impedance amplifier  Limits speed at which spectrum can be scanned - Less sensitive than electron multipliers

48 Atomic Mass Spectra and Interferences: Spectroscopic interferenceSpectroscopic interference  An ionic species in the plasma has the same m/z values as an analyte  Isobaric interference - Two elements that have isotopes with nearly the same mass - Differ by less than 1 amu  113 In + overlaps with 113 Cd + and 115 In + overlaps with 115 Sn + - Isobaric interference occurs with the most abundant and most sensitive isotope  40 Ar + overlaps with 40 Ca + (97%) need to use 44 Ca + (2.1%)  58 Ni + overlaps with 56 Fe + need to use 56 Fe + - Exactly predictable from abundance tables  Polyatomic ion interference - More serious than isobaric - Polyatomic species form from interactions with species in plasma, matrix or atmosphere - Several molecular ions can form and interfere - Typically observed for m/z < 82 amu - Potential interference: 40 Ar 2+, 40 ArH +, 16 O 2 +, H 2 16 O, 16 OH +, 14 N + H 2 16 O, 16 OH +, 14 N + - Serious interference: 14 N 2 + with 28 Si +, NOH + with 31 P +, NOH + with 31 P +, 16 O 2 + with 32 S +, 16 O 2 + with 32 S +, 40 ArO + with 56 Fe +, 40 ArO + with 56 Fe +, 40 Ar 2 + with 80 Se + 40 Ar 2 + with 80 Se + - Correct with blank

49 Atomic Mass Spectra and Interferences: Spectroscopic interferenceSpectroscopic interference  Oxide and Hydroxide species interference - Most serious - Oxides an hydroxides of analyte, matrix solvent plasma gases plasma gases - MO + and MOH + ions - Formation depends on injector flow rate, RF power, sampler skimmer spacing, sample orifice size, plasma gas composition, oxygen elimination solvent removal efficiencies  Matrix Effects - Occur at concentrations > 500 to 1000  g/ml - Reduction in analyte signal, sometimes signal enhancement - General effect - Minimized by using more dilute solutions, altering sample introduction procedure or by separating out offending species - Use internal standard to correct effect Affect of pesticide response to matrix effects Add protectant to remove matrix effect Analytical Sciences 2005, 21, 1291

50 Fourier Transform (FT) MS: Improves signal-to-noise ratiosImproves signal-to-noise ratios Greater SpeedGreater Speed Higher sensitivity and resolutionHigher sensitivity and resolution Requires an Ion TrapRequires an Ion Trap  Ions are circulated in well-defined orbits for extended periods  Ion cyclotron resonance (ICR) - Ions in a magnetic field circulate in a plane perpendicular to the direction of the field - Angular frequency of this motion  cyclotron frequency (  c )  Ion trapped in a magnetic field can absorb energy from an ac electric field - Frequency of field must match cyclotron frequency - Absorbed energy increases velocity of ion and radius without affecting  c

51 Fourier Transform (FT) MS: Detection of ICRDetection of ICR  Ions circulating between plates induces current between plates - Image current - Non-destructive - Decays over a few seconds through collision  Decay over time of the image current after applying an RF pulse is transformed from the time domain into a frequency domain Ions of two different m/q ratios excited on resonance for the same amount of time with the same excitation voltage. Ion [A] has the lower m/q ratio and thus has a higher cyclotron frequency. Ion [B] has the higher m/q ratio and thus a lower cyclotron frequency.


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