Atomic Mass Spectrometry

Atomic Mass Atomic Mass SpectrometrySpectrometry

Yongsik LeeYongsik Lee20042004

IntroductionIntroduction

► Atomic mass spectrometryAtomic mass spectrometry Versatile and widely used toolVersatile and widely used tool All elements can be determinedAll elements can be determined

► Advantage over AOSAdvantage over AOS Detection limits are 1000 times betterDetection limits are 1000 times better Simple spectraSimple spectra Measure atomic isotope ratiosMeasure atomic isotope ratios

►DisadvantageDisadvantage Cost – 100 or 1000 times expensiveCost – 100 or 1000 times expensive Instrumental drift – 5% per hourInstrumental drift – 5% per hour Interference effectsInterference effects

11A General Features11A General Features

► Atomization = same as AOSAtomization = same as AOS► Conversion to ions = same as AOSConversion to ions = same as AOS► Separation of ions by mass-to-charge ratioSeparation of ions by mass-to-charge ratio► Counting the number of ions of each typeCounting the number of ions of each type

Atomic weights in MSAtomic weights in MS

► Atomic mass unit (amu)Atomic mass unit (amu) = dalton (Da)= dalton (Da) Relative scale where mass of C12 is 12 amuRelative scale where mass of C12 is 12 amu Carbon-12 scaleCarbon-12 scale 1 amu = (1/12)(12g / Navo)1 amu = (1/12)(12g / Navo) = 1.66054 x 10-24 g/atom of C12= 1.66054 x 10-24 g/atom of C12

► Exact massExact mass CH4, CH3D, CH2D2, CHD3, CD4CH4, CH3D, CH2D2, CHD3, CD4

► Nominal massNominal mass A whole-number precision A whole-number precision

Chemical atomic weightChemical atomic weight

► Average atomic weightAverage atomic weight Fractional abundance x iFractional abundance x i

sotope masssotope mass►m/zm/z

Atomic or molecular maAtomic or molecular mass / charges of ionsss / charges of ions

If Z=+1, m/z = massIf Z=+1, m/z = mass

Atomic mass of ZrAtomic mass of Zr

► (51.5 x 90) + (11.2 x (51.5 x 90) + (11.2 x 91) + (17.1 x 92) + 91) + (17.1 x 92) + (17.4 x 94) + (2.8 x 96) (17.4 x 94) + (2.8 x 96) = 9131.8 = 9131.8

► The average mass of The average mass of these 100 atoms would these 100 atoms would be 9131.8 / 100 = 91.3 be 9131.8 / 100 = 91.3 (to 3 significant (to 3 significant figures).figures).

► 91.3 is the relative 91.3 is the relative atomic mass of atomic mass of zirconium.zirconium.

MS of chlorine moleculeMS of chlorine molecule► FragmentationFragmentation

When chlorine is passed into the ionisation chamber, an electron is knocked off the When chlorine is passed into the ionisation chamber, an electron is knocked off the molecule to give a molecule to give a molecular ion, molecular ion, Cl2+. Cl2+.

These ions won't be particularly stable, and some will fall apart to give a chlorine atoThese ions won't be particularly stable, and some will fall apart to give a chlorine atom and a Cl+ ion.m and a Cl+ ion.

► If the Cl If the Cl atomatom formed isn't then ionised in the ionisation chamber, it simply gets formed isn't then ionised in the ionisation chamber, it simply gets lost in the machine - neither accelerated nor deflected.lost in the machine - neither accelerated nor deflected.

► The Cl+ ions will pass through the machine and will give lines at 35 and 37, deThe Cl+ ions will pass through the machine and will give lines at 35 and 37, depending on the isotope.pending on the isotope.

► You will also record lines for the You will also record lines for the unfragmentedunfragmented Cl2+ ions. Cl2+ ions.► Both atoms could be 35Cl, both atoms could be 37Cl, or you could have one of Both atoms could be 35Cl, both atoms could be 37Cl, or you could have one of

each sort. That would give you total masses of the Cl2+ ion of:each sort. That would give you total masses of the Cl2+ ion of: 35 + 35 = 7035 + 35 = 70 35 + 37 = 7235 + 37 = 72 37 + 37 = 7437 + 37 = 74

► That means that you would get a set of lines in the m/z = 70 regionThat means that you would get a set of lines in the m/z = 70 region

MS of ChlorineMS of Chlorine

Types of atomic MSTypes of atomic MS

►Hyphenated methodsHyphenated methods ICP-MSICP-MS DCP-MSDCP-MS MIP-MSMIP-MS

► SSMS (spark source)SSMS (spark source)► TIMS (thermal ionization)TIMS (thermal ionization)►GDMS (glow discharge)GDMS (glow discharge)► LMMS (laser microprobe)LMMS (laser microprobe)► SIMS (secondary ion)SIMS (secondary ion)

11B Mass spectrometers11B Mass spectrometers

Ion deflectionIon deflection► Atoms can be deflected by magnetic fields - provided the atom is first turned intAtoms can be deflected by magnetic fields - provided the atom is first turned int

o an ion. o an ion. ► Electrically charged particles are affected by a magnetic field although electricallElectrically charged particles are affected by a magnetic field although electricall

y neutral ones aren't.y neutral ones aren't.► Stage 1: IonisationStage 1: Ionisation

The atom is ionised by knocking one or more electrons off to give a positive ion. This The atom is ionised by knocking one or more electrons off to give a positive ion. This is true even for things which you would normally expect to form negative ions (chlorinis true even for things which you would normally expect to form negative ions (chlorine, for example) or never form ions at all (argon, for example). Mass spectrometers ale, for example) or never form ions at all (argon, for example). Mass spectrometers always work with positive ions.ways work with positive ions.

► Stage 2: AccelerationStage 2: Acceleration The ions are accelerated so that they all have the same kinetic energy.The ions are accelerated so that they all have the same kinetic energy.

► Stage 3: DeflectionStage 3: Deflection The ions are then deflected by a magnetic field according to their masses. The lighter The ions are then deflected by a magnetic field according to their masses. The lighter

they are, the more they are deflected.The amount of deflection also depends on the nthey are, the more they are deflected.The amount of deflection also depends on the number of positive charges on the ion - in other words, on how many electrons were knumber of positive charges on the ion - in other words, on how many electrons were knocked off in the first stage. The more the ion is charged, the more it gets deflected.ocked off in the first stage. The more the ion is charged, the more it gets deflected.

► Stage 4: DetectionStage 4: Detection The beam of ions passing through the machine is detected electrically.The beam of ions passing through the machine is detected electrically.

Basic Principle - deflectionBasic Principle - deflection

► subject something to a subject something to a sideways force, it will sideways force, it will move in a curvemove in a curve deflected out of its original deflected out of its original

pathpath► The amount of deflection The amount of deflection

for a given sideways force for a given sideways force depends on the mass of the depends on the mass of the

ball. ball. ► If you knew If you knew

the speed of the ballthe speed of the ball the size of the force, the size of the force,

► Then you could calculate Then you could calculate the mass of the ballthe mass of the ball The less the deflection, the The less the deflection, the

heavier the ball.heavier the ball.

Mass analyzerMass analyzer

► separate ions to measure m/z and intensity

► resolutionresolution

Electron multipliersElectron multipliers

►General characteristicsGeneral characteristics Rugged and reliableRugged and reliable High current gainsHigh current gains Nanosecond response timeNanosecond response time Requires enough Kinetic energy ionsRequires enough Kinetic energy ions

►Acceleration required for quadrupolesAcceleration required for quadrupoles► typestypes

Discrete dynode empDiscrete dynode emp Continuous dynode empContinuous dynode emp

Electron multipliersElectron multipliers► Discrete dynode electron multiplier Discrete dynode electron multiplier

(figure 11-2 a)(figure 11-2 a) Similar to PMTSimilar to PMT Cu/Be surface Cu/Be surface

► Burst of electronsBurst of electrons► when struck by energetic ions or elewhen struck by energetic ions or ele

ctronsctrons 20 dynodes = gain of 10 million20 dynodes = gain of 10 million

► Continuous dynode electron multipliContinuous dynode electron multiplier (figure 11-2 b)er (figure 11-2 b) Trumpet shaped Trumpet shaped Made of glass of highly Pb dopedMade of glass of highly Pb doped Electrical potential across the length Electrical potential across the length

of transducerof transducer Gain of 100 thousand – 100 millionGain of 100 thousand – 100 million

Multi Cannel Plate (MCP)Multi Cannel Plate (MCP)

Faraday CupFaraday Cup► principleprinciple

Ions strike the collector Ions strike the collector electrodeelectrode

Charge of the positive ions is Charge of the positive ions is neutralized by a flow of neutralized by a flow of electronselectrons

Electron flow cause a Electron flow cause a potential drop over a resistorpotential drop over a resistor

High impedance amplifierHigh impedance amplifier► advantagesadvantages

Independent of the energy, Independent of the energy, mass, chemical nature of the mass, chemical nature of the ionion

Inexpensive and simpleInexpensive and simple► disadvantagesdisadvantages

Low response speedLow response speed Less sensitive than EMPLess sensitive than EMP

Photographic platePhotographic plate

►Photographic platePhotographic plate Silver bromide emulsion are sensitive to energetiSilver bromide emulsion are sensitive to energeti

c ionsc ions Frequently used for spark source instrumentFrequently used for spark source instrument Simultaneous observation of wide m/z possibleSimultaneous observation of wide m/z possible

Magnetic sector analyzerMagnetic sector analyzer

Analyzing ionsAnalyzing ions

►For fixed radius and charge can use permanent magnet, vary A and B

potential (V) or variable electromagnet, fixed A and B

potential (V)

Double focusing analyzerDouble focusing analyzer

►Single-focusing magnetic sector analyzers have Rmax < 2000 translational energy aberrations angular aberrations Addition of electrostatic analyzer simultaneously

minimizes both►Electrostatic analyzer focuses ions of unique

m/z at entrance slit to magnetic sector

Quadrupole analyzerQuadrupole analyzer► ions travel parallel to four r

ods opposite pairs of rods have r

apidly alternating potentials (AC)

ions try to follow alternating field in helical trajectories

stable path only for one m/z value for each field frequency

► Harder to push heavy molecule - m/zmax < 2000

► Rmax ~ 500

Time of flight analyzerTime of flight analyzer

►Generate pulse of ions (by laser, electrons) with same initial energy

►Ions travel down field-free tube separate according to mass light ions arrive first, heavy ions arrive later Unlimited mass range m/zmax > 100 kDa

►Poor resolution Rmax < 1000►Poor sensitivity

TOFTOF

SummarySummary► One of most powerful analytical tools:

sensitive (10-6 to <10-13 g) range of ion sources for different situations elemental composition for small and large MW- biomolecules limited structural information qualitative and quantitative analysis of mixtures composition of solid surfaces isotopic information in compounds

► But complex instrumentation expensive

► high resolution structure obtained indirectly complex spectra/fragmentation for hard ionization sources simple spectra for soft ionization sources

ICPMSICPMS

Spark source MSSpark source MS