Atomic Mass Spectrometry Nearly all elements in the periodic table can be determined by mass spectrometry Nearly all elements in the periodic table can

Atomic Mass Atomic Mass SpectrometrySpectrometry

Nearly all elements in the periodic Nearly all elements in the periodic table can be determined by mass table can be determined by mass spectrometryspectrometry

More selective and sensitive than More selective and sensitive than optical instrumentsoptical instruments

Simple spectraSimple spectra Isotope ratiosIsotope ratios Much more expensive Much more expensive

instrumentationinstrumentation

Still spectroscopy?

What is a mass spectrometer?

Illustration of the basic components of a mass spectrometry system.

IonizationSource

MassAnalzyer Detector

Inlet all ionsselectedions

DataSystem

Lets talk about mass!Lets talk about mass!

Atomic mass of CarbonAtomic mass of Carbon

Atomic mass of ChlorineAtomic mass of Chlorine

Atomic mass of HydrogenAtomic mass of Hydrogen

Lets talk about mass!Lets talk about mass!

Atomic mass of CarbonAtomic mass of Carbon 12.000000000000000000000000000 12.000000000000000000000000000

amuamu Atomic mass of ChlorineAtomic mass of Chlorine

35.4527 35.4527 amuamu Atomic mass of HydrogenAtomic mass of Hydrogen

1.00794 1.00794 amuamu

1amu = 1 dalton (Da) 1amu = 1 dalton (Da)

What about isotopes?What about isotopes?

Atomic mass of CarbonAtomic mass of Carbon 12.000 amu for 12.000 amu for 1212C but 13.3355 for C but 13.3355 for 1313CC

Atomic mass of ChlorineAtomic mass of Chlorine 34.9688 amu for 34.9688 amu for 3535Cl and 36.9659 for Cl and 36.9659 for

3737ClCl Atomic mass of HydrogenAtomic mass of Hydrogen

1.00794 amu for H and 2.0141 for D!1.00794 amu for H and 2.0141 for D!

Just for clarificationJust for clarification

Atomic massAtomic mass amuamu, atomic mass units (uma??), atomic mass units (uma??) ““Da”Da” or or DaltonDalton. . kD (kiloDalton for kD (kiloDalton for

macromolecules) macromolecules) 1 amu = 1.66056*101 amu = 1.66056*10-27-27 kg. kg. proton, mp = 1.67265*10proton, mp = 1.67265*10-27-27 kg, kg, neutron, mn = 1.67495*10neutron, mn = 1.67495*10-27-27 kg. kg.

Ways to define and calculate the Ways to define and calculate the mass of an atom, molecule or ion mass of an atom, molecule or ion

Average mass: Average mass: calculated using the atomic calculated using the atomic weight, which is the weighted average of the weight, which is the weighted average of the atomic masses of the different isotopes of atomic masses of the different isotopes of each element in the molecule.each element in the molecule.

Often used in stoichiometric calculations.Often used in stoichiometric calculations. Nominal mass: Nominal mass: calculated using the mass calculated using the mass

of the predominant isotopes of each element of the predominant isotopes of each element rounded to the nearest integer value that rounded to the nearest integer value that corresponds to the mass number. corresponds to the mass number.

Monoisotopic massMonoisotopic mass: calculated using the : calculated using the extract mass of the most abundance isotope extract mass of the most abundance isotope for each constituent element.for each constituent element.

Use monoisotopic mass if possible in MSUse monoisotopic mass if possible in MS

Differences between Differences between MassesMasses

C20H42 C100H202

Nominal: (20 x 12) + (42 x1) = 282 u (100x12) + (202x1) = 1402uMonoisotopic: (20 x12) + (42 x 1.007825) = 282.33 (100x12) + (202x1.007825) = 1403.5807Average: (20 x 12.011) + (42 x 1.00794) = 282.5535(100x12.011)+(202x1.00794) = 1404.7039

Exact Masses of Some Common Elements and Their Isotopes:Exact Masses of Some Common Elements and Their Isotopes:

Element Element Symbol Symbol Exact Mass (u) Exact Mass (u) Rel. Abundance % Rel. Abundance %

Hydrogen Hydrogen 1H 1H 1.007825037 1.007825037 100.0 100.0

Deuterium Deuterium 2H or D 2H or D 2.014101787 2.014101787 0.015 0.015

Carbon 12 Carbon 12 12C 12C 12.00000 12.00000 100.0 100.0

Carbon 13 Carbon 13 13C 13C 13.003354 13.003354 1.11223 1.11223

Nitrogen 14 Nitrogen 14 14N 14N 14.003074 14.003074 100.0 100.0

Nitrogen 15 Nitrogen 15 15N 15N 15.00011 15.00011 0.36734 0.36734

Oxygen 16 Oxygen 16 16O 16O 15.99491464 15.99491464 100.0 100.0

Oxygen 17 Oxygen 17 17O 17O 16.9991306 16.9991306 0.03809 0.03809

Oxygen 18 Oxygen 18 18O 18O 17.99915939 17.99915939 0.20048 0.20048

Fluorine Fluorine 19F 19F 18.998405 18.998405 100.0 100.0

Sodium Sodium 23Na 23Na 22.9897697 22.9897697 100.0 100.0

Silicon 28 Silicon 28 28Si 28Si 27.9769284 27.9769284 92.23 92.23



Phosphorus Phosphorus 31P 31P 30.9737634 30.9737634 100.0 100.0

Sulfur 32 Sulfur 32 32S 32S 31.972074 31.972074 100.0 100.0

Sulfur 33 Sulfur 33 33S 33S 32.9707 32.9707 0.78931 0.78931

Sulfur 34 Sulfur 34 34S 34S 33.96938 33.96938 4.43065 4.43065

Sulfur 36 Sulfur 36 36S 36S 35.96676 35.96676 0.02105 0.02105

Chlorine 35 Chlorine 35 35Cl 35Cl 34.968854 34.968854 100.0 100.0

Chlorine 37 Chlorine 37 37Cl 37Cl 36.965896 36.965896 31.9783631.97836

(a) only one chlorine atom (b) only one bromine atom

c) one chlorine and one bromine atom

3:1 1:1

3:4:1

35Cl: 75.7737Cl: 24.23

79Br: 50.6981Br: 49.31

Types of Atomic mass Types of Atomic mass spectrometersspectrometers

Atomic mass Atomic mass spectrometerspectrometer AMS

MMS

YO, Y(I), Y(II) EMISSION ZONESCOURTESY VARIAN

Dr. Houk Presentation, 2002

Sampler

Skimmer

Photo by A. L. Gray

Dr. Houk Presentation, 2002

AGILENT 7500 OMEGA LENS

Plasma Torches

8,000 to 10,000 oC

Ionization Source

Mass Analyzer (Quadrupole)Mass Analyzer (Quadrupole)

Skoog et al., 1999, Instrumental Analysis

Two pairs of rods:Attach + and - sides of a variable dc source Apply variable radio-frequency ac potentials to each pair of rods.

Ions are accelerated into the space between the rods by a small potential (5-10V)

Ions having a limited range of m/z value reach the transducer.

Ion trajectories in a QuadrupoleIon trajectories in a Quadrupole A pair of positive rods (as lying in the xz A pair of positive rods (as lying in the xz

plane).plane). In the absence of a dc potential:In the absence of a dc potential:

Positive half of the ac cyclePositive half of the ac cycle: Converge (ion in the : Converge (ion in the channel will tend to converge in the center of the channel will tend to converge in the center of the channel during the positive half of the ac cycle).channel during the positive half of the ac cycle).

Negative half of the ac cycleNegative half of the ac cycle: Diverge (ions will : Diverge (ions will tend to diverge during the negative half). tend to diverge during the negative half).

Skoog et al., 1999, Instrumental Analysis

Whether or not a positive ion strikes the rod will depend upon the rate of movement of ion along the z axis, its m/z, and the frequency and magnitude of the ac signal.

A pair of positive rods (Cont’d)A pair of positive rods (Cont’d)

With dc potential:With dc potential:

Heavier ionsHeavier ions: less affected by ac : less affected by ac (largely by dc).(largely by dc).

Lighter ionsLighter ions: deflected during negative : deflected during negative cycle of ac.cycle of ac.

The pair of positive rods: a high-pass The pair of positive rods: a high-pass mass filter for positive ions traveling in mass filter for positive ions traveling in the xz plane.the xz plane.

The pair of negative rodsThe pair of negative rods

In the absence of the ac potential:In the absence of the ac potential:All positive ions will tend to strike the All positive ions will tend to strike the rods. rods.

With ac potential:With ac potential:For the lighter ions, however, this For the lighter ions, however, this movement may be offset by the positive movement may be offset by the positive half cycle of ac potential.half cycle of ac potential.

Thus, the pair of negative rods operates Thus, the pair of negative rods operates as a low-pass mass filter.as a low-pass mass filter.

The mass that can be analyzed can be The mass that can be analyzed can be varied by adjusting the ac and dc varied by adjusting the ac and dc potential.potential.

How does it work?How does it work?

U + Vcoswt

...

.

.-U - Vcoswt

m/z=1000

m/z=1100

m/z=990

m/z=900

m/z=1010

Typical mass spectrumTypical mass spectrum

How good this is?How good this is?

Linear calibrations over 4 orders of magnitude Multi-elemental analysis of a standard

ICP-MS: a handy tool!ICP-MS: a handy tool!

Spectral Interferences?Spectral Interferences?

Isobaric overlapIsobaric overlapDue to two elements that have isotopes Due to two elements that have isotopes having having substantiallysubstantially the same mass the same mass

4040ArAr++ and and 4040CaCa++

PolyatomicPolyatomic

Due toDue to interactions between species in interactions between species in the plasma and species in matrix or the plasma and species in matrix or

atmosphereatmosphere 5656Fe and Fe and 4040ArAr1616OO4444Ca and Ca and 1212CC1616OO1616OO..

Isobaric interferences?Isobaric interferences?

Spectral Interferences?Spectral Interferences? Refractory oxideRefractory oxide

AAs a result of incomplete dissociation of the sample matrix or from recombination in the plasma tail

MOMO++, MO, MO2+2+, MO, MO3+3+

Doubly charged ionsDoubly charged ions

Matrix Effects?Matrix Effects? Space charge effectsSpace charge effects

Advanced Analytical Chemistry – CHM 6157Advanced Analytical Chemistry – CHM 6157 ® Y. CAI® Y. CAI Florida Florida International UniversityInternational UniversityUpdated on 9/13/2006Updated on 9/13/2006 Chapter 3Chapter 3 ICPMSICPMS

Isotope DilutionIsotope Dilution Isotope dilution is a super internal Isotope dilution is a super internal

standard addition method on the basis standard addition method on the basis of isotope ratios.of isotope ratios.

Add a known amount (spike) of a Add a known amount (spike) of a stable enriched isotope of the element stable enriched isotope of the element considered, which has at least two considered, which has at least two stable isotopes 1 and 2, to the samplestable isotopes 1 and 2, to the sample

Measure the isotope ratio of isotopes Measure the isotope ratio of isotopes 1 and 2 in the Spike, the unspiked 1 and 2 in the Spike, the unspiked sample and finally the spiked sample.sample and finally the spiked sample.

The concentration of the element of The concentration of the element of interest can then be deducted from interest can then be deducted from these isotopic ratios and from the these isotopic ratios and from the amount of spike added.amount of spike added.


Advantages:Advantages:

Simplified chemical and physical Simplified chemical and physical separation proceduresseparation procedures

Elimination (reduction) of matrix Elimination (reduction) of matrix effectseffects

Elimination of the effect of Elimination of the effect of instrumental driftinstrumental drift


Theory Theory

In principle, any element with at In principle, any element with at least two isotopes that can be least two isotopes that can be measured is suitable for measured is suitable for determination by isotope dilution. determination by isotope dilution. The two selected are designed 1 The two selected are designed 1 and 2.and 2.

Three solutions will be used:Three solutions will be used:

Sample (s)Sample (s) Standard (t)Standard (t) Spiked Spiked sample (m)sample (m)


11nnss is the number of moles of is the number of moles of isotope 1 in the sample.isotope 1 in the sample.

22nnss is the number of moles of is the number of moles of isotope 2 in the sample.isotope 2 in the sample.

11nnt t is the number of moles of isotope is the number of moles of isotope 1 in the standard.1 in the standard.

22nntt is the number of moles of isotope is the number of moles of isotope 2 in the standard.2 in the standard.

RRss is the ratio of isotope 1 to isotope is the ratio of isotope 1 to isotope 2 in the sample solution.2 in the sample solution.

RRtt is the ratio of isotope 1 to isotope is the ratio of isotope 1 to isotope 2 in the standard.2 in the standard.

RRmm is the ratio of isotope 1 to is the ratio of isotope 1 to isotope 2 in the spiked sample.isotope 2 in the spiked sample.


Assuming the molecular Assuming the molecular sensitivity sensitivity 11S/S/22S of the MS for S of the MS for isotope 1 and 2 are the same, isotope 1 and 2 are the same, then then

For the sample solution:For the sample solution:

RRss = = 11nnss//22nnss

[1][1]

For the standard solution: For the standard solution:

RRtt = = 11nntt//22nntt

[2][2]


For the spiked sample solution:For the spiked sample solution:RRmm = ( = (11nnss + + 11nntt)/()/(22nnss + + 22nntt)) [3][3]

Substitution of equations 1 and 2 into Substitution of equations 1 and 2 into equation 3:equation 3:RRmm = (R = (Rss

22nnss+ R+ Rtt22nntt)/()/(22nnss + + 22nntt))

[4][4]Rearranged to:Rearranged to:

22nnss = = 22nntt (R (Rmm-R-Rtt)/(R)/(Rss-R-Rmm)) [5][5]

Convert the number of moles of isotope 2 Convert the number of moles of isotope 2 in the sample to the total number of moles in the sample to the total number of moles of the elements in the sample.of the elements in the sample.

nnss =( =(22nntt/θ/θ22)(R)(Rmm-R-Rtt)/(R)/(Rss-R-Rmm)) [6][6]θθ22 is the isotopic abundance of isotope 2 in is the isotopic abundance of isotope 2 in

the sample.the sample.


The mass of the element in the The mass of the element in the sample is then given by:sample is then given by:

MMss = M( = M(22nntt/θ/θ22)(R)(Rmm-R-Rtt)/(R)/(Rss-R-Rmm))[7][7]

M is the molecular weight of M is the molecular weight of the element.the element.

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Atomic Mass Spectrometry Nearly all elements in the periodic table can be determined by mass spectrometry Nearly all elements in the periodic table can