Inductively Coupled Plasma Mass Spectrometry

Dr. Lloyd Allen and Dr. Stuart Georgitis

LECO Corporation3000 Lakeview AvenueSt. Joseph MI 49085

Principles of OperationSpectrometer Componenets

Sample Introduction System

Sample Cell

Optical Bench

Detector

Recorder or Computer

The ICP-MS Spectrometer

• ICP : Source of Ions

– Atmospheric Argon based plasma

– Operated from 2,500 to 8000 Kelvin to produce ions

– Requires interface to vacuum bench

• Mass Spectrometer : Mass Filter or Mass Analyzer

– Quadrupole

– High Resolution Double Focusing

– Ion Trap

– Time-of-Flight

The ICP-MS Spectrometer (2)

• Sample Forms Possible

– Solids : Conductive and Non-Conductive

– Liquids : Aqueous and Organic

– Gases

• Method Advantages

– Very Low Noise = Very High Signal to Noise Ratio

• excellent detection limits (ppt)

– Isotopic Analysis

– Few interferences compare to other atomic techniques

– Speed

The ICP-MS Spectrometer

• Qualitative Uses

– Semi quantitation in the absence of standards, solids.

– Concentration profiling

– Isotopic Ratios = Dating, Finger Printing, Finger Pointing

• Method Disadvantages

– TDS : typically < 0.2%

– High Sensitivity = Contamination during sample prep.

– Sequential systems have elevated RSD’s for Ratios

– High resolution systems : Resolution > = < Sensitivity

– Argide and Matrix Interferences

The Purpose of the Plasma

Desolvation : Drying Droplets

Vaporization : Particles to gas

Excitation : Emission

Atomization : Dissociation

Ionization : Electron Losse-

Radial Emission

Med Energy Required(NAZ) Mn Compromise

High Energy Required Short Wavelengths As, Se

Low Energy Required Long Wavelengths K, Cs, La,Li, Na, Sr

The Plasma of ICP

• AES or MS– Frequency

• 27 Mhz• 40 Mhz

– Matching Network• Crystal Controlled• Free Running

– Solid State– Mini-Torch or

Standard

• ICP-MS Only

– Secondary or Pinch Discharge

• Center Tapped• Interlaced Coils• Torch Shields

– X, Y, Z Control

ICP-MS Components: Interface

Ions must proceed from atmospheric pressure to an area of reduced pressure

required for MS

Plasma 1st stage 2nd stage Analyzer stage

AtmosphericPressure

~ 1 torr 10-4 torr 2 x 10-6 torr

Mechanical Pump(Interface)

Turbo Pump Turbo Pump

Mechanical Pump(Backing)

The ICP-MS Interface

Sampler Cone

Skimmer Cone

Barrel Shock

Mach Disk

SupersonicJet

Zone of Silence

Torch & ICP

SampleIon lenses andMass Analyzer

The ICP-MS Interface

Torch & ICP

Sample

Sampler Cone

Skimmer Cone

Secondary Discharge Oxides, Polyatomics,secondary excitation

Velocity Consideration4 eV, 100 amu ion 1964 m/s

14 eV, 100 amu ion 3674 m/s

A factor of 2 reduction in ions in extraction volume

Ion Energies for Shielded Load Coil

Detection Limits (10s, 3 s)Shielded vs. Top-grounded

ICP Sample Introduction Systems

• Solution Nebulizers

– Concentric

– High Efficiency C.

– V-groove

– Modified Liechte

– Cross Flow

– Burgener

• Spray Chambers

– Scott’s

– Cyclonic’s

– Inert or glass

• Ultrasonic Nebulizer

– Membrane Desolvator

• Direct Injection Nebulizer

• Arc-Spark

• ETV

• Laser

• Direct Insertion Nebulizer

• Hydride Generator

• Discrete Sampling

ICP-AES and ICP-MS

Inductively Coupled Plasma

Simultaneous Sequential

PMT

CID

CCD

PMTPMT’s

Magnetic Sector

Quadrupole

ICP-AES ICP-MSICP-AES ICP-MS

Magnetic Sector

Time of Flight

Ion Trap

The Two Major Approaches to ICP-MS Spectrometry

Sequential : Mass Filters

or

Simultaneous : Mass Analyzer

Quadrupole ICP-MSA Sequential Mass Filter

All m/z Values In

One m/z Value Out

+

-+

-

Separation Based on Stability of the m/z Value in the RF and DC Fields on the Quadrupole Rods

Quadrupole ICP-MS

RF1, DC1

RF3, DC3

Time1

Time2

Sequential Analysis Limitations: ICP-MS

• Sample throughput > = < a function of the number of

m/z values measured

– Transient Signals : very few isotopes analyzed

• ETV

• Chromatography

• Single spot laser ablation

– Can not obtain high precision isotope ratios

– Small volume samples: v. few isotopes analyzed

– Susceptible to cones plugging (TDS) by prolonged sample contact

TOF ICP-MS TheorySimultaneous Mass Spectra

KE = 1/2 mv2 = zV m/z = 2V/v2 m/z = (2Vt2)/(L2)

Velocity v = L / t

Flight Tube Length (L)

(+)

Accelerating Voltage (V)

++++++

Repeated up to 30,000 times per second

Requirements of TOF-ICP-MS

• Continuous ion beam requires modulation

• Detector must respond to fast ion events (ns)

• Data acquisition system must be able to handle TOF

speeds

• Matrix ions must be removed to avoid detector saturation

Right-angle/Orthogonal Injection

Repeller

Acceleration Field

Field-FreeFlight Region

Ion Lenses

Orthogonal TOF ICP-MSDisadvantages

• Transmission Efficiency at best 20%

• Sensitivity/Resolution Tradeoff

• Mass Dependent Optics in TOF due to mass dependent energies

Orthogonal Transmission

Detector Planeor

Ion Mirror

OriginalIon Packet

L= 0.5 to 0.75 m

23 mm dia.ion detector

vy / vx

y

x

Mass-Dependent Energies

Repeller

Acceleration Field

DetectorIon Mirror

Vsteer

Green - PbRed - Li

Ion beam

Orthogonal Mass Bias

0 256M/Z

CTS

Low Mass BiasLow Mass Bias

Mid Mass BiasMid Mass Bias

High Mass BiasHigh Mass Bias

Axial Mass Bias

0 256M/Z

CTS

On-Axis Ion Injection Advantages

• Improved Ion Transmission Efficiency

• Reduced Mass Bias

• Reduced Optical Maintenance

• Reduced Instrument Footprint

Schematic Diagram of Axial TOF ICP-MS

123

Acceleration

Extraction

Skimmer

Sampler

Detector

Ion Mirror

ICP Torch

Vacuum StagesFlight Tube

Simultaneous Mass Spectra Modulation

(+)

++++++

Repeated up to 30,000 times per second

Accelerate to TOF Accelerate to TOF Accelerate to TOF

RejectReject RejectReject RejecRejectt

38 Micro S38 Micro S38 Micro S38 Micro S 38 Micro S38 Micro S

Schematic Diagram of Axial TOF ICP-MS

123

Einzel 2

X-Steering

Y-Steering

Acceleration

Repeller

Modulation

Extraction

Skimmer

Sampler

Third Stage Orifice

Detector

Energy Barrier

Gridded Ion Mirror

Flight Tube

ICP Torch

Ion Optic 1

Einzel 1

Ion MirrorIon Mirror

Acceleration Field

Detector

Simultaneous Advantages• Transient Signals: complete multielement analysis

• High precision isotope ratios : Simultaneous Reads– no additive noise when employing corrections– no sample introduction or plasma noise

• Small volume samples: complete multielement analysis– minimum sample destruction– maximum spatial concentration profile capability

• Sample throughput =delivery and rinse time primarily

• Cone plugging via TDS exposure is minimized

Method Advantage :TOF Means Speed

30,000 Full Mass Spectra per Second

U = Mach 115

Detection LimitsAre They Signal To Background ? Or Signal to Noise?

0 256M/Z

CTS

Detection LimitsAre They Signal To Background ? Or Signal to Noise?

0 256M/Z

CTS

TOF ICP-MS Detection Limits (3)

Element DL (ng/mL) Element DL (ng/mL)

Ba 0.002 Rb 0.004

Co 0.004 Rh 0.002

Cu 0.004 Sr 0.002

Dy 0.009 Ta 0.006

Er 0.008 Tb 0.001

Eu 0.003 Th 0.005

Gd 0.005 Tl 0.008

Ho 0.002 Tm 0.002

La 0.003 U 0.004

Lu 0.002 W 0.004

Nd 0.009 Y 0.003

Pr 0.002 Yb 0.005

Mn 0.003 V 0.003

Short Term StabilityInternal Standard Results

(20 min. 10 ppb)

% RSD % RSD Limit

V 1.04 0.31

V/Y 0.51 0.42

Ba 0.70 0.33

Ba/Tb 0.51 0.43

U 1.44 0.3

U/Bi 0.59 0.46

0

1000

Sig

na

l (cp

s)

0

6000

204 206 208

4

m/z

Sig

na

l (m

v)

204 206 208

4

m/z

Dual-Mode Detection

1 ng/mL 100 ng/mL

Ana

log

Sig

nal (

mV

)Io

n C

ount

ing

Sig

nal (

cps)

Saturation

10 -3 10 -2 10 -1 100 101 102 103 104100

101

102

103

104

105

106

107

209

Bi

Concentration (ppb)

Co

un

tin

g S

ign

al

(cp

s)

10 -3

10 -2

10 -1

100

101

102

103

104

An

alo

g S

ign

al

Dynamic Range

Correlation CoefficientCounting Analog

Co 0.9993 1.0000In 0.9994 0.9999

Cs 0.9994 1.0000Ba 0.9999 0.9997Bi 1.0000 1.0000U 0.9991 0.9999

Dynamic Range (Counting and Analog)

1E-3 0.01 0.1 1 10 100 1000 10000 100000

0.1

1

10

100

1000

10000

100000

1000000

R=0.99954

Bis

mu

th R

ela

tive

Sig

na

l A

rea

Concentration (ppb)

LECO Patented Ion Counting/Analog Detection

Scheme

ETP AF831H 20 dB

gain switchingpre-amp

100 MHz Ion Counter

500 MHzflash A/D

Windowed Buffer

Dual Accumulator VMEBus

High Data Throughput• Data throughput from ICP-MS up to 750 Mbytes/sec

• reduction is necessary for practical analysis

• Buffer retains 2000, 2 ns bins from each spectra

• Individual spectra are summed and the data transferred to the host computer

•Max bandpass 0.75 Mbytes/sec

TOF-MS500 MHz A/D, 100 MHz Counter

30 kHz Spectral Rate

Buffer

Accumulation

Mass Mapping

Summation

Throughput(Mbytes/sec)

750

0.75

120

13

Mass Mapping

Mass

68 69 70

40Ar

14N 2

68Zn (impurity)

70Ge

139La

(2+)69

Ga

2 ppb Ga/Ge, 500 ppb La

Time Bins

Bin Summation

255 Summed to 1

Figures of Merit and Applications

• Spectral resolution and matrix deflection

• Detection limits and speed of analysis

• Multielement transient signal analysis

• Isotope ratios and internal standards

• Solid sample analysis by LA

Quadrupole Resolution

Low M/Z High M/Z

0.3 AMU 1.0 AMU

TOF Resolution

Low M/Z High M/Z

Unit Mass Baseline Resolved 1.0 AMU at Greatest Mass

< 0.3 AMU at Least Masswith No Sacrifice in Sensitivity

TOF Resolution

Low M/Z High M/Z

Lower Mass Resolving Power

66 67 68 69 70 71 72 73 74 75 76 77

40Ar

14N

2

68Zn (impurity)

76Ge

74Ge

73Ge

72Ge

71Ga

70Ge

139La

2+

69Ga 2 ppb Ga/Ge, 500 ppb La

2 ppb Ga/Ge

Mass

Resolving Power at High Mass

204 205 206 207 208 209 210

R50%

= 1270

R10%

= 475

Sig

na

l

Mass

50 ppt Pb, Bi

ResolutionSelected Spectral Regions Expanded

58 60 62 64

65Cu

+

64Zn

+

63Cu

+

62Ni

+

61Ni

+

60Ni

+

134 135 136 137 138

59Co

+

58Ni

+

138Ba

+

137Ba

+

136Ba

+

135Ba

+

134Ba

+

m/z

202 204 206 208

208Pb

+

207Pb

+

206Pb

+

205Tl

+

204Pb

+

203Tl

+

Quadrupole ICP-MSMatrix Filter

RF1, DC1

RF3, DC3

Time1

Time2

TOF and High Matrix

Low M/Z High M/Z

Coulombic Repulsion During Flight

TIME

+

+ +

+

TOF and High Matrix

Low M/Z High M/Z

Selectable Matrix Removal

Flight Tube

T.R.I.P.Transverse Rejected Ion

Pulse

AccelerationRepeller

Modulation

Background Species Deflection (T.R.I.P.)

NO+

Ar+

Ar+

NO+

O+, OH+

ICP-MS Speed Quadrupole vs TOF

* Theoretical 0.3 sec. dwell time, 5 replicates, 60 sec. rinse time

** 3 points/peak, 10 s quadrupole settle time

EPA 200.8 (Analytes, Interference Corrections, Internal Standards)

0.0

100.0

200.0

300.0

400.0

500.0

600.0

1 11 21 31 41 51 61 71 81 91

Number of m/z Values Measured

Tim

e (

se

co

nd

s)

Quadrupole**

TOFMS