Transcript Time of Flight ICP-MS: Many Sequential Barriers Disappear

Inductively Coupled Plasma
Mass Spectrometry
Dr. Lloyd Allen and Dr. Stuart Georgitis
LECO Corporation
3000 Lakeview Avenue
St. Joseph MI 49085
Principles of Operation
Spectrometer 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
eIonization : Electron Loss
Excitation :
Emission
Atomization : Dissociation
Vaporization : Particles to gas
Desolvation : Drying Droplets
Radial Emission
Low Energy Required
Long Wavelengths
K, Cs, La,Li, Na, Sr
Med Energy Required
(NAZ) Mn Compromise
High Energy Required
Short Wavelengths
As, Se
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
Atmospheric
Pressure
1st stage
~ 1 torr
Mechanical Pump
(Interface)
2nd stage
Analyzer stage
10-4 torr
2 x 10-6 torr
Turbo Pump
Turbo Pump
Mechanical Pump
(Backing)
The ICP-MS Interface
Barrel Shock
Zone of Silence
Mach Disk
Torch & ICP
Ion lenses and
Mass Analyzer
Sample
Supersonic
Jet
Skimmer Cone
Sampler Cone
The ICP-MS Interface
Secondary Discharge
Oxides, Polyatomics,
secondary excitation
Torch & ICP
Sample
Skimmer Cone
Sampler Cone
Velocity Consideration
4 eV, 100 amu ion
14 eV, 100 amu ion
1964 m/s
3674 m/s
A factor of 2 reduction in ions in extraction volume
12
Ion Energies for Shielded
Load Coil
11
10
9
Energy (eV)
8
7
6
5
4
3
2
1
0
50
100
150
Mass
200
250
Detection Limits (10s, 3 s)
Shielded vs. Top-grounded
D e te c tio n L im it (p p t)
100
90
80
70
60
TG
50
ST
40
30
20
10
0
Bi
Pb
Tl
Ba
In
Co
Element
Al
Mg
Na
Li
ICP Sample Introduction Systems
• Solution Nebulizers
– Concentric
– High Efficiency C.
– V-groove
– Modified Liechte
– Cross Flow
– Burgener
• Ultrasonic Nebulizer
– Membrane Desolvator
• Direct Injection Nebulizer
• Arc-Spark
• ETV
• Laser
• Spray Chambers
– Scott’s
• Direct Insertion Nebulizer
– Cyclonic’s
• Hydride Generator
– Inert or glass
• Discrete Sampling
ICP-AES and ICP-MS
Inductively Coupled Plasma
Simultaneous
ICP-AES
Sequential
ICP-MS
ICP-AES
PMT
CID
CCD
Time of Flight
Ion Trap
Magnetic Sector
PMT
PMT’s
ICP-MS
Quadrupole
Magnetic Sector
The Two Major Approaches to
ICP-MS Spectrometry
Sequential : Mass Filters
or
Simultaneous : Mass Analyzer
Quadrupole ICP-MS
A Sequential Mass Filter
One m/z
Value Out
+
All m/z
Values In
+
-
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 Theory
Simultaneous Mass Spectra
Repeated up to 30,000 times per second
+
+
+
+
+ +
(+)
Flight Tube Length (L)
KE = 1/2 mv2 = zV
Accelerating
Voltage (V)
m/z = 2V/v2
m/z = (2Vt2)/(L2)
Velocity v = L / t
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
Acceleration
Field
Repeller
Ion Lenses
Field-Free
Flight Region
Orthogonal TOF ICP-MS
Disadvantages
• Transmission Efficiency at best 20%
• Sensitivity/Resolution Tradeoff
• Mass Dependent Optics in TOF due
to mass dependent energies
Orthogonal Transmission
 vy / vx
y
23 mm dia.
ion detector

x
L= 0.5 to 0.75 m
Original
Ion Packet
Detector Plane
or
Ion Mirror
Mass-Dependent Energies
Detector
Acceleration
Field
Vsteer
Repeller
Green - Pb
Red - Li
Ion beam
Ion Mirror
Orthogonal Mass Bias
Mid Mass Bias
Low Mass Bias
High Mass Bias
CTS
0
256
M/Z
Axial Mass Bias
CTS
0
256
M/Z
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
Detector
Flight Tube
Vacuum Stages
3
2
1
ICP Torch
Ion Mirror
Sampler
Skimmer
Extraction
Acceleration
Simultaneous Mass Spectra Modulation
+
+
+ +
(+)
Accelerate to TOF
Accelerate to TOF
Reject
Reject
38 Micro S
38 Micro S
Accelerate to TOF
Rejec
t
38 Micro S
Repeated up to 30,000 times per second
Schematic Diagram of Axial TOF
ICP-MS
Flight Tube
Detector
Energy Barrier
3
2
1
Y-Steering
Einzel 2
X-Steering
ICP Torch
Gridded Ion Mirror
Sampler
Skimmer
Einzel 1
Acceleration
Repeller
Modulation
Extraction
Third Stage Orifice
Ion Optic 1
Ion Mirror
Ion Mirror
Detector
Acceleration
Field
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
U = Mach 115
30,000 Full Mass Spectra per Second
Detection Limits
Are They Signal To Background ? Or Signal to Noise?
CTS
0
256
M/Z
Detection Limits
Are They Signal To Background ? Or Signal to Noise?
CTS
0
256
M/Z
TOF ICP-MS Detection Limits
(3s)
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 Stability
Internal 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
Dual-Mode Detection
1 ng/mL
100 ng/mL
1000
6000
Ion Counting
S ig n a l ( c p s )
Signal (cps)
Saturation
0
4
Analog
S ig n a l ( m v )
(mV)
Signal
0
4
204
206
m /z
208
204
206
m /z
208
Dynamic Range
C o rre la tio n C o e ffic ie n t
Co
0 .9 9 9 3
7
10
A n a lo g
Bi
1 .0 0 0 0
0 .9 9 9 4
0 .9 9 9 9
Cs
0 .9 9 9 4
1 .0 0 0 0
Ba
0 .9 9 9 9
0 .9 9 9 7
Bi
1 .0 0 0 0
1 .0 0 0 0
U
0 .9 9 9 1
0 .9 9 9 9
10
10
10
10
10
10
10
6
10
5
10
4
10
3
10
2
10
1
10
0
10
-3
-2
-1
0
1
2
3
10 10 10 10 10 10 10 10
C oncentration (ppb)
4
3
2
1
0
-1
-2
-3
Analog Signal
In
4
209
Counting Signal (cps)
C o u n tin g
10
Dynamic Range (Counting and
Analog)
1000000
Bismuth Relative Signal Area
100000
R=0.99954
10000
1000
100
10
1
0.1
1E-3
0.01
0.1
1
10
100
Concentration (ppb)
1000
10000 100000
LECO Patented Ion
Counting/Analog Detection
Scheme
100 MHz Ion Counter
ETP
AF831H
20 dB
gain switching
pre-amp
Windowed Buffer
500 MHz
flash A/D
Dual Accumulator
VME
Bus
High Data Throughput
• Data throughput from ICP-MS up to
750 Mbytes/sec
• reduction is necessary for practical
analysis
T O F -M S
T h ro u g h p u t
( M b y te s/s e c )
5 0 0 M H z A /D , 1 0 0 M H z C o u n te r
3 0 k H z S p e c tr a l R a te
750
B u ffe r
120
•
Buffer retains 2000, 2 ns bins from
each spectra
A ccu m u la tio n
M a ss M a p p in g
• Individual spectra are summed and
the data transferred to the host
computer
13
S u m m a tio n
0 .7 5
•Max bandpass 0.75 Mbytes/sec
Mass Mapping
Ga
69
La
139 (2+)
Ge
70
2 ppb Ga/Ge, 500 ppb La
40
14
Ar N 2
68
Zn (impurity)
68
Mass
69
70
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
0.3 AMU
Low M/Z
1.0 AMU
High M/Z
TOF Resolution
< 0.3 AMU at Least Mass
with No Sacrifice in Sensitivity
Low M/Z
Unit Mass Baseline Resolved
1.0 AMU at Greatest Mass
High M/Z
TOF Resolution
Low M/Z
High M/Z
Lower Mass Resolving Power
69
2 ppb G a/G e, 500 ppb La
Ga
139
La
70
2+
2 ppb G a/G e
Ge
71
40
68
14
Ar N 2
Ga
76
Zn (im purity)
74
72
67
68
69
70
71
Ge
Ge
73
66
Ge
72
M ass
Ge
73
74
75
76
77
Resolving Power at High
Mass
R 10% = 4 7 5
R 50% = 1 2 7 0
Signal
50 ppt Pb, Bi
204
205
206
207
M ass
208
209
210
Resolution
Selected Spectral Regions Expanded
59
Co
+
138
63
Cu
Ba
205
+
Tl
+
+
208
58
Ni
Pb
+
+
203
65
Cu
Tl
+
+
206
Pb
+
207
60
Ni
+
+
64
Zn
+
137
136
135
62
61
58
Pb
60
Ni
Ni
+
134
+
62
64
Ba
Ba
+
Ba
Ba
+
+
204
+
134
135
136
m/z
137
138
202
Pb
+
204
206
208
Quadrupole ICP-MS
Matrix Filter
RF1, DC1
Time1
Time2
RF3, DC3
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
Acceleration
Repeller
Modulation
Background Species Deflection
(T.R.I.P.)
150
140
Ar+
130
120
110
100
90
Relative Intensity (mV)
80
70
60
50
NO+
40
O+, OH+
30
20
10
Ar+
2
NO
+
1
0
8
12
16
20
8
12
16
ICP-MS Speed Quadrupole vs TOF
600.0
500.0
EPA 200.8
(Analytes, Interference Corrections, Internal Standards)
Time (seconds)
400.0
300.0
Quadrupole**
TOFMS
200.0
100.0
0.0
1
11
21
31
41
51
61
71
81
91
Number of m/z Values Measured
* Theoretical 0.3 sec. dwell time, 5 replicates, 60 sec. rinse time
** 3 points/peak, 10 s quadrupole settle time