Inductively Coupled Plasma Mass Spectrometry Dr. Lloyd Allen and Dr

Inductively Coupled Plasma Mass Spectrometry Dr. Lloyd Allen and Dr
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
Ionization : 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 ICP-MS Only 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 Atmospheric Pressure ~ 1 torr 10-4 torr 2 x 10-6 torr Mechanical Pump (Interface) Turbo Pump (Backing)

The ICP-MS Interface Ion lenses and Mass Analyzer Torch & ICP
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 Torch & ICP Skimmer Cone Sampler Cone
Secondary Discharge Oxides, Polyatomics, secondary excitation Torch & ICP Sample Skimmer Cone Sampler Cone

Velocity Consideration
4 eV, 100 amu ion m/s 14 eV, 100 amu ion 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
Ultrasonic Nebulizer Membrane Desolvator Direct Injection Nebulizer Arc-Spark ETV Laser Direct Insertion Nebulizer Hydride Generator Discrete Sampling Solution Nebulizers Concentric High Efficiency C. V-groove Modified Liechte Cross Flow Burgener Spray Chambers Scott’s Cyclonic’s Inert or glass

ICP-AES and ICP-MS Inductively Coupled Plasma Simultaneous Sequential
PMT CID CCD Magnetic Sector Time of Flight Ion Trap Magnetic Sector Quadrupole PMT PMT’s

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 + Shown above is an example of how the quadrupole works. In this case, we see three different mass (small, medium and large) elements (in reality, ions of may m/z values, 1-250, will be present). Using the first set of RF and DC conditions, the lightest ion is transmitted. Next, the RF and DC conditions are switched to only transmit the heaviest ion in this case. Several things need to be pointed out here. The quadrupole is a sequential system which transmits only one m/z value at any given moment in time. Information about all other species is discarded. As a result, the amount of time spent analyzing a sample is directly related to the number of elements analyzed. The more elements analyzed, the longer the analysis will take (under a fixed set of acquisition conditions). This feature is particularly detrimental when dealing with small volume samples or as we will demonstrate, any sample that yields a transient signal - Separation Based on Stability of the m/z Value in the RF and DC Fields on the Quadrupole Rods

Quadrupole ICP-MS RF1, DC1 Time1 RF3, DC3 Time2
A quadrupole is a mass filter. All mass spectrometers require charged bodies (ions) in order to control the movement of the atoms involved. Quadrupole mass spectrometers use the ICP to generate a continuous ion beam that is directed into the filter fields of the mass spectrometer. Shown above is an example of how the quadrupole works. In the first case (Time1), three ions of different mass (small, medium and large) are pictured (in reality, ions of many m/z values, 1-250, will be present). The first set of RF and DC conditions allows only the lightest ion through the fields emitted by the rods where it then strikes the detector. All other ions that have a different m/z ratio repelled by the fields and they scatter without reaching the detector. The scattered ions are eventually removed completely by the vacuum system. The discrimination of the mass filter is really performed on the basis of the mass (m) to charge (z) ratio (m/z). It is not strictly correct to say that the ions are filtered by size or mass unless the are in the form of a +1 ion where mass divided by charge is equal to mass (m/1 = m). An interference can occur when a +2 charged ion of twice the mass is present since 2m/2 = m/1. 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) Accelerating Voltage (V) Shown above is a simple schematic of a TOF MS. Ions (shown at the left) are generated and extracted from the plasma exactly as with any other type of mass analyzer. Ions are then guided into an acceleration region where a large pulse in order to accelerate all the ions to the same kinetic energy. Since all ions are accelerated to the same kinetic energy, the lightest ions will travel faster that the heaviest ions which travel more slowely. In the flight tube, ions of different mass (m/z) will separate to there individual values. This process is analogous to throwing a bowling ball and a golf ball at a target. If timed, obviously the golf ball would arrive at the target first. Behavior at the atomic level is the same. KE = 1/2 mv2 = zV 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 There have been many types of Time-of-Flight spectrometers coming on to the market but most of them have been adapted to techniques that normally have pulsed ion sources. ICP is a continuous ion beam source which means the beam must be modulated in some manner to produce the discrete ion packets for acceleration in a TOF system. Other requirements include detectors that can clear signals very rapidly as the ion groups arriving from the flight tube nanoseconds apart. Logically, the data conversion from the detector signal must also be extremely fast to interpret signals up to the magnitude of 30,000 full mass range spectra per second. Finally, another fundamental difference between a mass filter and mass analyzer has to be addressed. A sequential mass filter can avoid detector overranges by simply skipping the RF/DC setting for very high concentrations such as the Argon at m/z 40 from the plasma or Silicon from digested quartz sample at m/z TOF systems capture these high concentration ions from the plasma or sample matrix along with all the other ions from the sample. Somehow these high concentrations in the acceleration segment must be eliminated to avoid a detector overrange event that causes the detector to be “blinded” for the next ions that arrive before the high detector signal has cleared.

Right-angle/Orthogonal Injection
Acceleration Field Field-Free Flight Region Repeller Ion Lenses The most common design for a TOF spectrometer is the Orthogonal or Right Angle design. The ions generated by the plasma move through the sampler and skimmer cones and into the ion optics in a more or less straight line. When the region in front of the accelerator optic is filled the acceleration pulse is imposed on this segment of the plasma ion beam. The segment of ions is punched into the flight tube at a right angle to the plasma ion beam. During the time required for the slowest ion of the segment to reach the detector, the ions from the plasma continue to move past the accelerator optic in a straight line. This is a simple and elegant way to control the extraction of a segment of ions from a continuous ion beam. The right angle approach is easier to design for unit mass resolution because the continuous ion beam does not have momentum in the right angle vector. However, there are some disadvantages to this approach.

Orthogonal TOF ICP-MS Disadvantages
Transmission Efficiency at best 20% Sensitivity/Resolution Tradeoff Mass Dependent Optics in TOF due to mass dependent energies Orthogonal TOF has been studied extensively in the last ten years and the limits to transmission efficiency have been theorized to be at best 20%. Therefore, despite the rapid duty cycle only one in five of the usable ions actually reach the detector to be counted. Although the orthogonal design is conducive to good resolution there is the traditional trade-off of sensitivity in order to attain that resolution. Recent publications have shown a maximum sensitivity of 1 million counts per second per ppm of analyte which is 30X lower than the average quadrupole on today's market. Finally it is necessary to have mass dependent optics to account for mass dependent energies. This will make more sense on the next slide.

Orthogonal Transmission
vy / vx y x 23 mm dia. ion detector  L= 0.5 to 0.75 m The plasma ions are moving in the Y direction and the acceleration is performed in the X direction. The result is a drift in the Y direction that is related to the ratio of the 2 velocities. Angle of divergence from the ideal X direction is mass dependent with larger masses diverging more since they have more time to drift. Picture the collision of two automobiles at an intersection. The larger the mass of the object (green ion) moving on the Y vector, the greater the angle of divergence resulting in the heavy objects missing the detector plane. In the absence of ion lenses or steering plates one would certainly expect more heavy mass objects to miss the detector plane and ion mirror than light mass objects. Original Ion Packet Detector Plane or Ion Mirror

Mass-Dependent Energies
Ion Mirror Detector Acceleration Field Vsteer Repeller Steering plates or other ion optic elements are often employed to control the mass bias but it is necessary to select a compromise condition since the movement of small ions is relatively greater than any larger ion under the influence of the same driving force. Using a fixed force, the compromise condition is often selected to favor efficiency of ion transmission in the middle of the mass spectrum thus resulting in losses of efficiency in the low and high mass extremes. Using a variable force with proper timing the mass bias can be significantly diminished in comparison to a fixed compromise condition but it can not be eliminated. The significant mass bias effect is a fact of life for today's right angle acceleration systems. If it were possible to build detectors with vast surfaces then this situation would be improved. However, the problem with large surface plate detectors is a loss in signal clearing speed and increased noise that is inappropriate for elemental Time-of-Flight mass spectrometry. Green - Pb Red - Li Ion beam

Orthogonal Mass Bias Mid Mass Bias Low Mass Bias High Mass Bias CTS
Historically right angle Time-of-Flight was developed more rapidly because of the difficulty in modulating an ion beam in a linear direction. With the improvements in the speed of electronics and computers it was possible to come back to axial orientations and overcome the obstacle of the low duty cycle. Now that the problem of duty cycle has been mastered the next consideration is the spread or range of ion energies that must be dealt with for modulation. Again, advances in electronics have made it possible to narrow the spread of ion energies in the plasma so the ion beam can be more purely repelled during the time acceleration is taking place and more quickly re-established to fill the acceleration region for the next segment of ions. Acceleration is made in the Y direction on an axial system and that is the same direction of movement of the plasma ions. The overall plasma gas pressures and plasma energetics influence the efficiency of ionization, the flux of the ions to the interface and the velocity of ions in the ion beam. The flux of ions determines the density of ions in the acceleration region and the speed of the ions influences the resolution. Therefore, the major advances in axial TOF are the gains in controlling the ion beam to obtain a high duty cycle and the establishing unit mass resolution despite the ion spreads and plasma dynamics. 256 M/Z

Axial Mass Bias CTS Historically right angle Time-of-Flight was developed more rapidly because of the difficulty in modulating an ion beam in a linear direction. With the improvements in the speed of electronics and computers it was possible to come back to axial orientations and overcome the obstacle of the low duty cycle. Now that the problem of duty cycle has been mastered the next consideration is the spread or range of ion energies that must be dealt with for modulation. Again, advances in electronics have made it possible to narrow the spread of ion energies in the plasma so the ion beam can be more purely repelled during the time acceleration is taking place and more quickly re-established to fill the acceleration region for the next segment of ions. Acceleration is made in the Y direction on an axial system and that is the same direction of movement of the plasma ions. The overall plasma gas pressures and plasma energetics influence the efficiency of ionization, the flux of the ions to the interface and the velocity of ions in the ion beam. The flux of ions determines the density of ions in the acceleration region and the speed of the ions influences the resolution. Therefore, the major advances in axial TOF are the gains in controlling the ion beam to obtain a high duty cycle and the establishing unit mass resolution despite the ion spreads and plasma dynamics. 256 M/Z

On-Axis Ion Injection Advantages
Improved Ion Transmission Efficiency Reduced Mass Bias Reduced Optical Maintenance Reduced Instrument Footprint Historically right angle Time-of-Flight was developed more rapidly because of the difficulty in modulating an ion beam in a linear direction. With the improvements in the speed of electronics and computers it was possible to come back to axial orientations and overcome the obstacle of the low duty cycle. Now that the problem of duty cycle has been mastered the next consideration is the spread or range of ion energies that must be dealt with for modulation. Again, advances in electronics have made it possible to narrow the spread of ion energies in the plasma so the ion beam can be more purely repelled during the time acceleration is taking place and more quickly re-established to fill the acceleration region for the next segment of ions. Acceleration is made in the Y direction on an axial system and that is the same direction of movement of the plasma ions. The overall plasma gas pressures and plasma energetics influence the efficiency of ionization, the flux of the ions to the interface and the velocity of ions in the ion beam. The flux of ions determines the density of ions in the acceleration region and the speed of the ions influences the resolution. Therefore, the major advances in axial TOF are the gains in controlling the ion beam to obtain a high duty cycle and the establishing unit mass resolution despite the ion spreads and plasma dynamics.

Schematic Diagram of Axial TOF ICP-MS
Detector Vacuum Stages Flight Tube 3 2 1 ICP Torch Pictured above is a schematic of the patented axial TOF manufactured by LECO corporation. The basic elements include a plasma torch for generation of a continuous ion beam, a sampler / skimmer / extraction cone series for sampling the ion beam while removing much of the neutral Argon load and an ion optical array to position the ion beam for accelerating segments and repelling the beam in a modulated fashion. Segments enter the flight tube and are directed to the ion mirror where they are refocused and reflected back to the detector. The overall flight path is 1.0 meters. The major advantage of the axial system is the lack of divergence of ions thereby minimizing mass bias. Ion Mirror Sampler Skimmer Extraction Acceleration

Simultaneous Mass Spectra Modulation
+ + (+) + Accelerate to TOF Reject 38 Micro S Shown above is a simple schematic of a TOF MS. Ions (shown at the left) are generated and extracted from the plasma exactly as with any other type of mass analyzer. Ions are then guided into an acceleration region where a large pulse in order to accelerate all the ions to the same kinetic energy. Since all ions are accelerated to the same kinetic energy, the lightest ions will travel faster that the heaviest ions which travel more slowely. In the flight tube, ions of different mass (m/z) will separate to there individual values. This process is analogous to throwing a bowling ball and a golf ball at a target. If timed, obviously the golf ball would arrive at the target first. Behavior at the atomic level is the same. Repeated up to 30,000 times per second

Schematic Diagram of Axial TOF ICP-MS
1 2 3 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 Einzel 1 Ion Optic 1

Ion Mirror Ion 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
U = Mach 115 30,000 Full Mass Spectra per Second

Detection Limits Are They Signal To Background ? Or Signal to Noise?
CTS Historically right angle Time-of-Flight was developed more rapidly because of the difficulty in modulating an ion beam in a linear direction. With the improvements in the speed of electronics and computers it was possible to come back to axial orientations and overcome the obstacle of the low duty cycle. Now that the problem of duty cycle has been mastered the next consideration is the spread or range of ion energies that must be dealt with for modulation. Again, advances in electronics have made it possible to narrow the spread of ion energies in the plasma so the ion beam can be more purely repelled during the time acceleration is taking place and more quickly re-established to fill the acceleration region for the next segment of ions. Acceleration is made in the Y direction on an axial system and that is the same direction of movement of the plasma ions. The overall plasma gas pressures and plasma energetics influence the efficiency of ionization, the flux of the ions to the interface and the velocity of ions in the ion beam. The flux of ions determines the density of ions in the acceleration region and the speed of the ions influences the resolution. Therefore, the major advances in axial TOF are the gains in controlling the ion beam to obtain a high duty cycle and the establishing unit mass resolution despite the ion spreads and plasma dynamics. 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 Here is an example of early detection limits for the LECO Renaissance TOF-ICP-MS. In general the limits are from 1-10 ppt which is about average in comparison to the quadrupoles available in today’s market. However, there is a huge difference in the length of time required for analysis to get these detection limits. These 5 second integration, “on peak” detection limits are calculated from 3 times the standard deviation of 10 replicates of the blank solution. Total acquisition time = 50s for all elements listed. Typical detection limit reporting for quadrupole instruments is based on 10 seconds integration per point per replicate and per m/z. To operate in the fastest data acquisition mode possible a quadruple will be operated in peak hopping mode with one point per peak. Using the above list, a quadrupole would spend 260 seconds + whatever dead time between DC/RF settings per replicate. A TOF spectrometer would spend only 10 seconds per replicate since all elements are read simultaneously and the duty cycle is about 2500 times faster. So detection limits as a figure of merit are more applicable to TOF since you can obtain the best signal to noise counting statistics in a very short period of time. Mass filter instruments rarely operate at detection limit capabilities because it requires too much time and sample to obtain favorable signal to noise counting statistics. Other pertinent differences will be discussed in the following slides. 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 13

Short Term Stability Internal Standard Results (20 min. 10 ppb)

Dual-Mode Detection 1 ng/mL 100 ng/mL Ion Counting Signal (cps)
Saturation Ion Counting Signal (cps) The signals on the top left are pulse counting signals where the concentrations of the analytes are within the sensitive realm of this detector mode. In this situation there is not enough signal for analog interpretation, bottom left. When concentrations increase the pulse counting mode can be overwhelmed and the signal reports are distorted, top right, whereas the analog mode is perfectly suited to these signal levels. Analog Signal (mV)

Dynamic Range This graphic shows clearly the cross over from pulse counting to analog detection. Using both detector modes it is possible to have a linear working range of 1 E6 to 1 E7. Either mode is linear within its preferential zone of sensitivity but offset values must be empirically derived to account for the shift of response between the two modes for quantitative analysis.

Dynamic Range (Counting and Analog)

LECO Patented Ion Counting/Analog Detection Scheme
100 MHz Ion Counter ETP AF831H 20 dB gain switching pre-amp The detector is a discrete dynode electron multiplier made by ETP. The detector has been modified so the pulse and analog signals are obtained simultaneously which is not the usual approach. Quadrupole mass spectrometers dwell on single m/z signals for extended periods of time so it is convenient for their detectors to sequentially shift from pulse mode to analog when the signal starts to increase. TOF instruments operate at a pace 1000x faster than mass filters so it is necessary to have both modes operating simultaneously since data loss would occur in a sequential process. Not pictured in this schematic is the rapid gain switching required to drop the preamplifier sensitivity quickly before massive signals from high concentration elements swamp the detector electronics. The patented LECO system has gain switching at 30 nanoseconds. Pulse counting is used for the most sensitive work near the detection limit while analog is used to extend the dynamic range. The detector is therefore a dual mode, dual range model. Windowed Buffer 500 MHz flash A/D VME Bus Dual Accumulator

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 A critical area of the Time-of-Flight spectrometer is the data acquisition system. The combination of duty cycle up to 30,000 full mass range spectra per second and an equally a fast responding detector creates an enormous amount of data. The patented system in use by LECO takes the 750 Megahertz data flow and splits it, simultaneously to carry the pulse counting and analog signals from the detector at the same time. Basically the data is trimmed by retaining only the spectral regions where peaks exist and condensed by a summation of individual spectra at each time bin. The result is manageable and understandable data that can be reported at a maximum rate of 200 points per second for profiling transient signals. Many applications do not require fast reporting so the software can report any time duration as a replicate or sample.

Mass Mapping 2 ppb Ga/Ge, 500 ppb La Mass Time Bins Ga La Ge Ar N
69 139 (2+) La Ge 70 2 ppb Ga/Ge, 500 ppb La 40 14 Ar N 2 68 Zn (impurity) Unit mass resolution is necessary for practical analysis by ICP-MS but describing the resolution in TOF is a little trickier than a description of a mass filter. A mass filter can usually have selectable resolution in the range of 0.5 AMU per peak up to 1.2 AMU. This is accomplished by using finer increments of DC/RF settings in the quadrupole with the trade-off of lower sensitivity with higher resolution. Whatever the choice, the same resolution will be in effect across the entire mass range. Really the only functional level of resolution obtainable for TOF is unit mass or one Atomic Mass Unit discrimination. The TOF spectrometer is designed to achieve this resolution at the greatest mass since it will have the widest peak at the base. In Time-of-Flight the lesser mass ions have greater velocity in the flight so they ultimately produce signal peaks that are narrower than their larger mass cousins. Also, the lesser the mass, the more widely separated the peaks. So a TOF system may have one AMU discrimination at high mass and 0.3 AMU at low mass. The spectra above illustrate the uniqueness of the TOF resolution. Normally a unit mass resolution will not allow the spectrochemist to view the double charged Lanthanum ions as a separate peak (La ++ = m/z = 139/2 = 69.5 AMU). In quadrupole instruments this peak would be an interference on Germanium and Gallium but in TOF it is resolved. 68 69 70 Mass Time Bins

Bin Summation 255 Summed to 1
Unit mass resolution is necessary for practical analysis by ICP-MS but describing the resolution in TOF is a little trickier than a description of a mass filter. A mass filter can usually have selectable resolution in the range of 0.5 AMU per peak up to 1.2 AMU. This is accomplished by using finer increments of DC/RF settings in the quadrupole with the trade-off of lower sensitivity with higher resolution. Whatever the choice, the same resolution will be in effect across the entire mass range. Really the only functional level of resolution obtainable for TOF is unit mass or one Atomic Mass Unit discrimination. The TOF spectrometer is designed to achieve this resolution at the greatest mass since it will have the widest peak at the base. In Time-of-Flight the lesser mass ions have greater velocity in the flight so they ultimately produce signal peaks that are narrower than their larger mass cousins. Also, the lesser the mass, the more widely separated the peaks. So a TOF system may have one AMU discrimination at high mass and 0.3 AMU at low mass. The spectra above illustrate the uniqueness of the TOF resolution. Normally a unit mass resolution will not allow the spectrochemist to view the double charged Lanthanum ions as a separate peak (La ++ = m/z = 139/2 = 69.5 AMU). In quadrupole instruments this peak would be an interference on Germanium and Gallium but in TOF it is resolved.

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 The following figures of merit and applications will be presented.

Quadrupole Resolution
0.3 AMU 1.0 AMU Low M/Z High M/Z

with No Sacrifice in Sensitivity
TOF Resolution < 0.3 AMU at Least Mass with No Sacrifice in Sensitivity Unit Mass Baseline Resolved 1.0 AMU at Greatest Mass Low M/Z High M/Z

TOF Resolution Low M/Z High M/Z

Lower Mass Resolving Power
Unit mass resolution is necessary for practical analysis by ICP-MS but describing the resolution in TOF is a little trickier than a description of a mass filter. A mass filter can usually have selectable resolution in the range of 0.5 AMU per peak up to 1.2 AMU. This is accomplished by using finer increments of DC/RF settings in the quadrupole with the trade-off of lower sensitivity with higher resolution. Whatever the choice, the same resolution will be in effect across the entire mass range. Really the only functional level of resolution obtainable for TOF is unit mass or one Atomic Mass Unit discrimination. The TOF spectrometer is designed to achieve this resolution at the greatest mass since it will have the widest peak at the base. In Time-of-Flight the lesser mass ions have greater velocity in the flight so they ultimately produce signal peaks that are narrower than their larger mass cousins. Also, the lesser the mass, the more widely separated the peaks. So a TOF system may have one AMU discrimination at high mass and 0.3 AMU at low mass. The spectra above illustrate the uniqueness of the TOF resolution. Normally a unit mass resolution will not allow the spectrochemist to view the double charged Lanthanum ions as a separate peak (La ++ = m/z = 139/2 = 69.5 AMU). In quadrupole instruments this peak would be an interference on Germanium and Gallium but in TOF it is resolved.

Resolving Power at High Mass
50 ppt Pb, Bi A very low concentration profile, 50 ppt in solution, is pictured above for Lead an Bismuth. First of all the profile looks the same whether you have a TOF or a quadrupole but in reality there are differences that will be described momentarily. The Pb, Bi profile represents the most common occurrence of adjacent high mass peaks encountered by the typical mass spectroscopists. Here the resolving power is calculated to be 1270 at 50% peak height. Some right angle TOF systems have higher resolving power such as 1500 at this portion of the spectrum but higher resolution beyond the minimum of baseline separation has no benefit at all. The hidden story of these peaks that look very much like a quadrupole spectra is this. A mass filter will step through typically either 10 or 40 settings to generate a peak profile. If 10 steps are used, each segment of the mass scale is 0.1 AMU wide. The first segment that shows signal increase is the finite m/z setting of the mass filter that allows a few ions through the filter to the detector. The apex of the peak is the setting that allows the most ions through the filter. The peak apex is composed of a mixture ions that appear at other m/z settings as well as those that were screened out. It’s very much like stepping an optical slit across a light beam in a stepwise manner from left to right. First a little light gets through, then more and more until the optimum and then the light diminishes as you pass the optimum. There are many factors that cause slight differences in the apparent m/z of similar ions that result in a spread of m/z within a single AMU.

Resolution Selected Spectral Regions Expanded
59 Co + 138 205 + Ba + Tl 63 Cu + 208 Pb + 58 Ni + 203 Tl + 65 Cu + 206 Pb + 207 Pb + 60 Ni + 64 Zn + 137 Ba + 136 Ba + 135 Ba + 62 Ni + 204 Pb + 134 Ba + 61 Ni + 58 60 62 64 134 135 136 137 138 202 204 206 208 m/z

Quadrupole ICP-MS Matrix Filter
RF1, DC1 Time1 Once adequate counting has occurred on the first m/z, the RF and DC conditions are switched, allowed to settle and then counting is initiated on the next m/z of interest (Time2). The field change, field settling and counting of ions is repeated until all the analyte ions, correction equation ions and internal standard ions are adequately counted. The lower the concentration, the greater the time for counting. Also, in a general sense, the greater the change in RF/DC field settings the longer the settling time. This process can be very rapid, particularly when the settings are directed such that only one point per peak (peak hopping) is used rather than 10 or more points to draw the profile of the peak. Quadrupole ICP-MS has been likened to simultaneous ICP emission spectrometry because the analytical speed is quite similar. Yet, the quadrupole is truly a sequential system which transmits only one m/z value at any given moment in time. Information about all other species in the ion beam at that moment is discarded. As a result, the amount of time spent analyzing a sample is directly related to the number of elements analyzed. The more elements analyzed, the longer the analysis will take (under a fixed set of acquisition conditions). This feature is particularly detrimental when dealing with small volume samples or as we will demonstrate, any sample that yields a transient signal. 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
T.R.I.P. Transverse Rejected Ion Pulse Flight Tube Acceleration Repeller Modulation

Background Species Deflection (T.R.I.P.)
Ar+ NO+ O+, OH+ Another key difference between quadrupole or mass filter instruments and Time-of-Flight instruments is the occurrence of large concentrations and how to prevent those high signals from saturating the detector. A mass filter can simply skip over the DC/RF settings that would let a matrix ion through to the detector. A Time-of-Flight instrument accepts the matrix ions into the acceleration segment and has to deal with them in another manner. TOF has to take the bad with the good whereas a mass filter can avoid the bad. Matrix ions such as Argon from the plasma or O, OH and NO from the aqueous sample can be popped out of the flight path with a sideways pulse much like the acceleration pulse of an orthogonal or right angle TOF. This function is called the Transverse Rejected Ion Pulse or T.R.I.P. T.R.I.P. can be set to pulse away several mass regions of different widths thereby protection the detector from over range signals from matrix ions while preserving the analyte ions for analysis. Ar+ NO+

ICP-MS Speed Quadrupole vs TOF
EPA 200.8 (Analytes, Interference Corrections, Internal Standards) Above is an illustration of the time dependence of a quadrupole mass analyzer with the number of element analyzed. Shown in white, the time spent on a sample increases linearly with the number of m/z values analyzed. However, shown in green, the TOF is completely independent of the number of m/z values measured. The illustration is given for USA EPA method which requires some 37 m/z values. More m/z values are measured than just the analyte list since internal standard and correction equation m/z points must be included for accuracy. Clearly the TOF offers vastly improved sample throughput in any multi-element analysis. * Theoretical 0.3 sec. dwell time, 5 replicates, 60 sec. rinse time ** 3 points/peak, 10 ms quadrupole settle time

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