Determination of Metals, Part 9 - Search for courses · 2017. 4. 3. · Indcutively Coupled Plasma...

Determination of Metals,Part 9

Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

Determination of Metals, Course # 55236, March 2016, Norbert Maier

Indcutively Coupled Plasma Mass Spectrometry ( ICP-MS )

• ICP-MS is effective technique for quantitative multielemental analysis.• Covers most of the elements and has very low detection limits.• Excellent for determination trace concentrations of the heavier elements.

• ICP-MS Agilent 7500ce, Octopole reaction system. Laser ablation unit New Wave UP213 with Nd:YAG deep UV (213 nm) laser.


Components of ICP-MS

1 bar

10-3 bar

10-9 bar 10-5 bar


Components of ICP-MS

• Sample introduction system• Plasma torch • Interface• Ion optics• (Collision/Reaction cell)• Mass separation device • Detector


Sample Introduction System & Plasma Torch

• Sample introduction systems used in ICP-MS are similar to ICP-AES.• The most common sample introduction system is the nebulizer connected

to the spray chamber.• Plasma torch is also similar to ICP-AES, but in ICP-MS sample is ionized in

the torch.

http://departments.agri.huji.ac.il/zabam/ICP.htmlDetermination of Metals, Course # 55236, March 2016, Norbert Maier

Interface-Region• The role of the interface is to transport the ions efficiently, consistently,

and with electrical integrity from the plasma, which is at atmosphericpressure (1.0 bar), to the mass spectrometer analyzer region, which is atultrahigh vaccum (10-9 bar).

• This is achieved with an interface region, which consists of two metalliccones with very small orifices. The interface is kept in vacuum of about10-3 bar.

• Ions exreacted from the plasma are passing through the sampler coneorifice (diameter 0.8–1.2 mm), and then further through the skimmercone (0.4–0.8 mm) to the mass spectrometry.

• The cones are usually made from nickel or platinum that is more tolerantto corrosive liquids.


Interface-Region


The Ion Focusing System• The ion focusing system, also known as ion optics, are comprised of one

or more ion lens components, which electrostatically steer the analyteions from the interface region into the mass separation device.

• Ion optics consists of a series of metallic electrodes in form of plates,barrels, or cylinders that have a voltage placed on them.

• The ion optics system takes ions from the hostile environment of theplasma at atmospheric pressure via the interface cones and steers theminto the mass analyzer, which is under high vacuum.

• The ion optics also avoid that particles, neutral species, and photons(that would cause signal instability) are introduced into the massanalyzer and eventually arrive at the detector. This is achieved usingshadow stop devices, which are placed directly between interface andmass analyzer, or with off-axis geometries.


The Ion Focusing System


The Mass Analyzer

• Separates the ions according to their mass/charge ratio (m/z).• Goal: separate the ions of interest from all the other nonanalyte, matrix,

solvent, and argon-based ions.

• There are three kinds of commercially mass analyzers1. quadrupole mass filters2. double focusing (magnetic/electostatic) sector field3. time-of-flight


Quadrupole Mass Filters• The most common mass analyzer in ICP-MS.

• Quadrupole ICP-MS technology is considered a very mature, routine, high through put, trace-element technique.

• A quadrupole consists of four cylindrical or hyperbolic metallic rods of the same length and diameter.

• They are typically made of stainless steel or molybdenum, and sometimes have a ceramic coating to ensure corrosion resistance.

• By placing a direct current (dc) field on one pair of rods and a radio frequency (rf) field on the opposite pair, ions of a selected mass are allowed to pass through the rods to the detector, while the others are ejected from the quadrupole.

• Ions differing from the preselected mass to-charge ratios are ejected from the quadrupole operation space through the spaces between the rods.


• The mass scanning process is then repeated at different mass-chargeratio until all the analytes of interest in multielement analysis have beenmeasured.

• In a multielement run, repeated scans are made over the entire suite ofanalyte masses.

Mass Scanning Process




• Resolution and abundance sensitivity govern the mass analyzers ability toseparate an analyte peak from spectral interference.

• Resolution is normally defined as the width of a peak at 10%-height.

• The ability to separate different masses with a quadrupole is determined by acombination of factors including shape, diameter, and length of the rods,frequency of quadrupole power supply, operating vacuum, applied rf-dcvoltages, and the motion and kinetic energy of the ions entering and exitingthe quadrupole. All these factors will have a direct impact on the stability of theions as they travel down the middle of the rods and thus the quadrupole’sability to separate ions of different mass-to-charge ratios.



• In theory, the resolution of a quadrupole mass filter can be variedbetween 0.3 - 3.0 amu, in practice 0.5 – 1.0 amu, resolving power 300-400.

• However improved resolution always compromises sensitivity.

• Sensitivity specifications are typically 10-50 million counts per second(mcps) per ppm with 10 cps background.

Mass Scanning Process: Sensitivity vs. Resolution


• Abundance sensitivity is related to the signal contribution of the tail ofan adjacent peak at one mass lower and one mass higher than theanalyte peak.

• The overall peak shape, particularly its low mass and high mass tail, isdetermined by the abundance sensitivity.

• It is affected by a combination of factors including the design of the rods,the frequency of the power supply, operating vacuum and the motion andkinetic energy of the ions as they enter and exit the quadrupole.

Mass Scanning Process: Abundance Sensitivity


Double Focusing Magnetic Sector Technology

• Offers better resolution than quadropole.• Two designs: Standard (forward)and Nier-Johnson (reverse) geometry. Both

consist of two analyzers — a traditional electromagnet and an electrostaticanalyzer (ESA). In the standard (sometimes called forward) design, the ESA ispositioned before the magnet, and in the reverse design it is positioned afterthe magnet.

• Ions are sampled from the plasma in a conventional manner and thenaccelerated in the ion optic region to a few kilovolts before entering the massanalyzer.

• The magnetic field is dispersive with respect to ion energy and mass; focusingall the sampled ions with diverging angles of motion.

• The electrostatic analyzer is only dispersive with respect to ion energy;focusing the ions onto the exit slit onto the detector unit(s).




• The resolving power of magnetic sector mass spectrometer is up to10,000.

• Sensitivity specifications are typically 100–200 million counts per second(mcps) per ppm with 0.1–0.2 cps background. It is therefore suitable forisotope analysis.

• Typically cost 2-3 times that of quadrupole ICP-MS instruments and aremore difficult to operate.

• Typical speeds for a full mass scan (0–250 amu) of a magnet are in theorder of 400–500 ms, compared with 100 ms for a quadrupole: Thismeans that a magnetic sector mass spectrometers are 4-5 times slower indata aquisition compared to quadropole instruments.



Time-of-Flight Technology

• Because, the kinetic energy of ions is directly proportional to its mass,the population of ions with different mass will have different velocitywhen given the same kinetic energy with acceleration voltage.

• Time-of-Flight Technology (TOF) separates different mass-charge ratiosaccording to time over fixed flight path distance.


• TOF is capable of sampling all ions generated in the plasma atexactly the same time which is an advantage in:

– Multielement determinations involving rapid transient signalsgenerated by sampling accessories such as laser ablation andelectrothermal vaporization devices; chromatographic techniques.

– High-precision, relative calibration techniques such as internalstandardization and isotope ratio analysis.

– Rapid multielement measurements, especially where sample volumeis limited.

• Resolving power 500–2000.

Time-of-Flight Technology


Detectors• The detector converts the sampled ions into electrical pulses, which are

then counted by its integrating measurement circuitry.

• The magnitude of the electrical pulses corresponds to the number ofanalyte ions present in the sample.

• In older models, most common detectors are channel electron multiplierand Faraday cup. In newer models, active film or discrete dynode electronmultiplier is the most common.

• Discrete dynode electron multiplier uses an array of multiple dynods. Ionshitting the first dynode liberate secondary electrons, which uponimpacting the second dynode release more electrons. This processrepeated at each dynode unit of the multiplier, generating a pulse ofelectrons that is ultimately captured by an amplifier unit, and transformedinto an electronically processable signal.


Interferences

• Matrix interferences:

Transport effectEffect on ionisation temperature

• Spectral interferences:

Polyatoms and moleculesOxides, hydroxides, hydrides and double charged ionsIsobaric interferences


Matrix Interferences

Transport effects

• Physical properties of the matrix may diminish the signal of the analyte.• Matrix affects the formation of the droplets in the nebulizer or the

distribution of the droplets in the spray chamber.• In organic matrix this is caused by the difference in the viscosity of the

sample matrix and the solvent.

Effect on ionisation temperature

• Matrix composition affects the ionisation temperature in the plasma.• High acid concentrations change the ionisation properties of the plasma.


Spectral InterferencesSpectral interferences are caused by polyatoms, molecules, isotopes and doublecharged ions that have the same mass-charge ratio as the analyte.

Polyatoms and molecules:

• The most common spectral interferences.

• Two or more ions form polyatoms or molecules that have the same mass-charge ratio as analyte.

• They are formed from plasma- and/or nebulising gas, solvents, sample matrix,hydrogen or oxygen from air and analytes.

• For example, argon isotope 40Ar can form in presence of oxygene argonoxide(40Ar16O), causing interference with the iron isotope 56Fe; similarily, inpresence of chloride, formed argonchloride (40Ar35Cl) may interfer witharsenic 75As.


Oxides, hydroxides, hydrides and double charged ions• Oxides, hydroxides and hydrides are formed when hydrogen, oxygen or

hydroxides from water or air react with elements in the sample. Theseproducts can have the same mass-charge ratio as the analyte.

• Molecules are usually formed in the cool part of plasma near the cones.• Douple charged ions are formed when element loses two electrones in

plasma. This causes the mass-charge ratio to drop in half.

Isobaric Interferences• Isobaric interference are caused by isotopes of other elements having the

same mass as the analyte. For example, the selenium isotope 78Se has the same mass as the krypton isotope 78Kr.

Spectral Interferences


Examples for Polyatomic

InterferencesFrequently Observedin ICP-MS


Removal of the Matrix Interferences

• The easiest way to reduce matrix interferences is to dilute the sample. However, this can cause issues with detection limits.

• Use suitable sample preparation techniques.

• Matrix of the sample and standard should be as similar as possible; use of matrix-matched standards.

• The most effective way is to use internal standard. The standard solutionof some element that is not present in the sample is added in the blanks, samples and the standards. Adjustments in the analytes intensity can bemade according to changes in the internal standard intensity.


Removal of the Spectral Interferences

• The easiest way is to use another isotope of the analyte, which does not suffer from spectral interference. Unfortunetely, this is not always possible.

• Formation of molecules in the plasma can be effectively reduced by adjusting the flow rate of the nebulizing gas, power of the radio frequency and the distance of the cones from the plasma.

• Removal of the matrix: accomplished using suitable sample preparation techniques prior to analysis; e.g., for determination of trace elements in seawater, SPE with chelating ion exchange resins to remove high concentrations of interfering sodium and magnesium chloride prior to cause spectral interferences.


High Resolution Analyser:

• The most effective way to reduce spectral interferences is to use highresolution mass-analyzer, such as a double focusing sector analyzer. Inmost cases, enhaced resolution allows to distinguish between themasses of interfering peaks and the analyte of interest.

Cold plasma:

• Cold plasma can be used to reduce polyatomic interferences caused byargon. Here plasma is used in 500-800 W RF power and 1.5-1.8 l/minnebulizer gas flow instead of normal 1000-1400 W and 0.8-1.0 l/min. Thischanges the ionisation properties of the plasma and reduces theinterferences. This method is good for the small group of elementsuffering interferences caused by argon (K, Ca, Fe…). Cold plasma reducesthe ionisation temperature of the plasma and increases the detectionlimits of the most elements.

Removal of the Spectral Interferences


Removal of the Spectral Interferences:Collision/Reaction Cell Technology

• Collision/reaction cell is used to reduce spectral interferences.• The cell can be based on quadrupole, hexapole, or octopole guiding

devices that are positioned between cones and mass analyzer.• This components are not used to separate masses but to

fragment/fractionate ions.• The cells are typically charged with a low concentration of a reaction gas,

inducing collisions of ionic species with gas molecules.• Most commonly used reaction gases are helium and hydrogen.• In the cell ion-molecules collide with reaction gas molecules and form

new products by several different mechanisms. Upon collision withreaction gas, interfering polyatoms tend to fragment into the desired ionicspecies; or are converted into species not interfering with analytical taskat hand, thus reducing background and improving analyte sensitivity.


• 40Ar16O interferes with 56Feanalysis. In the reaction withreaction gas ammonia, it breaksthe argonoxide interferenceeffectively, while leaving the56Fe ion unaffected.

• Note: In collision cells,interfering compounds formedin secondary reactions areremoved according to kineticenergy. This may increases background and reduce sensitivity.Because of this, high reactivegases can be used only in smallamounts.

• In the dynamic reaction cell,masses can be separated andgases with very high reactivitycan be used.

Collision/Reaction Cell Technology -Example: 40Ar16O/56Fe Interference


After Collision Cell PassageUsing He as Collision Gas


Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS): Priniciple

• Powerful analytical technology enabling highly sensitive elemental andisotopic analysis to be performed directly on solid samples.

• A laser beam focused on the sample surface to generate via dielectricbreakdown fine particles – a process known as Laser Ablation.

• The resulting particles are then transferred to the secondary excitationsource of the ICP-MS instrument for on-line digestion and ionization ofthe sampled mass.

• The ions produced in the plasma torch are subsequently introduced toa mass spectrometer detector for both elemental and isotopic analysis.


Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS): Advantages

• Ultra-highly sensitive chemical analysis down to ppb (parts per billion)level — without any sample preparation.

• Samples can be both conducting or non-conducting, and the analysiscan be performed in the air without the need for a complex vacuumsystem.

• Results are available within seconds; therefore LA-ICP-MS delivers thatfastest analysis speed of all analytical techniques with the limit ofdetection approaching ppb level.

• The sample mass size required for LA-ICP-MS analysis is sub-microscale — picograms to femtograms. Traditional liquid nebulizationapproaches for ICP-MS require the removal of milligrams of samplemass in order to be effective.


When applied with optimized laser ablation conditions and ICP-MS data acquisition protocols, LA-ICP-MS allows versatile solid sampling schemes that include:

• Bulk analysis

• Local inclusion and defect analysis

• Depth profiling

• Elemental/isotope mapping

The two most commonly used laser-based methods are bulk analysis with a typical laser spot size of 100 ~ 350 m and microanalysis with the laser spot size as small as a few microns.

Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS): Applications


Laser Ablation ICP-MS: Instrumentation

CETAC LSX 500

Thermo Finnigan Element

www.cetac.com

www.thermo.com


Nanosecond vs. Femtosecond Laser Ablation

• Nanosecond laser ablation is partly a thermal process

• Differences in the vaporization properties of elements leads to elemental fractionation

• With femtosecond pulses, the ablation process occurs by a mechanism far less dependent on thermal effects. Melting is not observed with pulse widths < 1 ps.



Crater Profiles

Crater profiles for 100 fs and 4 ns lasers after 50 pulses


HOW TO CALIBRATE LASER ABLATION?

COMPENSATION FOR MATRIX DEPENDENCE OF ABLATION PROCESS IS NEEDED

• MATCHED STANDARDS

• MEASURMENT OF ANALYTE RELATIVE TO MINOR ISOTOPE OF ELEMENT AT KNOWN CONCENTRATION

• CALIBRATE RELATIVE TO A SOLUTION AEROSOL (Aerosol Standard Addition)

Calibration of LA-ICP-MS with Dried Solution Aerosols

• Simultaneous introduction of particles from a LA cell and desolvated aerosol particles from a micro-flow nebulizer

Stotal = Ssolid + Ssolution

= RX,solid TLAt[X]solid + RX,soln VTneb[X]soln

RX isotope-specific response factor (signal/ng X)TLA transport from LA cell (ng solid/s)t time of ablation transient (s)[X]solid concentration of isotope in solid (ng X/ng solid)V volume of solution injected to ICP (L)Tneb nebulizer efficiency[X]soln isotopic concentration in solution standard (ng X/L)


pump

CETAC LASER ABLATION SYSTEM TSI PIEZOBALANCE

FINNIGANICP-MS

ESI NEBULIZER

Nd:YAG laser (266 nm)

camera

translation stage

argon inlet

zoom lens

impactorelectrostatic precipitator

calibration solution

waste90%

10%

20%

80%

ICP

ESA

magnet

argon inlet


0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 100 200 300 400 500 600 700

Time (s)

Sign

al (c

ount

s s-1

) / 1

06

60Ni+

51V+52Cr+

NIST STEEL

Solution

Ablatedsolid


Calibration of LA-ICP-MS with Dried Solution Aerosols

Data for Steel Reference Material (NIST 1264a Steel, 8 Elements)

• 193 nm ArF LASER• AVG. 3 SPOTS, TWO-POINT STANDARD ADDITION• 50 SHOTS PER SPOT, MEDIUM RESOLUTION• PARTICLE TRANSPORT MEAS. WITH MICROBALANCE

CONCENTRATION (wt %)MEASURED CERTIFIED

V (51V+) 0.115 0.011 0.106Cr (52Cr+) 0.078 0.036 0.066Co (59Co+) 0.137 0.035 0.150Ni (60Ni+) 0.139 0.108 0.142Cu (63Cu+) 0.277 0.201 0.250W (184W+) 0.108 0.013 0.102Pb (208Pb+) 0.021 0.004 0.024Bi (209Bi+) 0.0006 0.0001 (0.0009)


Comparison: ICP-MS, ICP-OES,Flame AAS, and GF-AAS



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