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Chem415Quantitative Bio-Element

Imaging Center (QBIC): Part II

APRIL 13, 2015

DIRECTOR: PROFESSOR THOMAS V. O’HALLORAN

MANAGING DIRECTOR: KEITH MACRENARIS, PH.D

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Analytical Advantages of ICP over AA?

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ICP is The Choice for Multi-Element Analysis

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Component Breakdown of ICP

Front End Integration• For Liquid Samples: autosamplers coupled to peristaltic pumps• For Separations/Speciation Analysis: Systems can be designed with GC, HPLC, IC or CE prior to

integration into the ICP• For Imaging: Laser ablation units can be integrate for quantitative elemental imaging/mapping

Sample Introduction• Nebuliser – Need to produce a fine mist for efficient sample introduction – Usually made of

quartz or PFA – concentric, self-aspirating, and cross-flow • Spray Chambers – Need to discern between large and small droplets from the nebulizer

prior to introduction into the plasma torch – bead impact, cyclonic

Plasma Torch• More efficient and reproducible vaporization, atomization, excitation, and ionization due to

high temperatures of the plasma 6000 K - 10000 K versus 3300 K for flames and furnaces• Uses mainly argon as carrier gas an torch source so low noise or contamination due to

electrode materials required in Flame AA or GFAAS

Detection• Optical Emission Spectrometer – Usually a echelle polychromator and CCD, CID, or CMOS

detector chips• Mass Spectrometer – Quadropole, Magnetic Sector Field, or Orbitrap with pulse counting

and analog detectors

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Autosampler/Peristaltic Pump

Autosamplers: Usually come in a variety of configurations with multiple rack types

Peristaltic Pumps are typically used due to low cost and consistency

2 issues with peristaltic pumps: 1) with ICP-MS you have to average out the peristalsis. 2) sample contamination of tubing is difficult to remove (also known as memory artifacts)

Newer valve/syringe assemblies allow for auto-dilution and decrease uptake and washout times while increasing accuracy and precision

Even using new valve and syringe-driven autosampling peristaltic pumps are still required for waste and washout

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Nebulizers

Nebulizers use 2 priniciple to create a fine mist for sampling:

1) Venturi effect whereby a fluid’s velocity must increase as it passes through a constriction

2) Pneumatic Induction whereby a gas is used as the driving force to convert a liquid into a fine spray

The main types of nebulizers used are concentric and burgener or enhanced parallel path nebulizers (usually for Highly corrosive samples such as HF)

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Spray Chambers

Purpose of a Spray Chamber is to remove droplets produced by the nebulizer that are > 8 µm in diameter, smooth out pulses produced by the peristaltic pump, and if cooled keep the sample thermally stable

Two main types are double-pass and cyclonic spray chambers

Use correct material and choose proper design for particular sample types

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Plasma Torch

Want to increase energy for more efficient atomization and ionization

ICP torches form the plasma by a tangential stream of argon gas flowing between 2 quartz tubes followed by pulsing AC of RF power through the coil producing an oscillating magnetic field (27.12 MHz usually)

An electric spark is applied to introduce free electrons into the gas stream which are then accelerated by the rapidly changing magnetic field

Accelerated electrons collide with argon atoms forcing the loss of an electron which in turn accelerate in the oscillating magnetic field

Results in multiple temperature zones within the produced plasma

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ICP-OES Optics

After sample passes through the plasma the atom is allowed to drop from a high energy state back to the ground state through emission of a photon

These photons are analyzed through a monchromator and echelle grating before hitting the detector

Allows readouts in 2 dimensions providing better resolution and faster analysis than old PMT methods

Can get full spectrum analysis for every sample (called full frame capture)

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ICP-OES: Axial versus Radial Viewing

Radial View: Off-axis plasma viewing provides highest upper linear ranges (~ 100 pm or greater depending upon intensity of spectral line)

Axial View: Views the light looking down the center of the torch providing better detection limits than radial view by up to 10-fold

Dual View: Allows viewing in both axes

Radial viewing is accomplished through a hole in the plasma torch allowing for faster transitions between axial and radial views (shorter run times)

11Advantages and Disadvantages for ICP-OES

Advantages Due to high temperature of ICP

plasma most species are broken into atoms or ions for excitation and subsequent emission

Of all analytical atomic spectrometry techniques has the fewest interferences

Can tolerate up to 20%-30% TDS

Multi-element capability due to CCD/CID chips

Disadvantages Prone to spectral interferences

Easily-ionize-element (EIE) effect – happens with elements with low ionization potentials such as alkaline elements which can suppress or enhance emission signals

Does not provide ultra-trace or real-time analysis

Does not provide information about the different naturally occurring isotopes of a particular element

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ICP-MS

ICP-OES was commercialized in 1974, whereas ICP-MS wasn’t commercialize until 1983

Use Mass Spectrometer to analyze samples due to atomization and more importantly ionization of elements in the plasma

Singly charged ions are being detected according to there mass-to-charge ratio

Have to get the system into a low vacuum state between 10-5 to 10-7 Torr which is accomplished using a sample and skimmer cone interface

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ICP-MS Detection: Cones

Sample Cone

Skimmer Cone

Cones are generally made out of Ni: Robust, minimal interferences, tolerate high matrix, relatively long-lasting and realtively low cost

Can be made out of platinum: More resistant to corrosion, longer lasting, no Ni interference, cost 5-10 times more than Ni cones

Helps restrict the amount of sample going into the ion lenses and quadrupole and helps decrease the pressure incrementally

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ICP-MS Detection: Lens Stack

Positioned between the skimmer cone and mass separation device

Consisting of multiple electrostatically controlled lens components

Steer the ions from the hostile environment of the plasma at atmospheric pressure and steer them into the mass analyzer at high vacuum

Ions with high kinetic energy will be transmitted in preference to ions with medium or low kinetic energy

Recent advancements have led to the design of a 90° lens prior to entry into the KED/CCT reaction cell or quadrupole

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Quadrupole Mass Analyzer

Using a quadrupole mass analyzer consisting of 4 rods that are 15-20 cm in length

Place a direct current on one pair of rods and a radio frequency field on the opposite pair ions of a selected mass are allowed to pass through to the detector while the others are ejected from the quadrupole

In the example 63Cu is repeatedely scanned as electrical pulses are stroed and counted by a multichannel analyzer

Scan rates are typically 2500 amu per second and can cove the entire mass range of 0-300 amu in about 0.1 s

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Mass Interferences

Common interferences are argides, oxides, and chloride

Collision cell and reaction cell technology were developed to handle major mass interferences

Another solution is for high resolution magnetic sector field ICP-MS which is cost prohibitive

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Advantages and Disadvantages for ICP-MS

Advantages Due to high temperature of ICP

plasma most species are broken into atoms or ions for excitation and subsequent emission

Low level detection down to ppt

Isotope abundance determination

Large linear range due to multiple detectors PC and analog

Disadvantages Prone to mass interferences

Low tolerance for TDS usually < 0.5%

Difficult to analyze complicated matrices

More maintenance and associated cost

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Laser Ablation ICP-MS

Information

For Microscopy and LA-ICP-MS email: Keith MacRenaris at keithmacrenaris2009@u.northwestern.edu

For STEM/EDS analysis email: Reiner Bleher at bleherreiner@gmail.com

QBIC website: http://qbic.facilities.northwestern.edu/

NUANCE website: http://www.nuance.northwestern.edu/