ICP MS + Atomic Absorption

Sample requirements for ICP Mass Spec

• Whole blood: royal blue top with EDTA

• Serum - royal blue top

• Urine - approved containers

Laboratory glassware is metals free or acid washed

ICP-MS Sample introduction system

• Composed of a nebulizer and spray chamber and provides the means of getting samples in the instrument

• Small droplets enter the torch while discarding the larger droplets

How does inductively coupled plasma (ICP) ionization work

Argon plasma heats up liquid droplets to dry them to a solid and then heated to gas. As the atoms continue their travel to the plasma, they absorb enough energy to release one electron to form singly charged ions

Use of interface for ICP-MS

Links the atmospheric pressure ICP ion source to the high vacuum mass spec

Use of universal cell in ICP-MS

Helps deal with interferences in analysis

How to deal with ICP-MS interferences without using a universal cell?

• Mathematical calculations used to negate interferences

  • Ex. Argon oxide can mimic iron as it has the same mass of 56. Causes a false positive

  • Argon chloride has a m/s of 75, which would cause false positives when measuring arsenic

3 modes of universal cell

Standard mode, Collision cell, Reaction cell

Universal cell standard mode

All ions pass through the mass spec

Universal cell collision cell (kinetic energy discrimination)

Interferant removal based on size of interferent. Interferants react with an inert gas more frequently than the analyte of interest.

The interferrants no longer have enough energy to pass through the energy barrierC

Cons of kinetic energy discrimination

Lose signal of analyte. There should be high concentrations of analyte

Universal cell reaction cell theory

Interferent removal based on chemical reaction. Interferent produces exothermic reaction with reacrtive gas (analyte produces endothermic reaction)

Interferent becomes neutral and ejected from cell before passing into quadrupole. Analyte signal is preserved

Use of ion optics in ICP-MS

Guides the desired ions into the quadrupole while assuring that neutral species and photons are discarded from the ion beams

Two examples of triple quad (QQQ) ICP-MS setups

Mass Shift and On Mass

Mass Shift ICP-MS QQQ

Example use with Arsenic (m/z = 75) and Interferent Sm2+ (m/z=75):

• Set Q1 to 75, gets rid of Sm+, but we still have Sm++

• Collision cell converts Aresenic to aresnic oxide, converts mass to 91

• Set Q3 to measure 91

On Mass ICP-MS QQQ

Example use with Cadmeium (m/z = 111) and MoO (111)

• Set product ion mass to 111 in Q1

• Q2 filled with reactive gas converts interferant into something new

• Q3 measures 111 still, and only Cd+ is measured

Atomic Absorption Light Source

Hollow-Cathode Lamps coated with the metal of interest. (Each element tested needs its own lamp)

The lamp is filled with a monoatomic gas at low pressure, and special glass is allowed for transmission of the proper wavelength

How does a Hollow-Cathode Lamp work

When light is turned on:

• Electrical potential built up between cathode and anode, impacts the inert gases in the lamp which get ionized

• Positively charged gases attracted to negatively charged cathode

• When gas hits cathode, the element of interest is released

• Arsenic atoms get excited, and then fall back down to ground state, releasing radiant energy that arsenic in the sample can absorb (different energy levels for different atoms)

Sputtering

Continual release of metal atoms from the inner surface of the cathode due to an electrical charge

Atomic Absorption Line Spectra

Radiant energy is measured In line spectra because aborption bands are thin of energy levels

Choose wavelength that Is best for analysis (usually more intense ones)

How does atomic absorption work?

Now that we have the light source is producing radiant energy, we have to get smaple between light source and detector. Done through a spray chamber/nebulizer

• Sample goes through nebulizer and hits glass bead, converts sample to a fine mist/aerosol

• Heavy droplets go to waste

• Sample is now introduced into flamed, and dissociates from chemical bonds (MgCl2 --> Mg2+ + 2e- from flame --> Mg)

  • Ions absorb electrons to become atoms

• Radiant energy released from light source that magnesium atoms can absorb in its ground state

• Light not absorbed is transmitted to detector

What is nebulization

As sample solution goes through the nebulizer, it is converted into a fine spray or aerosol while being introduced into the flame

Purpose of glass impact bead in Atomic absorption sample introduction

Produces fine spray for nebulization

2 types of burners for Atomic absorption sample introduction

Total consumption burner and premix burner

What is a total consumption burner

Fuel, oxidant, and sample are all passed through separate channels to a single opening from which the flame. The entire sample passes into the flame, but in the rapid transit of the droplets through the hot region only the smallest droplets have time to be dried and burned.

What is premix burner

Sample, fuel, and oxidant are mixed in a chamber before entering the flame. The large droplets in the mixing chamber are collected on the walls and pass down the drain, while only the small droplets travel to the flame

Flame type in Atomic Absorption

Premix: Air-acetylene or nitrous oxide acetylene

Total consumption: air-hydrogen

What is emission interference

During atomic absorption, the normal process occurs - ground state atoms absorb light and non-absorbed light is transmitted in

However, When atoms absorb light, they get excited, go back down to ground state and release their own energy! Which will falsely decrease results because more E is getting transmitted to detector (transmission converted to absorbance for measurement)

How to eliminate the issue of emission interference

Problem eliminated bt use of a chopper, which is placed between the lamp and the flame

• Detector only detects light that comes from light source, because chopper causes pulse beam of light

• The chopper “chops” up light

• Detector can distinguish between pulse beam and steady beam. Only measures pulse beam because emission interference produces steady beam

Two forms of flameless atomic absorption

1. Cold vapor generation assembly: used for mercury analysis

2. Graphite furnace

What is cold vapour generation assembly used for

Limited to analysis of mercury, as mercury can exist in a volatile free atomic state at room temperature

Cold vapour generation assembly steps

Chemical reactions are used to convert mercury into an atomic vapour

1. Sample is decomposed by acid digestion (nitric acid)

2. A reducing agent (sodium borohydride) converts mercury to elemental state

3. A stream of gas is bubbled through the apparatus pushing mercury vapour into a sealed cell with quartz windows in the optical beam

How does graphite furnace Atomic absorption work

1. Sample is placed on a solid support

2. Graphite tube is heated via electrical resistance sucessfully to different temps:

3. 100-120C drying stage - solvent removed from sample furnace

4. 400-800C ashing stage: removes organic molecules or inorganic material (removal of matrix components)

5. 2600C atomization stage - free atoms are generated within a confined zone. Signal is a peak, with the height/area being related to the amount of analyte present

Use of a monochromator

Isolates a pure radiant signal, keeping extraneous radiant energy (other atomic spectra wavelengths or flame light) from reaching the photomultiplier tube

Use of a photmultipler tube

The detector - measure the intensity of the signal isolated by the monochromator. It functions to convert radiant energy from the hollow cathode to a signal that amplifies this signal to drive a recorder or meter

Photomultiplier tube principle

• Light impinges upon cathode and frees an electron

• Electron is drawn towards first dynode by applied voltage

• Secondary electrons are released and pass on to successive dynodes, which are increasingly higher voltages

• Increasing numbers of secondary electrons are generated at each stage

Pros and Cons of ICP-MS over Atomic absorption

Pros:

• Multi-analyte analysis

• Increased sensitivity

• Less sample preparation

• Less sample needed

Cons:

• Initial purchase of instrument is costly