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ICP-OES- Inductively Coupled Plasma Optical Emission Spectrometry

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Inductively Coupled Plasma Atomic Emission Spectrometry(ICP-AES)

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Atomic spectrometry

Atomic Absorption

Mass Spectrometry

Atomic Emission

Light of specific characteristic wavelength is

absorbed by promoting an electron to a higher

energy level (excitation)

Light absorption is proportional to

elemental concentration

Light of specific wavelength

from Hollow Cathode Lamp (HCL)

Light and heat energy from high

intensity source (flame or

plasma)

Light and heat energy from high

intensity source (plasma)

High energy (light and heat) promotes an

electron to a higher energy level (excitation).

Electron falls back and emits light at

characteristic wavelength

Light emission is proportional to

elemental concentration

-

-

-

- -

-

-

High energy (light and heat) ejects electronfrom shell (ionization). Result is free electron

and atom with positive charge (Ion)

Ions are extracted and measured directly in

mass spectrometer

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ICP-MS

ICP-AES

GF-AAS

Ranges< ppt

100 ppb0.1 ppb

10 ppm1 ppb

100 ppb

Merit

Demerit

Detection mass emission absorbance

high sensitivity

multielement

multielement

damage from

high salinity

relatively low

sensitivity

high sensitivity

monoelement

Characteristics of instruments

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i. Need to break sample into atoms to observe atomic spectra

ii. Basic steps:a) nebulization solution sample, get into fine droplets by spraying through thin nozzle

b) desolvation - heat droplets to evaporate off solvent just leaving analyte and other

matrix compounds

c) volatilization convert solid analyte/matrix particles into gas phase

d) dissociation break-up molecules in gas phase into atoms.

e) excitation and ionization

with light, heat, etc. for spectra measurement.->cause theatoms to become charged

Atomization

Evaporation/ Vapouration /

Dissociation

Excitation and ionization/

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Inductively Coupled Plasma

Inductively Coupled Plasma (ICP)

Plasma generated in a device called a Ar

cools outer tube, defines plasma shape

Rapid tangential flow of argon cools outer

quartz and centers plasma

Rate of Argon Consumption 5 - 20 L/Min

Radio frequency (RF) generator 27 or 41

MHz up to 2 kW

Telsa coil produces initiation spark

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Torch

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Magnetic field

Ions forced to flow in closed

path, Resistance to flow

causes heating

Ar charges

by Tesla coil(high voltages at high frequency)

Temperature Regions

in Plasma Torch

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Plasma characteristics

Hotter than flame (10,000 K) -more complete atomization/excitation

Atomized in "inert" atmosphere

Ionization interference due to high

density of e- is very small Sample atoms reside in plasma for

~2 msec

Plasma chemically inert, little oxideformation

Temperature profile quite stableand uniform.

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Instrumentation of ICP-AES

1. Radio Frequency (RF) Generator

2. Sample Introduction System

3. Torch

4. Spectrometer (Polychromators)

5. Detector

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Schematic diagram of ICP-AES instrument

Radiofrequencygenerator

Spray chamber

To drain

Injectorgas

Samplesolution

Peristalticpurge

Sample capillaryNebulizer

Coating gas

Auxiliary gas

Coolant gas Torch

SpectrometerInduction cell

Fireball

Tail flame

PMT

ComputerComputer

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A. RF Generator

Radio-Frequency (RF) Generator is a device that is used toprovide the power required for the generation and sustaining of

the plasma discharge.

The power required for ICP-AES measurements ranges

between 600 and 2500 W

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B. Sample Introduction System

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Peristaltic Pump

(1) Peristaltic Pump

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(2) Spray Chamber

Scott chamber- traditional equipment

- large volume

Cyclonic chamber

more aerosolsmaller volume

better wash out

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Why we need Spray Chambers?

Separation of small droplets from large ones

- small droplets to the plasma

- large droplets to the drain

Compensate pulsation of the pump

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(3) Nebu lizer

Transform a liquid sample into an aerosol

- direct introduction of a liquid would extinguish plasma

- two types of nebulizers are commonly used

(a) pneumatic nebulizers

(b) ultrasonic nebulizers- mostly use of a peristaltic pump to transport

(a) sample to the nebulizers

(b) aerosol to the plasma

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Torch

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Plasma Torches

The plasma torch consists of three concentric quartz tubesthrough which the gas (normally argon) flows:

- The outer tubecontains the coolant gas flow, whichspirally flows tangentially through the torch at a high

velocity. This assists in cooling the torch and henceprevents damage.

- The middle tubecontains the auxiliary gas flow tokeep the plasma discharge away from the auxiliaryand nebulizer tubes

- The innermost tubehas the nebulizer gas flow whichcarries the sample aerosol to the plasma.

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4 S t t (P l h t )

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4. Spectrometer (Polychromator)

The role of a spectrometer is to isolate the analytical wavelengths of interest

from the light emitted by the plasma source.

The advantage of polychromators is:- capable of determining several analytes simultaneously.

- high sample throughput

- lower running cost

Echelle Polychromator

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5. Detectors

Commonly used detectors:

Photomultiplier tubes (PMT)

Solid-state detectors: Charge-coupled devices (CCD)arrays detector

Silicon photodiodes with thousands of individual

elements

Very sensitive, very well-suited to echelle gratingpolychromators, very fast

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Photomultiplier tubes (PMT)

Schematic diagram of

an end-on PMT

PMT consists of a photo-sensitive cathodeand a series of dynodes, which are set at successively

more positive potentials until an anode is reached. Light comes from the plasma, passes through

the transparent casing of the multiplier and strikes cathode. This then emits electrons, which are

accelerated down the dynode chain. Each time an electron impacts with a dynode, a number of

secondary electrons are emitted.

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CCD

Examples of the wavelength ranges covered

by individual CCD chips:

Chip 1 127,000 - 142,125 nm

Chip 2 141,285 - 160,681 nm

Chip 3 160,040 - 179,618 nm Chip 4 178,704 - 198,477 nm

Chip 5 197,023 - 216,947 nm

Chip 6 215,903 - 235,897 nm

Chip 7 234,792 - 254,788 nm

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K 766nm

Virtual Entrance Slit

Second Grating2400 l/mm

First Grating2924 l/mm

Entrance Slit

125nm

460nm

Li 670nm

Na 589nm

Multi CCD

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Detection Limits of ICP-OES

Typical detection

limits (Varian VistaMPX):

Considerations

include the number

of emission lines,

spectral overlap

Linearity can span

several orders of

magnitude.

Element Wavelength (nm)

Detection Limit

axial (ug/L)

Detection limit

radial (ug/L)

Ag 328.068 0.5 1

Al 396.152 0.9 4

As 188.98 3 12

As 193.696 4 11Ba 233.527 0.1 0.7

Ba 455.403 0.03 0.15

Ba 455.403 0.03 0.15

Be 313.107 0.05 0.15

Ca 396.847 0.01 0.3

Ca 317.933 0.8 6.5

Cd 214.439 0.2 0.5

Co 238.892 0.4 1.2

Cr 267.716 0.5 1

Cu 327.395 0.9 1.5Fe 238.204 0.3 0.9

K 766.491 0.3 4

Li 670.783 0.06 1

Mg 279.55 0.05 0.1

Mg 279.8 1.5 10

Mn 257.61 0.1 0.133

Mo 202.03 0.5 2

Na 589.59 0.2 1.5

Ni 231.6 0.7 2.1

P 177.43 4 25

Pb 220.35 1.5 8

Rb 780.03 1 5

S 181.972 4 13

Sb 206.83 3 16

Se 196.03 4 16

Sr 407.77 0.02 0.1

Sn 189.93 2 8

Ti 336.12 0.5 1

Tl 190.79 2 13

V 292.4 0.7 2

Zn 213.86 0.2 0.8

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Sampling and sample preparation

Are the samples representative of what you are trying to

measure?

Will any elements volatilize during sample preparation?

How much contamination can the sample tolerate during

preparation?

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Sample preparation

For many applications, the sample analyzed by ICP-AES will not be insuitable form . In order to transform solid samples into suitable form, sample pre-treatment is required.

The pre-treatment method used will be depend on the natureof the sample and the element which are to be determined.

Those methods commonly used are:- Dry-ash,

- Acid digestion,

- Fusion,

- Solubilization

- Microwave digestion.

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Analysis using ICP Sample preparation

Dry Ashing

samplecrucible

Mufflefurnace

Dry at 105 -100

Ash at 200-800

Dissolve the ashed sample inacids, usually HCl, H2SO4,

HNO3and HCl/ HNO3

Acid Digestion

Preparation is simple and widely applicable but sample lossesthrough volatilization and retention

Use of strongly oxidizing mineral acid, suchas HNO3, HF, H2SO4and HClO4 to oxidizethe resistant components, with gentle heat.

It can be carried out in closed or openreaction systems. Closed systems minimize

the losses in volatilization.

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Advantages:

Disadvantages:

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Salt Fusion

Sample is mixed in a platinum crucible with a fluxwhich attacks all the major rock-forming silicates.

Fused in a furnace.

Cooled to room temperature.

Dissolved in HNO3.

Fusion methods are commonly used with geological samples. Sample + Fluxcrucible

Mufflefurnace

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Advantages:

Disadvantages:

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Microwave digestion

Microwave sample preparationuses microwave power to heat

several samples at once, whichcan speed up a digestionprocesses.

Advantages: improved detectionlimits, low acid concentrationand a reduced need for dryashing or fusion.

Reaction pressure, temperatureand time are computer controlled

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Plug

Safety Disk

SealScrew Cap

Liner

Bomb Jacket

Vessel Base Plate

Supporting Vessel

Pressure vessel (TFM)

Microwave sample preparation system

MULTIWAVE (Anton Paar GmbH)

Acid digestion apparatus: microwave system

Preparation of Solid Sample

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Analysis using ICP Sample preparationMicrowave digestion

USEPA method 3051: MICROWAVE ASSISTED ACID DIGESTION OF SEDIMENTS,SLUDGES, SOILS, AND OILS

Digestion vessels carefully acid washed and rinsed with water before use.

Sample up to 0.5 g and 10 ml of concentrated HNO3are placed in microwave vessel.

(For soils, sediments, and sludge use no more than 0.50 g. For oils use no more than 0.25 g.) Sample vessel equipped with a single-port cap and a pressure relief valve.

The vessels are capped and heated by a suitable laboratory microwave unit.

The vessel contents are filtered, centrifuged, or allowed to settle and then

diluted to volume and analyzed.

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Advantages:

Disadvantages:

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ICP/AES INTERFERENCES

Spectral Interferences

Physical Interferences

Chemical Interferences

Memory Effect

S t l i t f

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Spectral interferences: caused by background emission from continuous or recombination phenomena,

stray light from the line emission of high concentration elements,

overlap of a spectral line from another element,

or unresolved overlap of molecular band spectra.

Corrections

Background emission and stray light compensated for by subtractingbackground emission determined by measurements adjacent to the analyte

wavelength peak.

Correction factors can be applied if interference is well characterized

Inter-element corrections will vary for the same emission line among

instruments because of differences in resolution, as determined by the

grating, the entrance and exit slit widths, and by the order of dispersion.

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The Example for Choosing Wavelength in ICP-AES

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The Example for Choosing Wavelength in ICP AES

Element wavelength

(nm)Element wavelength

(nm)

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Physical interferences

Cause effects associated with the sample nebulization and transport

processes.

Changes in viscosity and surface tension can cause significantinaccuracies,

especially in samples containing high dissolved solids

or high acid concentrations.

Salt build up at the tip of the nebulizer, affecting aerosol flow rateand nebulization.

Reduction by diluting the sample or by using a peristaltic pump,

by using an internal standard

or by using a high solids nebulizer.

Ph i l i t f

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Physical interferences

Wavelength calibration

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Chemical interferences:

include molecular compound formation

Normally, this effect is not significant with the

ICP technique.

Chemical interferences are highly dependent

on matrix type and the specific analyteelement.

C bi d Eff t

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Combined Effects

Compensation : (a) matrix of standards should be closely matched with that

of the samples (matrix-matched calibration)

(b) Matrix removal

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Memory interferences:

When analytes in a previous sample contribute to thesignals measured in a new sample.

Memory effects can result from sample deposition on the uptake tubing to the nebulizer

from the build up of sample material in the plasma torch andspray chamber.

The site where these effects occur is dependent on theelement and can be minimized by flushing the system with a rinse blank between samples.

High sal t concentrat ions can cause analyte signalsuppressions and confuse interference tests.

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ICP-AES S1

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