Atomic Emission - AES

Thermal excitation M → M*

Radiative decay to lower energy level M* → M + h

Emission signal directly proportional to concentration

Holy Grail of Atomic Spectroscopy

For one sample:

The ability to measure all elements at all ranges of concentration at one time.

Excitation Source

The atoms are excited by energy provided by the source.

The energy created by a flame can excite only a few atoms, e.g. alkali metals

Other atoms (especially non-metals) need much higher energy - plasma

If you only have a flame instrument, you can use AES for alkali metals (and a few others), otherwise you should use AAS to achieve good detection limits.

Types of high energy analytical plasmas

DC Arc 4000-5000 K

HV Spark 40,000

Direct Current Plasma 6000-10,000

Inductively Coupled Plasma (ICP) 6000-8000

Microwave Induced Plasma (MIP) electrodeless

5000-7000

Capacitively Coupled Microwave Plasma (CMP) electrode

5000-7000

PlasmasIonized gas that is electrically neutral

Very high temperature and energy

Contains ions, electrons, neutral atoms & molecules

Inductively Coupled Plasmas

Up to 20 mL/min Ar flow

Annual cost of several thousand dollars

Ionized Ar flow, sustained in a torch by the RF field generated by induction coils.

Characteristics of Plasma AES

Sufficient energy to excite all elements

Capable of doing solids, liquids, or gases-sample introduction via nebulizer, ETV, laser ablation, others

Tolerant to variety of solvents and solutions

Simultaneous multielement analysis

Large Linear Dynamic Range (LDR)

Low LOD

ICP-AES spectrum

ICP AES Calibration CurveIf you can interpret

your spectrum, you can get great quantitative results.

Calibration curve is plotted as log/log, because the LDR spans several orders of magnitude.

Internal Standard

An internal standard is used to compensate for various random (and even systematic) errors.

A big random error in plasma emission spectroscopy is power/intensity fluctuations of the plasma.

Reasoning: fluctuations effect on analyte will be the same as the effect on the internal standard.

Quantitative Analysis - Calibration with Internal Standard

Internal standard must be something not present in your standards or sample (in this example, Y)

The signal plotted is the ratio:

Intensity ratio = Analyte signal Yttrium signal

Homework problemy = 4.2781x + 7.1758

0

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0 20 40 60 80 100 120

Cu concentration (ppm)

intensity ratio

y = 0.7619x + 1.0441

0

0.5

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1.5

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-0.5 0 0.5 1 1.5 2 2.5

log concentration (ppm)

log intensity ratio

linear-linear plot log-log plot

When your LDR spans more than 2 orders of magnitude, it can be helpful to do a log-log plot so you can see your data points better.

ICPAdvantages

Analysis of solutions or dissolved solids

LDR spans several orders of magnitude

Detection limits in the parts per billion range

Multielement analysis: Determine up to 70 elements in two minutes per sample

Disadvantages

ionization leads to complex spectra

need high resolution monochromator

Expensive

Plasma source leads to messy background - fluctuations