lecture 3 aas&ir

Atomic Absorption Spectroscopy PHAM VAN HUNG, PhD Atomic Absorption Spectroscopy • AAS is commonly used for metal analysis • A solution of a metal compound is sprayed into a flame and va

Atomic Absorption

Spectroscopy

PHAM VAN HUNG, PhD

Atomic Absorption Spectroscopy

• AAS is commonly used for metal analysis

• A solution of a metal compound is sprayed into a flame and vaporises

• The metal atoms absorb light of a specific frequency, and the amount of light absorbed is a direct measure of the number of atoms of the metal in the solution

Metal Zn Fe Cu Ca Na

λ (nm) 214 248 325 423 589

Atomic Absorption Spectroscopy:

An Aussie Invention

• Developed by Alan Walsh (below) of the

CSIRO in early 1950s

Principles of AAS

• The metal vapor absorbs energy from an external light source, and electrons jump from the ground to the excited states

• The ratio of the transmitted to incident light energy is directly proportional to the concentration of metal atoms present

• A calibration curve can thus be constructed

[Concentration (ppm) vs Absorbance]

Absorption and Emission

Ground State

Excited States

Atomic Absorption

• When atoms absorb light, the incoming energy excites an electron to a higher energy level

• Electronic transitions are usually observed in the visible or ultraviolet regions of the electromagnetic spectrum

Atomic Absorption Spectrum

• An “absorption spectrum” is the

absorption of light as a function of

wavelength

• The spectrum of an atom depends on its

energy level structure

• Absorption spectra are useful for

identifying species

Atomic Absorption/Emission/

Fluorescence Spectroscopy

Atomic Absorption Spectroscopy

• The analyte concentration is determined from

the amount of absorption

Overview of AA spectrometer.

Sample Compartment

• Emission lamp produces light frequencies unique to

the element under investigation

• When focussed through the flame these frequencies

are readily absorbed by the test element

• The ‘excited’ atoms are unstable- energy is emitted

in all directions – hence the intensity of the focussed

beam that hits the detector plate is diminished

• The degree of absorbance indicates the amount of

• It is possible to measure the concentration of

an absorbing species in a sample by applying the Beer-Lambert Law:

Io

⎛

⎝

⎜ ⎞

⎠

⎟

Abs = εcb

Atomic Absorption Spectroscopy

• Instrumentation

• Light Sources

• Atomisation

• Detection Methods

Light Sources

• Hollow-Cathode Lamps (most common)

• Lasers (more specialised)

• Hollow-cathode lamps can be used to detect one or several atomic species

simultaneously Lasers, while more sensitive, have the disadvantage that they can detect only one element at a time

Hollow-Cathode Lamps

• The electric discharge ionises rare gas

(Ne or Ar usually) atoms, which in turn, are

accelerated into the cathode and sputter

metal atoms into the gas phase

Hollow-Cathode Lamps

Hollow-Cathode Lamps

• The gas-phase metal atoms collide with

other atoms (or electrons) and are excited to

higher energy levels The excited atoms

decay by emitting light

• The emitted wavelengths are characteristic

for each atom

Atomisation

• Atomic Absorption Spectroscopy (AAS) requires that the analyte atoms be in the gas phase

• Vapourisation is usually performed by:

– Flames – Furnaces – Plasmas

Flame Atomisation

• Flame AAS can only analyse solutions.

• A slot-type burner is used to increase

the absorption path length (recall

Beer-Lambert Law).

• Solutions are aspirated with the gas

flow into a nebulising/mixing chamber

to form small droplets prior to entering

the flame.

Flame Atomisation

Flame Atomisation

• Degree of atomisation is temperature

dependent

• Vary flame temperature by fuel/oxidant

mixture

Fuel Oxidant Temperature (K)

Acetylene Air 2,400 - 2,700

Acetylene Nitrous Oxide 2,900 - 3,100

Acetylene Oxygen 3,300 - 3,400

Hydrogen Air 2,300 - 2,400

Hydrogen Oxygen 2,800 - 3,000

Cyanogen Oxygen 4,800

Furnaces

• Improved sensitivity over flame sources

• (Hence) less sample is required

• Generally, the same temp range as flames

• More difficult to use, but with operator skill

at the atomisation step, more precise measurements can be obtained

• Enables much higher temperatures to be achieved Uses Argon gas to generate the plasma

• Temps ~ 6,000-10,000 K

• Used for emission expts rather than absorption expts due to the higher sensitivity and elevated temperatures

• Atoms are generated in excited states and

AAS - Calibration Curve

• The instrument is calibrated before use by testing the

absorbance with solutions of known concentration

• Consider that you wanted to test the sodium content of

bottled water (A = 0.650?)

• The following data was collected using solutions of

sodium chloride of known concentration

0.76 0.52 0.38 0.18

Absorbance

8 6 4 2

Concentration (ppm)

Calibration Curve for Sodium

Concentration (ppm)

A b s o r b a n c e

0.2 0.4 0.6 0.8 1.0

Use of Calibration curve to determine sodium

concentration {sample absorbance = 0.65}

Concentration (ppm)

A

b

s

o

r

b

a

n

c

e

0.2

0.4

0.6

0.8

1.0

∴Concentration

Na + = 7.3ppm

Sample Problem

• The nickel content in river water was determined by AA analysis after 5.00 L was trapped by ion with 25.0 mL of a salt solution released all of the nickel and the wash volume was adjusted

to 75.00 mL; 10.00 mL aliquots

of this solution were analyzed

by AA after adding a volume of 0.0700 μg Ni/mL to each A plot of the results are shown below Determine the concentration of the Ni in the river water.

Determination of Nickel Content by AA

y = 5.6x + 20

0 40 80 120

Volum e of Nickel Added(m L)

Answer: 0.375 μg/mL

Infrared Spectroscopy

What is Infrared?

• Infrared radiation lies between the visible and microwave portions

of the electromagnetic spectrum

• Infrared waves have wavelengths longer than visible and shorter than microwaves, and have frequencies which are lower than visible and higher than microwaves.

• The Infrared region is divided into: near, mid and far-infrared

– Near-infrared refers to the part of the infrared spectrum that is closest to visible light and far-infrared refers to the part that is closer to the microwave region

– Mid-infrared is the region between these two

• The primary source of infrared radiation is thermal radiation (heat).

• It is the radiation produced by the motion of atoms and molecules

in an object The higher the temperature, the more the atoms and molecules move and the more infrared radiation they produce

• Any object radiates in the infrared Even an ice cube, emits infrared

What is Infrared? (Cont.)

Humans, at normal body temperature, radiate most strongly in the infrared, at a wavelength of about 10 microns (A micron is the term commonly used in astronomy for a micrometer or one millionth of a meter) In the image to the left, the red areas are the warmest, followed by yellow, green and blue

(coolest)

The image to the right shows a cat in the

infrared The yellow-white areas are the

warmest and the purple areas are the coldest

This image gives us a different view of a

familiar animal as well as information that we

could not get from a visible light picture Notice

the cold nose and the heat from the cat's eyes,

mouth and ears

Infrared Spectroscopy

• Infrared spectroscopy is the measurement of the wavelength and intensity of the absorption of mid-infrared light by a sample Mid-mid-infrared is energetic enough to excite molecular vibrations to higher energy levels

• The wavelength of infrared absorption bands is characteristic of specific types of chemical bonds, and infrared spectroscopy finds its greatest utility for identification of organic and organometallic molecules

The high selectivity of the method makes the estimation

of an analyte in a complex matrix possible

Infrared Spectroscopy

The bonds between atoms in the molecule stretch and

bend, absorbing infrared energy and creating the

infrared spectrum

Symmetric Stretch Antisymmetric Stretch Bend

A molecule such as H2O will absorb infrared light when the vibration

(stretch or bend) results in a molecular dipole moment change

Infrared Spectroscopy

A molecule can be characterized (identified) by its molecular vibrations, based on the absorption and intensity

of specific infrared wavelengths

Infrared Spectroscopy

For isopropyl alcohol, CH(CH3)2OH, the infrared absorption

bands identify the various functional groups of the molecule

Capabilities of Infrared Analysis

™Identification and quantitation of organic solid, liquid or gas samples

™Analysis of powders, solids, gels, emulsions, pastes, pure liquids and solutions, polymers, pure and mixed gases

™Infrared used for research, methods development, quality control and quality assurance applications

™Samples range in size from single fibers only

Applications of Infrared Analysis

™Pharmaceutical research

™Forensic investigations

™Polymer analysis

™Lubricant formulation and fuel additives

™Foods research

™Quality assurance and control

™Environmental and water quality

analysis methods

™Biochemical and biomedical research

™Coatings and surfactants

™Etc

• Dispersive instruments: with a monochromator to be used

in the mid-IR region for spectral scanning and quantitative analysis

• Fourier transform IR (FTIR) systems: widely applied and

quite popular in the far-IR and mid-IR spectrometry

• Nondispersive instruments: use filters for wavelength

selection or an infrared-absorbing gas in the detection system for the analysis of gas at specific wavelength

Instrumentation

BRUKE TENSORTM

Series

Perkin ElmerTM

Spectrum One

Instrumentation

Dispersive IR spectrophotometers

Simplified diagram of a double beam infrared spectrometer

Modern dispersive IR spectrophotometers are invariably double-beam instruments, but many allow single-beam operation via a front-panel switch.

Double-beam operation compensates for atmospheric absorption, for the

wavelength dependence of the source spectra radiance, the optical

efficiency of the mirrors and grating, and the detector instability, which

are serious in the IR region.⇒single-beam instruments not practical.

Double-beam operation allows a stable 100% T baseline in the spectra.

Dispersive spectrophotometers Designs

Null type instrument

Sample preparation techniques

The preparation of samples for infrared spectrometry is often the most

challenging task in obtaining an IR spectrum Since almost all substances absorb

IR radiation at some wave length, and solvents must be carefully chosen for the

wavelength region and the sample of interest.

Infrared spectra may be obtained for gases, liquids or

solids (neat or in solution)

The end!