Spectroscopic Methods

Modern Electrochemistry - by J O'M Bockris, Amulya K N Reddy

Polymer Chemistry - by Malcolm P Stevens

Continuous Emission Monitoring - by James A. Jahnke

Spectroscopy

is the study of matter by investigating light, sound, or particles that are emitted, absorbed or scattered by the matter under investigation

Spectroscopy may also be defined as the study of the interaction between light and matter


Physical quantity measured

The type of spectroscopy depends on the physical quantity measured.

Normally, the quantity that is measured is an amount or intensity of something


Three main types of spectroscopy

Absorption spectroscopy uses the range
of electromagnetic spectra in which a
substance absorbs.

commonly used in atomic absorption spectroscopy, the sample is atomized and then light of a particular frequency is passed through the vapour

the amount of absorption can be related to the concentrations of various metal ions through the Beer-Lambert law

widely used to measure concentrations of ions such as sodium and calcium

Other types of spectroscopy may not require sample atomization

For example, ultraviolet/visible (UV/ Vis) absorption spectroscopy is most often performed on liquid samples to detect molecular content

Emission spectroscopy uses the range of
electromagnetic spectra in which a
substance radiates

The substance first absorbs energy and then radiates this energy as light

This energy can be from a variety of sources, including chemical reactions

Scattering spectroscopy

measures certain physical properties by measuring the amount of light that a substance scatters at certain wavelengths


Common types of spectroscopy

Flame Spectroscopy

Liquid solution samples are aspirated into a burner or nebulizer/burner combination, desolvated, atomized, and sometimes excited to a higher energy electronic state

The use of a flame during analysis requires fuel and oxidant, typically in the form of gases

Common fuel gases used are acetylene or hydrogen

Common oxidant gases used are oxygen, air, or nitrous oxide

methods are often capable of analyzing metallic element analytes in the part per million, billion, or possibly lower concentration ranges

Atomic Emission Spectroscopy

uses flame excitation; atoms are excited from the heat of the flame to emit light

commonly uses a total consumption burner with a round burning outlet

A higher temperature flame than atomic absorption spectroscopy (AA) is typically used to produce excitation of analyze atoms

Since analyte atoms are excited by the heat of the flame, no special elemental lamps to shine into the flame are needed



Atomic Fluorescence Spectroscopy

This method commonly uses a burner with a round burning outlet

The flame is used to solvate and atomize the sample, but a lamp shines light at a specific wavelength into the flame to excite the analyte atoms in the flame

The atoms of certain elements can then fluoresce emitting light in a different direction. The intensity of this fluorescing light is used for quantifying the amount of analyte element in the sample.

A graphite furnace can also be used for atomic fluorescence spectroscopy. This method is not as commonly used as atomic absorption or plasma emission spectroscopy.



Alternatively, a monochromator can be set at one wavelength to concentrate on analysis of a single element at a certain emission line

Plasma emission spectroscopy is a more modern version of this method.



Atomic Absorption Spectroscopy (often called AA)

commonly uses a pre-burner nebulizer (or nebulizing chamber) to create a sample mist and a slot-shaped burner which gives a longer pathlength flame

The temperature of the flame is low enough that the flame itself does not excite sample atoms from their ground state

The nebulizer and flame are used to desolvate and atomize the sample, but the excitation of the analyte atoms is done by the use of lamps shining through the flame at various wavelengths for each type of analyte

In AA, the amount of light absorbed after going through the flame determines the amount of analyte in the sample


A graphite furnace for heating the sample to desolvate and atomize is commonly used for greater sensitivity

The graphite furnace method can also analyze some solid or slurry samples.

Because of its good sensitivity and selectivity, it is still a commonly used method of analysis for certain trace elements in aqueous (and other liquid) samples.


Atomic Fluorescence Spectroscopy

This method commonly uses a burner with a round burning outlet

The flame is used to solvate and atomize the sample, but a lamp shines light at a specific wavelength into the flame to excite the analyte atoms in the flame

The atoms of certain elements can then fluoresce emitting light in a different direction. The intensity of this fluorescing light is used for quantifying the amount of analyte element in the sample.

A graphite furnace can also be used for atomic fluorescence spectroscopy. This method is not as commonly used as atomic absorption or plasma emission spectroscopy.



Spark or arc (emission) spectroscopy

can be used for the analysis of metallic elements in solid samples

An electric arc or spark is passed through the sample, heating the sample to a high temperature to excite the atoms in it

The excited analyte atoms glow emitting light at various wavelengths which could be detected by common spectroscopic methods

Since the conditions producing the arc emission typically are not controlled quantitatively, the analysis for the elements is qualitative.


Visible spectroscopy

Many atoms emit or absorb visible light. In order to obtain a fine line spectrum, the atoms must be in a gas phase. This means that the substance has to be vaporised.

Spectrum is studied in absorption or emission.


UV spectroscopy

All atoms absorb in the UV region because photons are energetic enough to excite outer electrons. If the frequency is high enough, photoionisation takes place.


Infra-red spectroscopy

In organic chemistry different types of interatomic bond vibrate at different frequencies in the infra-red part of the spectrum.

The analysis of IR absorption spectra shows what type of bonds are present in the sample.


Nuclear Magnetic Resonance (NMR) spectroscopy

NMR spectroscopy analyzes certain atomic nuclei to determine different local environments of hydrogen, carbon, or other atoms in the molecule of an organic compound or other compound. This is used to help determine the structure of the compound.




Use of Atomic Absorption (AA) Spectroscopic methods
Metals

Significance

Effects of metals in water and wastewater range from beneficial to dangerously toxic

Some metals are essential
Fe – blood
Cu, Zn – physical performance

Others may adversely affect water consumers, wastewater treatment systems, and receiving waters
Arsenic- cancer, toxic to aquatic spp
Asbestos – asbestosis ( ‘black' lung cancer)
Cadmium, chromium, copper – liver and kidney damage

Methods

Atomic absorption (AA)
Colorimetric methods
Sampling and sample preservation


What fraction to be analyzed
Dissolved
Suspended
Total
Acid-extractable


Preservation

Acidify with 1.5 ml (5ml for alkaline samples) HNO3 /L sample to pH < 2
Filter samples for dissolved metals before preserving
Store in fridge at 4oC
Stable for 6 months


Pre-treatment of samples

Total metals – all metals inorganically and organically bound, both dissolved and particulate

Colorless, odorless water - < I NTU – acidify with HNO3 to pH <2 and analyze directly


Dissolved metals

filter sample (0.45 micrometer membrane filter)
acidify filtrate with HNO3 to pH < 2
And analyze directly

Suspended metals

Filter sample (0.45 micrometer membrane filter)
Digest filter
And analyze

Acid-extractable metals

Extract metals
And analyze extract



Pre-treatment for Acid-extractable metals

extractable metals – lightly absorbed on particulate matter
At collection acidify sample with 5 ml HNO3/L sample
To prepare sample
Mix well
Transfer 100 ml to flask
Add 5 ml HCl
Heat 15 min on a steam bath
Filter thorough 0.45 micrometer membrane filter)
Adjust filtrate volume to 100 ml with water and analyze



Pre-digestion of metals

Reduce interference by organic matter and to convert metal associated with particulates to a form that can be determined by AA

HNO3 – digestion is adequate for clean samples or easily oxidized materials

HNO3-H2SO4 or HNO3-HCl – digestion is adequate for readily oxidizable organic matter

HNO3-HClO4 or HNO3-HClO4-HF – is for the difficult to oxidize organic matter or minerals

Nitric Acid Digestion

Mix sample
Transfer 50 to 100ml to 125 ml conical flask
Add 5 ml conc. HNO3 and few boiling chips
Boil and evaporate on hot plate to lowest volume (10 to 20 ml) before precipitation occurs
Continue heating and adding HNO3 until digestion is complete (light-coloured, clear solution)

Wash down flask walls with water and then filter
Transfer filtrate to 100 ml flask , cool, dilute to mark, mix and analyze.

Direct Air-Acetylene Flame Method

Applicable for Ca, Cd, Cr, Co, Cu, Fe, Mg, Mn, Ni, Zn, etc

Principle

The technique of flame atomic absorption spectroscopy (AA) requires a liquid sample to be aspirated, aerosolized, and mixed with combustible gases, such as acetylene and air or acetylene and nitrous oxide

The mixture is ignited in a flame whose temperature ranges from 2100 to 2800 oC

During combustion, atoms of the element of interest in the sample are reduced to free, unexcited ground state atoms, which absorb light at characteristic wavelengths