Sunday, January 6, 2013
The chemical reaction campaign has come to an end now that the new European Chemicals law, REACH, has come into force.
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another Classically, chemical reactions encompass changes that strictly involve the motion of electrons in the forming and breaking of chemical bonds between atoms, and can often be described by a chemical equation.
However, now the law is in place there is much to be done to ensure that it really does protect us from the worst chemicals; the following web sites can tell you more about what is happening now.
The "substitute it now" campaign against the worst chemicals
Monday, February 13, 2012
Scanning Probe Microscopy (SPM):
The scanning probe microscopy is a general term for a wide variety of microscopic
techniques, which measure the morphology and properties of surfaces on the atomic scale.
This includes the following:
Scanning Tunneling Microscopy (STM) – which studies the surface topography and electronic
structure, Atomic Force Microscopy (AFM) – which studies the surface topography, surface
hardness and elastic modulus, Lateral Force Microscopy (LFM) – which studies the relative
frictional properties, Scanning Thermal Microscopy (SThM) – which studies the thermal
conductivity, Magnetic Force & Electric Force Microscopies (MFM & EFM) – which study
the magnetic and electric properties.
The techniques of STM and AFM are discussed below, since these are widely used:
Principle: The general principle for all the scanning probe microscopes is that a sharper
probe (or a very fine tip) is used to scan the surface of the sample with much lower force and
obtain the topography and morphology information.
Scanning tunneling microscope: When a sharp tip made of a conducting material is brought
close to a conducting sample, overlapping of the electron clouds between the two surfaces
will occur. If a potential is given between them a current of electrons is formed, which is
often referred as “tunneling” current, and the effect is known as “tunneling” effect. This
effect is largely depended on the distance between the tip and the sample material. Hence, if
the scanning tip is controlled by a high precision motion device made of piezo-electric
material, the distance between the tip and the sample can be measured during a scanning
through a feedback loop control of the piezo-electric element. By this way the sample can be
scanned with sub-angstrom precision.
Atomic force microscope: This technique operates by measuring the forces between the
sample and the tip, and the sample need not be a conducting material. Here, the tip is brought
close enough to the sample surface to detect the repulsive force between the atoms of the tip
material and the sample. The probe tip is mounted at the end of a cantilever of a low spring
constant and the tip-to-sample spacing is held fixed by maintaining a constant and very low
force on the cantilever. Hence, if the tip is brought close to the sample surface, the repulsive
force will induce a bending of the cantilever. This bending can be detected by a laser beam,
which is reflected off the back of the cantilever. Thus by monitoring the deflection of the
cantilever, the surface topography of the sample can be tracked. Since the force maintained
on the cantilever is in the range of inter-atomic forces (about 10-9
Newton), this technique
derived the name “atomic force” microscopy.
AFM operates at two modes:
Repulsive or contact mode – which detects the repulsive forces between the tip and sample;
Attractive or non-contact mode – which detects the van der waals forces that act between the
tip and sample.
Scanning tunneling microscope: It mainly consists of a scanner, probe motion sensor
composed of piezo-electric material, micro probe, etc.
Atomic force microscope: It mainly consists of a scanner, cantilever, laser source, photo-
diode detector, micro-probe, etc.
Applications: Both STM and AFM find applications widely in material sciences especially
for surface studies on a nano scale range. While STM finds its applications in the
characterization of surface structure (including the electronic structure), AFM finds its
applications in measuring the hardness of materials. Sometimes, AFM can be used in the
study of “depth profile” of the deposited oxide layer on to a material.
Disadvantages: A limitation to STM is that it can study only the conducting samples, since
the technique is based on the tunneling current between two conducting areas. Hence, it
doesn’t lend itself to the study of non-conducting materials. In fact, the AFM had been
developed to encounter this problem. These methods require special sample preparation
techniques, which are tedious, like, thin sectioning, electo-polishing, various mechanical
cutting and polishing techniques, etc.
1:14 PM Analysis Through Microscopy, analytical chemistry, instrumental chemical analysis No comments
Transmission Electron Microscopy (TEM):
Principle: In this technique, a beam of high-energy electrons (typically 100 - 400keV) is
collimated by magnetic lenses and allowed to pass through a specimen under high vacuum.
The transmitted beam and a number of diffracted beams can form a resultant diffraction
pattern, which is imaged on a fluorescent screen kept below the specimen. The diffraction
pattern gives the information regarding lattice spacing and symmetry of the structure under
consideration. Alternatively, either the transmitted beam or one of the diffracted beams can
be made to form a magnified image of the sample on the viewing screen as bright-and dark-
field imaging modes respectively, which give information about the size and shape of the
micro-structural constituents of the material. High-resolution image, that contains
information about the atomic structure of the material, can be obtained by recombining the
transmitted beam and diffracted beams together.
Instrumentation: It comprises of a tungsten filament or LaB6 or a field emission gun as
source of electron beam, objective lens, imaging lens, CCD camera, monitor, etc.
Applications: Transmission electron microscopy is used to study the local structures,
morphology, and dispersion of multi-component polymers, cross sections & crystallization of
metallic alloys and semiconductors, microstructure of composite materials, etc. The
instrument can be extended to include other detectors like Energy Dispersive Spectrometer
(EDS) or Energy Loss Spectrometer (ELS) to study about the local chemistry of the material
similar to SEM technique.
Disadvantages: The instrumentation is complicated and needs high vacuum. Sample
preparation is very time consuming. Some materials, especially polymers, are sensitive to
electron beam irradiation which results in the loss of crystallinity and/or mass.
1:08 PM Analysis Through Microscopy, analytical chemistry, instrumental chemical analysis No comments
Scanning Electron Microscopy (SEM):
Principle: In this technique, an electron beam is focused onto the sample surface kept in a
vacuum by electro-magnetic lenses (since electron possesses dual nature with properties of
both particle and wave an electron beam can be focused or condensed like an ordinary light)
The beam is then rastered or scanned over the surface of the sample. The scattered electron
from the sample is then fed to the detector and then to a cathode ray tube through an
amplifier, where the images are formed, which gives the information on the surface of the
Instrumentation: It comprises of a heated filament as source of electron beam, condenser
lenses, aperture, evacuated chamber for placing the sample, electron detector, amplifier, CRT
with image forming electronics, etc.
Applications: Scanning electron microscopy has been applied to the surface studies of metals,
ceramics, polymers, composites and biological materials for both topography as well as
compositional analysis. An extension (or sometimes conjunction to SEM) of this technique is
Electron Probe Micro Analysis (EPMA), where the emission of X-rays, from the sample
surface, is studied upon exposure to a beam of high energy electrons. Depending on the type
of detectors used this method is classified in to two as: Energy Dispersive Spectrometry
(EDS) and Wavelength Dispersive Spectrometry (WDS). This technique is used extensively
in the analysis of metallic and ceramic inclusions, inclusions in polymeric materials,
diffusion profiles in electronic components.
Disadvantages: The instrumentation is complicated and needs high vacuum for the optimum
1:02 PM Analysis Through Microscopy, analytical chemistry, instrumental chemical analysis No comments
ANALYSIS THROUGH MICROSCOPY
The techniques described here are not for the simple, ordinary optical microscopes which use
light for the magnification. These employ electron beam and mechanical probes to magnify
the surfaces under study.
Tuesday, February 7, 2012
5:16 AM Analysis through X-RAY Techniques, analytical chemistry, instrumental chemical analysis 1 comment
X-Ray Diffractometry (XRD):
Principle: In this technique the primary X-rays are made to fall on the sample substance
under study. Because of its wave nature, like light waves, it gets diffracted to a certain angle.
This angle of diffraction, which differs from that of the incident beam, will give the
information regarding the crystal nature of the substance. The wavelength of the X-rays can
be varied for the application by using a grating plate.
Instrumentation : It consists of X-ray tube for the source, monochromator and a rotating
Applications : The diffraction of X-rays is a good tool to study the nature of the crystalline
substances. In crystals the ions or molecules are arranged in well-defined positions in planes
in three dimensions. The impinging X-rays are reflected by each crystal plane. Since the
spacing between the atoms and hence the planes can’t be same or identical for any two
chemical substances, this technique provides vital information regarding the arrangement of
atoms and the spacing in between them and also to find out the chemical compositions of
crystalline substances. The sample under study can be of either a thin layer of crystal or in a
powder form. Since, the power of a diffracted beam is dependent on the quantity of the
corresponding crystalline substance, it is also possible to carry out quantitative