MARINE AND COASTAL SURVEYS
LASER PARTICLE SIZE ANALYZER:
Introduction:
Every sediment is a mixture of grains of varying sizes, and grain size is
considered the most fundamental physical property of sediment particles, affecting their
entrainment, transport and deposition. The objectives of a grain-size analysis are to
accurately measure the size of the individual particles, to determine their frequency
distribution, and to calculate a statistical description that adequately characterizes the
sample.
To measure grain size, it is necessary to obtain the size of the clastic particles as
they were deposited. This is often difficult because of the presence of organic matter and
clustering of smaller particles into bigger lumps, especially clay minerals, which have
surface electrical charges. Thus it is necessary before any size analysis to separate all the
individual grains without smashing any of them and remove organic material, chemically
precipitated substances, dissolved salts etc. The ideal methodology for size analysis
includes the following steps.
1. Removal of organic matter by H2O2
The removal of organic matter may be necessary to achieve complete dispersion of the
clay and, in sediments with an elevated organic content (>3%), to prevent the organics from
being counted as part of the sample, which would bias the grain-size distribution. The sample
is placed in a 600-ml beaker and a small volume (~10 ml) of 30% hydrogen peroxide is
added. The sample is stirred and, if necessary, water is added to slow the reaction down and
prevent bubbling over. More hydrogen peroxide is added until the dark color of the organic
matter has largely disappeared; then the sample is washed three times with a NaOAc buffer of
pH 5 and once with methanol to remove the remaining released cations (Jackson, 1956).
2. Washing / removal of salts
This is done by mixing distilled water with the sample after H2O2-treatment and decanting
clear water after 24 hours. The process is repeated twice to ensure complete removal of
dissolved salts.
3. Dispersal (breaking up of particle agglomerates using a suitable dispersant)
Separation of clay lumps into individual flakes is very difficult as they have a negatively
charged ionic lattice and take-up positive ions from the solution leaving it swarming with
unsatisfied negative ions. The best way to deflocculate is to add exact amount of dispersant
(also called peptiser), which build up charges on the clay particles or provide a repulsive
coating around them. The common dispersants are conc. NH4OH, 0.2N Na2CO3, 0.1N
sodium oxalate or sodium hexametaphosphate.
4. Wet – sieving: Wet sieving is done to separate the material coarser than sand
(2mm).
The sand and finer fraction can be size-analysed with the help of the Laser Particle Size
analyzer MASTERSIZER2000 which is operative in the Petrology Laboratory since 2006.
LASER PARTICLE SIZE ANALYSER
Because of their small size, fine-fraction particles are difficult to measure by sieving. Before
the advent of laser diffraction technique, there were classical techniques like the hydrometer and
pipette methods to separate the different fractions of sediment. However, many sedimentation
laboratories have discontinued or limited the use of pipette and hydrometer techniques because of
inherent problems with settling (e.g. Brownian motion), thermal convection, irregular particle
shape, mass settling in density currents, rounding errors due to large multiplication factors, and,
especially, the time necessary to extend an analysis down into the clay range.
Laser diffraction, alternatively referred to as Low Angle Laser Light Scattering (LALLS), is the
most useful and modern method for volumetric particle size analysis. It is a non-destructive
analysis of wet or dry sediment, and has inherent advantages, which make it preferable to other
options. Laser diffraction based particle size analysis relies on the fact that particles passing
through a laser beam will scatter light at an angle that is directly related to their size. As particle
size decreases, the observed scattering angle increases logarithmically. Scattering intensity is also
dependent on particle size, diminishing with particle volume. Large particles therefore scatter
light at narrow angles with high intensity whereas small particles scatter at wider angles but with
low intensity.
It is this behaviour that instruments based on the technique of laser diffraction exploit in order to
determine particle size. A typical system consists of a laser, to provide a source of coherent,
intense light of fixed wavelength; a series of detectors to measure the light pattern produced over
a wide range of angles; and some kind of sample presentation system to ensure that material
under test passes through the laser beam as a homogeneous stream of particles in a known,
reproducible state of dispersion. The dynamic range of the measurement is directly related to the
angular range of the scattering measurement. The instrument MASTERSIZER200 analyzes
particles in the size range 0.02 to 2000 micron using a diffraction model based on Mie Theory.
Mie Theory provides a rigorous solution for the calculation of particle size distributions from
light scattering data for all particles, small or large, transparent or opaque and thus used in
modern instruments.
However, there are two important assumptions.
•
•
The particle is assumed to be spherical
This is important, as few particles are actually spherical. Laser diffraction is sensitive to
the volume of the particle. For this reason, particle diameters are calculated from the
measured volume of the particle, but assume a sphere of equivalent volume.
The suspension is dilute
The particle concentration is assumed to be so low that scattered radiation is directly
measured by the detector (i.e. single scattering) and not rescattered by other particles
before reaching the detector (i.e. multiple scattering).
For making measurements in MASTERSIZER2000 (Malvern Instruments), SOP’s are
prepared before any analysis. SOP’s (Standard Operating Procedures) are user-defined
procedures that are easily programmed into the software to allow the same type of sample to be
measured in consistent way with all the decisions on measurement strategy already been made.
When SOP is run, it will automatically run through the defined measurement procedure,
prompting the operator to perform tasks by giving instructions in a message box. Once enough
sample is added, the sample is measured and the result (size distribution) is automatically
recorded in a measurement file.
It is important to Note that the above method (Laser Diffraction) provides only volumetric
results, whereas pippetting method gives weight percent analysis. Thus there are chances of
differences between these results.
The results provided in different formats and at size intervals chosen by operator, can be used for
size classification as well as sedimentological studies.
Report Sheet for MASTERSIZER 2000
DIFFERENTIAL THERMAL ANALYSIS
Thermal analysis comprises a group of techniques (DTA, DSC, TG) in which a physical
property of a substance is measured as a function of temperature, while the substance is subjected
to a controlled temperature programme. In DTA (differential thermal analysis), the temperature
difference that develops between a sample and an inert reference material is measured, when both
are subjected to identical heat treatments. DSC (differential scanning calorimetry) relies on
differences in energy required to maintain the sample and reference at an identical temperature.
TG (Thermogravimetry) is a technique that monitors changes in the mass of the specimen
In Differential Thermal Analysis a sample (called reference sample, the standard) is
placed side by side with a sample of thermally inert material (the inert) in a suitable specimen
holder and the temperature difference between the two is continuously recorded as they are
heated. When no reaction occurs in the specimen there is no temperature difference between the
two, but as soon as a reaction commences the specimen becomes hotter or cooler than the inert
material and a peak develops on the curves for temperature difference against time or against
temperature.
A DTA curve can be used as a fingerprint for identification purposes, for example, in the
study of clays where the structural similarity of different forms renders diffraction experiments
difficult to interpret.
Apparatus
The key features of a differential thermal analysis kit are as follows.
1. Sample holder comprising thermocouples, sample containers and a ceramic or metallic
block.
2. Furnace.
3. Temperature programmer.
4. Recording system.
The essential requirements of the furnace are that it should provide a stable and
sufficiently large hot-zone and must be able to respond rapidly to commands from the
temperature programmer. A temperature programmer is essential in order to obtain constant
heating rates. The recording system must have a low inertia to faithfully reproduce variations in
the experimental set-up.
The sample holder assembly consists of a thermocouple each for the sample and
reference, surrounded by a block to ensure an even heat distribution. The crucible may be made
of materials such as Pyrex, silica, nickel or platinum, depending on the temperature and nature of
the tests involved. The thermocouples should not be placed in direct contact with the sample to
avoid contamination and degradation, although sensitivity may be compromised.
Experimental Factors
The packing state of any powder sample becomes important in decomposition reactions
and can lead to large variations between apparently identical samples. In some circumstances, the
rate of heat evolution may be high enough to saturate the response capability of the measuring
system; it is better then to dilute the test sample with inert material. The shape of a DTA peak
does depend on sample weight and the heating rate used. Lowering the heating rate and reducing
the sample weight, both lead to sharper peaks with improved resolution. The influence of heating
rate on the peak shape and disposition can be used to advantage in the study of decomposition
reactions, but for kinetic analysis it is important to minimize thermal gradients by reducing
specimen size or heating rate.
SDT Q600, TA Instruments : Petrology Division, M & CS, Kolkata has a thermal instrument
(SDT Q600, ) for clay mineralogical studies, generally used in conjunction with X-Ray
Diffraction studies. The SDT Q600 provides simultaneous measurement of weight change and
differential heat flow from ambient temperature to 1500 °C. SDT technology features a dual
beam thermo balance that compensates for beam growth and buoyancy contributions to baseline
drift; thermocouples that provide differential temperature measurements (DTA) within the dual
ceramic beams; and a purge gas system with digital mass flow control, gas switching capability
and a separate gas inlet for the option to deliver reactive gas to the sample. For analyzing samples
that tend to lose weight during heating, the new Q600 technology provides improved DSC
accuracy when the instantaneous weight, rather than the initial sample weight, is used in heat
flow integration. The DSC signal is also useful in providing higher temperature solid-state phase
and melting transitions where no weight loss occurs.
Limitations for qualitative work:
It is difficult to interpret the curves because of limitations imposed by four main factors •
•
•
•
Apparatus: Basically the method appears to require only a furnace, a specimen holder
with suitable wells for the specimen and the inert material and a thermocouple system so
arranged that the temperature difference between the two samples and the specimen and
inert material temperature could be recorded. In practice however, there are
complications. Thus for even reasonably accurate work it is essential that heating rate be
uniform over the whole range and reproducible.
Technique: The peak temperature which is normally used for reporting differential
thermal results as well as for identification is rather variable depending upon heating
rate, amount of active material packing of specimen, type of specimen holder etc. Thus
rigid standardization of technique is essential and result obtained on one apparatus is not
strictly comparable with those obtained on another unless one has considerable
knowledge of curve for the same samples obtained from the two apparatuses.
Minerals: For detection of any mineral it is of course, essential that it undergoes a
measurable energy change in the temperature range used, since most minerals undergo
such a change at some temperature, choice of an appropriate temperature range should
thus have priority. However, assuming a suitable range is used, variations occurring in
the minerals themselves may frequently lead difficulties in the interpretation of a curve.
Thus although certain kaolinite or hyolosite are characterized by a large endothermic
effect at 550-600◦C and a large exothermic peak at about 1000◦C, some ball clay or
fireclay may show only an ill-defined hump in place of the exothermic peak. Mineral
identification from a normal differential thermal curve is therefore very difficult, and is
frequently impossible.
Organic matter: Clay frequently contains organic matter which burns off in the region
above 250◦C interfering with the normal mineral peaks. Removal of such organic matter
by hydrogen per oxide treatment frequently leads, in the presence of calcium, to the
formation of calcium oxalate which gives endothermic peaks at about 200◦C and 770◦C
and a strong exothermic peak at about 470◦C. Its curve can therefore interfere with the
mineral curve almost as much as that for the original organic matter.
In spite of such a formidable list of limitations, the thermal methods have wide range of
applicability in that it gives information which is not obtainable by other methods.
References:
1. University of Cambridge, Materials Science & Metallurgy H. K. D. H. Bhadeshia
2. The differential Thermal Investigations of clays; Robert C. Mackenzie
3. Instrument Manual of SDT Q600.