elemental analysis for glass 1980s Flashcards
refractive index
manufacturers improved quality control, so refractive index was not as discriminating - greater discrimination power was needed
destructive
most elemental analysis of glass is destructive (apart from X-Ray Fluorescence, XRF)
physical and optical comparisons
Physical and optical comparisons are well established and widely accepted in courts internationally
tests
Tests are non-destructive
Tests use inexpensive instruments
Elemental analysis is done after all non-destructive tests and only when additional discrimination is needed
Differences in RI are due to major elements in glass
Classification of glass –> major elements
Discrimination of glass –> minor or trace elements
name two surface techniques for elemental analysis
SEM/EDX
XRF
both techniques are only semi-quantitive
SEM/EDX
Disadvantage: quantitative elemental determination not possible on irregular shaped glass fragments
EDX uses an electron beam to bombard the material otherwise similar to XRF
XRF
Regularly used as accurate, precise, quality control in glass manufacture
emission of characteristic “secondary” (or fluorescent) X-rays from a material that has been excited by bombarding with high-energy X-rays or gamma rays
SEM and XRF - surface not bulk
surface rather than a bulk technique that allow the chemical characterisation of minor and major elements
size of interaction varies with what - SEM nd XRF
sample and accelerating voltage.
what are typical penetration depths - SEM and XRF
between 2-5um or <1um3
what is SEM more useful for compared to XRF
SEM is more useful for getting x-ray analysis from a smaller area
x-rays have created penetrating power than
electrons
XRF is useful for what analysis
bulk analysis
Flameless Atomic Absorption Spectroscopy (FAAS) and Inductively Couples Plasma (ICP)
Both methods used to measure elemental composition of glass
Destructive
Glass has to be dissolved in very strong hazardous acids e.g. HF
ICP is an expensive technique as uses a lot of argon gas, but is multi-element and sensitive
AA is cheaper but much less sensitive and only one element at a time can be measured using suitable lamps
Elemental discrimination using ICP-OES
Flame (1000°C) produces excited atoms and ions that emit electromagnetic radiation at wavelengths characteristic of a particular element. Intensity is proportional to concentration.
ICP-Mass spectrometry
100 times sensitive than ICP-OES information on isotopic abundance
Allows smaller size of sample than ICP-OES
Gives information on isotopic abundance “Isotopic dilution method” but doubles sample preparation time
LA-ICP-MS
Laser Ablation of a solid sample produces droplets of glass from surface that are swept into the ICP-MS
Ablation is a progressive and superficial destruction of a material by melting, fusion, sublimation, erosion and explosion
advantages of LA-ICP-MS
No chemical sample prep involved – reduction time of analysis
Reduce sources of contamination – no solvent used
Reduce poly-atomic interferences – background signal lower
Eliminate risk of hazard HF
Not destructive technique, minimal sample amount
Cheaper as high-grade solvents not required
disadvantages of LA-ICP-MS
Laser parameters need to be optimised for each matrix
Quantification difficult as there are few solid calibration standards -needs to be matrix matched because Mass of sample laser ablated varies depending on matrix
important point LA-CIP-MS
Laminated windshields have different composition between inner and outer glass
Float glass will have a higher content of tin on float side
Tempered glass has different RI within a single fragment, due to exterior compression and interior under tension, but no differences in elemental composition
see powerpoint for
Comparison of discrimination power of EC and LA-ICP-MS
Laser induced breakdown spectroscopy – LIBS
• Laser produces an ionised gas which excites atoms which then give off characteristic light in an emission spectrum.
Advantages of LIBS over ICP techniques
cheaper and portable
Can analyse gasses, liquids and solids
Non-destructive
Minimal sample preparation time
Easier to operate
disadvantages of LIBS over ICP techniques
Interference effects
detection limits not as low, but still very good for discrimination for glass.
Less mature.
Neutron activation analysis
sensitive multi-element analytical technique
discovered in 1936 by Hevesy and Levi
based on nuclear transitions
the specimen is bombarded with neutrons from uranium fission with high flux and low kinetic energy
Results in an excited nuclear state to create artificial radioisotopes of the elements present which decay with emission of gamma rays characteristic of the element from which they were emitted.
Uses small sample ~50 mg and very sensitive, accuracy 5%
can detect up to 74 elements - better for heavier elements from Mg upwards
ICP-AES and PIXE, NAA was the standard analytical method for performing multi-element analyses in the sub-ppm range
Less popular now as samples remain radioactive and there are less nuclear reactors making technique more expensive
Discrimination potential of elemental composition analysis
first use of neutron activation analysis as a method of comparing physical evidence in a British court.
Coleman and Goode (1973) used Neutron Activation Analysis looking at 25 elements
Were able to distinguish all but two pairs from 539 different glass samples
Generally glass composition does not vary much over the dimensions of a window pane and glass is manufactured in only a few places (California Law 1971)
Al, As, Ba, Ca, Hf, Mn, Na, Rb, Sb, Sc, and Sr provided the most discrimination
Concentrations of main elements of glass are
controlled
Concentrations of trace elements often
uncontrolled
trace elements can be introduced to glass
as impurities in the raw materials
from deterioration of the glass furnace
Glass composition analysis can differentiate between glasses made by different manufacturers, glasses from different production lines of the same manufacturer, and glasses made over a period of time in a single production line (Koons 2002).
elemental analysis of glass is performed infrequently because
methods are destructive and samples not large enough
instrumentation expensive to purchase and maintain, with few other applications.
Complex calculations, Bayesian statistical analysis including compositional data is extremely difficult to apply.
