elemental analysis for glass 1980s

Question 1

refractive index

Answer 1

manufacturers improved quality control, so refractive index was not as discriminating - greater discrimination power was needed

Question 2

destructive

Answer 2

most elemental analysis of glass is destructive (apart from X-Ray Fluorescence, XRF)

Question 3

physical and optical comparisons

Answer 3

Physical and optical comparisons are well established and widely accepted in courts internationally

Question 4

tests

Answer 4

Tests are non-destructive

Tests use inexpensive instruments

Elemental analysis is done after all non-destructive tests and only when additional discrimination is needed

Question 5

Differences in RI are due to major elements in glass

Answer 5

Classification of glass –> major elements

Discrimination of glass –> minor or trace elements

Question 6

name two surface techniques for elemental analysis

Answer 6

SEM/EDX
XRF

both techniques are only semi-quantitive

Question 7

SEM/EDX

Answer 7

Disadvantage: quantitative elemental determination not possible on irregular shaped glass fragments

EDX uses an electron beam to bombard the material otherwise similar to XRF

Question 8

XRF

Answer 8

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

Question 9

SEM and XRF - surface not bulk

Answer 9

surface rather than a bulk technique that allow the chemical characterisation of minor and major elements

Question 10

size of interaction varies with what - SEM nd XRF

Answer 10

sample and accelerating voltage.

Question 11

what are typical penetration depths - SEM and XRF

Answer 11

between 2-5um or <1um3

Question 12

what is SEM more useful for compared to XRF

Answer 12

SEM is more useful for getting x-ray analysis from a smaller area

Question 13

x-rays have created penetrating power than

Answer 13

electrons

Question 14

XRF is useful for what analysis

Answer 14

bulk analysis

Question 15

Flameless Atomic Absorption Spectroscopy (FAAS) and Inductively Couples Plasma (ICP)

Answer 15

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

Question 16

Elemental discrimination using ICP-OES

Answer 16

Flame (1000°C) produces excited atoms and ions that emit electromagnetic radiation at wavelengths characteristic of a particular element. Intensity is proportional to concentration.

Question 17

ICP-Mass spectrometry

Answer 17

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

Question 18

LA-ICP-MS

Answer 18

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

Question 19

advantages of LA-ICP-MS

Answer 19

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

Question 20

disadvantages of LA-ICP-MS

Answer 20

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

Question 21

important point LA-CIP-MS

Answer 21

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

Question 22

see powerpoint for

Answer 22

Comparison of discrimination power of EC and LA-ICP-MS

Question 23

Laser induced breakdown spectroscopy – LIBS

Answer 23

• Laser produces an ionised gas which excites atoms which then give off characteristic light in an emission spectrum.

Question 24

Advantages of LIBS over ICP techniques

Answer 24

cheaper and portable

Can analyse gasses, liquids and solids

Non-destructive

Minimal sample preparation time

Easier to operate

Question 25

disadvantages of LIBS over ICP techniques

Answer 25

Interference effects

detection limits not as low, but still very good for discrimination for glass.

Less mature.

Question 26

Neutron activation analysis

Answer 26

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

Question 27

Discrimination potential of elemental composition analysis

Answer 27

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

Question 28

Concentrations of main elements of glass are

Answer 28

controlled

Question 29

Concentrations of trace elements often

Answer 29

uncontrolled

Question 30

trace elements can be introduced to glass

Answer 30

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).

Question 31

elemental analysis of glass is performed infrequently because

Answer 31

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.