elemental analysis of glass

Question 1

Elemental Analysis for glass -1980s

Answer 1

 Initially, classifying specimens as to source to rule out or confirm alibis.
 manufacturers improved quality control so refractive index was not as discriminating - greater discrimination power was needed
 most elemental analysis of glass is destructive (apart from X-Ray Fluorescence, XRF)
 Physical and optical comparisons are well established and widely accepted in courts internationally

Question 2

advantages

Answer 2

 Tests are non-destructive

 Tests use inexpensive instruments

Question 3

when is elemental analysis done

Answer 3

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

Question 4

Differences in RI are due to

Answer 4

major elements in glass:

Si ~30% Na~ 8% Ca~ 8% Mg ~ 2% K ~ 1.5%

Question 5

Glasses may have same RI but differ in minor elements:

Answer 5

Al ~ 1% Fe ~ 0.3%

Question 6

Or differ in trace elements:

Answer 6

Ba Mn Ti Sr Zr ~ 0.1%

Question 7

Classification of glass -

Answer 7

major elements

Question 8

Discrimination of glass -

Answer 8

minor or trace elements

Question 9

Techniques for elemental analysis

Answer 9

Surface techniques: SEM/EDX or XRF

look at the ppt for diagram of instruments

However, both techniques are only semi-quantitative

Question 10

SEM

Answer 10

• Surface rather than a bulk techniques that allow the chemical characterization of minor and major elements.
• The size of the interaction volume varies with sample and accelerating voltage.
• Typical penetration depths are between 2-5 micrometers or < 1 micrometer cubed
• SEM is really more useful for getting x-ray analysis from a smaller area

Question 11

XRF

Answer 11

• X-rays have a greater penetrating power than electrons, therefore XRF samples a much larger volume than is possible with an electron gun (SEM), (20 micrometers cubed, approx.. hundreds of micrometers deep)
• XRF analysis is isefule for bulk analysis

Question 12

Elemental composition of glass can also be measured by

Answer 12

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

Question 13

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

both methods are

Answer 13

 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 an be measured using suitable lamps

Question 14

Elemental Discrimination using ICP-OES

Answer 14

• Flame (1000°C) produces excited atoms and ions that emit electromagnetic radiation at wavelengths characteristic of a particular element.
• Intensity is proportional to concentration.
  FBI dependable method to determine Al, Ba, Ca, Fe, Mg, Mn, Na, Sr, Ti, Zr which gave great discrimination e.g. auto side window glass
   Probability of two glass panes from different cars being indistinguishable was 1 in 1080 using ICP-OES
   Using just RI this was 1 in 5.
  Refractive Indices of Auto Side Windows

Question 15

ICP-Mass Spectrometry

Answer 15

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 16

LA-ICP-MS

Answer 16

Laser Ablation of a solid sample produces droplets of glass from surface that are swept into the ICP-MS

Question 17

Laser Ablation

Answer 17

Progressive and superficial destruction of a material by melting, fusion, sublimation, erosion and explosion

Question 18

Advantages of LA-ICP-MS

Answer 18

• No chemical sample prep involved- reduces analysis time
• Reduce sources of contamination (reagents, airborne particulates)- no solvents used
• Reduce poly-atomic interferences- background signal lower
• Eliminate risk of hazard HF
• Not destructive technique, minimal sample amount
• Cheaper as high grade solvents used

Question 19

laser ablation instrument

Answer 19

look on ppt

Question 20

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

Answer 20

look at ppt

Question 21

Elemental ration profile for windshield

Answer 21

look on ppt

Question 22

Comparison of elemental profile: different sources

Answer 22

look on ppt

Question 23

Disadvantages of LA-ICP-MS:

Answer 23

 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 24

Some important points :

Answer 24

 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 25

Laser induced breakdown spectroscopy-LIBS

Answer 25

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

look at ppt

Question 26

Advantages of LIBS over ICP techniques:

Answer 26

	cheaper and portable
	Can analyse gasses, liquids and solids
	Nondestructive
	Minimal sample preparation time
	Easier to operate

Question 27

Disadvantages of LIBS over ICP techniques:

Answer 27

 Interference effects
 detection limits not as low, but still very good for discrimination for glass.
 Less mature.
Becoming very popular.

Question 28

Neutron activation analysis

Answer 28

• 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

look on ppt

Question 29

Discrimination Potential of Elemental Composition Analysis

Answer 29

 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 30

in summary:

Answer 30

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).

Question 31

HOWEVER it 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.