Analytical Methods:Microprobe


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[edit] Element micro-analysis

Summary table
ScaleA specialist laboratory technique only requiring specialist equipment and technician
Samples and storageSpecific to the question, usually samples receive a degree of processing prior to analysis depending on the analytical technique used and the questions being asked.
Time and costThis is a high cost technique the turnaround time depends on the sample preparation, if impregnated polished samples are required this can be more than 9 months.
General commentsThis technique is best used to answer very specific questions about specific deposits or artefacts. Where bulk analysis is feasible multi-element analysis is likely to be a cheaper and quicker option.

Microprobe analysis produces element concentration data on a microscopic scale.

[edit] Questions

Microprobe analysis

  • Provenance and mineralogy
  • Material identification
  • Post-depositional soil processes (Ca-Fe-PO4)

Case studies include:

[edit] Sample preparation

The samples for analysis need to be very specific as space in the electron microscope is limited and it can be hard to locate the area of interest on a large polished sample.

SEM-EDX and SEM-WDX: Polished thin sections are ideal but if semi-quantitative analysis is sufficient then powders, dried or fresh samples may be used depending on the capabilities of the electron microscope. For EDX and WDX analyses samples also need to be coated with carbon to prevent the build up of electrical charging at the surface.

No special sample preparation is needed for LA-ICP-MS, PIXIE or XRF analyses.

[edit] Analysis

An SEM-EDX system in use
An SEM-EDX system in use

A number of microanalytical systems exist; the most commonly used are SEM-EDX, SEM-WDX, PIXE, XRF and LA-ICP-MS.

Advantages and disadvantages of Microanalysis techniques
EDXFull spectral analysis means that potentially all elements from Beryllium can be analysed simultaneously. Very rapid analysis though mapping may take a few hours to produce a good image. Usually non-destructive. High detection limits typically more than 1000 mg kg-1.
WDXLower detection limits (typically between 100 and 500 mg kg-1), can produce element maps as well as point analysesMaximum of five elements can be analysed in any one run. Slower than EDX. May cause sample surface damage.
XRFNon-destructive, low detection limits, can produce element maps as well as point analysesTypically low spatial resolutions of 50 μm or more, analysis of elements heavier than Na.
PIXENon-destructive, full spectral analysis from sodium onwards, better trace element resolution than EDX, can produce element maps as well as point analysesNot suitable for light element analysis (C, N, O).
LA-ICP-MSLow detection limits, gives structural/molecular and isotopic informationDestructive, point/spot analysis only

Synchrotron Radiation induced X-ray analysis has also been used increasingly over the last decade. However, the cost of this high energy technique generally limits it use to research projects only. The main benefit of synchrotron radiation analysis is increased spatial resolution, its ability to provide fine structural analysis of materials and molecules, and lower element detection limits.

[edit] Data and interpretation

The type of data produced will depend on the way in which the analysis was carried out. Maps of relative element concentrations Semi-quantitative element concentrations - point and area analyses, Fully quantitative - point and area analyses with standards.

Things to remember and assumptions: Relative concentration maps % concentrations (mass or atomic)

Variability and scaling issues Interference and overlap Operating conditions Standards

[edit] Related techniques

[edit] External links

Oxford Instruments microanalysis web pages [1]

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