EDS Flashcards
Electron transitions are….
The movement of electrons between shells.
How can EDS be used in a SEM? (Detector)
A further detector is required to be fitted to the SEM, in addition to the secondary and backscatter detectors, to detect X-rays generated by electron interactions.
EDS can be used in a SEM to determine….
The elemental composition of samples.
How can SEM-EDS be used in art?
Can determine chemical composition of unknown inorganic pigments.
How can the energy of the X-ray determine element composition?
Energy of generated X-ray is characteristic of the element it originated from.
X-rays are detected in a SEM by and EDS which is housed in….
A SEM chamber on a mechanical arm.
The EDS system involves a detector made of….
A semi-conductor crystal, eg silicon. This is a silicon drift detector, SDD.
Two advantages to using silicon drift detectors SDD….
Doesn’t require cooling by liquid nitrogen.
Give a higher wavelength resolution.
The SDD absorbs incoming X-rays and it’s atoms become ionised, this liberates….
Free electrons from accessible energy shells and generates an electric current.
The generated voltage is proportional to the energy of the xray and therefore the….
Element is originated from.
How is the EDS spectrum formed?
X-ray emissions are generated simultaneously and arranged according to magnitude into a spectrum.
What does each peak on the spectrum show?
The energy of each X-ray and therefore each peak is specific to a specific element.
SEM-EDS tells you which element is present (qualitative), it can only tell you concentration (Quantitive) if….
The instrument is calibrated against a standard.
Composition info can be obtained by EDS in two ways….
Spot analysis and elemental mapping.
Obtaining composition info by spot analysis…..
The beam can be focused onto a particular area of the sample. It acquires a full spectrum for the area under the beam.
Obtaining composition by elemental mapping…
The beam is rastered, as it would be to produce an image, this collects X-rays from the area of interest.
An advantage of elemental mapping is….
It can provide info about spatial distribution of elements in a sample.
EDS is unsuitable to determine….
Organic pigments, instead a GC-MS should be used.
How is the resolution of an element map determined?
Determined by beam size. X-ray response depends how how long the beam dwells on a particular area. The longer the dwell time, the more info collected.
The final element map can be given a false colour to…..
Enhance the signal.
Element maps may be overlaid to identify….
Particular multi element phases.
Once composition features have been identified on an elemental map…..
Spot analysis may be used in the zone of interest.
For quantitive elemental analysis, the sample is polished flat and the system is calibrated against….
A suite of mineral standards (measured annually) plus a metal standard (primary standard).
The used of standards in a SEM-EDS allows…..
Identification of element peaks and absolute concentrations of elements when analysing unknown minerals.
How are standards analysed?
In a sample chamber under the same conditions as the sample.
