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Morphologi G3-ID : What particle size range can it measure?

12 October 2012 No Comment

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Were very happy to see that many hundreds of you tuned in for the Morphologi G3-ID launch last week. There were so many questions that Debbie and myself were unable to answer them all in the time we had – so the Blog comes to the rescue!

One question that was asked by quite a few people was:

Over what particle size range can the Morphologi G3-ID do chemical ID measurements?

The honest answer to this question is that it all depends upon the sample you are looking at and which objective lens you used for the particle size and shape measurements.

Objective lens

The instrument will automatically collect Raman spectra from particles using the 50x objective regardless of the objective lens used for the morphological (particle size and shape) measurements as this gives the best signal-to-noise. The instrument automatically makes adjustments for this, however reliable targeting of very small particles ie. 1um-20um can only be achieved by using the 50x objective for the morphological measurements. The maximum and minimum particle size which can reliably be used for targeting for morphological measurements made with each of the objective lenses is shown in the table below.

Therefore if you have a very polydisperse sample such as soils for example, you will need to take a decision on which particle sizes are most important to the question you are trying to answer from a chemical ID perspective. It should also be noted that for chemical ID it is not possible to merge morphological measurements from different objectives and separate measurements will need to be made for each objective.

Raman scattering from the sample

In addition to the ability to accurately target the particles, the minimum size range may also depend upon the Raman scattering efficiency of the sample. If the sample is a strong Raman scatterer, then reliable chemical ID of 1um particles is possible, whereas for a weak Raman scatterer there may not be sufficient signal to noise.

This is especially true for particles smaller than the diameter of the laser spot (approx. 3um) , as the Raman signal to noise becomes progressively less as you will be sampling both particle and background as illustrated in the diagram below for 7um(blue) and 2um (green) particles of the same chemical composition.

So the bottom line is – it all depends!

Hopefully that gives a more complete answer than I was able to give during the webinar. If you have any particular queries regarding the suitability of the Morphologi G3-ID for your application don’t hesitate to get in touch!