'2DTraC' - a new tool for spatial referencing of samples during microscopy and microanalysis

This report describes a mechanism that has been developed to enable easy tracking and relocation of points of interest between different microscopes and microbeam instruments. This is a requirement when a small area or series of points of interest need to be examined by several microbeam techniques. The report first considers some generic problems associated with the spatial referencing/co-ordinate systems of different microscopes/instruments, and then:
1) evaluates the best procedure for marking a sample with reference points that can be used on any piece of equipment.
It is recommend that the sample surface be initially marked with two ink-crosses, one in the bottom-left corner and one at some other point on the sample surface (ideally diametrically opposed to the first point). A small pit (10-20 microns) should then burned at the approximate centre of each cross using the laser of the Laser Ablation-ICP-MS instrument.
This combination provides a quick means of identifying the general locale of the reference point, and a sufficiently small reference point such that operator bias/error in identification of its centre is reduced.
2) evaluates the accuracy and reproducibility with which an operator is able to measure the location of reference points and points of interest on each piece of analytical equipment.
Tests indicate that the precision with which an operator can relocate the position of reference points and points of interest is largely controlled by the size of the feature of interest. An operator can usually relocate a laser pit of approximately 30 microns diameter with a precision of within 10 microns. The stages of all the microscopes/instruments tested are sufficiently precise to return to within a few microns of a specified pair of co-ordinates.
3) describes an Excel Add-in ‘2DTraC’ that has been developed to convert co-ordinates measured on one piece of equipment to co-ordinates appropriate to the piece of equipment to which the sample is being transferred.
2DTraC (2-Dimensional Transform Calculator) automatically determines the combination of mirror, rotational, stretching and translational transforms that will move the co-ordinates of the reference points as measured using the co-ordinate system of one microscope/instrument into the co-ordinate system of another microscope/instrument. These transforms are then applied to a list of points of interest.
The development of this add-in has removed the need for development and prototyping of highly engineered sample holders for each piece of equipment. However, in order to generate precise transforms, 2DTraC requires that the reference co-ordinates are known sufficiently precisely - errors of 10 microns on each of the reference point measurements will readily compound to give errors in the calculated results of >50 microns. This project initially aimed to be able to relocate points of interest on different microscopes/instruments to within 100 microns. This has been achieved provided that the reference points can be located sufficiently precisely (with care it is possible to relocate the reference points to within 10 microns).