Low Energy Analysis with a Specialised Detector

Light element analysis using Oxford Instruments' Ultim Extreme Detector
OI Extreme
Oxford Instruments' Ultim Extreme energy dispersive spectroscopy (EDS) detector is installed on our JEOL IT800 FE-SEM and is designed for high quality spatial resolution at low energy. This means that PEMC can conduct elemental compositional analysis at lower voltages than previous with better detection of light elements, allowing a greater range of samples to be anaysed. Our Extreme detector can work concurrently with the Ultimax 65 mm2 detector on the same instrument.
Improved Spatial Resolution
The size of the features resolvable during electron microscopy analysis depends on a number of factors, one of which is the "interaction volume" of the beam. Although we are analysis the surface of the sample, the electron beam interacts with a larger area and the characteristic X-rays detected by the EDS detector originate from beneath the sample surface. Larger accelerating voltages produce larger interaction volumes which can obscure small features and is typically mitigated by lowering the accelerating voltage used. However, compositional analysis (EDS) requires enough energy to excite the sample to release characteristic X-rays and can result in a balancing act between sufficient energy for good quality data, the interaction volume generated, and the time required for an experiment. 
 
The Extreme detector is designed to work at low voltages, thus minimising the interaction volume, and allows researchers to collect compositional data of small features that may otherwise not be resolvable during conventional EDS analysis. This has applications across multiple scientific fields including biology, geology, and materials science. 
The example below demonstrates one of the uses of this low energy EDS analysis on a meteorite sample, giving improved compositional detail in an impact melt and revealing the nucleation of tiny crystals that began to form as the melt cooled.
Extreme EDS
Layered element map of an impact melt in a meteorite collected by the Extreme detector. Pink = feldspar, blue-teal = pyroxene, yellow = troilite, green = iron. This image is 0.24 mm across.
Impact melts form when two planetary bodies collide and are preserved in many meteorites. This style of melting occurs very quickly, as does the subsequent cooling which means that minerals have very little time to grow. The smallest features in these melts are tiny minerals that had only just started to crystallise before the melt solidified, and resolving these under typical EDS conditions can be difficult. 
By analysing impact melts with the Extreme detector at low voltages we are not only able to observe subtle compositional zoning in the larger crystals, but also identify the mineralogy of the small newly formed crystals. In this particular test example, the smallest features resolved are under 200 nm. Optimisation of the instrument and detector can improve this. 
This sort of analysis will also be beneficial to geoscience researchers studying volcanic glasses, samples with fibrous crystal habits, and so on. 
Light Element Analysis
As well as improved spatial resolution, the Extreme detector is designed to measure the characteristic X-rays of light elements released during analysis. The Extreme is suitable for biological samples due to its sensitivity for light elements such as nitrogen, and the lower working voltages can reduce sample damage and charging effects.
Electron microscopy-based compositional analysis for biological material is usually conducted through scanning transmission electron microscopy (STEM) EDS - available on our JEOL 7001F FE-SEM and Zeiss Crossbeam 550 FIB-SEM - and requires specialised sample preparation. However, as the characteristic X-rays measured during EDS analysis originate from a deeper area in the sample and TEM/STEM samples are prepared as electron transparent slices, it can be difficult and/or time consuming to generate a sufficient number of counts.

 
Analysis using the Extreme detector does not require the same preparation as for transmission electron microscopy (TEM) or STEM-EDS, but TEM grids can be mounted in such a way to be analysed in the JEOL IT800. Additionally, compositional analysis of biological material using the Extreme does not require prior staining or coating of the sample.
Imaging of the sample can be conducted at the same operating conditions as Extreme EDS analysis. Although the imaging modes available are different from those on the JEOL 1400 TEM, the low voltages and subsequent small interaction volume can reveal key details about a sample. 
Depending on the sample and the goals of the research, the team at PEMC will recommend the best instrument for your particular needs.
Barley leaf
Layered element map of the cell structure in a barley leaf. This image is 0.07 mm across.
We've tested our Extreme detector on a number of samples including the meteorite shown above and this barley leaf. 
This barley leaf sample was prepared as a thick slice that would be too thick to image properly with TEM, and without heavy element staining. This layered element map shows the combined distribution of calcium, oxygen, and nitrogen. We were able to image the sample without adding a conductive coating as is common practice for EDS analysis, and minimal damage to the sample or the resin it is mounted in was observed which allows for further analysis at a different time.
Tests like this have given us chance to develop protocols and standard operating conditions for biological samples so that we are best able to support the students and researchers who access our facilities.