PREPARATION OF TEPHRA SAMPLES FOR ELECTRON
MICROPROBE ANALYSIS
(revised 23rd April 2007)
Pre-requisite
The tephra sample should have previously been processed to remove all residual organic
material i.e. it should consist only of tephra +/- mineral grains.
Equipment required
Glass slides (46mm x 22mm (or wider, but not longer than 47mm)
Buehler Epo-Thin resin and hardener, top-loading balance, hotplate
sharp pencil, razor blade, tweezers, cocktail sticks, pipettes (at least one of each per
tephra sample), lollypop sticks,
carborundum grinding papers with a range of grades (180, 400, 800, 1200, 2500, 4000
grit).
6 μm and 1 μm diamond laps and 6μm and 1μm polishing compounds, Kemet polishing
lubricant. Beakers, ultrasonic bath, Decon90 and petroleum ether for cleaning.
Reflected light microscope
A.
Grain mounting procedure
i. Take a clean glass slide of an appropriate size to fit in the electron microprobe sample
holder and the polishing chucks. Gently grind one face on rough carborundum paper (180
grit) to produce a frosted surface. This will allow the resin to bond properly with the
slide. Ultrasonically clean the slide for a few minutes in water to remove any residual
coarse carborundum.
ii. Using a sharp pencil mark out areas on the slide to scatter the tephra sample within and
add sample names. Several samples can be placed on the one slide but ensure there is a
gap between the samples to prevent cross contamination. A suggested layout for 3 or 4
samples is shown below in Figure 1. 6 samples can be mounted with care by making the
‘boxes’ smaller. Be sure to leave a 2-3mm gap between the ends of the slide and the
sample/your name areas.
Sample name
Your name
Sample name
Your name
Figure 1.
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iii. Mix up a batch of resin. The resin should be mixed in the proportion 100:39 by
weight resin to hardener (e.g. 100g:39g or 1g resin to 0.39g hardener). Each individual
sample will require ~4-6 drops. Cover any sample details with a thin layer of resin. Be
sure to leave a gap of ~4 mm at the top and bottom of the slide - this is important for
correct (level) mounting in the e-probe sample holder.
Sample mounting options:
PMcD: Spread a few drops of resin within a sample area. Sprinkle the tephra on top of
the resin then mix with a clean cocktail stick to distribute the glass throughout the volume
of resin.
JC: Spread a thin film of resin over sample area then sprinkle shards onto resin film.
Allow to dry until tacky then add more drops of resin on top. This method might stop
shards from ‘floating’ to surface meniscus, and might keep shards at the same relative
height (depth) for ease of polishing.
(But also see DAS comment below)
Repeat for other samples until the slide is full. Try to leave a gap between the blocks to
prevent cross contamination.
iv. Leave slide at room temperature for 1-2 hours (but can be left overnight). Then
transfer the slide to the hot plate for 60 minutes to allow the resin to fully cure/set.
Remove from hotplate and allow to cool.
B.
Grinding
The sample must be ground to expose the tephra shards on a flat surface prior to
polishing. This should be done progressively and carefully to produce a uniform, flat
surface, with minimal loss of material.
i.
Grinding should be done sequentially through the following grades: 400, 800,
1200, 2500 and finally 4000: coarser grades cut, finer grades polish. Ensure the papers
are wet. Spend enough time on the 400 grade to expose tephra shards on the surface of
the resin (but do not grind to a sample thickness of less than 0.004”/100um/0.1mm).
Exposed shards will be highly reflective relative to the resin when viewed under the
reflected light microscope. Then 2-3 minutes on each grade should be enough.
Geography modification (per Anthony Newton/Julie Mitchell):
a) using a micrometer measure glass slide thickness.
(NOTE: This step no longer used - see DAS comment below……b) grind slide to slide
thickness PLUS 0.006” (= 150um/0.15mm) using 180 grade paper)
b) grind to slide thickness PLUS 0.004” (= 100um/0.10mm) using 400 grade paper
c) grind to slide thickness PLUS 0.003” (= 75um/0.075mm) using 800 grade paper
d) grind to slide thickness PLUS 0.002” (= 50um/0.050mm) using 1200 grade paper
e) polish sequentially using 2500 and 4000 grade papers. Check grain polish after a
minute or so on each……
NOTE: Use micrometer frequently to monitor sample thicknesses and ‘bias’ pressure
during polishing to maintain relative equal sample thicknesses over section.
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DAS comment: The Geography method minimises overgrinding (and surface shard loss)
using (180 and) 400 grade papers, but is obviously dependent on where/how the shards
are initially loaded into the resin. If shards are uniformally very small (<40um) it is
possible that they may not be sufficiently exposed using the “JC” loading method. The
PMcD method would disperse/mix the shards more evenly throughout the resin but has
the apparent drawback of potentially more shard loss during the initial stages of grinding.
An additional concern is 180 grade paper is probably too coarse and leads to
scouring/plucking of grains from the resin. Therefore use only 400 grade for initial
grinding down to slide thickness plus 0.004”.
ii.
Following grinding put the slides in a beaker of pet ether and place in the
ultrasonic bath for 5 – 10 minutes to remove grinding media and any loose but trapped
sample cuttings.
C.
Final Polishing
Samples should be fine polished first on the 6 μm diamond polishing lap and then the 1
μm polishing lap for 15 minutes each. Ultrasonically clean the slides in pet-ether
between the 6 and 1 μm polishing grades and after the 1 μm polishing step. Check that
the tephra shards are well polished using the reflected light microscope before carbon
coating the samples prior to analysis.
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