lecture 1. sample preparation

8/10/2019 Lecture 1. Sample Preparation

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Pr epar at ion of Sam ples for Reflected Light M icroscopy andElectron Microprobe Analysis

Er ich U. PetersenMa r ch 31, 1998

The following procedures are recommended for the preparation of samples suitable

for electron microprobe analyses. Typically you will prepare one of two types of samples:

A) 1" round plastic mounts or B) polished thin-sections (1" round or rectangular). The

objective is to produce a surface for analysis that is absolutely flat, highly polished, and

squeaky clean. All sample preparation can be accomplished in MINES 116 (Rock

Preparation Laboratory, Quinten Saharatian, 1-7209).

Round P lastic Mounts

A1 . Cutting of Specimens to Size

The rock specimen should be cut using one of the diamond saws to make a rock chip

that is 1.0+ 0.5 cm thick. The area of interest on the slab should be marked and cut to fit

within the 1" mold (casting cup) used for sample mounting. There should be at least 1/8"

clearance at all points between the cut specimen and the mold. The surface to be analyzed

should be ground the 45 or 15 micron cast aluminum lap until all saw marks and

irregularities are removed. This will minimize later grinding and give you a good idea of

what your sample will look like when completed.

A2 . Mounting the Specimen

Place the specimen face down in a 1" plastic casting cup. Pour Titan Casting Resin in

one of the small (capacity about 90 cc; 3 oz.) paper cups, filling the cup with the amount of

resin needed. Use a new blank paper cup each time. Do not measure out resin in the

calibrated measuring cup. Each 1" casting cup will hold about 10 cc or 1/3 oz of resin (of

course your sample will take up some room). Add 12 drops of the resin hardener for each

30 cc or 1 oz. of resin and stir thoroughly (be precise and thorough or you will get poor

results). Pour the prepared resin into the casting cup over the specimen filling the mold to

the rim. Do this slowly to avoid trapping air bubbles. At this point you may wish to drop a

small label into the resin, identifying your specimen; alternative labeling procedures are

given below. Let the resin set for at least 24 hour sin a relatively warm room. A heat

treatment on a hot plate or in an oven, even for thermally insensitive materials, is neither

advisable nor recommended. If the resin is still tacky after a full day the best procedure is


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merely to give it more time; there is simply no substitute for giving the sample the time it

needs.

A3 . Labeling the Specimen

As indicated above you may have placed a label in the resin itself. Some times it ispossible to write the sample number directly on the rock chip before casting it in resin.

Alternatively you can use a tungsten-carbide scriber or engraving tool to mark the back or

side of the sample once it has hardened.

A4 . Grinding the Sp ecimen

The sample surface, now mounted in epoxy, should be ground on the 15 micron

cast aluminum lap for about 2 - 10 minutes or until the surface is free of excess epoxy and

irregularities. The objective is to obtain a fine sheen over the entire ground surface (dry).

To determine whether a surface is sufficiently flat and free of irregularities you should look

for defects on a dry surface. Apply an even pressure to the sample as you grind to keep the

surface flat. Use the entire lap wheel moving the specimen counter clockwise against the

flow of the wheel. Perhaps the most important point to be stressed here is that if your

specimen is porous or if there are any air bubbles or holes in the ground section you should

not proceed without consulting Quinten. These holes will collect the coarse grit and at a

later stage will destroy your polish as well as destroy the very expensive diamond abrasive

used in later steps. These holes can be eliminated by a second stage coating of resin.

Grind the back of the sample also. This will remove the sharp edges and save much wearand tear on your fingers later on. In addition, these irregular edges will create problems

when you ultimately come to examining the specimen on the microscope stage. As a final

step, consider using the 6 micron cast aluminum lap. Clean the specimen thoroughly in

the ultrasonic cleaner, using water and a drop or two of detergent.

A5 . The Initial Polish (0.3 micron alumina on cloth)

Polish the sample on cloth using 0.3 micron alumina. You will have to experiment

with the consistency of the alumina slurry. Be sure to rotate the sample in place to obtain a

uniform polish. Ninety (90) seconds of polishing should be sufficient for most samples.

When you are done the sample should look shiny as glass when dry. Clean your sample in

the ultrasonic cleaner. Give the sample to Quinten for the Final Polish (A6).

A6 . The Final Polish (1 micron diamond, polishing room)

Clean the sample in the ultrasonic cleaner again. Your sample must be absolutely


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clean to avoid contaminating the diamond lap (One jar of diamond paste costs

approximately $55 and any unfortunate soul who does not follow the instructions to clean

their sample thoroughly will be presented with the bill). Place your sample upside down on

the sample aligner and place a 1" sample holder over it. Tighten the set screw sufficiently

so that the sample will not come loose. Place the sample on the Syntron. See "SyntronProcedures" for instructions on operating the Syntron. After polishing, wash your sample

with soapy water and clean it in the ultrasonic cleaner. Rinse in distilled water. Blow-dry

and do not touch the surface of the sample with you fingers. A light buffing on a chamois

cloth and blow-drying will remove any remaining contamination. Put your sample in a dust

free box.

Thin-Sections (Round or Rectangular)

B1 . Cutting of Specimens to Size

Follow the same procedures used in producing a regular thin-section. If you have

never had the pleasure of making your very own thin-sections here is how: The rock

specimen should first be cut using one of the diamond saws to make rock chip (slab) that is

no thinner than 1.0 cm thick. The area of interest in the slab should be marked and cut to fit

within the perimeter of the 1" diameter round glass plate to which it will later be affixed or,

in the case of rectangular sections, approximately 2 cm x 4 cm.

B2 . Rough Grinding the SpecimenThe chip surface should be ground on the 45 micron cast aluminum lap for about 2 -

10 minutes or until all the saw marks or irregularities are removed from the surface. To

determine whether a surface is sufficiently flat and free of irregularities you should look for

defects on a dry surface. Apply an even pressure to the sample as you grind to keep the

surface flat. Use the entire lap wheel moving the specimen counter clockwise against the

flow of the wheel. After all the cutting and grinding has been completed, clean the

specimen thoroughly. Give your chip to Quinten.

B3 . Mounting the Ch ip on the Round Glass Plate and Preparation for Polishing

Quinten will mount the chip on a glass plate, label the glass plate, cut the chip off at

approximately the right thickness, thin the section to first order yellow on the 45 and 15

micron laps, and return the section to you. Sand the rim of the glass plate so that it no

longer feels sharp. This is very important because it will prevent the polishing cloth from

tearing and possibly destroying your section in the process. Next, thin your sample with


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1000 grit on a glass plate. Do this carefully because it is very easy to wear through the

section. The main point of this step is to reduce the surface roughness of the sample and

dramatically reduce polishing time. Clean your sample in the ultrasonic.

B4 . The Initial Polish (0.3 micron alumina on cloth)Use the appropriate plastic sample holder (round or rectangular) for your sample.

Polish your sample on cloth using 0.3 micron alumina. You will have to experiment with

the consistency of the alumina slurry. Be sure to rotate the sample in place to obtain a

uniform polish. Ninety (90) seconds of polishing should be sufficient for most samples.

When you are done the sample should look shiny as glass when dry. Clean your sample in

the ultrasonic cleaner. At this point you may have the most uniform, but not necessarily the

best possible polish on your sample. Consider whether it is worth going on to step B6 . A

diamond polish is desirable, but because this step removes so little material, taking this step

will result in a surface with areas of excellent polish and adjacent areas with no apparent

improvement. The reason is that thin-section preparation by hand cannot produce an

optically flat surface. So, if you want to try the syntron, give your sample to Quinten for

the Final Polish (B6).

B6 . The Final Polish (1 micron diamond, polishing room)

Clean the sample in the ultrasonic cleaner again. The sample must be absolutely clean

to avoid contaminating the diamond lap (One jar of diamond paste costs approximately

$55. and any unfortunate soul who does not follow the instructions to clean their samplethoroughly will be presented with the bill). Use a 1 1/4" sample holder and a 1" metal

insert. Tighten the set screw enough so that it will not come loose. Place a drop of oil on

the back of your round thin-section and press the section into the metal insert. Surface

tension will hold it there. Carefully place the sample holder in the Syntron. See "Syntron

Procedures" for instructions on operating the Syntron. After polishing your sample, wash

it with soapy water and clean it in the ultrasonic cleaner. Rinse in distilled water. Blow-

dry and do not touch the surface of the sample with you fingers. A light buffing on a

chamois cloth and blow-drying will remove any remaining contamination. Put your sample

in a dust free box.

lecture 1. sample preparation

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