Sample Pellet Preparation Procedures

Sample Pellet Preparation Procedures

Introduction

Note that the procedures presented here are at least as important to the generation of good data as the collection of the original hand sample. It is particularly vital that the weighing steps described be carried out patiently and with scrupulous attention to the degree of precision and accuracy required. The best security we can provide for the labware used in the major element fusion technique is anonymity. My own practice is to tell curious passers by that these articles are made of a "stainless alloy." This is true without being foolishly accurate. For similar reasons, this labware should never be left out in the open when no-one is present to watch over it: Put it away before you leave! You should not neglect to close and lock the lab door if you are working alone late at night!

The flux/sample mixture used in the fusion techniques emits toxic fumes when heated as a normal part of the procedure. ALWAYS be certain that the fume hood is on and operating properly before you attempt to make fused pellets! The switch is identical with the one that operates the hood in Dr. Berg's inner office. If you do not have unlimited access to his office, then you must make arrangements with someone who does to turn the hood on before you begin!

Fused Major Element Pellets

1. Weigh out 0.4000 g sample and 3.6000 Li2B4O7 into glass vials with plastic snap-on caps. Be certain not to use the thin-walled vials which are used to store pulverized samples; these will shatter in the following step. Use Al-foil weighing "papers," carefully cleaned with methanol and a kimwipe, to minimize static, and weigh both powders on the same paper to a constant sum. Fold the edges of the weighing "paper" under to prevent contamination from fragments of Al, and to keep ragged edges from trapping sample when it is poured into the vial. Always use an indelible marker to identify your samples; the moisture on your hands will remove non-waterproof labels!

2. Lock the vial in the mixer/mill and mix for two minutes. The danger here is probably slight, but it makes good sense to lock the cover shut while the mill is in operation. Place a 1-liter beaker filled with 3:1 dilute HCl on the small hotplate and heat it at a medium setting. When the acid solution begins to emit steam, and the beaker is hot to the touch, turn the adjusting knob to the lowest setting.

3. Transfer the powder to a clean Pt crucible and add 1 or 2 drops of LiI solution. This material acts as a release agent, by increasing the surface tension of the liquid, so that the pellet will not adhere to the mold. Note that if you make new LiI solution from the stock bottle in the hood, you must dilute the stock solution 9:1 before you use it! Failure to do this will result in bizarre melt behavior further on in the procedure.

MAKE CERTAIN THAT THE FUME HOOD IS OPERATING PROPERLY BEFORE YOU CONTINUE!!

4. Use Ni-chrome tongs to place the loaded Pt crucible over a Meker burner set for a strongly oxidizing flame, and cover with the Pt lid (mold) using the same tool. It is helpful to hold the tongs reversed in this and subsequent steps. Note that Pt in or near the flame should always be handled with Ni-chrome tongs; when wet with acid or water, use standard tongs. The use of Ni-chrome wire tongs is required when handling hot Pt because of the solubility of Fe in Pt, which would then contaminate your sample.

5. Heat for about 6 to 7 minutes, and then use a set of Ni-chrome tongs to "slosh" the liquid in the crucible to make sure that it is well mixed and to scavenge any droplets adhering to the walls. Do not allow the crucible to cool at all during this step, and keep the lid heated (Ni-chrome tongs) the while. Place the crucible back in the wire triangle, replace the lid, and heat for an additional 4 to 5 minutes.

6. "Slosh" the liquid as before, again keeping the lid (mold) cherry red on the second flame. Quickly and carefully pour the liquid from the crucible into the lid, and then use a circular motion to slosh the liquid in the lid over the flame to insure homogeneity. When this has been achieved, and only then, quench the glass pellet on the steel heat sink provided. Keep the crucible mildly heated until the pellet has been quenched.

7. Remove the crucible from the flame, and, as the cherry red color just fades, plunge it into a 1 liter beaker full of distilled water. This will shatter the glass remaining in the crucible and make it much easier to remove. Pick up the crucible using standard (not Ni-chrome) tongs and clean the glass shards out with your finger. Do not use any hard object to clean the crucible at any time! The crucible will be irreparably damaged if you do. If any beads of glass adhere too strongly to the crucible to be removed with your finger, then gently heat the crucible adjacent to the bead, holding it with Ni-chrome tongs, and the bead will commonly pop off as the water beneath it flashes to steam. Place the crucible in a second 1 liter beaker containing 6N HCl to further clean it, again using standard tongs. The water in the first beaker should be changed approximately every 10 samples.

8. After the pellet in the Pt lid has cooled, tap the glass disc out onto a kimwipe and place the Pt lid into the HCl along with the crucible. Being certain that nothing, neither your fingers nor the marker, touches the convex side (which was against the mold) of the glass disc, inspect it for inhomogeneities (e.g. crystallites, large bubbles, bits of undissolved sample) which would necessitate recasting; label the concave side (which was the upper surface of the liquid); put it on a kimwipe cushion in a polycon pillbox, and store it in a dessicator. You will probably want to label the pillbox also for convenience. Always use an indelible marker to identify your samples; the moisture on your hands will remove non-waterproof labels!

9. Taking extreme care that the mold surface of the lid is not scratched, remove the crucible and the lid from the acid bath using the standard tongs, and rinse them in the beaker of distilled water. Use the Ni-chrome tongs to gently dry them over the flame, and allow them to cool on the heat sink; it is best not to get the crucible or lid any hotter than is necessary to dry them, lest the cooling time be made unnecessarily long. Place the lid on the crucible to cool -- never place it directly on the heat sink where particles of iron oxide may adhere to it, to be dislodged into the next sample, or Fe may diffuse into the Pt metal and also contaminate your sample.

Cellulose Acetate Backed Trace Element Pellets

1. Weigh 1.0000 g sample powder and 0.5000 g microcrystalline cellulose, using Al-foil weighing papers, and weighing both powders on the same paper to a constant sum. Step 1 of the major element pellet method describes the preparation of and rationale for the use of Al-foil weighing papers.

2. First put a clean acetate ball into the snap-cap plastic vial, and then add the powders weighed in step 1. Mark the vial with the sample name in two places on the side, and always use an indelible marker to identify your samples; the moisture on your hands will remove non-waterproof labels! Agitate vials and their contents in the Spex mixer/mill some four to five at a time, for 10 minutes; use a rubber band to make them easier to handle. The danger is probably slight, but it makes good sense to lock the cover of the mill down while it is operating.

3. Assemble the base to the die cylinder and then put the bottom die in place (You will only forget once -- it makes quite a mess). Put a single, level, scoop of cellulose acetate into the die, level it with the spatula, and press it flat with the plunger.

4. Add the sample powder from the plastic vial, being as careful as possible that the acetate ball does not crater the cellulose acetate. Remove the acetate ball with forceps and put it into the "dirty" beaker.

5. Carefully level the surface of the sample powder without digging into the underlying cellulose acetate. If you do inadvertantly disturb the cellulose acetate, the pellet is ruined and you must begin again with step 1!

6. Clean and install the top die, good face down, and place the plunger in position on top of it. Put the assembly into the press and apply 20 tons pressure for at least 5 minutes. (It is this step which will damage the die cylinder if your sample is insufficiently pulverized.) When the time has elapsed, release the pressure with the bleed screw very, very slowly. A too-rapid release of pressure will cause the layer of sample to separate from the cellulose acetate. Leaving the sample in the press longer than 5 minutes causes no immediate problems, except that very long times (> 15 minutes) can make the pellet difficult to remove from the cylinder intact.

7. With the plunger still in place, turn the die cylinder over and remove the base. Still holding the cylinder inverted, and using the tips of the fingers to hold the plunger tightly against the top (now lowermost) die, rap the plunger sharply against the cardboard cushion to drive out the finished pellet and both dies.

8. Never allow anything but the XRF sample holder to touch the sample side of the pellet! Inspect the sample surface of the pellet for foreign materials (e.g. Cr or Al flakes), surface dilution, excessive separation from the cellulose acetate backing, or evidence of insufficient pulverization; discard the pellet if any of these conditions exist. If it passes inspection, label the pellet on the back and place it on a kimwipe cushion in a polycon pillbox. You will probably also want to label the pillbox. Store in a dessicator.

Boric Acid Backed Trace Element Pellets

1. Clean all equipment with methanol and a Kimwipe before beginning and between samples. Never trust the last person who used the kit -- even if it was you. Preheat the drying oven by turning it on, making sure that the dial is set between 1.0 and 1.1 to obtain the desired temperature of approximately 60 to 65 degrees C. Lay out a clean petri dish, with its top in place, ready to receive the partially prepared sample. You may wish to mark the petri dish top and bottom with the sample name at this point.

2. Rap the base of the sample vial smartly on the counter two or three times to be certain that the sample powder is compacted sufficiently, and then use the smaller of the two measuring spoons to put two heaping scoops of powder into the agate mortar. (This should yield approximately 5 grams total.) Make a mound with it.

3. Use an automatic pipette to put 0.75 ml of 4% Mowiol solution into the mortar immediately adjacent to the mound of sample powder, not on top of it. (This is one of the "tricks of the trade.") Use the pestle to spread the rock powder carefully over the liquid and sop it up. When this has been accomplished, use the pestle with a grinding motion to mix the Mowiol and the rock powder intimately. Note that the idea is not to grind the powder more fine, but to mix it with the glue. Do not pound with the pestle on the mortar, both are fragile and expensive! You have finished this step when the mixture of powder and Mowiol is uniform in shade and consistency. If you don't sop up the Mowiol with the powder as suggested, you will get a gooey mess adhering to the pestle which is difficult to work with.

4. Use the stainless steel spatula to scrape the mixture into the petri dish, put the lid in place, mark the sample name on the top and bottom of the dish without turning it over if you haven't done so previously, and then put it in the drying oven for approximately 55 minutes. You can set a timer at this point to keep track of the time.

5. Clean the equipment with methanol and a Kimwipe. I find it useful to wipe everything down with a dry kimwipe, left over from the previous sample to remove the big pieces, and then do a final cleaning with a single Kimwipe moistened with methanol. You get better results, and use fewer Kimwipes. Save the "wet" Kimwipe to use as the "dry" Kimwipe later.

6. Prepare another sample in the same fashion and put it in the drying oven with the first, continuing in this way until the 55 minutes required to dry the first sample have elapsed. I can usually have eight samples completed and ready for the next step in this amount of time. It is important not to overdry the samples, so don't take the lids off of the petri dishes during the drying step, and work as smoothly and quickly as possible during both parts of the procedure. If you do happen to overdry a sample, you can add a small amount of distilled, de-ionized, water to it to reconstitute the glue in step number 8.

7. Use methanol and a Kimwipe to clean the die cylinder and ancillary equipment. Be careful in cleaning the aluminum sleeve, because aluminum abrades from it easily and can contaminate other parts of the kit. Clean it last. Assemble the die set, and place a tungsten carbide die in the cylinder, good face up. Examine it carefully to see that there are no contaminating particles on its surface -- if there are, take it out and clean it again. Put the aluminum sleeve in place.

8. When the 55 minutes have elapsed, take the sample out of the drying oven and use the stainless steel spatula to scrape it back into the mortar. There should still be a fair amount of moisture in the sample; use the pestle to distribute it evenly as before. Scrape the re-mixed sample into the aluminum sleeve in the die cylinder, being careful not to spill or to jostle the sleeve so as to allow sample to escape below and get between it and the die face. Hold the sleeve down firmly, and use a chopping motion with the spatula to tamp the sample and distribute it as evenly as is reasonably possible.

9. Insert the stainless steel plunger into the aluminum sleeve while holding the latter firmly down. If you don't, escaping air will cause it to rise and sample will escape from under it, ruining the pellet. Press down on the plunger with your body weight to compress the sample into a semi-solid mass.

10. There is a trick to getting the stainless steel plunger and aluminum sleeve out of the die cylinder without disturbing the sample. It is important to do it correctly because the sample must be concentric within the finished pellet. While holding the sleeve down, twist the plunger gently from side to side, lift it a short distance free from the sample pellet, and then let it back down again. Using your thumb and first two fingers on the plunger and sleeve as you would with a syringe, gently increase the pressure until you raise the sleeve away from the sample. Let it back down again until it just contacts the upper part of the pellet, and then break the plunger free as before. Using the sleeve, now lift both sleeve and plunger free of the sample and die cylinder. Look down the bore of the cylinder to see that all is well. If the sample pellet comes away with the plunger, put the whole back in the cylinder carefully and gently repeat the procedure outlined here.

11. Take the larger of the two measuring spoons, cleaned of course, and use it to put a single scoop of boric acid crystals on top of the sample in the die cylinder. Pour it in such a way that you are sure to fill the annular space around the sides of the pellet that was occupied by the aluminum sleeve. Use the stainless steel spatula to spread the boric acid evenly over the top of the sample pellet, put in the top die good side down, and then insert the large steel plunger.

12. Place the whole in the press and apply just less than 8 tons of pressure, reading the 1 1/4 inch scale on the dial. Leave it there for one minute, while you clean equipment. Check the pressure on the ram and keep it just above 7 tons if it drops below that value. Do not vary the pressure from that given here: The degree of compaction of the powder in a pellet affects the analytical signal that the pellet generates.

13. When the minute has elapsed, release the pressure on the assembly slowly, and then remove the cylinder from the press. Turn the cylinder over, holding the large plunger in place with your fingers, and remove the base. Still holding the cylinder inverted, and using the tips of the fingers to hold the plunger tightly against the top (now lowermost) die, rap the plunger sharply against the cardboard cushion to drive out the tungsten carbide bottom die. Do not let it bounce against the working face of the pellet, but remove it from the assembly as soon as you can get your fingers on it. Rap the plunger again against the cushion to drive the pellet out.

14. Never allow anything but the XRF sample holder to touch the working face of the pellet! Inspect the working face for foreign materials (e.g. Cr or Al flakes, or other foreign objects), surface dilution, intrusion of the boric acid along the margins of the sample, or evidence of insufficient pulverization; discard the pellet if any of these conditions exist. If it passes inspection, use an indelible marker to label the pellet on the back and place it face up on a Kimwipe cushion in a labelled polycon pillbox. Leave the pillbox open for the time being.

15. Continue in this manner until all of the samples in the drying oven have been finished; then start again from the beginning, as necessary. Dry the pellets face up in air overnight, and then for 20 minutes in the drying oven (set at 60 to 65 degrees C, as before) the next day. Take them out of their pillboxes and put them in petri dishes (four each) before putting them into the oven. After the second drying step, you can put them face down on the Kimwipe, and close the pillbox. Place them in a vacuum dessicator overnight before they are analyzed, but do not store boric acid backed pellets under vacuum or in a dessicator for periods of weeks or months. I am told that they tend to come apart under those conditions.

Low-Dilution Fusion Pellets

1. Weigh 2.0000 g sample powder, 0.8000 g lithium tetraborate, and 3.2000 g lithium metaborate, using Al-foil weighing papers and weighing all three powders on the same paper to a constant sum. Do not pre-mix the fluxes; they have different physical characteristics and will separate during storage and handling. Put the weighed materials into thick-walled vials with plastic snap-on caps. Be certain not to use the thin-walled vials which are used to store pulverized samples; these will shatter in the following step which requires use of the Spex mixer-mill. Step 1 of the major element pellet method describes the preparation of and rationale for the use of Al-foil weighing papers. Always use an indelible marker to identify your samples; the moisture on your hands will remove non-waterproof labels!

2. Before you continue operations, turn on the large hotplate and adjust the control knob to the second of the two marks. This will produce a temperature in the vicinity of 250 degrees centigrade in the center of the surface of the plate. Turn on the fume hood, and place a 1-liter beaker filled with 3:1 dilute HCl on the smaller hotplate and heat it at a medium setting. When the acid solution begins to emit steam, and the beaker is hot to the touch, turn the adjusting knob to the lowest setting. The fume hood switch is identical with that which controls the fume hood in Dr. Berg's office. If you do not have unlimited access to his office, then you must make arrangements with someone who does in order to get the fume hood turned on.

3. Lock the vial in the mixer/mill and mix for two minutes. The danger here is probably slight, but it makes good sense to lock the cover shut while the mill is in operation.

4. Transfer the powder to a clean Pt crucible and add six drops of LiI solution. This material acts as a release agent, by increasing the surface tension of the liquid, so that the pellet will not adhere to the mold. Note that if you make new LiI solution from the stock bottle in the hood, you must dilute the stock solution 9:1 before you use it! Failure to do this will result in bizarre melt behavior further on in the procedure.

MAKE CERTAIN THAT THE FUME HOOD IS OPERATING PROPERLY BEFORE YOU CONTINUE!!

5. Use Ni-chrome tongs to place the loaded Pt crucible over a Meker burner set for a strongly oxidizing flame, and cover with the Pt lid (mold) using the same tool. It is helpful to hold the tongs reversed in this and subsequent steps. Note that Pt in or near the flame should always be handled with Ni-chrome tongs; when wet with acid or water, use standard tongs. The use of Ni-chrome wire tongs is required when handling hot Pt because of the solubility of Fe in Pt, which would then contaminate your sample.

6. Heat for about 6 to 7 minutes, and then use a set of Ni-chrome tongs to "slosh" the liquid in the crucible to make sure that it is well mixed and to scavenge any droplets adhering to the walls. Do not allow the crucible to cool at all during this step, and keep the lid heated (Ni-chrome tongs) the while. Place the crucible back in the wire triangle, replace the lid, and heat for an additional 4 to 5 minutes.

7. "Slosh" the liquid as before, again keeping the lid (mold) cherry red on the second flame. Quickly and carefully pour the liquid from the crucible into the lid, and then use a circular motion to slosh the liquid in the lid over the flame to insure homogeneity. When this has been achieved, and only then, quench the lid and glass pellet on the steel heat sink provided. Keep the crucible mildly heated the while.

8. Remove the crucible from the flame, and, as the cherry red color just fades, plunge it into a 1 liter beaker full of distilled water. This will shatter the glass remaining in the crucible and make it much easier to remove.

9. When the incandescent glow has just faded from the glass pellet, quickly place it in the center of the hotplate for two minutes. At the end of this time, move it to a cooler corner of the hotplate for another two minutes, and then return it to the steel heat-sink until it is cool enough to handle. The durations of the steps in this annealing operation may have to be modified depending upon the composition of your sample. Granitic compositions may require little annealing at all, whereas rocks of basaltic, even more mafic, or unusual compositions may require somewhat longer times. The rule of thumb is: The more dark the color of the cooling glass pellet, the longer it should spend in the center of the hot plate, to a maximum of four to five minutes for pellets which are completely opaque. It does not appear to be necessary to increase the time spent on the corner of the plate. If the pellet cracks but sufficient volume remains, it is possible to remelt and recast it so long as you don't touch the fragments with your hands or in some other way contaminate them.

10. Pick up the crucible from the quench beaker using standard (not Ni-chrome) tongs and clean the glass shards out with your finger. Do not use any hard object to clean the crucible at any time! The crucible will be irreparably damaged if you do. If any beads of glass adhere too strongly to the crucible to be removed with your finger, then gently heat the crucible adjacent to the bead, holding it with Ni-chrome tongs, and the bead will commonly pop off as the water beneath it flashes to steam. Place the crucible in a second 1 liter beaker containing 6N HCl to further clean it, again using standard tongs. The water in the first beaker should be changed approximately every 10 samples.

11. After the pellet in the Pt lid has cooled, tap the glass disc out onto a kimwipe and place the Pt lid into the HCl along with the crucible. Being certain that nothing, neither your fingers nor the marker, touches the convex side (which was against the mold) of the glass disc, inspect it for inhomogeneities (e.g. crystallites, large bubbles, bits of undissolved sample) which would necessitate recasting; label the concave side (which was the upper surface of the liquid); put it on a kimwipe cushion in a polycon pillbox, and store it in a dessicator. You will probably want to label the pillbox also for convenience. Always use an indelible marker to identify your samples; the moisture on your hands will remove non-waterproof labels!

12. Taking extreme care that the mold surface of the lid is not scratched, remove the crucible and the lid from the acid bath using the standard tongs, and rinse them in the beaker of distilled water. Use the Ni-chrome tongs to gently dry them over the flame, and allow them to cool on the heat sink; it is best not to get the crucible or lid any hotter than is necessary to dry them, lest the cooling time be made unnecessarily long. Place the lid on the crucible to cool -- never place it directly on the heat sink where particles of iron oxide may adhere to it, to be dislodged into the next sample, or Fe may diffuse into the Pt metal and also contaminate your sample.