Many analytical samples cohere well under pressure; those samples which crumble or ablate after pressing require a binder. Binders are usually blended with the sample after pulverizing and before pressing, but can also serve as a pellet matrix, supporting a thin layer of sample.

Binders can be liquids or powders, and range from commonly available reagents to brand name products with secret formulas. Their use should lead to a stable, crumble-proof sample disc achieved with a minimum of dilution, contamination, and effort.

Generic binders include cellulose, paraffin, boric acid, and graphite. Of these, cellulose and paraffin are available through SPEX SamplePrep in extra-pure, finely powdered form, ideal for blending and pelletizing. PrepAid Cellulose, catalog number 3642, has a particle size of less than 20 microns, and blends quickly and completely with samples at 10% by weight; it can also be used undiluted as a samplematrix with the sleeve-and-plunger technique (refer to p. 165) It will
neither stick to a die nor contaminate a sample, and does not cake in the bottle during storage. Discs stabilized with cellulose should, however, be kept in a desiccator if they are to be retained, as with time they can absorb moisture and slowly swell and crack.

UltraBind, an exclusive SPEX SamplePrep product, satisfies our notions of the perfect all-around binder: blend, contamination-free, self-lubricating, strong when pelletized, and moisture-resistant. A fine (20 micron), powder, it blends easily with samples to yield a disc which is spectro-graphically clean, easy to remove from the die, and durable enough to withstand rough handling. It also resists cracking and swelling in storage, and is an excellent pellet matrix when used with the Sleeve-and-Plunger technique, described on p.165. UltraBind is catalog number 3644 and is described on p.162 with other PrepAid products.

Sample discs formed with 10% to 20% paraffin are air-stable, as the waxy binder seals the surface against moisture. Powdered paraffin is also offered by SPEX SamplePrep as a PrepAid, catalog number 3646. As with cellulose, the fine (‹30 micron) powder blends evenly with samples and does not contaminate them. However, paraffin should not be used undiluted with the sleeve-and-plunger technique, as in this concentration it will stick to the die.

Boric acid is commonly available in pure form, and can be used diluted or undiluted to make a stable sample disc. However, it is much more hygroscopic than cellulose and hence it typically supplied in granular form, requiring grinding in a mill or mortar-and pestle to become a fine enough powder to blend evenly with a sample. Some analysts add boric acid to the sample during the final stage of grinding to achieve this. Sample discs bonded with boric acid must be kept in a desiccator if they are to be preserved.

Graphite powder makes sample discs unaffected by moisture, and blends rapidly with any sample. It also serves a its own lubricant, making it easy to free discs from the pellet die. Its disadvantage for XRF is that it must be used in a proportion of at least 50% of the sample disc by weight.

Graphite was used extensively in the development of sample-disc pressing techniques, but that was because of its high conductivity. The 31 mm sample disc accepted as the standard size by most XRF manufacturers and users was originated for the point-to-plane technique in arc/spark optical emission spectroscopy, where a high-voltage current is passed through the sample. Sample discs pressed with graphite are also used for spark ablation techniques.

Solid binders also include powdered wax, pulverized acrylic plastic, and others. Liquid binders are also used; they are blended with a sample prior to pressing. Most appear to be a solution of polymer(s) in solvent(s); their reputed advantage is that during blending, all the sample particles receive a light coating of the polymer, so that subsequent pelletizing is more direct and efficient.

There are also materials which are touted as aids for both grinding and binding. Generally these are in tablet form, so that a uniform amount can be easily added to the sample just before grinding. The classic example of this genre is aspirin, but there are more sophisticated, prop-rietary grinding/binding tablets available. Of the use of binders in general, it should be remarked that their most conspicuous property is that of caking under pressure, or at least of flowing under pressure and then caking when the pressure is relieved. Hence there is a good chance that adding a binder to the sample before grinding will lead to caking in the grinding container. In this connection it should be noted that a very finely ground sample (e.g. cement-mix ground to below 10 microns) is more likely to bind well under pressure than the same sample ground to moderate fineness, and that grinding aids are often needed to attain very fine particle size.