A Simple Micropelleting Technique W. J. de Klein and K. Ulbert 1 Laboratory of Organic Chemistry, State University of Leiden, Leiden, The Netherlands
PELLETING TECHNIQUES are long established in chemical spectroscopy. Dies of various diameter have been described in the literature (I, 2) or are available commercially. Micro samples require a special pelleting technique. The usual procedure is to use a metal disk holder, drill a hole in its center, and fill it with the material to be pressed. In most cases a steel disk holder is used which has the disadvantage of needing a special and expensive die-e.g., the Perkin-Elmer Ultra Micro die. A simple method is described which can be carried out with a normal die. The first part of this paper describes a method that can usefully be employed in infrared spectroscopy, where the metal disk holder is used to hold the micro pellet in the beam of infrared radiation. The second part describes an adaptation which offers the possibility to withdraw the pellet from the metal holder in cases where the metal may influence other types of measurement-e.g., conductivity measurements on the pressed material. The disk holder is a circular piece of lead, 16 mm in diameter, 1 mm thick with a hole of 1-1.5 mm drilled in its center. It is prepared in the following way. A piece of lead is rolled into a flat bar of 1 mm thickness. From this bar lead pellets of 16-mm diameter are punched with a hollow punch. A round steel pin with a sharp point serves to make a hole of the required diameter in the center of the lead pellet. This hole is carefully cleaned with a file and pipe-rag. The disk holder is now put on the upper die of the die-assembly (type RIIC D-0.3, for 16-mm pellets) which is turned upside down to facilitate manipulation with the lead disk holder. Next, the lower die is placed on the lead disk holder and firmly pressed by hand to ensure complete flatness of the lead disk holder and good contact between the lead and the upper die. (A poor contact would spill sample material when filling the hole.) The lower die is removed and the pellet holder is now ready to be filled. It is our practice, when preparing KBr pellets, to first fill the hole of the holder with a small amount of pure KBr powder so that the bottom of the hole is just covered. Now the hole is filled with KBr containing the sample-e.g., eluted from a TLC-spot. A hole of 1.5 mm in diameter requires about 5 mg KBr. The KBr is tamped into the hole with a small rod with a flat end-e.g., a broken sewing needle. When the hole is filled with the sample containing KBr a small pile of pure KBr is built over the orifice of the hole. (If some sample containing KBr is left over it can of course also be used to build the pile.) The amount of pure KBr needed to build a 1.5-mm high pile over a hole with a bore of 1.5-mm diameter is about 10 mg. This pile is indispensable for it serves to press the KBr into a clear pellet and prevents the hole from being pressed together when pressing the pellet. The pile is thus comparable with the steel pin used in most micro dies. The lower steel die is put on and firmly pressed by hand onto the KBr pile. The bottom piece of the die assembly is put on and the whole die is turned and put into a press in its normal way.
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Figure 1. a. Insert and sample lie in the lead pellet; b. deformation of the Teflon (Du Pont) insert under pressure causes a deformation of the pressed material; c. the pellet as it lies loose in the lead holder after pressure has been released
‘On leave from the Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, Prague 6, Petriny, Czechoslovakia.
A vacuum is applied for about 2 minutes and a pressure of 8-9 tons/cm2 is applied for 3-4 minutes. In this way micro pellets of excellent optical transparency can be fabricated. Good quality infrared spectra were obtained in this way with the 1.5-mm diameter pellet holder from 1-4 pg solid material using a refracting beam condenser with KBr optics. With an ordinate scale expander good spectra could be obtained with less than 1 pg of solid sample. When applying pressures higher than 8-9 tons/cmz the pellet holder is deformed and the hole is squeezed together thus considerably reducing the diameter of the pellet. At a pressure of 8-9 tons the original bore of 1.5 mm is squeezed together by less than 10% when a KBr pile of adequate body is used. When it is necessary to withdraw the pellet from the holder the following adaptation is made. The modification is based on the use of a Teflon (Du Pont) insert which is placed in the hole and has the same height as the lead holder (Figure la). The sample is slightly precompressed in the insert with the flat-ended rod. The die is assembled as described above and a pressure of about 10 tons/cmz is applied. Under pressure the insert is deformed as shown in Figure 16. After releasing the pressure the Teflon insert recovers to almost the original shape with the diameter only slightly smaller (Figure IC). The pellet now lies loose in the Teflon holder and can be pushed out easily. For most applications the distortion of the cylindrical shape is of no harm. The ratio of the insert’s wall thickness to its diameter and height must be found by trial and error. It depends to a certain degree on the sample used as the pellet and on the precompression. This method was used in preparing micropellets from the iodine complex of the leucobase of Bindschadlers’ green (organic semiconductor). The following dimensions of the lead holder and Teflon insert were used: lead holder, diameter 13 mm, thickness 1 mm; and Teflon insert, 0.d. 2.1 mm, i.d. 1.5 mm, length 1 mm. The applied pressure was 10 tons/cmz. The resulting pellet had a diameter of 1.4 mm (1.35 mm in the middle part, thus having a hyperbolic shape distortion.) It appeared to be possible to press metal electrodes from lead or gold powder on the surface of the pellet. This was done in one operation as the pellet was pressed. It also appeared possible to make electrodes on the surface of the pellet by a metal evaporation technique; this has to be done before removing the pellet from the holder to protect the side surface of the actual pellet from becoming metallized. In this way the conductivity of the sample as a pellet could be measured.
(1) F. E. Resnik, L. S . Harrow, J. C. Holmes, M. E. Bill, and F. L. Greene. ANAL.CHEM.,29, 1874 (1957). (2) F. Bissett, A. L. Bluhm, and L. Long, [bid.,31, 1927 (1959).
RECEIVED for review October 4, 1968. Accepted December 6, 1968.
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ANALYTICAL CHEMISTRY
