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Examination of Very Small Samples, using Emission Spectrography
Examination of Very Small Samples, using Emission Spectrography D. M. ELLEN Metropolitan Police Forensic Science Laboratory, 2, Richbell Place, London, W.C.l E. Martin, in 1963, described a method in which a sample i s compressed in the t i p of a graphite pellet and burnt in the arc of a n emission spectrograph. An apparatus produced by Messrs. Research and Industrial Instrument Company, which produces similar pellets i s described here. T h e results obtained from the use of this apparatus have bee% found to improve the consistency and strength of the emission spectra of very small samples of paint and other materials. Spectra of fragments of paint weighing 5ovg are shown to indicate the consistency of the method. Introduction The emission spectrograph is well established as an important tool for the forensic scientist. Its use extends from the examination of residues from organs for metallic poisons to the comparison of small fragments of paint and other materials whose inorganic content is sufficiently variable to be of value for comparison purposes. One of the special problems which occurs in comparisons of this nature is the handling of very small particles which are often the only material available for examination. The standard method of obtaining emission spectra is the use of a cupped carbon electrode containing the sample, and a pointed upper electrode. This method usually produces good results for samples of the order of 4 to lmg or more, but there is some risk of the sample falling out of the electrode before it is completely burned. This hazard increases with smaller samples and it is very difficult to get consistent results from samples of about 50pg in weight. Other methods such as the wet oxidation of the sample and followed by the absorption of the solution in graphite powder (Nickolls 1956) do not improve the reproducibility appreciably. An elegant method for the spectrography of very small fragments was described by E. Martin in 1963. The fragment to be examined is compressed into the top of a graphite pellet which is then arranged as one of the electrodes in the arc of the spectrograph. The fragment is then held securely in the electrode until it is completely burned and thus the maximum emission is obtained from the inorganic elcmcnts in the sample.
A new method of sample preparation An apparatus similar to that used by Martin to produce suitable compressed graphite pellets has been made by Messrs. Research and Industrial Instrument Company, Stannary Street, London, S.E. 11. I t is an adaptation of the evacuable die normally used to prepare potassium bromide discs for infra red absorption methods, and is shown in figure 1. It consists of an outer case, two inner cylinders, one of wide bore (13mm) and one of narrow bore (5mm), a plunger and a specially shaped lower die. This has a cavity ground in it which produces a rounded point on the graphite pellet which is produced. I t is this rounded point which contains the sample to be analysed.
Figure 1
Apparatus for the production of compressed graphite pellets
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Figure 2 Arrangement of component parts of apparatus
Figure 3 Spectra of five samples of the same common titanium oxide based paint (approx. 80% T i 0 2 and approx. 50 vg per sample)
Figure 5 Holder for graphite pellet
A small quantity of graphite, just sufficient to cover the surface, is placed in the cavity of the die (a) and the sample is placed in the centre on the layer of carbon. This can be done under a stereo microscope for greater precision and ease of handling. The lower inner cylinder (b) is then placed over the die and about 200mg of pelletable graphite powder (Johnson Matthey, J. M., 2251) is placed in the centre of the cylinder. I t is necessary to tamp down the graphite gently with the plunger (d) to ensure that about 200mg of graphite is used. The upper inner cylinder (c) is then placed on the lower cylinder and the plunger (d) inserted in the centre of the cylinders. The outer case (e) is then placed upside down over the plunger and cylinders so that the apparatus is arranged as shown in figure 2. A pressure of about $ ton is then applied to the top of the apparatus, i.e., the bottom of the outer case, for a few seconds. This will produce a compressed graphite pellet (f) as shown in figure 1 containing the sample in the tip of the protruding nipple. The graphite pellet is removed by first removing the outer case of the apparatus and then removing the die (a). This can be done by hand, or if this is difficult it can be removed by compression on the plunger (d) in the press after a perspex ring, supplied with the apparatus, has been placed round the die (a). The graphite pellet is then removed from the lower inner cylinder (b) by sliding the cylinder down the plunger until the pellet is free. This can also be done in the press with the aid of the perspex ring. The use of about 200mg of graphite powder produces a pellet with a stem of about 4mm in length. A special holder (g) also shown in figure 1 has been produced by the Research and Industrial Instrument Company, and consists of a brass rod with a brass collar fitted closely round one end. The graphite pellet is fitted by first sliding the collar down the rod, then holding the flat surface of the pellet on to the top of the rod and sliding the collar up past the pellet so that about lmm of the stem is gripped by the collar. This will ensure that about 3mm of the stem protrudes above the holder, which avoids the risk of the brass of the holder contaminating the arc. (See figure 5). The holder can then be fitted to the lower electrode holder of an emission spectrograph and an arc produced between the pellet in the holder and a pointed upper electrode. The sample embedded in the tip of the pellet is then completely burned and its emission spectrum recorded. Martin used a further refinement, the "Air Jet" of Stallwood (1954) but satisfactory results can be obtained without this. The apparatus has been used in the Metropolitan Police Laboratory for some months and has given consistently good results especially on very small paint fragments. A direct current arc of 3 amps at 200 volts for 5 seconds is used on a Hilger Medium Quartz spectrograph. Other materials such as plaster, brick dust, minerals and some powders have also been examined in this way and generally the results are more consistent and stronger than the results by the more conventional method, especially on very small samples. Care must be taken with very hard materials which can damage the surface of the die.
Results from the new method An illustration of the consistency of the results obtained by this method on very small samples is shown in figures 3 and 4. Figure 3 shows part of the spectra of five samples of the same common Titanium Oxide based paint (approx. 80% TiOz). Each sample weighed about 5 0 ~ gand was spectrographed by the use of a specially cut cavity electrode. The results show spectra which are not consistent and in most cases too weak to be of value ; a result not unexpected in samples of this size. Figure 4 shows part of five spectra obtained by the method described above, of fragments of the same paint each ,197
weighing approximately 50vg. These are consistent in intensity of the spectral lines and all are stronger than the strongest of spectra produced by the normal method. In each of the illustrations the sixth spectrum is of Johnson Matthey R.U. (raies ultimes) powder. Stronger spectra can be produced from larger fragments, but it can be seen from figures 3 and 4 that adequate spectra can be consistently obtained from about 50vg of paint ; representing about 40vg of inorganic material. I would like to thank Mr. M. Mosley, of Research and Industrial Instruments Company, for his work in the production of the apparatus ; Superintendent W. R. Cramb and Mr. J. D. McCafferty, for producing the photographs,; and Dr. H. J. Walls, Director of the Metropolitan Police Forensic Science Laboratory, for encouragement in the writing of this paper.
References MARTIN, E. (1963), Die Bedeutung Der Emmissions~ectrogra~hie in der Kriminalistic Chemie No. 73, August, 263 NICKOLLS, L. C. (1956), The Scientific Investigation of Crime. London, Butterworth STALLWOOD, B. J. (1954),J. Opt. Soc. America, 2, 174
A new method of sample preparation An apparatus similar to that used by Martin to produce suitable compressed graphite pellets has been made by Messrs. Research and Industrial Instrument Company, Stannary Street, London, S.E. 11. I t is an adaptation of the evacuable die normally used to prepare potassium bromide discs for infra red absorption methods, and is shown in figure 1. It consists of an outer case, two inner cylinders, one of wide bore (13mm) and one of narrow bore (5mm), a plunger and a specially shaped lower die. This has a cavity ground in it which produces a rounded point on the graphite pellet which is produced. I t is this rounded point which contains the sample to be analysed.
Figure 1
Apparatus for the production of compressed graphite pellets
-
Figure 2 Arrangement of component parts of apparatus
Figure 3 Spectra of five samples of the same common titanium oxide based paint (approx. 80% T i 0 2 and approx. 50 vg per sample)
Figure 5 Holder for graphite pellet
A small quantity of graphite, just sufficient to cover the surface, is placed in the cavity of the die (a) and the sample is placed in the centre on the layer of carbon. This can be done under a stereo microscope for greater precision and ease of handling. The lower inner cylinder (b) is then placed over the die and about 200mg of pelletable graphite powder (Johnson Matthey, J. M., 2251) is placed in the centre of the cylinder. I t is necessary to tamp down the graphite gently with the plunger (d) to ensure that about 200mg of graphite is used. The upper inner cylinder (c) is then placed on the lower cylinder and the plunger (d) inserted in the centre of the cylinders. The outer case (e) is then placed upside down over the plunger and cylinders so that the apparatus is arranged as shown in figure 2. A pressure of about $ ton is then applied to the top of the apparatus, i.e., the bottom of the outer case, for a few seconds. This will produce a compressed graphite pellet (f) as shown in figure 1 containing the sample in the tip of the protruding nipple. The graphite pellet is removed by first removing the outer case of the apparatus and then removing the die (a). This can be done by hand, or if this is difficult it can be removed by compression on the plunger (d) in the press after a perspex ring, supplied with the apparatus, has been placed round the die (a). The graphite pellet is then removed from the lower inner cylinder (b) by sliding the cylinder down the plunger until the pellet is free. This can also be done in the press with the aid of the perspex ring. The use of about 200mg of graphite powder produces a pellet with a stem of about 4mm in length. A special holder (g) also shown in figure 1 has been produced by the Research and Industrial Instrument Company, and consists of a brass rod with a brass collar fitted closely round one end. The graphite pellet is fitted by first sliding the collar down the rod, then holding the flat surface of the pellet on to the top of the rod and sliding the collar up past the pellet so that about lmm of the stem is gripped by the collar. This will ensure that about 3mm of the stem protrudes above the holder, which avoids the risk of the brass of the holder contaminating the arc. (See figure 5). The holder can then be fitted to the lower electrode holder of an emission spectrograph and an arc produced between the pellet in the holder and a pointed upper electrode. The sample embedded in the tip of the pellet is then completely burned and its emission spectrum recorded. Martin used a further refinement, the "Air Jet" of Stallwood (1954) but satisfactory results can be obtained without this. The apparatus has been used in the Metropolitan Police Laboratory for some months and has given consistently good results especially on very small paint fragments. A direct current arc of 3 amps at 200 volts for 5 seconds is used on a Hilger Medium Quartz spectrograph. Other materials such as plaster, brick dust, minerals and some powders have also been examined in this way and generally the results are more consistent and stronger than the results by the more conventional method, especially on very small samples. Care must be taken with very hard materials which can damage the surface of the die.
Results from the new method An illustration of the consistency of the results obtained by this method on very small samples is shown in figures 3 and 4. Figure 3 shows part of the spectra of five samples of the same common Titanium Oxide based paint (approx. 80% TiOz). Each sample weighed about 5 0 ~ gand was spectrographed by the use of a specially cut cavity electrode. The results show spectra which are not consistent and in most cases too weak to be of value ; a result not unexpected in samples of this size. Figure 4 shows part of five spectra obtained by the method described above, of fragments of the same paint each ,197
weighing approximately 50vg. These are consistent in intensity of the spectral lines and all are stronger than the strongest of spectra produced by the normal method. In each of the illustrations the sixth spectrum is of Johnson Matthey R.U. (raies ultimes) powder. Stronger spectra can be produced from larger fragments, but it can be seen from figures 3 and 4 that adequate spectra can be consistently obtained from about 50vg of paint ; representing about 40vg of inorganic material. I would like to thank Mr. M. Mosley, of Research and Industrial Instruments Company, for his work in the production of the apparatus ; Superintendent W. R. Cramb and Mr. J. D. McCafferty, for producing the photographs,; and Dr. H. J. Walls, Director of the Metropolitan Police Forensic Science Laboratory, for encouragement in the writing of this paper.
References MARTIN, E. (1963), Die Bedeutung Der Emmissions~ectrogra~hie in der Kriminalistic Chemie No. 73, August, 263 NICKOLLS, L. C. (1956), The Scientific Investigation of Crime. London, Butterworth STALLWOOD, B. J. (1954),J. Opt. Soc. America, 2, 174