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A Simple Density Gradient Technique for the Comparison of Glass Fragments
A Simple Density Gradient Technique for the Comparison of Glass Fragments J. R 1;. LT-OYI) Home Ofice Formsic Science Laboratory, Priory House, Goock Street North, Birmingham 5, England
A method for the generation o f a linear and stable density gradient column for the measurement of s ~ e c i j cgravity of glass fiarticles i s described. Althougl~density gradient methods for the comparison of glass fragments are capable of improving considerably the exactness of the commonly used flotation methods (Green and Rurd, 1965; Kind and Summerscales, 1966), such techniques have not been widely adopted for forensic purposes. In this laboratory a density gradient method of comparison has been in routine use for over two years. The basis of the method is well known (Tung and Taylor, 1955), but it is thought that the straightforward construction, rapidity of use and effectiveness may commend the apparatus to those dissuaded from the use of density gradients 1)y the specialised equipment often required, or by the sometimes unstable and highly nonlinear nature of thermally-generated gradients.
Fig. 1. Density gradient apparatus.
The apparatus, filled with an aqueous solution of eosin to facilitate photography, is shown in Fig. 1. The burette serves as the input vessel of total 115
voliime about i 5 ml. The burette jet passes through a rubber bung into the mixifig vessel, of about 25 ml capacity, wliich is equipped with a magnetic stirrer. The mixing vessel is connected by a glass capillary, passing through the bung, and by 'teflon' tubing of 1mm internal diameter to the bottom of the gradient column. The column is 50 cm in length and 6 mm in internal diameter ; it is protected from sudden changes in temperature by a large water jacket, and the whole apparatus is positioned away from sunlight or any other source of heat likely to cause thermal effects. The gradient is prepared from mixtures of bromoform and bromobenzene ; we have not encountered the difficulties described by Greene and Burd (loc. cit.) with such solvents in thermally-generated gradients except when glass has been powdered. Gradients appropriate to the comparison of glass particles having densities in the region of 2.5 g/ml a t room temperatures are prepared in the following way. By flotation a mixture of approximately 50 ml of bromoform and bromobenzene (that retained from previous experiments and kept under sodium metabisulphite and anhydrous magnesium sulphate is suitable) is adjusted in composition until its density is equal to that of a control sample of glass. This adjustment is not critical. 20 ml of the fluid mixture is transferred to the mixing vessel, 0.2 ml of bromobenzene is added and the stirrer is started. To the rest of the mixture 0.75 ml of bromoform is added for every 10 ml and the input vessel is filled with this fluid. The rubber bung, carrying a t this stage only the connection to the jacketed gradient tube, is fitted into the mixing vessel. The jet of the burette is pushed into the bung, which forces a small amount of the mixture out of the vessel into the bottom of the gradient tube. The contents of the input vessel are now run drop by drop into the mixing vessel and the gradient column forms simultaneously ; the process takes about twenty minutes. The column is isolated with a screw clip and disconnected from the mixing vessel. After drying and fitting with a new gradient tube the apparatus is ready for re-use.
T)EPTII IS COI.1'JIS
(crns)
Fig. 2. Variation of specific gravity with depth in a density gradient column. Curve 1 (full line) is a calculated gradient, curve 2 (broken line) is an experimental one.
116
Glass particles, cleaned in hot detergent, may now by introduced into the column. Particles from the same source generally descend to within 1 cm. of each other, unless they are lighter than about 0.1 mg or very irregular in shape when high positions may be occupied. Particles are recovered from the columns by filtration or, if they are insufficiently differentiated in form or weight to permit their subsequent identification, by the addition of further mixture from the mixing vessel when the displaced particles and mixture can be removed from the top of the column with a bulb pipette. The gradient generated by the apparatus may be represented by part of a curve that approaches the density of the mixture in the input vessel exponentially :d3 = dz+ (dz-dl) exp(-v/V) where the densities of the mixture in the input vessel, in the mixing vessel initially, and in the effluent are dl, d2 and d3 respectively, v is the volume of effluent and V the volume of liquid in the mixing vessel. Provided that v/V is less than 1, as in the apparatus described, deviation from linearity of a gradient so generated is small (Cherkin et al., 1953). This is shown in Fig. 2, where curve 1 is calculated from the above equation and experimental parameters assuming dl =2.5250 and d2=2.4870 (specific gravities at 20°C), and curve 2 is plotted from experimental data. Curve 2 is slightly less linear than curve 1, probably because the effective volume of liquid in the mixing vessel is less than 20 ml, but the deviation from linearity is negligible over separations of less than about 15 cm. I t follows that these gradients may be used for the determination of densities of glass fragments by extrapolation from positions taken up in the columns by fragments of known density. This use and its accuracy are demonstrated in the next article (Crockett and Taylor, 1969). The development of this method owes much to its criticism and use by my colleagues to whom I am very grateful. I wish to thank Mr. T. R. Watson for Fig. 1, and Miss H. M. Ratcliffe for the experimental data plotted in Fig. 2.
References CHERKIN,A., MAKTIKEZ,F. E., and L)UNN, M. S., 1953, J. Amer. Chem. Soc., 75, 1244. CROCKETT,J. S., and TAYLOR, M. E., 1969, J. For. Sci. Soc., this issue. GREENE, R. S., and BURD,D. G., 1965, J. Forensic Sci., 10, 52. KIND,S. S., and SUMMERSCALES, L., 1966, Analyst, 91, 669. W. C., 1955, J. Polymer Sci., 17, 441. TUNG,L. H., and TAYLOR,
A method for the generation o f a linear and stable density gradient column for the measurement of s ~ e c i j cgravity of glass fiarticles i s described. Althougl~density gradient methods for the comparison of glass fragments are capable of improving considerably the exactness of the commonly used flotation methods (Green and Rurd, 1965; Kind and Summerscales, 1966), such techniques have not been widely adopted for forensic purposes. In this laboratory a density gradient method of comparison has been in routine use for over two years. The basis of the method is well known (Tung and Taylor, 1955), but it is thought that the straightforward construction, rapidity of use and effectiveness may commend the apparatus to those dissuaded from the use of density gradients 1)y the specialised equipment often required, or by the sometimes unstable and highly nonlinear nature of thermally-generated gradients.
Fig. 1. Density gradient apparatus.
The apparatus, filled with an aqueous solution of eosin to facilitate photography, is shown in Fig. 1. The burette serves as the input vessel of total 115
voliime about i 5 ml. The burette jet passes through a rubber bung into the mixifig vessel, of about 25 ml capacity, wliich is equipped with a magnetic stirrer. The mixing vessel is connected by a glass capillary, passing through the bung, and by 'teflon' tubing of 1mm internal diameter to the bottom of the gradient column. The column is 50 cm in length and 6 mm in internal diameter ; it is protected from sudden changes in temperature by a large water jacket, and the whole apparatus is positioned away from sunlight or any other source of heat likely to cause thermal effects. The gradient is prepared from mixtures of bromoform and bromobenzene ; we have not encountered the difficulties described by Greene and Burd (loc. cit.) with such solvents in thermally-generated gradients except when glass has been powdered. Gradients appropriate to the comparison of glass particles having densities in the region of 2.5 g/ml a t room temperatures are prepared in the following way. By flotation a mixture of approximately 50 ml of bromoform and bromobenzene (that retained from previous experiments and kept under sodium metabisulphite and anhydrous magnesium sulphate is suitable) is adjusted in composition until its density is equal to that of a control sample of glass. This adjustment is not critical. 20 ml of the fluid mixture is transferred to the mixing vessel, 0.2 ml of bromobenzene is added and the stirrer is started. To the rest of the mixture 0.75 ml of bromoform is added for every 10 ml and the input vessel is filled with this fluid. The rubber bung, carrying a t this stage only the connection to the jacketed gradient tube, is fitted into the mixing vessel. The jet of the burette is pushed into the bung, which forces a small amount of the mixture out of the vessel into the bottom of the gradient tube. The contents of the input vessel are now run drop by drop into the mixing vessel and the gradient column forms simultaneously ; the process takes about twenty minutes. The column is isolated with a screw clip and disconnected from the mixing vessel. After drying and fitting with a new gradient tube the apparatus is ready for re-use.
T)EPTII IS COI.1'JIS
(crns)
Fig. 2. Variation of specific gravity with depth in a density gradient column. Curve 1 (full line) is a calculated gradient, curve 2 (broken line) is an experimental one.
116
Glass particles, cleaned in hot detergent, may now by introduced into the column. Particles from the same source generally descend to within 1 cm. of each other, unless they are lighter than about 0.1 mg or very irregular in shape when high positions may be occupied. Particles are recovered from the columns by filtration or, if they are insufficiently differentiated in form or weight to permit their subsequent identification, by the addition of further mixture from the mixing vessel when the displaced particles and mixture can be removed from the top of the column with a bulb pipette. The gradient generated by the apparatus may be represented by part of a curve that approaches the density of the mixture in the input vessel exponentially :d3 = dz+ (dz-dl) exp(-v/V) where the densities of the mixture in the input vessel, in the mixing vessel initially, and in the effluent are dl, d2 and d3 respectively, v is the volume of effluent and V the volume of liquid in the mixing vessel. Provided that v/V is less than 1, as in the apparatus described, deviation from linearity of a gradient so generated is small (Cherkin et al., 1953). This is shown in Fig. 2, where curve 1 is calculated from the above equation and experimental parameters assuming dl =2.5250 and d2=2.4870 (specific gravities at 20°C), and curve 2 is plotted from experimental data. Curve 2 is slightly less linear than curve 1, probably because the effective volume of liquid in the mixing vessel is less than 20 ml, but the deviation from linearity is negligible over separations of less than about 15 cm. I t follows that these gradients may be used for the determination of densities of glass fragments by extrapolation from positions taken up in the columns by fragments of known density. This use and its accuracy are demonstrated in the next article (Crockett and Taylor, 1969). The development of this method owes much to its criticism and use by my colleagues to whom I am very grateful. I wish to thank Mr. T. R. Watson for Fig. 1, and Miss H. M. Ratcliffe for the experimental data plotted in Fig. 2.
References CHERKIN,A., MAKTIKEZ,F. E., and L)UNN, M. S., 1953, J. Amer. Chem. Soc., 75, 1244. CROCKETT,J. S., and TAYLOR, M. E., 1969, J. For. Sci. Soc., this issue. GREENE, R. S., and BURD,D. G., 1965, J. Forensic Sci., 10, 52. KIND,S. S., and SUMMERSCALES, L., 1966, Analyst, 91, 669. W. C., 1955, J. Polymer Sci., 17, 441. TUNG,L. H., and TAYLOR,