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Infrared spectroscopy of ballpen paste
Forensic Science ZnternatiunaJ 46 (19901105- 109 Elsevier Scientific Publishers Ireland Ltd.
105
INFRARED SPECTROSCOPY OF BALLPEN PASTE
BEATA TRZCINSKA Institute of Forensic Research, Cmcow lPoland)
Summary Infrared spectroscopy has been selected for the examination of samples of ballpen paste. An FTS spectrometer C’Digilab”) was used for testing the samples of ballpen paste collected from one of the local manufacturers (“ZZG Niegossowice”). It was established that: (al mixing of a sample of paste (before extraction1 and the length of mixing time does not affect the intensity of spectrum in a significant way, and (bl the effect of length of extraction time is also negligible. The characteristic quantity selected does not allow for the determination of the effect of mixing on an IR spectrum. For selected strips the values of confidence intervals were calculated and on this basis samples of ballpen paste were divided into three groups for the purpose of further examination. The results obtained so far seem to be encouraging. Key words: Infrared spectroscopy; Ballpen paste: Identification
Introduction In the criminal investigation of a handwritten document both the written line and writing ink used are considered as a trace. In Poland, the most often used writing agents are: ballpoint ink, writing ink and drawing ink. A ballpoint pen is the instrument used regularly by almost everybody, a nib pen and various kinds of markers are employed by few persons, others using them from time to time. One of the features of the writing agents under consideration is their colour. In this respect, irrespective of the agent, the most widespread inks are those of navy-blue, blue-violet, then black, green and red. Taking the above mentioned under consideration, the Institute of Forensic Research in Cracow entered upon a study of blue ballpoint inks. The aim of the study was criminal identification. From the criminalistic point of view the best result would be the possibility of reaching an individual identification. In the case of the group of investigated traces comprising blue ballpoint ink individual identification can mean only the possibility of differentiation of the ink in the limits of the given group. In practice, it resolves itself to the possibility of differentiation of inks characterized by slightly different formulations. The problem arises when a document is suspected to be fraudulent. 03794733/90/$03.50 0 1990 Elsevier Scientific Publishers Ireland Ltd. Printed and Published in Ireland
106
It is obvious that a forger would try not only to forge writing but also to use writing ink the physical features of which, especially the “appearance” of the written line, would be close to the appearance of the lines on the remaining portion of a document. Forensic chemists have been interested in this group of traces for a long time. Examinations have been carried out using various methods such as paper chromatography, electrophoresis, thin layer chromatography. A number of non-destructive methods such as observation under visible light using narrow-band filters, inspection under ultraviolet and infrared light, fluorescence under visible light, infrared and ultraviolet luminescence etc have also been developed. It is unquestionable that the employment of nondestructive methods is the only right solution. However, since this is not always possible, for many reasons, there is a need to search among destructive methods for the ones distinguished by other advantages. For example, thin layer chromatography requires a microquantity of examined ink, and can be extremely valuable when used in criminal investigations. It is however of little use when the examined samples differ quantitatively other than qualitatively. One must remembr that both the kind and amount of components used for ballpoint ink production are limited as well as that a production batch contains a few thousand ballpoint pens. It can happen therefore that writing ink, matched by a forger in respect of colour, could originate from the same manufacturer or from another who produces ink according to a similar recipe and processing. So it is necessary to have a method which could differentiate inks with similar formulations and infrared spectroscopy can serve as this method. Spectra of pure chemical substances in the medium infrared region is rightly compared to fingerprint marks and therefore it makes possible an individual interpretation. In the case of mixtures, identification of mixture components becomes slightly more difficult because the appearance of bands originating from the given component in the spectrum, which makes possible its identification, depends not only upon the amount of the component in a sample but also on the relative absorbance of bands coming from particular components of the mixture. In this case linkage of a spectrometer with a computer and effective software is of great assistance. In the case of questioned document examination it is very often not essential to know what kind of ink was used to write a document (although the answer to the question would be very valuable1 but it is essential to determine whether the questioned part of the document has been filled in the same writing ink as the remaining part of the text. The obtained spectra ought to be therefore considered not only qualitatively, i.e. which chemical compounds in the ballpoint ink components gave the existing bands but also whether the spectra have quantitative parameters: the bands location, their absorbance,.area fields. If the questioned parts of a document were filled in the same kind of writing ink, qualitative identification would not show differences between them.
107
The studies carried out presently in the Institute are connected with the possibility of discrimination of blue ballpoint inks with similar formulations by infrared spectrometry, thus it is a trial of a quantitative approach to the spectra of this trace group. Experimental The studies are of model character and are accomplished for Polish ballpoint ink. The ink is manufactured in a plant near Cracow (ZZG Niegoszowice), which makes it possible to obtain, if necessary, data concerning any changes in a recipe and processing as well as data on the final product standard properties. The examined samples comprised a dozen or so various batches produced during a few months. It is necessary to explain at this point that in the case of ballpoint ink production, as it is in many organic syntheses, there is a possibility of obtaining from the same kind of raw materials in similar conditions the same final product but different amounts of various by-products. Since the ballpoint ink production does not require special purity of raw materials or very severe processing conditions, particular batches differ one from another. So it is necessary to discover whether infrared spectrometry makes it possible to mark these differences and on this basis to differentiate particular batches. A document examiner, however, seldom has pure ballpoint ink to examine. It is mostly its trace in the form of a line on a substrate which, in the case of a document, may be different sorts of paper. Therefore, concurrently with pure ink examination, studies on given ink placed on a substrate were carried out. Since the studies are of model character, there is no danger in minimalizing a sample amount at this stage. Several dozen milligrams of pure ballpoint ink were taken for examination; the amount of ink placed on paper was in the range 3-7 mg and the area of the substrate was about 0.5 cm2. Possibilities of reducing the amount of ink placed on the substrate to a single line a few millimetres in length are real and will be worked out at further stages of the study. IR spectra of both groups of samples were.measured on a Digilab Fourier transform infrared spectrometer FTS-15B. For pure ballpoint ink analysis, a technique of sample absorption onto the potassium bromide pellets was used. As to the ballpoint ink placed on a substrate, the spectra measurements were preceded by a process of ink liberation from the substrate (extraction). The apparatus set for this purpose has been designed in the Institute. The main part of the extraction apparatus comprises a capillary with sintered glass arranged vertically. This capillary is filled with finely ground potassium bromide. The part of the capillary tube which contains BKr is heated and maintained at elevated temperature. The upper part of the capillary is water-cooled. The lower part, below the sinter, is connected with a vacuum system. A paper fragment with ballpoint ink is placed onto the potassium bromide (it may slightly cover it up). From above, at specified intervals, specified
108
amounts of solvent are added by means of a micro-syringe. Ballpoint ink is dissolved and the solution moves into a bromide layer. Since the layer is heated to the temperature near the solvent boiling point, the extract is thickened (one must remember that besides the ballpoint ink some constituents of paper such as fillers and clays may also be extracted). Ballpoint ink ‘settles at a definite height, and solvent remnants are removed by the vacuum system. A part of the packing containing the settled ballpoint ink is used for the preparation of pellets for IR spectra measurement. Any contamination present or paper constituents extracted settle at the other areas on the capillary column. It is possible to extract six samples at the same time, which eliminates any differences that would have been originated as a result of unexpected change in the extraction conditions resulting from non-stabilized heating and cooling. Results
The obtained spectra of ballpoint ink and its extract differ slightly. The differences are related only to the absorption bands which originate from the solvent used for ballpoint pen production. In the extract spectrum, these bands almost do not occur at all if they originate from the component, or their intensity is lowered considerably if the occurrence of the given band in the spectra is connected with other components. StatisticalAnalysis of Results Statistical methods were used to evaluate these small quantitative differences existing in the spectra and to differentiate the ballpoint inks on this basis. To implement the evaluation, sets of parameters (features for testing1 were selected on the basis of spectra of any pure ballpoint ink and its corresponding extract. In the case of IR spectra analysis the feature is the ratio of the intensity of two bands because it is a value independent of concentration. Ratios having the relative standard deviation (RSD) not more than 5% were selected; the quantity of the features in the set was in accordance with the testing requirements. The calculation of the features for both spectra showed that the sets are not identical so, after elimination of the dependant features, the following sets for testing of the samples pairs were selected: Set No 1: ballpoint ink (1) - ballpoint ink (21 Set No 2: extract of ballpoint (11 - extract of ballpoint (21 Set No 3: ballpoint ink (11 - extract of ballpoint (1)
109
Set No 3 was formed with the participation of common features as well as features of the least RDS values belonging to sets No 1 and No 2. Hotelling’s T2-test, which is a test of significance was used as an instrument for testing. It is used for verification of a hypothesis which postulates that the differences between comparable values are insignificant. It is a multidimensional equivalent of Student’s t-test - and therefore it is suited for evaluation of spectroscopic analysis results. Moreover, it takes into consideration the precision of the determinations and the correlation between the features as well as the fact that it can be used if data is lacking on variance and covariance of measured features. Statistical testing was performed on a commodore 64 computer. In the first stage an attempt to determine the effect of stirring and time of stirring and extraction on the IR spectra was made. Hence the measurements of the IR spectra for the ballpoint ink not stirred and stirred for various periods of time (15-45 mini and for the extract of the ballpoint ink not stirred and stirred were performed. For all these groups, the given sets of features were computed and on their basis the statistical testing for all possible combinations of the sample pairs was performed. If the calculated value of the T2-statistic is lower than the critical value connected with the given number of features and the confidence interval assumed, then there is no reason to reject the null hypothesis postulating that the mean values of the measured features did not differ significantly. It is possible, therefore, to assume that the samples are of the same population and cannot differ from each other. However, if the calculated value Tzstatistic exceeds the critical value, the null hypothesis must be rejected. Assuming the stirring has no influence, set No 1 was used for testing. Then, assuming that the stirring can accelerate the process of solvent evaporation, set No 3 which eliminates the features connected with the solvent bands, was tested. It was only set No 3 which gave the T2 values lower than the critical values. Comparison of the ink samples of different time of stirring, both for set No 1 and No 3, gave the T2 values lower than the critical value. For all the other sample combinations, when using the proper set of features, lower values were also obtained. On this basis it was assumed that the stirring time in the studied range has no effect on the ink spectrum whereas the effect of extraction may be included by using the proper set of features (No 31, the former being connected mainly with solvent evaporation. The studies we have carried out hitherto did not allow for explicit determination of the stirring effect; it is only possible to state that the effect is similar to that of extraction. Studies of particular batches are now being carried out. The batches have been divided into thret+groups on the basis of the calculated confidence intervals for the selected features.
105
INFRARED SPECTROSCOPY OF BALLPEN PASTE
BEATA TRZCINSKA Institute of Forensic Research, Cmcow lPoland)
Summary Infrared spectroscopy has been selected for the examination of samples of ballpen paste. An FTS spectrometer C’Digilab”) was used for testing the samples of ballpen paste collected from one of the local manufacturers (“ZZG Niegossowice”). It was established that: (al mixing of a sample of paste (before extraction1 and the length of mixing time does not affect the intensity of spectrum in a significant way, and (bl the effect of length of extraction time is also negligible. The characteristic quantity selected does not allow for the determination of the effect of mixing on an IR spectrum. For selected strips the values of confidence intervals were calculated and on this basis samples of ballpen paste were divided into three groups for the purpose of further examination. The results obtained so far seem to be encouraging. Key words: Infrared spectroscopy; Ballpen paste: Identification
Introduction In the criminal investigation of a handwritten document both the written line and writing ink used are considered as a trace. In Poland, the most often used writing agents are: ballpoint ink, writing ink and drawing ink. A ballpoint pen is the instrument used regularly by almost everybody, a nib pen and various kinds of markers are employed by few persons, others using them from time to time. One of the features of the writing agents under consideration is their colour. In this respect, irrespective of the agent, the most widespread inks are those of navy-blue, blue-violet, then black, green and red. Taking the above mentioned under consideration, the Institute of Forensic Research in Cracow entered upon a study of blue ballpoint inks. The aim of the study was criminal identification. From the criminalistic point of view the best result would be the possibility of reaching an individual identification. In the case of the group of investigated traces comprising blue ballpoint ink individual identification can mean only the possibility of differentiation of the ink in the limits of the given group. In practice, it resolves itself to the possibility of differentiation of inks characterized by slightly different formulations. The problem arises when a document is suspected to be fraudulent. 03794733/90/$03.50 0 1990 Elsevier Scientific Publishers Ireland Ltd. Printed and Published in Ireland
106
It is obvious that a forger would try not only to forge writing but also to use writing ink the physical features of which, especially the “appearance” of the written line, would be close to the appearance of the lines on the remaining portion of a document. Forensic chemists have been interested in this group of traces for a long time. Examinations have been carried out using various methods such as paper chromatography, electrophoresis, thin layer chromatography. A number of non-destructive methods such as observation under visible light using narrow-band filters, inspection under ultraviolet and infrared light, fluorescence under visible light, infrared and ultraviolet luminescence etc have also been developed. It is unquestionable that the employment of nondestructive methods is the only right solution. However, since this is not always possible, for many reasons, there is a need to search among destructive methods for the ones distinguished by other advantages. For example, thin layer chromatography requires a microquantity of examined ink, and can be extremely valuable when used in criminal investigations. It is however of little use when the examined samples differ quantitatively other than qualitatively. One must remembr that both the kind and amount of components used for ballpoint ink production are limited as well as that a production batch contains a few thousand ballpoint pens. It can happen therefore that writing ink, matched by a forger in respect of colour, could originate from the same manufacturer or from another who produces ink according to a similar recipe and processing. So it is necessary to have a method which could differentiate inks with similar formulations and infrared spectroscopy can serve as this method. Spectra of pure chemical substances in the medium infrared region is rightly compared to fingerprint marks and therefore it makes possible an individual interpretation. In the case of mixtures, identification of mixture components becomes slightly more difficult because the appearance of bands originating from the given component in the spectrum, which makes possible its identification, depends not only upon the amount of the component in a sample but also on the relative absorbance of bands coming from particular components of the mixture. In this case linkage of a spectrometer with a computer and effective software is of great assistance. In the case of questioned document examination it is very often not essential to know what kind of ink was used to write a document (although the answer to the question would be very valuable1 but it is essential to determine whether the questioned part of the document has been filled in the same writing ink as the remaining part of the text. The obtained spectra ought to be therefore considered not only qualitatively, i.e. which chemical compounds in the ballpoint ink components gave the existing bands but also whether the spectra have quantitative parameters: the bands location, their absorbance,.area fields. If the questioned parts of a document were filled in the same kind of writing ink, qualitative identification would not show differences between them.
107
The studies carried out presently in the Institute are connected with the possibility of discrimination of blue ballpoint inks with similar formulations by infrared spectrometry, thus it is a trial of a quantitative approach to the spectra of this trace group. Experimental The studies are of model character and are accomplished for Polish ballpoint ink. The ink is manufactured in a plant near Cracow (ZZG Niegoszowice), which makes it possible to obtain, if necessary, data concerning any changes in a recipe and processing as well as data on the final product standard properties. The examined samples comprised a dozen or so various batches produced during a few months. It is necessary to explain at this point that in the case of ballpoint ink production, as it is in many organic syntheses, there is a possibility of obtaining from the same kind of raw materials in similar conditions the same final product but different amounts of various by-products. Since the ballpoint ink production does not require special purity of raw materials or very severe processing conditions, particular batches differ one from another. So it is necessary to discover whether infrared spectrometry makes it possible to mark these differences and on this basis to differentiate particular batches. A document examiner, however, seldom has pure ballpoint ink to examine. It is mostly its trace in the form of a line on a substrate which, in the case of a document, may be different sorts of paper. Therefore, concurrently with pure ink examination, studies on given ink placed on a substrate were carried out. Since the studies are of model character, there is no danger in minimalizing a sample amount at this stage. Several dozen milligrams of pure ballpoint ink were taken for examination; the amount of ink placed on paper was in the range 3-7 mg and the area of the substrate was about 0.5 cm2. Possibilities of reducing the amount of ink placed on the substrate to a single line a few millimetres in length are real and will be worked out at further stages of the study. IR spectra of both groups of samples were.measured on a Digilab Fourier transform infrared spectrometer FTS-15B. For pure ballpoint ink analysis, a technique of sample absorption onto the potassium bromide pellets was used. As to the ballpoint ink placed on a substrate, the spectra measurements were preceded by a process of ink liberation from the substrate (extraction). The apparatus set for this purpose has been designed in the Institute. The main part of the extraction apparatus comprises a capillary with sintered glass arranged vertically. This capillary is filled with finely ground potassium bromide. The part of the capillary tube which contains BKr is heated and maintained at elevated temperature. The upper part of the capillary is water-cooled. The lower part, below the sinter, is connected with a vacuum system. A paper fragment with ballpoint ink is placed onto the potassium bromide (it may slightly cover it up). From above, at specified intervals, specified
108
amounts of solvent are added by means of a micro-syringe. Ballpoint ink is dissolved and the solution moves into a bromide layer. Since the layer is heated to the temperature near the solvent boiling point, the extract is thickened (one must remember that besides the ballpoint ink some constituents of paper such as fillers and clays may also be extracted). Ballpoint ink ‘settles at a definite height, and solvent remnants are removed by the vacuum system. A part of the packing containing the settled ballpoint ink is used for the preparation of pellets for IR spectra measurement. Any contamination present or paper constituents extracted settle at the other areas on the capillary column. It is possible to extract six samples at the same time, which eliminates any differences that would have been originated as a result of unexpected change in the extraction conditions resulting from non-stabilized heating and cooling. Results
The obtained spectra of ballpoint ink and its extract differ slightly. The differences are related only to the absorption bands which originate from the solvent used for ballpoint pen production. In the extract spectrum, these bands almost do not occur at all if they originate from the component, or their intensity is lowered considerably if the occurrence of the given band in the spectra is connected with other components. StatisticalAnalysis of Results Statistical methods were used to evaluate these small quantitative differences existing in the spectra and to differentiate the ballpoint inks on this basis. To implement the evaluation, sets of parameters (features for testing1 were selected on the basis of spectra of any pure ballpoint ink and its corresponding extract. In the case of IR spectra analysis the feature is the ratio of the intensity of two bands because it is a value independent of concentration. Ratios having the relative standard deviation (RSD) not more than 5% were selected; the quantity of the features in the set was in accordance with the testing requirements. The calculation of the features for both spectra showed that the sets are not identical so, after elimination of the dependant features, the following sets for testing of the samples pairs were selected: Set No 1: ballpoint ink (1) - ballpoint ink (21 Set No 2: extract of ballpoint (11 - extract of ballpoint (21 Set No 3: ballpoint ink (11 - extract of ballpoint (1)
109
Set No 3 was formed with the participation of common features as well as features of the least RDS values belonging to sets No 1 and No 2. Hotelling’s T2-test, which is a test of significance was used as an instrument for testing. It is used for verification of a hypothesis which postulates that the differences between comparable values are insignificant. It is a multidimensional equivalent of Student’s t-test - and therefore it is suited for evaluation of spectroscopic analysis results. Moreover, it takes into consideration the precision of the determinations and the correlation between the features as well as the fact that it can be used if data is lacking on variance and covariance of measured features. Statistical testing was performed on a commodore 64 computer. In the first stage an attempt to determine the effect of stirring and time of stirring and extraction on the IR spectra was made. Hence the measurements of the IR spectra for the ballpoint ink not stirred and stirred for various periods of time (15-45 mini and for the extract of the ballpoint ink not stirred and stirred were performed. For all these groups, the given sets of features were computed and on their basis the statistical testing for all possible combinations of the sample pairs was performed. If the calculated value of the T2-statistic is lower than the critical value connected with the given number of features and the confidence interval assumed, then there is no reason to reject the null hypothesis postulating that the mean values of the measured features did not differ significantly. It is possible, therefore, to assume that the samples are of the same population and cannot differ from each other. However, if the calculated value Tzstatistic exceeds the critical value, the null hypothesis must be rejected. Assuming the stirring has no influence, set No 1 was used for testing. Then, assuming that the stirring can accelerate the process of solvent evaporation, set No 3 which eliminates the features connected with the solvent bands, was tested. It was only set No 3 which gave the T2 values lower than the critical values. Comparison of the ink samples of different time of stirring, both for set No 1 and No 3, gave the T2 values lower than the critical value. For all the other sample combinations, when using the proper set of features, lower values were also obtained. On this basis it was assumed that the stirring time in the studied range has no effect on the ink spectrum whereas the effect of extraction may be included by using the proper set of features (No 31, the former being connected mainly with solvent evaporation. The studies we have carried out hitherto did not allow for explicit determination of the stirring effect; it is only possible to state that the effect is similar to that of extraction. Studies of particular batches are now being carried out. The batches have been divided into thret+groups on the basis of the calculated confidence intervals for the selected features.