ANALYTICAL SCIENCES JUNE 1995, VOL. 11

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1 483 Microanalysis Textile Fibers in by Forensic Science: Total Reflection Characterization X-Ray Fluorescencet of Single Andreas PRANGE*tt, Ulrich REDS*, Helga BODDEKER*, Rainer FISCHER** and Franz-Peter ADOLF** * GKSS-Forschungszentrum, Max-Planck-Str. D Geesthacht, **Bundeskriminalamt, Thaerstr. 11, D Wiesbaden, Germany Germany Fibers of different textile materials, such as polyesters, viscose, and wool, were analyzed using TXRF, yielding a "fingerprint -type" trace element pattern. This pattern can be compared with quantitative results obtained from weighable amounts, e.g. 50 µg of the material. It can be shown that, besides the omnipresent elements Ca, Fe and Zn, some of these materials contain specific elements, such as P, Mn or Sb, and that an identification is possible using these elements even when very small amounts of sample are involved (i.e. sub-microgram). Since many synthetics contain Ti as a matting additive, the Ti concentration can be used to further differentiate the sample material. Keywords Total-reflection X-ray fluorescence spectrometry, TXRF, textile fiber microanalysis, trace element, forensic science, One of the most striking features of TXRF is its ability to quantify trace elements, even with the smallest amounts of sample. Obviously, this fact has created enormous interest in the TXRF technique by those scientists working in the field of forensic science, particularly where microsamples are very common. Several specific applications have been developed and published in the past concerning micro-analytical items, for example in the analysis of tiny hair samplesl or in the analysis of pigments2 and forensic samples.3 This contribution focusses on a classical, and yet very actual, problem in forensics: namely, the characterization of textile fibers. The unambiguous correlation of fibers, found at the site of a crime, to a certain piece of clothing or tissue is an ideal forensic scientists have, but which is hardly ever realized. The aim of this study was to find answers to the following questions: can various fibers made of the same material be distinguished by their trace element content?. is a single fiber of 2 mm length (corresponding to about 0.5 µg) sufficient for such an analysis and differentiation? S are the differences in the trace element content significant enough to enable, or to support, the correlation of single fibers to one specific source (i.e. from a variety of possible sources)?. what is the influence of external contamination of the Presented at the 5th Workshop on Total Reflection X-Ray Fluorescence Spectroscopy and Related Spectroscopical Methods, October 17-19,1994, Tsukuba, Japan. To whom correspondence should be addressed. fibers, and how can this be excluded or the contamination removed? To date, we can present answers to the first three questions. Although the problem of contamination has not yet been studied in detail, it is not necessarily as important as one might expect. Experimental The following five different groups of materials have been investigated:. Trevira, shiny (the same as Terylene, a polyester made of terephthalic acid and glycol). Fourteen different samples were investigated.. Trevira, opaque (which contains larger amounts of white pigments, mostly consisting of Ti02). Nine different samples were used for this analysis.. Polyester (of varying composition and varying Ti02 content) was also analyzed using three samples,. three samples of Viscose (i.e. modified cellulose), and six samples of different pretreated wool (which is a protein) were also finally analyzed. Sample preparation (Fig. 1) In order to obtain an element pattern for these different materials, the best approach is to analyze a larger and representative amount of sample by dissolving and preparing it with an internal standard; briefly, follow the sample preparation which is common to TXRF, and which is called here "method A". This has been used for wool, viscose, and two of polyester samples using a

2 484 ANALYTICAL SCIENCES JUNE 1995, VOL. 11 Fig. 1 Sample preparation for bulk material of fibers and for single fibers. sample input of about 5 mg and heating it with acid to 100 C for 1 h. After a first measurement, the sample matrix is removed by exposing the support to an oxygen cold plasma for about 30 min, which is then followed by another measurement of the sample. The second measurement, after matrix removal, yields of course much better detection limits. However, a so-called "direct" measurement is necessary in order to determine sulfur, which may to a greater extent be removed by the plasma. Unfortunately, this preparation method is not suited for trevira fibers for two reasons. Firstly, the high Ti02 content of some of these samples yields irreproducible results, because Ti02 does not dissolve in nitric acid. Secondly, the terephthalic acid set free upon decomposition, resists any further attack by nitric acid, and forms a white precipitate, so that the sample solution is no longer homogeneous. We have therefore used "method B" for these samples, which means that about half a milligram of fiber material is prepared (and weighed) directly on a support. Predigestion with one drop of nitric acid will soak the fibers and glue them to the support; it is also used to add the internal standard to the sample. A direct measurement in this state is necessary if the samples contain sulfur, which was actually not the case. The matrix is then removed by plasma ashing, and the samples are measured as previously described. Finally, individual fibers of the materials were simply analyzed by preparing one or several pieces of 2 mm length on a sample support, and subsequently measuring them. We have estimated the individual sample mass by assuming a fiber weight of 350 µg/m, which, in combination with a standard element, yielded quantitative results. The fiber weight was determined empirically by weighing some larger fibers using a microbalance; the results varied from 200 to 500 µg/m depending on the fiber material. Fig. 2 TXRF spectrum of a bulk sample (Trevira No. 6). Gallium is used as internal standard, its concentration is 50 µg/g. Detection limits (e.g. for Pb) are at about 0.2 µg/g. Results and Discussion Figure 2 shows a typical spectrum of one of the socalled "bulk" samples, a Trevira sample with a moderate Ti02-content. Gallium was used as the internal standard, with a concentration of 50 µg/g. Element concentrations range from 320 µg/g for Ti down to 0.2 µg/g for Pb. Bulk samples of this type have been analyzed for all five materials investigated in this work, yielding a survey of element abundances. Element patterns Table 1 gives a closer look at the element patterns, and shows which elements were found in the different samples. The concentrations are given only as orders of magnitude; anything below 1 sg/g has been left out. It is obvious that K and Ca are always present, and that their concentrations do not show much fluctuation. In addition, these two elements also have a high probability of appearing as contaminants, which is also the case for Fe and Zn. These elements were therefore left out of first step identification procedures. Cr and Cu were found only at low levels, if at all, and were therefore of no practical use. Those elements, however, which are highlighted by a darker background, show either an interesting pattern or a large variation in their concentrations. For example, Ba is only present in the viscose samples; Sb is either present with 200 to 300 ppm or not at all. Similarly, Mn is present either at a concentration of about 30 ppm or near zero. Sulfur is found in the percent range in wool (which is no surprise), but is also found together with P in viscose samples. On the other hand, in all other samples these two elements are one to two orders of magnitude lower in concentration. Unfortunately, we did not determine sulfur for most of

3 485 Table 1 Trace element patterns in fiber samples Fig. 3 Element ratios of the determined from bulk analysis. Trevira opaque fibers as the Trevira samples because of the experimental difficulties mentioned earlier; thus no results can be given here. From the data which we have, however, typical values seem to be in the range of about 100 to 1000 ppm. Finally, Ti appears to be the most prominent element in many of the samples. Without any further information, the order of magnitude of the concentration of Ti points to a certain group of probable materials, therefore excluding most of the other groups. Element ratios Further differentiation, within one group of materials or for all samples, can be achieved by looking at the element ratios. Figure 3 shows three such ratios between the elements P, Ti, Mn and Sb. The nine samples in this group can be clearly distinguished by the use of these ratios. If we take the first three specimens, they are completely different in their element patterns: No. 1 has no P and no Sb; Nos. 2 and 3 are similar in Mn/Ti and Sb/Mn ratios, but differ by two orders of magnitude in the P/Ti ratio. There are also noticeable differences in the other two ratios (Sb/Mn of No. 2 and No, 3 differ by a factor 2!). Similar considerations can be made for all of the other samples. However, even if these ratios should not be sufficient for an identification, the absolute concentration of Ti, which is easy to determine, will allow further differentiation, since it varies between 1500 to 6500 µg/ g within this group of samples. Singlefiber results Figure 4 shows the TXRF-spectrum of a 3 mm piece of an "unknown" fiber measured without standard; and all we knew about it was that it had to be one of the 35 samples involved in this study. The measuring time was 3000 s in order to obtain better detection limits. Besides the elements of interest (mentioned above), also Fe, Cu and Zn were present, which were probably contaminants. It should be mentioned that the preparation of single fibers, which are much thinner than a human hair, is not easy, particularly if there is no suitable equipment for micromanipulation available. This is why samples can become contaminated with some picograms of some of the more "common" elements. In any case, there is a rather strong Ti peak, which indicates that the sample belongs to the Polyester or to the Trevira opaque group (see Table 1). Polyester can be excluded because of the presence of P, which was not very clearly seen in the spectrum, but was found in the evaluation. We are thus back to the same group of fibers which we have described before, namely Trevira opaque; and we must now try to further specify our unknown fiber. The blank bars in Fig. 5 indicate the element ratios, or more precisely the range of possible values for these

4 486 ANALYTICAL SCIENCES JUNE 1995, VOL. 11 Table 2 Quantitative results of a single fiber (values in µg/g) Fig. 4 TXRF spectrum of an "unknown" single fiber of 3 mm length, measured directly without internal standard. ratios. The next experiment was carried out with the addition of an internal standard and yielded the results given in Table 2. As could be expected from the data shown before, the results for Ti, Mn and Sb leave open two possibilities; however the P concentration obtained here definitely excludes sample No. 3, even when a 3Q criterion is applied. The unknown fiber is thus identified as coming from the Trevira opaque sample No. 9. The same kind of single-fiber measurement was performed for the other samples in order to answer such questions as to whether an unambiguous identification is the rule, or is more exceptional. The results for all 35 samples are: definite assignment: 2 candidates left: 3 candidates left: 23 samples 9 samples 3 samples (65%) (26%) (9%) We call any assignment definite when three element ratios or two ratios plus one absolute value are in agreement, and there is no other candidate to fit these data within three standard deviations. This requirement is fulfilled in two thirds of all cases. The double and triple candidate cases originate mostly from the group "Trevira shiny", where some samples are very similar, or, even worse, have no trace elements at all in a useful range of concentration. Fig. 5 Element ratios of a single fiber (black intervals) compared to the ratios obtained from analysis of Trevira bulk samples. ratios, when a 1Q uncertainty is applied. These data were derived from the spectrum of the unknown fiber. The P/Ti value excludes all samples except for Nos. 3 and 9. From the two other ratios, sample No. 9 is clearly favored, although we can not state this unambiguously. Thus, the number of candidates has been reduced from 35 to 2 by just identifying elements and comparing In conclusion, it can be stated that there are in fact trace element patterns in textile fibers, which allow us to clearly distinguish different fiber materials. Even within groups of fibers made of the same material, differences are, in many cases, large enough for this purpose. The identification of very small fragments of single fibers is possible. The smallest pieces analyzed in this study were about 3 mm in length and had a mass of less than 1 µg. For a quantitative analysis of these fragments, however, a reliable determination or estimate of their individual mass is needed. Since the assignment can be made in most cases by simply comparing element patterns, quantitative data are only required when the pattern alone is not conclusive. On the basis of these data, 23 out of 35 samples could be definitely assigned in the present study. For the application of this technique to the field of forensic science, it is of special importance to note that, since it is nondestructive, precious pieces of evidence do not have to be sacrificed for the analysis. Two major points still have to be studied in connec-

5 487 tion with textile fiber examination. First, there is the problem of surface contamination and how it can be removed from the samples; we have discussed this problem in the context of elements such as Ca, Fe or Zn. Second, since all of the fibers investigated in this work were uncolored, the influence of dyes remains to be studied, as most fibers in "real life" will origin from colored materials. This influence may complicate the identification procedure, or may even facilitate it by introducing additional specific elements. References A. Prange and H. Schwenke, Adv. X -Ray Anal., 35, 899 (1992). R. Klockenkamper, A. V. Bohlen, L. Moens and W. Devos, Spectrochim. Acta, 48B, 239 (1993). T. Ninomiya, S. Nomura, K. Taniguchi and S. Ikeda, Anal Sci., 11, 489 (1995). (Received March 3, 1995) (Accepted May 8, 1995)