Anal Bioanal Chem DOI 10.1007/s00216-015-9126-z
PAPER IN FOREFRONT
Forensic examination of electrical tapes using high resolution magic angle spinning 1H NMR spectroscopy Torsten Schoenberger 1 & Ulrich Simmross 2 & Christian Poppe 1
Received: 10 July 2015 / Revised: 21 September 2015 / Accepted: 16 October 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract The application of high resolution magic angle spinning (HR-MAS) 1H NMR spectroscopy is ideally suited for the differentiation of plastics. In addition to the actual material composition, the different types of polymer architectures and tacticity provide characteristic signals in the fingerprint of the 1H NMR spectra. The method facilitates forensic comparison, as even small amounts of insoluble but swellable plastic particles are utilized. The performance of HR-MAS NMR can be verified against other methods that were recently addressed in various articles about forensic tape comparison. In this study samples of the 90 electrical tapes already referenced by the FBI laboratory were used. The discrimination power of HR-MAS is demonstrated by the fact that more tape groups can be distinguished by NMR spectroscopy than by using the combined evaluation of several commonly used analytical techniques. An additional advantage of this robust and quick method is the very simple sample preparation. Keywords Polymers . Spectroscopy . Instrumentation . NMR
Introduction Electrical (or insulating) tapes are used for electrical and nonelectrical applications. They can be found in every household
* Torsten Schoenberger
[email protected] 1
Bundeskriminalamt - KT12, Thaerstraße 11, 65193 Wiesbaden, Germany
2
Bundeskriminalamt - KT14, Thaerstraße 11, 65193 Wiesbaden, Germany
and are also widely used in a criminal context, e.g., in the course of making improvised explosive devices (IED), shackling or gagging victims, producing packages of illegal drugs. Comparative electrical tape examinations are performed to explore the possibility of an evidentiary link between a suspect and a crime or between different items or scenes [1]. An electrical tape usually consists of two major parts: the backing (predominantly based on PVC, plasticizers, flame retardants, fillers and stabilizers) and the adhesive layer (mostly rubber based). There are several analytical techniques available for the analysis and discrimination of electrical tapes. Visual/ microscopic examination, infrared (IR) or Raman spectroscopy, pyrolysis–gas chromatography/mass spectrometry (PyGC/MS), scanning electron microscopy/energy-dispersive spectroscopy (SEM⁄EDS), or even Pabst’s method have proved to be very effective in the course of forensic examination and comparison [1–6]. However, the application of NMR spectroscopy in this context has not been described in detail until now. In the Forensic Science Institute of the Federal Criminal Police Office (Bundeskriminalamt, BKA), NMR spectroscopy has been employed in tape analysis and discrimination for more than 15 years, in particular, by using high resolution magic angle spinning (HR-MAS) NMR spectroscopy [6]. However, the NMR results remained more or less unpublished, apart from occasionally mentioning them during presentation about the BKA’s NMR performance features in general. Whereas many soluble polymers have already been characterized by HR-NMR without problems but admittedly with broader signals, HR-MAS NMR even manages insoluble materials if they are swellable. The resulting spectra are nearly as well resolved as solution spectra. Therefore, the method is perfectly applicable for various
T. Schoenberger et al.
Fig. 1 HR-MAS sample preparation
polymers, even certain cross-linked materials like natural or synthetic rubbers. The HR-MAS NMR method used at the BKA has been accredited according to ISO 17025 since 2009. Extensive validation experiments were performed in order to reveal the robustness of the HR-MAS analysis of adhesive tapes. Several parameters, such as swelling time or temperature, have no relevant influence on the NMR spectrum [7]. The material homogeneity within an adhesive tape roll was controlled multiple times as well. In each case all NMR spectra of different sampling points were indistinguishable. The discrimination power of HR-MAS was tested by taking the set of 90 black PVC tapes provided by the laboratory of the Federal Bureau of Investigation (FBI) and comparing the results with those from several studies which had already been conducted on this set [1, 2].
Materials and method At least a duplicate measurement was performed for each individual tape. Ten milligrams (±1 mg) of each tape sample was placed in a 4-mm ZrO2 rotor and filled up with approximately 70 μL CDCl3 without using an insert (Fig. 1). Fig. 2 Most significant 1H NMR spectrum region of a typical PVC tape with structural assignment
The NMR measurements were carried out on a Bruker Avance 500 spectrometer using a 4-mm HR-MAS probe with z gradient. Exclusively single pulse 1H NMR experiments with 16 scans were performed at room temperature. The samples were rotated with a frequency of 4000 Hz. The observing frequency was 500.13 MHz. Hierarchical cluster analysis (HCA) was performed by using the software STATISTICA version 12. Ward’s method was used as merger rule and Euclidian distance as distance function.
Results and discussion The complete organic compositions of the majority of average PVC-based electrical tapes are revealed by just one 1H NMR spectrum. Figure 2 shows the assignment of the signals to the major components belonging to the backing and the adhesive layer. The shapes of the polymer signal are very diverse. They depend on the type of polymerization. For example, the polymerization of 1,3-butadiene can lead to cis-1,4-, trans-1,4-, or 1,2-polybutadiene units. The ratio of these units and the connection to each other affect the signal shape as well as the tacticity (iso-, syndio-, heterotactic). Examples of different polybutadiene types and different NMR signal shapes are shown in Fig. 3. The large number of options for the presence of tape components and for the type of polymerization is detected by NMR. So the 1H NMR spectrum gives a kind of fingerprint of the tape. Preliminary groups were formed by applying HCA for all spectra of the 90 black electrical tape samples (Fig. 4). The first grouping was helpful for further evaluation but not absolutely reliable. On the one hand concentration varieties, especially those of plasticizers, caused by different sample treatment can lead to subgrouping. In contrast to that, small
Forensic examination of electrical tapes using high resolution Fig. 3 Examples of different types of polymerization and different 1H NMR signal shapes of polybutadiene of the first four electrical tapes
differences in the initial material composition do not automatically build a new group. Because of the issues discussed above, all spectra had to be compared visually. The duplicate measurement of each tape supported the decisions whether tapes were really distinguishable or not. Figure 5 gives an impression of the variety of the resulting NMR spectra. Sometimes only small but reproducible differences lead to the distinction of samples as shown in Fig. 6. This figure illustrates the NMR spectra of five tapes which cannot be distinguished by other analytical techniques [1, 2, 6]. Differences in signal intensities were not taken into account. In particular, the concentrations of plasticizers and the adhesive layer components can vary considerably depending on the treatment and storage of the tape samples. The comparison of the 90 tape samples by HR-MAS NMR spectroscopy leads to the 59 groups listed in Table 1.
As shown in Fig. 7, tapes from the same product can consist of slightly different compositions as well. This is consistent with our experience in material analysis. Producers of technical products are (more or less) interested in steady product characteristics and not in absolutely unique compositions. Analogous grouping of the same set of 90 tapes was performed by Mehltretter et al. by applying different analytical techniques [1, 2]. There the combined evaluation of all the analyses with separate measurement of adhesives and backings resulted in a discrimination of 44 groups. The details and our generated NMR performance data are listed in Table 2. The discrimination power (DP) was also used by Mehltretter et al. for the specification of the analytical techniques’ discrimination capability. DP is calculated as follows:
Fig. 4 Preliminary grouping of all spectra by applying HCA
Fig. 5 HR-MAS 1H NMR spectra of the first four electrical tapes
number of indistinguishable pairs DP ¼ 100% 1− total number of comparison pairs
T. Schoenberger et al.
have been regularly analyzed in the BKA by using this method. Very good results have also been obtained with HR-MAS for the distinction of expanding foams and car tires.
Conclusion
Fig. 6 1H NMR comparison. Small but reproducible differences of very similar tapes that cannot be distinguished by other analytical techniques
DP can be understood as the likelihood of discrimination by taking two random samples out of the set of 90. The discrimination power of the HR-MAS NMR analysis was calculated as 98.4 % (for comparison, the DP all FBI techniques combined is 96 %). Certainly, the exact DP value is only valid for this limited set of samples of one product type. But it provides at least an assessment for the probative value if material conformity can be observed in forensic case work. The application HR-MAS is not limited to electrical tapes. Other types, such as duct tapes and packing tapes,
Table 1
It has been shown that HR-MAS NMR is a feasible technique for the discrimination of electrical tapes. An extraordinary discrimination power can be achieved just by one single measurement of an unseparated small amount of tape. Other techniques such as IR or Py-GC/MS can also be used to determine the organic chemical material composition and at least IR can detect varieties in stereochemistry in principle. However, it could be shown that HR-MAS NMR is superior regarding DP in comparison to techniques typically used. Techniques such as SEM/EDX or Pabst’s method are mainly based on the analysis of inorganic compounds, which are generally not detectable by 1H NMR. Thus they can be a good addition to the method presented in this work. The discrimination of HR-MAS NMR is even better than the combination of a number of other analytical techniques with separated measurement of adhesive layer
Grouping of 90 electrical tapes by means of HR-MAS NMR
Group no.
Sample roll
Brand name
Product
Origin
1 1 2
1 5 2
Marcy Enterprises, Inc. Tape It, Inc. Advance®
MA 750 E-60 AT7, BS3924, 31 ⁄ 90Tp
Taiwan Taiwan England
3 4 5 6 7
3 4 6 7 8
Work Saver™, a Royal Tools brand Tesa Tape, Inc. Qualpack® Marcy Enterprises, Inc. Manco®
Stock no. 55, 5 color PVC tape assortment 40201, no. 111 E52811A 1346, 6-Color MA 750 200 MPH, AE-66
China Taiwan China Taiwan Taiwan
7 7 8 9 9 9 10 11 11 11 11 11 11
38 67 9 10 23 24 11 12 15 18 19 20 25
Manco® Manco® Archer® (packaged for Radio Shack) 3M Scotch™ 3M Scotch™ 3M Scotch™ 3M Scotch™ 3M Scotch™ 3M Scotch™ 3M Scotch™ 3M Scotch™ 3M Scotch™ 3M Scotch™
P-660 P-66 64-2349 Super 88, 054007-06143 Super 88, 054007-06143 Super 88, 054007-06143 Super 33+, 10414 NA Super 33+, 10455 NA Super 33+, 10455 NA Super 33+, Cat. 195NA Super 33+, Cat. 194NA Super 33+, 10414 NA Super 33+, 054007-06132
Taiwan Taiwan Taiwan USA USA USA USA USA USA USA USA USA USA
Forensic examination of electrical tapes using high resolution Table 1 (continued) Group no.
Sample roll
Brand name
Product
Origin
11 11
39 41
3M Scotch™ 3M Scotch™
Super 33+, 3744NA Super 33+, 200NA
USA USA
12
13
3M Scotch™
Super 33+
USA
12 13
54 14
3M Scotch™ Frost King®
Super 33+ on core, 03404NA on packaging ET60
USA Taiwan
14
16
3M
Tartan™ 1710, part no. 054007 49656
USA
14 15
40 17
3M 3M Scotch™
Tartan™ 1710, part no. 054007 49656 Super 88 054007-06143
USA USA
16 17
21 22
Manco® Manco®
P-66 667 Pro Series™
Taiwan Taiwan
18
26
3M Scotch™
Super 33+, 054007-06132
USA
19 19
27 28
3M 3M
Tartan™ 1710, part no. 054007 49656 Tartan™ 1710, part no. 054007 49656
USA USA
20 20 21
29 30 31
3M 3M Regal®
Temflex™, 1700, 54007-69764 Temflex™, 1700, 54007-69764 Model ET-6
USA USA Taiwan
22 23
32 33
GE 3M Scotch™
GE2472-3DD Cat. 190
Taiwan USA
24
34
3M
Tartan™ 1710, part no. 54007-49656
USA
25 26
35 36
Frost King® 3M
ET60 Tartan™ 1710, part no. 054007 49656
Taiwan USA
27
37
National
All-purpose grade
Taiwan
28 28 28 29 29
42 51 53 43 44
National National National 3M 3M
All-purpose No. 101, E52811A No. 101, E52811A Tartan™ 1710, part no. 054007 49656 Tartan™ 1710, part no. 054007 49656
Taiwan Taiwan Taiwan USA USA
29 30 30 31 32
47 45 58 46 48
3M Calterm® Frost King® Manco® Tape-It
Tartan™ 1710, part no. 054007 49656 49605 ET60 P-20 36-T
USA Taiwan Taiwan Taiwan USA
32 33 34 35 36 37 38 38
49 50 52 55 56 57 59 60
Tape-It General Electric Frost King® Manco® Victor Automotive Products, Thermoflex United Tape Company Tuff™ Hand Tools Tuff™ Hand Tools
36-T GE2472-31D ET60FR 1219-60 33-UL60, no. 101 E52811A UT-602
USA Taiwan USA Taiwan Taiwan Taiwan China China
39 39 40 40 40 40 40 41 42
61 63 64 68 76 77 83 62 65
3M Scotch™ 3M Scotch™ 3M Scotch™ 3M Scotch™ 3M Scotch™ 3M Scotch™ 3M Scotch™ Nitto Denko 3M Scotch™
88T Super 88, 054007-06143 Super 33+, 10455NA Super 33+ Super 33+, cold weather electrical tape, 16736NA Super 33+, 054007-06132 Super 33+, 10414NA No. 228 700 commercial grade, 054007-04218
USA USA USA USA USA USA USA Taiwan USA
T. Schoenberger et al. Table 1 (continued) Group no.
Sample roll
Brand name
Product
Origin
43 44
66 69
3M Scotch™ 3M
19453 Tartan™ 1710, part no. 054007 49656
Taiwan Taiwan
45
70
Tyco Adhesives National Tape Products
No. 101, E52811A
Taiwan
46 47
71 72
Qualpack® Nitto Denko
1346, 6-color Nitto® no. 228
China Taiwan
48
73
Frost King®, Thermwell Products Co., Inc.
ET60FR
China
49 49
74 79
3M Scotch™ 3M Scotch™
700 commercial grade, 054007-04218 700 commercial grade, 054007-04218
USA USA
50 51
75 78
3M Scotch™ 3M
Linerless electrical rubber splicing tape, 2242, 06165 Tartan™1710 general use, 054007-49656
USA Taiwan
52
80
3M Scotch™
Super 88, 054007-06143
USA
53 53
81 82
Ace (Imported for Henkel Capital) Ace (Imported for Henkel Capital)
All weather Weather resistant
Taiwan Taiwan
53 54 55
86 84 85
Duck, Henkel Consumer Adhesives 3M Frost King®, Thermwell Products Co., Inc.
Vinyl electrical tape Tartan™ 1710, general use, 054007-49656 ET60FR
Taiwan Taiwan China
56 57
87 88
Nitto Denko Frost King®, Thermwell Products Co., Inc.
No. 21E ET60FR
China China
58
89
Power Pro Craft
ETF
China
59
90
Duck, Henkel Consumer Adhesives
Extra wide electrical tape
China
and backing. Furthermore, the NMR preparation and measurement takes only about 20 min. This means that the introduced solid state application is a very fast and effective method for the discrimination of electrical tapes when having access to an NMR spectrometer equipped with an HR-MAS unit. The current study of a detailed and well-reported comparison of the discrimination power for a technical
Fig. 7 NMR spectra of tapes with the same label (BFrost King, ET60, Taiwan^) but different compositions
product is extremely unique. Typically the forensic scientist has to cope with numerous different polymer products. It is not possible to perform comprehensive studies such as this one every time. Based on our experience the present results for the discrimination of electrical tapes are a very good example for the analysis of technical polymer products in general.
Forensic examination of electrical tapes using high resolution Table 2 Comparison of the number of groups divided by the application of different analytical techniques Number of groups Technique
Adhesive
Stereomicroscopy and physical measurement
3
24
FTIR Py-GC/MS
8 16
14 12
SEM/EDS Techniques combined
5 23
15 40
Adhesives and backings combined
44 59 64
NMR and Pabst’s method NMR and FBI combined and Pabst’s method
62 65
Semmler A (2000) Entwicklung einer Methode zur Untersuchung von Kunststoffen mit Hilfe der Kernresonanzspektroskopie (NMR) / HR-MAS. Diploma thesis, Fresenius University of Applied Sciences, Idstein
Backing
FBI laboratory
NMR (only) NMR and FBI combined
7.
Torsten Schoenberger is head of the NMR unit of the Forensic Science Institute, Federal Criminal Police Office (Bundeskriminalamt, BKA), Germany. His latest research fields cover quantitative NMR spectroscopy. In addition to quantification of all kinds of organic substances, one of his current main tasks is the structure elucidation of new psychoactive substances. This current work also covers the detection of counterfeit pharmaceuticals as well as the analysis of explosives and polymer-based technical products.
We are convinced that HR-MAS NMR is broadly applicable for the purpose described. The application is easy, very fast, and leads to outstanding results. Acknowledgments The authors thank Andria H. Mehltretter for her consent to use the 90 tape samples for further studies and Silke Cox and Niksa Blonder for proofreading. Compliance with ethical standards Conflict of interest The authors declare no conflict of interest.
Ulrich Simmross is an organic chemist. In 1991 he received his Ph.D. in Polymer Chemistry from the Max Planck Institute for Polymer Research. He works for the Forensic Science Institute of the BKA as a forensic practitioner for material analysis. He chairs the Working Group BPaint and Glass^ (EPG) of the European Network of Forensic Science Institutes (ENFSI).
References 1.
2.
3.
4.
5.
6.
Mehltretter AH, Bradley MJ, Wright DM (2011) Analysis and discrimination of electrical tapes: part I. Adhesives. J Forensic Sci 56: 82–94 Mehltretter AH, Bradley MJ, Wright DM (2011) Analysis and discrimination of electrical tapes: part II. Backings. J Forensic Sci 56: 1493–1504 Goodpaster JV, Sturdevant A, Andrews K, Brun-Conti L (2007) Identification and comparison of electrical tapes using instrumental and statistical techniques: I. Microscopic surface texture and elemental composition. J Forensic Sci 52:610–629 Goodpaster JV, Sturdevant A, Andrews K, Brinley E, Brun-Conti L (2009) Identification and comparison of electrical tapes using instrumental and statistical techniques: II. Organic composition of the tape backing and adhesive. J Forensic Sci 54:328–338 Dietz ME, Stern LA, Mehltretter AH, Parish A, Mc Lasky V, Aranda R 4th (2012) Forensic utility of carbon isotope ratio variations in PVC tape backings. Sci Justice 52:25–32 Henning S, Schoenberger T, Simmross U (2013) The Pabst’s method: an effective and low-budget tool for the forensic comparison of opaque thermoplastics – part 1: additional discrimination of black electrical tapes. Forensic Sci Int 233:387–392
Christian Poppe is a business chemistry student at the Fresenius University of Applied Science. He worked on the described project during his internship at the BKA.
