Introduction

Pressure-sensitive adhesive tapes are encountered in criminal investigations as evidence from a crime scene, in document examinations as residues or contaminants on the documents in question, and as contaminants in food, pharmaceutical, and other consumer products. In these cases, it is necessary to be able to identify the type and possible source of the tape in question, often from a very small sample. In addition, certain sampling techniques, such as dust sampling, require the use of adhesive tapes to lift samples of particles from a solid substrate. The analyst must then choose a suitable solvent to remove the adhesive from the particles of interest, which requires some knowledge about the kind of tape that was used and its solubility properties.

A reliable method has been developed for preparing cross-sections from tape samples as small as 1 mm², which can then be photographed, analyzed by any suitable means, and compared to known reference samples. The method involves immersing the sample briefly in liquid nitrogen, and cutting cross-sections by hand while the sample is still partially frozen. The advantage of this method is that the resulting specimen will maintain its original cross-sectional dimensions, without any dragging or deformation of the tape layers. Cross-sections that are prepared by this method are also much thinner than those prepared at room temperature, on the order of 10-50 μm compared to 100-200 μm for room temperature preparations. 

Description of Method

For all of the steps in this procedure, the slides are placed on top of a glass block that measures 3 x 1 x 1/2", which elevates the slide so that one can easily work at hand-level. The glass blocks are described in detail in The Microscope Vol 50:4, pp.159-168 (2002).

The choice of cutting blade will greatly affect the quality of the finished cross-sections. A good blade should be sharp, thin, and stiff, with a cutting edge parallel to its length. Micro-surgical blades are thin and sharp, but they bend at the tip, and their cutting edge, 15 and 45 degrees from horizontal, makes these blades hard to position for sectioning. Double-edged generic razor blades have all of the desired qualities. Four cutting blades can be prepared from one double-edge razor blade with a pair of heavy-duty scissors, as shown in Figure 1a. Each blade will fit into an X-Acto knife holder, and should be oriented so that the curved corner of the blade is used for cutting. The blade should not protrude more than 5 mm from the holder (see Figure 1b), or the blade will bend too easily. Note that the blades are fragile. The cutting tip, less than 1 μm, must be handled gently, and after a few cuts the blade should be replaced with a fresh one.  To check if a blade is still sharp, turn it upside down and focus on the cutting edge at about 20X magnification under a stereomicroscope. A bright line, no matter how fine, indicates that the cutting edge of the blade has been bent.  An undamaged cutting edge is difficult to see at 20X magnification.

Figure 1a

Figure 1b

Step 1 - The Preparation Slide

The preparation slide allows the analyst to cut a 1.5 mm-wide strip of the subject tape, anchor it, and transfer it to the sectioning slide for Step 2. The preparation slide is a standard glass microscope slide to which a 2” long strip of double-sided tape has been attached (See Figure 2). At one end of the slide, a stack of tape squares (Scotch Magic™ Tape #810), about 1/2" x 1/2" and about 15 layers thick, is placed adhesive side up. The stack of tape squares is made by slicing a block of tape from the roll, and cutting a beveled edge at one corner of the stack so that it is easy to pick up one layer of tape at a time. If the subject sample consists of double-sided tape, you will also need a stack of about 50 plastic wrap squares, about 1/4" x 1/4", next to the Magic™ Tape Squares. For the plastic wrap squares, we found that generic wrap purchased from Jewel Food Stores worked best; Saran Wrap™ was too oily and did not adhere well to the subject tape samples.

Figure 2

A small piece of the subject tape, about 1.5 x ≥3.0 mm in size, is placed adhesive side down on the stack of Magic™ Tape squares, so that one third of the subject tape extends off of the Magic™ Tape stack., as shown in Figure 2.  If the subject sample consists of double-sided tape, a square of plastic wrap is removed from the stack and placed on top of the subject tape. The top layer of the Magic™ Tape squares, with the subject tape attached, is peeled from the Magic™ Tape stack and transferred to the Sectioning Slide.