Forensic Microscopy: An Introduction

• Forensic microscopy is an introduction to forensic science.

Objectives

• Becoming familiar with the parts of an optical microscope is important.
• Resolution and depth of focus are some optical properties to consider.
• It's important to learn the variations of optical microscopes and when they are useful.
• Useful optical microscope variations include polarized light, stereomicroscope, and comparison scope.
• A non-optical microscope can be used for high resolution.

Trace Evidence

• Trace evidence can be anything and remembering Locard's Exchange Principle is important.

Overview

• Evidence is usually small or microscopic in size.
• Primary evidence examples are soil from a gravesite.
• Secondary evidence examples are dog hair transferred from a couch or car seat.

Microscopy

• Microanalysis is the application of a microscope and microscopical techniques to observe, collect, and analyze micro-evidence.
• Microanalysis can be used for milligram (mg) and Microgram (µg) amounts.
• Microscopy can be used to determine possible associations as well as the strength of the associations.
• Microscopy aids in the comparison of materials found and the conclusions of common origins.
• Properties observed under the microscope are morphology and analytical data.
• Types of analytical data include optical properties, molecular spectra, and elemental analysis.

Instruments of Microanalysis

• Basic microscopy can include visible and infrared spectrophotometry via a microscope.
• Scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDS) is another instrument.

Microscopy Labs

• Large labs typically handle fiber, hair, mineralogy, paint, serology, controlled substances, and firearm analysis.
• Smaller labs typically handle chemistry, biology, microscopy, drugs, and ballistics.
• The instrument employed for the collection and first evaluation of small evidence is a microscope.

Basic Microscopy

• Stereo binocular microscopes are employed in the preliminary evaluation of submissions for location and recovery of particles and materials from substrates.
• Total magnification is determined by the power of the objective (OBJ), first, lens multiplied by the power of the eyepiece (EP), finial lens.
• Stereo binocular microscopes generates a three-dimensional figure, with a good depth of focus and wide field of view.
• Stereomicroscopes are good for screening evidence like soil/sand, glass, paint, fabric, hair, documents, projectiles, plastics, plant morphology, and tool marks.
• Stereomicroscopes are used to analyze the distribution of particles such as explosive residues.
• Compound binocular microscopes are typically used in schools and medical laboratories.
• Compound binocular microscopes obtain morphological or structural information, and transmits bright light through the sample
• Total Magnification (TM) with a compound binocular microscopes = OBJ x EP
• This magnifies an image from 25-2300 times, however forensic Laboratories generally utilize 40-400 times

Compound Light Microscope

• The compound light microscope components are:
  • Ocular (eyepiece)
  • Body tube
  • Objective
  • Stage
  • Coarse and fine focus
  • Condenser arm
  • Field lens (diaphragm)
  • Base
• Total magnification = magnification of ocular lens multiplied by magnification of objective lens

Resolution

• Resolution is the degree to which a microscope can distinguish fine details or the ability to distinguish two very small and closely spaced objects as separate entities.

Depth of focus

• To focus in different planes, move the stage up or down.

Microscope application

• Fiber, hair, trace evidence, microorganisms, and biological samples can be seen through an optical (compound) microscope.

Polarizing Microscopy

• Light that is confined to a single plane of vibration is said to be plane-polarized.
• The examination of the interaction of plane-polarized light with matter is made possible with polarizing microscopes.
• Polarizing microscopy is useful for studying birefringent materials, which are materials that split a beam of light in two, each with its own refractive index value.
• Determining refractive index data helps to identify minerals present in a soil sample or the identity of a man-made fiber.
• Polarizing microscope is useful for ID of birefringent samples such as fibers, minerals and crystals and drug ID.

Comparison Microscopy

• Comparison microscopy uses two microscopes linked by an optical bridge allowing observer to view two independent images in one field, each from a separate objective.
• Comparison can perform superimposition of two images on macroscopes like large tool marks and fabrics.
• Comparison microscopes can be used in Firearm ID.
  • Class characteristics are rifling marks (rifling specifications).
  • Individual characteristics are tool marks (imperfections, irregularities, other effects of use).
  • Firing pin impression is another characteristic.
• Comparison microscope can be used to analyze projectiles.
• Comparison microscope can be used to analyze hair.

The Microspectrophotometer

• The microspectrophotometer is a spectrophotometer coupled with a light microscope.
• Under a microscope, the examiner can simultaneously obtain the visible absorption spectrum or IR spectrum of the material being observed.
• This instrument is especially useful in the examination of trace evidence, paint, fiber, and ink evidence.
• The microspectrophotometer can be used for non-destructive single fiber technique for color analysis, where two different blue fibers can be differentiated.

Scanning Electron Microscopy (SEM)

• SEM has greater magnification and resolution, up to 10-100,000 times.
• Energy Dispersive x-ray spectrometer (EDS) generates spectra revealing the elemental composition of the object.
• SEM can detect Back scattered electrons (BSEs) and Secondary electrons (SES), that emitted from the surface and converted to electrical signal
• SEM can used to detect characteristic x-rays, where energy released is quantized, identifiable to specific atoms, and hence useful for qualitative analysis.
• Human eye can Resolve objects 0.2 mm apart, but Scanning Electron Microscopes allow to see smaller details.

EDS and SEM

• EDS = Energy Dispersive X-ray Spectroscopy.
• Peak heights generally correspond to relative abundance.
• Energy is characteristic to the elements that emitted the x-ray.