20-Bio-B10 Biomechanical Device Design & Human Factors · December 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, December 2015 — 04-Bio-B10 Analytical Biochemistry. Three hours, closed book, any non-communicating calculator. Six questions of equal value (20 marks each); five constitute a complete paper and only the first five appearing in the answer book are marked. All six are solved here, because this set is a study resource rather than an examination script. Every question is essay/descriptive (technique principle, interpretation of an instrument trace or image), with no numerical calculation on this sitting.
Reference texts (the books a candidate should have reviewed for this subject):
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
An FTIR spectrometer is built around four functional stages in series: a broadband infrared source (e.g., a Globar silicon-carbide element) generates continuous mid-IR radiation; this beam passes through a Michelson interferometer (a beamsplitter plus one fixed and one continuously moving mirror), which encodes every wavelength's intensity into a single time-domain signal called an interferogram; the resulting beam passes through the sample compartment, where the sample absorbs radiation at wavenumbers corresponding to its molecular vibrations; and the transmitted (or reflected) beam finally reaches a detector (commonly a pyroelectric DTGS or a more sensitive cooled MCT detector), whose electrical output — the interferogram — is digitized and passed to a computer that applies a Fourier transform to convert it into the familiar intensity-vs-wavenumber IR spectrum.
FTIR bands are identified by their position on the wavenumber axis, expressed in reciprocal centimetres (cm−1) — the number of wave cycles per centimetre, equal to 1/λ when wavelength λ is expressed in cm. Wavenumber is preferred over wavelength in IR spectroscopy because it is directly proportional to photon energy and to vibrational frequency, so band positions and spacings on a wavenumber axis map linearly onto molecular vibrational energies.
A Michelson interferometer is an optical device that splits an incoming beam of light into two paths using a beamsplitter: roughly half the light is transmitted toward a fixed mirror and reflected straight back, while the other half is reflected toward a moving mirror and likewise reflected back. The two returning beams recombine at the beamsplitter and travel on together to the detector, but because one mirror is continuously displaced, the two beams have travelled slightly different total path lengths (an "optical path difference" or retardation). Depending on that path difference relative to each wavelength present in the source, the two beams interfere constructively or destructively at the detector for that wavelength; scanning the moving mirror through a range of positions therefore produces a detector signal — the interferogram — that is the superposition of many interference patterns, one per wavelength component in the source beam, each modulated at a different rate as the mirror moves. Because all wavelengths are measured simultaneously in this single time-domain scan (the multiplex or Fellgett advantage, one of FTIR's key benefits over older dispersive scanning instruments), a computer must mathematically decompose the interferogram back into its individual wavelength (or wavenumber) components afterward — which is exactly what the Fourier transform accomplishes, converting intensity-vs-mirror-position data into the conventional intensity-vs-wavenumber IR spectrum.
