20-Bio-B6 Analytical Biochemistry · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Exams, December 2013 — 04-Bio-B6 Bioinstrumentation. Three hours, open book, non-communicating calculator permitted. Six questions of equal value (25 marks each); four constitute a complete paper and only the first four appearing in the answer book are marked. All six are solved here as a complete study resource. Every question is a design/essay question (block-diagram instrumentation-system design, or descriptive explanation).
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.
Approach. A flash stimulator provides the t=0 timing reference; the corneal/reference/ground electrode trio feeds a high-input-impedance, electrically isolated differential front end sized for the 0-900 µV, 0-50 Hz signal, followed by digitisation and a peak-detection stage that reports a-wave/b-wave latency and amplitude against the pre-flash baseline.
The active electrode is a gold or stainless-steel ring resting on the corneal surface (contact-lens style), the reference is an Ag-AgCl electrode on the forehead, and the ground is on the neck — all as specified. Because the ring electrode is a moving mechanical contact directly on the eye, a drop of topical anaesthetic and a wetting/conductive gel or saline bridge are used both for patient comfort and to stabilise the corneal contact impedance (an unstable, high and drifting contact impedance is the dominant noise source in ERG recording).
An isolated instrumentation amplifier with very high input impedance (≥1010 Ω, comparable to or higher than the corneal-electrode source impedance) and high CMRR (>100 dB) is used, both to avoid loading/attenuating the small corneal signal and, being isolated, to protect the patient (Question 5 applies directly here: the eye is a sensitive structure and the electrode is a direct, low-impedance contact). Gain is set so the largest expected ERG (900 µV) uses the ADC's input range without clipping: a gain of 5000× brings 900 µV to 4.5 V, fitting comfortably inside a ±5 V rail with headroom for larger flashes or artifact. A bandpass of 0.1-100 Hz is used: the low corner (0.1 Hz, not DC) blocks slow electrode-offset drift without distorting the a-wave/b-wave shapes (whose energy lies at Hz-scale, well above 0.1 Hz), and the high corner gives an anti-alias roll-off margin above the stated 50 Hz signal bandwidth ahead of the ADC's own anti-alias filter.
The ADC samples at 1 kHz — ten times the 100 Hz Nyquist rate implied by the 50 Hz bandwidth — so that a-wave/b-wave peak-timing resolution is about 1 ms, fine enough given the whole trace is only 150 ms long. Each sweep is recorded from a short pre-flash baseline (e.g. 20 ms before t=0) through the full 150 ms post-flash window; the pre-flash segment supplies the zero-amplitude reference. Software finds the first trough (a-wave) and the following peak (b-wave), reporting each one's time-to-peak (measured from the t=0 flash trigger) and its amplitude measured from the pre-flash baseline — exactly the two features the question asks for. Averaging several flash repetitions (with the subject re-adapted between flashes) is offered as an optional SNR improvement, though the 900 µV ERG is large enough relative to typical ocular/EOG noise that a single well-filtered sweep is usually already measurable.
The system displays the raw filtered waveform in real time (for the technician to confirm electrode contact quality) and, after each flash, a numeric read-out/table of a-wave time-to-peak and amplitude and b-wave time-to-peak and amplitude, stored alongside the flash intensity/colour used for that trial.
Because the active electrode contacts the cornea directly, isolation (battery-powered or isolation-amplifier front end, per the devices described in Question 5(iv)) is mandatory to eliminate any hazardous current path through the eye. Comfort measures include topical anaesthetic before electrode placement, a lightweight low-mass electrode/lid speculum to minimise blink interference and irritation, dim, controlled ambient lighting for consistent dark/light adaptation between flashes, and limiting the number/duration of flash trials with rest breaks.
