Apple or Pear?

Apple or Pear? You won't have any problems distinguishing it by smell. But for an electronic nose this could be a problem. But a team from the Polytechnic University of Valencia in Spain and the University of Gävle in Sweden was able to rigg an array of 32 commercially available sensors that can sniff a bit of crushed fruit and tell if the source was an apple or a pear.

The sensors can detect a wide variety of odorous chemicals, including natural gas, carbon monoxide, alcohol, propane, toluene, by measuring  conductance changes across a narrow gap between two electrodes as trace gases adsorb to the electrode’s surface. Different gasses produce distinct curves of voltage over time, and each type of sensor has different response characteristics. There is, some sensor-to-sensor variation in response and changing the sensor’s operating temperature further modifies its response curves.

The team characterized each of the 32 resulting signals by three dynamic parameters: transient slope (the rate of initial rapid increase when the gas is first detected), saturation slope (the slower rate of voltage increase as the senor reaches its greatest response), and maximum slope  (measured when the sensor is closed off from the sample).

The investigators subjected each of these parameters to principal component analysis, so that each electronic sniff generated patterns of 96 points in three-dimensional space for each of 20 sample runs. They then fed the results into 10 different pattern recognition programs* and asked each one, in effect, “Is it an apple, or is it a pear?”  All but one of them could correctly identify apples or pears nine times out of 10, or better. And one pattern-reader—IB1, a nearest-neighbor algorithm— got it right on the nose 100% of the time. And this is better than most people could do.