Chronoprints: Identifying Samples by Visualizing How They Change over Space and Time

The modern tools
of chemistry excel at identifying a sample, but
the cost, size, complexity, and power consumption of these instruments
often preclude their use in resource-limited settings. In this work,
we demonstrate a simple and low-cost method for identifying a sample
based on visualizing how the sample changes over space and time in
response to a perturbation. Different types of perturbations could
be used, and in this proof-of-concept we use a dynamic temperature
gradient that rapidly cools different parts of the sample at different
rates. We accomplish this by first loading several samples into long
parallel channels on a “microfluidic thermometer chip.”
We then immerse one end of the chip in liquid nitrogen to create a
dynamic temperature gradient along the channels, and we use an inexpensive
USB microscope to record a video of how the samples respond to the
changing temperature gradient. The video is then converted into several
bitmap images (one per sample) that capture each sample’s response
to the perturbation in both space (the y-axis; the
distance along the dynamic temperature gradient) and time (the x-axis); we call these images “chronological fingerprints”
or “chronoprints” of each sample. If two samples’
chronoprints are similar, this suggests that the samples are the same
chemical substance or mixture, but if two samples’ chronoprints
are significantly different, this proves that the samples are chemically
different. Since chronoprints are just bitmap images, they can be
compared using a variety of techniques from computer science, and
in this work we use three different image comparison algorithms to
quantify chronoprint similarity. As a demonstration of the versatility
of chronoprints, we use them in three different applications: distinguishing
authentic olive oil from adulterated oil (an example of the over $10
billion global problem of food fraud), identifying adulterated or
counterfeit medication (which represents around 10% of all medication
in low- and middle-income countries), and distinguishing the occasionally
confused pharmaceutical ingredients glycerol and diethylene glycol
(whose accidental or intentional substitution has led to hundreds
of deaths). The simplicity and versatility of chronoprints should
make them valuable analytical tools in a variety of different fields.