28 chromatography/spectroscopy
The power to choose expands
possibilities for measurement
The future of molecular analysis lies in speed,
property of the sample itself, namely surface tension,
accuracy, and ease-of-use of microsample
to hold itself in place during the measurement cycle.
Combining fiber optic technology with the inherent
technologies. Philippe Desjardins reports.
surface tension of liquids resulted in the development
of a unique sample retention system that is the basis of
A
new system configured for both NanoDrop technology.
microvolume pedestal measurements The Thermo Scientific NanoDrop 2000c
as well as cuvette measurements gives Spectrophotometer employs this sample retention
scientists the flexibility to use the unique system by using a pair of optical pedestals to hold the
capabilities of a novel microsample retention system sample during measurement. The user places a droplet
as well as those of traditional cuvettes. This dual of sample (usually 1ul of sample for aqueous solutions
functionality is ideal as microsample analyses continue of nucleic acids) onto the lower optical pedestal (Fig. 1)
to push the limits of detection and molecular techniques and lowers the lever arm. The sample makes contacts
continue to evolve that require the flexibility to perform with both optical surfaces, forming a vertical liquid bridge
conventional and unconventional quantitation methods. (Fig. 2). Light from a xenon flash lamp fires through an
Devices are being developed that integrate optical fibre embedded in the upper pedestal, then passes
microsample technologies with simple ‘sample-in through the sample, and is collected by another optical
answer-out’ capability. As sequencing, PCR (polymerase fiber embedded in the lower optical pedestal. The light
chain reaction), microarrays and other molecular then continues to an internal CCD detector to provide
techniques continue to use ever smaller amounts of the requisite data. The software displays a full UV-Vis
sample, the quality control of these samples is essential. spectrum as well as a calculated concentration of the
In order to keep pace with the miniaturisation of sample being measured.
sample volume throughout all of molecular biology, new The absence of a solid containment barrier allows
quantification methods had to be developed. To meet this the distance between the optical surfaces to change in
challenge, Dr Charles Robertson, then a physicist at the real time. The system finds the correct transmittance of
E.I DuPont Experimental Station, developed an elegant light through the sample column by rapidly changing
solution – remove the containment device altogether the distance or path length from 1 mm to 0.2mm,
(ie cuvettes). The central idea is based on the concept 0.1mm, and 0.05mm (Fig. 3). According to the basic
that the containment device itself was a limiting factor laws of spectroscopy, shorter path lengths allow for
in reducing volume. Removing the containment device higher concentrations to be measured. By having four
in effect removes the limitation of having to fill a certain incrementally shorter path lengths, the new system
minimum volume in order to take a measurement. But has the broadest dynamic measurement range of any
how is this done? The answer lies in using the physical spectrophotometer, essentially eliminating the need to
➠
Fig. 1. A microvolume (1uL) droplet of sample is pipetted Fig. 2. The sample is held in place by surface tension between
directly onto the lower optical pedestal. the upper and lower optical pedestals during measurement.
www.scientistlive.com
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