Abbott i-STAT Alinity Operation Manual page 129

Closure
The closure creates an airtight seal necessary for proper fluid movement within the cartridge. The
closure also ensures that calibrant and sample remain contained within the cartridge during the testing
cycle and subsequent disposal.
Cartridge on the left in the illustration contains natural rubber latex on the snap closure. The cartridge
shown on the right is not made with natural rubber latex.
Heating elements
All i‑STAT cartridges require thermal control at 37°C (98.6°F), and include heating elements on the
underside of the sensor chips which are contacted and heated by the instrument's thermal probes.
Standardization and calibration
Standardization is the process by which a manufacturer establishes "true" values for representative
samples. A multi-point calibration curve, the slope or sensitivity of which is defined by coefficients in the
CLEW software, is derived for each sensor by this standardization process. These calibration curves are
stable over many lots.
A one-point calibration is performed each time a cartridge requiring calibration is used. During the first
part of the testing cycle, the calibrant solution is automatically released from its foil pack and is
positioned over the sensors. The signals produced by the sensors' responses to the calibrant solution are
measured. This one-point calibration adjusts the offset of the stored calibration curve. Next, the
instrument automatically moves the sample over the sensors and the signals produced by the sensors'
responses to the sample are measured. While coefficients are used rather than graphic calibration
curves, the calculation of the result is equivalent to reading the sample's concentration from an adjusted
calibration curve.
Types of cartridge sensors
Sensors are thin film electrodes microfabricated onto silicon chips. Sensing functionality is imparted to
each electrode by a number of chemically sensitive films coated over the active region of the electrodes.
The cartridges have three different types of sensors built in: potentiometric, amperometric, and
conductometric.
Potentiometric sensors
In potentiometric measurements, the difference in potential that exists between an indicator
electrode and a reference electrode is measured. Ion-selective electrodes (ISE) are examples of
potentiometric sensors. The indicator electrode is designed to be sensitive to a particular ion in a
solution. In cases where other ions are sensed by the system, selectivity coefficients can be used
to correct for this interference. An enzyme can be added to an ISE to produce ions from analytes
of interest that are not themselves ions.
Potentiometric sensors utilize two important concepts. The first concept is the Nernst Equation
which relates the measured potential to the activity of the ion being measured. It is written as:
E = E° + RT/nF ln a
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Rev. Date: 02-Nov-2022