Abaxis VetScan vs2 - Principles of Operation

9-2 Principles of Procedure and Operation
9.2 Principles of Operation
Chemical reactions occur between reagent beads, the diluent contained in the reagent rotor, and
the added sample. These reactions produce chromophores that are measured photometrically by
the VS2. The microprocessor then calculates the concentrations of the analytes.
The measurement optics include a stroboscopic xenon lamp, a wavelength selection system, and a
multiple-wavelength detector. Light from the lamp is directed by a mirror to pass through each
cuvette. Some light is absorbed by the cuvette contents, and the remainder travels through two
apertures and is then collimated by a lens. The collimated light is divided by beam splitters,
passed through interference filters at wavelengths of 340, 405, 467, 500, 515, 550, 600, 630, and
850 nm, and measured at each wavelength by photodiodes.
The chemical reactions used in the tests produce chromophores that absorb light at known wave-
lengths. The photometer measures the light transmitted through each chromophore-containing
cuvette (the reaction cuvette). This transmitted light, when corrected for flash-to-flash variability
and electronic offset, is indirectly related to the analyte concentration of the sample. Transmit-
tance measurements are then converted to absorbance according to the relationship
absorbance = log (transmittance).
System errors and factors that can interfere with sample result calculations are eliminated by mea-
suring light intensity at four locations on the reagent rotor: the method-specific cuvette containing
test reagent, sample, and diluent; the sample blank cuvette containing sample blank reagent, sam-
ple, and diluent (used in endpoint reactions — see below); the open cuvette, which allows all light
to pass through; and the dark cuvette, which blocks the passage of light.
Light passing through a reaction cuvette is measured at the wavelength absorbed by the chro-
mophore (I
O1RC
), and also at a wavelength not absorbed by the chromophore (I
O2RC
). The ratio
of these two measurements corrects for cuvette optical quality and flash-to-flash variability in the
light source. The intensity of light transmitted through the open cuvette (I
O1OC
, I
O2OC
) is mea-
sured at the same two wavelengths as the light transmitted through the reaction cuvette. A correc-
tion for electronic offset is made at the same two wavelengths by measuring the residual signal
when the dark cuvette is in the optical path (I
O1
0
, I
O2
0
).