Principles Of Operation - Abaxis VETSCAN VS2 Manual Del Usuario

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 mul-
tiple-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 elec-
tronic offset, is indirectly related to the analyte concentration of the sample. Transmittance measure-
ments 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 measur-
ing 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, sample, 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 chromophore
(I ), and also at a wavelength not absorbed by the chromophore (I ). The ratio of these two
1RC 2RC
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 , I ) is measured at the same two
1OC 2OC
wavelengths as the light transmitted through the reaction cuvette. A correction 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 , I ).
1 2
0 0
Principles of Procedure and Operation
9-2