8.2
Principles of Operation
Chemical reactions occur between reagent beads, the diluent contained in the reagent disc, and the
added sample. These reactions produce chromophores that are measured photometrically by the Pic-
colo Xpress. 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 aper-
tures and is then collimated by a lens. Collimated light is split by beam splitters and interference fil-
ters at pre-determined wavelengths (340, 405, 467, 500, 515, 550, 600, 630, and 850 nm), and
captured at each wavelength by photodiodes.
The chemical reactions utilized in the tests are designed to produce chromophores that absorb light at
known wavelengths. The photometer measures light that is transmitted through a chromophore-con-
taining cuvette (the reaction cuvette). The transmitted light, when corrected for flash-to-flash variabil-
ity and electronic offset, is indirectly related to the analyte concentration of the sample. Transmittance
measurements are converted to absorbance using the relationship: absorbance = –log (transmittance).
System errors and factors that may interfere with sample result calculations are eliminated by measur-
ing light intensity at four locations of the reagent disc: 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 the light to pass
through; and the dark cuvette, which blocks the passage of light.
Light passing through a reaction cuvette is measured both at the wavelength that is absorbed by the
chromophore (I
) and at a wavelength not absorbed by the chromophore (I
). The ratio of these
λ
λ 2RC
RC
1
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
, I
) is measured at the same two
λ 1OC
λ 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
).
λ 1
λ 2
0
0
8-2
Operating Principles