Jenway 7305 - Manual (Page 15)

inner chamber (the area filled with sample) must be wider than the aperture in the sample holder or
light will reach the detector without passing through the sample. In this case, semi-micro or micro
cuvettes with self-screening black surrounds must be used or, alternative holders for these cuvettes
should be used.
e) Glass test tubes and other sample tubes should be used with care. Where possible, matched tubes
should be used and any index mark set to the correct position before measurements are made.
f) Ensure any sample containers used are compatible with the constituents of both the samples and
standards they are to hold. Plastic cuvettes are not compatible with organic solvents.
g) All sample containers must be handled with care; by the top, bottom and non-optical surfaces only.
Any finger marks evident must be removed by a suitable cleaning process.
h) Flow-through cuvettes must be selected with care and consideration for the sample type, sample
volume, pumping system, rinse, sample and waste handling to be used.
5. Samples and standards should not be stored in open cuvettes or sample containers as evaporation will
change the value and lead to staining of the walls which may be irreversible. If stored in stoppered and
sealed cuvettes, they should be filled with little or no air space and the values regularly checked against
a reference standard or quality control material.
6. Samples should be allowed to equilibrate to ambient temperature before measurement (unless a
suitable temperature controlled sample holder is in use). Temperature change during measurement may
cause air bubbles to form on the walls of the sample holder. This is a common cause of drift during
measurement.
7. In the preparation of samples and standards high grade borosilicate glass and AR grade chemicals
and reagents must be used. Good quality deionised water or other suitable solvents must be used for
dissolving or diluting samples, chemicals and reagents.
8. All measurements require calibration to a blank, for maximum accuracy this should be prepared
with care using the same deionised water or solvent used for dissolving or diluting the sample. Where
reagents are added to the sample to produce a colour proportional to its concentration a ‘sample based’
blank should be used. In this case the blank should consist of all reagents or chemicals to be used,
except the sample which will produce the colour to be measured.
9. Deviations from the Beer-Lambert Law may occur at high and low concentrations giving non-linear
response during sample concentration measurements. For all new methods a linear range should be
defined by the preparation of a calibration curve.
10. Cuvettes and sample holders must be filled to a minimum level which covers the light path. All
Jenway spectrophotometers have a beam height of 15mm.
11. The instrument must be calibrated to zero absorbance/100% transmittance prior to taking readings.
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