12
3.3
Good Practice Guidelines
1. For optimum performance all spectrophotometers should be sited in a clean, dry, dust free atmosphere. When
in use ambient temperature and light levels should remain as constant as possible.
2. If required adherence to Standard Operating Procedures (SOP) and Good Laboratory Practice (GLP) should be
monitored with regular calibration checks and a suitable Quality Control (QC) programme.
3. The sample chamber lid must be fully closed during measurement and before any readings are recorded or
printed.
4. The correct selection of sample containers is imperative for accurate and reproducible results:
a) Check that the material of the sample container is compatible with the wavelengths to be used for
measurement. In general glass can only be used down to 360nm or 320nm depending on quality.
Standard plastic cuvettes can be used down to 320nm. Special UV versions can be used down to 260nm.
Below this level quartz cuvettes must be used.
b) Plastic disposable cuvettes should only be used ONCE.
c) Glass cuvettes should be thoroughly cleaned after use. Discard when scratches become evident on optical
surfaces.
d) Care should be taken when selecting semi-micro or micro cuvettes. The cuvette window on the 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.
