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 Polarimeter Tubes and Sample Cells

 

Optical Activity Limited, Bury Road Industrial Estate, Ramsey, Cambridgeshire, PE26 1NF, United Kingdom
Tel: +44 (0)1487 813913   Fax: +44 (0)1487 812789
E-mail:  sales@opticalactivity.com   Web:  www.opticalactivity.com

The superb range of quality polarimeter sample tubes manufactured by Optical Activity can be used in almost all polarimeter applications, and so, if you need to measure the tiniest amount of an expensive essential oil, or, if you have litres of sample, one of our tubes is sure to be suitable. 
Our range is extensive and has been continually added to for over 20 years, but if you still do not
see the tube you need, we will design and make a special - just let us know your requirements.

Our tubes and cells use the internationally standard 30mm diameter collars.  Tubes are to class A standard, as specified by  I.C.U.M.S..A.  Where the tube material is stainless steel, this is in all cases Type EN 58 (316) quality stainless steel.  End windows are made from highly annealed strain free Crown glass, held in position by Nitrile O-rings (Viton O-rings are available to special order for use with solvents such as chloroform, which attack Nitrile).  The seal is glass to metal or glass to glass; essential for accurate tube length determination.  The O-rings simply relieve pressure on the end windows, preventing strain, and do not come into contact with the sample in ordinary use.

To obtain the best results from your polarimeter, it is important to select the most appropriate tube for the particular sample to be measured.  If your polarimeter will accept a wide variety of types and sizes of tubes, it makes the instrument more versatile.  All Optical Activity polarimeters can be used with tubes as long as 200mm, or as short as 5mm, with various tube bores (we offer from 8mm down to 1.5mm as standard), single sample or flow tubes, which may be temperature controlled or not, as required.  If this choice proves confusing at first, the information provided on our specification pages are intended to assist you in choosing the best tube for your own use.

Adapters are available to allow Optical Activity tubes to be used in other manufacturers' instruments which are designed to take non-standard diameter tubes.

Tubes with Temperature Sensor

If you have an Optical Activity PolAAr 3 series polarimeter, these instruments have the facility for accepting a sample tube temperature sensor.  All the following flow tubes (series A2, A4, F5 or P6) are optionally available with fitted temperature sensor.  For single sample tubes (series A1 or G7), a dipping temperature sensor is available.

SAMPLE TUBE SELECTION GUIDELINES

Tube Type

For a comparitively small number of samples brought for measurement one at a time, a SINGLE SAMPLE tube is ideal - generally less expensive than a flow type tube and often useful for measuring odd samples if a flow system is normally used.

With a large number of samples, especially large batches of similar samples, a FLOW TUBE, connected for rapid sampling, will save time. Samples can be poured one after another into a funnel without removing the tube from the polarimeter.  This system works well with all samples except those that are highly viscous.

Dissimilar samples, such as those with different solvents, are probably best measured in single sample tubes.  A flow system can be used, provided steps are taken to thoroughly clean and dry the tube between samples.

Quantity of sample available.  If you have plenty of sample with flow tube connected for rapid sampling, you can use one sample to displace the previous one - this is the quickest method. However, you will obviously need more sample than simply to fill a clean, dry tube.

For an auto-sampling system, or continuous monitoring of flowing fluids, a flow type tube MUST be selected.

Path Length

Optical rotation is directly proportional to path length; the longer the tube, the greater the overall accuracy.  It is therefore desirable to use the longest tube possible, however, there are other considerations:

Sample Quantity - less fluid is needed to fill a shorter tube.

Dark Samples - a shorter tube will allow you to get sufficient light through to make a measurement (transmission decreases logarithmically with path length, optical rotation directly, thus halving the path length will give much more light while reducing the accuracy by only 50 per cent).

Flowing Samples - a long path length may integrate the changes you are looking for

Comparison With Other Work - it may be convenient to select a particular tube length to allow direct comparison (provided the accuracy is adequate) with published figures.  The 'standard' length for specific rotation is 1dm (100mm); the sugar industry uses 200mm in the definition of the International Sugar Scale.

Tube Bore

The main considerations are sample quantity, darkness and viscosity.  Smaller bore tubes obviously require less sample, however, larger tubes are generally easier to fill (and clean), especially as the viscosity of the sample increases.

If the bore of the sample tube is smaller than the beam diameter of your polarimeter, light will be lost by beam obstruction.  In Optical Activity polarimeters the beam diameter is 4mm.  Tubes with bores less than 4mm will partially obstruct the beam and, while not affecting the accuracy of the instrument, will reduce the optical density tolerance.  Tubes with bores of 2.5mm or 1.5mm cannot therefore be considered suitable for dark, coloured or cloudy samples.  A 4mm or 5mm bore tube is a good compromise to minimise the amount of sample, while remaining easy to fill and not obstructing the beam.

Temperature Control

For the most accurate work, we generally advise controlling the temperature of the sample tube by use of a chiller thermocircular, for which a jacketed tube is necessary (IF the temperature coefficient is known, an alternative is to measure the sample temperature and apply the necessary correction),  however, consideration of the overall accuracy required may indicate that this is not essential.  It will depend on the rate that the optical rotation of the sample (at the concentration being measured) varies with temperature; some samples are not very temperature dependent, others vary rapidly with the temperature.