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Servicing Guide - Replacing a Deuterium Lamp
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What are Deuterium Lamps and what do they do?

A deuterium lamp produces UV radiation, and unlike a mercury lamp, covers the whole UV spectrum (190-350/400nm). Its does have a low emission into the visible region, but not enough for serious HPLC, and if you require detection over 400nm you should obtain a tungsten lamp if available for your detector. Light output much below 190nm is not transmitted because of the UV absorption of the quartz envelope.

There are only four major deuterium lamp manufacturers in the world and hence it is relatively easy to obtain an original replacement lamp. It is important to do so because there are several differences in construction and the software in a UV detector will be set up to generate a linear response at all wavelengths for the manufacturers chosen brand of deuterium lamp. Variations include the cathode aperture, affecting the light intensity, the voltage required to run the lamp (normally 2.5 or 12 volts) and the voltage to start the lamp. Some have built in filters to prevent drift and reduce noise, and the construction materials and design dramatically affect the life expectancy of the lamp.

Operating life is defined as the time taken for the lamp output to fall to 50% of its original value at a specified wavelength, and this will vary considerably depending upon the wavelength used.

There are three major causes of reduction in the intensity of light output:

1) Evaporation of tungsten, molybdenum and the filament coating, causing a reduced output and ignition problems

2) Reaction of the coating material with the quartz envelope,

3) Irradiation of quartz at 200-250nm causes it to build up an absorption at that wavelength.Lamp life expectancy is also inversely proportional to the number of ignitions of the lamp.

How to tell if a lamp is faulty.

If a lamp has blown there will be several symptoms. If you can get access to the lamp housing there will be no tell-tale blue glow coming from round the edges of the casing. With the lamp turned off, you can sometimes tell if you remove the bulb and the quartz envelope is darkened.

Please be aware of the dangers of UV radiation to the human eye. ALWAYS use UV safety glasses if you think you may be exposed to UV light. Low energy long wavelength light from a tube in a disco is OK in limited doses but a high energy low wavelength flash can do serious damage to your retina, and is not recommended if you wish to be able to see to change your lamp!

Should you be using a UV detector and suspect a lamp has blown, the signs from the detector output trace are a seriously drifting baseline (it is subtracting nothing from nothing relatively unsuccessfully) and no response to peaks.

Most detectors have a lamp energy button so that the lamp energy may be displayed. It is good practice to monitor the energy in the sample and reference cells, so that you are familiar with what is a normal response for your detector. The output of a deuterium lamp decreases slowly during its lifetime, but when it is approaching the end of its days, the rate of decrease in light output increases quite rapidly until the bulb finally fails to start. Hence it is possible to receive some advance warning of impending doom and prepare a replacement lamp.

How to replace a deuterium lamp.

Alignment of the lamp is critical. The cathode aperture is sometimes only one or even half a millimetre, and so even a very slightly mispositioned lamp can result in a significant loss in light energy reaching the detector. To overcome this, most lamps are now supplied pre-aligned on a bracket. It is a little more expensive than buying a bulb and aligning it yourself, but infinitely better value.

There are two wires to disconnect, usually fitted with on one-way connector to prevent connection the wrong way round, and the bracket is held in by one or sometimes two screws. Remove the cable before undoing the screws, and connect it last once the new lamp is installed.

If your detector lamp is not pre-aligned such as on old LDC detectors, the new lamp will have to be aligned with extreme care. Instructions for how to adjust its position will be contained in the manual, but you should consider the services of a service engineer if you are not sure. Nothing will explode if you get it wrong (assuming you do not get it that badly wrong!) but you could have very low light energy entering the detector and hence have poor sensitivity. If you do get to this stage and do not want to pay for a service visit, package the detector, send it to us by a secure and insured means, and we will sort it out for you.

How not to destroy a perfectly good deuterium lamp.

There are a few things you can do (or not do) which will help to give the optimum life from a deuterium lamp.

Do not touch the lamp with your fingers. It gets very hot and the grease etc. from your fingers will burn on the quartz, form a dark pigment and block the light output.

When the lamp is hot, do not subject it (or the detector with the lamp inside) to any physical jolt or shock. This can damage or break the filament.

Allow the lamp at least 30 seconds to cool down before turning it back on. The ignition power is quite high and the filament is best able to cope with this when it is cool.Having said all that, turning the lamp on and off actually does it a lot less damage than you might think. However leaving the lamp on overnight increases its use by 300% (24 hours instead of 8 per day) and so it is a good idea to turn the lamp off overnight, and especially at weekends. Should you be tempted to turn it off over a lunch break, though, bear in mind that it will take a further 30-60 minutes to warm up and settle down when you get back!

Don't forget. If you have problems during the repair or wish to discuss it first, please call us on 01634-294001, or email us at: stuart@laserchrom.co.uk. We'll be pleased to help if we can.

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