The configuration of the sampling system used on the BWB-XP is different to that used on some competitive instruments and it might be interesting to understand the rationale behind our design approaches.

During the design phases of the BWB-1 nothing was taken for granted – we examined all the options with a view to choosing approaches that would give optimum performance.  This meant revisiting some of the established folklore associated with flame photometry recognising that new technologies now make viable approaches not possible when most competitive units were designed.  A key part of this thinking was the sampling system comprising the nebuliser and mixing chamber assemblies. – an area that is the most critical regarding flame stability and reproducibility.

Nebuliser

The nebuliser is a key component; if this is not performing well then results will never be satisfactory.  The primary function of the nebuliser is the production of a fine mist consisting of very small droplets of a consistent size.  (In fact the word “nebule” means small cloud.)  The nebuliser (or nebulizer) used in flame photometers was originally called an atomiser, but this term, being scientifically incorrect, was dropped many years ago.

Traditionally, the nebuliser has been concentric in construction.  This approach was pioneered by EEL in the 1950’s on their Model 100 and the same design continues in use to this day.  There were attempts to use a right-angled configuration in some clinical models, but this approach was considered unsuccessful.  The design of a nebuliser is always going to involve compromise as the production of a fine spray from a sample of limited size must utilise fine bore tubing that is prone to blocking.  This is particularly relevant as most flame photometer samples contain salts that can easily precipitate and cause restrictions.

When we looked at the design options it was realised that anything we could do to minimise blockages and also minimise unit to unit variations would be an advantage.  Extensive testing with precision machined configurations proved that we could make nebulisers as good as those currently available, but without any significant advantages.  We then reviewed moulding options and recognised that if we could identify a robust polymer, improvements in performance would be possible.  Many industrial samples analysed on flame photometers are aggressive, with strong acids, alkalis and solvents being routinely encountered so the choice of moulding material was seen as critical.  Fortunately polymer chemistry has advanced in recent years and a suitable material was identified. Following some intensive testing, this was found to be adequately resistant to attack.

In designing the moulded nebuliser we looked to avoid unnecessary restrictions where blockages could occur and sharp edges that could lead to salt accretion.  We also saw the opportunity to improve the spray quality by rethinking the nozzle that sits in the mixing chamber.  Traditionally the nozzle is stepped, a limitation imposed by machining technology and we recognised that a more complex shape, possible only by moulding, called TIR (total indicated radius), would offer advantages.  We also reviewed the fuel gas/air/sample mixing regime and, following detailed testing, we found that introducing the fuel gas into the nebuliser spray prior to mixing chamber entry enhanced flame stability.

Of course moulding also gives great consistency, with each unit being exactly the same as its predecessor; something that is of value if a nebuliser needs changing in the fullness of time.

We are confident that our moulded nebuliser design offers significant advantages over traditional machined units both in terms of performance and cost. 

Mixing Chamber

The mixing chamber on a flame photometer, as the name suggests is primarily concerned with the efficient mixing together of the sample aerosol from the nebuliser with the fuel gas.  It is also important that the aerosol mist reaching the burner consists of only the smallest droplets and an additional function of the mixing chamber is to eliminate larger droplets and direct them immediately away to waste.

We know from history that goes back to the 1950’s that the cylindrical chamber fitted with baffles can work adequately and many thousands of units are installed around the world that use this approach.  History also tells us (remember that BWB has access to flame photometer specialists who were there at the time) that when sophisticated flame photometers were being developed in the 1970’s for the clinical market, much research was carried out by market leaders such as IL and Corning on mixing chamber configurations.  The optimum with respect to stability was found to be the centrifugal whirlpool option where the sample is introduced tangentially into a vertical chamber without baffles. Many thousands of units using that technology found their way into hospitals around the world.

There was a move to introduce the whirlpool chambers onto industrial units at that time, but the aggressive nature of many industrial samples precluded the use of the materials used on the clinical models.  No polymers were available with the necessary resistant characteristics, so the easy and low cost option was to stick with tradition even although there was some trade-off in performance.

When BWB were looking at mixing chamber design we revisited the 1970’s research and again confirmed the benefits of the whirlpool chamber.  As well as stability advantages we also found that drainage, always an issue with cylindrical chambers, was much improved.  The whirlpool chamber also makes obsolete the baffles used in tubular chambers; it uses gravity instead. There is a significant advantage when looking at speed of response as it takes time for droplets hanging on the baffles to be flushed through the system. Also, where clinical samples are being analysed, the baffles are a site where protein build-up can occur; cleanliness is always important for accurate results.

Using the polymers identified during our nebuliser design phase we now have a chamber that have superbly resistant to all normal (and most abnormal) industrial samples – in fact better than the materials used by our competitors. 

The BWB-XP sampling system is different because it has been designed in the 21st century using technology that was not available to competitors when their products were designed.  The system you see on the BWB-XP has been carefully optimised and offers the best design for stability and reproducibility, the best design for drainage efficiency, the best design for cleaning and is manufactured using the most resistant material available. 

Martyn J Fall