Low Flow Liquid Control

July 5, 2016 James Walton
Laboratory tubes

Ultra low flow splitter techniques for Analytical Chemistry

I have been working with analytical chemists for a long time, whenever that happens it gives you a unique insight into the challenges they face and the methods employed to overcome them. There is a particular technique that causes problems the world over, it is hard to do and it can have varying results and levels of success. However, people do persevere with it and it currently has no long term successor, of course I am talking about ultra-low flow splitter techniques.

People I frequently speak to are challenged to deliver solutions for low, repeatable flow measurement, especially when employing hybrid techniques. These combinatory analytical techniques put multiple instruments in-line to be used in the detection and separation of chemicals from a solution, of course this means that the sample, unless split out, could be destroyed and with it any further chance to re-analyse the sample.

Splitting the sample has long been possible and is used in analytical chemistry to try and preserve original samples where possible, there are 2 main techniques currently in use and they both have issues.

Resistance splitting
Resistance splitting is one of the oldest and most reliable methods of continuous flow splitting employed by laboratories; the split is created by using a tube of smaller diameter as a split off from the main flow, creating a known resistance, or delta P, and a fairly accurate continuous split of sample.

Time splitting
Time splitting is similar to resistance splitting but with the added advantage of being able to collect a particular specific compound from a sample. The method is built in much the same way as the earlier resistance split, however the time part of the method title refers to the time taken for your specific compound of interest to pass through the separation column. Once you know this it allows you to give a delta T to a flow switch following the column so that it splits the flow at the right time to split the flow and collect the now separated compound in an allocated vessel.

As I am sure you can see there are multiple problems with both of the above techniques. The set-up of both these methods is very static and there is not much room for flexibility. In conversation these techniques I have found that to change the fluid profile by using a different mobile phase or defining a different compound of interest it is possible to render the original set-up redundant.

While it is possible to re-time the separation of the new compound of interest, or same compound different mobile phase, it may be necessary to re-cut the tubes and re-make the connections from scratch.

I have found that along with being hard to run as a live technique it also creates problems for routine service and maintenance, any replacement of tubes on a calibrated system needs the tubes to be of exactly the same type and length, any change, particularly at low flows can have a huge effect on the velocity of a fluid in a tube and therefore changes in the performance of the system as whole.

Other techniques have been released onto the market, some include a fast acting piezo valve but the cost to performance balance has never lead to a surge away from the old tried and tested methodology.

It is an area that people are showing a great amount of interest in, I am sure it is on the development plan of end users and manufacturers alike, people are asking for an improvement and it is now up to the industry to deliver it.

With the ever increasing use of fluid independent flow technology, please see upcoming blog about utilising the Coriolis force for metering and control, and its ability to increase the simplicity of such an under developed method. New and dynamic advances are surely over-due in this important field of analytical chemistry. I am sure that it will be a hot topic over the next 18-24 months and one that I will revisit to provide a more detailed and in-depth look at the future of analytical flow splitter technology.

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