Pressures in post-column flow cell

Discussions about HPLC, CE, TLC, SFC, and other "liquid phase" separation techniques.

10 posts Page 1 of 1
Hi,

I'm currently designing an UV/vis absorbance detector for HPLC, and I am considering re-purposing the spectrometric detector from a derelict combiflash columning machine.

My worry is using the lower-pressure flow cell at HPLC pressures, but I haven't been able to find specific information on what pressures to expect in a flow cell post-column. The main flow restrictor in the system will always be the column, so I suppose the pressures are fairly low.

Thanks in advance.

Chris
A flow cell for HPLC use should be able to withstand about 1000psi. It also needs to have as close to coherent flow as possible and it needs to have a very low internal volume.
The longer the path, the better. These all conspire to severely limit the amount of light you can pass through a fairly small hole that has to be aligned very carefully if you expect to get any signal at all out of it.

In other words, I do not hold a lot of hope for a good result using a repurposed instrument... I suspect that your photomultiplier/amp setup will not be up to the task and that your lamp/cell arrangement will not help either. It's king of like wanting to see cellular organs and all you have handy is a telescope.
Thanks,
DR
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I agree with the above comment that the task you have set for yourself and the lack of basic HPLC knowledge may lead to a disappointing result. Under normal conditions, an HPLC flow cell would be expected to show only a few bars of pressure at their outlet. The column outlet is essentially at atmospheric pressure so the only back pressure placed on the flow cell inlet comes from the tubing used (between connections) and any backpressure created against it from the mobile phase (viscosity being the issue) and flow rate. Only clogs or partial obstructions would test this concept (over time, most users will do things with their HPLC systems that are not advised and clog a flow cell).

In my opinion, some of the best flow cell designs use an expanding spring system which allows the flow cell to open up and leak when a pressure max is reached, then exceeded, then close back up when the pressure drops below the threshold value (e.g. most Agilent spring-style flow cells have this feature). It is extremely rare to ever replace one of these cells due to clogging. Most other types of flow cells simply rupture or crack, requiring replacement of the flow cell ($$$$, not the best design). Most industry flow cells used for UV/VIS applications are designed with a standard high pressure rating of between 40 and 120 bars, with optional "high-pressure" cells offered which can withstand full pump pressure. These specialized high pressure cells are only needed when the flow cell is position BETWEEN two detectors or columns, so may "see" extremely high pressure if the flow cell ahead of it (or column) becomes obstructed. The std cells are more than adequate for most applications.

As noted, flow cell path-length and volume must be matched to the exact application and method. It is critical to get these parameters correct. Major instrument manufacturers have already spend a great deal of time designing their cells and offering them in a range of sizes to fit most needs. Designing or re-purposing one from another detector may not be the best use of time when plenty of used instruments (and flow cells) are available on the market today. Depending on need, as little as $250 USD can buy a basic used detector with flow cell.
Thank you for the feedback.

So in summary: If the out port on the flow cell is venting to atmospheric pressure, it will likely survive, but a bigger problem is that it is dimensioned for a different set of flow/concentration conditions than those seen in HPLC.

I may have formulated my question poorly though. I am not going to attempt re-purposing the flow cell in another detector. I was planning on using the entire optical module from the FPLC, complete with flow cell, deuterium lamp, grating, lenses etc. and detector.

The detector requirements are basically none, since I am doing this in my spare time. The flow cell does not mention volume, but has a path lenght of 100 µm, which might be a bit on the small side for the very dilute solutions encountered in anylytical HPLC.

Another option I'm considering is to construct a fiber optical flow cell from a PEEK cross fitting, with a fiber to a photodiode and one to a spectroscopic light source. Something like this has been reported in literature, I believe.

But I agree, finding a second hand detector is definitely easiest.

--Chris
In other words a path length of 0.1 mm ! That cell has applications in Preparative HPLC chromatography only, not analytical HPLC. I would suggest you forget the whole thing and just purchase a used HPLC detector. You will learn more.

While I admire anyone who tries to build their own module for the purpose of learning and experience, I also understand when something just makes no sense too. If you want to learn about these things, start with by reading some of the classic texts on HPLC, then work on some of the older modules (available for very little money worldwide).
The PDA that's made me happiest in my career was the old Surveyor one sold by Thermo. It had a path length of 50mm. (Light-pipe design, in effect, a cleverly planned version of the fiber-optic flow cell). Just a real pity about the Surveyor autosampler...
I thought that the Surveyor's major problem was a nasty habit of the coating peeling off the interior of the flow cell, rendering them unusable after a time.
Great design concept, poor execution.
Thanks,
DR
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maybe if you had them for a very long time? We had ours from Autumn 2003 until Spring 2015 without any problem, but we don't use particularly aggressive solvents - maybe that would affect them too? If the flow-cell went, it was extremely easy to change, but I suspect it would have cost a fortune for the new cell.
My feeling about the Surveyor in general was that the autosampler was a catastrophe - its syringe drive was particularly sensitive to dirt, with a weird 2-diameter syringe which meant if the drive got confused about where it was, it would squirt the sample clean through the loop and into the waste. If the drive was dirty or the needle getting a bit blocked, it wouldn't fail completely, instead the syringe would just lose its position gradually over 15 runs, which was very annoying. It also had by far the worst carry-over I've ever seen, even when it as behaving itself. The pump was OK but nothing to write home about. The PDA was the saving grace. Those were the only three modules we had, so I can't judge the rest of the Surveyor range. Back in those days, Thermo were brilliant at making mass specs and not so hot on the LC side; acquiring Dionex made very good sense.
or we were lucky!
So, thanks for all the insight, but it turns out I can get my Waters 2996 running afterall.

Millennium 32 will happily operate the 2996 if you set it to 996 in the instrument menu. I had no idea but apparantly they're very very similar. Maybe just differing in firmware revisions.

--Chris
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