U.S.A. Stopped-Flow vs. Others:
What Is Relevant and What Is Spurious?

Volume per Shot or Volume per Final Result?

If final results were easy, by definition, volume per shot would be truly irrelevant. However, especially in fluorescence and circular dichroism, good results can be difficult to obtain... signals can be small and light levels low.

Every Olis spectrophotometer and each associated stopped-flow mixing unit is designed and optimized for sensitivity (= best results). Higher volumes produce stronger signals: one sees more signal looking through 100 µl than through 20 µl.

Those Olis clients with our rapid-scanning spectrophotometers (the RSM 1000) have the extraordinary luxury of obtaining hundreds of wavelengths—or "all the data you want"—with each stopped-flow shot. As has been noted about the Olis system: "The first shot fills the tubing, the second shot gives partial kinetics, the third shot is the keeper, the fourth shot confirms the third." No one complains about using 50, 100, or even 150 µl per shot to get "massive amounts of data, sufficient for gorgeous and unique data."

How ironic that our competitor makes such an issue of volume per shot. Especially when you talk with some of the owners of their systems who cry about "the tremendous amount of time and sample that is required."

The 'volume per shot' from any particular Olis stopped-flow depends upon the pathlength (20, 10, 4, and 2 mm) and upon the diameter of the tubing between the syringes and the observation cell. The final volume for final results can be 160 to 500 µl, depending upon the pathlength/tubing diameter issue, signal strength of sample, difficulty of wavelength range, and other parameters of the experiment which must also be considered with "hard" numbers such as deadtime and 'volume per shot.' We put quotes around "hard" numbers, because such a number is meaningless unless considered in context with other parameters.

Tying in with volume per shot is priming and drive volumes. Again, the absolute amounts vary from one Olis stopped-flow to the other, as we have the freedom of choice when building your stopped-flow to select among pathlengths and tubing diameters. The first shot does the priming with part of the second perhaps being required to complete it. After that, each shot you take is 'final results' quality.

Automatic vs. Manual Preparation for Next Shot?

Related to this issue of having to take a few shots versus tens of shots, our competitor touts their 'automatic' firing. Presumably, someone faced with taking 6-10 shots per wavelength (at numerous wavelengths) suggested that too much effort was required to get data and so some automation was added. We have not added this electronic and mechanical component because none of our clients need it; and it would be completely spurious when used with our rapid-scanning spectrophotometer. Flushing the stop syringe a few times a session is hardly asking too much of the user!

Horizontal or Vertical Drive Syringe Orientation?

Another spurious issue that our competitor has raised for years was their choice of orienting the drive syringes in a vertical rather than classically horizontal position. We thought the vertical orientation rather good when we first saw it, as the idea is that bubbles will rise to the top. When we used the system, we found the orientation annoying. Firstly, anyone the least bit skilled in filling syringes (and what scientist isn't) will not introduce bubbles in the first place. Secondly, vertically oriented syringes force the operator to tilt his head sideways to look at the syringe both to note whether bubbles are present or not and to gauge the volume left in the syringe. Simply looking down at the syringes is much easier. When we built the first Olis USA stopped-flow, we tested horizontal, vertical, and 45° drive syringe positions and concluded that horizontal had more advantages and fewer disadvantages.

Temperature Range?

A new issue our competitor has raised is a 'broader temperature range.' This is patently false. Olis has an exclusive contract with a company that machines the inner parts of our stopped-flow from ceramic (fired sapphire). This extraordinary dense (anaerobic, inert, and temperature impervious) material allows us to build stopped-flows with a temperature range of -10° to over 100° C.

Deadtime: 1 vs. 2 Milliseconds?

Deadtime is another spurious issue. Because we optimize for sensitivity and minimal shots to be averaged, we sacrifice a little in deadtime. However, we also allow you to cool your sample. Cooling a sample slows it. This simple control has far more consequence on the rate of reaction and on capturing 'all' the reaction than one will ever realize between a mixing unit with a 1 or a 2 millisecond deadtime. To quote an owner of an Olis USA stopped-flow, "After all, we are talking a factor of 2 not a order of magnitude. For any of the "fast" stuff ["too fast for a 2 millisecond deadtime"] they should not be using a mixing system anyway. I believe there was a Nobel prize associated with this realization a while back."

There is nothing unique about our competitors' products which Olis could not duplicate. However, we each have our own philosophy. At Olis, this philosophy can be described as "best results, minimal effort; best performance, minimal complexity; best reliability, minimal repair needs; best support, minimal need for it." And those of our clients with the Olis RSM 1000 rapid-scanning spectrophotometer have the luxury of capturing entire spectral scans as data points during a reaction, rendering all other discussion spurious.