Wednesday, April 14, 2010

Modulation

I'm beginning to see a pattern here, and I like it. Electrical engineers do it all day and night (as does anyone listening to a radio or watching a TV), but as chemists/physical scientists, we don't do enough with intentionally modulating our data signals. And it's to our loss. Consider these examples of what happens when it is done:

1) Dynamic mechanical analysis. It's one of the most fundamental measuring techniques in rheology. A modulated strain (or stress is you have a newer machine!) is applied to the sample, and out pops the storage and loss moduli (or the corresponding compliances or tan delta or viscosity or whatever function your heart desires). Without the modulation, you can only get the viscosity - as in a capillary viscometer.

2) Modulated DSC. By adding a sinusoidal overlay on the temperature ramp, you are able to separate out reversible and irreversible transitions, and do it with more sensitivity than with a linear temperature ramp.

I've been aware of these previous two examples for nearly 20 years, but a ran across a new one yesterday [1].

3) US Patent Application 2006/0228806, "Method for detecting the modification of a characteristic of a sample caused by an enviromental influence". This is a technique for determining degradation in polymers due to natural/accelerated weathering. A very common technique is to use a spectrophotometer which gives the standard L,a,b values (or CIE values or ...). This new method applies a mask to the sample during the exposure time, providing geometric modulation. During testing, the mask is removed and the entire samples is scanned on a flatbead scanner, a much cheaper and more common device than a spectrophotometer, although not without its disadvantages too.[2] Using correlation techniques, changes in the sample can be detected much quicker than by using absolute measurements. The equipment has been commercialized;[3] I've not used it so I can't say anything about it.

So now that I see a three-peat, I can see the bigger picture: modulate whatever you can whenever you can.


[1] The new Atlas-MTS Newsletter, "Sunspots", Spring 2010 in the lead article mentions the techique as a "Ten Times Faster to Result" method. The Public PAIR site shows that the application is still active after. Applications were also filed in Europe too and I don't know what the status is on those.

[2] Spectrophotometers are essential for field work. You can't use a flatbed scanner to look at color changes occuring in an installed window, a common occurance for us and our parent corporation.

[3] The Catscope, from ERT-Optik. Warning: the site is in German, but for us old school chemists who had to learn German, that's not a problem, is it?

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