Noise reduction.

After I knew, that the device worked, I started to think of what can reduce noise and improve the performance. Obviously, the vacuum pump was not suitable. The mobile phase must be pressed through the system, not sucked. Luckily Sebastian found an old peristaltic pump which was not used in the laboratory, it was not the perfect solution because an HPLC-Pump would be better, but it was what we had at the moment.

Anyway, after the “new” version of the device was built I noticed a strong increase of noise, it basically showed a sinus-function, as it can be seen in the basis-line:

and an injection of 200μl 1% 4-Methoxybenzaldehyde solution gave this chart:

It is possible to recognize the substance-peak but the noise is too high. To solve that problem I have connected Arduino directly, without the breadboard to the sensor and isolated Arduino in a box warped in aluminium foil (what I actually needed to do at the beginning). I also noticed, that moving the cable, which goes from the sensor, results in big signal fluctuations:

Even opening and closing the box in which Arduino was placed had a very big influence. Here is an example of changes in the signal after opening and closing the cover of the box:

Shielding made the signal better but it was not enough, so I decided to use a “Smoothing filter”- it collects readings, calculates the average of the last M readings and sends it to the computer. In my case, I found out that with a 50-millisecond delay between readings and M=50, you get the best results.

This chromatogram shows the separation of 50μl 150μmol/ml Triphenylphosphine + Dihydroxyacetophenone  (flow rate 9ml/min) mixture with smoothing filter:

And this one shows the separation of the same mixture with the same conditions but without the filter:

Obviously, it was better- the curves looked much more reliable but it was still not exactly the same as in the commercially available chromatographs. In the next step, the filter was improved by temporarily saving readings as floats (the first version used integers- which gave not so accurate results) - thanks, Aleks! Now the signal had good strength and the basis line had “noise” of just ±0,25.

This chromatogram shows the separation of 40μl 150μmol/ml Triphenylphosphine + Acetophenone + Furfural (flow rate 9ml/min):

All 3 peaks are recognizable and it looks almost like it needs to be.

Update

The final version of the  code can be found here