Public Lab Research note

Idea for measurement of airborne pollutants (or other stuff)

by gbathree | April 08, 2016 03:47 08 Apr 03:47 | #12938 | #12938

This came out of the GOSH 2016 conference after meeting Marco Mauro's amazing little quartz crystal microbalance. Ok, so here it is:

The QCM is quite a simple machine, and extremely robust (go read about it in wiki). It uses the impact of mass on the oscillation of a piece of quarts to measure the weight of an object down to the billionth of a gram or better. We were taking measurements of our breath at GOSH while a really loud band was playing in the same room - yeah, it's awesome :)

Anyway, it gets used for a wide range of interesting things in labs, but because of it's robustness and the relative simplicity of the actual measurement... but after talking with Macro I think there's unexplored applications in the field. Imagine this as the simplest possible example:

1) Let's say I put a piece of active carbon (which absorbs everything out of the air) on the QCM, and put in in a room and took a reading every minute. If there were periods of lots of dust or soot in that room, I would see the rate of increase in mass of the active carbon go up during those periods. I could even quantify in pretty exact terms the amount of particulate in the air based on the rate of accumulation.

2) Ok - so that's cool but very non-specific. To get a bit (only a bit) more fancy, what if I had a piece of functionalized active carbon (or some other polymer or something), where the functionalized component only absorbed my pollutant of interest... So the rate of mass gain was proportional to the amount of that specific material in the air. In this case, depending on the amount of pollutant and rate of absorption, one could imagine having this out in the field for quite some time before it began to saturate my functionalized material (this is calculatable).

Again, what's cool is this is relatively cheap, relatively simple technology which could be designed to hold up in the field, and is kind of infinitely expandable to a variety of different possible pollutants based on the method.

So that's it - what do you guys think?


I think this is a super interesting idea. There would be, of course, a lot of caveats and assumptions made, but I'd be interested in exploring them. My first two skeptic's flags (because my brain quickly goes from "neat!" to "here's a litany of issues to think through" back to "neat!") are: (1) even if a substance isn't chemically sorbing, there can be physical sorption simply due to impact or settling (i.e. a particle could still land on it, even if it were chemically modified to select for certain gaseous molecules) that could impact weight measurements (2) even the best functionalization only gets selective to a certain extent, so it'd be more likely to work for a "class of molecules" rather than a specific chemical itself. That's more of a detail about communication of results though. I like the idea!

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This is a good idea. There are a series of PM, NOx and SOx monitoring tools built around this idea that require a balance such as this.

I found his work while looking for passive monitors. I dismissed it as too difficult for a community science project based on the cost of a balance. there are two such projects listed here:

Martin Ferm is one developer in Sweden, here's one of the projects he's worked on for NOx. He deploys strings of NOx/SOx sensitive materials and raw filters in housings, and weighs them:

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An inexpensive microbalance also changes the costs of running filter-based PM monitors, some of which are Federal Reference Methods for PM2.5/10. Currently its $15-20 a filter to do lab measurement + blanks for mailing, etc.

Having a balance on site can potentially cut transport out of the equation and speed local analysis. Making a DIY filter system still requires designing low cost impactors (or cyclones) and a smooth, consistent pump running off DC power, both of which are non-trivial tech developments, but the most expensive component is the balance.

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Hmmm... those are interesting! Actually in further discussion here we came to a similar idea that if the filter/carbon/whatever filled up with material too fast you could seal it and actively pump outside air in in a sampling regime (like 2 minutes of every 60 minutes) to extend the life... similar to the systems you described. I think that adds a lot to the design.

We had another set of ideas that also estimates weight, but in a more direct way and with a wider range of applications on a single 'scale' (ie single set of electronics and piece of quartz). This is Robert Zegarac's idea BTW that came out of a discussion I had with him.

Imagine a star shaped piece of quartz (or other high Q oscillating material), where the tip of each point on the start had a chemical treatment which reacted to different pollutants or chemicals of interest (obviously a lot of discussion about what that would be... I'm addressing the smirks of the chemists reading this right now :). Each of the points on the star will add to the overall quarts oscillation in a unique way, which will come out using FFT (fast fourier transform) of the return signal. So basically, the spikes on the FFT correspond to the points of the star, and as the points of the star change weight (ie react), the spikes on the FFT produce a corresponding shift in resonance. (see attached image for what is hopefully a more clear description).

We discussed more elegant potentially longer term ideas (like same idea but using MEMS technology similar to that used in an accelerometer), but here's why I like the quarts idea:

1) it's human visible, and therefore it's easier to begin testing on (human beings could add the chemicals to the tips of the star, you can pick this up and assemble it by hand, unlike MEMS).

2) it's relatively low cost, and except for the question of how to get a start shaped piece of quartz, it's testable like today.

3) in the long term, once it's proven, you could move to MEMS as a lower cost, mass manufacturable version.

One could imagine choosing the 5 most important air quality components to measure, creating robust chemistries to apply to the oscillator, and using the occassional sampling method you described to have a pretty robust, easy to change, long term measuring device at relatively low cost. Obviously technology to be developed there, but it feels like a pretty compelling idea that's in the realm of possibility. IMG_20160415_155112.jpg

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Happy new year! Any updates on this interesting project?

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Happy new year to you too!

No, haven't made progress, but I think it's still worth exploring!

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