I have some PNIPAM (Poly(N-isopropylacrylamide)) aqueous solution in which PNIPAM are crosslinked to form spherical microspheres. I want to know the concentration of these microspheres in solution. One way I can think of is to dry the solution out and measure the weight followed by calculating the concentration based on density and volume. Unfortunately, I don't have much solution therefore the weight might be too small to measure. I wonder is there any other method to measure the concentration? Thanks a lot!
If you can find a TGA (e.g. from Mettler) in a lab near you, then you will need very little sample. Typically a sample of 50milligram can be measured accurate to tenths of a microgram, so you will get excellent accuracy for the mass fraction, by drying your solution. I assume PNIPAM is the only dissolved matter.
What exactly do you want to know? The concentration (mol/L) or the mass concentration (g/L)? If you are interested in the concentration in mol/L, you could estimate the mass concentration of course with TGA as suggested by Alan, you could also try UV-Vis, but for that of course you´d need either the correct molar extinction coefficient or you need to make a calibration.
Also, you could measure static and dynamic light scattering, with which you get Mw and Mn which together with the mass concentration will give you the particle concentration. If these are big-enough microspheres, you could also try a FACS machine or a cell counter from a biology lab. You should be certainly able to find one a the UW.
The answer given by Robert can be tried if sample can be measured by using light scattering experiment. Particle size analyzer can be tried also to get size distribution. The density measurement will give the weight from the determined size..
Thanks Prof. Luxenhofer! I want to know the mol concentration (mol/L). I don't have enough samples to do TGA measurement given that I tried to dry 20% of my sample and it only weigh 0.5 mg. The sphere size is about 160 nm to 330 nm -- does it worth trying a FACS machine or a cell counter?
When you only want the molar concentration, it should be possible to obtain from mass balance according to preparation protocol.
Dear Zhaoxia Qian,
no, for such small systems a FACS machine is not likely to be suitable. It depends of course on the buildt, but when I checked a few years back, I was told that below 500-1000µm it is unlikely that the FACS will register.
I think your best shot may still be static/dynamic light scattering in combination with UV-Vis
Best
Robert
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