Hi there.
Welcome to Part 2 in our UV-Vis series.
Today I'll discuss the importance of using an analytical balance to prepare your sample,
and I'll walk you through the process.
Using Beer's Law, you can actually calculate the mass concentration of a substance suspended
in a solution that has strong absorbance properties.
For this purpose, the method of UV-Vis sample prep is very important.
In Part 1, you watched me prepare a qualitative measurement, using pipetters to measure the
volumes of the silver colloid and the water that it's being diluted down with.
If you were to try and use that data from that measurement to calculate mass concentration
of your substance, you would only get approximate values.
When you prepare the sample on an analytical balance, you increase the accuracy of the
volume significantly.
We use this practice very frequently when we prepare dilutions of nanomaterials and
need to verify that the target concentration after the dilution was achieved.
I'll explain that process in a bit more detail later.
Now to prepare what we call a quantitative sample, you first need an analytical or semi-analytical
balance.
These are balances that measure weights in the sub-milligram range.
An ideal balance for this would be one that measures as low as 0.1 milligrams, because
oftentimes the nanoparticle sample volume can be quite low, in the 10 to 100 uL range,
which for most aqueous colloids translates to milligrams directly since the density of
the liquid is essentially the same as that of water, which is 1 gram per milliliter.
Now before preparing the sample dilution you'll need to figure out what volumes are going
to be needed in order to prepare a solution that has an absorbance or optical density
of about 1, since that's the optimal OD for measuring on the spectrophotometer.
It's quite straightforward, if the absorbance is known.
For example, all of our nanomaterials come with a Certificate of Analysis which includes
the optical properties such as peak wavelength and absorbance, so if the reported optical
density of the material was 150 you'd need to prepare a 150-fold dilution.
In this case you need to measure out 100 uL of the nanoparticle solution and bring it
up to about 15 mL with water.
If the absorbance of the colloidal solution is not known a qualitative sample which we
covered in part 1 will need to be prepared and measured in order to approximate the optical
density.
Then, the subsequent quantitative sample can be prepared and measured based on those results.
So now that you're ready to prep your sample, you'll want to weigh out the nanoparticle
solution in a vial or bottle large enough to accommodate the final dilution volume.
And then that's going to be pretty substantial because as you can see were working here with
concentrated, it's really dark so this is concentrated silver BioPure 20 nm.
And as you can see it's pretty dark, this has an OD or absorbance of about 166.
So, we're going to have to do with 166-fold dilution to measure on the UV-Vis spectrophotometer
accurately.
So, we're going to start off by placing our scint vial inside the balance.
Make sure you tare it, and I've pre-calculated my solutions because I already know the concentration
of this particular material and the OD.
If you didn't know you would have to prep a qualitative sample like we did in part 1
in order to approximate the OD, and then you can subsequently prepare an analytical sample
for the quantitative measurement.
So, for this I'm going to need about 50 uL of this BioPure solution and I'm going to
dilute it up to about 9 1/2 mL with water.
And now I'm going to write that number down, and next I'm going to add the necessary amount
of water with this stripette here.
Let that remain stable and then we'll write down that number, and now you're pretty much
ready to measure it on the UV-Vis spectrophotometer.
Once again, you'll blank it and then you'll use the solution directly for the measurement.
And that concludes part 2 of our UV-Vis series.
In part 3, I'll explain how to analyze your data to calculate mass concentration and track
stability of a material over time.
I'll also show you examples of spectral data from other plasmonic materials such as these,
and I'll demonstrate how the optical properties change depending on their shape and size.
Thanks for watching!
And please feel free to leave comments below.