[PASCO Polarimeter: Introduction]
Hi. I'm Tom Loschiavo, the Chemistry Education Manager at PASCO scientific.
I'm here today to talk about the PASCO Polarimeter.
The polarimeter is a device used to measure
the rotation of plane polarized light by optically active compounds.
Before I get into the polarimeter,
I just want to tell you a little bit about what that sentence means.
Assume this is a compound with a central atom
and four different atoms attached to it.
This is the same central atom with the same four different atoms attached to it.
But these are actually different compounds.
These are non-superimposable mirror images of each other.
They look the same in mirror image, but I cannot superimpose the two.
These are called enantiomers.
These two enantiomers happen to have a chiral center.
That chiral center will rotate the plane polarized light
either this way or this way depending on the compound.
So, what does that mean, "plane polarized light"?
I can demonstrate that over here.
I have a light source in the back.
I have a polarizing film.
I have a sample of water
and I have another polarizing film.
You will notice that when I turn the first polarizing film,
that the sample and the film around it get darker.
The water is not having any effect on the brightness of the light.
But if I take that same light
and I run it through a sample with a chiral center,
in this case Karo syrup,
then you will notice that some wavelengths of are light getting blocked,
while others are getting through.
Now, as this is getting darker and this is getting brighter,
different colors are starting to appear
because of the different wavelengths of light
that are being blocked or let through by that particular sample.
That is what I mean by rotation of plane polarized light.
This is rotating the light.
This is not.
Because there are chiral centers in the compound.
What we want to do is we want to quantify that.
To quantify that we're going to use our polarimeter.
The polarimeter has a simple turn on power button right here.
It can be connected via Bluetooth or USB.
Inside there is a sample cell holder.
Here's a sample cell.
This sample cell just has water in it.
But it's a 100 mm sample cell with quartz ends,
so it won't affect any of the light.
In the polarimeter, there is a 589 nm monochromatic light
going through a polarizing film on this end
and a polarizing film on this end in front of a detector
that I can rotate around just like I could rotate this around.
I have monochromatic light that's getting plane polarized,
going through my sample, and hitting a detector.
What I am going to use is some PASCO software.
You can use either SPARKvue or Capstone.
In this case, I'm using Capstone.
I am going to try to figure out the exact angle of rotation
where I get the most intense light to come through my water sample.
Then I will do that again with the sample containing a chiral compound.
We'll see how we can measure that angle of rotation
based on the chiral compound that is in there.
Over here I have my polarimeter attached to the software.
I have a graph of light intensity versus the angle.
So we will be able to measure the angle rotation.
I'm going to hit Record.
And I'm going to start spinning the wheel.
You will see that there's my plot of intensity versus angle of rotation.
A nice plot.
What I'm going to do is go from zero to 360 degrees a couple of times,
just rotating around, to make sure I have lots of good data to get a good fit on,
so I can really pinpoint the angle at the highest intensity.
I'll spin it around a little more.
There we go, a couple of cycles through. We got through our 360.
I am going to hit Stop now.
Now I am going to take this sample out.
If you are doing this as an actual experiment,
you will probably want to use the same sample holder each time.
For this purpose, I'm just going to use a different sample holder.
I am going to put this in.
This has a sucrose solution in it.
I am going to start recording again.
I am just going to spin the wheel. There we go.
Notice the intensity is a little different.
That makes sense, there's something in there.
But also notice that the peak intensity
is shifted slightly from where it was before,
because, just like this, there is an optically active chiral compound in there
that will shift that plane polarized light.
I will spin this around a little bit.
The wheel on here has a notch in it,
so you know where you start and end to make sure you are going around at least once.
You can also look at the graph to see we are going from zero to 360.
Once I get to 360, a couple of times to round that out,
I am going to hit Stop.
Now I'm going to do some analysis. This is where the software comes in.
I'm going to try to find the peak intensity here and here.
To do that, I am going to highlight an area.
This is my Run #2.
I am going to apply a Gaussian fit to this.
This Xo is my angle at the highest intensity for that particular set of data.
Now I am going to do the same thing with my first run,
which was the blank, which was water.
I am going to highlight some data.
I am going to turn on my fit again.
Now I have my Gaussian fit for both of those samples.
So I had my blank water.
The sort of zeroing or blanking based on the water was at 75.9 degrees.
The rotation for the sample of sugar was at 81.9 degrees.
So it looks like based on the concentration of sugar in there,
I had a 6.0 degree rotation.
The other nice thing about this is that you can do this with multiple concentrations.
This is actually a good way to introduce concentration.
The specific rotation of your sample will depend on the path length,
for which we have used industry-standard sample cells
to make sure you have a very specific path length.
It will also be based on the concentration of the sample.
So you can look at concentrations of samples as a function of optical rotation
as a novel way to introduce concentration.
The polarimeter is a great way to talk about concentration.
It is also a great way to talk about the differences in molecular structure.
So using the PASCO Polarimeter
and some PASCO software, like Capstone and SPARKvue,
you can really get a handle on optical rotation of chiral compounds.
Thank you very much.
This has been Tom Loschiavo.
You can reach me at chemistry@pasco.com.
I'd like to encourage you to "Think Science".