The Observation of Molecular
Emission (OH) from an Induction Coupled Plasma
(a demonstartion project)
In this experiment, a commercial
(and expensive) Atomic Emission Spectometer, a Varian Vista CCD Simultaneous
ICP-AES, will be used to observe the optical emission from a highly-excited
gas sample. The Spectrometer itself is usually used to detect trace
amounts of elements, which after being atomized and heated, show characteristic
sharp emission lines. We saw such lines in the spectrum of our Hydrogen
lamp in a previous lab.
The spectrometer itself is an
unfriendly looking box with few visible controls:
Figure 1: The Vista ICP AES
and, of course, is completely
under the control of a computer:
Figure 2: The Vista Console, complete with Spectroscopist
The instrument itself requires
a setup taking approximately 1/2 an hour, which (hopefully) the TA will
have performed for you. Your job begins with
Check to make sure that the Argon
tank is open and has at least 500 psi in the tank and delivers 80psi to
the instrument.(Ar is the major constituent of the plasma)
Check that the Heat Exchanger is
Figure 3: The Neslab(TM) Heat Exchanger
Check that the exhaust hose is moving
air and that the room lights are on (This increases airflow in the hood
Make sure that the sample tubing
is configures properly on the peristaltic pump and that the Inlet tube
(front) has slightly less pressure that the outlet tube. The sample
inlet system looks like this:
Figure 4: The Sample Inlet of the Vista
Sample prep: Our sample will
be quite exotic, pure water! Why? Because we should
be able to see, H atoms (which we know all about), O atoms, a lot of Ar*
and Ar+ lines, and if we are lucky, OH molecular emission!
We will detect identify this emmission by its spectral characteristics,
and, to do this, we will use two different spectrometers (hense two computer
systems). One spectrometer will be the internal CCD/Eschelle spectrometer
of the Vista itself, and the other is a PChem lab addition: an Ocean Optics
micro-spectrometer coupled to the VISTA instrument via a fiber optic.
The Dell computer hooked up to
the Ocean Optics Spectrophotometer has an icon on the desktop labeled "Ocean
Optics Spec." Double click on this icon to open the Ocean Optics software.
Once the program has opened, click on the button labeled "Data Acquisition".
(It looks like a spectra with three blue peaks). This will tell the spectrophotometer
to begin acquiring data
The Ocean Optics Spectrophotometer
has two measurement channels, Master and Slave. These are located on the
side of the small Ocean Optics Spectrometer box. The data is obtained through
a fiber optic cable. You will be using the "Master" channel, which has
wavelength range of approx. 205nm to 520nm. The "Slave" channel has a wavelength
range of 365-900, which is too high to see a lot of the OH emission spectra.
Once the Ocean Optics software has started acquiring data, you can watch
the real time emission on the Ocean Optics software.
On the desktop of the HP computer
associated with the Vista spectrometer, you must double-click on the icon
labeled "vista.exe." This will start the program that operates the plasma
spectrometer. A window will open with three buttons to choose from: Worksheet,
Instrument, and Exit. At the bottom of the screen, inside of the grey border,
will be the ongoing actions taken by the plasma spectrometer. As you will
note, the ICP torch has a Argon purge delay that must be completed before
the torch is lit. This will take approximately 20 minutes.
When the Argon purge delay is
finished, click on the "Worksheet" button. It will prompt you to choose
if you want a new or an existing worksheet. Select the open option and
open the file named "OH emission.vws." This file has a wavelength for Al
displayed in the "summary" window. The Vista series spectrometer does not
allow you to perform a broadband scan. You are only allowed to look at
certain "windows" for various elemental emission wavelengths. There is
no way to tell this window to look at a molecular line. In this case, we
shall be looking at emission lines for Al that have OH lines in close proximity
When you have 'prepared' your sample,
turn the Plasma ON. This is accomplished with the Instrument Settings
window by clicking on the Plasma Icon. After a few seconds ( There
are serveral warm-up delays before the plasma is actually ignited ) you
should see this...
Figure 5: The Plasma Torch
To take a spectra with the Vista,
click on the "Edit Method" button. Select the "Conditions" tab. You will
now see a listing of the operational parameters of the instrument. To measure
a spectra, click on the "Read Spectrum" icon. (It looks like a set of 3
blue peaks under a black arrow) The Vista will now obtain a spectra. Once
it has finished, look in the Al window off to your right. The molecular
emission peak will most likely be very broad and centered around 309.23
nm. To zoom in, click and drag a box around the region you would like to
see magnified. This can be repeated several times to get the desired level
of magnification. To unzoom, right-click with your mouse and select the
"Unzoom" feature. It is also possible to place the cursor over a desired
region of the spectrum and read off the nm and intensity values for that
spectral region in the grey bar below the spectra boxes.
You may now wish to modify some of
the instrumental parameters to obtain a better signal. You may modify Power,
Plasma Flow, Auxillary flow, Nebulizer Flow, and viewing height. Please
don not lower the power below 0.7 kW. Note that each time you change or
modify a parameter, you must click on the "Read Spectrum" icon for the
parameters to be changed.
You can note the changes each parameters
has on the broadband spectra measured by the Ocean Optics Spectrometer.
Adjust the parameters of the plasma to maximize your OH signal. To zoom
in on a specific region on the Ocean Optics Spectra, click on the "Set
Scale" button. (It resembles two crossed red arrows) This will open up
a window that will allow you to change the min and max values for the x-axis(wavelength).
This will zoom in on the wavelength region specified by the new max and
min wavelength values. To unzoom, repeat the above procedure, but type
in the old min and max values for the x-axis.
You are now ready to save your broadband
scan from the Ocean Optics of your optimized OH emission signal. Click
on "File", and then select "Save." You are given the choice to save as
either a sample or an experiment. If you save as a sample, the file is
able to be opened in Excel for data manipulation. By choosing the "sample"
option, a new window will open prompting you to enter the name of the save
file. Type in the desired name in the blank area labeled "File Name". Click
on the Okay button to save your file. It is important to note that you
are only saving what you currently see on the screen and not the current
complete scan. If you have zoomed in on a region, the software will save
the spectra of that zoomed-in region. To save the complete spectra, make
sure you Unzoom before saving.
To close the software, click on
"File" and select "exit". Once you are finished using the Ocean Optics
Spectrophotometer, you are ready to turn off the ICP torch and shut down
the instrument. Close the "Method Editor" window. A prompt will ask you
if you want to update the window, click "OK". Now click on the icon that
resembles an ICP torch with a red X through it. (Or press F4) This will
turn the plasma off. Now close the main window. If a prompt asks you if
you want to save changes, click "YES". It is very important to turn off
the gas before you leave. Leave the heat exchanger running for approximately
5-10 minutes to cool down the system.
The plasma is hot! So hot
that most molecules are completely dissociated into atoms. This contributes
to the analytic utility of the the instrument and method. But because
there is so much water in the plasma, a few OH molecules survive and emit
in the UV region of the electromagnetic spectrum. As opposed to atoms,
molecules emit in bands:
Figure 6: A simulated spectrum of OH radicals in a 4000K Plasma
When you look at greater detail,
the spectrum appears more 'atomilike' in that it has a bunch of sharp lines
(maybe too many)
Figure 7: A portion of the OH spectrum shown at 0.02 nm resolution.
The OH spectral data corresponding to this figure
is in ASCII (wavelength, Intensity) columns.
I give you this data because it
is not very easy to get a 'big picture' of the emission observed in our
VISTA spectrometer and the Ocean Optics device does not have the greatest
resolution. The Vista instruments' best view of the spectrum comes
in what can be called an eschelle spectrum, or eschellogram, that your
TA will help you find in the instrument menu. What you see in this
is a large number of sharp lines, almost all of which are atomic.
A few molecular lines creep in, and are considered 'interferences'.
OH is a common interference, because of the solvent, water. But OH
lines are weaker than atomic lines because the intensity of the electronic
emission is spread out over vibrational and rotational states (see Figure
6,7), unlike that in atoms.
Lots of information is available
about the transition you have observed in OH
Get familiar with the spectrometer.
Explore molecular spectroscopy. Read up on OH. Then, in your
paper, make sure to include the following points:
1) Use the VISTA
to find at least one (hopefully a few) OH emission lines in the water/Ar
plasma, with the proviso that it is in the range of wavelengths covered
in Figure 7. Identify the exact wavelength of the emission, and redraw
Figure 7 with stars (or other symbols) marking the lines you have seen.
2) Assign the transitions
in part 1) in terms of ALL all the quantum numbers in the
upper and lower states. You might need help from a spectroscopist
in this assignment of the rotational quantum numbers of the upper and lower
states as the details of the rotational energy of the upper and lower states
in the transition are quite different from one another. Here
is a tabulation of the transition wavelengths (in
air) of the A-X (0,0) band of the OH radical if both the upper and
lower states are treated as Hund's case (a) (J, the total angular momentum
is used as a quantum number), and here is a tabulation
if they are both treated as Hund's case (b) (N, the intertial rotational
angular momentum is used as a quantum number). Warning:
case (b) is not for the faint of heart.
3) Describe the function
(in the VISTA) of
4) Describe the following
The Eschelle grating
The RF power generator
The Peristaltic Pump
That's all you have to do!
2S -> 2P