Using the Ocean Optics Spectrometer
The Ocean Optics spectrometer is a small analysis tool for the spectrum of gas discharges. Emitted light from the tube is carried via a fiber cable to the unit which then samples the spectrum and displays the spectrum in real-time on the PC display.
Begin by reading the manual for the spectrometer, available via the link below.
The spectrometer is activated via an icon on the lab computer. After initialization a display similar to that seen in figure 1 will be seen. The spectrometer is real-time and displays the spectrum as it is analyzed.
Two controls of importance are the Integration Time and Average parameters. Integration Time is used as a light control. By increasing this parameter the spectrometer collects more light and so peaks become larger. If this parameter is set to '1' and peaks are still too large to display the spectrometer is saturated and an attenuating filter must be placed between the source and the fiber probe. The Average parameter allows the spectrometer to average the reading shown over many samples. Setting this to a larger number like '10' stabilizes the display which would otherwise fluctuate rapidly due the the flickering of the spectrum tube driven by an AC source (which turns the tube off 120 times per second).
To save an image of the spectral output use the Copy Graphical Data option from the Edit menu. The image may then be pasted into any word processing (e.g. Word) or Paint-type package (e.g. Paint) and saved to disk as a file for later use in writing a lab report.
Use In Analysis Of Gas Discharges
Consider the following spectral output of a gas discharge tube filled with helium gas:
From the Hyperphysics web site the known helium lines are listed as:
Indeed, as expected, we find peaks at all of these wavelengths ... and a few more to boot! The question begs "Where in $%#& did the line around 545nm come from?". It certainly did not come from helium so it must be from some contaminant. We begin by using the NIST webbook to search the persistent spectral lines for common contaminants in a vacuum system, namely Nitrogen (from a leak), Oxygen (liberated when electrodes heat) and Hydrogen (from water vapour). A quick check reveals that none of these elements exhibit a line near 545nm. What about mercury?? That sounds odd, since it isn't found commonly in our atmosphere (let's hope not, at least :), but suppose the prototype tube (there are several in the lab) was previously used with mercury?
Sure enough, the NIST webbook shows that mercury has a strong line at 546.0nm. Let's NOT take this at face value though ... PROVE it is mercury. This is done by checking that mercury also has a strong line at 615nm ... which also shows as a peak on the output. One would then systematically identify all unknown peaks as to the impurity that caused them. Be forewarned that there may well be more than one impurity in the tube ... anything from traces of the last lab group's experiment to whatever was in the air that day. To identify a weak peak, one may well want to increase the integration time so that while larger peaks are saturated (with flat tops) weaker lines are clearly exhibited for analysis. Save both traces as 'strong' lines and 'weak' lines for later analysis. For precise wavelength determination, the OO spectrometer can also save the raw spectral data to a file for later analysis ... the accuracy of such wavelengths will be much higher than reading it from a screen. Save spectral data using the Copy Spectral Data option from the Edit menu. Copy All Spectral Channels then open an editor such as Notepad in Windows and Paste to get a list like this ...
Master WL Master Data 339.10 0.000 339.49 0.000 339.87 0.000 340.26 0.000 340.64 0.000 341.02 0.000 341.41 0.000 341.79 0.000 .... 1029.35 0.000 1029.63 0.000 1029.91 0.000 1030.19 0.000Note that the wavelengths (left column) are in nanometers and feature a resolution of better than 0.5nm. Now, simply SAVE this file as something like "HeliumData.txt". The data can be later opened with a text editor or pasted into an Excel spreadsheet for graphing or analysis.