Introduction

The class will be split into two groups with each group performing both parts of the lab alternately.

Pre-Lab

• Research the operation of a spectrograph using the Scientific American CD-ROM collection (available for sign-out from Central Supply in L-18 as well as in the LRC). Specifically look at articles from Jan. 1975 and May 1968 for example designs. For a more general view search for the word "Spectroscopy".
• Read the Operation manual for the COLEMAN 20 and GeneSys-20 units in our lab. These are on the Standard Operating Procedures (SOP) page.
• Read the Operation manual for the Lambda-3B unit on the SOP page.

PART A: Single-beam Spectrometers

Examine the operation of the Coleman-20 unit by setting the wavelength dial to 500nm and inserting a strip of paper into the sample holder. Rotate the wavelength knob while observing light on the paper. The optics in this unit consist of a monochromator followed by the sample holder and then a photocell detector. This is a single-beam type of unit.

Zero the unit as per the procedure in the manual then insert a filter into the sample holder and measure the transmission of this filter at various wavelengths 25nm apart. Remember to set 100%T on the unit for each wavelength selected! When all data is collected graph the transmission of the filter using a spreadsheet program. Repeat this procedure for both filters provided. Make hard copies of both transmission graphs.

If time permits, analyze one filter using the GeneSys-20 spectrometer. This unit operates much like the Coleman-20 however provides digital selection of wavelength and readout of transmission. Like the Coleman unit, the unit must be set for 100%T for each sample with no sample installed, then the sample installed and the transmission determined.

PART B: Dual-beam Spectrometers

Setup the Lambda-3B unit as per the procedure (set 100%T as well) then insert the same filters used in Part A and run transmission curves for these filters through the entire range of 400nm through 700nm (or the same range as you had used in Part A). Repeat this procedure for a narrow-band dielectric filter - you will manufacture filters of this type in Vacuum and Thin Film Technology next year.

Assignment Questions (not all are worth the same marks)

Part A and B:

1. What is a monochromator?
2. Contrast the differences between a single-beam (like the Coleman-20) and a dual-beam (like the Lambda-3B) spectrometer in terms of user involvement and explain WHY must the user set 100%T on the single-beam unit for each new wavelength.
3. What is the major advantage of a dual-beam unit in chemical analysis where, for example, a sample is dissolved in a solvent which itself has an optical absorption curve.
4. Assuming a chemical to be analyzed was dissolved in a known solvent (i.e. two cells are provided: one with the solvent+unknown chemical, the other with just pure solvent) what is the procedure for EACH type of machine (single- and dual-beam) and why must this procedure be performed to isolate the unknown?
5. What is the advantage of using a diffraction grating over a prism (and vice-versa) for a spectrometer?
6. What is a photomultiplier tube (PMT) and why is it preferred over a simple photocell?
7. Why is it necessary to switch-in a filter (the right knob) in our Coleman-20 unit when using UV or IR ranges? (interestingly, the Lambda-3B also has filters which are inserted into the light beam - automatically of course)

Hand-In graphs made using the Coleman 6-20 (and the GeneSys), the same filter(s) made using the Lambda-3B, the dielectric filter, and answers to the above questions. A formal lab write-up is not required for this lab however a basic discussion of the results (e.g. comparison of the same filter done using the manual and Lambda-3B units) is required. Contrast the differences seen in the optical resolution of the two types of units used here (in terms of ability to resolve closely-spaced features).