The Chem 106 Lab Reports -- Summary and Checklist for Core Experiments

Fall 2004 Version 9/7/2004

 

Experiment 9-1 and 9-4 Atomic Spectroscopy

 

Note: This can be a very long experiment, especially for the first group who have to make up the stock solutions. The First Group should check with the instructor about shortening some of the sections. Recommendations for Group 1: Shorten 9.1 C 1 to two elements; D 1 only (not 1 and 2); F 2 only, not 1 and 2; 9-4: Do at least A; do A and B if there is time. The Groups that follow are expected to do all parts as stated.

 

Experiment 9.1 Atomic Absorption

 

  A. Instrument Parameters

 

        1.  Burner Height

        2.  Fuel/Air Ratio

         

  B.  Quantitative Analysis of Copper

 

1.   Ordinary Calibration

2.   Calibration by Standard Addition

 

  C.  Sensitivity. Comparison of Different Elements

 

1.   Cu, Mg, Sn, and Mn each measured at 5.0 µg/ml (Actually, 5 microgram/mL may not work. Be sure to use a concentration you can SEE!)

 

2.   Sensitivity defined by SHB as that conc that will give an absorbance of 0.0044. (0.0044 = log 100%T - log 99%T). In S&L (or SHN) we have Signal = sensitivity x concentration + Signal of Blank; so sens = (signal - blank)/(concentration) which is just the slope of the working curve. Please report the slope of the working curve, in the correct units, when stating the sensitivity.

 

D.  Effect of solvents

 

1.   Miscible organic solvents (1) water, (2) 20% ethanol or propanol in water, (3) 50% alcohol/water.

 

2.   Solvent extraction of Cu into MIBK. Several concs. Working curve in MIBK.

 

  E.  Detection Limits

 

1.   Do for Cu. Compute the conc that would be 2 x the std dev of the Cu readings.

 

  F.  Interferences

 

1.   Measure Ca in the presence of K.

 

2.   Measure Ca in the presence of phosphate.

 

G.  Hollow Cathode Lamp emission spectrum. (Added to SHB. See Core experiments in Syllabus.) [Because of the length of the AAS experiment, this Hollow Cathode Lamp Spectrum exercise has been placed with the Molecular Absorption and Optics experiment below.]

 

Experiment 9-4 Determination of Na (or K), Li, and Ca by Atomic Emission

 

A.    Working curve for Na by the direct intensity method.

 

B.    Working curve for Na using Li as an internal standard.

 

C.    Effect on Na emission of 0, 10%, and 50% ethanol, then 50% glycerine.

 

D.   Working curve for Ca using (a) CaCl2, (b) Ca(NO3)2, (c) CaCl2 w phosphate, (d) CaCl2 w Al.

 

*************************************

GLC

 

Added Fall 2004 GCMS

 

As of the data above, we have the GCMS working. Mr. Kleiwer, Dr. Dormedy, and Dr. Gump along with students with experience in GCMS have gotten us started. Here is what we plan to do:

 

First Group: Get all members of the group familiar with how to make a basic run using a solution of BHT in THF supplied by Dr. Gump. Set temperature program and detector turn-on time to eliminated having the solvent (THF) peak swamp the detector. In the runs we saw, the solvent peak was over about 3 minutes into the run, with the BHT (and a few other) peaks showing up within 20 minutes. ID the peaks with Mass Spec. Record all operating procedures in preparation for Show and Tell Day. Then prepare BHT standards and do a quantitative analysis of a sample provided by Dr. Gump. If there is time remaining, begin some of the sample workup for the Arson experiment.

 

Subsequent Groups:

 

Do an initial injection of BHT in THF, separate the peaks, and do a MS ID of the peaks. Once you are familiar with the instrument, proceed to “Who Set the Fire? Determination of Arson Accelerants by GC-MS in an Instrumental Methods Course,” by David A. Sodeman and Sheri J. Lilard, J. Chem. Educ. 2001, 78, 1228-1230.

 

From Semester previous to Fall 2004 and before GCMS (in case we need a Plan B)

 

The first group will be occupied with learning how to cope with the Agilent GLC and Chemstation software. To this end they will prepare two sets of solutions to work with the two columns in the instrument.

 

(Note: A handout with more detail on the Core Experiment with the Agilent GLC is available. This is an evolving document, so get the latest version.)

 

Set 1: A mixture of normal hydrocarbons. We could try the mixture referred to in Experiment 12-3 below.

 

Set 2: A mixture of alcohols of increasing molecular mass.

 

Determine the optimum conditions for separation of these two mixtures using the appropriate megabore column on the Agilent GLC. Parameters to be studied include:

 

1.     Injection temperature

2.     Detector temperature

3.     Column temperature isothermal

4.     Column temperature programmed

5.     Column flow rate

6.     Makeup gas flow rate

7.     Sample size of mixture as injected

8.     Samples diluted in a carrier solvent, then injected

 

Create a set of procedures that leads the user through the maze of options available on the Chemstation software.

 

Learn some of the post-processing analysis techniques available in the Chemstation software.

 

If time permits, the first group can do one or more of the SHB exercises listed below (or portions of some of the exercises).

 

Experiment 12-2 Quantitative Analysis of Mixtures is changed from SHB.

 

(Note: This experiment is very similar to that done now in Chem 102, except the Standard Addition part.)

 

Instead of the binary mixture in SHB, use a mixture of water, methanol, ethanol, and isopropanol. Choose one of these as an internal standard and keep it constant. One of the others is chosen as the unknown. Vary it by standard addition. Make up to volume with a third choice. Compute the concentration of an unknown made by the other student on the experiment. The student will have to:

 

A.    Choose a proper column and operating conditions to resolve the four components.

B.    Work up the data with internal standard correction and standard addition to measure the unknown.

 

Experiment 12-3 Resolution and Qualitative Identification of Hydrocarbons by GLC.

 

A.    Instead of setting the flow rate and doing a full Van Deemter plot, run the mixture at three very different flow rates, compute the HETP for each peak and comment on the effect, if any, on the resolution obtained. What is the best trade-off between analysis time and resolution?

B.    Get individual chromatograms of the pure hydrocarbons (pentane, hexane, heptane, octane, nonane)

C.    Get chromatogram of an unknown sample made from these.

D.   Get chromatogram of unleaded gasoline.

E.    Make log retention time vs. carbon number plot.

F.    Make log retention time vs. boiling point plot.

G.   Identify the peaks in the hydrocarbon unknown.

H.   Calculate resolution of adjacent peaks in the HC unknown R = (t2-t1)/[0.5(W1+W2)]

I. "I.D." as many peaks as possible in the gasoline sample. (Note: this is what you have to resort to if you can't use a GC-MS.)

 

 

*****************************************

HPLC

 

Instead of the experiment below, use the handout prepared for the Determination of Pain Relievers. For those students who may have already done that determination, you may choose a different HPLC experiment, perhaps in consultation with Dr. Dormedy or Dr. Gump, or you could do the experiment below as modified from SHB.

 

Experiment 13-1 Determination of Caffeine

 

  A.  Caffeine Standards in 20% MeOH/80% H2O at pH 3.50

 

       Five concentrations from 25 mg/L to 125 mg/L

       Plot peak area vs. concentration.

 

  B.  Caffeine in Coffee or Tea

 

       Measure concentration by direct calibration.

       Measure concentration by standard addition (or internal standard?)

 

  C.  Caffeine in Cola

 

       Direct

       Standard Addition

 

*********************************************

 

Molecular Spectroscopy and Optics Experiment (UV-VIS)

 

Experiment 6-1 Basic Exercises

 

Part 1. Operation and response of the spectrophotometer.

 

  A.  Spectral Response & B. Visible Spectrum

 

       -- Collect response vs. wavelength on the Heath Spectrophotometer.

       -- Plot response curve vs. wavelength.

       -- Prepare a Sensitivity vs. Wavelength table for the 1P28A photomultiplier tube from the S-5 response curve from the Heath manual.

       -- Compute the lamp intensity vs. wavelength.

       -- Plot lamp intensity vs. wavelength.

       --  Plot colors seen (part B) on spectrum or table.

 

Part 2. Absorption Spectrophotometry (use Diode Array Spectrometer).

 

       --  Get the absorption spectra for Cr (III) at several concentrations. Save each in a different register on the computer and save to floppy disk.

       -- Get the absorption spectra for Co (II) at several concentrations. Save each in a different register on the computer and save to floppy disk.

 

Part 3. Do Beer's Law plots

 

       --  using a spreadsheet and the data saved in Part 2 at six different wavelengths for Cr (III)

       --  using a spreadsheet and the data saved in Part 2 at six different wavelengths for Co (II)

 

Part 4. Two-component mixture of Cr & Co.

 

       --  Spectrum of mixture.

       --  Compare to graph where Co & Cr spectra are added.

       --  Solve for components of mixture by simultaneous equations. Use matrix methods on spreadsheet if possible.

 

Supplementary -- Additional 2-component mixture

 

       --  Chose Co/Ni or Dichromate/Permanganate system.

 

Hollow Cathode Lamp Spectrum

 

Now that you are familiar with the "Heath" (GCA/McPherson) scanning monochromator system, use the Hollow Cathode Lamp enclosure, a Heath Monochromator, the Photomultiplier Module, the Readout Module and a strip chart recorder to measure the spectra of the hollow cathode lamps used in the Atomic Absorption Spectroscopy (AAS) experiment. You will need to vary the following parameters to get a suitable spectra:

 

-- Wavelength region: Choose regions near the analytical lines used in AAS. Our ultimate goal will be to determine the optimum wavelength and bandwidth to set for doing AAS. The slit must be wide enough to maximize the HCL signal, but narrow enough to exclude any nearby emission peaks from the lamp. Check with the AAS group, or look at the AAS experiment above to determine which elements to find and lamps to use.

 

-- Slit width: The Heath monochromator has a dispersion figure of 0.02 nm bandwidth/micron slit width. Make your scan at as small a slit width as you can. Remember that our AAS instrument can be set to bandwidths between 0.2 nm and 2.0 nm, with most settings at the low end. Remember, the smaller the slit width the more sensitive the detector must be. See Gain settings below.

 

-- Lamp current: Use typical lamp current settings. If no specific values are given, use 20% of the maximum current on the lamp. You may need to increase this if you need increased intensity from the lamp.

 

-- Scan rate: Scan the monochromator slowly enough so you can easily resolve the peaks in the vicinity of the HCL analytical lines. Choose chart speed rates that give good hard copy.

 

-- Gain Settings: (Photomultiplier current, Readout Module Gain, and Chart Recorder Gain) The recommended voltage for the photomultiplier (PM) tube is 600 volts. Higher voltages up to 1000 volts are routinely used to increase PM sensitivity. Used the supplied chart as a guide to setting higher voltages if needed. The Readout Module has a series of gain settings, as does the chart recorder itself. Explore various combinations of gains and voltage to get good sensitivity (high signal), while keeping noise to a minimum. Noise generally increases with increased readout/chart recorder gain and increased PM voltage. Try for the best signal-to-noise ratio and be prepared to defend your choices in your lab report.

 

--Report: (A) Report your choices of all the parameters mentioned above and defend the choices you made. (B) Show the spectra, choose the best slit width to recommend for AAS, and defend your choices.