Embedded Files
PROCEDURAL VIDEOS TO WATCH:
PROCEDURAL VIDEOS TO WATCH:
REQUIRED CHEMICALS (FOUND ON SHELF UNLESS OTHERWISE NOTED):
REQUIRED CHEMICALS (FOUND ON SHELF UNLESS OTHERWISE NOTED):
REQUIRED VOLUMETRIC GLASSWARE:
REQUIRED VOLUMETRIC GLASSWARE:
250-mL volumetric flask (1)
100-mL volumetric flasks (4)
50-mL volumetric flasks (5)
25-mL glass pipet (1)
15-mL glass pipet (1)
10-mL glass pipet (1)
5-mL glass pipets (2)
2-mL glass pipet (1)
SPECIAL EQUIPMENT (FOUND ON SHELF UNLESS OTHERWISE NOTED):
SPECIAL EQUIPMENT (FOUND ON SHELF UNLESS OTHERWISE NOTED):
MARSXpress vessels for microwave digestion (4; on the MARS6 cart)
Carrying rack for Xpress vessels (on the MARS6 cart)
Mortar and pestle
PROCEDURE:
PROCEDURE:
A. Preparation of Cereal Samples (NOTE: The MARS 6 microwave oven requires a minimum of eight samples for a single digestion run, but can hold any even number of samples up to 24. You will only be preparing four separate cereal samples for analysis. So you must either load your samples with those of one or more other groups, or prepare your steel samples for the ICP-AES lab at the same time as your cereal samples since the cereal and steel samples require the same digestion procedure. You can store the digested steel samples until you are ready to complete that experiment at a later date.)
The cereal samples will be digested in Teflon-lined MARSXpress vessels. The components of these vessels are shown in the figure below. Obtain four clean, dry Xpress vessels, stoppers, and caps, along with a tightening tool and a carrying rack.
2. Use a mortar and pestle to crush ~3 g of cereal.
3. Use an analytical balance to accurately weigh ~0.5 g of crushed cereal into an Xpress vessel using a plastic weighing funnel (found in the container above the balances) and the "weighing by difference" method shown in this video. Record the mass of cereal and repeat for three more cereal samples - each in a separate Xpress vessel. Place the vessels in the carrying rack, using the markings on the carrying rack to keep track of the vessels. (Don't write on the Xpress vessels themselves.)
4. Take the carrying rack to a fume hood. In the hood, use a 10-mL graduated cylinder to carefully add ~10 mL of concentrated HNO3 (trace-metal grade) into each of the Xpress vessels. Make sure that all of the cereal pieces are in the acid at the bottom of the vessels since any pieces stuck to the sides may "superheat" and cause permanent damage to the vessel.
5. For each vessel, insert a tapered stopper (with the tapered end down so that it fits securely), screw on the cap, and tighten using the tightening tool. Make sure the tool is seeded all the way down on the cap, then turn the tool until you hear the click. This ensures that the cap is sealed to the correct tightness. (VIDEO) Place each vessel back in its original location in the carrying rack.
6. Take the carrying rack with all four samples to the MARS6.
7. Open the oven door and remove the turntable. Place each of the vessels into the turntable using the proper configuration. Use the diagram below to load your samples. Make sure all vessels are placed into the Kevlar tubes and that each vessel is pushed all the way down. (VIDEO)
8. Turn on the power to the MARS 6. (The power switch is located on the lower right-hand side of the instrument toward the rear.) It will take 1-2 minutes to boot up. Make sure the vent tube from the back of the MARS 6 is secured under the glass sash of the fume hood and that the fume hood is turned on and running. (VIDEO)
9. Carefully lift the turntable and place it inside the MARS 6 in the proper orientation (with the Xpress logo facing you). The turntable should be locked in place and not able to rotate freely. Close the door and make sure the handle is securely latched. (VIDEO)
10. When the instrument is ready, the display screen will give you two options. Choose the "One-Touch" button. Scroll down the list of methods and find the method called Chem 330. To look at information about the method, hold your finger on the icon for a few seconds. A bar will appear in the upper right-hand corner. Press the lower case "i" to display information.
11. To run the method, simply touch the Chem 330 icon. To start the digestion, press the "Start" button in the lower right-hand corner of the screen. While your samples are running, you can monitor the temperature and time (and other parameters) by clicking on the different graphs on the display screen. The entire process will take ~45 minutes to complete.
12. While your samples are running, begin parts B and C below. (You'll need the solution from part B to dilute your samples when the digestion is complete.)
13. When the digestion is complete, open the microwave door. Remover the turntable and place it on the lab bench. Making sure to grab the individual Xpress vessels by the top, place each of them back in the carrier in their original location and take them back to the analytical lab.
14. Take the vessels to the hood and put on gloves. Bring a squirt bottle of ultrapure water and a beaker of ultrapure water to the hood as well. The contents of the vessels are under high pressure, so open each vessel by pointing it away from you INTO THE HOOD and using the tightening tool to remove the cap. You should hear the depressurization as you open each vessel. Quantitatively transfer the contents of each vessel into separate (labeled 1-4) 100-mL volumetric flasks, dilute each to the mark with ultrapure water, and mix well. You'll probably see some solution on the stoppers, so you'll want to use tongs to rinse them into the flask. (WATCH THIS VIDEO!! Note the video doesn't show the flasks being mixed, but be sure to invert the flask 20-30 times to mix the solution.)
15. Wash the Xpress vessels with soap and water, rinse 2-3 times with DI water, and leave them to dry.
Sample Prep Summary
B. Preparation of of 2% HNO3 Solution
Obtain a 1-L plastic bottle and cap from your lab drawer and label the bottle "2% HNO3". Fill it approximately 3/4 full with ultrapure water.
In the hood, use a graduated cylinder to carefully measure 20 mL of trace-metal grade concentrated nitric acid (HNO3). Slowly add this to the water in the plastic bottle, and swirl gently to mix.
Fill the bottle to the fill line with ultrapure water as shown in the figure below. Screw the cap on tight, and shake to mix. You will use this solution for many of the sample dilutions outlined below.
C. Preparation of Standard Solutions
Using a clean glass pipet, pipet 5 mL of the 1000 µg/mL Fe standard solution into a clean 250-mL volumetric flask and dilute to the mark with the 2% HNO3 you created in part B. This is your standard stock solution.
Obtain five clean 50-mL volumetric flasks and label them 1-5. Pipet 2 mL, 5 mL, 10 mL, 15 mL, and 25 mL of standard stock solution into flasks 1-5, respectively. Dilute to the mark with the 2% HNO3 created in part B and invert 20-30 times to mix well. These are now your standard solutions.
You are now ready to perform the AA analysis, but it is important that you first calculate the Fe concentration (in mg/L) in each of your five standard solutions since you'll need to enter these values in the instrument software. Show these calculated values to me or a TA before continuing.
Standard Prep Summary
D. Instrument Preparation
1. Bring all nine of your volumetric flasks (your five standards plus your four sample solutions) to the AA instrument. You’ll also need to bring the remainder of your 2% HNO3 solution (in a 250-mL Erlenmeyer flask) to use as the blank. Bring a second 250-mL Erlenmeyer flask filled with 200 mL of RO water to use as a rinse solution. (Be sure to label the flasks so you don’t mix them up.)
4. Make sure the instrument is turned on. The green LED in the lower left-hand corner should be lit.
5. It is important that both the lamp and burner be aligned for maximum signal sensitivity. In the interest of time, however, either I or a TA will have already taken care of this step ahead of time.
E. Setting up the Software
1. Log on to the instrument with USERNAME: CHM4498\chemstu and PASSWORD: C12H22O11 (caps are important).
2. Open the SpectrAA software by clicking on the desktop icon.
3. Click the Worksheet icon to begin creating your method. Select New. In the dialog box that appears (see picture below), specify a filename, your name (Analyst), and any comments you wish to add. You can also specify that you are only running four samples. Click Ok.
4. When the worksheet opens up, you’ll notice four tabs: Filing, Develop, Labels, and Analysis. You’ll want to begin developing your method under the Develop tab. Click the Add Methods button. Select Fe from the list of elements, as well as Cookbook and Flame (see figure below), then click Ok. This will load a standard flame method for iron that we can edit for our particular analysis.
5. Now we are ready to edit our method, so click Edit Methods from the Develop tab. This will open the window below. From the Type/Mode tab, select the Manual sampling mode (no autosampler) and the Absorbance instrument mode. Also make sure the Air/Acetylene flame type is selected. (The gas flow rates are only important if your instrument has automatic gas control, which ours does not.)
6. Under the Measurement tab and make sure the selections are consistent with the screen shot below.
7. Click on the Optical tab. Make sure the Lamp Position 1 is selected and that the Lamp Current is set to 10.0 mA. (Double check the recommended lamp current printed on the side of the lamp, and make sure the lamp selection lever is pointed toward lamp 1. The door to the lamps opens by simply pulling it open from the bottom.) The monochromator settings should be the default settings for Fe from the cookbook method file, so you shouldn’t have to make any changes.
8. Skip the SIPS tab and go to the Standards tab. Here’s where you need the concentrations of the standards. Fill in the concentration values of your five standard solutions (in mg/L, as indicated). Also, the Lower Valid Concentration should be the concentration of the least concentrated standard, and the Upper Valid Concentration should be the concentration of the most concentrated standard. Sample measurements calculated beyond these limits will be reported as out-of-range.
9. We won’t make any changes to the Calibration or QC Tests tabs. The Sampler tab is only relevant if we’re using the autosampler. You may want to make some notes under the Notes tab. (Your unknown number would be a good thing to put in here.) Do check out the Cookbook tab: the Notes will give you some useful information regarding the method, and the Graph will show you what you can expect your calibration curve to look like.
10. Click Ok to save the method.
11, Now we need to make some changes to our sequence. This will allow us to have the software do some things for us automatically. From the Develop tab, click the Edit Sequence Parameters button.
12. We will only focus on the Options and Reports tabs. Follow the images below. The important points you will need to check and possibly change are circled on the images below. The Reports feature is especially nice. A complete report will be printed at the end of the run with all your calibration and sample measurement results.
13. Click Ok to save the sequence changes.
14. Look at the top of the software screen. Click on the Options tab, and at the very bottom there should be a ‘Select Printer…’ option. Click on this and make sure that the Microsoft Print to PDF printer is selected. If not, select it and press Ok.
15. Click the Labels tab. Only four rows should be visible. (This can be changed with the Total Rows button if need be.) Enter a name for your sample in the Sample Labels column.
F. Acquiring the Data
1. Click the Analysis tab. Then click the Flame Facilities button in the upper toolbar to open the Flame Facilities window.
2. Make sure the door to the lamp compartment on the instrument is closed, then click the Turn on Lamps button. You should see the lamp on the instrument light up through the tinted window. You can now close the Flame Facilities window.
3. Locate the Ignition button (shown below) on the front of the instrument.
4. Making sure once again that the exhaust fan is on and that the flame door in front of the AA burner is closed, push and hold the Ignition button until the flame is lit. (This should only take 2-3 seconds.) If you open the flame door, you should see a flame like that in the image below. Leave the flame on for at least 5 min before you begin making measurements.
5. Place the sample tube in your flask of RO water for ~10 s to rinse out the burner. (NOTE: Be very careful when handling the sample tube. It crimps very easily and this will prevent sample from being sucked into the burner. Try to hold the tube stationary when introducing and removing the sample flask.) (VIDEO)
6. Fill a 10-mL graduated cylinder with RO water and place the sample tube in the cylinder. Measure the rate at which the water is sucked into the burner. (This is the aspiration rate.) It should be approximately 4 mL/min. Let me or a TA know if the rate is significantly less.
7. Begin the data acquisition by clicking the Start button in the program. When the Analyst Checklist appears, click Ok.
8. Place the sample tube in your 2% HNO3 solution and click Ok to zero the instrument.
9. Cycle through each of the standards and samples as directed by the software, clicking Read after the sample tube has been placed in the appropriate solution. Use a paper towel to gently wipe any excess solution off of the sample tube before putting it into a new solution. (Be careful not to pull on the sample tube, however.) After reading each cereal sample, rinse with ~30 s of RO water to prevent solids from plugging the sample tube.
10. When all the data has been collected, you will be prompted for a location to save your report (which will be a pdf file). Choose a location that will allow you to then easily upload the file to the Data page in your LabArchives notebook. (Use a flash drive to do this.)
G. Shutting Down the Instrument
Click the Flame Facilities button, followed by the Turn Off Lamps button. You should see the lamp go out.
Rinse the system with water for ~1 min, then remove the water flask and let air be drawn though the system for ~30 s. You may now stop the flame by hitting the Flame Stop button on the front of the instrument.
3. Show me your data before closing the software.
4. Close the SpectrAA software.
H. Waste Disposal / Clean Up
Collect all unused solutions, including leftover Fe standard, in a single container (I recommend a large Erlenmeyer flask). Put the container on a stir plate and drop a magnetic stir bar in the solution. Add ~2 mL of universal indicator and begin stirring, then neutralize the solution with sodium carbonate or sodium hydroxide (found in the hood) until the color is green or yellow. Pour the solution down the drain to dispose.
Rinse the test tube that held the Fe standard solution put it in the empty sample container bin in the hood.
Dump any leftover cereal in the trash, take the label off the container and put the empty container in the empty sample container bin in the hood.
Place all used pipets in the "Pipets to Be Washed" bin.
Rinse all volumetric flasks 5-6 times with tap water and then 2-3 times with RO water, and return to the shelf. (Use acetone to wash off any markings.) Similarly rinse all glassware from your drawer and return it to your drawer.
Use a sponge to wipe up your bench area.
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