2.6: Flame Photometer

RELATED READING: Pages 5-7, 9-11, 97. See Methods in CD-ROM for Sodium and Potassium

OBJECTIVES

Upon completion of this exercise, appropriate discussion, and related reading, the student will be able to:

1. Identify the aspirator, burner, and photodetectors and explain the function of each.
2. Properly “start up” the flame photometer.
3. Dilute samples using a semi-automated diluter.
4. Calibrate the flame photometer by adjusting flow rate, Li sensitivity, Li blank, and Na/K standard.
5. Determine Na and K values on 4 unknowns with 90% accuracy.
6. Properly shut down the instrument.

PRINCIPLE

When Na and K are atomized into a flame, their valence electrons are excited to a higher energy level. As these electrons return to their normal (ground state) energy level, they emit photons of light energy. Each element emits light of a characteristic wavelength, the intensity of which is directly proportional to it’s concentration. Photodetectors connected to readout devices are used to detect the emitted light and quantitate Na and K levels. This same principle can be applied to other elements if appropriate filters and detectors are used. This type of instrument is no longer used for routine measurement of sodium and potassium, however induced plasma arc devices, which are used for measuring a wide variety of a variety of metals, use the same principles of excitation but at much higher temperatures.. This experiment is designed to demonstrate the principles of this technology. To minimize the variance of the procedure an internal standard, lithium, is used. Similarly for the induced plasma arc technique, internal standards are used to reduce the variance.

GLOSSARY

Aspirator: device that draws material, usually a liquid, up into an instrument for additional processing.

Flame ignitor: an electronic device that produces a spark to ignite the gases in a flame photometer.

Sample cups: small plastic cups used with many automated instruments, into which small volumes of sample are placed. The instruments usually aspirate sample directly from the sample cups.

MATERIALS

• Flame Photometer
• Diluter-dispenser
• Distilled water
• Na/K standard unknown samples
• Controls
• Lithium internal
• Sample cups
• Standard

DILUTION PROCEDURE

1. Run the diluter through several cycles of fill and dispense, ending with a dispense cycle,. Set the diluter for a 1:100 dilution.
2. Press down on the diluter control knob and allow the diluter to aspirate air.
3. Place a clean disposable sample cup labeled “Blank” under the diluter tip and press the control knob again. Allow the diluent to flow down the side of the sample cup to minimize splashing. Set this cup aside to be used later.
4. Put the stock Na/K standard under the diluter probe tip and press the control knob to aspirate an aliquot of standard.
5. Wipe the tip off gently.
6. Place a clean sample cup labeled “Standard” under the tip and dispense the diluted (1:100) standard into the cup.
7. Wipe the tip gently.
8. Repeat steps #3 through #6 to make a duplicate dilution of the stock standard.
9. Proceed to make duplicate dilutions of the controls and each unknown sample you were assigned.

FLAME IGNITION

1. Open propane shut-off valve located on top of the propane cylinder by turning the valve two turns counterclockwise (or as indicated on the instrument).
2. Turn on the air supply unit or open air valve fully.
3. Turn instrument switch to the ON position.
4. A faint clicking sound should be heard and should cease within a few seconds. The clicking sound is from the electronic flame ignitor. The “Flame On” indicator should then come on.
   If ignition does not take place within fifteen seconds, turn the Instrument off and repeat the ignition procedure.

ASPIRATION RATE

1. Pipet 1.0 mL of distilled water into a sample cup.
2. Put the aspirator tip Into the 1.0 mL water sample and measure the time it takes the Instrument to aspirate the 1 mL completely.
3. If this time is not 45 seconds (±5 seconds) the aspiration rate must be adjusted. Ask the laboratory instructor for assistance.
4. Observe the inside of the atomizer bowl for the formation of fine droplets.

CALIBRATION/OPERATION

1. Aspirate lithium blank (diluent) for approximately fifteen seconds.
2. Adjust the sensitivity control (Li Response) for maximum sensitivity (Maximum = center of the arrow). Note: If the sensitivity drifts significantly from this point during testing, the source of the problem must be identified and corrected.
3. Adjust the zero controls for Na and K to obtain a reading of 0.0 on each scale.
4. Aspirate diluted Na/K standard for approximately 15 seconds.
5. Adjust the balance (standard) control to achieve the Na and K values for the standard being used.
6. Check these values with those of the second dilution of the standard. If the values do not repeat (±2 Na, ±0.05 K), an additional set of dilutions should be made using the same diluter. If reproducible results are still not obtained, the diluter and flame photometer should be evaluated for malfunction.
7. Aspirate each of the dilutions to be tested and record each result on the data sheet. For any sample that does not give consistent results (Na ± 2, K ± 0.05), make additional dilutions until acceptable values are obtained.
8. After all samples have been tested, aspirate distilled water for a minimum of 1 minute to flush the aspirator and burner.

OPTIONAL ACTIVITIES

1. Under the guidance of the instructor, observe the color of the burner flame while aspirating distilled water, lithium diluent and separate solutions of sodium and potassium salts. Record your results on the data sheet.
2. To demonstrate the effect of contamination on flame photometry, place a lithium blank under the aspirator and zero the instrument. Remove the blank and “contaminate” it by touching it with your finger tip, stirring it with an applicator stick or some other method approved by the instructor. Sample the “contaminated” blank(s) and observe for changes from the zero values.
3. Re-adjust the aspiration rate so that it is approximately 55 sec. Reanalyze one of the samples for Na and K. Repeat, but after adjusting the aspiration rate to approximately 35 sec. Record Na and K values and actual aspiration rates on data sheet.

SHUT DOWN

1. Close the propane valve by turning in a clockwise direction.
2. When the “Flame On” indicator goes off, turn off the air supply, then the instrument power switch.

DATA SHEET, EXERCISE #6

NAME: ___________ DATE: ___________

RESULTS:

	Sodium (Na), mmol/L		Potassium (K), mmol/L
Lithium Blank	Dilution 1	Dilution 2	Dilution 1	Dilution 2
Na/K Standard
Normal Control
Abnormal Control
Unknown 1
Unknown 2
Unknown 3
Unknown 4

OPTIONAL ACTIVITY

1. Flame Color:

Water	________
Lithium Diluent	________
Na	________
K	________

2. Lithium blank after contamination

Na	______mmol/L
K	______mmol/L

C.

Aspiration rate (mL/min)	Na, mmol/L	K, mmol/L

Discussion Questions

1. If the Lithium diluent is changed during a run, would the instrument need to be recalibrated? Why or why not?
2. How are variances in flame “quality” corrected for in flame photometry?
3. How are variances In aspiration rate corrected for in flame photometry?
4. What is the advantage to running samples in duplicate?
5. What are two reasons, other than to obtain a Li blank, for running the diluter through a cycle before making your first dilution?
6. Why Is It important to note the formation of droplets in the atomizer bowl?