Introduction to Graphite Furnace Atomic Absorption Spectrometry.

Introduction

You have already seen how atomic absorption can provide low level measurements of elements using a flame atomization source. Replacing the flame with a electrically heated carbon tube increases the concentration of atoms in the light path which in turn allows much lower detection limits. Some disadvantages of the technique are the long analysis times and the number of variables that must be optimized. Due to time limitations, we will use a cookbook method to determine lead concentrations in a variety of aqueous samples.
 
 

Safety Precautions

The chemicals used in this experiment pose little hazard provided that routine laboratory precautions are taken to avoid ingestion and skin contact.
 
 

Procedure

1. Preparation of Solutions

(Note: we are dealing with very low concentrations of a ‘sticky’ element, therefore thorough cleaning and careful preparation of solutions is essential).

i. Calibration Standard. The use of an autosampler allows standards to be made from a simple master standard. This certainly saves a lot of time, however, if the master standard is incorrectly prepared all of the results will be invalid. Prepare a master solution of 50 ppb lead from the 1,000 ppm stock standard and deionised water.

ii. Modifier. Due to the volatile nature of lead, a matrix modifier is required. The cookbook method calls for 50 ?g of ammonium nitrate per firing. The autosampler will add 5?L of matrix modifier per firing. Calculate the concentration of modifier that you will need and prepare this solution in deionized water.

iii. Samples. All samples will be provided.
 
 
 
 
 
 

1. Instrument Setup and Optimization

i. Switch on Instrument and Gases. Open the argon gas cylinder to 15psi (this should already be set so just open the main cylinder valve). Switch on the main part of the spectrometer, using the switch at the back, and the power supply for the graphite furnace (smaller box to the right). Switch on the water chiller by pressing the i/o button. Ensure that DI water is in the wash bottle and that the waste bottle is not full. Open the Solaar software and click on the wizard button.

(Take a note of gas cylinder pressure at the beginning and end of your experiment and notify the instructor if it is low)
  1. Optimization. Open the ‘analysis wizard’ and follow the directions given. Make sure the method ‘chem352Pb’ is loaded and hit ‘next’. Hit ‘optical setup’ and ‘aurozero’ and proceed to the next page. You can assume that the furnace position is already optimized. Click on the ‘GFTV’ button and select ‘analysis’. This will open a program that allows you to view the inside of the graphite cuvet. Now we can ‘align probe’ and view the probe position in the GFTV screen. If the probe is correctly aligned it should look like:

If the probe does not enter the cuvet you will need to adjust the autosampler position using the control on the top and right side of the autosampler unit until the probe reliably enters the cuvet. The depth of the probe can be adjusted by a control on top of the probe itself. To clean the system perform the following: ‘park autosampler’, ‘wash autosampler’, ‘purge autosampler’ and finally ‘clean cuvet’ which will heat the furnace to burn off any contaminants. On the next screen select the ‘ASLG’ button and load the autosampler tray as indicated.
 
 

2. Performing the analysis

Once the optimization is complete, you can start the analysis. The instrument is totally automated so you don’t need to watch it, however, keep an eye on the results periodically so that problems can be identified and corrected as early as possible. 3. Shut Down and Print Out Switch off the spectrometer, power supply and water chiller. Close of the argon cylinder by the main valve. Your results can be printed by selecting ‘options’ under the ‘results’ pull down menu and making sure that your results only are in the right-hand column. Under the file menu select ‘print options’ and check boxes for ‘large font’, ‘method’, ‘description’, ‘resample results’, and ‘calibration graph’only. You can now print your results. 4. Report In your report make sure to answer the following questions: