Operating the GBC 902
Atomic Absorption Spectrometer

Part 3: Analyzing the Samples

The first sample we set the instrument up for is the BLANK. This can either be distilled water, or the background solvent for our remaining samples.
A noise-reduction method that integrates the signal over time will be shown in the next several steps. Do not confuse this Integration with the Peak Area button shown at the left of the graphic. Peak readings are used for certain non-flame methods where a small sample is burned in a special furnace. When using a flame, follow the directions below.
The flame and hollow cathode lamp have a lot of flicker, causing the reading of the light passing through the flame to vary rapidly with time. To smooth out these variations the instrument samples the light every 0.02 seconds, then gives us an average reading over a time we can specify. To specify this Integration Time, begin by pressing the INT TIME button.
A value of 5 seconds is shown here by pressing "5" on the numeric keypad, then pressing READ. Each time you press READ, the instrument will add up all the readings in this interval, then present you with the average value.
If the Single Int lamp is lit, the instrument will give you a reading only if you press the READ button. If the Single Int lamp is not lit, the instrument will give you a reading every 5 seconds without pressing the READ button each time. Press the Single Int button to toggle the setting.
Press READ while aspirating the blank. The instrument will probably not read zero absorbance. To set it to zero absorbance, press ZERO. After the five seconds of integration have elapsed, the instrument's on-board computer will adjust the reading to 0.000. You are now ready to read your samples.
You can aspirate your samples directly from the volumetric flasks that contain your standards and unknowns. Remember to run your samples and unknowns all a the same time to maintain constant instrument performance for all your samples. Let the sample aspirate for a few seconds before reading it. It is a good idea to recheck the zero setting with your blank every few samples and at the end of your analysis.
If you are running in Single Integrate mode, press the READ button to start the Integration period.
At the end of the integration period the absorbance will appear. Be sure you have enough significant figures for good statistics. For example, if your middle standard shows only 0.003, you need to make stronger standards. Absorbances greater than 1.000 mean you need to dilute your standards. If you are just learning how to use atomic absorption, repeat this integration reading two more times, recording the absorbances in your laboratory notebook. From the variations observed in this number you can estimate the error in your reading. If later readings are all different from the first one, it means you didn't wait long enough after the start of aspiration to wash the previous sample out of the nebulizer. This is known as the Memory Effect.
Repeat this sequence of readings for all your standards and for your unknown. It is quite common for the working curve in Atomic Absorption to have some curvature. You may need a second order fit. (Or you can draw the line in by hand.) Don't force a straight line through these values if they have an obvious curve.
Here the curve is shown correctly fitted with a second order polynomial using the Excel 5.0 Trendline... feature. By the way, the values shown here are somewhat exaggerated to show the curvature clearly. Real samples will be curved, but probably not this much.
Here we show an improper use of a Linear Least Squares Line to fit these data. Note the error in concentration that results.
When you are through running all of your samples make sure to aspirate plenty of distilled water through the nebulizer to remove all traces of your samples from the instrument before shutting it down.
To turn the flame off, just turn off the FUEL by turning the FUEL valve clockwise until it seats. The flame will go out quietly. You can then turn off the Air using the oxidizer master control as shown. Go to Part 4 to learn how to completely shut down the instrument if you are the last one to use it for your laboratory section.