Real-time Spectral Plot Diagnostics

The GC-AED ChemStation Real-Time Spectral Plot can be used to diagnose leaks, bad gases, column bleed, contamination and other potential AED system problems using the steps described below. This information is taken from Diablo Analytical's GC-AED Training Course.

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Configure the Real Time Spectral Plot

Switch to the ChemStation "Method and Run Control" View.
Make sure that the AED Real-Time Spectral Plot is displayed. If it is not displayed, select "AED Spectral Plot on/off" from the "View" menu.
Make sure that the "Microwave Power" is on, the AED Plasma is lit, and a spectrum is displayed in the window.
Under the "Valves" section of the Real-Time Spectral Plot window, turn off all valves except "O2", as shown above.
Under the "Spectrometer Position" section, enter "181" into the "nm" text box as shown above, and then press the "Go To" button below it.
Wait for the spectrometer to move to 181 nm (this can take a few minutes). When the spectrometer arrives at 181 nm, a spectrum similar to the one shown above should be displayed. You may need to "Zero" the spectrum, adjust the attenuation ("Attn"), and spectral display width ("nm") using the controls in the upper left corner of the display.

Evaluate the Real-Time Spectral Plot

Find the Nitrogen 174 peak, the broad Oxygen molecular band around 185 nm, and the Carbon 193 peak.
The following peak ratios can be used to evaluate the current status of your instrument:

The N174 Peak Height should be 1/2 or less than the height of the O2 Band

The C193 Peak Height should be 2x or less than the height of the O2 Band

Diagnose any Problems

If the N174 Peak is higher than 1/2 the height of the O2 band, this is usually a sign of a leak in your gas delivery system, in the AED, or in the GC (or a bad cylinder of gas). Try setting the GC column head pressure to 0. The N174 peak height should decrease if the leak is coming from the GC. Try changing gas cylinders to make sure that you don't have a bad cylinder of Helium.
If the C193 Peak is high compared to the O2 band (and the N174 Peak is OK), this is usually a sign of column bleed, or hydrocarbon contamination. Try setting the GC column head pressure to 0. The C193 peak height should decrease if the carbon source is from the GC column.