Troubleshooting Calcium (240FS, nitrous oxide)

Any tips on analysing calcium on a 240FS with a nitrous oxide/acetylene flame?

I set up our new nitrous burner and have been exploring calcium at 239.9nm. First time, the signal looked awful (0.1-0.2 absorbance units of noise bouncing around), instrument wouldn't calibrate, and the gain was spiked in the 90+% range. The next day, I did a little cleaning of the burner head (minimal), made some very minor optimization changes (burner, lamps, signal), and the signal was beautiful with gain in the 35-40% range. Then I stepped away for a week or so…

The past couple days I've been looking at higher concentration calcium samples at 239.9nm, and I'm back to a noisy signal with 90+% gains and poor calibration. I have added 2200ppm potassium to try and improve the calcium signal in the most recent samples.

I'm planning to take everything apart and give it a good cleaning in the morning, but if anyone else has tips I'd appreciate any input. I am able to get the absorbance readings from the calcium standard, and although it isn't the prettiest calibration curve, it at least looks like it's trying to be linear... the noisy signal is just throwing it off.

Thanks again. That one magical day with pristine data got my hopes up.

  • Hi bodwinjj,

    Based on the above, I would first focus on the hollow cathode lamp signal. Before you turn on the flame, is the gain >90%? A few things to look out for - make sure the light is not clipping the burner head, remember to rescale and continue optimizing if the signal goes above scale, while off make sure the lamp is correctly and securely fit into to the socket, and that the socket isn't loose or completely out of alignment. It's also possible the Ca HCL has aged and needs to be replaced

    Below is a helpful video produced by Agilent which goes through the lamp optimization process. Have a great day

    Atomic Absorption (AAS) Trouble Shooting and Maintenance Part 1: Hollow Cathode Lamps - YouTube

  • Thanks for the help. An update...

    1. The light is not clipping the burner head. I've aligned it with the card multiple times.
    2. The lamp is secure in the socket.
    3. When optimizing/aligning the lamp (flame off), the gain is 93-94%. I optimize to max, rescale, continue to optimize up, rescaling as needed, until I hit a max and start to drop, then back up to the max
    4. The lamp (serial # starts with "20", it's the original lamp from our install) only has ~2000mAh on the counter in the software, so it shouldn't be used up but...
    5. I ordered a new Ca/Mg HCL, installed it, reset the counter, aligned, optimized, and the gain is still at 93-94%. 
    6. When optimized, the signal for Ca is very noisy. On the optimization screen, it will jump around over a range of 0.1 Abs unit or more. Despite the questionable optimization, I went ahead and did a calibration and ran a sample. The signal is shown below and it's wildly noisy.
    7. Mg (the same lamp) behaves much better, although the gain at optimization is still 72% (seems a bit high...). The Mg signal is much more stable (only bounces around ~0.01 Abs unit).
    8. Cu & Zn both optimize well, with gains 52-56%. (That also seems a little higher than I might expect...)


    At this point, I'm not sure what to try next. I feel like I've checked off all the more obvious user errors, and swapping lamps had no effect, so that doesn't seem like it should be the problem. I've optimized signals both above and below the Ca detector wavelength, so there shouldn't be a limit problem with the monochromator...

    Thanks again for any help. I'm still hoping that I'm missing some relatively simple user error, those are often hard to find but easy to fix...

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