Why the same sample gives different results
In Flame Photometry a stable flame is vitally important. The open flame of a lighted candle will flicker wildly if someone walks by, but can change even if someone opens a door on the opposite side of the room.
This is why the flame is carefully shielded within the device—to avoid this obvious source of interference. More importantly, the burning gas is under fixed and carefully controlled pressure to maintain flame stability at the burner head.
Even so, environmental factors can alter results. Consider airborne oxides, such as CO2, NO2, or Sulphur Dioxide, whose presence can affect flame temperature and how brightly the flame burns. These pollutants will dull the flame because the oxygen is bound too tightly to react, reducing the locally available oxygen. Indeed even airborne water (humidity) can change your results, as could airborne CH4.
Controlling Influences
Ideally we would build our laboratories far away from highways, factories, and farms but since that isn’t generally practical there is a much easier way. We have to be vigilant about our calibration curves.
Calibration curves take all these factors into account, and create a baseline for your location. It is, however, not a “once-and-done” setting. The calibration ought to be repeated several times in a long session to compensate for changing humidity, temperature, etc., or each time you change the materials you are testing for.
How to Calibrate
The emitted light from the flame is read by photodiodes as each species is being aspirated into the flame. Using previously prepared molar solutions of, for example, 10ppm, 20ppm, or 50ppm, you can calibrate your flame photometer for the expected range of your test substances. Equally important for calibration purposes is de-ionised (DI) water so you can set your absolute zero mark (or Blank).
First, however you must allow the instrument to warm up according to the manufacturer’s instructions. Instrument warm up time will vary from brand to brand but for BWB instruments we suggest 20 minutes, all the while ensuring that you are aspirating DI water to keep that burner at a steady temperature.
Calibration processes will vary per manufacturer so we’ll concentrate our example on the BWB range of simple to use instruments.
Select calibrations from the main menu and select the elements you wish to calibrate, you can select just the one element or all of them.
Now select how many calibration points you wish to generate your curve from. Remember that the Blank is excluded, most of our instruments offer up to 10 points of permissible calibration, in this example we’ll work with a 3 point calibration.
Once the calibration configuration has been completed the machine will prompt you to aspirate the Blank calibration standard, in most cases this is the zero and is therefore de-ionised water. Ensure the system is aspirating this from a sample cup and press accept. The instrument will automatically monitor and store a stable reading and prompt you for the next point.
Time to begin calibrating your curve, start with calibration standard 1, ensuring that the concentrations throughout your calibration standards increase in concentration, so point 1 needs to contain the lowest concentrations, here we’re going to use 10ppm. Aspirate the calibration standard and press accept. Again the instrument will monitor the reading for a stable result and store this to begin generating your curve.
The instrument will prompt you for calibration standard 2, swap over your sample cups and ensure that you are aspirating the correct standard before pressing accept.
Lastly you’ll be asked to aspirate calibration standard 3, again ensure the correct sample is aspirating and press accept.
Upon completion the machine will revert to the main menu whereby you can view your calibration data or get on with running samples.
You are now properly calibrated and may begin testing your samples. Don’t forget to sample DI water between each sample (for a few seconds) to make sure a prior sample doesn’t interfere with the next one.
Time to get to work!
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