Photometry by its very description is the measurement of light.
Therefore as we live in a world surrounded by light of both natural and artificial sources, background sources of light are obviously going to interfere with the reading of Emission Spectroscopy. As the method of detection for light waves comes from photodiodes which detect specific wavelengths of light, background light emissions will push the reading of concentration of the ions detected higher than is to be expected if they are not accounted for in the reading of concentration.
The most important part of calibrating a Flame Photometer is to have a point of reference for a reading.
Without a point of reference for a specific quantity of light absorbed by the ions, the results are meaningless and are nothing more than a measure of current through a photodiode. It’s like trying to measure how tall a house is without knowing the concept of standardised lengths.
To calibrate anything, you need to know what their meaning is from a known point of comparison. The smaller the range of points and the more points of comparison there are, the higher the precision of the measurement. A good explanation of why this happens is that it is harder to measure something that is 3.5cm with a meter stick compared to measuring 3.5cm with a 10cm ruler.
When the precision of results is imperative to be as close to the true value of concentration of a solution, a ballpark measurement of what its expected concentration range is always a good starting point. From there it is easier to hone in on the measurement of its true value with a pinpointed calibration range. For example, if you had no idea what the expected concentration of sodium in a solution was, diluting it down in a 1:10 ratio then making standard calibration solutions of 0 to 1000ppm solutions in increments of 100ppm of sodium would give a ballpark figure of what the true concentration is. Diluting the solution down to below 100ppm is ideal for flame photometry as it makes the reading more precise. After diluting the sample down to this range, a calibration standard series of 0-100 in 10ppm increments would give a precise result for the true value of sodium in this solution.
If you were measuring multiple samples of this solution and one sample gave a reading of 110ppm, the precision of this result would be significantly lower than the readings of the other data in this series of samples. This feature is common to modern photometers, and whilst there isn’t a calibration range for the reading of 110ppm, the reading is taken by extrapolating the calibration curve out of its known range and then the measured values are input into the calibration curve for a new measurement of concentration. It would be more precise to make a few more calibration standards and remeasure the sample.
One of the most important parts of calibration however comes down to what you are using to calibrate your flame photometer with. Going back to the length analogy, imagine trying to measure a 96cm object with a meter stick, but the meter stick is only 90cm. Therefore the utmost care should be taken in both the dilution of your samples, as you need to apply a dilution factor to make the sample back up to its true value, as well as accurately diluting or weighing your standards.
If you already own or are interested in owning a professional flame photometer from BWB, we offer 24/7 support to help with calibration.