Can someone explain the difference or how to tell between internal/external standard, standard addition and how it relates calibration standards?
2 answers
http://en.wikipedia.org/wiki/Standard_addition
Suppose we determine metal, M, in a sample by color at a specific wavelength. If we make standards and graph absorbance vs concn, that is the calibration curve. The only standards are the one we prepare to make the calibration curve. Some call that external standards but I don't think that is the spirit of what an external standard is. An external standard is some material of known concn added to a SEPARATE sample and analyzed separately. How that sample behaves with regard to absorbance will tell you, you hope, how to adjust the concn you read from the calibration curve for the actual sample. An internal standard is some material which is added to all standards and samples alike, in known concn, and the absorbance of both the sample "peak" at the appropriate wavelength and the internal standard "peak" at the appropriate wavelength is read. Then instead of graphing the A vs concn, one graphs the ratio of Asample/Ainternal std vs concn. The idea behind this is that if some outside factor affects the A of the sample that it will affect the absorbance of the internal to the same degree so that the ratio Asample/A i.s. will not be affected. Internal standards are very effective in setting up calibration curves for flame photometry, emission spectroscopy, fluorescent spectroscopy, x-ray spectroscopy, and chromatography. External standards are used in chromatography. You can google internal standards and get a broader picture of this concept. Personally, I have used internal standards in flame photometry, emission spectroscopy, and x-ray spectroscopy. IN emission spectroscopy, for example, the precision for an elemental analysis for copper in the 1 nanogram range was quite horrible with no internal standard but could be performed with about 5% precision on a routine basis with an internal standard. That may sound like poor analytical technique; however, at the +/-10% level, that is 0.0000000009 g to 0.0000000011 g for a sample in which the amount present is 0.000000001 g.