Can someone explain the difference or how to tell between internal/external standard, standard addition and how it relates calibration standards?
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.
Certainly! The concepts of internal/external standard, standard addition, and calibration standards are essential in analytical chemistry, particularly in quantitative analysis. Let's break down each concept and see how they differ from one another and their relationship to calibration standards.
1. Internal/External Standard:
- Internal Standard: An internal standard is a known amount of a substance that is added to both the standard solutions and the sample. It helps to compensate for any variations that may occur during sample preparation, analysis, or instrument response. The internal standard should ideally be chemically and physically similar to the analyte but differ significantly in mass or structure. By comparing the signal of the internal standard to that of the analyte, accurate quantitative measurements can be obtained.
- External Standard: In contrast, an external standard involves preparing a separate set of standard solutions with known concentrations of the analyte but without the addition of any internal standard. The signal responses from these external standard solutions are used to construct a calibration curve or calibration graph, which relates the concentration of the analyte to the measured signal response. The concentration of the analyte in the sample can then be determined by comparing its signal response to the calibration curve.
2. Standard Addition:
- Standard Addition: Standard addition is a technique used for determining the concentration of an analyte in a sample that contains unknown or interfering substances. Instead of preparing a range of standard solutions, a set amount of standard solution is added step-wise to different aliquots of the sample. After each addition, the sample is measured, and the resulting signal response is recorded. By comparing the increase in signal response with the added standard concentration, the concentration of the analyte in the original sample can be calculated.
3. Calibration Standards:
- Calibration Standards: Calibration standards are solutions with known concentrations of the analyte. These standards are used to establish a relationship between the measured signals and the concentrations of the analyte. The calibration standards can be prepared by diluting a stock solution of the analyte or by weighing specific amounts of the analyte and dissolving them in a solvent. The number of calibration standards and their concentrations depend on the desired analytical range and accuracy.
In summary, the key differences between internal/external standard, standard addition, and calibration standards are as follows:
- Internal/External standard: Internal standard is added to both standards and samples, while external standard is prepared without an internal standard and used to construct a calibration curve.
- Standard Addition: A technique where a known amount of standard solution is added to a sample to determine the analyte's concentration.
- Calibration Standards: Solutions with known analyte concentrations used to establish a relationship between the measured signals and concentrations.
Please note that these techniques may vary slightly depending on the specific analytical method or instrument used. Always refer to specific protocols or procedures provided by your analytical instrument or method to ensure accurate and precise results.