This follows on more from previous posts and hopefully gives some practical answers to the calibration question.
Calibration is a tricky area. A lot depends on the method, the instrumentation used, the conditions, the analytes being detected etc
XRF is a relatively easy system to calibrate. As discussed earlier, it uses fluorescence, so as long as you know how much energy goes into the sample matrix, you always get the same amount out depending on the atom the X Ray hits. If you have a good excess of X Ray energy, the calibrations performed in the factory remain constant. Using a sample containing a known concentration of metals as a field reference is a good idea in theory. The problem here is that it is difficult to get certified reference samples that remain stable . The problem is that even though metals are usually non volatile and insoluble, they do slowly get lost from the reference, especially if they are supplied as a powder (the most common form) I would like to see metals of interest at appropriate concentrations embedded in polyethylene. This would be a much more stable reference.
As the reference sample degrades, so the user may conclude the analyser is not working that well. If XRF were to become more widely used, the reference sample producers would look into making more practical and stable reference samples. Personally, I am now happy to rely on a simple calibrator that confirms the XRF can get the correct concentration value for a single metal such as the nickel content in a certified stainless steel. If the nickel value is correct, it is 95% certain the others are too.
As has been noted. XRF really is very accurate and repeatable and you can get a lot of analysis done in a fairly short time period.
For organics, life becomes much more complex. For anything that is affected by temperature or timing, the calibration must be carried out at the same time. This includes all colourimetric, turbidometric and immunoassay bases systems. Portable GC methods are also affected by temperature and timing and require quite complex calibration procedures. The next challenge is selecting a calibrator that is appropriate for what is being looked for. For TPH, the differences in aliphatic/aromatic ratios, the ratio of C10 to C20 hydrocarbons etc as the hydrocarbon degrades makes a universal calibrator difficult to select. In many cases the dynamic range of the method is such that a calibration curve is just that, a curve. This means a minimum of 3 calibration solutions must be used, preferably 5. Diluting standards to create calibration curves is hard enough in the lab and experience shows almost impossible in the field. Other organics such as chlorinated solvents can be easier because you are looking for a single compound, but on some sites there are mixtures of different solvents and degradation also occurs with the various breakdown products have different response factors. All of these methods have some form of chemical reaction (or physical chemistry interactions) going on. This means the operator must be consistent in their timings to ensure the calibrator is relevant to the sample analysis.
Infra red systems, especially Fourier Transform methods also have stable calibrations, but unfortunately are not that good at identifying the type of hydrocarbon present. This makes selecting the correct calibrator difficult, which is why infra red, even from labs, fell out of use.
This is why fluorescence for TPH/PAH is a better choice. It is significantly less affected by temperature and time and can identify the hydrocarbon type, so can select the most appropriate calibrator. Just like XRF. As no chemical reactions are occurring, it does not matter how consistent the operator is when performing the analysis. The master calibrations carried out by the manufacturer remain stable just like the calibrations in XRF. Selecting a single surrogate calibrator to measure the energy output is therefore much easier. The biggest problem is ensuring the reference calibrator is the correct concentration and has not been contaminated. Fluorescence can once again help here because it is possible to cross reference several parameters when running the reference calibrator to ensure it matches the initial manufacturer derived parameters. Analysers such as the QED use extensive artificial intelligence algorithms to continuously check the analyser, field calibrator and sample performance to ensure everything is working correctly. The best analysers have a facility to compare the calibration carried out at the start of a sample batch to the same calibration at the end of the batch. Provided the calibrator solution matches the required proof that it is the correct concentration, is the correct material and is contamination free, a variance of less than +/-15% between the first and final calibration check should provide a good level of confidence that the analyser has remained stable (the effect on the results of a 15% variation is actually only around 5% for the reported concentration)
There are test kits for inorganic compounds such as cyanide, Chrome 6, chloride, ammonia etc. These are usually colourimetric, but rely on an end point where a final coloured product is produced. Once the analyte of interest has been consumed to make the coloured product, it does not matter about time or temperature. In these cases, calibration is carried out by comparing the colour intensity or absorbtion at a set wavelength to find the concentration. Once again it is a factory calibration curve that is set up and stored in the reader.
I all of these cases, the manufacturer strives to ensure any reference materials they use come from traceable and certified reference materials and have sufficient evidence to back up the procedures used to construct the calibrations.
Having a good verification scheme in place would allow manufacturers to demonstrate the stability of their system and give users confidence that the data is of a suitably high quality. I hope it will happen
As to the question, how does NASA calibrate their instruments. Just as I have outlined above. For UV fluorescence they have a solid fluorescent block with a known concentration of compound in. They were looking at a form of calcite, which has a natural and very stable fluorescence
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