NIST Traceable Reference Standards for Spectroscopy
Hey, everybody! It’s FireflySci’s second in command here, Gordon. I’m happy to make my debut article about a topic I deal with frequently when it comes to talking with customers interested in our spectroscopy reference standards. NIST Traceability!
The most common question that we hear about standards is “are these NIST traceable?” or “can you claim traceability to NIST.” Interestingly enough, many of the customers themselves aren’t 100% sure about what this entails. Mostly, they are just told that this is a requirement from their Quality Control department and it’s left at that. However, at FireflySci, one of our missions is to provide the maximum amount of information to our customers so that they can make the best, most informed decisions possible.
Let’s quickly discuss NIST and traceability. Firstly, NIST is the US government bureau tasked with providing quality control benchmarks for industrial processes, experimental control samples and so forth. Essentially, NIST is the gold standard. If you have a potassium dichromate liquid standard from them and you are using it to test your spectrophotometer for photometric accuracy there is no question that you can trust your results. However, as NIST either charges extremely high amounts of money for their standards or they have discontinued them (such is the case with the potassium dichromate: see here: https://www-s.nist.gov/srmors/view_detail.cfm?srm=935A)
Definition of Traceability
If you don’t have standards directly from NIST, nothing to fear! Traceability is your friend. What is traceability? According to ISO (the international standards control body), traceability is the “Property of the result of a measurement or the value of a standard whereby it can be related to stated references usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties.” Basically, if you have a NIST filter you can use it to transfer traceability to another filter if you are using the same spectrophotometer.
FireflySci leverages this concept to sell you NIST-traceable reference standards. As we have NIST standards (i.e. NIST 930, NIST 2031, NIST 2034), which we use with our calibration-grade spectrophotometers, we are then able to claim traceability for ALL of the standards that we sell. For instance, our FUV filters are benchmarked and made traceable to NIST in our calibration laboratory via SRM-930e-2350 and SRM-2031-171. At the end of those numbers are the serial #’s for NIST,which makes the traceability complete.
NIST Traceability: What to Look for in a Calibration Report
So what are the nuts and bolts of NIST traceability? Typically, two things are necessary for a claim of traceability. Firstly, the process of measurement being used to establish the claim needs to be laid out and secondly a description of the chain of comparisons used to establish a connection to a particular stated reference.
So what would this look like in a calibration report? There are a couple of things to look out for: Firstly, the identification of the filters being used (in our case, the FUV series filters); the conditions of measurement, which includes the type of machine being used, the settings, the temperature parameters and the date of measurement; thirdly, a description of the results obtained, which are typically laid out in a table; the traceability chain (in our case we have the NIST serial numbers laid out) and finally, the uncertainties or margins of error.
This full report will give no question that your results are 100% NIST-traceable.
Can our Standards be Used to Transfer NIST Traceability?
A question that we recently heard from a customer was a good one. “If we use your standards could we pass along your NIST traceability to a standard that we already have from another vendor?” The quick answer is no. NIST does not allow this and considers this a break in the link of traceability. Let’s say that you have one of our FUV standards and you want to know if you can make your own homemade nicotinic acid standard NIST traceable. The reason this is not possible is because NIST requires a direct comparison of the nicotinic acid that is directly compared to a NIST standard. The only traceability that the nicotinic acid would have would be to our FUV filter. If you did want to make the nicotinic acid standard traceable you would need to procure a NIST filter and analyze them in parallel.
The FireflySci team
A great source of material for this is a presentation NIST put out about a decade ago on making your own NIST traceability materials, or MYO. You can see the original presentation here: http://www.cstl.nist.gov/strbase/pub_pres/KlineNISTTraceabiliy2003.pdf