Welcome back from Thanksgiving Break!!
This week we will crack open the analytical textbook and cover more math!
We are going to figure out how to go low. I’m not talking about dropping it low on the dance floor….or limbo, although if anyone is interested in some friendly competition I’m sure we can arrange for that to happen at next year’s FELC. Any takers!? 😊
How Low Can You Go??
Sure, I know those instrument companies are big on posting lower detection limits for all of their instruments, but you can’t take that answer as the gospel truth. Those posted detection limits are created under the most ideal operating conditions. It’s the same as car companies who post that their brand new, super shiny, fresh off the production line vehicle has an average 48 MPG HIGHWAY…..we all know that’s not true! Maybe it’ll happen once if the wind is blowing in the right direction….but it really isn’t a fair indicator of how your vehicle is going to operate day in and day out. Standard detection limits on instruments are the same way,it might happen, but it probably won’t.
Setting detection limits is a journey you and your instruments are going to have to go on together.
IYou may or may not have noticed, but several of the tests we run on ethanol, at least finished product ethanol, are searching for an answer very near to 0.0…..or as close as we can realistically get.
These are all test results that typically you’d typically expect to see very low-levels on. How do you know that your instrument is capable of seeing levels that are that low effectively? Low level detection is one of the most difficult things we ask of our instruments, so it’s important that we know exactly what we can and cannot expect.
Calculating your Detection Level, or DL is a pretty easy and straightforward process. First things first you’ll need to run a good calibration curve, but let’s assume you’ve read all the blogs and you’ve already got that step done! Find a standard with low values, similar to where you hypothesize your DL might be. You’re going to analyze that standard 7-10 times. I don’t recommend doing this over time, just run them back to back. Unlike Control Limits which are calculated based on instrument shift over time, DL can be calculated based on a snapshot of the instrument.
For the math portion, start by taking the standard deviation across all injections for all the components. My table also shows the average, but you won’t directly need that for DL calculation, but it is good practice to check your repeatability recovery….the average over the known value*100 will get you there. If your repeatability recovery isn’t inside of your control levels we discussed in Common Calibration Conundrums and Other Laboratory Queries Part 4, you’ll want to rerun the study with a higher standard level. It could be that you’re too close to your instrument’s detection level.
The Detection Limit is then calculated as the Standard Deviation value times 3.143.
There statistically are several ways of calculating a DL,but this is the easiest and for most laboratory purposes will work just fine. In my above example, my IC can see sulfate peaks down to 0.0026mg/L and chloride peaks down to 0.0084mg/L. Now, that’s absolute bottom low as you can go level on my instrument. Do I routinely analyze samples at that level….no. There is a practical level for using your instrument. In my case, I don’t consider my instrument practically capable of analyzing samples below 0.25mg/L, and I wouldn’t report any levels lower than that.
Hopefully this will help you dial in the lower limits of your instrument systems, and guide you toward some practical levels of analysis and reporting.