Part 3: How to Know if Your Calibration Curve is Correct

You’ve bought some standards and you made a multi-point curve.  You might think your work to improve laboratory accuracy is over, but then doubt starts to creep into your mind.  What if something’s gone wrong!?!  What if you run a sample and you expect one answer and your instrument tells you another?!  Is the sample bad, or is there a problem with your calibration curve?  Has something gone wrong somewhere in your plant’s process, or do you have a bad batch of calibration standards!?

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The first step in ensuring that your calibration curve is up to par is checking the Correlation Coefficient.  I know, I know….more math….but stick with me here because it’s important!  The R2 value of your calibration curve has a large amount of statistical power.  No one wants to talk about statistics, but it has a lot to do with how well your instrument is calibrated.

In a nutshell, the correlation coefficient gives you an idea of how well the standards relate to each other.  R2 values statistically can range from -1.0 to +1.0, but on your instruments, you should look for a value as close to 1.0 as possible!  Values of R2=0.9987, R2=0.99943, or R2=0.9963 are examples of what you are looking for.  The closer to 1.0 your R2 value is, the stronger the relationship between your standards.  A strong relationship between standards leads to more confidence in the reported results for your samples.  Remember that your standard value points hug your samples, and no one likes crappy, wet noodle armed hugs! It’s listed somewhere in your instrument’s software, you might just have to look around for it.

So you’ve got a strong curve, but calibration curves can be strong….but wrong!  How can you check the accuracy of the standards you used to make the curve?

There is a way to prove that you standards and instrument are operating correctly.  We are going to employ the Dr. Sheldon Cooper of standard….the validation standard!  Just like Dr. Copper, the validation standard knows more than you, and it isn’t afraid to tell you when you’re calibration is wrong!

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The validation standard should be a standard SEPARATE from your curve!  Buy it from an alternative supplier, buy a different lot of a standard you use in your curve, or buy 4 standards and just pick one to use only as a validation….just don’t use your validation standard as part of your calibration curve.

If you just analyze one of your calibration standards again as a sample, you will get the right answer 100% of the time….because somewhere in the software you’ve entered those values in as the answer.  This might look promising, but it tells you ZERO useful information about the accuracy and correctness of your calibration curve.  The validation standard’s “job” is essentially to be an unknown sample, that you secretly know the answer to.

A good basic rule of thumb is that the difference between the known value of your validation standard and the instrument’s reported result based on your curve should vary no more than 10%.  We’ll call this “Validation Recovery” for ease of terminology.  For most applications, 10% is much larger than the recovery should be, but if you’re just started down the path of improving your laboratory accuracy….it’s a good basic place to start.

From here we are going to have to dive into a bit more math, and I think we can all agree that one math topic per post is frankly too much math!

Next week I’ll cover the basic statistics of calculating a more precise validation recovery range for your instrument, tracking validation recovery, and how you can use that information to access the performance of your instrument!

I’ve Got A Brand New Blog

I was asked to start writing a science blog for work.  I’m not sure how much interest there will be, or how interested any of my current readers might be in this info, but I thought I might start sharing them here as well.

A few fun facts and some science knowledge never hurt anyone after all!  Currently, the plan is that I’ll post for work weekly.  That might taper off, but we have some events coming up and we wanted to drum up some interest and talking points with some customers before the events.  I will also be keeping up on my “personal” blog posts….which I have been bad about but hopefully all of this extra blogging will keep me on a schedule!

So….without further ado….I present my new blogging topic–

Common Calibration Conundrums and Other Laboratory Queries

Hey!

I’m coming at you to discuss some common science questions and issues I get asked in the lab frequently.  I thought I would start with a little mini-series on calibration issues, and then hopefully we can move on to other issues and questions you face in the lab on a daily basis.

If there is a specific topic or question you liked me to address, just leave a comment on this post and I’ll get back to you!

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Part 1: When to Calibrate

You should calibrate as often as practically feasible for your laboratory process and timelines!  It will not hurt your instruments to calibrate them too often, but it will hurt your results if you do not calibrate often enough.

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I know you might think that calibrating often is a waste of time…….and you’re busy…and the plant operators want their results…and the results seem fine so you’ll just ignore recalibrating your instruments until something bad happens.

Just take a minute and think about all the parameters around your instruments that change….even weekly.

Did you consider factors like-

  • Did you change mobile phase?
  • Standards will gradually degrade over time.
  • Wear and tear on the instrument components themselves

Even one of those small changes can compound and lead to catastrophic effects on your results over time.

Calibrating an instrument is a bit like potty training a new puppy.  Puppies only know what you tell them.  Imagine you tell the puppy one day to go outside.  The puppy might remember that knowledge for a day….maybe two.  Eventually, if you don’t remind the puppy what good behavior is, that puppy is going to forget.  When that happens, you’re going to come home to a disaster zone!!

Instruments only know what area counts corresponds to want concentration because you tell them….with standards and a calibration curve!  The instrument might be able to hold onto that curve and that knowledge for a day….or two, but eventually it’s going to drift too far to be accurate.  When that happens you wouldn’t be able to supply anyone with valid data, and you’ll end up behind a backlog of samples trying to figure out where it all went wrong.

I’ll be covering more about calibration data in the next couple posts so be sure to keep an eye out for those!  In the meantime, if you have any specific questions or you would like me to cover another lab related issue you find yourself facing, don’t hesitate to contact me about it!