Analytics

What Did You Just Say to Me?—Part 1

Deciphering the Jargon of the Analytical Lab—Chapter 1

You know that language humanity has created for texting? Those weird combinations of letters, like the one an undergrad used in a lab report she turned into me, likely b/c she was in a hurry? To the uninitiated (which may be a godsend), the language might be like deciphering cuneiform. It’s not just millennials who have crafted new languages like conversing in chat acronyms, emojis and/or GIFs. And as language is deconstructed to save some seconds, will it come full circle back to the grunt or evolve to mental communication like an ET?

Show me an industry, and I’ll show you jargon that no one else understands, perhaps not even many of the folks in the industry that created it. Analytical laboratories and the regulators and auditors who oversee them are masters of a foreign tongue. This article will cover three, as will others in this trilogy.

And the first will be the LOD. The Limit of Detection is a term used to describe how low an instrumental method can go — the analytical limbo. It defines the bare minimum signal that a method can detect. The problem with this metric is that it’s not an adequate enough signal to make any analytically sound conclusions. So, the LOD doesn’t do much for you.

Which is why analytical chemists really care about the LOQ. The Limit of Quantitation describes the analytical bottom of the barrel, the lowest point that can be accurately measured. The LOD and LOQ are numbers with sidekick units like milligrams per gram (mg/g), or grams per liter (g/L).

An LOD, for example, can refer to a signal detection limit, which confers the smallest signal that the instrument and the method being used can differentiate from that of a blank measurement. [1] A signal LOD can be calculated by measuring a blank at least seven times and taking the average. Then add to this average value the product between the calculated standard deviation (s) and the Student’s t value, which can be found in the tables of analytical chemistry books or online.

One can also calculate a concentration detection limit. This value can be generated using the signal detection limit and a calibration curve. Multiply the t-value and standard deviation calculated for the signal detection limit, and divide this product by the slope of the calibration curve. [1] Another fairly universal benchmark for LOD is to take the average signal of the blank and multiply by the product of 3*s. The LOQ can be determined using a similar equation, but where the averaged blank signal is multiplied by 10*s.

Our third trip into the tedium unveils specificity, and this shouldn’t be too troublesome for you to see. For, the root word “specific” is contained therein, and you know what that means. We have specific tastes in music, or food, or beer, or cultivars, or people, and we pride ourselves on this. The analytical chemist can feel pride inside regarding many things, and one of these things is their method. They strive to make their method as specific as possible. “Look how well I detected caryophyllene,” you’ll hear them speak, or “Look at the baseline resolution of that peak.”

“At that concentration?” another analyst asks, incredulous, aghast. An LC method is considered specific if it can easily pluck the targeted molecule from the myriad of others in the matrix.

The proper definition of specificity is the “ability of an analytical method to distinguish the analyte from everything else that might be in the sample.” [1] The analyst can prep a matrix blank that contains all of the ingredients, even including the analyte in question. They can quantify the concentrations of these constituents, and then, can spike in a known amount of the analyte. Their ability to accurately measure the analyte, within a defined statistical threshold, unveils their specificity.

There are other acceptable ways to go about determining specificity, LOD, and LOQ. And there are other statistics required to prove out a method besides just these few. So, join us for tales of robustness, precision, and accuracy in chapter 2.

Reference

[1] Harris, D. Quantitative Chemical Analysis, New York, WH Freeman and Company, 2003.

Image Credit: Red Bubble

About the author

Jason S. Lupoi, Ph.D.

Jason S. Lupoi, Ph.D.

Leave a Comment