Empower Tip: Baseline Noise

In this tip for Empower Chromatography Data System (CDS), we will begin a new series on Baseline Noise.

Since baseline noise is needed for the signal-to-noise calculation, let’s take a closer look at how it is measured starting with manual calculations.

Typically, noise is measured in an area of the chromatogram where the baseline is relatively flat and there are no spurious chromatographic peaks. This representative section of baseline over which noise will be measured is commonly proportional to the chromatographic run time.

When noise is measured to calculate signal-to-noise, it is common that the noise region selected be proportional to the peak width of the chromatographic peaks of interest.

Using the displayed section of baseline, let’s explore some of the variants that make baseline noise measurements so difficult to pin down.

The use of various calculations in different chromatography software packages can be particularly frustrating if you base critical validated factors like Limit of Detection (LOD), or Limit of Quantitation (LOQ) upon the noise calculation.

In laboratories where multiple vendors chromatography software is used routinely, it is not uncommon to find chemists resorting to hand calculations of chromatographic noise to help eliminate the differences between the various software packages.

In environments where a single vendor’s software is used, it is common for chemists to rely on the software calculations for noise. This can cause problems for these environments if they need to transfer methods to labs where a different software is used, or if the lab decides at some point to switch to a different vendors software.

Manual calculations are subjective and time consuming. Automated calculations depend upon which vendors software is used. What should laboratories do to incorporate consistency? The answer is to understand the calculations used by each of the software products in your laboratory, and in laboratories that will be the recipient of methods you develop, and to configure.

If you were to measure noise manually, you might get a room full of chemists to agree on where the bottom of the noise is in the example chromatogram. Where to draw the upper line though could be a subject of greater debate. Where should we measure the magnitude of the noise? Should we do it on the right side of the chromatogram or the left side of the chromatogram? Should we make it a requirement that the upper and lower lines be parallel to each other to eliminate this problem?

Where do you place the parallel line? Do you eliminate the spike at 26.6 minutes as an outlier? If you include it, are you overstating the measured noise in this chromatogram? Maybe you should choose a different section over which to measure noise where you don’t have these outliers.

How about this section? I’m not actually suggesting that this is where noise should be measured. This might be too extreme to be what chemists in your lab would do, but it does demonstrate the highly subjective nature of using manual calculations without proper analyst training, and without specific variables like the time region to use and the y-axis scaling to be used being written in the analytical method.

As I previously mentioned, manual calculations are largely subjective, and may cause problems with borderline results in a validated laboratory environment. Next time, we will discuss the use of automated calculations such as peak-to-peak noise.

It’s that easy!