# Get Empowered: Reader Question on Retention Time Difference | Tip #75

By June 27, 2018

## Tip #75, Reader question: Why is my retention time slightly different than the retention time of UV apex spectrum?

Welcome back to Get Empowered! In the last Empower tip-of-the-week post for Empower Software, we answered a submitted reader question and learned how Empower calculates the ratio of peak height to valley height when there is a valley point between peaks (Tip #74).

In this week’s tip, I will answer another common question.

#### Question: Why is the retention time of my peak slightly different than the retention time of the UV apex spectrum for the peak?

Great question – and there is a simple explanation.

Let me show you how it is done.

### Step 1

Once I extract a wavelength from a 3D PDA channel of data and integrate the chromatogram, I see the retention times for the peaks are different than the retention times for the spectra extracted at their apices (Figure 1).

Figure 1

Peak retention time is determined by fitting a quadratic curve to the highest point in the peak and 2 points on either side of that highest point, and then determining the inflection point of that quadratic curve. The reason this is done is because if the peak is misshapen and you have collected data at a low sampling rate (set in the instrument method) the true apex of the peak may be between 2 actual data points. The retention time of the apex spectrum is simply taken from the highest point in the peak.

### Step 2

#### Follow up Question: Can I set up Empower so that the retention time of the peak and the apex spectrum match?

The answer is YES – provided you use the Traditional peak detection and integration algorithm. There is a system policy called Use v2.XX Style Retention Time Calculations, found on the Data Processing Tab of the System Policies (figure 2). If enabled Empower will determine peak retention time as the highest point in the peak which is what was done in Millennium v2.XX.

Figure 2

### Step 3

We see the same chromatogram after the system policy has been enabled and the peak retention times match the apex spectra retention times (figure 3).

Figure 3

It’s that easy!

Final Notes:

1. The change in system policy only works for the Traditional peak detection and integration algorithm. The Apex Track algorithm is not affected by this system policy.
2. This procedure can be followed using the QuickStart or Pro interface.

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Next week in Empower Tip #76 – More answers to questions from our readers

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