# Get Empowered: Reader Question on Impurity Response with Two APIs | Tip #76

By July 3, 2018

## Tip #76, Reader question: How do you calculate impurity response when there are two APIs in the sample?

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 why the retention time of a peak is slightly different from the retention time of the UV apex spectrum for the peak (Tip #75). (See the link at the bottom of our post to ask your own question, or to provide a tip of your own!)

In this week’s tip, I will answer a question on calculating impurities.

#### Question: How do you calculate impurity response when there are two APIs in the sample?

Another great question.

Let me show you how it is done.

### Step 1

In this chromatogram, we see two API peaks – API A and API B. We also see some impurity peaks, some related to API A and some related to API B (Figure 1). The goal is to calculate the %Area against the Main Component where the Main Component could be API A or API B.

Figure 1

### Step 2

On the Components tab of the Processing Method, we assign the CCompRef1 to be the API, which corresponds to each impurity peak (Figure 2).

Figure 2

### Step 3

We create a custom peak field that calculates %Area against the Main Component with a simple formula using the area of the CCompRef1 peak (Figure 3).

Figure 3

### Step 4

Back in the Processing Method, we go to the Impurity tab and set the Impurity Response to the name of the custom field (Figure 4).

Figure 4

### Step 5

We integrate/quantitate and see the Impurity Response calculated for the impurity peaks using the proper API (Figure 5).

Figure 5

It’s that easy!

Final Note:  This procedure can be followed using the QuickStart or Pro interface.

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Next week in Empower Tip #77 – More answers to questions from our readers

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