Extraneous Peaks in Chromatgraphic Analyses

The unknown chromatographic peaks related to the system, excipients, or contamination but not related to the product are known as Extraneous Peaks.

By the definition of the FDA, drug impurities are components found in a drug product that are neither its drug substances nor excipients. When exceeding certain thresholds, the identification of these unknown impurities is required by regulatory guidelines and their toxicity and/or safety may also need to be evaluated dependent upon the nature and amount of the identified species. According to their origins, the commonly observed drug product impurities can be classified into several categories: 

1. degradation products of drug substance (or active pharmaceutical ingredient, API)
2. synthetic process impurities
3. extractable/leachable components from packaging materials
4. external contaminants. The external contaminants may be introduced during sample preparation steps of drug analysis. Since the external contaminants are usually not structurally related to the drug substance and/or the excipients, their identification can be challenging due to the lack of relevant structural information.

The unknown chromatographic peaks related to the system, excipients, or contamination but not related to the product are known as Extraneous Peaks.

Chromatography is a popular analytical technique that separates compounds on the basis of their properties and how they interact with stationary phase present in the column.

This separation happens when a mixture of compounds passes through a chromatographic column and individual peaks of different compounds show in the form of chromatographic peaks. Some compounds interfere in the chromatographic system during the separation of compounds in different ways that produces unwanted peaks in the chromatogram. These unwanted and unidentified peaks are known as extraneous peaks.

It is important to identify those peaks because these can ruin the actual results. This article will explain the cause of extraneous peaks and the tips to identify them easily. 

What are extraneous peaks? A chromatographic peak that is not a result of the compound being analysed but appears unknowingly is known as an extraneous peak. These peaks are not a part of product and appears on the graph like hump rather than a smooth base line. These peaks can alter your chromatographic results and make it difficult to draw an accurate conclusion. Sometimes these peaks are too wide and looks like a hump on the graph rather than a peak and makes it difficult to determine what compounds are present in the sample.

 It is very common for chromatography results to have extraneous peaks. It occurs due to a lot of causes that depend on the sample itself and the equipment being used for analysis.

 If you are using hydrocarbon solvents in the analysis, it may attract some hydrocarbon impurities from the air that can cause the unknown peaks in the chromatographic results. 

Other causes of extraneous peaks include 

• contamination from the air, such as dust, solvent vapours or even oils from your fingertips.

Sometimes, solvents that you are using in your sample dilutions can also cause retention time issues and show pumps on your chromatograph rather than peaks.

 Which peaks can be classified as extraneous peaks? The most common extraneous peaks are those that represent the component of the equipment rather than the compound in the sample. The peaks that are considered extraneous peaks include: 

1. Solvent Peaks: These peaks arise from the mobile phase or diluent used to prepare the sample. These peaks may occur due to methanol, acetonitrile or water present in the mobile phase. 

2. Reagent or Excipient Peaks: Sometimes, excipients used in formulation may appear in the form of peaks in the chromatography analysis. 

3. Contamination Peaks: Carryover from previous injection or contamination from glassware may be the cause of extraneous peaks. 

4. Column Bleed Peaks: Column bleed is the breakdown product from the HPLC columns. This contamination from column stationary phase generally occurs in GC columns or old HPLC columns. 

5. Environmental Contaminants: Air from the atmosphere or handling of the sample may cause contamination that appears in the form of extraneous peaks during the chromatographic analysis.

6. Mobile Phase Impurities: When the mobile phase is not filtered or degassed properly can cause baseline noise or unexpected peaks. 

IDENTIFICATION

A. The best way to identify the extraneous peaks is to compare the chromatogram with the previously analysed product chromatogram by this way you can easily identify the newly appeared peaks in the current sample.

B. You can run a system blank or a solvent blank by injecting solvent or mobile phase without the analyte. If any peak appears that may be from solvent, glassware contamination, mobile phase or any system carryover. This helps to eliminate any non-sample related source of peaks. 

C. You can also inject placebo to determine the peaks other than the active ingredients or other interfering substances. 

D. You can inject standards of API, known impurities or solvents and compare their retention times to the unknown peaks. If these do not match with any known standards then you can consider it an external speak. 

There is not any specific regulatory guideline for extraneous peaks that can help the industry to identify the extraneous peak in the chromatogram. Pharmaceutical firms have no specific reporting threshold for extraneous peaks. Some firms consider the extraneous peaks if any peak has 1% area of the main peak and they leave the peaks having an area below 1% and some have their limit of 2% area, which means there is no specific criterion for it and there is no justification for this 1% or 2% reporting threshold.