Secrets of Chiral Gas Chromatography (Chiral GC): How to Understand

Table of Contents

• Introduction and outcome
• Chiral Stationary Phases in Chiral GC
• Principle and Chiral separation mechanism
• Selection of Chiral stationary phase/column
• Advancements in Chiral GC
• Can Achiral compounds can be separated along with Chiral isomers?
• Limitations of Chiral gas chromatography
• Conclusion

Introduction and Outcome: Chiral Gas Chromatography (Chiral GC)

Chiral Gas Chromatography (Chiral GC) is widely used to determine the optical purity of volatile chiral pharmaceuticals. More than 80% of active pharmaceuticals are chiral. The compounds in the human body are also chiral. Therefore the human body often responds differently to enantiomers, one form may produce the desired effect while the other may be ineffective or toxic. Therefore, it is crucial to separate enantiomers in the pharmaceuticals to ensure the desired therapeutic outcomes. Several chromatographic techniques are used for chiral separation and one of them is Chiral gas chromatography. It is widely used for volatile pharmaceuticals due to its unique selectivity, fast analysis and lower analysis cost. In this article, I will discuss the GC chiral stationary phases, the selection of the chiral stationary phases and their advantages. After reading this article you will understand the Secrets of Chiral Gas Chromatography.

Chiral Gas Chromatography (Chiral GC)

When GC with a capillary column containing a chiral stationary phase is used for the separation of Chiral volatile pharmaceuticals then it is called Chiral Gas Chromatography (Chiral GC)

Chiral GC Columns or Chiral GC Stationary Phases

A chiral stationary phase is prepared by binding a chiral selector onto a support material, typically silica. The enantiomers interact with the chiral selector in the CSP, forming diastereomeric complexes with differing physicochemical properties, which can then be separated. The following stationary phases are used in Chiral GC :

• Cyclodextrin derivative
• Amino acid derivatives, and
• Metal coordination complexes.

Among the above CSPs, cyclodextrin-based stationary phases are the most widely used and popular. Cyclodextrins are formed by glucose units joined together, and their derivatives can efficiently separate a wide range of polar and non-polar chiral compounds. Different derivatized cyclodextrin phases are commercially available for the separation of various compounds.

Principle and Chiral separation mechanism

Both the architecture and chemistry of Cyclodextrin contributes to enantiomers separation. Separation is achieved by Incursion complexing and Hydrogen bonding

Selection of Chiral stationary phase/column

Selection of the CSPs depends upon the polarity and nature of the molecules (e.g. volatile nature of pharmaceuticals)

• Use amino CSPs for amino compounds
• Use cyclodextrins CSP for hydroxyl Chiral compounds

Can Achiral impurities be separated along with Chiral isomers?

The answer is yes and it is two-dimensional GC technique .

The two-dimensional GC technique is a powerful tool for the separation of Achiral impurities as well as Enantiomers of a Chiral pharmaceuticals. For this, two columns are connected in the series using union. First column is an achiral column and the second column is a chiral column (typically cyclodextrin-based). This heart-cutting technique allows for the selective separation of the target enantiomeric compounds as well as Achiral impurities. In a single run one can get both Achiral impurities profile and chiral purity of the Enantiomeric molecules.

Advancements in Chiral GC

In addition to cyclodextrin derivatives, other types of chiral stationary phases have been developed and applied in chiral GC. These include cyclofructans, metal-organic frameworks, porous organic cages, and chiral ionic liquids. Each of these stationary phases offers unique selectivity and has specific applications in the separation of different types of enantiomeric compounds.

Case Studies: Chiral purity of Menthone

Following is the structure of menthone:

Menthone has two chiral centres and hence it will have four chiral isomers. All four chiral isomers can be separated by Chiral gas chromatography using Beta Cyclodextrin CSPs.

Advantages and Applications of Chiral GC

Chiral GC offers several advantages for enantiomeric separations. It provides high efficiency, fast analysis, sensitivity, and reproducibility. Additionally, the coupling of chiral GC with mass spectrometry (GC-MS) allows for the unambiguous identification and detection of enantiomers, even at trace levels.

Advantages of Chiral gas chromatography

The following are the advantages of the Chiral GC:

• Fast analysis
• Low analysis cost
• Reproducible result
• Better resolution between the isomers (R)
• High column efficiency (N)
• Sharp and symmetrical peaks
• Acceptable by Regulatory agencies
• Easy to validate the method
• Suitable for volatile molecules
• High sensitivity or low detection limit
• The separation of complex samples containing multiple enantiomeric pairs can be achieved through two-dimensional GC (2D-GC) techniques.

Limitations of Chiral gas chromatography

Not suitable for non volatile Chiral molecules like steroids, amino acids, Carbohydrates’, Antibiotics…

Conclusion

Chiral gas chromatography is a powerful technique for the separation of Enantiomers of various pharmaceuticals. Chiral gas chromatography, particularly with the use of chiral stationary phases, has revolutionized the separation of enantiomers in pharmaceutical applications. The development of highly enantioselective chiral stationary phases, such as cyclodextrin derivatives, has enabled the accurate analysis and separation of chiral compounds. Chiral GC offers numerous advantages, including high efficiency, sensitivity, and rapid analysis, making it an indispensable tool in pharmaceutical research, development, and quality control.

Hope this post has cleared all your doubts related to Chiral gas chromatography and you can replicate the same during Analytical method development for chiral molecules. Write your opinion or questions related to this article in the comment sections.

FAQs

What is chiral gas chromatography?

The separation of volatile chiral pharmaceuticals/molecules using chiral capillary stationary phases (CSPs) is called chiral gas chromatography.

What is the principle of chiral chromatography?

The separation of chiral compounds in chiral GC occurs due to the interaction of different isomers between the CSPS and the carrier gas. Separation is achieved by inclusion complexing and hydrogen bonding.

What is the mechanism of chiral chromatography?

In chiral chromatography, separation is achieved by the inclusion complexing and hydrogen bonding while using cyclodextrin CSPs

Can chiral separation be achieved using GC?

This can be achieved by GC using chiral capillary column

What are the limitations of Chiral gas chromatography?

Not suitable for non volatile Chiral molecules like steroids, amino acids, Carbohydrates and Antibiotics.

Abbreviations:

• GC: Gas chromatography
• CSPs: Chiral stationary phases

References:

• USP
• Analytical chemistry Gary D christian
• https://pharmaknowledgeforum.com/why-is-chiral-separation-so-important/
• Practical high-performance liquid chromatography: Veronica R. Meyer
• Separation of Enantiomers with ChiraDex^®