Gas Chromatography – An Introduction

October 30, 2023 Bob Brousseau

A Brief History of Gas Chromatography
Gas chromatography (GC) is one of the most important and prevalent analytical tools available to chemists. It was invented in 1952 by A.T. James and A.J.P. Martin as an outgrowth of research dating back to the previous decade.^1-6 Their early techniques on adsorption and partition enabled some of the later developments that A.J.P. Martin spoke about at the 1957 Lansing Symposium, entitled, “Past, Present and Future of Gas Chromatography.” He concluded his address with the following prediction: “If we tie the gas chromatograph to other pieces of laboratory equipment, we have the possibility of almost the automatic chemist…”² While the idea of an “automatic chemist” hasn’t quite come to fruition, Martin’s belief “that the uniting instrument of the gas chromatograph in the center” of his lab of the future certainly has.² With the meteoric rise of GC⁷, its adoption and adaptation continued unabated in the ensuing decades. Then, “The introduction of robust, efficient, and reproducible fused-silica capillary columns and the provision of relatively inexpensive but reliable equipment for GC-MS provided a crucial new impetus in the 1980s.”¹ Interestingly, the GC column saw some major advancements within just a few miles of AMPAC Analytical Laboratories. At that location, Walter Jennings and the company he co-founded, J&W Scientific, were instrumental in the development and manufacturing of capillary GC columns. The company was eventually purchased by Agilent in 2001.^8,9

GC Capabilities
Some of the analyses that GC can do include the separation of compounds in mixtures based on the polarity of the compounds, testing for purity – or for impurities, e.g., and detection of residual solvents. Conversely, with a technique known as preparative chromatography, GC can be used to prepare pure compounds from a mixture. In pharmaceutical analysis, there are additional applications:

Analysis of various functional groups.
Determining purity of pharmaceutical compounds.
Analysis of drugs that are commonly abused.
Determination in pharmaceutical R & D the identity of natural products that contain complex mixtures of similar compounds.
Use in metabolomics studies.10

GC for Testing Residual Solvents
The testing of residual solvents is necessary to ensure potency and, with some solvents, to determine their potential negative effects. GC is an excellent choice to do this. Residual solvents are separated into three classifications by the ICH (International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use):

Class 1 solvents: Solvents to be avoided – Known human carcinogens, strongly suspected human carcinogens and environmental hazards. The Permissible Daily Exposure (PDE) of these solvents used in pharmaceutical products can range from 2 to 1500 parts per million (PPM).

Class 2 solvents: Solvents to be limited – Non-genotoxic animal carcinogens or possible causative agents of other irreversible toxicity such as neurotoxicity or teratogenicity and solvents suspected of other significant but reversible toxicities. The PDE for these solvents ranges from 50 to 3880 ppm.

Class 3 solvents: Solvents with low toxic potential – low toxic potential to man requiring no health-based exposure limit. Class 3 solvents have PDEs of 50 mg or more per day.¹¹

Within these three classifications, some of the most commonly used solvents are listed:

Benzene (class 1)
Acetonitrile, Cyclohexane, Hexane, and Methanol (class 2)
Acetic Acid, Acetone, and Heptane (class 3).^11,12

Headspace GC (HSGC) and Direct Injection GC
In addition to the numerous GC developments such as column type, phase and coating techniques, and multidimensional GC, two types of sampling methods arose: direct injection (“DI”) and headspace (“HSGC”). In the former, the sample is injected “directly” into a typical sample injector of the GC column. In HSGC, when heated, “the more volatile compounds will tend to move into the gas phase (or headspace) sample. The more volatile the compound, the more concentrated it will be in the headspace. Conversely, the less volatile (and more GC-unfriendly) components that represent the bulk of the sample will tend to remain in the liquid phase.“¹³

Therefore, by extracting the “headspace vapor and injecting it into a gas chromatograph, there will be far less of the less-volatile material entering the GC column, making the chromatography much cleaner, easier, and faster.”¹³ Generally, HSGS is much cleaner and results in less wear on the column. For more on comparative techniques with DI and HSGC, please see the first entry in the “Resources” section below.

We Can Assist with your GC Needs

AMPAC Analytical has years of experience and numerous experts in Gas Chromatography who can assist with method development for analyses for both common and atypical sample matrices that will allow you to stay ahead of evolving regulatory concerns. Please contact us with any specific questions or to receive a quote for your GC needs.

References

Resources

Blog

Chiral Purity Analysis – The Need to Know What Both Hands Are Doing

October 10, 2023 Bob Brousseau

Background on Chiral Purity
Chirality refers to the phenomenon that occurs when a mirror image cannot be superimposed. It is sometimes called “optical rotation”.

The origin is from the late 19th-century Greek word kheir (‘hand’) and is one of the easiest demonstrations of the concept. Although a person’s hands may appear virtually identical, if they were switched, the outcome would be very different. Amino acids and sugars are the chiral building blocks of larger molecules such as peptides, proteins, and nucleic acids. Therefore, those polymers, in turn, are chiral as well.¹ Molecules with chiral centers may have a different therapeutic impact, and this guides the need to test and control chiral purity. The effects of chiral impurities can result in horrific outcomes, as evidenced by the infamous birth defects associated with Thalidomide² or as benign as Aspartame and sugars (the D/L sugars) that, when superimposed, can create different taste sensations (sweet versus sour, etc.) or metabolic activity. Each chiral center can generate two enantiomers. “Enantiomers or optical isomers are chiral molecules which are non-superimposable mirror images of each other,”³ and multiple chiral centers can generate diastereomers (non-mirror image isomers).

Determining Purity
HPLC has been the primary technique for determining chiral purity, with gas chromatography used occasionally. Measurement of optical rotation is a legacy technique that is fast but not as accurate. Historically, HPLC methods used under normal phase conditions could limit the type of molecules that could be analyzed. However, now modern chiral columns are compatible with reverse phase conditions.

Simulated Moving Bed (SMB) Chromatography as an Option for Chiral Purity Analysis
Do you have a partner for chiral separations? AMPAC Analytical has the expertise in performing chiral purity testing, along with the equipment and techniques. Additionally, our parent company, AMPAC Fine Chemicals, has decades of experience conducting chromatographic separations at a commercial scale in a highly regulated environment. Our services include SMB screening, method development, proof-of-concept demonstration, and production. We operate the largest CGMP Simulated Moving Bed (SMB) chromatography unit in the United States. These technologies and expertise are part of a one-stop shop (from 10-millimeter columns up to 1000mm). Our SMB processes can be developed in a few weeks and are easily scalable. In many cases, scale-up from gram to multi-ton quantities can be achieved in fewer than six months. Our facilities include kilo-scale and pilot-scale units to support smaller quantities, also under CGMP conditions. The SMB facilities have been inspected and approved by the FDA for the manufacturing of APIs. AFC has registered four products with regulatory authorities (FDA/EMA) using SMB technology. Along with chiral separations, we can also perform the separation of diastereomers & regioisomers.

Contact us today for information on how we can assist with your raw material, amino acid, drug product, and API chiral purity testing or to learn more about our SMB processes.

References

Ampac Analytical

Monthly Archives: October 2023

Gas Chromatography – An Introduction

Chiral Purity Analysis – The Need to Know What Both Hands Are Doing