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Investigating gas chromatography.

Experiment #8 from Organic Chemistry with Vernier

Introduction

Gas Chromatography is a technique widely used to separate complex mixtures of substances. Compounds present in a volatile liquid or gaseous solute are isolated after traveling through a coated column based on the substance’s size and intermolecular interactions. If a compound tends to bind to the column through intermolecular interactions, it takes a longer time to emerge compared with a compound that does not tend to stick onto the column. The level of binding experienced between the substances and the column is determined based on the number and strength of intermolecular interactions between the two species. Substances that pass quickly through the column exhibit fewer intermolecular interactions with the column.

The Vernier Mini GC uses a metal column with a nonpolar coating, called the stationary phase. A sample, consisting of one or more compounds, is injected into the column and is carried through the stationary phase by atmospheric air, which acts as the mobile phase. The nonpolar coating of the stationary phase most strongly retains solutes of the same polarity. Organic compounds flowing out of the chromatography column are then detected by a chemical sensor that produces electrical responses proportional to the concentration of the compounds. The presence of such a chemical at the detector is seen as a peak on a chromatogram. The unique time it takes for a compound to exit the column after it is injected is called the retention time. With a gas chromatograph, a compound can be identified from a mixture by its retention time.

Several factors can affect a compound’s retention time. More volatile compounds (i.e., compounds with a lower boiling point) will move through the column faster because they are flowing in the mobile phase and not strongly bonded with the stationary phase. The surface functional groups present on the compound are also a factor. For example, alcohols may weakly bond with a polar stationary phase more than esters because alcohols are capable of forming hydrogen bonds. The molecular weight of a compound may also play a role to a slight extent, although it is not a direct relationship that the heavier the molecule, the slower it will travel through a GC column.

As you will discover in this experiment, the instrument settings also affect a compound’s retention time. When separating compounds with a wide range of boiling points and polarities, it helps to raise the column temperature during the separation. Temperature programming reduces elution times of highly retained compounds. Adjusting the pressure will have a similar affect; higher pressures cause greater strain on the intermolecular interactions between the compound and stationary phase, ultimately reducing the retention time.

In this experiment, you will gain experience with the Vernier Mini GC by injecting a known sample into the device. The sample contains five compounds that will separate under the proper conditions. You will test this one mixture of compounds repeatedly and vary the profile of the Mini GC operation to obtain the best possible separation of this mixture.

In this experiment, you will

• Measure and analyze the chromatogram of a mixture of five compounds as they pass through a Vernier Mini GC.
• Vary the temperature-pressure profile of the Mini GC and observe how the chromatogram is affected by such changes.
• Determine the best temperature-pressure profile to obtain clear separation of all five compounds.

Sensors and Equipment

This experiment features the following sensors and equipment. Additional equipment may be required.

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This experiment is #8 of Organic Chemistry with Vernier . The experiment in the book includes student instructions as well as instructor information for set up, helpful hints, and sample graphs and data.

Gas Chromatography - What It Is and How It Works

Introduction to Gas Chromatography

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• Ph.D., Biomedical Sciences, University of Tennessee at Knoxville
• B.A., Physics and Mathematics, Hastings College

Gas chromatography (GC) is an analytical technique used to separate and analyze samples that can be vaporized without thermal decomposition . Sometimes gas chromatography is known as gas-liquid partition chromatography (GLPC) or vapor-phase chromatography (VPC). Technically, GPLC is the most correct term, since the separation of components in this type of chromatography relies on differences in behavior between a flowing mobile gas phase and a stationary liquid phase .

The instrument that performs gas chromatography is called a gas chromatograph . The resulting graph that shows the data is called a gas chromatogram .

Uses of Gas Chromatography

GC is used as one test to help identify components of a liquid mixture and determine their relative concentration . It may also be used to separate and purify components of a mixture . Additionally, gas chromatography can be used to determine vapor pressure , heat of solution, and activity coefficients. Industries often use it to monitor processes to test for contamination or ensure a process is going as planned. Chromatography can test blood alcohol, drug purity, food purity, and essential oil quality. GC may be used on either organic or inorganic analytes, but the sample must be volatile . Ideally, the components of a sample should have different boiling points.

How Gas Chromatography Works

First, a liquid sample is prepared. The sample is mixed with a solvent and is injected into the gas chromatograph. Typically the sample size is small -- in the microliters range. Although the sample starts out as a liquid, it is vaporized into the gas phase. An inert carrier gas is also flowing through the chromatograph. This gas shouldn't react with any components of the mixture. Common carrier gases include argon, helium, and sometimes hydrogen. The sample and carrier gas are heated and enter a long tube, which is typically coiled to keep the size of the chromatograph manageable. The tube may be open (called tubular or capillary) or filled with a divided inert support material (a packed column). The tube is long to allow for a better separation of components. At the end of the tube is the detector, which records the amount of sample hitting it. In some cases, the sample may be recovered at the end of the column, too. The signals from the detector are used to produce a graph, the chromatogram, which shows the amount of sample reaching the detector on the y-axis and generally how quickly it reached the detector on the x-axis (depending on what exactly the detector detects). The chromatogram shows a series of peaks. The size of the peaks is directly proportional to the amount of each component, although it can't be used to quantify the number of molecules in a sample. Usually, the first peak is from the inert carrier gas and the next peak is the solvent used to make the sample. Subsequent peaks represent compounds in a mixture. In order to identify the peaks on a gas chromatogram, the graph needs to be compared to a chromatogram from a standard (known) mixture, to see where the peaks occur.

At this point, you may be wondering why the components of the mixture separate while they are pushed along the tube. The inside of the tube is coated with a thin layer of liquid (the stationary phase). Gas or vapor in the interior of the tube (the vapor phase) moves along more quickly than molecules that interact with the liquid phase. Compounds that interact better with the gas phase tend to have lower boiling points (are volatile) and low molecular weights, while compounds that prefer the stationary phase tend to have higher boiling points or are heavier. Other factors that affect the rate at which a compound progresses down the column (called the elution time) include polarity and the temperature of the column. Because temperature is so important, it is usually controlled within tenths of a degree and is selected based on the boiling point of the mixture.

Detectors Used for Gas Chromatography

There are many different types of detectors that can be used to produce a chromatogram. In general, they may be categorized as non-selective , which means they respond to all compounds except the carrier gas, selective , which respond to a range of compounds with common properties, and specific , which respond only to a certain compound. Different detectors use particular support gases and have different degrees of sensitivity. Some common types of detectors include:

When the support gas is called "make up gas", it means gas is used to minimize band broadening. For FID, for example, nitrogen gas (N 2 ) is often used. The user manual that accompanies a gas chromatograph outlines the gases that can be used in it and other details.

• Pavia, Donald L., Gary M. Lampman, George S. Kritz, Randall G. Engel (2006).  Introduction to Organic Laboratory Techniques (4th Ed.) . Thomson Brooks/Cole. pp. 797–817.
• Grob, Robert L.; Barry, Eugene F. (2004).  Modern Practice of Gas Chromatography (4th Ed.) . John Wiley & Sons.
• Harris, Daniel C. (1999). "24. Gas Chromatography". Quantitative chemical analysis  (Fifth ed.). W. H. Freeman and Company. pp. 675–712. ISBN 0-7167-2881-8.
• Higson, S. (2004). Analytical Chemistry. Oxford University Press. ISBN 978-0-19-850289-0
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Gas Chromatography-Mass Spectrometry

A national historic chemical landmark.

Dedicated at the H Hotel in Midland, Michigan, on June 8, 2019

In Star Trek , Mr. Spock’s hand-held tricorder can instantly tell what something is made of. We don’t have tricorders yet, but we’re getting close. Portable devices just a little too big to hold in one hand are used today to analyze samples at crime scenes, fires, and other places where time is of the essence.

The technology had its start 60 years ago in Midland, Michigan, with the pairing of two powerful analytical techniques — gas chromatography (GC) and mass spectrometry (MS). By coupling the ability of GC to separate a chemical mixture with the ability of MS to identify its components, the new, combined technique proved revolutionary. GC-MS is now routinely used for speedy analysis in forensics, environmental monitoring, drug testing of athletes, and other applications. 

Early mass spectrometry

• Gas chromatography's origins

GC and MS pair up

Further development, landmark dedication and acknowledgements, research resources.

The origin of MS dates to the early 20 th century, when Sir Joseph John “J. J.” Thomson of the University of Cambridge was studying the structure and behavior of atoms and molecules. Building on his and others’ previous research, Thomson in 1907 developed a device that created an electric arc in a container holding a small amount of a gas. The electrical discharge stripped electrons from the gas molecules, creating a variety of positively charged ions with a range of masses.

In the presence of an electric field, the ions could be accelerated and manipulated. When pushed through a magnetic field, the stream of ions bent and separated like light through a prism, Thomson discovered. The ions then struck a fluorescent screen or photographic plate at locations dictated by their mass-to-charge ratios, creating bright streaks where they landed. The resulting patterns were different for different materials, so Thomson could identify pure materials by their unique patterns.

Given this background, some historians credit Thomson as the inventor of MS. Most others look to his assistant, Francis W. Aston, who made multiple improvements and won a Nobel Prize in Chemistry in 1922 for the development of the first workable mass spectrograph.

By the mid-20 th century, more-advanced mass spectrometers became commercially available. For each sample analyzed, the ions yielded a chart or “mass spectrum” from which the original molecule’s structure could be inferred.

If analyzed under identical conditions, any given compound always produces the same family of ions, creating a unique mass spectrum for each compound. When two or more compounds are present, the mass spectrum is a combination of the spectrum of each component. The result may be so messy it can’t be used to identify the components, meaning MS works well for pure materials, but not so well for mixtures.

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“GC-MS is the synergistic combination of two powerful microanalytical techniques. The gas chromatograph separates the components of a mixture in time, and the mass spectrometer provides information that aids in the structural identification of each component.”— Gas Chromatography and Mass Spectrometry: A Practical Guide

Gas chromatography's origins

The first widely noticed introduction of GC was made in 1951-52 by Anthony T. James and Archer J. P. Martin of the National Institute for Medical Research, in London. Commercial instruments soon followed. The technique built on earlier chromatography research by multiple scientists, including work that earned Martin and Richard L. M. Synge the 1952 Nobel Prize in Chemistry .

GC relies on the differing affinities of vapor components for surfaces. In a gas chromatograph, a mixture is first vaporized and picked up by an inert gas. This carrier gas is then pushed into a tube or “column” that in early days was packed with small, solid particles. Due to their different chemical properties some compounds interact with the solid surfaces more strongly than others and are slowed in their race through the column. At the end of the column is a specialized detector that produces a signal as each compound exits the column, with the signal intensity corresponding roughly to the relative amount of each component. Plotting the signal on graph paper (or in later years, on a computer screen) gives a peak for each component in the mix. The pattern of peaks, or “chromatogram,” is reproducible for any given sample, assuming it’s run through the column in the same way.

Many GC columns separate compounds approximately by boiling point. Low-boiling substances move faster and have lower retention times than higher-boiling substances. However, boiling points aren’t unique, so different chemicals can have the same retention time. That means chromatographic retention time alone isn’t enough to unambiguously identify a component in a mixture.

“GC-MS is indispensable in the fields of environmental science, forensics, health care, medical and biological research, health and safety, the flavor and fragrances industry, food safety, packaging, and many others.”— Gas Chromatography and Mass Spectrometry: A Practical Guide

In 1950, Fred McLafferty and Roland Gohlke, two Dow Co. researchers, dramatically enhanced the analytical power of GC by coupling it with MS. Adding MS allowed each component exiting the gas chromatograph to be analyzed separately.

Taken together, the mass spectra and the chromatographic peaks allowed unambiguous identification of each component. For an unknown mixture, the mass spectrum for each peak can narrow the possible identity of each component. Known standards can then confirm the identifications if both retention time and mass spectra match.

In coupling GC with MS, Gohlke and McLafferty overcame many issues. Chromatography columns weren’t commercially available, so they had to make their own. GC operates under pressure, whereas MS operates in vacuum. They had to devise a valving arrangement that would leak only a little of the total material coming from the gas chromatograph, without altering retention times. They also had to rapidly capture the fleeting mass spectrum for each compound: Lab computers didn’t exist at the time, so they photographed each spectrum as it briefly appeared on an oscilloscope.

After making a gas chromatograph and valve they thought would work, the researchers met with William C. Wiley and Daniel B. Harrington at Bendix Aviation Corp in Southfield, Michigan. There, McLafferty and Gohlke coupled their gas chromatograph with a very fast mass spectrometer that Wiley and his Bendix colleagues had developed. In short order they produced spectra of acetone, benzene, carbon tetrachloride, and toluene from a mixture of these compounds.

After this first successful demonstration of a paired GC-MS instrument in the winter of 1955-56, McLafferty and Gohlke convinced Dow to buy a Bendix mass spectrometer. Gohlke continued the GC-MS experiments at Dow’s spectroscopy lab with numerous colleagues. He and McLafferty first presented their results at the American Chemical Society’s April 1956 national meeting. Gohlke published the first journal article about their GC-MS work in Analytical Chemistry in 1959.

Around this same time, Joseph C. Holmes and Francis A. Morrell of Philip Morris Inc. also coupled GC and MS using a slower spectrometer made by Consolidated Engineering Corp., an approach the Dow scientists had rejected. Holmes and Morrell initially announced their findings at an American Society for Testing and Materials MS committee meeting in Cincinnati in May 1956. They wrote up their findings in a 1957 paper in Applied Spectroscopy .

Holmes and Morrell are credited by some for the development of GC-MS due to the independent but near-simultaneous demonstration. None of these four scientists patented the technology, leaving other researchers and companies free to adapt and improve on the method.

Mass spectrometers work on several different principles. The Bendix spectrometer used by Gohlke and McLafferty was a “time-of-flight” instrument that produced a spectrum based on the time it took ions to traverse a long tube. Bendix began marketing a GC-MS device in 1959, but the first commercial success was LKB Instruments Inc.’s Model 9000, which debuted in 1965. The LKB instrument used a magnet to disperse the ions just like Thomson did years earlier. Other companies followed suit, including Finnigan Instruments, Perkin Elmer, and Hewlett Packard, now Agilent.

Several other advances paved the way for GC-MS to go mainstream. The instruments became smaller and less expensive. With developments in computing power, libraries of mass spectra could be compiled and computers could identify chromatographic peaks.

GC-MS is an essential technology in modern analytical chemistry labs. Applications include development of new pharmaceuticals and analysis of their purity, detection of chemical warfare agents and explosives, screening of athletes’ urine for banned performance-enhancing substances, and analyzing soil samples on Mars. Portable units can now be carried in one arm for on-site analysis, bringing us closer than ever to Star Trek’s vision.

Landmark dedication

The American Chemical Society (ACS) honored The Dow Chemical Company’s innovation in combining gas chromatography and mass spectrometry with a National Historic Chemical Landmark (NHCL) in a ceremony at the H Hotel in Midland, Michigan, on June 8, 2019. The commemorative plaque reads:

In 1955-56, Dow Chemical scientists Fred McLafferty and Roland Gohlke first demonstrated the combination of gas chromatography (GC) and mass spectrometry (MS) to identify individual substances in a mixture. This was the first coupling of a separation technology with a spectrometry technique to provide rapid characterization of chemical components. GC-MS remains one of the most powerful, flexible, and widely used tools for analyzing chemical mixtures in drug screening, forensic, environmental, and trace analysis, as well as other applications.

Acknowledgements

Written by Mark Jones.

The author wishes to thank contributors to and reviewers of this booklet, all of whom helped improve its content, especially members of the ACS NHCL Subcommittee.

The nomination for this Landmark designation was prepared by the Midland Section of the ACS and The Dow Chemical Co.

Further reading

• "Gas Chromatography and Mass Spectrometry: A Practical Guide"

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