Scientists usually travel much slower than low

Scientists interested in identifying the individual chemicals in a mixture use an instrumental method called gas chromatography mass spectrometry or GC-MS. Continue reading to learn about the ins and outs of this fascinating chemistry technique.

Identifying Chemical Compounds: GC-MS Overview

Have you ever grabbed an item from the grocery store shelf only to wonder exactly how much of each ingredient that item contained? Scientists are often faced with a similar dilemma. They may need to know all of the ingredients (or chemicals) and in what amount are used to create a chemical mixture. One instrumental method called gas chromatography mass spectrometry can be used to help tackle this issue.

Example of a GC-MS Machine
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Gas chromatography mass spectrometry, or GC-MS, is an analytical technique used to separate, identify, and quantify chemical mixtures. Just by looking at its name you may have gathered that GC-MS is a combination of two techniques.

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  1. The first part is gas chromatography, which is used to physically separate your mixture into pure individual chemicals according to their boiling point (the temperature at which a liquid transitions to a gas or vapor.
  2. The second part, mass spectrometry, is used to identify and quantify each chemical that is separated by gas chromatography.

What samples are commonly analyzed by a GC-MS? Typically complex mixtures such as the air we breathe and water we drink are commonly analyzed for compounds in very small or trace amounts. By using GC-MS you can discover not only what is present in a sample, but compare it to a standard.

A standard is a sample that has a known amount or concentration.

Understanding the Gas Chromatograph

One characteristic of a sample used in GC-MS is its ability to be volatile in nature and stable at high temperatures. Gas chromatography is built from five main components: mobile phase, injection port, stationary phase, detector, and a data recording system.

  • The mobile phase or ‘moving’ phase is a gas, usually helium (but sometimes nitrogen or hydrogen) that is used to carry the sample mixture to the stationary phase.
  • The injection port is where your sample is injected for analysis. At this site the temperature is around 150-250C so that the liquid sample can become volatiles and turn from a liquid to a vapor.

    Why should the liquid be a vapor at this point? To make sure the carrier gas can carry this volatile sample through the GC.

  • The stationary phase is a solid substance inside a tubing called a column. It is what the liquid mixture (carried by the mobile phase) can be adsorbed onto. Adsorbption, not to be confused with absorption, occurs when a liquid or gas molecule sticks onto a solid surface. The rate at which the sample travels through the column is dependent on the physical properties of molecular weight and boiling point. For example, heavy compounds with high boiling points usually travel much slower than low molecular weight compounds with low boiling points.

Example of Liquid Molecules Passing Through A Column
gcms stationary phase
  • The detector tube is what measures the individual chemicals that are released from the column.
  • The recorder plots out what is detected from the detectors based on signals that are received. This recording output is called a chromatograph.

Schematic Design of How GC-MS Works
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If gas chromatography is used to separate the compounds and report these findings as a chromatograph, what can we use to identify and quantify these findings? This is where the mass spectrometer comes to the rescue!

All About The Mass Spectrometer

Mass spectrometry is a technique used to measure the mass to charge ratio (MZ ratio) of molecules that are present in a sample. After an individual compound leaves the GC column it enters the ionization source in a mass spectrometer. While here, a stream of electrons attacks the compound.This electron stream causes the compound to break into several large or small fragments, which are charged ions that have a certain mass. Dividing this mass by its respective charge allows you to determine the M/Z ratio. This ratio enables a scientist to use the mass spectrometer to identify and quantify compounds that are separated by the GC.

Results from the mass spectrometer are placed onto a graph called a mass spectrum. This spectrum can be compared to a library database of spectra when trying to identify a sample. Thus, the mass spectrum can be thought of as a chemical fingerprint for the sample being analyzed. By comparing it to a library of known fingerprints, a scientist can unravel the mystery behind what the unknown sample in a mixture is.

Lesson Summary

Gas chromatography mass spectrometry or GC-MS is an analytical technique used to separate, identify, and quantify unknown chemical mixtures.

  • The gas chromatography is responsible for separating compounds by their boiling point.
  • Mass spectrometry uses the mass to charge ratio (MZ ratio) to help identify and quantify chemicals that are separated by GC.

Gas chromatography has 5 stages:

  1. The mobile phase is when a gas, usually helium, carries the sample mixture to the stationary phase.
  2. The injection port is where your sample is injected for analysis.
  3. The stationary phase is when the sample travels and adsorbs (sticks to) to a column that sits inside an oven. The rate of travel depends on the physical properties of molecular weight and boiling point.

    (Heavier weight and high boiling point travels slower)

  4. The detector tube measures the individual chemicals that are released from the column.
  5. The recorder plots out what is detected using a chromatograph.

The output for mass spectrometry is a mass spectrum (graph). Standards (samples with known amount or concentrations) are useful in determining both what the sample is and how much is present.

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