The Fundamentals of GC-MS

The Fundamentals of GC-MS Gas chromatography mass spectrometry (GC-MS) is a powerful analytical approach that combines the utility of two very different, but complementary, techniques: gas chromatography and mass spectrometry. This infographic will outline the general principles of GC-MS, including a comparison of common mass analyzers used in GC-MS and GC-MS/MS systems. By combining these two approaches, the GC-MS technique can output threedimensional data that enables the identification of unknown compounds, as well as outputting a chromatogram that can be used for qualitative and quantitative analysis. In a standard workflow, a sample mixture is injected into the gas chromatograph, where it is separated out into its constituent parts before those analytes are eluted into the MS for detection. Sample injection Vaporization Column chromatography Separation Mass analyzer Ion detector Sample MS detector Sample Injection Depending on the exact sample being tested, there may be a need to do some initial sample clean-up before the sample is introduced to the apparatus. Once prepared, the sample can be injected manually or via an autosampler into the gas chromatograph, where it is vaporized. Step 1 Elution With the molecules separated, they are sent through a heated transfer line into a connected mass spectrometer. Step 3 Mass analysis These ions are separated according to their mass-to-charge ratio (m/z) using a mass analyzer. There are many distinct mass analyzers that are commonly used for GC-MS, each with a slightly different working principle. Step 5 • Another of the most common mass analyzer types used • Consists of four parallel conducting rods spaced around a central axis. Each opposing rod pair is connected, with a radio frequency voltage and fixed direct current (DC) applied. • Setup ensures that only ions of a certain m/z will reach the detector over a given range of voltages; others will simply collide with the rods. • Varying the applied voltage allows the operating analyst to scan through a wide range of m/z values Quadrupole mass analyzer • Tandem mass spectrometry methods involve the use of more than one mass spectrometer at once. This helps to reduce background from co-eluting compounds that might have the same m/z but different chemical structures. • In gas chromatography with tandem mass spectrometry (GC-MS/MS), the first mass analyzer selects a so-called precursor ion that is fragmented further in an additional step. The resulting product ions are separated using the second mass analyzer, before being fed into the ion detector. Tandem mass spectrometry Ionization Once the molecules arrive in the mass spectrometer, they are ionized by electron ionization. This ionization process will ionize the analyte molecules as they come in from the chromatograph, but it can also lead to molecules fragmenting. This results in a very large number of ions with different masses entering the mass spectrometer. Step 4 Chromatography A typical gas chromatograph consists of an injection port, a spooled chromatography column, a carrier gas inlet, heaters for controlling the temperature of the column and a detector. Step 2 Retention time, explained Gas chromatography separates compounds based on their retention time – a measure of the time taken for that compound to fully pass through the chromatography column. A compound’s retention time will depend on: • Volatility A compound with a high boiling point will spend nearly all its time in the column condensed as a liquid, resulting in long retention times. • PolarityIf the polarity of the compound and the column stationary phase are similar, the retention time will increase due to strong polar interactions between the two. • Column temperatureExcessively high column temperatures will result in very short retention times for all compounds, but at the expense of poor overall separation. Once the sample is injected and vaporized, it will be carried through the chromatographic column by an inert carrier gas. Compounds will travel through the column at differing rates depending on their physical and chemical properties, which causes the sample mixture to separate out into its various components. Retetion times depend on a compound's: • Boiling point/vapor pressure • Polarity Ion detector Once the ions have been separated by the mass analyzer based on their m/z, they are sent to the ion detector. This signal is what is recorded by acquisition software and outputted in the form of a chromatogram and a mass spectrum for each data point. Step 6 The mass analyzer is the heart of the mass spectrometer apparatus. There are many different mass analyzer types available that work well with GC-MS analysis. Common mass analyzers for GC-MS • One of the most common mass analyzers. • Separates by m/z based on the length of time taken for ions to travel through a flight tube and reach a detector. • Ions with a larger m/z will travel the slowest and arrive at the detector last. • Good mass resolution, with a very wide range of m/z values able to be collected in parallel. Time-of-flight (TOF) mass analyzer • Similarly to how a glass prism disperses light, a magnetic sector mass analyzer uses magnetic fields to alter the trajectory of ions according to their m/z ratios. • Magnetic sector mass analyzers are most commonly used for isotope ratio analyses. Magnetic sector mass analyzers • Consists of two outer electrodes and a spindle-like central electrode. • When the voltage on the inner electrode is ramped up, ions will begin to orbit inside the trap. • According to their specific m/z ratio, the ions will spread into rings orbiting around the inner spindle. • Measuring the oscillation frequency induced by ions on the outer electrons allows a mass spectrum to be acquired. • Orbitrap mass analysis can achieve a very high mass resolution and accuracy. Orbitrap mass analyzers Applications of GC-MS and GC-MS/MS