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Gas chromatography-Gas chromatography-mass spectrometry mass spectrometry (Additional (Additional

Reading)Reading)

prof. azaprof. aza

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IntroductionIntroduction

Gas chromatography-mass spectrometryGas chromatography-mass spectrometry ((GC-MSGC-MS) is a method that combines the features ) is a method that combines the features of of gas-liquid chromatographygas-liquid chromatography and and mass spectrometrymass spectrometry to identify different to identify different substances within a test sample. Applications of substances within a test sample. Applications of GC-MS include GC-MS include drugdrug detection, fire investigation, detection, fire investigation, environmental analysis, and explosives environmental analysis, and explosives investigation. GC-MS can also be used in airport investigation. GC-MS can also be used in airport security to detect substances in luggage or on security to detect substances in luggage or on human beings. Additionally, it can identify trace human beings. Additionally, it can identify trace elements in materials that were previously elements in materials that were previously thought to have disintegrated beyond thought to have disintegrated beyond identification.identification.

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Introduction, continuedIntroduction, continued

The GC-MS has been widely heralded as a "The GC-MS has been widely heralded as a "gold standardgold standard" for " for forensicforensic substance substance identification because it is used to perform a identification because it is used to perform a specific testspecific test. A specific test positively identifies . A specific test positively identifies the actual presence of a particular substance in the actual presence of a particular substance in a given sample. A a given sample. A non-specific testnon-specific test, however, , however, merely indicates that a substance falls into a merely indicates that a substance falls into a category of substances. Although a non-specific category of substances. Although a non-specific test could statistically suggest the identity of the test could statistically suggest the identity of the substance, this could lead to false positive substance, this could lead to false positive identification.identification.

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1 History1 History 2 Instrumentation2 Instrumentation 3 Analysis3 Analysis 4 Applications4 Applications

4.1 Environmental Monitoring and Cleanup4.1 Environmental Monitoring and Cleanup 4.2 Criminal Forensics4.2 Criminal Forensics 4.3 Law Enforcement4.3 Law Enforcement 4.4 Security4.4 Security 4.5 4.5 AstrochemistryAstrochemistry 4.6 Medicine4.6 Medicine

5 See also5 See also 6 External links6 External links 7 References7 References

ContentsContents

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HistoryHistory

The use of a mass spectrometer as the detector in gas The use of a mass spectrometer as the detector in gas chromatography was developed during the 1960s. These chromatography was developed during the 1960s. These sensitive devices were bulky, fragile, and originally sensitive devices were bulky, fragile, and originally limited to laboratory settings. The development of limited to laboratory settings. The development of affordable and affordable and miniaturizedminiaturized computerscomputers has helped in the has helped in the simplification of the use of this instrument, as well as simplification of the use of this instrument, as well as allowed great improvements in the amount of time it allowed great improvements in the amount of time it takes to analyse a sample. In 1996 the top-of-the-line takes to analyse a sample. In 1996 the top-of-the-line high-speed GC-MS units completed analysis of fire high-speed GC-MS units completed analysis of fire accelerants in less than 90 seconds, whereas first-accelerants in less than 90 seconds, whereas first-generation GC-MS would have required at least 16 generation GC-MS would have required at least 16 minutes. This has led to their widespread adoption in a minutes. This has led to their widespread adoption in a number of fields.number of fields.

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InstrumentationInstrumentation

The GC-MS is composed of two major building blocks: The GC-MS is composed of two major building blocks: the the gas chromatographgas chromatograph and the and the mass spectrometermass spectrometer. The . The gas chromatograph uses the difference in the chemical gas chromatograph uses the difference in the chemical properties between different properties between different moleculesmolecules in a in a mixturemixture to to separate the molecules. The molecules take different separate the molecules. The molecules take different amounts of time (called the retention time) to come out amounts of time (called the retention time) to come out of the gas chromatograph, and this allows the mass of the gas chromatograph, and this allows the mass spectrometer downstream to evaluate the molecules spectrometer downstream to evaluate the molecules separately in order to identify them. The mass separately in order to identify them. The mass spectrometer does this by breaking each molecule into spectrometer does this by breaking each molecule into ionizedionized fragments and detecting these fragments using fragments and detecting these fragments using their mass to charge ratio. Each molecule has a specific their mass to charge ratio. Each molecule has a specific fragment fragment spectrumspectrum which allows for its detection. which allows for its detection.

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InstrumentationInstrumentationThese two components, used together, allow a These two components, used together, allow a much finer degree of substance identification much finer degree of substance identification than either unit used separately. It is possible to than either unit used separately. It is possible to make an accurate identification of a particular make an accurate identification of a particular molecule by gas chromatography or mass molecule by gas chromatography or mass spectrometry alone. The mass spectrometry spectrometry alone. The mass spectrometry process normally requires a very pure sample process normally requires a very pure sample while gas chromatography can be confused by while gas chromatography can be confused by different molecular types that both happen to different molecular types that both happen to take about the same amount of time to travel take about the same amount of time to travel through the unit (through the unit (i.e.i.e. have the same retention have the same retention time). time).

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Instrumentation, continuedInstrumentation, continued

Sometimes two different molecules can also Sometimes two different molecules can also have a similar pattern of ionized fragments in a have a similar pattern of ionized fragments in a mass spectrometer (mass spectrum). Combining mass spectrometer (mass spectrum). Combining the two processes makes it extremely unlikely the two processes makes it extremely unlikely that two different molecules will behave in the that two different molecules will behave in the same way in both a gas chromatograph and a same way in both a gas chromatograph and a mass spectrometer. So when an identifying mass spectrometer. So when an identifying mass spectrum appears at a characteristic mass spectrum appears at a characteristic retention time in a GC-MS analysis, it is usually retention time in a GC-MS analysis, it is usually taken as proof of the presence of that particular taken as proof of the presence of that particular molecule in the sample.molecule in the sample.

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AnalysisAnalysis

The primary goal of chemical analysis is to identify a The primary goal of chemical analysis is to identify a substance. This is done by comparing the relative substance. This is done by comparing the relative concentrations among the atomic masses in the concentrations among the atomic masses in the generated spectrum. Two kinds of analysis are possible, generated spectrum. Two kinds of analysis are possible, comparative and original. Comparative analysis comparative and original. Comparative analysis essentially compares the given spectrum to a spectrum essentially compares the given spectrum to a spectrum library to see if its characteristics are present for some library to see if its characteristics are present for some sample in the library. This is best performed by a sample in the library. This is best performed by a computercomputer because there are a myriad of visual because there are a myriad of visual distortions that can take place due to variations in scale. distortions that can take place due to variations in scale. Computers can also simultaneously correlate more data Computers can also simultaneously correlate more data (such as the retention times identified by GC), to more (such as the retention times identified by GC), to more accurately relate certain data.accurately relate certain data.

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Another analysis measures the peaks in relation Another analysis measures the peaks in relation to one another, with the tallest peak receiving to one another, with the tallest peak receiving 100% of the value, and the others receiving 100% of the value, and the others receiving proportionate values, with all values above 3% proportionate values, with all values above 3% being accounted for. The parent peak normally being accounted for. The parent peak normally indicates the total mass of the unknown indicates the total mass of the unknown compound. This value can then be used to fit to compound. This value can then be used to fit to a chemical a chemical formulaformula containing the various containing the various elementselements assumed to be present in the assumed to be present in the compound. The compound. The isotopeisotope pattern in the spectrum, pattern in the spectrum, which is unique for elements having many which is unique for elements having many isotopes, can also be used to identify the various isotopes, can also be used to identify the various elements present. elements present.

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Once a chemical formula has been matched to the Once a chemical formula has been matched to the spectrum, the molecular structure and bonding can be spectrum, the molecular structure and bonding can be identified, and needs to be consistent with a substance identified, and needs to be consistent with a substance with the characteristics recorded by GC/MS. The fitting is with the characteristics recorded by GC/MS. The fitting is normally done automatically by programmes which come normally done automatically by programmes which come with the machine, given a list of the elements which with the machine, given a list of the elements which could be present in the sample.could be present in the sample.A “full spectrum” analysis considers all the “peaks” within A “full spectrum” analysis considers all the “peaks” within a spectrum. However, selective ion monitoring (SIM) a spectrum. However, selective ion monitoring (SIM) which looks only at a few characteristic peaks associated which looks only at a few characteristic peaks associated with a candidate substance, can also be done.with a candidate substance, can also be done.

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This is done on the assumption that at a given This is done on the assumption that at a given retention time, a set of retention time, a set of ionsions is characteristic of a is characteristic of a certain compound. This is a fast and efficient certain compound. This is a fast and efficient analysis, especially if you have some prior analysis, especially if you have some prior information about a sample or are looking for a information about a sample or are looking for a specific compound. When the amount of specific compound. When the amount of information collected about the ions in a given information collected about the ions in a given gas chromatographic peak is reduced, the gas chromatographic peak is reduced, the sensitivity of the analysis goes up. So, SIM sensitivity of the analysis goes up. So, SIM analysis allows a smaller quantity of a analysis allows a smaller quantity of a compound to be detected and measured, but the compound to be detected and measured, but the degree of certainty about the identity of that degree of certainty about the identity of that compound is reducedcompound is reduced

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ApplicationsApplications

Environmental Monitoring and CleanupEnvironmental Monitoring and Cleanup

Criminal ForensicsCriminal Forensics

Law EnforcementLaw Enforcement

SecuritySecurity

AstrochemistryAstrochemistry

MedicineMedicine

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Environmental Monitoring and CleanupEnvironmental Monitoring and Cleanup

GC-MS is becoming the tool of choice for GC-MS is becoming the tool of choice for tracking tracking organic pollutantsorganic pollutants in the environment. in the environment. The cost of GC-MS equipment has fallen The cost of GC-MS equipment has fallen significantly, and the reliability has increased at significantly, and the reliability has increased at the same time, which has contributed to its the same time, which has contributed to its increased adoption in environmental studies as increased adoption in environmental studies as cost is always a major consideration in this kind cost is always a major consideration in this kind of work. There are some compounds for which of work. There are some compounds for which GC-MS is not sufficiently sensitive, including GC-MS is not sufficiently sensitive, including certain pesticides and herbicides, but for most certain pesticides and herbicides, but for most organic analysis of environmental samples, organic analysis of environmental samples, including many major classes of pesticides, it is including many major classes of pesticides, it is very sensitive and effectivevery sensitive and effective

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Criminal ForensicsCriminal Forensics

GC-MS can analyze the particles from a GC-MS can analyze the particles from a human body in order to help link a criminal human body in order to help link a criminal to a to a crimecrime. The analysis of . The analysis of firefire debris debris using GC-MS is well established, and using GC-MS is well established, and there is even an established American there is even an established American Society for Testing Materials (ASTM) Society for Testing Materials (ASTM) standard for fire debris analysis.standard for fire debris analysis.

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Law EnforcementLaw Enforcement

GC-MS is increasingly used for GC-MS is increasingly used for detection of illegal narcotic, and may detection of illegal narcotic, and may eventually supplant drug-sniffing dogseventually supplant drug-sniffing dogs

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SecuritySecurity

A post-A post-September 11September 11 development, development, explosivesexplosives-detection -detection systems have become a part of all systems have become a part of all USUS airportsairports. These . These systems run on a host of technologies, many of them systems run on a host of technologies, many of them based on GC-MS. There are only three manufacturers based on GC-MS. There are only three manufacturers certified by the certified by the FAAFAA to provide these systems (citation to provide these systems (citation needed), one of which is Thermo Detection (formerly needed), one of which is Thermo Detection (formerly Thermedics), which produces the Thermedics), which produces the EGISEGIS, a GC-MS-based , a GC-MS-based line of explosives detectors. The other two line of explosives detectors. The other two manufacturers are Barringer Technologies, now owned manufacturers are Barringer Technologies, now owned by Smith's Detection Systems and Ion Track by Smith's Detection Systems and Ion Track Instruments, part of General Electric Infrastructure Instruments, part of General Electric Infrastructure Security Systems.Security Systems.

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AstrochemistryAstrochemistry

Several GC-MS have left earth. Two were Several GC-MS have left earth. Two were brought to brought to marsmars by the by the Viking programViking program..[1][1] VeneraVenera 11 and 12 and 11 and 12 and Pioneer VenusPioneer Venus analysed analysed the atmosphere of the atmosphere of venusvenus with GC-MS. with GC-MS.[2][2] The The Huygens probeHuygens probe of the of the Cassini-HuygensCassini-Huygens mission mission landed one GC-MS on landed one GC-MS on SaturnSaturn's largest moon, 's largest moon, TitanTitan..[3][3] The material in the The material in the cometcomet 67P/Churyumov-Gerasimenko67P/Churyumov-Gerasimenko will be analysed will be analysed by the by the RosettaRosetta mission with a chiral GC-MS in mission with a chiral GC-MS in 2014. 2014. [4][4]

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MedicineMedicine

In combination with isotopic labelling of In combination with isotopic labelling of metabolic compounds, the GC-MS is used metabolic compounds, the GC-MS is used for determining metabolic activity. Most for determining metabolic activity. Most applications are based on the use of C13 applications are based on the use of C13 as the labelling and the measurement of as the labelling and the measurement of C13/C12 ratios with an isotope ratio mass C13/C12 ratios with an isotope ratio mass spectrometer (IRMS); an MS with a spectrometer (IRMS); an MS with a detector designed to measure a few select detector designed to measure a few select ions and return values as ratios.ions and return values as ratios.

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MedicineMedicine

In combination with isotopic labelling of In combination with isotopic labelling of metabolic compounds, the GC-MS is used metabolic compounds, the GC-MS is used for determining metabolic activity. Most for determining metabolic activity. Most applications are based on the use of C13 applications are based on the use of C13 as the labelling and the measurement of as the labelling and the measurement of C13/C12 ratios with an isotope ratio mass C13/C12 ratios with an isotope ratio mass spectrometer (IRMS); an MS with a spectrometer (IRMS); an MS with a detector designed to measure a few select detector designed to measure a few select ions and return values as ratios.ions and return values as ratios.

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ReferencesReferencesEiceman, G.A. (2000). Gas Chromatography. In R.A. Meyers (Ed.), Eiceman, G.A. (2000). Gas Chromatography. In R.A. Meyers (Ed.), Encyclopedia of Analytical Chemistry: Applications, Theory, and Encyclopedia of Analytical Chemistry: Applications, Theory, and InstrumentationInstrumentation, pp. 10627. Chichester: Wiley. , pp. 10627. Chichester: Wiley. ISBN 0-471-97670-9ISBN 0-471-97670-9 Giannelli, Paul C. and Imwinkelried, Edward J. (1999). Drug Identification: Giannelli, Paul C. and Imwinkelried, Edward J. (1999). Drug Identification: Gas Chromatography. In Gas Chromatography. In Scientific EvidenceScientific Evidence 22, pp. 362. Charlottesville: , pp. 362. Charlottesville: Lexis Law Publishing. Lexis Law Publishing. ISBN 0-327-04985-5ISBN 0-327-04985-5. . ^̂ The Development of the Viking GCMSThe Development of the Viking GCMS ^̂ V. A. Krasnopolsky, V. A. Parshev (1981). "Chemical composition of the V. A. Krasnopolsky, V. A. Parshev (1981). "Chemical composition of the atmosphere of Venus". atmosphere of Venus". NatureNature 292292: 610 - 613. : 610 - 613. DOIDOI::10.1038/292610a010.1038/292610a0. . ^̂ H. B. Niemann, S. K. Atreya, S. J. Bauer, G. R. Carignan, J. E. Demick, H. B. Niemann, S. K. Atreya, S. J. Bauer, G. R. Carignan, J. E. Demick, R. L. Frost, D. Gautier, J. A. Haberman, D. N. Harpold, D. M. Hunten, G. R. L. Frost, D. Gautier, J. A. Haberman, D. N. Harpold, D. M. Hunten, G. Israel, J. I. Lunine, W. T. Kasprzak, T. C. Owen, M. Paulkovich, F. Raulin, Israel, J. I. Lunine, W. T. Kasprzak, T. C. Owen, M. Paulkovich, F. Raulin, E. Raaen, S. H. Way (2005). "The abundances of constituents of Titan’s E. Raaen, S. H. Way (2005). "The abundances of constituents of Titan’s atmosphere from the GCMS instrument on the Huygens probe". atmosphere from the GCMS instrument on the Huygens probe". NatureNature 438438: : 77-9-784. DOI:10.1038/nature04122. 77-9-784. DOI:10.1038/nature04122. ^̂ Goesmann F, Rosenbauer H, Roll R, Bohnhardt H (2005). Goesmann F, Rosenbauer H, Roll R, Bohnhardt H (2005). "COSAC "COSAC onboard Rosetta: A bioastronomy experiment for the short-period comet onboard Rosetta: A bioastronomy experiment for the short-period comet 67P/Churyumov-Gerasimenko". 67P/Churyumov-Gerasimenko". AstrobiologyAstrobiology 55 (5): 622-631. (5): 622-631. DOI:10.1089/ast.2005.5.622. DOI:10.1089/ast.2005.5.622.

Retrieved from "http://en.wikipedia.org/wiki/Gas_chromatography-Retrieved from "http://en.wikipedia.org/wiki/Gas_chromatography-mass_spectrometry"mass_spectrometry"

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Gas Chromatography-Mass Gas Chromatography-Mass SpectroscopySpectroscopybackgroundbackground

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IntroductionIntroductionGas chromatography-mass spectroscopy (GC-MS) is Gas chromatography-mass spectroscopy (GC-MS) is one of the so-called hyphenated analytical techniques. one of the so-called hyphenated analytical techniques. As the name implies, it is actually two techniques that As the name implies, it is actually two techniques that are combined to form a single method of analyzing are combined to form a single method of analyzing mixtures of chemicals. Gas chromatography separates mixtures of chemicals. Gas chromatography separates the components of a mixture and mass spectroscopy the components of a mixture and mass spectroscopy characterizes each of the components individually. By characterizes each of the components individually. By combining the two techniques, an analytical chemist can combining the two techniques, an analytical chemist can both qualitatively and quantitatively evaluate a solution both qualitatively and quantitatively evaluate a solution containing a number of chemicals.containing a number of chemicals.The uses for GC-MS are numerous. They are used The uses for GC-MS are numerous. They are used extensively in the medical, pharmacological, extensively in the medical, pharmacological, environmental, and law enforcement fields. This environmental, and law enforcement fields. This laboratory exercise will look at how environmental laboratory exercise will look at how environmental chemists evaluate samples containing the pollutants chemists evaluate samples containing the pollutants called PAHs.called PAHs.

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SRIF's GC-MS SystemSRIF's GC-MS System

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Gas ChromatographyGas Chromatography

In general, chromatography is used to separate In general, chromatography is used to separate mixtures of chemicals into individual mixtures of chemicals into individual components. Once isolated, the components components. Once isolated, the components can be evaluated individually. can be evaluated individually.

In all chromatography, separation occurs when In all chromatography, separation occurs when the sample mixture is introduced (injected) into a the sample mixture is introduced (injected) into a mobile phasemobile phase. In liquid chromatography (LC), . In liquid chromatography (LC), the mobile phase is a solvent. In gas the mobile phase is a solvent. In gas chromatography (GC), the mobile phase is an chromatography (GC), the mobile phase is an inert gas such as helium. inert gas such as helium.

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The mobile phase carries the sample The mobile phase carries the sample mixture through what is referred to as a mixture through what is referred to as a stationary phasestationary phase. The stationary phase is . The stationary phase is a usually chemical that can selectively a usually chemical that can selectively attract components in a sample mixture. attract components in a sample mixture. The stationary phase is usually contained The stationary phase is usually contained in a tube of some sort. This tube is in a tube of some sort. This tube is referred to as a column. Columns can be referred to as a column. Columns can be glass or stainless steel of various glass or stainless steel of various dimensions. dimensions.

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The mixture of compounds in the mobile phase The mixture of compounds in the mobile phase interacts with the stationary phase. Each interacts with the stationary phase. Each compound in the mixture interacts at a different compound in the mixture interacts at a different rate. Those that interact the fastest will exit rate. Those that interact the fastest will exit (elute from) the column first. Those that interact (elute from) the column first. Those that interact slowest will exit the column last. By changing slowest will exit the column last. By changing characteristics of the mobile phase and the characteristics of the mobile phase and the stationary phase, different mixtures of chemicals stationary phase, different mixtures of chemicals can be separated. Further refinements to this can be separated. Further refinements to this separation process can be made by changing separation process can be made by changing the temperature of the stationary phase or the the temperature of the stationary phase or the pressure of the mobile phase.pressure of the mobile phase.

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Our GC has a long, thin column containing Our GC has a long, thin column containing a thin interior coating of a solid stationary a thin interior coating of a solid stationary phase (5% phenyl-, 95% dimethylsiloxane phase (5% phenyl-, 95% dimethylsiloxane polymer). This 0.25 mm diameter column polymer). This 0.25 mm diameter column is referred to as a capillary column. This is referred to as a capillary column. This particular column is used for semivolatile, particular column is used for semivolatile, non-polar organic compounds such as the non-polar organic compounds such as the PAHs we will look at. The compounds PAHs we will look at. The compounds must me in an organic solvent. must me in an organic solvent.

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The capillary column is held in an oven that can The capillary column is held in an oven that can be programmed to increase the temperature be programmed to increase the temperature gradually (or in GC terms, ramped). this helps gradually (or in GC terms, ramped). this helps our separation. As the temperature increases, our separation. As the temperature increases, those compounds that have low boiling points those compounds that have low boiling points elute from the column sooner than those that elute from the column sooner than those that have higher boiling points. Therefore, there are have higher boiling points. Therefore, there are actually two distinct separating forces, actually two distinct separating forces, temperature and stationary phase interactions temperature and stationary phase interactions mentioned previously. mentioned previously.

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As the compounds are separated, they elute As the compounds are separated, they elute from the column and enter a detector. The from the column and enter a detector. The detector is capable of creating an electronic detector is capable of creating an electronic signal whenever the presence of a compound is signal whenever the presence of a compound is detected. The greater the concentration in the detected. The greater the concentration in the sample, the bigger the signal. The signal is then sample, the bigger the signal. The signal is then processed by a computer. The time from when processed by a computer. The time from when the injection is made (time zero) to when elution the injection is made (time zero) to when elution occurs is referred to as the occurs is referred to as the retention timeretention time (RT). (RT).

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While the instrument runs, the computer generates a While the instrument runs, the computer generates a graph from the signal. (See figure 1). This graph is called graph from the signal. (See figure 1). This graph is called a a chromatogramchromatogram. Each of the . Each of the peakspeaks in the in the chromatogram represents the signal created when a chromatogram represents the signal created when a compound elutes from the GC column into the detector. compound elutes from the GC column into the detector. The x-axis shows the RT, and the y-axis shows the The x-axis shows the RT, and the y-axis shows the intensity (abundence) of the signal. In Figure 1, there are intensity (abundence) of the signal. In Figure 1, there are several peaks labeled with their RTs. Each peak several peaks labeled with their RTs. Each peak represents an individual compound that was separated represents an individual compound that was separated from a sample mixture. The peak at 4.97 minutes is from from a sample mixture. The peak at 4.97 minutes is from dodecane, the peak at 6.36 minutes is from biphenyl, the dodecane, the peak at 6.36 minutes is from biphenyl, the peak at 7.64 minutes is from chlorobiphenyl, and the peak at 7.64 minutes is from chlorobiphenyl, and the peak at 9.41 minutes is from hexadecanoic acid methyl peak at 9.41 minutes is from hexadecanoic acid methyl ester.ester.

Figure 1:Figure 1: Chromatogram generated by a GC. Chromatogram generated by a GC.

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If the GC conditions (oven temperature ramp, If the GC conditions (oven temperature ramp, column type, etc.) are the same, a given column type, etc.) are the same, a given compound will always exit (elute) from the compound will always exit (elute) from the column at nearly the same RT. By knowing the column at nearly the same RT. By knowing the RT for a given compound, we can make some RT for a given compound, we can make some assumptions about the identity of the compound. assumptions about the identity of the compound. However, compounds that have similar However, compounds that have similar properties often have the same retention times. properties often have the same retention times. Therefore, more information is usually required Therefore, more information is usually required before an analytic al chemist can make an before an analytic al chemist can make an identification of a compound in a sample identification of a compound in a sample containing unknown components.containing unknown components.

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Mass SpectroscopyMass SpectroscopyAs the individual compounds elute from the GC As the individual compounds elute from the GC column, they enter the electron ionization (mass column, they enter the electron ionization (mass spec) detector. There, they are bombarded with spec) detector. There, they are bombarded with a stream of electrons causing them to break a stream of electrons causing them to break apart into fragments. These fragments can be apart into fragments. These fragments can be large or small pieces of the original molecules. large or small pieces of the original molecules. The fragments are actually charged ions with a The fragments are actually charged ions with a certain mass. The mass of the fragment divided certain mass. The mass of the fragment divided by the charge is called the mass to charge ratio by the charge is called the mass to charge ratio (M/Z). Since most fragments have a charge of (M/Z). Since most fragments have a charge of +1, the M/Z usually represents the molecular +1, the M/Z usually represents the molecular weight of the fragment. weight of the fragment.

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A group of 4 electromagnets (called a A group of 4 electromagnets (called a quadrapolequadrapole, , focuses each of the fragments through a slit and into the focuses each of the fragments through a slit and into the detector. The quadropoles are programmed by the detector. The quadropoles are programmed by the computer to direct only certain M/Z fragments through computer to direct only certain M/Z fragments through the slit. The rest bounce away. The computer has the the slit. The rest bounce away. The computer has the quadrapoles cycle through different M/Z's one at a time quadrapoles cycle through different M/Z's one at a time until a range of M/Z's are covered. This occurs many until a range of M/Z's are covered. This occurs many times per second. Each cycle of ranges is referred to as times per second. Each cycle of ranges is referred to as a a scanscan. . The computer records a graph for each scan. The x-axis The computer records a graph for each scan. The x-axis represents the M/Z ratios. The y-axis represents the represents the M/Z ratios. The y-axis represents the signal intensity (abundance) for each of the fragments signal intensity (abundance) for each of the fragments detected during the scan. This graph is referred to as a detected during the scan. This graph is referred to as a mass spectrummass spectrum (see Figure 2). (see Figure 2).

Figure 2:Figure 2: Mass-spectrum generated by an MS. Mass-spectrum generated by an MS.

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The mass spectrum produced by a given chemical The mass spectrum produced by a given chemical compound is essentially the same every time. Therefore, compound is essentially the same every time. Therefore, the mass spectrum is essentially a fingerprint for the the mass spectrum is essentially a fingerprint for the molecule. This fingerprint can be used to identify the molecule. This fingerprint can be used to identify the compound. The mass spectrum in Figure 2 was compound. The mass spectrum in Figure 2 was produced by dodecane. The computer on our GC-MS produced by dodecane. The computer on our GC-MS has a library of spectra that can be used to identify an has a library of spectra that can be used to identify an unknown chemical in the sample mixture. The library unknown chemical in the sample mixture. The library compares the mass spectrum from a sample component compares the mass spectrum from a sample component and compares it to mass spectra in the library. It reports and compares it to mass spectra in the library. It reports a list of likely identifications along with the statistical a list of likely identifications along with the statistical probability of the match probability of the match

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GC-MSGC-MS

When GC is combined with MS, a When GC is combined with MS, a powerful analytical tool is created. A powerful analytical tool is created. A researcher can take an organic solution, researcher can take an organic solution, inject it into the instrument, separate the inject it into the instrument, separate the individual components, and identify each individual components, and identify each of them. Furthermore, the researcher can of them. Furthermore, the researcher can determine the quantities (concentrations) determine the quantities (concentrations) of each of the components. of each of the components.

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Figure 3 represents a three-dimensional graph Figure 3 represents a three-dimensional graph generated when the GC is combined with the generated when the GC is combined with the MS. Try to visualize how the chromatogram MS. Try to visualize how the chromatogram combines with the mass spectrum to produce combines with the mass spectrum to produce this image. It is important for you to be able to this image. It is important for you to be able to picture this 3D image and translate it into the picture this 3D image and translate it into the previous 2D graphs. (This image is not made previous 2D graphs. (This image is not made from the same compounds in the previous from the same compounds in the previous figures.) Note that you can create either a mass figures.) Note that you can create either a mass spectrum or a chromatogram by making the spectrum or a chromatogram by making the appropriate cross section of this 3D image. Try appropriate cross section of this 3D image. Try to visualize which cross section would produce a to visualize which cross section would produce a spectrum and which would produce a spectrum and which would produce a chromatogram. chromatogram.

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Figure 3:Figure 3: 3D Depiction of GC-MS output 3D Depiction of GC-MS output