GAS CHROMATOGRAPHY-MASS SPECTROMETRY(GC-MS)

A.SWETHA

SEMINAR IN ANLYSIS

M.Pharm(1st year)

OUCT

GC-MS COMBINATION

GC = separation

MS = identification

When GC is combined with MS, a powerful analytical tool is created.

The symbiotic relationship of these two methods makes an interesting study in the development of analytical instruments ,as the first of a growing class of “ hyphenated” techniques.

HISTORY

The use of a mass spectrometer as the detector in gas chromatography was developed during the 1950s by Roland Gohlke and Fred McLafferty.

PRINCIPLE OF GC-MS

The GC works on the principle that a mixture will separate into individual substances when heated.

Sample introduced into GC inlet vaporized at 250 °C , swept onto the column by He carrier gas & separated on column.

Sample components emerge from column, flowing into the capillary column interface connecting the GC column and the MS (He removed).

The computer drives the MS, records the data, and converts the electrical impulses into visual displays and hard copy displays. 

Identification of a compound based on it's mass spectrum relies on the fact that every compound has a unique fragmentation pattern.

 A large library of known mass spectra is stored on the computer and may be searched using computer algorithms to assist the analyst in identifying the unknown

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GC/MS can handle mixtures.

  The GC effluent is directed into the mass spectrometer, where a

spectrum of each component is obtained as it elutes from the column.

  Just as an FID gives a response based on the number of ions

formed as sample elutes from the column, so does GC/MS, in the form of a total ion chromatogram(TIC).

  The difference is that each point in the chromatogram is actually a

mass spectrum.

  Thus a mass spectrum can be retrieved for any peak in the

chromatogram.

  Conversely, a selected ion chromatogram can be retrieved for any

mass.

The GC portion of GC-MS provides high resolution separation of volatile organic solutes in a mixture in the gas phase.

 

As each solute exits the GC column, it is diverted into a mass spectrometer which is capable of both monitoring the amount of and identifying the chemical nature of the solute.

In this way, both quantitative and qualitative information about the mixture can be obtained.

The MS portion of the system takes each gaseous solute exiting the GC and ionizes it in an electron beam.

The ions formed by a specific solute will depend on the nature of the bonds in

the molecule, and both ionized molecules and ion fragments of the molecule are possible

The ions are then directed down a separator which isolates and counts the ions according to mass.

The sequence and relative intensity of the mass peaks give information about the chemical identity of the solute.

The absolute intensity of the peaks provides information about the amount of substance present.

INSTRUMENTATION OF GC-MSSamplesDesired characters To be suitable for GC analysis, a compound must have sufficient volatility and

thermal stability. (All or some of compound molecules are in the gas or vapor phase at 250-

350°C or below, do not decompose at these temperatures.)State Organic compounds must be in solution for injection into the gas

chromatograph. The solvent must be volatile and organic (for example, hexane or dichloromethane).

Amount Depending on the ionization method, analytical sensitivities of 1 to 100 pg per

component are routine.Preparation Sample preparation can range from simply dissolving some of the sample in a

suitable solvent to extensive cleanup procedures using various forms of liquid chromatography.

 Analysis Time In addition to sample preparation time, the instrumental analysis time usually

is fixed by the duration of the gas chromatographic run, typically between 20 and 100 min. Data analysis can take another 1 to 20 hr (or more) depending on the level of detail necessary.

GC & MS FLOW CHART

Gas chromatography

GCMS INTERFACE

Incompatibility of GC and MS GC operate at atmospheric pressure and the

MS ion source at 10-5 Torr. 108 fold pressure difference Need of the interface

The carrier gas must be removed and GC peak components transferred to the MS ion source

RHYPHAGE CONCENTRATOR

BIEMANN CONCENTRATOR

GAS CHROMATOGRAPHY (GC) Injection port – One microliter (1 µl, or 0.000001 L) of

solvent containing the mixture of molecules is injected into the GC and the sample is carried by inert (non-reactive) gas through the instrument, usually helium. The inject port is heated to 300° C to cause the chemicals to become gases.

Oven – The outer part of the GC is a very specialized oven. The column is heated to move the molecules through the column. Typical oven temperatures range from 40° C to 320° C.

Column – Inside the oven is the column which is a 30 meter thin tube with a special polymer coating on the inside. Chemical mixtures are separated based on their votality and are carried through the column by helium. Chemicals with high volatility travel through the column more quickly than chemicals with low votality.

MASS SPECTROMETER (MS)

Ion Source – After passing through the GC, the chemical pulses continue to the MS. The molecules are blasted with electrons, which cause them to break into pieces and turn into positively charged particles called ions. This is important because the particles must be charged to pass through the filter.

Filter – As the ions continue through the MS, they travel through an electromagnetic field that filters the ions based on mass. The scientist using the instrument chooses what range of masses should be allowed through the filter. The filter continuously scans through the range of masses as the stream of ions come from the ion source.

Detector – A detector counts the number of ions with a specific mass. This information is sent to a computer and a mass spectrum is created. The mass spectrum is a graph of the number of ions with different masses that traveled through the filter.

GC-MS

GCMS

INTERPRETATION OF THE RESULTS

Through GC – a chromatogram is obtained.

Through MS – a spectrum is obtained.

GC-MS gives a 3D graph which has both chromatogram and a spectrum to each separated component in chromatogram.

 

CHROMATOGRAM GENERATED BY A GC.

While the instrument runs, the computer generates a graph from the signal. This graph is called a chromatogram.

  Each of the peaks in the

chromatogram represents the signal created when a compound elutes from the GC column into the detector.

  x-axis shows the RT y-axis shows the intensity

(abundance) of the signal.

MASS-SPECTRUM GENERATED BY AN MS.

The computer records a graph for each scan. This graph is referred to as a mass spectrum.

The mass spectrum produced by a given chemical compound is essentially the same every time.Therefore, the mass spectrum is essentially a fingerprint for the molecule and can be used to identify the compound.

  The computer on GC-MS has a library of

spectra that can be used to identify an unknown chemical in the sample mixture.

  The library compares the mass spectrum

from a sample component and compares it to mass spectra in the library.

  It reports a list of likely identifications along

with the statistical probability of the match

3D DEPICTION OF GC-MS OUTPUT

It has both the chromatogram and spectrum on Y-axis it represents intensity/abundance.

On X-axis it is retention time. The graph between retention time and abundance is a

chromatogram. The graph between m/z and retention time is mass spectrum of the

individual peaks of the chromatogram.

GENERAL USES

Identification and quantitation of volatile and semivolatile organic compounds in complex mixtures

Determination of molecular weights and (sometimes) elemental compositions of unknown organic compounds in complex mixtures

Structural determination of unknown organic compounds in complex mixtures both by matching their spectra with reference spectra and by a priori spectral interpretation

 

COMMON APPLICATIONS

Environmental Monitoring and Cleanup

Criminal Forensics

Law Enforcement

Security

Food, Beverage and Perfume Analysis

Astrochemistry

Medicine

SPECIFIC APPLICATIONS

Identification of unknown organic compounds in hazardous waste dumps

Identification of reaction products by synthetic organic chemists

Analysis of industrial products for quality control To analyze a urine sample for tetrahydrocannabinol, (THC)

the principle psychoactive component of marijuana, the organic compounds are extracted from urine, purified, concentrated and injected into the GC-MS.

Analysis of Anabolic Steroids in Biological materials. Quantitation of pollutants in drinking and wastewater

using official U.S. Environmental Protection Agency (EPA) methods

Quantitation of drugs and their metabolites in blood and urine for both pharmacological and forensic

It follows that the use of GC/MS has become a popular analytical procedure in forensic chemistry, toxicology and environmental studies.

The separation and identification of degradation products of organic and organometallics making the elucidation of their structures.

The routine analysis of substances present in minute quantities

The evaluation of pronounced metabolic disturbances

The identification of noxious and toxic compounds and their quantitation in emergency cases

The diagnosis of inborn errors of metabolism especially in new borns

LIMITATIONS OF GC-MS 

General  Only compounds with vapor pressures exceeding about 10 torr

Many compounds with lower pressures can be analyzed if they are chemically derivatized

Determining positional substitution on aromatic rings is often difficult.

Certain isomeric compounds cannot be distinguished by mass spectrometry

False positives and false negatives are possible.

If MS feed is impure it results in background "noise" in the mass spectrum.

Also the MS suffers from the inexact practice of interpreting mass spectra.

 Accuracy

Qualitative accuracy is restricted by the general limitations cited above.

Quantitative accuracy is controlled by the overall analytical method calibration.

Using isotopic internal standards, accuracy of ±20% relative standard deviation is typical.

 

Sensitivity and Detection Limits

Depending on the dilution factor and ionization method, an extract with 0.1 to 100 ng of each component may be needed in order to inject a sufficient amount.

PRECAUTIONS

Inject the specimen into the septum rapidly and smoothly to attain good separation of the components in a specimen. 

If the technician injects the specimen too slowly, the peak may be broad or overlap.  A twin peak may result from the technician hesitating during the injection. 

A smoothly performed injection, without abrupt changes, should result in a smoothly formed peak.  A twin peak may also indicate that the technician injected two specimens consecutively.

EARLY GC-MS

Dow gas chromatography and Bendix TOF mass spectrometer in the Dow Spectroscopy Laboratory, 1957.

LATEST GC-MS

Agilent 7000 Series Triple Quadrupole GC/MS 

CONCLUSION

REFERENCES Instrumental methods of chemical analysis, Gurudeep R

Chatwal, Sham K. Anand , Pages 2.699 Practical Pharmaceutical Chemistry fourth edition-Part two,

Edited by A.H.Beckett, J.B.Stenlake, Pages 474-477 Gas Chromatography Mass Spectrometry, Ronald A. Hites,

Indiana University School of Public and Environmental Affairs and Department of Chemistry, Chapter 31, Pages 609-626 

Journal of the American Society for Mass Spectrometry Volume 4, Issue 5, May 1993, Pages 367-371

http://www.labcompare.com/Mass-Spectrometry/154-Gas-Chromatograph-Mass-Spectrometer-GC-MS-Instrument/

http://www.chem.agilent.com/enUS/Products/Instruments/ms/7000triplequadrupolegcms/Pages/default.aspx

http://www.chemistry.nmsu.edu/Instrumentation/GC_MS.html

http://www.chem.arizona.edu/massspec/intro_html/intro.html

http://www.waters.com/waters/nav.htm?locale=en_US&cid=514259

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