Interfacing Gas Chromatography with Mass Spectroscopy and Infra Red Spectroscopy Is this a large expensive detector…. Or a separation prior to analysis….

Early Use of Mass Spectroscopy Quantitative methods for determination of the components in complex hydrocarbon mixtures Later used for the identification and structural analysis of complex compounds Method requires samples that are “clean” or interpretation is confusing

Principles of measurements As an identification method: –When a given molecular species is impacted with an electron beam, a family of positive particles are produced –The mass distribution of the particles are characteristic of the parent species

Interfacing GC with Spectroscopic Methods - Early eluates from column collected as separate fractions after being detected - composition measured by Mass Spectrometry or IR Limitation - small (micromolar) composition of the solute procedure still useful for qualitative analysis of multi-component

Application of a Selective Detector - Modern The detector monitors the column effluent continuously Need computers to control instruments and store spectral data for display of spectrum and chromatograms

Interfacing Gas Chromatography and Mass Spectroscopy (GC/MS)

GC/ Mass Spectrometry GC equipment can be directly interfaced with rapid-scan Mass Spectrometers The flow rate is usually small enough to feed directly into the ionization chamber of the Mass Spectrometer Packed columns use a jet separator, which removes the carrier gas for the analyte

GC/ MS Increase momentum of heavier analyte molecules so that 50% or more go into the skimmer Lighter helium molecules are deflected by vacuum and pumped away Use to identify components present in natural and biological systems –odor/flavor of foods - pollutants

What is GC/MS? Gas chromatography/mass spectrometry (GC/MS) is the synergistic combination of two powerful analytic techniques. The gas chromatography separates the components of a mixture in time The mass spectrometer provides information that aids in the structural identification of each component

What is GC/MS?

The GC/MS Interface Transports the effluent from the gas chromatograph to the mass spectrometer Analyte must not condense in the interface Analyte may not decompose before entering the mass spectrometer ion source The gas load entering the ion source must be within pumping capacity of the mass spectrometer

GC/MS Interfaces Capillary Columns Macrobore and Packed Columns

Capillary Columns Insert exit end of column into ion source Under normal operating conditions, the mass spectrometer can handle the entire effluent of the column Must heat the capillary column to prevent condensation Surface of columns must be inactive

Macrobore and Packed Columns Effluent must be reduced before entering ion source Splitting the effluent results in a loss of sensitivity Enrichment devices are used –Jet Separators are most common

Jet Separator Two capillary tubes aligned with a small space between them. (1 mm) A vacuum is created between the two tubes using a rotary pump The GC effluent enters the vacuum region, those molecules which continue in the same direction enter the second capillary tube and continue to the ion source

Jet Separator The carrier gas molecules are more easily diverted from the linear path by collisions The analyte molecules are much larger and carry more momentum The surface of the separator must be inactive and a reasonably even temperature Prone to leaks

Resolution and Mass Accuracy With a modern mass spectrometer, it is possible to measure the mass of an ion to 1ppm with a resolution of 100,000 or better GC/MS scanning conditions are limited to 5-10 ppm mass accuracy and resolution is only between 2,000 and 10,000. These limitations are usually sufficient to allow for only a few reasonable and possible compositions

Resolution and Mass Accuracy Resolution can be increased by restricting the height and the width of the ion beam A compromise must be made between minimizing mass interference and signal intensity for low levels of material Gas chromatograph eliminates most compounds that cause mass interference. Principle cause of peak overlap is the internal mass standard.

Uses for GC/MS May separate, analyze and identify unknown mixutres May separate, and analyze known mixtures For sample GC/MS experiments check out: –

Complex Mass Spectrometer Detectors Display modes - real time or computer reconstructed Each has a choice of total ion current chromatogram or selected ion current chromatogram Each can be generated on to a computer screen for print out

Ion Trap Detector compact - less expensive than quadropole simplest mass detector for use in GC ions are created form eluted sample by electron impact or chemical ionization stored in radio-frequency field ions injected from the storage area to a detector

ITD Ejection is controlled so the scanning of mass to charge ratio is possible

Gas Chromatography Infrared Spectrometry (GC/IR) Instrumentation/Interface Advantages Problems/Cons Solutions Practical Applications

Infrared Spectrometry Is especially useful for qualitative analysis of functional groups and other structural features measuring concentrations is possible establish identity of unknown compound with standard

Instrumentation/Interface Infrared Spectrophotometer determines the relative strengths and positions of the infrared region and plots the information on calibrated paper Gas Chromatograph partitions the sample as it passes through the column The two can be linked through glass column or vacuum tubes and other devices on more expensive equipment

Fourier-Transform Infrared Spectroscopy (FTIR) Overcomes the problem of scanning for a collected sample or monitoring one wavelength

Fourier Transform IR Mechanically simple Fast, sensitive, accurate Internal calibration No tracking errors or stray light

FTIR Analyze all wavelengths simultaneously signal decoded to generate complete spectrum can be done quickly better resolution more resolution However,...

Gas Chromatography / Infrared Spectrometry Capillary GC with IR specs can enable the separation and identifying the compounds The interface between the column and the detector is the main detail Small pipe (length cm, diameter 1-3 mm) connected to column by narrow tubing Transmission of radiation occurs by multiple reflection off the wall

GC/ IR Light pipe is heated in order to rid condensation and maximize path length for enhanced sensitivity This also minimizes the dead volume to lessen band broadening Detector - highly sensitivity, liquid nitrogen cooled Scanning is started and a brief delay is needed for compound to travel form the detector region to the IR cell

More on General GCIR Very sensitive very expensive sample recovery

Practical Uses Pharmaceutical Industrial DNA Analysis of blood samples, other fluids many others

INTERFACE to Multiwavelength UV / VIS Detectors Monitor several specific wavelengths set by colored dyes attached (DNA) Flow through a multiwavelength detector and optical multichannel analyzer

Conclusion Gas chromatography can be effectively coupled with uv/vis detectors for monitoring dye labels, and infra-red spectroscopy and mass spectroscopy to more effectively analyze mixtures. This is also true for liquid chromatography, although the interfaces present different problems.