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?

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:– http://www.lehigh.edu/~ingcms/ingcms.html

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 IRFourier 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 10-40 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.