Gas Chromatography / Mass Spectrometry
Interfaces
Explore the correct interface types for use with different GC Columns and MS Instrument types
Explain the working principle of the most important GC/MS interfaces currently available
Describe some of the more common problems affecting the interface in GC/MS equipment
Why use GC-MS ?
GC-MS combines the separating power of Gas Chromatography (GC), with the detection power of mass spectrometry (MS)
Why use MS detector ?
Gives a definitive identification of the separated compounds (the most important)
Fast response
Mass spectrometers show high sensitivity for volatile compounds
Library searching
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It truly provides a second dimension of information to the chromatographic analysis.
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Parallel between characteristics of GC and MS analytical techniques.
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An adequate pressure drop
Maximize the throughput of sample while maintaining a gas flow rate compatible with the source operating pressure.
Low dead volume at the column exit.
Analyte must not condense in the interface
Analyte must not decompose before entering the mass spectrometer ion source (Remain the chemical constitution of the sample).
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General requirement of GC-MS interfaces
The main interface types developed for Gc/MS coupling:
Based on sample enrichment (separator)
The classical jet separator (or molecular jet) interface
Permselective membrane interface
Molecular effusion (or watson- Bieman) interface
Open split Interface
Direct Introduction Interface
GC/MS interfaces
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Column flow rate (20-60 ml/min)
Interface for high gas flow (packed column)
The performance of any type of molecular separator is characterized in terms of its separation factor (enrichment) N and separator yield (efficiency) Y.
Y = (WMS/WGC) x 100
WMS : the amount of sample entering the MS
WGC : the amount of sample entering the interface or from GC
Separator yield represents the ability of the device to allow organic material to pass into the source of the MS.
Molecular Separator
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The separation factor N is defined as the ratio of analyte concentration in the sample entering the MS and the concentration from GC.
VGC is the volume of carrier gas entering the separator.
VMS is the volume of the carrier gas entering the MS.
Get Separator Interface
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Removes about 90% of carrier gas. About 60% of the sample reach the MS.
Get Separator Interface
Advantage
Relative low cost and easy to use.
Disadvantage
Gap between the column and the transfer line which is held under vacuum
Some analyte molecules (especially the more volatile species) may be lost to the vacuum.
Plugging problem
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Membrane separator
The silicone membrane separator works on the principle of differential permeability for the transmission of organic solutes compared to carrier molecules.
The transmission ability of organic molecules is much higher than those for carrier gas (two orders of magnitude).
Transmission Efficiency
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Membrane area
Thickness
Contact time
Temperature
Sample Size
Methods of membrane preparation
fundamental factor
permeability of the membrane
permeability equation from Crank and Park
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The effect of temperature
Fractions of Solutes Not Transmitted by the Separator from 1μ L Injections of 10% Solutions of MacReynolds Compounds in Decane
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Plot of the permeability coefficient as a funciton of temperature for several .gases through a silicone membrane
P= D*S
Premselective Membrane
Very effective enrichment procedure
Membrane selectivity based on polarity & highMW compound Slow to respond
Suffers from discrimination effects with more polar analytes and produces significant band broadening of their chromatographic peaks.
memory effect
Only a small fraction of analytes actually permeates through membrane
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Effusion separator
Effusion rates are different between sample and carrier gas F = 1/(MW)1/2
Molecular Effusion (Watson- Biemann) Interface
Porous glass frit including microscopic pores
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Disadvantage
High dead volume added and its high surface area (Band broadening peak).
Rate of diffusion is molecular weight dependent -selectivity based on MW (Low molecular weight)
Molecular Effusion (Watson- Biemann) Interface
This types of connection suffer from the drawback that the end of the column is at an undefined pressure lower than atmospheric : The separating power of column is lowered The comparison of chromatograms measured in or outsides a GC/MS combination is complicated
Consequence
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Direct Introduction Interface
Typical Flow rates for selected GC-MS columns
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Direct Introduction Interface
Usually heated >10oC above the final oven temperature program and at a lower temperature than the ion source
The heat applied to the interface must prevent condensation but must also avoid thermal decomposition of labile analytes , to prevent possibly adsorption on the column walls at the entry of the capillary into the ion source
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Ferrule Vespel/ Graghit
Direct Introduction Interface
Advantages:
Column is inserted directly into the mass spectrometer ionization chamber (the lack of dead volumes which reduce the separating power of high resolution column)
This interface gives the highest Sensitivity (100‰ yield)
Residue analysis
Disadvantages:
Changing columns is laborious and time consuming
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End of capillary column
Teflon Shrinking tube
Mass spectrometer inlet capillary
Mantling tube
Scavenge flow capillary
Heated guiding tube
Instrumental Open Split Separator
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Instrumental Open Split Separator
Advantage
This connection device is suited to various types of column (wide bore, megabore and even packed column). The exceess eluate not sucked into the mass spectrometer
A rather high yield (< 100)
The choice of GC conditions (column length, column diameter flow rate) can be optimized to give the best possible separation independent of the mass spectrometer
Optimal reliability because the open split requires no vacuum tight seal of the column to the spectrometer
Open Split Separator
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Advantage
Capillary columns can be changed rapid without venting the mass spectrometer vacuum system
The retention time of classical detector, such as FID or ECD, are retained in the GC/MS and allow a direct comparison chromatogram
At the end of column a split can allow the use of an additional to that given by the mass spectrometers
Open Split Separator
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Disadvantage
The split point is at atmospheric pressure. To prevent the penetration of air the split point must be flushed with the carrier gas. The additional screw joints can give rise to damaging leaks
If, because of balance of column flow and suction capacity of the mass spectrometer, there is a positive split ratio, the sensivity of the system decreases
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Comparative performance of GC/MS interfaces
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Inertness of Material in the Interface
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Silanized glass component in GC/MS systems can improve an inert system
Assessment of the degree of inertness of the GC interface has been described in experiments using :
Free cholesterol
Pyrrolidine
Pesticide endrin
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Conclusion
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Future Work
Infinite enrichment
Yield (100‰)
Low dead volume
No delay
Inertness
All carrier gas
A wide variety of compounds
Reference
J. Abian, J. Mass Spectrom. 34, 157-168 (1999)
B. J. Gudzinowicz, M. J. McFadden, Marcel Dekker , New York, 1-7 (1977-1980)
C. Dass, Fundamental of Contemporary Mass Spectrometry, Wiley, New York (2007).
D. Henneberg, U. Henrichs, G. Schomburg, 8, 449-451 (1975)
W. Mcfadden, John Wiley, New York (1973)
G. M. Message, John Wiley, New York (1984)
F. W. Karasek, R. E. Clement, Basic Gas Chromatograghy- Mass spectrometery, Elsevier Science B.V, (1988)
J. P. Lehman, Anal. Chem, 49 (3), 518-520 (1997)
W. Fock, Anal. Chem, 47(14), 2447-2450 (1975)
G. A. Junk. Int. J. Mass Spectrom. Ion Phys. 8, 1 (1972)
C. Casper. Greenwalt, Kent J. Voorhees, Jean H. Futrell, Anal. Chem, 55, 468-472 (1983)
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Thank you