6.10:
Mass Spectrometry: Complex Analysis
The analysis of complex mixtures requires the combination of separation methods and mass spectrometry in sequence, termed hyphenated methods.
GC–MS is the most common hyphenated method, where a thermally stable volatile mixture is separated by gas chromatography, and the column output is continually sent to the mass spectrometer for ionization and analysis.
Similarly, LC–MS is liquid chromatography coupled with a mass spectrometer.
Typically, a mixture with nonvolatile polar or ionic compounds is separated by liquid chromatography, and the output is ionized by electrospray ionization before being sent to the mass analyzer.
However, mixtures with less-polar molecules often use other atmospheric-pressure ionization methods.
Tandem mass spectrometry is another hybrid method for analysis, where two mass analyzers are connected in series.
Unlike GC–MS and LC–MS, tandem mass spectrometry offers separation in milliseconds, and it is more sensitive and selective.
Interestingly, highly complex mixtures can be analyzed by coupling GC or LC with tandem mass spectrometry, forming GC–MS/MS and LC–MS/MS.
6.10:
Mass Spectrometry: Complex Analysis
Mass spectrometry is an important technique for the identification of pure compounds. However, it has some limitations for the analysis of complex mixtures, often due to excessive fragmentation making the spectrum too complicated to decipher. Mass spectrometry can be combined with suitable separation methods in sequence, forming hyphenated methods, which are useful in the analysis of complex mixtures.
GC–MS is a powerful hyphenated method commonly used in forensics and environmental laboratories for precise analysis of mixtures. Gas chromatography uses narrow capillary columns to separate components of a thermally stable volatile mixture and passes them to the mass spectrometer for analysis.
LC–MS is another hyphenated method that couples liquid chromatography with a mass spectrometer to analyze nonvolatile mixtures. To make liquid chromatography compatible with a mass spectrometer, suitable pressure-maintaining ionization interfaces or atmospheric-pressure ionization techniques like electrospray ionization, which is applicable for polar and ionic compounds, or atmospheric-pressure chemical ionization, which applies to less polar molecules, are used.
Another hybrid method, called tandem mass spectroscopy, uses multiple mass analyzers in sequence. Compared to other hyphenated methods, tandem spectroscopy is faster, more sensitive, and more selective due to smaller chemical noise. Tandem spectroscopy can be further combined with separation techniques to form GC–MS/MS or LC–MS/MS for more complex mixture analysis.
Capillary electrophoresis–mass spectrometry is a very sensitive technique commonly used to analyze large biomolecules like DNA, proteins, and polypeptides. This method feeds quadrupole mass analyzers with capillary effluents after passing through an electrospray ionization interface.