Question

Explain why it is more difficult to couple liquid chromatography with mass spectrometry than it is to couple gas chromatography with mass spectrometry.

Answer 1

Comparing to GC/MS, the coupling of LC and MS has not been easy to accomplish. From the beginning, the main problem was that the high liquid flow make it difficult for maintaining the high vacuum required for the mass spectrometer. The gas flow entering the mass spectrometer from a GC capillary column is in the range 0.5 to 2 ml/min. In the case of conventional LC, about 1 ml/min of methanol or water has to be introduced. This liquid flow produces gas flow-rates as high as 350 ml/min for methanol and more than 1000 ml/min in the case of water. Only in the case of very low liquid flow-rates (below 10 μl min^-1), the gas flow would be in the same range as that for GC.