Nitrogen and hydrogen as carrier and make-up gases for GC-MS with Cold EI

Gas chromatography-mass spectrometry (GC-MS) with Cold EI is based on interfacing GC and MS with a supersonic molecular beam (SMB) and sample compounds ionization with a fly-through ion source as vibrationally cold compounds in the SMB (hence the name Cold EI). We explored the use of nitrogen and hydrogen as carrier and make-up gases with Cold EI and found: Nitrogen is very effective in cooling compounds in SMB and while helium requires 60 ml/min nitrogen provides effective cooling with only 7-8 ml/min combined column and make-up flow rate. Hydrogen is less effective than helium and requires higher flow rates. The transition from helium to nitrogen (or hydrogen) is simple and fast and requires just closing the helium valve and opening the nitrogen valve. The same column used with helium can be used with nitrogen or hydrogen. The same elution times could be obtained with nitrogen or hydrogen as with helium. The GC separation with nitrogen was reduced compared with helium and peak widths were increased by an average factor of 1.5 for similar elution times. Hydrogen provided ~0.7 narrower peak widths than helium. The signal with nitrogen was reduced compared with helium by an average factor of 3.3 and the signal loss was reduced with higher compounds mass. With hydrogen the signal loss was about a factor of 1.5 but the baseline noise was higher thus with similar S/N as with nitrogen. USEPA 8270 semivolatile mixture was easily analyzed with both nitrogen and hydrogen carrier gases.