Cold EI is electron ionization of vibrationally cold molecules in supersonic molecular beams (SMB) using a fly-through EI ion source. Cold EI is characterized by enhanced molecular ions which are the most characteristic ions for sample identification and quantification. The magnitude of this important enhancement was measured for a series of linear chain hydrocarbons from n-C9H20 to n-C40H82 with staggering results. The measured absolute enhancement of the molecular ion was a factor of 13 for n-C9H20 with pure exponential increase up to a factor of 2500 for n-C40H82. This strong effect is being reported here for the first time.
|Figure 1. Cold EI mass chromatogram (upper trace) of a mixture of linear chain hydrocarbons from n-C9H20 to n-C40H82 (5 ng each on-column) obtained with the Aviv Analytical 5975-SMB GC-MS with Cold EI. A 5 m short column (0.25 mm ID) was used with column flow program in the 2-24 ml/min range and GC oven temperature program of 50-320ºC at 50ºC/min. The middle trace is the Clod EI mass spectrum of n-C20H42 while the standard EI mass spectrum of this compound, obtained with Agilent 5975C MSD with 250ºC ion source temperature, is shown at the bottom trace. (Click to Enlarge)|
This exponential decline of the molecular ion abundance (both relative and absolute) can be explained in the following way. The process of electron ionization is local and thus its deposited vibrational energy is independent on the compound size. The sample compound heat capacity on the other hand is linearly increased with the number of atoms. Thus, the amount of excess vibrational energy above the onset of molecular ion bond breakage is linearly increased with the carbon number for thermal compounds that were not cooled by SMB hence are at the ion source temperature. If we assume that as commonly encountered in chemistry the molecular ion dissociation rate is exponentially increased with the available excess molecular ion vibrational energy we should expect exponential reduction of the molecular ion abundance with the increase of carbon number as observed in figure 2. This observation also suggests that only a few vibrational modes and not all are active in the dissociation and thus the RRK theory is preferable over RRKM. While this observation of figure 2 can be rationalized as above it is still surprising in its magnitude. We note that the molecular ion relative and absolute abundances in Cold EI are relatively carbon number independent since the molecular ion is stripped of all its thermal heat and only a carbon number independent portion of the ions have electron deposited energy above some onset thereby can dissociate. Above a certain carbon number the efficiency of Cold EI vibrational cooling starts to reduce due to increased velocity slip effect. Thus, above C27 the absolute Cold EI molecular ion abundance starts to slowly decline while the relative abundance always remains 100% meaning that the molecular ion is always the dominant ion in the Cold EI mass spectra.