How enhanced molecular ions in Cold EI improve compound identification by the NIST library

Abstract

This behavior which was previously observed by analyzing Cold EI mass spectra can be explained by the fact that high-mass ions, and especially the molecular ion, characterize a compound more than low-mass ions and therefore carries more weight in library search identification algorithms. These ions are uniquely abundant in Cold EI, which therefore enables enhanced compound characterization along with improved NIST library based identification.

The pre‐separation of oxygen containing compounds in oxidised heavy paraffinic fractions and their identification by GC‐MS with supersonic molecular beams.

“…The data format of the cold-EI mass spectra are fully compatible with NIST library identification, and whilst the matching factors are reduced the identification probabilities are often increased. 47 We note, as shown above, that the larger the compound, the greater the gain in molecular ion enhancement with the molecular cooling in cold-EI. The oxygenates were also successfully separated from the oxidised paraffinic matrix by SPE which allowed the subsequent identification by GC-MS with cold-EI.…”

“…Whilst one can also use low electron energy cold‐EI to further increase the relative abundance of the molecular ion in cold‐EI as reported in Amirav et al, 70‐eV cold‐EI combines the availability of molecular ions together with fragment ions that enables NIST library identification with good signal strength as shown throughout this study. The data format of the cold‐EI mass spectra are fully compatible with NIST library identification, and whilst the matching factors are reduced the identification probabilities are often increased . We note, as shown above, that the larger the compound, the greater the gain in molecular ion enhancement with the molecular cooling in cold‐EI.…”

Soft Cold EI – approaching molecular ion only with electron ionization

“…Furthermore, despite the enhanced molecular ion and high mass fragment ions the NIST identification actually improved and resulted in 79.7% identification probability which is very high for large hydrocarbons. The reason for this improved library identification is that, while the matching factors are reduced due to the enhancement of the molecular ion, the matching factors of other candidates are reduced even more since the molecular ion and high mass fragment ions are the most sample‐characteristic ions . This Cold EI mass spectrum was obtained with 84 mL/min helium make‐up gas flow rate plus 4 mL/min column flow rate for a total helium flow rate at the supersonic nozzle of 88 mL/min which is higher than our usual 60 mL/min combined make‐up and column flow rate.…”

“…The reason for this improved library identification is that, while the matching factors are reduced due to the enhancement of the molecular ion, the matching factors of other candidates are reduced even more since the molecular ion and high mass fragment ions are the most sample-characteristic ions. [8] This Cold EI mass spectrum was obtained with 84 mL/min helium make-up gas flow rate plus 4 mL/min column flow rate for a total helium flow rate at the supersonic nozzle of 88 mL/min which is higher than our usual 60 mL/min combined make-up and column flow rate. Since squalane is a highly branched hydrocarbon it requires improved cooling to optimize its molecular ion abundance.…”

New Advances in Toxicological Forensic Analysis Using Mass Spectrometry Techniques

“…The use of a miniaturized analytical method is the aim during the development of new analytical methods, and the analysis of drugs of abuse by mass spectrometry is not the exception. Most recently, GC-MS methodologies have used cold EI to analysis of heroin and cocaine [ 15 , 16 ]. Here, the GC has an interface known as a supersonic molecular beam (SMB) where the ionization vibration cold sample is in an axial fly-through ion source configuration ( Figure 1(a) ), providing mass spectra with enhanced molecular ions that are compatible with reference libraries, and the range of compounds are amenable to GC-MS compounds.…”