Chemical ionisation in the ion trap: a comparative study

Model for ion injection into a quadrupole ion trap to assess the distribution of initial conditions of confinement

A pulsed ion-injection mode for a quadrupole ion trap is described. Switched direct current (d.c.) potentials are applied to the source and trap electrodes to inject the ions into the trap and slow them down. The injection time is sufficient to ensure a steady distribution of the injected ions at the beginning of the confinement. An elementary uni-dimensional model is detailed giving the axial positions and velocities of the ions injected into the trap. The ion distribution in phase space, the number of injected ions and the number of injected ions that will be trapped are also given. These expressions depend on ion position and velocity at the creation, applied potentials and spatial location of the source and trap electrodes. This model is validated by comparing simulation and experimental results. For this purpose the number of confined ions is plotted versus the slowing-down potentials applied on the ring and the upper end-cap of the trap. The size of the area of removable ions in the source is deduced from these results.

Does the reagent gas influence collisional activation when performing in situ chemical ionization with an ion trap mass spectrometer?

Users of ion trap mass spectrometers frequently develop methods that associate chemical ionization with tandem mass spectrometry detection. With apparatus using internal ionization, the chemical reagent is present in the trap during the collision induced dissociation (CID) step and one may wonder if the reagent influences the fragmentation ratios in MS/MS. We report a comparison of the fragmentation ratios of protonated molecules when using the most common reagents (methane, ammonia, methanol, acetonitrile, isobutane) for performing in situ chemical ionization. Four molecules were chosen in the medical field to serve as models: alprazolam, diazepam, flunitrazepam and acetaminophen. In the non-resonant CID mode, the influence of the reagent mass is clearly seen in spite of its low partial pressure in the ion trap; the reagent acts as a "heavy target": the degree of fragmentation increases with the molecular weight of the reagent. In the resonant CID mode, there is no evident correlation between the fragmentation ratio of MH+ ions and the nature of the CI reagent; a slight shift of the secular frequency of the precursor ion, which tends to reduce the CID efficiency, could compensate for the "heavy target" effect underscored in the non-resonant mode.

In situ chemical ionization in ion trap mass spectrometry--the beneficial influence of isobutane as a reagent gas

We report a comparison of the ionization yields provided by the most common reagents (methane, ammonia, methanol, acetonitrile, isobutane) performing in situ chemical ionization with an ion trap mass spectrometer. Four molecules were chosen in the medical field to illustrate experimental results: alprazolam, diazepam, flunitrazepam and acetaminophen. Under usual operational conditions, relative abundances of protonated ions appreciably depend on the reagents. The greatest abundance of MH+ ions was obtained with isobutane while observed intensities for MH+ ions varied from 73% for methanol and ammonia to about 23% for acetonitrile and methane. Results were temptatively rationalized comparing energies of formation of the reagent ions and storage efficiency in the trapping field.

Low pressure chemical ionization in ion trap mass spectrometry

This article presents the specificities of low pressure chemical ionization in ion trap mass spectrometry. One main feature is the ability to perform chemical ionization with liquid reagents as readily as with "conventional" gases (methane, isobutane and ammonia). The reactivities and analytical applications of gas and liquid reagents are summarized from literature data and are compared when possible.

Cluster chemical ionization for improved confidence level in sample identification by gas chromatography/mass spectrometry

Upon the supersonic expansion of helium mixed with vapor from an organic solvent (e.g. methanol), various clusters of the solvent with the sample molecules can be formed. As a result of 70 eV electron ionization of these clusters, cluster chemical ionization (cluster CI) mass spectra are obtained. These spectra are characterized by the combination of EI mass spectra of vibrationally cold molecules in the supersonic molecular beam (cold EI) with CI-like appearance of abundant protonated molecules, together with satellite peaks corresponding to protonated or non-protonated clusters of sample compounds with 1-3 solvent molecules. Like CI, cluster CI preferably occurs for polar compounds with high proton affinity. However, in contrast to conventional CI, for non-polar compounds or those with reduced proton affinity the cluster CI mass spectrum converges to that of cold EI. The appearance of a protonated molecule and its solvent cluster peaks, plus the lack of protonation and cluster satellites for prominent EI fragments, enable the unambiguous identification of the molecular ion. In turn, the insertion of the proper molecular ion into the NIST library search of the cold EI mass spectra eliminates those candidates with incorrect molecular mass and thus significantly increases the confidence level in sample identification. Furthermore, molecular mass identification is of prime importance for the analysis of unknown compounds that are absent in the library. Examples are given with emphasis on the cluster CI analysis of carbamate pesticides, high explosives and unknown samples, to demonstrate the usefulness of Supersonic GC/MS (GC/MS with supersonic molecular beam) in the analysis of these thermally labile compounds. Cluster CI is shown to be a practical ionization method, due to its ease-of-use and fast instrumental conversion between EI and cluster CI, which involves the opening of only one valve located at the make-up gas path. The ease-of-use of cluster CI is analogous to that of liquid CI in ion traps with internal ionization, and is in marked contrast to that of CI with most other standard GC/MS systems that require a change of the ion source.

Detection of chlordanes by positive ion chemical ionization in an ion trap: a comparative [correction of comparitive] study of the non-conventional reagents acetonitrile, acrylonitrile and dichloromethane

The detection of some chlordane compounds (heptachlor, cis-/trans-chlordane and cis-/trans-nonachlor) by positive ion chemical ionization (PICI) in an ion trap was studied using acetonitrile, acrylonitrile and dichloromethane as non-conventional reagent gases. These reagent gases initiated specific fragmentation reactions and resulted in different response factors. All reagent gases enabled detection limits in the low-pg range for heptachlor, whereas the detection limits of cis-/trans-chlordane and cis-/trans-nonachlor were in the mid-pg range. Additionally, the acetonitrile and dichloromethane PICI mass spectra of the cis- and trans-stereoisomers of chlordane and nonachlor were different.