Determination of trace compounds and artifacts in nitrogen background measurements by proton transfer reaction time-of-flight mass spectrometry under dry and humid conditions.

A qualitative analysis was applied for the determination of trace compounds at the parts per trillion in volume (pptv ) level in the mass spectra of nitrogen of different qualities (5.0 and 6.0) under dry and humid conditions. This qualitative analysis enabled the classification and discovery of hundreds of new ions (e.g., [Sx ]H+ species) and artifacts such as parasitic ions and memory effects and their differentiation from real gas impurities. With this analysis, the humidity dependency of all kind of ions in the mass spectrum was determined. Apart from the inorganic artifacts previously discovered, many new organic ions were assigned as instrumental artifacts and new isobaric interferences could be elucidated. From 1140 peaks found in the mass range m/z 0-800, only 660 could be analyzed due to sufficient intensity, from which 463 corresponded to compounds. The number of peaks in nitrogen proton transfer reaction (PTR) spectra was similarly dominated by nonmetallic oxygenated organic compounds (23.5%) and hydrocarbons (24.1%) Regarding only gas impurities, hydrocarbons were the main compound class (50.2%). The highest contribution to the total ion signal for unfiltered nitrogen under dry and humid conditions was from nonmetallic oxygenated compounds. Under dry conditions, nitrogen-containing compounds exhibit the second highest contribution of 89% and 96% for nitrogen 5.0 and 6.0, respectively, whereas under humid conditions, hydrocarbons become the second dominant group with 69% and 86% for nitrogen 5.0 and 6.0, respectively. With the gathered information, a database can be built as a tool for the elucidation of instrumental and intrinsic gas matrix artifacts in PTR mass spectra and, especially in cases, where dilution with inert gases plays a significant role.

Elucidation of artefacts in proton transfer reaction time-of-flight mass spectrometers.

We present an effective procedure to differentiate instrumental artefacts, such as parasitic ions, memory effects, and real trace impurities contained in inert gases. Three different proton transfer reaction mass spectrometers were used in order to identify instrument-specific parasitic ions. The methodology reveals new nitrogen- and metal-containing ions that up to date have not been reported. The parasitic ion signal was dominated by [N2 ]H+ and [NH3 ]H+ rather than by the common ions NO+ and O2 + . Under dry conditions in a proton transfer reaction quadrupole interface time-of-flight mass spectrometer (PTR-QiTOF), the ion abundances of [N2 ]H+ were elevated compared with the signals in the presence of humidity. In contrast, the [NH3 ]H+ ion did not show a clear humidity dependency. On the other hand, two PTR-TOF1000 instruments showed no significant contribution of the [N2 ]H+ ion, which supports the idea of [N2 ]H+ formation in the quadrupole interface of the PTR-QiTOF. Many new nitrogen-containing ions were identified, and three different reaction sequences showing a similar reaction mechanism were established. Additionally, several metal-containing ions, their oxides, and hydroxides were formed in the three PTR instruments. However, their relative ion abundancies were below 0.03% in all cases. Within the series of metal-containing ions, the highest contribution under dry conditions was assigned to the [Fe(OH)2 ]H+ ion. Only in one PTR-TOF1000 the Fe+ ion appeared as dominant species compared with the [Fe(OH)2 ]H+ ion. The present analysis and the resulting database can be used as a tool for the elucidation of artefacts in mass spectra and, especially in cases, where dilution with inert gases play a significant role, preventing misinterpretations.