A high-resolution mass spectrometer to measure atmospheric ion composition

A high-resolution mass spectrometer to measure atmospheric ion composition H. Junninen, M. Ehn, T. Petäjä, L. Luosujärvi, T. Kotiaho, R. Kostiainen, U. Rohner, M. Gonin, K. Fuhrer, M. Kulmala, and D. R. Worsnop Department of Physics, P.O. Box 64, 00014, University of Helsinki, Helsinki, Finland Department of Chemistry, P.O. Box 55, 00014, University of Helsinki, Helsinki, Finland Division of Pharmaceutical Chemistry, P.O. Box 56, 00014, University of Helsinki, Helsinki, Finland Tofwerk AG, 3600 Thun, Switzerland Aerodyne Research Inc, Billerica, MA 01821, USA


INTRODUCTION
Atmospheric aerosol formation consists of a complicated set of processes that include the production of nanometer-sized clusters from gaseous vapours, the growth of these clusters to detectable sizes, and their simultaneous removal by coagulation with the pre-existing aerosol particle population (e.g.Kulmala and Kerminen, 2008).The recent development of physical nano condensation nuclei measurements (Kulmala et al. 2007, Mirme et al. 2007, Sipilä et al. 2008, Sipilä et al. 2009, Iida et al. 2008) has pushed the detection limit of these instruments down to the sizes where nucleation is occurring.The results show that, in addition to the more easily detectable ions, there seems to be also neutral molecules and clusters present at these sizes (Kulmala et al. 2007, Zhao et al., 2009).For resolving the participating compounds in atmospheric nucleation, chemical composition measurements need to be improved.On one hand, mass spectrometric methods can provide detailed information on the composition of atmospheric trace gases , atmospheric ions and even neutral clusters.On the other hand, recent development in the measurement methods of aerosol chemical composition has increased our capability to determine aerosol composition of smaller and smaller particles, down to 10 nm (Smith et al. 2009).There is still, however, a gap between the aerosol and gas phase instruments.The aim of this study is to fill part of this gap with an atmospheric pressure interface (APi) connected to a time-of-flight mass spectrometer (TOF).

METHODS
The APi-TOF consists of a time-of-flight mass spectrometer (TOF) coupled to an atmospheric pressure interface (APi) which guides the sampled ions from atmospheric pressure to the TOF while pumping away the gas (Figure 1).The APi is only an interface to the TOF, and should not be confused with atmospheric pressure ionization, as the APi-TOF in our context does not by default contain any ionization method.The APi-TOF has three differentially pumped chambers, the first two containing short segmented quadrupoles used in ion guide mode, and the third containing an ion lens assembly.The flow rate into the instrument is ~0.8 l min-1, regulated by a critical orifice (300 µm) at the instrument inlet.The first chamber is pumped down to ~5mbar by a scroll pump which can also be used as the backing pump for the turbo pump.The turbo pump has three stages, each pumping a different chamber as seen in Figure 1.The final pressure in the TOF is typically 10-6 mbar.
The API-TOF is manufactured by Tofwerk AG, Thun, Switzerland.It can be configured to measure either positive or negative ions, and can be run in either of two modes, V or W, the letters symbolizing the flight path of the ions inside the instrument.With the shorter flight path (V mode) which was used in this study, the resolving power (R) is specified to 3000 Th/Th and the mass accuracy to better than 20ppm (0.002%).Resolving power is defined as R=M/ M, where M is mass/charge and M is the peak width at its half maximum.In laboratory setup for testing instrument performance the API-TOF was connected in parallel with an electrometer, both sampling from a Herrmann nano differential mobility analyzer (HDMA, Herrmann, 2000).It takes advantage of high flow rates (sheath flow rate of up to 2000 l min-1 and sample flow of 15 l min-1) and can classify ions from 2.15 to 0.02 cm2V-1s-1 in electrical mobility corresponding to diameter of 0.8 to 10 nm.Ions were produced by electrospraying tetra-alkyl ammonium halides, which are commonly used positive ion mobility standards (Ude and Fernandez de la Mora 2005).

RESULTS
Tetra-heptyl ammonium bromide (THAB) was electrosprayed into a Herrmann DMA, which scanned the mobility range from 0.3 to 1.3 cm2 V-1s-1, and the output was measured by an electrometer and the API-TOF.The results are depicted in Figure 2. The electrometer counts all the ions coming out of the HDMA, and this concentration is plotted as the black dashed line in Figure 2. From previous ion mobility studies (Ude and Fernandez de la Mora 2005) we know that the peak at mobility diameter 1.47 nm corresponds to the THAB monomer, and the peak at 1.78 nm to the dimer.This was also clearly verified by the API-TOF.The horizontal axis in Figure 2 shows the mass, mobility, and mobility diameter scales.The three different axes are not universally interchangeable, but they are plotted here for reference to show rough relations between three very commonly used quantities.
The total ion count seen by the API-TOF is depicted by the solid black line.Electrometer counts are plotted on the left axis, and API-TOF counts on the right.The other lines correspond to the ion counts related to the different THAB peaks.As an example, the most abundant isotope of the tetra-heptyl ammonium cation (THA+, in the following denoted as "monomer") has a mass of 410.47 Da.The measured isotopic pattern can be found in the monomer inset figure, and the shape is mainly due to the 13C isotope.Summing up the total signal in the three major isotopes, yields the total monomer signal and this corresponds to the red line in Figure 2. The dimer consists of a neutral THAB clustered with THA+, yielding a maximum mono-isotopic mass of 901.86 Da.Bromide has two isotopes of roughly equal abundance at masses 79 and 81 Da, and thus the isotopic pattern detected by the API-TOF is very distinctive as can be seen in the dimer inset figure.Again, summing the signal in all of these peaks yields the total dimer signal, resulting in the light blue line in Figure 2.
Since we are able to detect THAB clusters up to pentamers, no considerable fragmentation of the those clusters happened inside the API-TOF.If this would have been the case, and pentamers and higher clusters would fall apart inside the API-TOF, we should instead detect the smaller fragments which retained the charge, but this was not the case.This is, however, probably the case with the peak at 1.6 nm which shows up in the API-TOF as pure monomer although it had a larger size when passing through the HDMA.This implies that it had been clustered with some impurity compound, which was lost before entering the extraction region in the TOF.

CONCLUSIONS
The recently developed atmospheric pressure interface time-of-flight mass spectrometer (APi-TOF) has been shown to be powerful instrument in studying atmospheric ions and clusters.We have demonstrated in laboratory that the new instrument is capable of measuring clusters in the size range of that is relevant for new particle formation event.We have also shown that no significant fragmentation is occurring inside APi-TOF.

Figure 1 .
Figure 1.Schematic of APITOF.First inlet chamber is pumped with separate scroll pump, other chambers are pumped with 3-stage turbo pump.Pressure drop in chambers is from 5mbar in the first chamber to 10-6 mbar in the time-of-flight region.Red bars represent two quadruple ion guides and green bars ion lens stack to guide ions to TOF.