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Development of a highly sensitive and versatile mass spectrometer system for laboratory and atmospheric measurements
von Sascha AlbrechtTrace gases in the upper troposphere and the lower stratosphere (UTLS)
strongly influence our climate and even can affect tropospheric weather. Therefore
a better understanding of the processes governing trace gas distributions
and transport is crucial. Sensitive and accurate atmospheric measurements are
thus essential to improve and verify Chemical Climate Models (CCM’s) and
ultimately enable more reliable forecasts of future climate.
Two especially important trace gases in the UTLS are sulphuric acid, H2SO4
and its precursor SO2. Although H2SO4 is present at sub-ppt mixing ratios it
plays a central role in the formation of the stratospheric aerosol layer, which
is a crucial component of the global radiative budget [52]. Very selective and
sensitive measurements of H2SO4 and many other trace species are possible using
mass spectrometers in combination with chemical ionization (CI) at atmospheric
pressure (AP). However, to reach a favorable detection limit, improvements
of the ion source and the ion transfer stages are needed. Therefore the ion
transmission through each transfer element needs to be optimized. Additional
sensitivity may be gained using a “brilliant” ion source.
The cluster chemistry in the ion source and transfer stage plays an important
role for the sensitivity and the reliability of the measurement. In order to
gauge the impact of cluster chemistry a transfer stage was developed that
provides realistic mass spectrometer measurements of ion bound clusters and
their reactions in the ion source. This transfer stage with tunable electrostatic
fields provides full control of the ion energy, to the extent that additional
reactions, induced by heating the ions in electrostatic fields, can be prevented.
A thermally sampling atmospheric pressure ionization mass spectrometer
has been constructed. The ion transfer stage offers the capability to sample
cluster ions at thermal equilibrium. Fundamental processes in the transfer
stage possibly affecting the cluster distribution can be readily identified. The
performance of the setup is demonstrated with regard to the proton-bound
water cluster system. The omnipresent water cluster chemistry occurs in every
AP ion source and is also part of most AP CI reaction cascades.
strongly influence our climate and even can affect tropospheric weather. Therefore
a better understanding of the processes governing trace gas distributions
and transport is crucial. Sensitive and accurate atmospheric measurements are
thus essential to improve and verify Chemical Climate Models (CCM’s) and
ultimately enable more reliable forecasts of future climate.
Two especially important trace gases in the UTLS are sulphuric acid, H2SO4
and its precursor SO2. Although H2SO4 is present at sub-ppt mixing ratios it
plays a central role in the formation of the stratospheric aerosol layer, which
is a crucial component of the global radiative budget [52]. Very selective and
sensitive measurements of H2SO4 and many other trace species are possible using
mass spectrometers in combination with chemical ionization (CI) at atmospheric
pressure (AP). However, to reach a favorable detection limit, improvements
of the ion source and the ion transfer stages are needed. Therefore the ion
transmission through each transfer element needs to be optimized. Additional
sensitivity may be gained using a “brilliant” ion source.
The cluster chemistry in the ion source and transfer stage plays an important
role for the sensitivity and the reliability of the measurement. In order to
gauge the impact of cluster chemistry a transfer stage was developed that
provides realistic mass spectrometer measurements of ion bound clusters and
their reactions in the ion source. This transfer stage with tunable electrostatic
fields provides full control of the ion energy, to the extent that additional
reactions, induced by heating the ions in electrostatic fields, can be prevented.
A thermally sampling atmospheric pressure ionization mass spectrometer
has been constructed. The ion transfer stage offers the capability to sample
cluster ions at thermal equilibrium. Fundamental processes in the transfer
stage possibly affecting the cluster distribution can be readily identified. The
performance of the setup is demonstrated with regard to the proton-bound
water cluster system. The omnipresent water cluster chemistry occurs in every
AP ion source and is also part of most AP CI reaction cascades.