A high resolution dual mass gate for ion separation in laser desorption/
ionization time of flight mass spectrometry
Carsten W. Stoermer, Stefan Gilb, Jochen Friedrich, Detlef Schooss,
and Manfred M. Kappes
Institut fu
¨
r Physikalische Chemie, Universita
¨
t Karlsruhe, D-76128 Karlsruhe, Germany
͑Received 8 August 1997; accepted for publication 26 January 1998͒
A dual mass gate is described which allows for high resolution separation of ions under realistic
laser desorption/ionization and matrix assisted laser desorption/ionization time of flight mass
spectrometry conditions at 10 kV kinetic energy. The device consists of two interleaved comb units
sequentially positioned in the ion beam path. This allows significant improvement in resolution
relative to one unit by separation of ‘‘low pass’’ and ‘‘high pass’’ functions. Principal limits as
given by mechanical dimensions of the device and electrical properties of the switches are also
discussed. © 1998 American Institute of Physics. ͓S0034-6748͑98͒05004-7͔
I. INTRODUCTION
Matrix assisted laser desorption/ionization time of flight
mass spectrometry ͑MALDI-TOFMS͒
1–3
is now a routine
method in organic mass spectrometry. A major application is
the analysis of high mass biochemical and to a lesser extent
organic polymer molecules. There is also much interest in
developing MALDI-TOFMS methodology for the character-
ization of large fragile inorganic cluster complexes.
A common experimental configuration consists of a
nanosecond pulsed laser focused onto a desorption target
mounted in the end plate of a two stage ion acceleration
region ͑repeller and extractor͒. Because of the small tempo-
ral and spatial extent of ion formation, this ion source can be
simply configured to comprise the repeller plate at high dc
potential and the extractor at ground. While mass resolution
can be enhanced with a reflectron setup, many MALDI-
TOFMS machines used to study fragile samples ͑i.e., mol-
ecules which extensively fragment during and after accelera-
tion͒ consist of a linear drift tube and on-axis
multichannelplate detector. Compared to a reflectron con-
figuration this provides somewhat lower performance ͑at sig-
nificantly lower complexity͒ while still allowing for the de-
tection of fast neutral fragments with the same on-axis MCP
detector. With our machine, a mass resolution of 3500 at
mass 1000 full width at half maximum ͑FWHM͒ can be
obtained using a 1.8 m linear drift tube.
In LDI- and MALDI-TOFMS the development of highly
resolving mass gates based on flight time is of interest to-
wards: ͑i͒ shielding the detector from intense unwanted ions
and ͑ii͒ being able to apply secondary analysis methods in
the context of tandem mass spectrometry ͑MS/MS͒. In the
latter application, mass gated species may, for example, be
subjected to collision induced dissociation ͑CID͒, surface
impact induced dissociation ͑SID͒, photodissociation ͑PD͒,
and ͑photo͒electron detachment ͓͑P͒ED͔.
In this contribution we describe the construction and per-
formance of a simple and effective ‘‘double selector’’ which
provides such mass gating. While based in part on published
designs,
4,5
our device improves on previous specifications
and more importantly makes routine mass gating possible
under the high voltage ion extraction conditions typically
used in MALDI-TOFMS.
II. OVERALL EXPERIMENTAL CONFIGURATION
Experiments were carried out in a linear LDI-TOFMS
apparatus which is shown schematically in Fig. 1. The ma-
chine consists of a differentially pumped load lock chamber
to quickly introduce samples from atmospheric pressure, an
ion source region, a drift tube containing the mass gate de-
vice, and a detection region comprising a MCP detector. Ion
source and detector regions were pumped, respectively, by
220 and 50 ls
Ϫ1
turbomolecular pumps. During operation the
mass spectrometer was maintained at pressures of about
10
Ϫ7
mbar. The most important instrumental descriptors
were as follows:
͑i͒ Ion source: A configuration was used consisting of
three 8ϫ8 cm square plates ͑repeller, extractor, and
grounded entrance to a 1.8 m drift tube͒ mounted parallel to
each other at separations of 9 and 6.35 mm, respectively. The
repeller plate had a central hole of 10 mm diameter into
which ͑electrically contacting͒ polished stainless steel target
disks could be fit so that their surface was mechanically at
the level of the plate to within 50
m. Disks were mounted
on an electrically insulated translation/rotation feedthrough
and inserted through the load lock. Samples were either in
the form of ͑a͒ solutions dried onto the disk surface, ͑b͒
pellets pressed into indentations on custom fabricated disks,
or ͑c͒ individual crystallites attached to the target disk sur-
face via epoxy glue. The extractor plate ͑0.5 mm thick stain-
less steel͒ comprised a central circular aperture onto which a
90% transmission nickel grid was mounted. Furthermore this
plate had an off-axis hole to allow irradiation of the target
disks with a pulsed laser incident at 45° relative to the sur-
face normal ͑see Fig. 1͒. The drift tube entrance aperture was
15 mm in diameter and similarly covered with a 90% trans-
mission nickel grid.
͑ii͒ Laser desorption/ionization and ion extraction: For
desorption/ionization a pulsed N
2
laser ͑MSG 405 TD, LTB,
REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 69, NUMBER 4 APRIL 1998
16610034-6748/98/69(4)/1661/4/$15.00 © 1998 American Institute of Physics