We have demonstrated in a number of recent studies that RF-trapping of massselected electrosprayed protonated peptides or oligonucleotides can provide targets
sufficiently dense for VUV [15, 16], and soft X-ray [17, 18], photofragmentation
studies. A similar technique has also been applied to VUV [19] and soft X-ray [20]
photoionisation of larger multiply protonated proteins. Dugourd and coworkers have
focused on deprotonated peptides and their radicals [21, 22]. Peptides and proteins
are particularly interesting for photoionisation studies. The fact that peptides are built
up from a construction kit of 20 canonical amino acids which are chemically very
different, allows the synthesis of systems for virtually any experimental purpose.
In this chapter, we will first describe the experimental technique, i.e. the homebuilt tandem mass-spectrometer featuring an RF ion trap and how it is interfaced
with third generation synchrotron beamlines for VUV and soft X-ray photons. After
this, the response of protonated peptides after VUV and soft X-ray photoabsorption
will be discussed.
11.2 Experimental Technique
To generate beams of singly or multiply protonated peptides, a home-built
electrospray ionisation (ESI) source is fed with the respective peptide solution.
Typically, ~30 μmol methanol solutions with 1 % formic acid are used for a peptide
such as leucine enkephalin (YGGFL, m ¼ 555.62). The electrosprayed particles
enter the vacuum chamber through a capillary which is 20 cm in length and has an
inner diameter of 0.125–0.5 mm (see Fig. 11.1, bottom left). An RF ion-funnel is used
for phase-space compression of the positive ions and focuses into an RF-only
quadrupole, which acts as a second phase-space compressor. A quadrupole mass
filter discriminates undesirable masses before a clean beam of mass selected biomolecular ions enters the 3D RF trap through one of the hyperbolic end caps until
sufficient ion density is reached. Continuous injection into the trap is only possible,
when the injected ions are collisionally cooled before they can escape the trap. To this
end, a He-buffer gas pulse is injected into the trap through a bore in the ring electrode.
The pressure in the trap increases to about 10
À3 mbar. Typically injection periods are
a couple of 100 ms. At the end of the loading cycle, the ESI beam is deflected by
means of a static electric field. The resulting trapped target typically has a diameter of
about 300 μm and contains a few 1,000 protonated peptides.
For exposure of the target to a beam of energetic photons, the apparatus is
interfaced with an appropriate synchrotron beamline. For the VUV photoionisation
experiments presented in this chapter, the U125/2 beamline and the 10 m focal
length normal incidence monochromator (NIM) at the BESSY II facility in Berlin
(Germany) were used [23]. The soft X-ray studies were performed using the U49/
2 beamline and a plane grating monochromator (PGM) at the BESSY II facility and
the i411 beamline at MAXlab in Lund (Sweden). In all cases, relatively low
resolution gratings were employed in order to have maximum photon fluxes of
the order of 10
13 s
À1 . The photon beam crosses the RF-trap centre through two
bores in the ring-electrode. Typical photon beam diameters in the centre of the trap
11 Action Spectroscopy of Gas-Phase Peptide Ions with Energetic Photons
211
sufficiently dense for VUV [15, 16], and soft X-ray [17, 18], photofragmentation
studies. A similar technique has also been applied to VUV [19] and soft X-ray [20]
photoionisation of larger multiply protonated proteins. Dugourd and coworkers have
focused on deprotonated peptides and their radicals [21, 22]. Peptides and proteins
are particularly interesting for photoionisation studies. The fact that peptides are built
up from a construction kit of 20 canonical amino acids which are chemically very
different, allows the synthesis of systems for virtually any experimental purpose.
In this chapter, we will first describe the experimental technique, i.e. the homebuilt tandem mass-spectrometer featuring an RF ion trap and how it is interfaced
with third generation synchrotron beamlines for VUV and soft X-ray photons. After
this, the response of protonated peptides after VUV and soft X-ray photoabsorption
will be discussed.
11.2 Experimental Technique
To generate beams of singly or multiply protonated peptides, a home-built
electrospray ionisation (ESI) source is fed with the respective peptide solution.
Typically, ~30 μmol methanol solutions with 1 % formic acid are used for a peptide
such as leucine enkephalin (YGGFL, m ¼ 555.62). The electrosprayed particles
enter the vacuum chamber through a capillary which is 20 cm in length and has an
inner diameter of 0.125–0.5 mm (see Fig. 11.1, bottom left). An RF ion-funnel is used
for phase-space compression of the positive ions and focuses into an RF-only
quadrupole, which acts as a second phase-space compressor. A quadrupole mass
filter discriminates undesirable masses before a clean beam of mass selected biomolecular ions enters the 3D RF trap through one of the hyperbolic end caps until
sufficient ion density is reached. Continuous injection into the trap is only possible,
when the injected ions are collisionally cooled before they can escape the trap. To this
end, a He-buffer gas pulse is injected into the trap through a bore in the ring electrode.
The pressure in the trap increases to about 10
À3 mbar. Typically injection periods are
a couple of 100 ms. At the end of the loading cycle, the ESI beam is deflected by
means of a static electric field. The resulting trapped target typically has a diameter of
about 300 μm and contains a few 1,000 protonated peptides.
For exposure of the target to a beam of energetic photons, the apparatus is
interfaced with an appropriate synchrotron beamline. For the VUV photoionisation
experiments presented in this chapter, the U125/2 beamline and the 10 m focal
length normal incidence monochromator (NIM) at the BESSY II facility in Berlin
(Germany) were used [23]. The soft X-ray studies were performed using the U49/
2 beamline and a plane grating monochromator (PGM) at the BESSY II facility and
the i411 beamline at MAXlab in Lund (Sweden). In all cases, relatively low
resolution gratings were employed in order to have maximum photon fluxes of
the order of 10
13 s
À1 . The photon beam crosses the RF-trap centre through two
bores in the ring-electrode. Typical photon beam diameters in the centre of the trap
11 Action Spectroscopy of Gas-Phase Peptide Ions with Energetic Photons
211