The Ion Source

The ion extraction region not only needed to provide access for the laser and camera optics, it also played a key role in the generation and collection of ions produced by laser desorption ionization. The initial source design was based on a classical Wiley-McLaren geometry [230,231], consisting of two flat grid electrodes placed at distances of d₁ ≈1 cm and d2 ≈1 cm from the sample plane, as is shown in fig. 4.3a. Following acceleration in this two-stage extraction region, the ions traversed aD≈1 m field-free drift region before impinging on the detector. The total flight timet=t1+t2+tD through all three regions

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4.1 Instrumental Design

can be estimated from the equations of motion as t1 = v1−v0

a₁ withv1 =^qv²₀+ 2a1(d1−x0) (4.1) t₂ = v2−v1

a₂ withv₂ =^qv²₀+ 2a₁(d₁−x₀) + 2a₂d₂ (4.2) tD = D

v₂ , (4.3)

where ai = −Q∆Ui/dim is the acceleration of an ion with mass m and charge Q in a homogeneous electric field in regioni with a voltage difference ∆U_i over a distanced_i. A spread in arrival time is observed because of different initial ion velocitiesv0, and variations in the point of ion creation x₀. Initial ion velocities in MALDI have been measured to amount to several hundred meters per second, but are typically below 1000 m/s [93, 231].

A spread in initial ion position can be caused by sample tilt, irregular surface morphology (e.g. large MALDI crystals), or removal of sample with consecutive shots. Especially in IR-MALDI, where the per-shot ablation volume is large, significant distributions ofx0 can be expected even for a single laser pulse. The use of the two-stage extraction region allows for a compensation of this position spread by varying the magnitude of the acceleration potential in the first acceleration region while leaving all other parameters, including the total acceleration potential, constant. Based on the flight distances, the optimum ratio G²= (∆U1+ ∆U2)/∆U1 of the total acceleration voltage to the one in the first region can be estimated numerically (to first order) from the relation

0 =−G+d₂ d1

1−G 1−G²

+ D

2d1G², (4.4)

although fine tuning of this value is typically necessary during the experiment. Much better mass resolution can be obtained if, in addition, the initial velocity spread is compensated.

This can be achieved by use of a reflectron or ion mirror, in which the ions are reflected eletrostatically: because faster ions penetrate deeper into the decelerating electric potential, they traverse a longer distance than slower ions. When adjusted correctly, the velocity focus, which is the plane at which the fast and slow ions arrive simultaneously, is placed onto the detector. However, because some amount of ions will fragment during their flight through the mass analyzer due to the internal energy they acquired during ionization (i.e.

post-source decay of ions in metastable energy states), and because a conventional reflectron introduces additional (lossy) grid electrodes, such a design usually has a lower sensitivity.

Alternatively, the ion extraction field in the ion source can be pulsed, a technique known as delayed extraction (DE). There, the ablation plume is allowed to expand for several hundred

Chapter 4 In-Vacuo Laser Desorption Ionization Time-of-Flight Mass Spectrometry

b) gridless design

S E G L G

S E G

d₁ d₂

a) Wiley-McLaren

d₁ d₂

Figure 4.3: a) A two-stage extraction region of the Wiley-McLaren type was used initially to achieve space-focusing. b) It was replaced by a gridless design which introduced less transmission losses and left room for the laser and camera optics. Two extraction regions and an Einzel lens were formed by the sample surface (S), the extraction electrode (E), two grounded apertures (G) and the lens electrode (E).

nanoseconds without an applied field, such that initially faster ions will be accelerated over a shorter distance once the field is switched on, again producing a velocity focus. As an additional benefit, this field-free expansion has been shown to reduce energy loss and fragmentation due to in-plume collisions, which can increase both the ion yield and the mass resolution [231]. As a consequence, the ablation under DE conditions can be performed at higher laser fluences, which would otherwise lead to very dense plumes and significant peak broadening under continuous extraction. However, while the electrostatic focusing performed in the two-stage extraction and reflectron design are mass-independent, delayed extraction only focuses ions in a certain mass range, depending on the delay time and field strengths.

The results presented here were performed in a linear TOF design and under continuous extraction. A flat front surface of the sample holder extending beyond the sample area was desirable to keep the extraction field well defined. Figure 4.2b shows the holder used here.

Most notably, there were no protruding screws or indentations on the front side. To achieve this, the coverplate was mounted on top of the 18 mm×18 mm silicon target by tightening four nuts from the backside of the holder, using four bolts welded to the coverplate.

A simple source design was chosen here to be able to concentrate on the underlying physics of ablation and ionization, without obfuscating results by an unnecessarily high number of parameters. This way, the ion yield for different sample preparations and laser fluences could be determined more directly. However, the simple two-grid extraction region was replaced with a gridless design based on that of the commercial Bruker machines, which is schematically depicted in fig. 4.3b. This had several advantages: a gridless design helped

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4.1 Instrumental Design

to avoid losses due to scattering at the electrode, and also focused the ion beam early on so that better collimation could be achieved. Besides the inherent lensing effect of the gridless acceleration, an Einzel lens was included in the design to control the amount of beam collimation. The new design also made sure that proper alignment could be ensured with the help of an alignment laser, i.e. several removable pinholes were included along the flight path. Most importantly, the new design featured access points for two lasers and a camera, so that experiments could also be performed in the more conventional front-ablation geometry.