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«Zur Erlangung des akademischen Grades eines Dr.rer.nat vom Fachbereich Bio- und Chemieingenieurwesen der Universität Dortmund genehmigte ...»

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Compared with pulsed lasers, cw diode lasers have very stable and accurately tuneable power outputs, which is an important preconditions for experimental studies of laser desorption/ionization. However, diode lasers have never played any role in this scope until now. Most diode lasers emit in the near-infrared range, and few materials resonantly absorb in this range. The power output of a cw diode laser is also not capable to induce ionization. Accordingly, if wanting a diode laser is going to be used for laser desorption/ionization, more power from the laser beam should be coupled to the surface and the desorption and ionization steps should be decoupled.

A graphite surface can work well as a photon absorbing material. Kim et al applied MALDI to a graphite target that works as an energy transfer mediator for visible light.72 This group showed that the graphite surface also works well for a near-infrared laser beam. A desorption of biochemical molecules with moderate molecular weight deposited on a graphite plate can be realized with a power density in the order of 104 W/cm2, which is 100 times lower than the power density which is needed to desorb from a white surface.18 The decoupling of desorption and ionization was realized by LD-APCI technique, which was recently presented by Coon et al.73-76 As shown in Fig. 3-1, a corona needle was positioned above the target to ionize the desorbed molecules rather than normal AP-MALDI process. In this work, —————————————————————————————————

- 26 Diode laser induced desorption in combination with APCI/MS on graphite substrate ————————————————————————————————— the signal intensities were enhanced by 2 to 3 orders of magnitude for different analytes when compared with measurements without corona discharge, which means that the desorption and ionization steps were decoupled.

Based on above theories, a study to evaluate the applicability of desorption from a graphite surface by a continuous wave diode laser is presented here.

Figure 3-1. Schematic representation of LD-APCI source73

3.1 Chemicals and sample preparation The first evaluation was accomplished with reserpine (from Sigma), a calmative and antihypertensive drug. It is often used for the calibration of the LC-MS system and is accordingly suitable to give a well-defined signal (for structure, see Fig. 3-2).


- 27 Diode laser induced desorption in combination with APCI/MS on graphite substrate —————————————————————————————————

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Sphigomyelin (SPM, from bovine brain) and Lecithin (L-αphosphatidylcholine, PC, from fresh egg yolk) were also employed as analytes (both from Sigma). They are the most abundant phospholipids inside living bodies. Sphingomyelin is a group of phosphosphingolipids made by transferring choline from lecithin to a ceramide, which make up a —————————————————————————————————

- 28 Diode laser induced desorption in combination with APCI/MS on graphite substrate ————————————————————————————————— significant portion of the membrane lipids of the myelin sheath of nerve tissue. Lecithin is a group of glycerophospholipids that are involved in the metabolism of several lipid compounds. Determination of the lecithin/sphingomyelin ratio in the amniotic fluid is a viable method of monitoring and assessing fetal lung maturity. This is particularly important in high-risk pregnancies, for which early confinement is to be expected.

They both are a group of phospholipids that have similar structures but different masses based on the length of respective carbon chain and the amounts of unsaturated carbon bonds, as shown in Fig. 3-3a, b.

Other chemicals used in this study include methanol (from Merck, Germany), acetic acid (from Merck, Germany), chloroform (99.8 %, from Aldrich), n-Hexane (for GC, from Merck, Germany), isopropanol (for GC, from Merck, Germany) and ethanol (99.8 %, for chromatography, from Merck, Germany). No further treatment or purification was performed for the materials.

The reserpine solutions were prepared by dissolving reserpine in an aqueous solution of 50 % methanol with 0.1 % acetic acid. The sphingomyelin and the lecithin solutions were prepared by dissolving their powders in a mixture of chloroform/methanol/n-hexane (13:18:69) and ethanol/n-hexane (70:30), respectively.

3.2 Design of ion source and experimental setup Fig. 3-4 shows a picture and its sketch of the ion source used for the following experiments. A stainless steel target with a diameter of 4 mm was mounted on a xy-stage (DS40-xy, Newport, USA). The distance of the target to the front of the heated capillary inlet (transfer capillary) of a LCQ Classic ion trap mass spectrometer (ThermoFinnigan, USA) and the position in reference to the axis could be changed by the xy-stage. A potential of 1.5 kV was applied to the target by an additional power supply —————————————————————————————————

- 29 Diode laser induced desorption in combination with APCI/MS on graphite substrate ————————————————————————————————— in order to improve the ion transmission efficiency. The corona needle was positioned on the axis of the transfer capillary and above the rear edge (in reference to the capillary inlet) of the target. The needle could also be moved along the axis, therefore allowing an adjustment of the needle-toplate distance for optimal APCI conditions. The standard APCI power supply of the LCQ system was connected to the needle to generate the corona discharge. A diode laser (GBL981000G from Roithner Lasertechnik) with a wavelength of 985 nm and a maximum power output of 1 W was used to desorb the analyte molecules. For all experiments the laser beam was focused on the surface with a calculated spot diameter of approximately 0.1 mm, resulting in a maximum power density in the order of 104 W/cm2.

Figure 3-4. Ion source


- 30 Diode laser induced desorption in combination with APCI/MS on graphite substrate ————————————————————————————————— The parameters of the mass spectrometer were optimized according to the analytes, individually. For reserpine, the capillary voltage was set to 6 V, the tube lens voltage offset was set to 57 V; for sphigomyelin and lecithin, they are 41.5 V and 43 V, respectively. The capillary temperature was maintained at 150 °C in all cases. The ion injection was set to AGC (Automatic Gain Control) mode, because the synchronization between laser and mass spectrometer is not necessary due to the use of a continuous wave diode laser system.

3.3 Optimization of experimental conditions The first step of the following experiments was to find the best position of the corona discharge in reference to the entrance of the transfer capillary in order to realize ionization of desorbed molecules and an efficient transportation by the electric field. A short distance between the target and the inlet of the mass spectrometer is helpful to improve the transmission efficiency. However, if the target is too close to the capillary inlet, electric arc will appear in between. A distance of 3 mm from the front edge of the target to the inlet was evaluated as the optimized parameter at last whereas the target was 3 mm underneath the axis of the transfer capillary. This ensured the cloud from desorbed materials formed on the axis. It was found that the target can influence the corona discharge if it is too close to the axis of the electric field. The tip of the corona needle was positioned on axis and above the rear edge of the target, 1-2 mm offset from the spot of the laser beam, which ensured that the electric field contained the whole region of the desorbed molecules in gas phase. It has to be noted that when the corona needle is moved above the front part of the target, only a very weak sample signal can be measured, and when the position is beyond the front edge, no signal can be obtained. Such a geometry layout assures that a well-build electric field is set to form the steady ion current to carry —————————————————————————————————

- 31 Diode laser induced desorption in combination with APCI/MS on graphite substrate ————————————————————————————————— ionized sample molecules into the mass spectrometer. In other studies about LD-APCI,73,74 long distance among each part of the source (1.5 or 3 cm from heated capillary inlet to the tip of corona needle) was applied and for efficient ionization in such a distance, a high discharge voltage (5.3 or

8.1 kV) was utilized. In our case, a compact source layout was employed to improve the transmission efficiency, and the discharge voltage was dropped to around 3 kV, which results in a current of 0.5 µA. Higher discharge currents were also used, but no obvious enhancement of the signal intensity could be observed, and the risk of a plasma breakthrough between corona needle and target is high. A potential of 1.5 kV on the target was found to enhance desorption and also improve ion transmission efficiency.

3.4 Analysis of a compound with moderate molecular weight Figure 3-5. Total ion counts obtained with (a) only corona discharge on (b) both corona discharge and laser on (c) only laser on.


- 32 Diode laser induced desorption in combination with APCI/MS on graphite substrate ————————————————————————————————— A reserpine solution of 2 µl (0.6 µg reserpine) was deposited on a piece of graphite plate that was placed on the metal target. The 1 W diode laser with maximal power output was used to desorb sample molecules after the sample solution was dried for 5 min at room temperature. The total ion counts in the m/z range from 150 to 700 were measured by the mass spectrometer.

As shown in Fig. 3-5, no signal of reserpine was measured when only the corona discharge was in operation (a), obviously due to the fact that reserpine was not evaporated from the surface of the graphite plate. After about 1 min, also the diode laser was switched on (b), and an intensive signal appeared. Finally, after about 2 minutes, the corona discharge was switched off (c) and the signal intensity decreased immediately to zero.

Therefore, analyte ions can only be measured when both the laser and the corona discharge are in operation. It is obvious that the increase of the signal initiated by laser irradiation was less slow than the decrease caused by stopping the corona discharge. The slow increase of the signal might be explained by the time of the heat transfer. The instantaneous decrease is due to the fact that the ionization process is stopped and no more ions enter the mass spectrometer.

In the case when only the laser was on (c), it is obvious to assume that the power from the diode laser was not high enough for ionization.

However, even if a few sample molecules are ionized, the absence of an electric field in the entrance region of the mass spectrometer resulting from the corona discharge reduces the transmission efficiency of ions.

Therefore these ions cannot be detected. In order to clarify this, the voltage of the corona discharge was decreased to the extent that the corona discharge expired, but attendant ions could still be transported into the capillary inlet. Nevertheless, there was still no measurable signal.

Therefore, desorption and ionization are well decoupled in this technique.


- 33 Diode laser induced desorption in combination with APCI/MS on graphite substrate —————————————————————————————————

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