A laser desorption ionization/matrix‐assisted laser desorption ionization target system applicable for three distinct types of instruments (LinTOF/curved field RTOF, LinTOF/RTOF and QqRTOF) with different performance characteristics from three vendors

Rationale We have developed a target system which enables the use of only one target (i.e. target preparation set) for three different laser desorption ionization (LDI)/matrix‐assisted laser desorption ionization (MALDI) mass spectrometric instruments. The focus was on analysing small biomolecules with LDI for future use of the system for the study of meteorite samples (carbonaceous chondrites) using devices with different mass spectrometric performance characteristics. Methods Three compounds were selected due to their potential presence in meteoritic chondrites: tryptophan, 2‐deoxy‐d‐ribose and triphenylene. They were prepared (with and without MALDI matrix, i.e. MALDI and LDI) and analysed with three different mass spectrometers (LinTOF/curved field RTOF, LinTOF/RTOF and QqRTOF). The ion sources of two of the instruments were run at high vacuum, and one at intermediate pressure. Two devices used a laser wavelength of 355 nm and one a wavelength of 337 nm. Results The developed target system operated smoothly with all devices. Tryptophan, 2‐deoxy‐d‐ribose and triphenylene showed similar desorption/ionization behaviour for all instruments using the LDI mode. Interestingly, protonated tryptophan could be observed only with the LinTOF/curved field RTOF device in LDI and MALDI mode, while sodiated molecules were observed with all three instruments (in both ion modes). Deprotonated tryptophan was almost completely obscured by matrix ions in the MALDI mode whereas LDI yielded abundant deprotonated molecules. Conclusions The presented target system allowed successful analyses of the three compounds using instruments from different vendors with only one preparation showing different analyser performance characteristics. The elemental composition with the QqRTOF analyser and the high‐energy 20 keV collision‐induced dissociation fragmentation will be important in identifying unknown compounds in chondrites.

LDI is also useful for analytes that cannot be dissolved easily in common solvents that are regularly applied to dissolve standard MALDI matrices.
Despite the much higher thermal load transferred to the molecules of interest in LDI, it is often a successful approach. It is often also difficult to select one individual appropriate MALDI matrix for chemically different analytes, because not every matrix works satisfactorily with the selected sample. [5][6][7] There are further differences in MALDI matrix selection for one given analyte between classical high-vacuum MALDI (with a TOF-MS instrument) and intermediate-pressure MALDI (typically with ion traps, hybrid quadrupole/reflectron TOFs and FT-ICR-MS instruments). 8,9 The UV LDI-MS method typically requires a higher laser fluence to desorb analyte ions from the target surface than MALDI-MS. As different manufacturers of MALDI/LDI instruments usually provide different target plates in terms of format, and surface structures including surface chemistry and fine structure surface only useful for one particular instrument, a serious comparison of one sample preparation is usually not easy. On the other hand, the ability to work on different MALDI/LDI instruments with only one target plate ensures that the sample is always prepared on the same surface and that no additional contamination can occur, which can be the case when using multiple target plates (despite careful cleaning procedures). This approach could be helpful if only small sample amounts are available and thus only one or two preparations, i.e. sample spots, can be made.
The switching from one type of ion source (e.g. intermediate pressure to high vacuum or different entry angles of the laser beam or different laser wavelengths) and/or of differently performing analysers can be done in a straightforward manner only if part of the sample is consumed (which is usually the case except perhaps in mass spectrometric imaging).
In this paper we compare the mass spectrometric behaviour of three selected small-molecule analytes (tryptophan, 2-deoxy-D-ribose and triphenylene) deposited on a standard Waters MALDI-MS target mounted on a home-built target adapter/holder using three different instruments (Axima TOF 2 , ultrafleXtreme and Synapt G2) in both LDI and MALDI modes. For this study our goal is the evaluation of the desorption/ionization properties of these model analytes utilizing one target with one preparation (including replicates) on different performing instruments (e.g. different laser wavelength applied in LDI mode, different LDI beam shapes, different laser pulse rates, different ion source pressures, different mass spectrometric accuracy and resolution, and high-energy versus low-energy collision-induced dissociation) with a home-built target adapter for three instruments.

| Preparation of samples and solutions
The analyte solutions for the LDI and MALDI experiments were  The target adapter was modified by using a CNC (computer numerical control) milling machine and is shown in Figure 1A with the Waters LDI/MALDI target. The dimensions of the target adapter with the modifications are given in Figure 1B.

| Stainless steel target adapter
The LDI/MALDI target is inserted from the top into the modified target adapter/MTP target and is held there by a cylindrical magnet mounted in the target adapter cavity ( Figure 1B, central circle marked with M) for insertion of the target. The target adapter can then be inserted either horizontally into the Shimadzu Axima TOF 2 instrument or vertically into the Bruker ultrafleXtreme mass spectrometer ( Figures 1C and 1D). As mentioned above, because the LDI/MALDI target plate originates from Waters, there is no need for the target adapter when using the Waters instrument, i.e. the target can be removed easily and put in the holder of the Waters Synapt G2 system.
With the Waters instrument the target is inserted vertically into the ion source ( Figure 1E).
The LDI/MALDI target was examined by means of the absolute digimatic indicator ID-C125B (Mitutoyo, Kawasaki, Japan, measurement accuracy of 3 μm) to evaluate any thickness variations.
No differences in the thickness of the target were observed within the measurement accuracy of the instrument. A coordinate measuring machine (Crysta-Plus M544, Mitutoyo) was used to measure the flatness (GD&T) of the target without the target adapter as well as when the target was mounted into the adapter system. After the instrument calibration with parallel jaws (Helios-Preisser, Gammertingen, Germany, measurement accuracy of 4 μm), the flatness of the target was 21 μm.
After inserting the target into the target adapter system the overall flatness increased to 24 μm due to the adaptation of the target adapter by CNC milling. For these measurements, eight measuring points were

| Bruker Daltonics ultrafleXtreme
The high-vacuum (2.1 × 10 −6 mBar) LDI/MALDI ultrafleXtreme mass spectrometer uses a 2 kHz, so-called smartbeam-II, laser were acquired in the m/z range 20 to 500. The method was optimized in order to cover an area of 1000 μm × 1000 μm with 10 × 10 raster spots. Ten laser shots per raster spot were fired yielding 1000 laser shots per experiment. The applied laser fluence was adjusted manually for each experiment. Again, a mixture of SA and sodium chloride was used for the calibration of the ultrafleXtreme. A volume of 0.5 μL of matrix solution was spotted onto the target plate and measured using an automatic acquisition method developed for the samples. All measurements were performed in positive ion reflectron and negative ion reflectron modes.

| Tryptophan
Applying  The errors in mass determination expressed in ppm (achieved with the Synapt G2 device) are shown in parentheses. ND: not detected.

| Triphenylene
Triphenylene, a polycyclic aromatic hydrocarbon, was the only measured compound that did not exhibit any protonated ion, adduct ion or deprotonated ion. Only an abundant radical cation at m/z 228.1 was detected in all experimental settings. The LDI and MALDI mass spectra of triphenylene are shown in Figure 5. Positive-ion LDI reflectron mass spectra of triphenylene utilizing the Axima TOF 2 (A1), the ultrafleXtreme (A2) and the Synapt G2 (A3) instruments and positive-ion MALDI reflectron mass spectra obtained with the Axima TOF 2 (B1), the ultrafleXtreme (B2) and the Synapt G2 (B3) using the MALDI matrices, DHB and THAP, respectively FIGURE 4 Positive-ion LDI reflectron mass spectra of 2-deoxy-D-ribose utilizing the Axima TOF 2 (A1), the ultrafleXtreme (A2) and the Synapt G2 (A3) instruments and positive-ion MALDI reflectron mass spectra obtained with the Axima TOF 2 (B1), the ultrafleXtreme (B2) and the Synapt G2 (B3) using THAP as MALDI matrix for all three instruments radical cation yields the most abundant signal compared with the MALDI matrix ion when using THAP. The mass accuracies obtained for the triphenylene radical cation are displayed in Table 1.

| CONCLUSIONS
A Bruker target was modified to function as an adapter for use with a  [14][15][16] and their influence on the development of organics on earth.