Useful information regarding the service measurements

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• Sample delivery

  A small amount of the sample (usually few mg, at least 10 µg) in a tightly closed vial is deposited together with a completely filled sample form at our delivery point next to the main entrance of the MSC (ground floor, room 1E29). Please make sure that the labels on the vials are clearly readable and permanent. We prefer solid samples; in the case of solutions, the solvent and analyte concentration need to be stated on the form. If not otherwise arranged, the samples are stored at room temperature and, if possible, dissolved in ACN/MeOH/H2O (49.5:49.5:1) for the measurement. Instability in one of these solvent components should thus be mentioned. In the case of short-lived samples and time-sensitive or urgent measurements (e.g. reaction controls), we ask you to make an appointment with one of our technicians in advance.

  Unless the compound is completely unknown, please make sure to provide a correct sum formula when requesting high-resolution measurements.
• Safety

  Radioactive and explosive substances as well as biohazard materials are not allowed in the MSC. To protect our employees, it is mandatory to label in a clearly readable way all samples that are known or assumed to be toxic, strongly reactive or posses other hazardous risks. It also has to be stated if special requirements have to be taken upon disposal.
• Sample purity

  The routine service measurements are performed without hyphenated chromatographic or electrophoretic separation. This usually makes sense only for pure compounds or well-defined, very simple mixtures. Salts, detergents and other ionic or easily ionizable substances (e.g. amines) present next to the compound of interest can interfere with the measurement already in very low concentrations and lead to uninformative or difficult to interpret mass spectra. Mass spectrometry, in contrast to most spectroscopic techniques, requires the sample to directly enter the instrument. High concentrations of the above mentioned accompanying substances in the samples can thus cause contamination of or even damage to the highly sensitive instruments, which again leads to long down-times and in the worst case high costs for instrument maintenance.

  Consequently, we request to only deliver purified samples. In the case of questions regarding the suitability of samples for MS analysis or for appropriate cleaning procedures, feel free to ask a member of the MSC team.
• Return of samples

  If you would like to get the remaining sample back, tick the “no” box on the sample form next to the question whether the sample shall be disposed of. After the measurement has been completed, you will find the remaining sample at the delivery point. If you want us to discard the sample, do not specify anything or do not pick up your sample within two weeks, it will be disposed by the MSC.
• Mass spectra interpretation (ESI-MS)

  In the majority of the cases, the interpretation of the ESI-MS full scan spectra is simple. The main compound in the sample is usually clearly visible in the mass spectra as M+ (for compounds with a fixed charge), [M+H]+, or [M+Na]+ in positive ion mode and as M- (for compounds with a fixed charge) or [M-H]- in negative ion mode. In the presence of salts in the sample, other so called adduct ions can be formed, such as [M+NH4]+, [M+Li]+, [M+Cl]-or [M+CH3COO]-. Ionic compounds usually dissociate and the counter ion is not detected – or only in a separate measurement. In some cases, multiply charged ions (e.g., [M+nH]n+) can be detected, whereby the number of charges can be determined from the measured m/z-difference between the isotopes.

  However, not all compounds can be measured with electrospray ionization (ESI). Compounds that are volatile, very apolar, or very badly soluble in rather polar solvent mixtures might not be accessible with this technique. Furthermore, the analytes need to be ionizable under ESI conditions, i.e. they need to possess a certain gas phase basicity, acidity or affinity to Na+. Although ESI is one of the softest ionization methods, it can happen that the produced ions are unstable and show spontaneous “in-source” decay. In all the above mentioned cases, the compound of interest might not be detected or only with very low intensity, while even relatively small amounts of easily ionizable and stable contaminants might show up with much higher intensity in the mass spectrum.

  The interpretation of mass spectra of metal-organic/inorganic complexes can occasionally be more tricky. In the case of complexes, ions can be formed by dissociation of one or more of the ligands. Since electrochemical reactions occur in the ESI source, it is also possible that the metal centre undergoes oxidation or reduction (e.g. Cu(II) to Cu (I)).

  Due to high workload, we are not able to interpret all the acquired mass spectra. However, we will do our best to answer your questions and help you with the interpretation of complex mass spectra.
• Mass spectra interpretation (EI-MS)

  In electron ionization (EI), ionization and fragmentation of the volatile molecule in the gas phase are coupled. It can in fact happen that the intact M+ dissociates completely and is not detected at all. On the other hand, EI mass spectra do usually provide a lot of structural information. If the accurate mass of the intact M+ is of essence, we will try to optimize the instrument parameters accordingly.
• Negative or unexpected results

  Despite very high sample throughput, our ambition is to provide mass spectra of high quality. However, as outlined above, some compounds cannot be ionized or detected for various reasons when analyzed with our routine methods. If the data from our service measurements are in conflict to other evidence (e.g. NMR data) for the identity and purity of your compound, please get in touch with a member of the technical staff or with Martin Zehl. We will be happy to suggest alternative experiments or techniques to obtain the required MS data for your compound.