Ionization Techniques DOI: 10.1002/anie.200804106 A Universal Ionization Label for the APLI-(TOF)MS Analysis of Small Molecules and Polymers** Ralf Schiewek, Ren Mnnikes, Volker Wulf, Siegmar Gb, Klaus Josef Brockmann, Thorsten Benter, and Oliver Johannes Schmitz* The drastically increased demand for qualitative and quanti- tative determinations of increasingly complex samples repre- sents a tremendous challenge to modern instrumental anal- ysis.Currently,for complex organic samples only chromato- graphic or electrophoretic separation with subsequent mass spectrometric detection (MS) fulfills these requirements. For MS analysis coupled with separation by gas chromatography (GC), liquid chromatography (LC), or capillary electropho- resis (CE), the ionization of the separated analyte molecules must be as quantitative aspossible.Different ionization methods have to be used for analytes of different molecular weight and polarity. [1] For polar compounds, electrospray ionization (ESI) is the gold standard with respect to mass spectrometric analysis of smallmolecules and, because of multiple charging, of large biomolecules, as well. [2] Analytes of up to several thousand daltons (kDa) and of moderate polarity are favorably ionized by atmospheric-pressure chemical ionization (APCI), [3] while atmospheric-pressure photo-ionization (APPI) and dopant- assisted (DA)-APPI are usually applied for the ionization of non-polarsubstances. [4] Atmospheric-pressure laser ioniza- tion (APLI), recently developed by our groups, shows an outstanding sensitivity for moderately to non-polar aromatic compounds. [5] This selectivity towards aromatic compounds arises from the ionization mechanism of APLI: Multiphoton excitation of matrix compounds is minimized by adjusting the laser power density close to the threshold ofresonantly enhanced (1 + 1) multiphoton excitation, in other words, to roughly 1 MW cm 2 . Linear absorption of most matrix com- pounds becomes negligible when photons with a wavelength of 248 nm are used for excitation. Readily available small- footprint excimer lasers are sufficiently powerful light sour- ces. With respect to efficient resonant two-photon ionization, the spectroscopic featuresof aromatichydrocarbonsare rather unique:They display strong linear absorption cross sections at 248 nm, long-lived intermediate electronic states, and highly vertical ionization transitions. Since hardly any other compound class exhibits such features, APLI is specific for arenes.On the other hand,this specificity may limit analyticalapplications. However,MS analysisof complex samples would largely benefit from selective ionization of analytes since ion suppression and the resulting failure in the quantitative determination is considerably reduced. [6] Cur- rently this is only possible either with a high-performance preseparation by means of hyphenated techniques, such as GCxGC, LCxGC, and LCxLC, [7] which ensure a baseline separation of all matrix components, or with stable-isotope- labeled standards (SILS). [8] Hyphenated techniques, though, are expensive, time-consuming, and maintenance-intensive, while SILS are expensive and available only for a rather limited number of analytes. In this work we presenta derivatization strategy that facilitates selective ionization of polar and non-polar com- pounds in complex matrices without hyphenated techniques or SILS. With this procedure the analytes are detected by the mass analyzerwithout noticeableinterferencefrom the matrix. For analytical applications,derivatizationsare widely used.As an example,methylation and silylation reagents are used to facilitate GC analysis of organic acids and alcohols. [9] In LC and CE, fluorescence markers are often applied to labelanalyte molecules in complex samples to realize a selective and also highly sensitive detection. [10] However,when several analytes must be derivatized with a single fluorescence marker, quantitative analysis requires correction factors to take into account the negative influence of the analytes on the fluorescent properties of the marker (fluorescence quenching). [10] To our knowledge, with the exception of applications in REMPI-MS, [11] derivatizations are used in MS analysis only 1) to form GC-accessible analytes and 2) to increase the ionization efficiency. [9] In each case there is a drawback: the requirement of a time-intensive calibration or the addition of a SILS for a quantitative determination. Here, we show an alternative approach to circumvent this problem. In analogy to fluorescence markers, APLI markers(anthracene-9-yl- methoxyacetic acid (1) and anthracene-9-ylmethanol (2)) are used to derivatizealcohols,amines,and organic acids (Scheme 1) for analysis by APLI as described above. Starting with the commercially available alcohol 2, carboxylic acid 1 was synthesized by reaction with bromoacetic acid. [12] APLI is a soft ionization method. Therefore, the spectra in Figure 1 always show the signals of the radical cations as the base peak of the derivatization products of 1 or 2 with ethanol, cholesterol, dodecyl amine, and oleic acid. There are [*] R. Schiewek, R. Mnnikes, V. Wulf, Prof. Dr. S. Gb, Dr. O. J. Schmitz Department of Analytical Chemistry, University of Wuppertal Gaußstrasse 20, 42119 Wuppertal (Germany) Fax: (+ 49) 202-439-3915 E-mail: olivers@uni-wuppertal.de Homepage: http://www.oliver-schmitz.de Dr. K. J. Brockmann, Prof. Dr. T. Benter Department of Physical Chemistry, University of Wuppertal Gaußstrasse 20, 42119 Wuppertal (Germany) [**] This work was financed by Deutsche Forschungsgemeinschaft (DFG), Cognis GmbH, Kratos Analytical PLC (Shimadzu Group Company), and Bruker Daltonics GmbH. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200804106. Angewandte Chemie 9989 Angew. Chem.Int.Ed.2008,47,9989 –9992 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim