Using oxidized carbon nanotubes as matrix for analysis of small molecules by MALDI-TOF MS

Chensong Pan 1 2 Songyun Xu 1 2 Ligang Hu 1 2 3 Xingye Su 1 2 Junjie Ou 1 2 Hanfa Zou 1 2 Zhong Guo 1 2 Yu Zhang 1 2 Baochuan Guo 0 1 2 0 Also at the Department of Chemistry, Cleveland State University , Cleve- land, OH 44115, USA 1 Published online April 15, 2005 Address reprint requests to Dr. H. Zou, National Chromatographic Research and Analysis Center, Dalian Institute of Chemical Physics, Chinese Academy of Sciences , 457 Zhongshan Road, Dalian 116023, China 2 National Chromatographic Research and Analysis Center, Dalian Institute of Chemical Physics, Chinese Academy of Sciences , Dalian, China 3 Also at the Research Center for Eco-Environmental Science, Chinese Academy of Sciences , Beijing 100085, China Oxidized carbon nanotubes are tested as a matrix for analysis of small molecules by matrix assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS). Compared with nonoxidized carbon nanotubes, oxidized carbon nanotubes facilitate sample preparation because of their higher solubility in water. The matrix layer of oxidized carbon nanotubes is much more homogeneous and compact than that of nonoxidized carbon nanotubes. The efficiency of desorption/ionization for analytes and the reproducibility of peak intensities within and between sample spots are greatly enhanced on the surface of oxidized carbon nanotubes. The advantage of the oxidized carbon nanotubes in comparison with -cyano-4-hydroxycinnamic acid (CCA) and carbon nanotubes is demonstrated by MALDITOF-MS analysis of an amino acid mixture. The matrix is successfully used for analysis of synthetic hydroxypropyl -cyclodextrin, suggesting a great potential for monitoring reactions and for product quality control. Reliable quantitative analysis of jatrorrhizine and palmatine with a wide linear range (1-100 ng/mL) and good reproducibility of relative peak areas (RSD less than 10%) is achieved using this matrix. Concentrations of jatrorrhizine (8.65 mg/mL) and palmatine (10.4 mg/mL) in an extract of Coptis chinensis Franch are determined simultaneously using the matrix and a standard addition method. (J Am Soc Mass Spectrom 2005, 16, 883- 892) 2005 American Society for Mass Spectrometry - Mof flight mass spectrometry (MALDI-TOFatrix assisted laser desorption/ionization time MS) [1, 2] could provide not only the molecular weight of large molecules, such as protein, DNA/ RNA, polysaccharides, polymer and so on [3 8], but also the structural information of molecules to identify the sequence by the post source decay (PSD) [9 12] technique with high speed, accuracy, and sensitivity. Recently, MALDI-TOF-MS has also been developed to analyze small molecules [13] successfully by using the different matrix substances, such as desorption/ionization on porous silicon (DIOS) [14 17], matrix with high molecular weight [18, 19], surfactant suppressed matrix [20], inorganic materials [2125], etc. Though considerable works of quantitative determination using MALDI have been well documented and reviewed [26], quantitative analysis by MALDI is still considered as a relatively unreliable technique for the limiting factor of reproducibility within and between sample spots. Recently, Siuzdak and coworkers [27] developed a method for quantitative analysis of small molecules with desorption/ionization on porous silicon using electrospray deposition of analytes, and a quite good quantitative result with linear calibrations R2 0.99 and values of RSD 7% was achieved. It is believed that MALDI could be a powerful technique to provide both qualitative and quantitative determination of small compounds. Carbon nanotubes have been attracting wide interest [28, 29] of scientists since they were initially discovered by Iijima et al. [30, 31]. Recently, Carbon nanotubes have been developed as the matrix for MALDI-TOF-MS for analysis of small molecules [25], in which carbon nanotubes function both as the energy receptacle for laser radiation and as the energy transporter for desorption/ionization of analytes with the minimization of interference signals caused by matrix ion. The effectiveness of the method as matrix has been demonstrated by several compounds with low molecular weight. However, the low solubility of carbon nanotubes in water or organic solvent makes it hard to deposit carbon nanotubes onto the sample target and to form a homogeneous layer of matrix, leading to the relatively poor reproducibility and resolution of peaks for analytes. Moreover, after most of the solvent was evaporated, carbon nanotubes could not attach to the sample target tightly and hence, easily fly off from the sample target under vacuum, which may cause the contamination of the ion source. In the end, the efficiency of desorption/ ionization for analytes on matrix layer of carbon nanotubes is still not satisfactory for the impurities in raw product of carbon nanotubes, such as amorphous carbon, graphite pieces, and catalytic metal particles. These impurities may block the energy transferring for desorption/ionization of analytes. The oxidation and further chemical functionalization of carbon nanotubes pioneered a new way to the research and application of oxidized carbon nanotubes [3236]. Recently, Ugarov et al. [35] showed that the acidic derivatized fullerenes and single wall nanotubes could be efficient matrices for MALDI-TOF-MS. Here, we developed oxidized carbon nanotubes as the matrix for MALDI-TOF-MS for the analysis of small molecules. After the oxidation procedure, carbon nanotubes were purified and cut into shortened nanotubes attached with carboxylic groups (-COOH, -COONa, or -COOK) [32, 33]. Compared with nonoxidized carbon nanotubes [25],oxidizedcarbonnanotubesfacilitatesamplepreparation because of their higher solubility in water. The efficiency of desorption/ionization for analytes and the reproducibility of peak intensities within and between sample spots are greatly enhanced on the surface of oxidized carbon nanotubes. Consequently, reliable quantitative analysis of jatrorrhizine and palmatine with a wide linear range and good reproducibility of relative peak areas are achieved using this matrix. Finally, the concentrations of jatrorrhizine and palmatine in an extract of Coptis chinensis Franch are determined simultaneously using the matrix and the standard addition method. Chemicals and Materials Multiwalled carbon nanotubes were kindly offered by Professor F. Wei (Tsinghua University, Beijing, China). Matrix of -cyano-4-hydroxycinnamic acid (CCA) and trifluoroacetic acid (TFA) was purchased from Sigma (St. Louis, MO). Amino acids of Asn, Glu, His, and Phe were purchased from Fluka (Buchs, Switzerland). Quinine, -cyclodextrin ( -CD) and epoxy propane were purchased from Shanghai Chemical Factory (Shanghai, China). Berberine, hydrochloride, jatrorrhizine hydrochloride, and palmatine hydrochloride were purchased from the National Institute for the Control of Pharmaceutical and Biological Products (Beijing, China (...truncated)