The work presented here describes the development of an ion source based on atmospheric pressure chemical ionization (APCI) that allows the direct mass spectrometric analysis of solid and liquid samples. The ion source makes use of a direct inlet probe (DIP)-system which automatically introduces the samples into the source region using a temperature-programmed push rod. The programmed heating of the push rod leads to a time-shifted vaporization of sample components as a result of their different vapor pressures.
The DIP-APCI ion source was coupled to a high-resolution Q-TOF-MS (Agilent Technologies) and an Ion Trap-MS (Bruker Daltonik) using two different source chambers manufactured at the machine shop of the University of Wuppertal. Gas-flows inside the ion source as well as the position of the probe tip in the ion source were optimized for both the DIP-APCI-Q-TOF-MS and the DIP-APCI-Ion Trap-MS coupling. In positive ionization mode all substances analyzed were detected as protonated molecules ([M+H]⁺). Ionization in negative ionization mode was made possible by supplying sufficient oxygen to the ion source. In negative ionization mode the formation of [M-H] -- and [M+O₂] ∙--ions was observed. Using both couplings a linear concentration response was obtained with caffeine as test analyte. This shows the possibility to perform quantitative analyses using the DIP-APCI ion source.
Employing the Q-TOF-MS the reproducibility of DIP-APCI-MS analyses was shown by the repeated analysis of an extract from Radix Angelicae sinensis. The possibility to separate sample components by the programmed heating of the push rod was demonstrated by the analysis of hexanoic acid methyl ester and dodecanoic acid methyl ester. The two fatty acid methyl esters were separated almost completely as a result of their differing vapor pressures.
The DIP-APCI-Q-TOF-MS coupling was applied to the qualitative analysis of plants used in Chinese Herbal Medicine (CHM). The Chinese Radix Angelicae sinensis and the Korean Radix Angelicae gigas could be differentiated by direct analysis of the powdered plants and augmented detection of coumarins in Radix Angelicae gigas. Processed in Xiao Yao pills, a preparation of several plants used in CHM, the application of the European Radix Angelicae archangelica instead of the Chinese Radix Angelicae sinensis could also be shown via the detection of coumarins. This demonstrates possible applications of DIP-APCI-MS analyses in authenticity tests and in the quality control of plants and preparations used in CHM.
The possibility of performing quantitative analyses using the DIP-APCI ion source was shown by the determination of coumarin in cinnamon employing the DIP-APCI-Q-TOF-MS coupling. The DIP-APCI-MS determination of coumarin in cinnamon following extraction was validated and showed satisfying linearity, recovery and reproducibility, both using deuterated coumarin as internal standard and without the use of an internal standard. When deuterated coumarin was used as internal standard, the coumarin contents determined in different cinnamon samples by DIP-APCI-MS were in good agreement with the results obtained by a complementary LC-MS analysis. When no internal standard was used high coumarin contents were overestimated in the LC-MS analysis and underestimated in the DIP-APCI-MS analysis. This shows that the use of deuterated coumarin as internal standard is advisable to obtain accurate data with both methods. The determination of coumarin in woodruff-flavored beverages showed that, using ambient ionization mass spectrometry, special attention has to be paid to possible spectral interferences. By means of LC-MS, coumarin could not be detected in the woodruff-flavored liquor analyzed. Due to the influence of the high temperature applied to the woodruff-flavored liquor during DIP-APCI-MS analysis an isobaric artefact was formed. The temperature-programmed vaporization in combination with the use of deuterated coumarin as internal standard helped in recognizing this spectral interference.
Using the DIP-APCI-Ion Trap-MS coupling a non-destructive analysis of phthalates and other plasticizers in plastic articles was performed. The identification of plasticizers was accomplished by MS/MS- and MS³-experiments and comparative analysis of plasticizer standards. In this way plasticizers could be identified in toys, plastic articles intended to come into contact with food and other plastic articles of daily use. The DIP-APCI-Ion Trap-MS coupling was also applied to the analysis of active ingredients in analgesics and antibiotics. The active ingredients of analgesics (acetylsalicylic acid, diclofenac, ibuprofen, paracetamol and naproxen) and antibiotics (clarithromycin, rifampicin and ethambutol) could all be detected in the tablets and ointments analyzed. The identities of the active ingredients were confirmed by MS/MS-experiments. In addition to the development of the DIP-APCI ion source a second focus of the work presented here was the analysis of phloem sap. By means of stylectomy phloem sap was gained from maize and rapeseed plants. The direct analysis of phloem sap using DIP-APCI-Q-TOF-MS enabled the detection of twelve components of maize phloem sap and twenty components of rapeseed phloem sap and the deduction of possible molecular formulae from the accurate m/z-values detected. By means of DIP-APCI-Ion Trap-MS, MS/MS-experiments were performed to elucidate the structures of the components detected in phloem sap. Due to the low molecular weight of substances detected in phloem sap as well as low signal intensities and nonspecific fragmentation it was only possible to identify the amino acids valine and leucine, which are known to be contained in phloem sap, by comparative analysis of their respective standards.