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«Zur Erlangung des akademischen Grades eines Dr.rer.nat vom Fachbereich Bio- und Chemieingenieurwesen der Universität Dortmund genehmigte ...»

-- [ Page 1 ] --

Thin-layer chromatography combined with diode laser

induced desorption/atmospheric pressure chemical ionization

mass spectrometry

Zur Erlangung des akademischen Grades eines

Dr.rer.nat

vom Fachbereich Bio- und Chemieingenieurwesen der Universität Dortmund

genehmigte Dissertation

vorgelegt von

M.Sc.-Chem. Song Peng

aus

Nanchang City, Jiangxi Province, China

Tag der mündlichen Prüfung: 18/08/2005

1. Gutachter: Prof. Dr. Andreas Manz

2. Gutachter: Prof. Dr. Thorsten Hoffmann Dortmund 2005 Contents ————————————————————————————————— Abstract………………………………………………………………………...…3

1. Introduction……………………………………………………………..……4

1.1 Projective intention………………………...…………………….………..……4

1.2 Conceptions………………………………………………………………...……6

2. Theoretical background…………………………………...………..………10

2.1 Basic techniques involved in this work……………..……...…………...…….10 2.1.1 Diode laser……………………………………………………………...10 2.1.2 APCI/MS ………………………………………………………………13 2.1.3 TLC……………………………………………………………...……..15

2.2 TLC-MS………………………………………...………………………………16 2.2.1 EI and CI…………………………………………………………….....17 2.2.2 SI, FAB and LSI…………………………………………………….….18 2.2.3 ESI……………………………………………………………………...19 2.2.4 LD, MAILDI and SALDI………………………………………………21 2.2.5 Scanning device………………………………………...………………23

3. Diode laser induced desorption in combination with APCI mass spectrometry on graphite substrate…………………………………….….26

3.1 Chemicals and sample preparation……………………………………...……27

3.2 Design of ion source and experimental setup...………………………………29

3.3 Optimization of experimental conditions……………………………………..31

3.4 Analysis of a compound with moderate molecular weight…………………..32

3.5 Analysis of complex samples..…………………………………………………35

4. Thin-layer chromatography combined with diode laser desorption/APCI mass spectrometry……………………………………………..……………41

4.1 Preparation of graphite-covered TLC plates……………………...…………42

4.2 Analysis on graphite-covered TLC plates…...……………………………..…43 4.2.1 Positive ion mode……………..………………………………………...43 4.2.2 Negative ion mode………..…………………………………………….45

4.3 Influence of glycerol……………………………………………………...…….46

4.4 Influence of the amount of graphite…………………………………….…….47

4.5 Influence of laser power…………………………………………………….…49 —————————————————————————————————

-1Contents —————————————————————————————————

5. A new interface to couple mass spectrometry for thin-layer chromatography full-plate scanning……………………………...……….51

5.1 Instrumental setup of the plate scanning system…………………..………...52

5.2 Chromatography………………………………………………………...……..53

5.3 Scanning on untreated TLC plates…………………………….………..…….54 5.3.1 Scanning of a single compound after development…….………………54 5.3.2 Scanning of a mixture after separation………………….……………...56

5.4 Scanning on the TLC plate with graphite assistance…………………….…..58 5.4.1 Graphite covered plates…………………..…………………….……….58 5.4.2 Graphite embedded plates………………..……………………………..60

5.5 Influence of scanning speed………………………………………………..…..63

5.6 Rapid screening………………………………………………………………...64

6. Quantification with the plate scanning device……………………….……67

6.1 Quantification in mass spectrometry…………………………………………67

6.2 Quantification in TLC-MS………………………………………………….…68

6.3 Quantification for the TLC plate scanning system…………………………..69

7. Conclusions………………………………………………………………….74

8. References……………………………………………………………...……76 Acknowledgements………………………………..……………………………86 —————————————————————————————————





-2Abstract —————————————————————————————————

–  –  –

An analytical technique to combine TLC with MS was developed. The initial study was carried out on a graphite plate, which functions as a photon absorbing material. A continuous wave diode laser replaced traditional pulsed lasers as the desorption source, which was employed for this purpose for the first time. The thermally desorbed analyte molecules are ionized in the gas phase by a corona discharge device at atmospheric pressure and detected subsequently by a mass spectrometer, by which both essential processes ― the desorption and the ionization of analyte molecules, which are often performed in one step - are separated. The technique was subsequently applied to thin-layer chromatography (TLC) to realize the combination of TLC and mass spectrometry. A graphite suspension was employed to couple the laser energy and improve the desorption efficiency. In this case, the necessary power density for desorption was decreased by two orders of magnitude. In addition, a TLC plate-scanning device was developed, by which the chromatography on a TLC plate can be recovered, and rapid screening for numerous analytes on a TLC plate can be achieved. The device can also be applied for the identification of unknown compounds or to recognize overlapping sample spots. Finally, a quantification method for this system was developed. An internal standard was added into the mobile phase to yield a ‘background’ signal, which was used as a reference signal for the quantification. In this way, a wide range of compounds can be chosen for this purpose.

—————————————————————————————————

-3Introduction —————————————————————————————————

1. Introduction

1.1 Projective intention Thin-layer chromatography (TLC), as an economic and handy chromatographic separation technique, has been extensively used for many years. The attractive features of TLC include parallel sample processing for high sample throughput; accessibility of the sample for postchromatographic evaluation free of time constraints; detection in the presence of the stationary phase independent of mobile phase properties;

and the stationary phase is normally used only once. It is generally agreed that thin-layer chromatography is most effective for the low-cost analysis of samples requiring minimal sample clean-up, or where thin-layer chromatography allows a reduction in the number of sample preparation steps (e.g. the analysis of samples containing components that remain sorbed to the stationary phase). Thin-layer chromatography is also preferred for the analysis of substances with poor detection characteristics requiring post-chromatographic treatment for detection. Since all sample components are located in the chromatogram, thin-layer chromatography is the most suitable technique for surveying sample properties. The popularity of TLC owes not only to the modest demands on instrumentation, but also to its sensitivity, general applicability, and, last but not least, its flexibility.

TLC can be viewed as the complementary technique in contrary to column chromatography, such as high-performance liquid chromatography (HPLC) due to their different attributes, resulting in a preference for one approach over the other independent of the application. However, the separated analytes are mostly detected with visual methods such as staining techniques, ultraviolet (UV) absorption and fluorescence —————————————————————————————————

-4Introduction ————————————————————————————————— extinction, which causes the inability to identify the analytes and a low specificity with partially overlapping substances in TLC. It does not obstruct a technique like HPLC because the combination of HPLC and mass spectrometry (MS) has been developed to a reliable and robust technique in the last years. Obviously, mass spectrometry is an excellent technique for identification. Therefore, the full complement of identification and quantification tools available to column liquid chromatography is the desirable goal for thin-layer chromatography. It somehow means an interface for scanning and recording in situ mass spectra.

The introduction of mass spectrometry is mostly realized by a ‘scrape and elute’ mode, i.e. scrape the stationary phase containing analytes;

elute/extract analytes from the stationary phase to a solvent; finally introduce the solvent into the ion source of the mass spectrometer in the regular way. Analytical strategies of TLC-MS are still widely carried out in this mode at present. However, as described, the whole procedure is time consuming and destructive for chromatography. In fact, it is not a way to really ‘couple’ mass spectrometry to thin-layer chromatography, but just a sample preparation process for mass spectrometry. It is suitable for the analytical work that has strict time limitation in method development or have a requirement for post-chromatographic treatment.

However, the advantage of high throughput analysis is lost. In contrast, a modern mass spectrometer coupled to a liquid chromatographic system can acquire data automatically and has large advantages in the analysis of complex samples. Moreover, several robust ionization methods have been developed for LC/MS coupling. However, its method development and separation process are normally time-consuming, and often not suitable for high throughput analysis. If a TLC/MS system can be realized ‘instrumentally’, it will have advantages of quick experimental procedure, —————————————————————————————————

-5Introduction ————————————————————————————————— large capacity for sample handling, and powerful identification ability at the same time.

There are three major factors in an ‘instrumental’ TLC/MS system:

sampling, transport and ionization. Sampling mode decides how to acquire analytes from the bulk of TLC material. Transport part delivers molecules/ions from the TLC plate, after they are somehow obtained, to the mass spectrometer. Finally, ionization has to be achieved for the mass spectrometric analysis. These three factors influence each other and determinate the structure and performance of an ‘instrumental’ TLC/MS system.



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