«Zur Erlangung des akademischen Grades eines Dr.rer.nat vom Fachbereich Bio- und Chemieingenieurwesen der Universität Dortmund genehmigte ...»
- 34 Diode laser induced desorption in combination with APCI/MS on graphite substrate ————————————————————————————————— decreased while the laser power was higher. Therefore, 0.73 W is an optimized power for some experiments in which the exhaustion of sample is necessary.
3.5 Analysis of complex samples Further experiments on graphite plate were carried out with two phospholipids — sphingomyelin and lecithin. Both are mixtures of a group of compounds with similar structures, and their spectra are more complicated than that of reserpine. In this study, 1 µl sample solutions of sphingomyelin and lecithin (each containing 100 ng analyte) were used.
The spectra are shown in Fig. 3-8, 3-9. In the spectra, the expression, such —————————————————————————————————
- 35 Diode laser induced desorption in combination with APCI/MS on graphite substrate —————————————————————————————————
- 37 Diode laser induced desorption in combination with APCI/MS on graphite substrate ————————————————————————————————— Compared with the spectrum of sphingomyelin, the background noise is higher in the spectrum of lecithin. Furthermore, molecular ions (ions without the label ‘Frag’) were observed in the higher mass range. Another difference is that the secondary fragments were also observed in the low mass range. These ions can be assumed to formed from the molecular ions by loss of the phosphocholine group and subsequently the loss of one side chain, i.e. RCOO+58. Here, RCOO is the fatty acid, and 58 is the mass of glycerol backbone. To prove that, a MS/MS experiment was carried out
with m/z 596, which can be assumed to be the hydrated fragment of 34:1
lecithin after losing the head group. The fragmentation process is shown in Fig. 3-11.
Figure 3-11. Fragmentation of lecithin molecules.
The MS/MS spectrum is shown in Fig. 3-12. The collision energy was set to a value that was not so high to destroy all the parent ions, so that the signal from the parent ions (m/z 596) could still be observed. As expected, a signal at m/z 578 was observed, which was the result of the parent ion losing a H2O. This signal was also observed in the spectrum of lecithin (Fig. 3-9) as the base peak. The parent ion can also lose one of two fatty acids in the side chains to form two kinds of secondary fragment ions, i.e.
- 38 Diode laser induced desorption in combination with APCI/MS on graphite substrate —————————————————————————————————
- 39 Diode laser induced desorption in combination with APCI/MS on graphite substrate —————————————————————————————————
Figure 3-13. Fragmentation by MS/MS.
In this study, diode laser induced desorption were performed on a graphite target, which was applied as a photon absorber to couple the near-infrared laser power. In combination with APCI-MS, biochemicals with moderate molecular weight can be well desorbed /ionized. Compared with traditional laser desorption/ionization, a 100 times lower power density was applied. The decoupling of desorption /ionization gives the opportunity to apply continuous wave diode lasers in laser desorption techniques. The decoupling also allows an individual optimization of these two steps. The results and experiences from the work presented in this chapter gave important instructions for the following steps of the experimental work.
- 40 Thin-layer chromatography combined with diode laser desorption/APCI mass spectrometry —————————————————————————————————
4. Thin-layer chromatography combined with diode laser desorption/APCI mass spectrometry In the last few decades mass spectrometry coupled to separation techniques like liquid and gas chromatography has become the most versatile detection technique in many fields of applications. Coupling of mass spectrometry to thin-layer chromatography (TLC), which has been extensively used for many years as an economic and handy separation technique, is still a challenge. Thin layer chromatography in conjunction with mass spectrometric detection could be a useful tool for rapid screening in many fields of applications.
Laser desorption (LD) allows to sample in a small and defined area and therefore guarantees higher spatial resolution than the TLC separation itself. This property offers the possibility to characterize partially overlapping components after separation on TLC plates. In order to desorb analytes from a commercially available TLC plate, the power density of the radiation should be about 106 W/cm2 and not exceed 107 W/cm2.78 This power density is valid for desorption of analytes from a white surface.
According to the initial study demonstrated before, a graphite surface can function well to couple the laser power, resulting in a 100 times lower requirement for power density. This advantage also can be applied to a TLC plate, if covering it with a graphite suspension, which has an almost wavelength independent absorption of the laser radiation.71 Such as in the initial experiment, graphite particles on the surface absorb the laser power and convert it to heat, causing a rapid evaporization under the laser spot.79 Graphite has the advantage that not only the absorption coefficient is relatively high, but also the heat conductivity.
- 41 Thin-layer chromatography combined with diode laser desorption/APCI mass spectrometry ————————————————————————————————— In this study, a graphite suspension was applied on a piece of TLC plate after depositing the analyte. The TLC piece was obtained by cutting it directly from a commercial TLC plate. LD-APCI/MS was carried out in the same way as on the graphite plate. However, in this case the sample molecules are underneath the graphite layer. Furthermore, the affinity between the analyte molecules and the TLC materials also hampers the desorption. In addition to the 1 W laser used before, a more powerful 4 W laser was also applied to probe the influence under higher laser power density.
4.1 Preparation of graphite-covered TLC plates The graphite suspension was prepared by diluting glycerol (99.5 %, photometric grade) in methanol and afterwards adding graphite powder.
For obtaining a well-mixed suspension, the mixture was then mechanically shaken for 30 min.
The purpose of the presented paper is to evaluate the applicability of desorption from a TLC-plate by continuous diode laser radiation.
Therefore, the samples were deposited on small pieces of TLC plates without actual separation. The pieces were obtained by cutting a commercial TLC plate (glass, 20 cm x 20 cm, Merck Art. 5721, Germany) to a size of 3 mm x 3 mm. Afterwards, a TLC-piece was fixed onto the sample holder by using double-sided adhesive tape. The sample solution was deposited on the TLC piece, and subsequently dried for 5 min at room temperature. 2 µl graphite/methanol/glycerol mixture was topped on the TLC piece and left in room air for 10 min in order to obtain a dry and flat surface. Finally, the experiment was started and the protonated analyte was detected by the LCQ Classic ion trap mass spectrometer.
- 42 Thin-layer chromatography combined with diode laser desorption/APCI mass spectrometry —————————————————————————————————
- 43 Thin-layer chromatography combined with diode laser desorption/APCI mass spectrometry ————————————————————————————————— The experimental demands when working with TLC plates are more complicated and challenging than with graphite plates. It is inevitable to cause a higher background noise and a decreased spectral resolution.
Furthermore, the signal intensity will also decrease because the desorption of the analyte molecules is more difficult. Fig. 4-2a, b shows the difference between the measurement on a graphite plate and on a piece of TLC plate with the same laser power. For the latter, the peak intensity is decreased about 16 times. The background noise is more pronounced, however, it is limited to the low mass range and consequently do not disturb the analyte signal. Furthermore, the graphite suspension also gives a background noise. Glycerol related ions, including glycerol ions, and glycerol/Cn+ cluster were observed especially in the lower mass range (as shown in Fig. 4-3), which was also reported in other work.80 —————————————————————————————————
- 44 Thin-layer chromatography combined with diode laser desorption/APCI mass spectrometry ————————————————————————————————— 4.2.2 Negative ion mode Molecules with acidic groups or electronegative elements can easily produce negative ions. Benefits of the negative ion mode APCI are efficient ionization, higher sensitivity and less fragmentation. There is also a greater selectivity for certain environmentally or biologically important compounds. Here, β-Alanine (0.1 µg) was chosen as a test compound in the negative ion mode APCI.
M-H] Relative Abundance
The analysis was first carried out in the positive ion mode. However, no signals were observed in this case. Therefore, the mass spectrometer was switched to the negative ion mode, and an intensive signal from β-Alanine and a related ion signal were observed, as shown in the spectrum (see Fig.
4-4). Except for the quasi-molecular ion [M-H]-, the dimer of the oxidized product of [2M+2O-H]- was also observed.
- 45 Thin-layer chromatography combined with diode laser desorption/APCI mass spectrometry —————————————————————————————————
4.3 Influence of glycerol The purpose of glycerol was described several times. One of the reasons is that the viscosity of glycerol is necessary to get a homogenous surface.
The results of the investigation of the signal dependence on the glycerol concentration are presented below. Graphite suspensions were prepared by diluting glycerol in methanol, and thereafter adding 14 mg/ml graphite powder. Different proportions of glycerol and methanol (60/40, 50/50, 30/70 and 0/100) were used.