«Zur Erlangung des akademischen Grades eines Dr.rer.nat vom Fachbereich Bio- und Chemieingenieurwesen der Universität Dortmund genehmigte ...»
- 46 Thin-layer chromatography combined with diode laser desorption/APCI mass spectrometry ————————————————————————————————— Fig. 4-5 shows 4 mass spectra measured with different amounts of glycerol. With the decrease of the amount of glycerol also the signal intensity decreases. Although the protonated molecular ion of reserpine could still be obtained without glycerol (Fig. 4-5d), the intensity was about 2 times weaker. In addition, the background noise increases when compared to those with 60 % glycerol (Fig. 4-5a). Therefore, higher glycerol concentration is helpful to obtain a sharp signal peak, which corresponds to a high mass spectrometric resolution. When the amount of glycerol was less than 30 %, the resulting graphite layer had not a glossy and uniform surface. Obviously, this led to the reduced mass spectrometric resolution.
4.4 Influence of the amount of graphite The purpose of graphite is to absorb the laser power and to convert it into heat, causing rapid thermal desorption of the analyte from the TLC plate.
Differing from other techniques in which pulsed lasers are employed, here graphite is not only for the purpose of improving the experimental results, but is necessary to obtain a signal.
Five different graphite concentrations 84 mg/ml, 42 mg/ml, 14 mg/ml,
5.6 mg/ml and 2 mg/ml were investigated to obtain an optimized result. A high concentration of graphite leads to a high absorption of the laser power, but on the other hand, a thick graphite layer keeps part of the analyte inaccessible to laser desorption. As indicated in Table 4-1, the signal intensities were smaller and the resolution was lower measured with 84 and 42 mg/ml graphite concentration than that with 14 mg/ml.
Furthermore, at higher graphite concentrations the danger of igniting the graphite layer increases. Strong analyte signals and ‘clear’ spectra were obtained with a concentration of about 14 mg/ml. Analysis at graphite —————————————————————————————————
- 47 Thin-layer chromatography combined with diode laser desorption/APCI mass spectrometry ————————————————————————————————— concentrations lower than 14 mg/ml were performed as well, however, the signal intensity started to decrease rapidly due to reduced absorption of laser power. In addition, the intensity of background signals stayed almost constant. When the graphite concentration was decreased to 2 mg/ml, only a faint gray shadow was visible on the surface of the TLC plate. Only a weak signal could be obtained in this case because the surface was not dark enough to couple necessary laser power for desorption. The data measured with a graphite concentration of 2 mg/ml shown in Table 4-1 are obtained by averaging 100 spectra. The average intensity is about 100 times weaker than that at 14 mg/ml graphite concentration, but the peak resolution is higher.
The choice of different graphite and glycerol proportion is a critical parameter for the desorption process. If only methanol and graphite are used, methanol penetrates quickly into the porous surface of silica gel, and graphite powder has not enough time to diffuse and form a smooth layer.
Glycerol keeps the graphite powder in the liquid phase and results in a uniform distribution. In general, a more homogeneous surface was —————————————————————————————————
- 48 Thin-layer chromatography combined with diode laser desorption/APCI mass spectrometry ————————————————————————————————— obtained when more glycerol is used. If the glycerol amount is small, the surface was not suitable for desorption. If the graphite concentration was less than 2 mg/ml, the resulting surface is not dark enough to absorb adequate power for desorption, and no stable sample signal could be obtained.
4.5 Influence of laser power In the beginning, the measurements were carried out by using the maximum output of the 1 W diode laser. Interestingly, the analyte signal disappeared even if the laser power was just reduced by 10%. Therefore, 1 W is presumably near to the ‘threshold’ for desorption for this method.
A stronger 4 W diode laser was utilized to reach higher signal intensity in the following experiments. As shown in Table 4-2, experiments were performed with the laser power of 4 W and 2 W. Spectra with better resolution and similar intensity were obtained at 2 W. The result at 4 W shows a marginal improvement in signal intensity, which was often cancelled by poor sample-to-sample reproducibility, but resulted in a notable increase of background noise and a decrease in mass spectrometric resolution. Furthermore, a very strong laser power causes much more material desorbed simultaneously including impurities from silica gel. Too many ions introduced into the ion trap can result in ‘space charge effects’, which broadens analyte peak and causes mass errors. Laser power also has an influence on the duration of the signal. It is reasonable that lower power causes longer signal duration. The duration of the signal measured at 4 W is much shorter than that at 2 W, which means more sample molecules are vaporized in the same time range at 4 W than at 2 W, but the signal intensity was nearly the same. One explanation for this observation is that many ions might be lost during transmission.
- 49 Thin-layer chromatography combined with diode laser desorption/APCI mass spectrometry ————————————————————————————————— Table 4-2. Influence of laser power on analyte signals Laser Power (W) Intensity Duration of Signal (min) Resolution
A novel technique, thin-layer chromatography combined with diode laser induced desorption/atmospheric pressure chemical ionization, is demonstrated in this study. The use of graphite suspension and the decoupling of desorption and ionization allows diode lasers to be used in TLC-MS for the first time. It supplies a possible fast and easy separation and detection method in the future. Graphite suspension plays a role of energy absorber and transfer medium, and it is proved to be a suitable material to couple the power of the continuous diode laser. Glycerol is used to improve signal intensity and decrease background noise, and it is important for obtaining a satisfying graphite layer. A corona discharge is employed to ionize the molecules desorbed by diode laser that is not strong enough to achieve the ionization step. Matrix related peaks are limited at low mass range, and no fragments are observed.
- 50 A new interface to couple mass spectrometry with thin-layer chromatography for full-plate scanning —————————————————————————————————
5. A new interface to couple mass spectrometry with thin-layer chromatography for full-plate scanning A TLC plate-scanner coupled with the mass spectrometer gives benefits for rapid screening and high-throughput analysis. It is also useful for the detection of unknown substances and overlapping spots.
In the preceding study,81 a method of TLC-MS by using a diode laser to desorb molecules and a corona needle to ionize the desorbed molecules with the assistance of graphite was demonstrated. The decoupling of desorption and ionization enable the transfer of the desorbed molecules for a certain distance, followed by the ionized in the ion source of the mass spectrometer without any modification of the spectrometer. Because the ionization is realized by an AP technique, the transporting system can work without a requirement for vacuum environment. All these advantages can be applied to a plate scanning system, which is uncomplicated but highly efficient.
Accordingly, a plate scanning system based on the same principle as described before was built by using a gas jet pump for the transport of the analyte molecules and a motorized movable stage for scanning. An optical fibre was used to guide the laser beam, which makes the experimental arrangement more flexible. The loss of molecules during transportation can be controlled by heating up the transport part. As described before, graphite can effectively reduce the demand for laser power and improve desorption efficiency. In this case, the graphite layer was produced by spraying the graphite suspension with a sprayer on the TLC plates after development. However, for some measurements, if a faster analysis is required, the pretreating procedure is not appropriate. Therefore, a stronger diode laser was employed, which could desorb molecules without the —————————————————————————————————
- 51 A new interface to couple mass spectrometry with thin-layer chromatography for full-plate scanning ————————————————————————————————— assistance of graphite. Two desorption modes: direct LD and LD with graphite-assistance are tested in this study.
5.1 Instrumental setup of the plate scanning system Figure 5-1. Schematic diagram of the experimental arrangement A sketch of the experimental arrangement used for the following experiments is shown in Fig. 5-1. The TLC plate was placed on a motor operated xy-stage that was controlled by a waveform generator (FG-5000, WAVETEK). A short glass tube (2.5 cm in length, 5.5 mm i. d., 7.5 mm o.
d.) was installed 2 mm above the plate. An inlet hole with a diameter of 2 mm was located at the bottom side of the short glass tube for sampling the desorbed molecules. The tube was heated up to 200 °C with a coiled heating wire. One end of the glass tube was closed, and the other was connected to a gas jet pump. A gas flow of 1.5 l/min N2 was introduced to the pump to supply a suction flow of 1.1 l/min. The gas was then —————————————————————————————————
- 52 A new interface to couple mass spectrometry with thin-layer chromatography for full-plate scanning ————————————————————————————————— transported to a transfer glass tube of 10 cm length and 7.5 mm inner diameter, which was positioned several millimeters in front of the capillary inlet of a LCQ Classic ion trap mass spectrometer (ThermoFinnigan, USA). A coiled heating wire maintained the temperature of the transfer tube at 350 °C to reduce the sample loss during transfer.
A laser beam from a diode laser (OTF 30P-40 from OPTOTOOLS) with a wavelength of 808.8 nm guided by an optical fiber, was aligned through the sampling inlet and desorbs analytes from the surface of the TLC plate.
The laser beam with a maximum power of 16.8 W was focused on the surface with a calculated spot diameter of 0.05 mm resulting in a maximum power density in the order of magnitude 106 W/cm2. The desorbed molecules were transferred to the area in front of the heated capillary inlet of the mass spectrometer and then ionized by a corona needle with a discharge current of 1 µA and a potential of 4 kV supplied by the LCQ system. The parameters of the mass spectrometer were optimized for each analyte, individually, as in the initial work.
5.2 Chromatography The analytes were separated on commercial silica gel TLC plates (aluminum-backed, 5 cm x 10 cm or 10 cm x 10 cm, Merck, Germany).
The separation was carried out with a chloroform/methanol/water (20:7:1) solvent system for 15 min at room temperature. If multi-samples were developed on the same plate simultaneously, the track distance in between was 1 cm. In the experiments with graphite-assistance, the graphite layer was produced by spraying the graphite suspension with a sprayer on the TLC plates after development, which can be monitored with a microscope.
The graphite suspension was prepared by adding graphite powder in —————————————————————————————————
- 53 A new interface to couple mass spectrometry with thin-layer chromatography for full-plate scanning —————————————————————————————————