«Zur Erlangung des akademischen Grades eines Dr.rer.nat vom Fachbereich Bio- und Chemieingenieurwesen der Universität Dortmund genehmigte ...»
- 19 Theoretical Background ————————————————————————————————— applied to the needle. The sample solution is sprayed into the source chamber to form droplets. The droplets carry charges when they exit the capillary. After the solvent is vaporized, the droplets disappear and leave highly charged analyte molecules.48,49 In contrast to APCI, ESI normally produces multiply charged ions, which allows the analysis of high molecular weight molecules using analysers with a lower nominal mass limit. However, the sample must be soluble in low boiling solvents and stable at very low concentrations, i.e. 10-2 mol/l. Very often HPLC methods have to be modified for ESI when they use high concentrations of buffers.18 The first work about TLC-ESI coupling was the development of a micro solvent extraction device.50 The device composes of an inner capillary with a sheath of absorbent material that is contained by a metal tube. A suitable solvent is transferred through the capillary to the area containing analyte on the TLC plate, diffuses a short distance, and then drawn up by capillary action into the absorbent material. The analyte is subsequently eluted from the sorbent for ESI-MS analysis. Luftmann presented another device based on a similar principle, but without absorbent material.
Furthermore, the solvent was injected continuously, which enable direct coupling with an ESI mass spectrometer to perform ‘on-line’ analysis.51 Berkel, et al. built a computer-controlled movable probing system that allows ‘on-line scanning’ of the whole TLC plate and, as a result, the investigation of unknown mixtures.52 Apart from the ‘extraction /transmission’ mode mentioned above, another approach that realizes electrospray ionization directly on the edge of the plate was developed.53 Here, the electrospray process happens at one sharp end of a TLC strip, which is connected to a power supply.
- 20 Theoretical Background ————————————————————————————————— 2.2.4 Laser desorption/ionization (LD/I), matrix-assisted laser desorption/ ionization (MALDI) and surface-assisted laser desorption/ionization (SALDI) In a laser desorption/ionization process, laser pulses desorb material from the surface and create a microplasma of ions and neutral molecules, which may react among themselves in the dense vapor phase near the sample surface. The laser pulse realizes both the vaporization and ionization of the sample.18 Because of the small sampling area of the laser, the introduction of laser systems to TLC gives the advantage of a higher spatial resolution, and also the possibility of repeatable analysis due to the unchanged bulk material. However, laser desorption/ionization always produces fragmentation products, especially when a UV laser is applied.54 A partial solution of the problem is the use of an infrared (IR) laser for desorption followed by multi-photon ionization (MPI).55 The LD/I process depends critically on the specific physical proprieties of the surface. Therefore, more laser power is required to desorb analytes from a TLC plate compared with desorption from a metal plate (sample-substrate affinity).56 Recently, MALDI is widely used in the analysis of large biological molecules such as proteins or DNA fragments.57 Differing from other LD/I techniques, MALDI involves the use of a matrix. The analyte molecules are dispersed in a solid matrix crystal so that they are isolated from each other. The rapid heating by the laser beam causes localized sublimation of the matrix crystals together with analyte molecules. It is the matrix that absorbs the most energy so that little internal energy is transferred to the analyte molecules.58,59 However, the ionization process in MALDI is still not fully understood,60 and the most widely accepted ion formation mechanism involves gas-phase proton transfer in the expanding plume with photoionized matrix molecules, as shown in Fig. 2-4. MALDI is a soft ionization technique and allows the determination of the molecular weight of molecules up to 500 kDa. Moreover, the MALDI process is —————————————————————————————————
- 21 Theoretical Background ————————————————————————————————— independent of the absorption properties and size of the analyte so that it is not necessary to adjust the laser wavelength to match the analyte.18
This technique was also introduced to TLC recently.61 However, one problem, compared with normal MALDI on a metal substrate, is that the analyte molecules must be ‘extracted’ to the surface of the plate from the bulk of the TLC material before they enter the matrix crystal. For this purpose, an ‘extraction’ solvent is applied on the sample spot after separation. Then, matrix is added to the extraction solvent gradually after the extraction step is finished, and at last forms a structure as normal MALDI.62-64 However, the lateral diffusion of separated compounds is inevitable in this method due to the ‘extraction’ solvent. Therefore, a protocol was developed to solve the problem. A matrix layer is prepared on a stainless steel plate first, and then the layer is transferred onto the developed TLC plate prewetted with the ‘extraction’ solvent. With a gentle pressure, the analyte is incorporated into the matrix structure, and then the stainless steel is discarded, followed by laser —————————————————————————————————
- 22 Theoretical Background ————————————————————————————————— desorption/ionization.65,66 More recently, a hybrid plate with a silica gel layer and an adjacent MALDI layer on a normal backing was developed to obtain nearly 100 % analyte recovery.67 In this method, after analytes are developed in the silica gel layer, the plate is rotated 90°, and the separated compounds are eluted to the MALDI layer, followed by normal MALDI analysis.
Another similar method, named surface-assisted laser desorption /ionization (SALDI) was developed lately.68,69 Instead of using a matrix, particle suspensions are employed on the plate surface to couple the laser energy resulting in a thermal induced desorption/ionization process.
Activated carbon or graphite particles are normally used in the technique, and the latter with a diameter of 2 µm shows the best capability for signal yield.70 SALDI can also be performed with a pencil drawing on a TLC plate.69 The extracting step is not necessary in the method, and it opens the possibility to choose lasers with different wavelength because particle suspensions have an almost wavelength independent absorption ability.71 2.2.5 Plate scanning device Most methods to couple TLC with MS are based on the ‘scrape and elute’ mode, but an interface for TLC plate scanning is attractive for TLC-MS. A plate-scanner coupled with a mass spectrometer gives the chance to reconstruct the chromatogram based on either single ion monitoring or the total ion current. If the plate is scanned along the lane in which the sample is developed, the detection can be carried out without the information of migrating distances for all the analytes. It is possible to detect component of unknown samples and overlapping spots. Moreover, the coloration step is not necessary in this mode. Furthermore, the scan can be also carried out orthogonally to the direction of development and on the level corresponding to a special substance. Since many samples are developed —————————————————————————————————
- 23 Theoretical Background ————————————————————————————————— parallelly on a TLC plate, the ‘screening’ of the substance in samples can be carried out with a high-throughput.
For any scanner, the TLC plate can be moved linearly past the source of desorption or extraction, or the source can be moved linearly along the plate. The former is a simpler method and has the fewest disadvantages.
Furthermore, the ionization of desorbed or extracted sample molecules also can happen before or after transfer, i.e. transporting ions or molecules.
The first solution is cumbersome, requiring major modifications to the spectrometer but provides better spectra. The other solution is simpler, and its setup can be coupled with a wide variety of mass spectrometers, but it includes the danger of samples loss or breakdown during transfer.
Because there were no AP ionization methods for mass spectrometry several years ago, the transfer of desorbed materials was complicated. It was necessary to maintain vacuum in the transferring tube and the chamber in which the plate was placed. Furthermore, there was a pumping time necessary for each analysis after the plate was placed inside the chamber and before the mass spectrometer was started. Ramaley et al.
built up such a device that was briefly described in the chapter 220.127.116.11,38 Another scanner was designed based on liquid secondary ionization,47 by which a stripe of TLC plate can be scanned. Because desorption and ionization happen simultaneously in liquid secondary ionization, ions are transferred rather than molecules, which is different from the case in Ramaley’ work. As a result, the ion source had to be modified.
Besides the fact that API techniques are softer than vacuum ionization techniques, API techniques give also the benefit of easier sample handling.
With API techniques, TLC plates can be placed on a movable stage directly without a sealed chamber and additional waiting time for vacuum.
Disadvantage is the possible loss of sample molecules during transfer. A scanner based on ESI was developed recently, in which analytes were —————————————————————————————————
- 24 Theoretical Background ————————————————————————————————— extracted and transported with a solvent.52 In this case, the sample recovery can be improved by choosing the suitable solvent and decreasing scan speed.
In all the experimental setups described above, the sample molecules were removed from the TLC material by thermal or laser desorption, bombardment with ions or extraction with solvent. Among them, laser desorption have the advantage of a smaller sampling area, which results in a higher spatial resolution. If the laser beam is conducted by an optical fibre, the experimental arrangement is even more flexible. However, in the techniques adopting traditional pulsed lasers, desorption and ionization are not decoupled, and both happen ‘in situ’ on the TLC plate. In this case, ions rather than molecules have to be transported. Moreover, the output of a traditional pulsed laser is usually unstable from pulse to pulse. It possibly varies from a power density high enough for ionization or even fragmentation of sample molecules to a level not enough for desorption without changing any working parameters. The decoupling of desorption and ionization is therefore quite difficult for pulsed lasers. These disadvantages can be eliminated by using a cw diode laser. A cw diode laser has very stable and accurately tuneable power output. In combination with the adequate irradiation time, the energy density can be accurately set to desorb molecules without ionization and less fragmentation.
- 25 Diode laser induced desorption in combination with APCI/MS on graphite substrate —————————————————————————————————
3. Diode laser induced desorption in combination with APCI/MSon graphite substrate
Diode lasers are reliable, compact, costs effective, easy to use and have high efficiency. According to inherent advantages of a diode laser, it can be anticipated that they will be extensively used in routine analytical tasks in the future.