TLC-DESI

Chapter 11, titled Drug Analysis by TLC-DESI MS, introduces basic information on technical solutions needed for an efficient coupling of TLC with the DESI MS detection. The authors attribute primary importance to the quality of the TLC... 

Desorption electrospray ionization (DESI) was used to obtain mass spectra of dyes, directly from TLC plates. TLC/DESI fundamentals and applications were demonstrated using rhodamines (6G, B, and 123) and federal food, drug, and cosmetic... 

Goldenseal and related alkaloids were qualitatively identified and quantitatively determined using TLC/DESI analysis. The TLC separations were performed using aluminum sheets precoated with silica gel 60 F254, or using glass-backed silica gel 60 plates. After the application of analytes, the plates were developed in ascending... 

In addition to MALDI, there are additional desorption MS techniques, whereby desorption electrospray (DESI) MS seems particularly versatile [15]. Readers who are particularly interested in DESI MS are referred to the work of Zoltan Takats and his group or another paper dedicated exclusively to TLC/DESI MS of lipids [16]. 

Fig. 13.8. Fully computer-controlled TLC-DESI unit attached to an ESI interface using the curtain gas design. Reprinted with permission from Ref. [23]. The American Chemical Society, 2005.

Example TLC-DESI was developed in the van Berkel group [23]. The sprayer was positioned about 4 mm from the curtain plate of the mass spectrometer at a 50° angle relative to the TLC plate surface. The TLC plates were cut to align the sanple bands with the DESI plume. Methanol was sprayed at about 5 pi min while a x-y-z robotic platform and control software were used to move the TLC plate relative to the stationary DESI emittea- at about 50 pm min The position of the TLC plate relative to the sprayer was monitored with a camera, the image of which was output to a PC so as to correlate staining, position, and DESI spectral data (Fig. 13.8). 

Fig. 3.11. Positive-ion SRM ion current profiles for 1 (m/z 443—415 black trace), 2 (mJz 443 - 415, red trace), and 3 (m/z 345-285, blue trace) obtained during development lane scans of replicate development lanes of the RP C2 TLC separation of a mixture (50 ng each) of rhodamines 6G (1), B (2), and 123 (3) at surface scan rates of (a) 19, (b) 44, and (c) 190 jum/s using a DESI solvent (methanol) flow rate of 0.5 //Emin. Dwell time was 100 ms for each transition. Signal levels were normalized to the signal in panel (c). Chromatographic resolution, R, calculated from the data is shown in each respective panel. Reprinted with permission from G. J. Van Berkel et al. [89].

In 2005, desorption electrospray ionization (DESI) was transferred to TLC [65]. The DESI emitter set at 4 kV was directed in an angle of about 55 degrees onto the zone on the layer. The sampling capillary touched the layer and was about 2 mm apart from the impact point of the DESI plume. The proper positioning of the DESI emitter, the TLC layer, and the MS orifice were... 

In Chapter 4, titled Principles of Mass Spectrometry Imaging Applicable to Thin-Layer Chromatography, the authors first introduce a budding analytical approach known as imaging mass spectrometry (IMS) strategy and then present some successful examples of its practical applications. Then, they introduce in detail three mass spectrometric techniques as those routinely used within the framework of IMS. These are secondary mass spectrometry (SIMS), matrix-assisted laser desorption/ ionization (MALDI-IMS), and desorption electrospray ionization (DESI). Finally, the authors discuss the advances and bottlenecks of these techniques when applied to TLC. 

Chapter 6, titled Selection of Ionization Methods of Analytes in the TLC-MS Techniques provides an overview of mass spectrometric techniques that can be coupled with TLC and act as specific detectors in this hyphenated approach. The mass spectrometric techniques discussed in this chapter are secondary mass spectrometry (SIMS), liquid secondary ion mass spectrometry (LSIMS), fast atom bombardment (FAB), matrix-assisted laser desorption/ionization (MALDI), atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI), electrospray ionization (ESI), desorption electrospray ionization (DESI), electrospry-assisted laser desorption/ionization (ELDI), easy ambient sonic spray ionization (EASI), direct analysis in real time (DART), laser-induced acoustic desorption/electrospray ionization (LIAD/ESI), plasma-assisted multiwavelength laser desorption/ionization (PAMLDI), atmospheric-pressure chemical ionization (APCI), and dielectric barrier discharge ionization (DBDI). For the sake of illustration, the authors introduce practical examples of implementing TLC separations with detection carried out by means of individual mass spectrometric techniques for the systematically arranged compounds belonging to different chemical classes. 

In the case of typical TLC plates, DESI seems to be a very useful technique of visualization due to its ambient working conditions. The limited spatial resolution of this source is not important for a majority of the TLC plates, including high-performance TLC (HP-TLC). However, for some applications, it is recommended to apply another ion source providing better imaging capabilities, such as MALDI-IMS. 

Mass spectrometer equipped with desorption electrospray (DESI) ion source is capable of detection and identification of different substances from the surface. Moreover, the information about spatial distribution of those substances is retained. Connection of this technique with TLC not only allows for the measurement of retention times for separated chemicals (spatial distribution), but also for their unambiguous identification, based on the molecular weight of certain substances and their fragmentation spectra. Additionally, because DESI works under ambient conditions, there is no need to apply a high vacuum system for the sample introduction. Moreover, samples analyzed by DESI practically do not require any kind of preparation (e.g., covering with matrix prior to MALDI analysis), thus connection of those two techniques is relatively easy. Certainly, not all the substances, due to their chemical features, may be detected with this technique. Only compounds, which are able to ionize in this type of ion source, may be analyzed. 

Combination of DESI MS with TLC separation was proposed for the first time by Van Berkel et al. [1]. In their study, apart from showing the possibility of such connection, the authors demonstrated practical application of this method during analyses of the FD C dyes (food dyes) and a mixture of aspirin, acetaminophen, and caffeine from a tablet of Extra Strength Excedrin. In the following year, this systan was further upgraded and improved by automating the process of analysis [2]. It was then used to separate the Goldenseal (Hydrastis canadensis) and related alkaloids from... 

During optimization of the procedure we have tested 100% methanol 70% methanol 30% water with 0.1% formic acid (FA) 100% methanol with 0.1% FA 100% acetonitrile 100% ethanol and 70% methanol 30% acetonitrile, as DESI solvents sprayed on the TLC plate surface. Finally, 100% methanol with a flow rate of 3 L min was used as an optimal solvent, providing best results. Other mass spectrometer acquisition settings were as follows mass spectra scan range 70-500 m/z ion accumulation time 200 ms heated capillary temperature 280°C voltage between capillaries 3200 V. Positively charged ion acquisition mode was applied. 

In the case of combining DESI ion source with TLC separation technique for its analysis, it is advisable to use good quality TLC plates. During the ionization process, the stationary phase might be detached from the plate surface and, in extreme cases, may clog the MS inlet. TLC plates with aluminum support seem to be more prone to such process than are those with glass support. 

As DESI works under ambient conditions, preparation of the TLC plate is usually limited to its firm attachment to the moving table by the aid of double-sided adhesive tape. Another crucial step is to find the exact starting position for the analysis. To achieve this, it is advisable to mark the point near the start of the separation lane with an easily ionizable substance. A small dot of Rhodamine B (MW = 442) from a red... 

Choosing an optimal solvent for TLC plate analysis is also very important. Solvent in the DESI source is delivered to the analyzed surface via nebulizing capillary, as a... 

Figure 11.2 presents signals generated from amphetamine, methamphetamine, paramethoxy-N-methylamphetamine, and benzydamine spotted on the surface of the TLC plate and analyzed using methanol (100%) and ethanol (96%) as solvents. As shown in Figure 11.2, methanol was obviously the best DESI solvent for this type of chemicals. 

Summarizing, DESI due to its ability to work under ambient conditions allows for convenient connection of simple and low-cost TLC separation and highly specific MS analysis. Of the currently popular MS imaging techniques, DESI seems to be the most comprehensive. This type of ion source does not cause extensive fragmentation of the sample as in the case of SIMS. Additionally, since there is no need to use any kind of matrix or high vacuum, as in the case of MALDI, the molecules of low molecular weight, such as drugs and their metabolites, may be easily analyzed. 

In recent years, TLC was successfully combined with different ionization techniques, matrix-assisted laser desorption/ionization (MALDI), ESI, atmospheric pressure chemical ionization (APCI), desorption electrospray ionization (DESI), electrospray-assisted laser desorption ionization (ELDI), and LDI for identification and quantification of organic and biomolecules. In this section, the interfacing of TLC techniques with MALDI-ESI/MS, DESI-MS, ELSI-MS, and LDI-MS will be described, performance will be discussed, and selected applications in the separation and identification of lipids, gangliosides, dyes, drugs, and medicinal compounds will be presented. 

An Overview of Research Involving TLC Combined with ESI, DESI, ELSI, and Other Ionization MS Techniques for the Separation and Identification of Lipids, Carbohydrates, Drugs, and Natural Products... 

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