INTRODUCTION:

Chroma and Graphein are two Greek phrases from which the term 'chromatography' was generated. Those terminologies stand for ‘color’ and ‘to write’ respectively. Sir Mikhail Tswett, first coined this term in 1906. The IUPAC (International Union of Pure and Applied Chemistry) concept of chromatography was suggested by Ettre (nomenclature of chromatography), which expresses it as a physical technique needed for the separation of components from a mixture with the aid of the stationary and mobile phase.^1-3Chromatography can be viewed as a collection of mobile and stationary phase balances.

For the calculation of the relative relationship between the two phases of the solvent, the partition (K) or distribution (D) coefficient (the solvent concentration ratio in the stationary phase of the solvent concentration in the mobile phase) is used. Based on multiple criteria, the chromatography technique may be graded.

Classical thin-layer chromatography has some privileges. Simplicity and relatively low operational costs are the benefits of this technique. Inferior separation ability, productivity and repeatability comparing to HPLC have been reported as limitations of it. Usually there are two clarifications for this state of matters. The first one is the mode of moving of mobile phase. The second element is chromatography device un-equilibration. These problems have been solved with the invention of the FFTLC process, a pumped stream chromatography method that uses a planar 2D column set-up.^4-5By allowing multiple samples to be analyzed side-by-side in a single run, by using the entire column width on a single sample to provide additional load strength, and by improving the efficiency of separation by enabling two-dimensional (2D) chromatography directly on a square column, this will improve throughput. FFTLC also has multi-dimensional capacity. It is limited not only to the separation process itself, but also to the multitude of modern sample implementations and detection methods. Kala'sz et al. used the chromatographic technique of forced-flow displacement and elution-displacement production to separate and detect ecdysteroids (steroidal compounds) from plants.^6-7

History of forced-flow planar chromatography:

In1976, a comparatively compact set-up for FFTLC was patented, while the indication of forced-flow thin-layer chromatography (FFTLC) was originated to cover the stationary phase, thereby eradicating the vapour phase.⁸

Basic principle of TLC:

Adsorption is the fundamental concept of TLC. TLC uses a thin (0.22mm) stationary phase sheet that is connected to an inert support. A glass plate (full plate area of 20cm × 20cm, half plate area of 20cm × 10cm, quarter plate area of 20cm × 5cm) is also the backing. It is easy to use cardboard sheets and aluminium foil as well. A broad range of sorbents, including chemically enhanced silica, are commercially available for pre-coated plates of varying layer thicknesses. Any of the following techniques can be used to prepare TLC plates: pouring, dipping, spraying, and spreading. The best methodology where TLC propagators are used is among these spreading techniques.^9-17Silica gel, alumina, diatomaceous earth, kieselguhr, and cellulose, are commonly used TLC sorbents. Thin-layer separations in the standard and reverse phases which must be carried out as modified silica for TLC can involve polar or nonpolar groups. Micropipettes or capillary tubes are used to add samples to the plate. Spots should be reserved upstairs 2cm. The mobile process streams through the plates under the control of capillary motion. High-performance thin-layer chromatography (HPTLC) is better than TLC as it permits better separations in shorter times. In the TLC technique, a development tank is used. It is perfectly saturated with solvent vapor. This is essential so as to avoid unequal solvent evaporation.^18-19It can be recognized by using the TLC process whether or not the unwanted organic compounds are present as impurities in pharmaceuticals such as hydrazine in carbidopa, morphine in APO-morphine hydrochloride, etc. Determination of ninhydrin positive mixtures in official amino acids (leucine, glutamic acid, aspartic acid) can be achieved by TLC method resourcefully. Two kinds of adsorbents primarily are used in TLC- 1. Inorganic and 2. Organic adsorbents. The inorganic adsorbents used in TLC are- Al₂O₃, Al₂H₂O₄, CaCO₃, bentonite, calcium silicate, fuller’s earth, silica gel, kieselguhr. The organic adsorbents used here are- cellulose and acetylated cellulose, charcoal and activated carbon, dextran gel, cellulose ion-exchange powder, polyamide, polyethylene powder, sucrose. The main cellulose ion-exchange powders are used DEAE-cellulose (Diethylamino ethyl cellulose), ECTEOLA- Cellulose (Epichlorohydrin linked triethanolamine cellulose), PEI-Cellulose (Polyethyleneimine cellulose).^20-21

Solvent system in TLC:

The outline of the solvent system in TLC describes a basic solvent or solvent combination that is governed exclusively by two variables: 1. If it is polar or non-polar, the precise character of the 'atomic constituent' must be distinguished. 2. Adsorption and partition chromatography are the precise nature of the specific technique being implemented.^22-25The rate of chemical material movement depends on the solvent system. The solvent's elution capacity is the key element to be weighed here. CCl₄, CH₃COCH₃, C₆H₆, CHCl₃, and C₂H₅OH are solvents contained in TLC. In the standard phase chromatography process, the stationary phase is hydrophilic (silica gel) in nature, and the mobile phase is hydrophobic in nature. The compounds with less preference for the stationary phase can be easily differentiated here. They're more easily eluted. The selection of the movable phase hinges onmany considerations, such as the shape of the material to be disconnected, the configuration of the stationary phase, the chromatography mode, and the discontinuation to be carried out. For the identification of various types of chemicals, multiple sensing agents are used.^26-28

These are as following-

HPTLC:

It is one example of classy and impulsive form of TLC. As a wide variety of stationary phases is possible in the case of HPTLC (normal phase-silica and reverse phase-C18, C8, C4), automatic sampling is applicable, new method of production chamber is accessible where solvent requirement is smaller, it is more advanced than TLC.^29-35UV, visible, fluorescence scanner, refractive index detector can be used here to scan the entire chromatogram. Two types of pumps are used in HPTLC.^36-38These are pneumatic pump and reciprocating pump. Here, a check-valve is used to control the stream frequency of solvent and back pressure. A significant element of HPTLC is the analytic column, which has stainless steel and heavy glass in its structure. The column length is 5-30centimeter, and the column diameter is 50 mm. The important characteristics of HPTLC are as stated following: a. General aspects- it includes clean-up operation, ‘analytic sample’ application, pre-chromatographic derivatization of samples, chromatogram development, analytic mixtures determination quantitatively, documentation and storage of analytical data., b. Quantitative application of HPTLC- it offers a fully automated analytical technique. It includes reproducible ‘off-line’ technique which is more reliable.³⁹In order to achieve extremely active layers that can inevitably give rise to the undesirable decomposition of the materials, high-temperature initiation over a longer period should often be avoided. Therefore, the ' HPTLC plates' can instead be used efficiently to bypass those traditional instances. This procedure can be used to test various medications such as aspirin, ketorolac tablets, itraconazole, and beta blockers.^40-44

Principle of FFTLC:

As with TLC, the fundamental concept of FFTLC is capillary motion. Furthermore, the outside pressure is the force controlling solvent motion in FFTLC. Depending on the ideal mobile step velocity, operating pressures of up to 50 bars will currently be used. The vapor process is completely extinguished in FFTLC; the chromatography plate is covered under external pressure by an elastic membrane; thus, the separation can be performed under controlled conditions. In the optimization of the solvent system, the lack of any vapor space must be considered.⁴⁵

Multilayer FFTLC:

FFTLC is ideal for the simultaneous enhancement of many chromatography plates. Several samples can be isolated by a single chromatographic run with this multi-layer technique (ML-FFTLC). This technique helps one to isolate 360 samples of plant extracts in 150 seconds. Using ML-FFTLC, in which the same or changed forms of stationary phase can be used for the creation of more chromatography plates, the speed and reliability of the forced flow thin layer chromatographic separation of complex samples can be increased.^46-49

Instrumentation:

The numbers in figure 1 are going to represent those components, respectively- 1: base plate made up of plastic, 2: stationary phase [silica or a bonded-phase media], 3: layer film, 4: way to introduce the mobile phase, 5: gas or liquid pressing the covering sheath to the stationary phase, 6: access position of gas or liquid pressing the casing film to the stationary phase. Here, one pump is used over the stationary phase to push the mobile phase. The figure 2 represents the exact flow of sample from starting point to last elution point. In its column housing construction, the uniqueness of FFTLC allows flat planar columns to be used in the same phase as cylindrical glass or stainless-steel columns⁵⁰In the FFTLC production chamber, there are three elementary configurations: a planar column, a cassette constructed from a machined PTFE sheet and an aluminium brace, and an electronically operated hydraulic press, validated the characteristic features of FFTLC and NIR (near infrared spectroscopy) by the separation of pharmaceutical preparations, containing acetylsalicylic acid, caffeine and paracetamol.

The stationary and mobile phases were HPTLC silica plates and C₆H₁₄, C₂H₅OC₂H₅, CH₃COOH, and CH₃OH mixture ratio (50:30:19:1, v/v), respectively. The Personal FFTLC 50 system was used. UV densitometry allows for quantitatively testing, and the identification of the compound structure is qualitatively used by NIR-spectra.

Columns:

The columns are about 20cm long and these are of different breadths like 5, 10 and 20cm. It relies on the capability of columns. Usual thicknesses of columns are 200 micrometer and 500 micrometers. At the periphery of each flatbed column, an elastomeric seal is located. For the coating of the columns, regular silica and mutual attached phases such as C₈, C₁₈ nitrile, chiral and amino phases are used.⁵¹

Cassettes:

the two main purposes of the cassettes are - a. formation of solvent-tightness under compression and b. resolve the solvent movement within the sorbent bed. An aluminium base plate holds a cassette and the cover is made of PTFE. The 1 mm holes in the PTFE sheet make flow to the stationary bed. For various column widths, special cassettes are available. Very commonly, method creation is done on the smallest column, 5cm across while intermediate and full-width columns often conduct screening and semi-preparative function. A modern technology has been used lately for web or semi-preparative purposes. It is known as flowing eluent wall (FEW). FEW is conceptually a stream of sample-free eluent which is pushed adjacent the elastomer seal to the side partition of the column. The sampling is confined to a mid-point of the column by this element and there is no direct interaction amongst the sample flow and the wall. It also tends to boost the form of the band (peak). FEW does not require any extra facilities. The solvent source is broken predominantly upstream from the injector. One stream runs to the injector and to the part of the column where the sample is to be detached, whereas the other stream is carried directly to the side walls of the column.

Stationary phase:

Here, TLC and HPTLC silica plates were utilized. Three of the boundaries of the plates were soaked using paraffin candle wax or Silpress. N. Gocan categorized the wide-ranging scale of stationary phases for planar chromatography. Superior stationary phases with a circular form and small particle sizes (example, 3mm) have been prepared for extra high pressure. The spots on the plate are isolated using planar chromatography. However, FFTLC with continuous development permits to monitor the effluent series. A calibration curve can be plotted taking HETP (height equivalent to a theoretical plate) and eluent velocity along the Y and X axis, respectively. With the use of internal reference compounds, the desired compound may be quantitatively calculated. The formula for HETP is: HETP = L/N = L/16(W/t)², where L is the length of the chromatographic column (centimetres), N is the number of theoretical plates, W is the peak width, and t is the distance from the injection point to the peak maxima. If HETP is lower, the column is more efficient and vice versa.⁵²

Figure 1. Instrumentation of FFTLC

Figure 2. Forced Flow Thin Layer Chromatography, an advanced chromatographic procedure.

Management of FFTLC instrument:

An HPLC framework can be installed in a simple FFTLC unit. It can be run in the same manner as a regular cylindrical HPLC and HPTLC alike, both online and off site. In the case of off-line mode, the sample is inserted directly into the column. Much more sample preparation is not required here, and the quantity of samples is not limited. There is no need for column equilibration or renovation. The sample is injected through the injection loop as in HPLC. We can apply formulated products or crude extracts directly to the stationary phase. Sample preparation steps are not required. The active ingredients among the formulating agents can be calculated in several instances. The pump's upper pressure limit is randomly set by the instrument such that when the production is done, the pump stops. Samples can either be maintained on a stationary stage and analyzed by HPTLC detectors until production is finished, or they can be eluted to an effective detector. A fluorescence indicator is used at 254 nm for the purpose of identification. To image the sample components on the panel, we may also use a spraying reagent solution. A full-spectrum scanning densitometry instrument, or more precisely, a single-wavelength desktop scanner, defines the UV-active compounds. In combination with OPLC, radio-detection, spectroscopy, and bio-autography may also be used. The column is extracted as a final stage, preserved with experimental documents and if possible, recovered at a later date or at an alternate location for study.

The distribution profile of the particle size of HPTLC and TLC using silica gel 60 as a sorbent-The plot of particle size (micrometer) versus volume (%) along the X and Y axis respectively, we get one graph in which HPTLC curve is relatively sharp prominent where TLC peak is blunt. This is because the mean particle size of sorbent used in HPTLC is half than the size of sorbent used in TLC. Therefore, on the surface of the HPTLC plate, it has a larger number of particulate materials per unit area.⁵³

Comparison of flow of mobile phase in saturated, unsaturated and FFTLC through graphical representation (figure 3)- By taking two parameters-distance and time along the Y and X axes respectively, one graph is plotted. Linearity is not maintained in saturated and unsaturated chamber. But it is maintained in FFTLC.⁵⁴The graphical representation is as following: -

This graph reveals that FFTLC achieves constant flow velocity of mobile phase.

Calculation of mobile phase flow-Movement of mobile phase front can be characterized by using this equation: Z_f²= (k×t). On the other hand, for the classical TLC, the formula is Z_f = (k×t). Here Z_f = front distance, t= time, and k= constant characteristic of the system. R_F ideals can be converted in linear development from circular by the following equation-

R_{F (linear)}= (R_{F circular})².^55-56

Figure 3. Time and distance are plotted along X and Y axis respectively. Here curve 1 is for saturated chamber, curve 2 is for unsaturated chamber and curve 3 is for FFTLC.

Advantages of FFTLC:

1. It is a more sophisticated approach than the TLC methodology. The bulk of TLC shortcomings have been addressed here in a clean and elegant way. Both unwanted effects associated with the evaporation of the mobile phase from the plate surface to the inner chamber room, such as composition changes during production in the unsaturated chamber or the generation of evaporative flow, can be prevented by covering the plate with the assistance of a pressurized lid. 2. Of FFTLC, linearity in mobile phase velocity can be preserved alongside the overall improvement of the chromatogram. 3. Instead of traditional TLC and HPTLC process, Chromatogram can be generated dramatically at a shorter period. 4. Many samples can be evaluated at a time in one step during offline operation of FFTLC. 5. The construction distance can be specifically planned and can also exceed 60cm when using multi plate mode. 6. Both on-line (using a traditional HPLC loop) and off-line (using a TLC applicator) implementation modes are available. 7. Mobile step gradient composition can also be achieved.

Disadvantages of FFTLC:

1. The plates in FFTLC are more lavish compared to regular TLC plates since their boundaries must be sealed in the case of the linear chromatogram device mode, which is the most common nowadays. 2. There are also some technological shortcomings in the FFTLC methodology, such as the mobile phase flow velocity of external strain. 3. Un-equilibrated chromatographic conditions can be generated in the case of off-line technologies, resulting in lower separation conditions relative to HPLC.^57-58

Application:

The FFTLC process has been used in the fields of pharmacognosy (plant research), pharmaceutical development, drug metabolism, and assay of drug molecules. However, many specific applications have been discovered over the past 20 years. This involves the identification in animal tissues of medications and metabolites (homogenates, urine) and of potentially active ingredients obtained from plant extracts, the formulation of topical agents such as lipsticks, shampoos, and creams, the presence of poisonous compounds in foods, as well as chemical ingredients in crude reaction mixtures. Detection limits vary between 0.1 and 1000 nanograms. It depends upon the product and the detection techniques employed.^59-60

Use in pharmacy field:

FFTLC has widespread uses in the pharmacy field. It can detect impurities in samples efficiently. FFTLC has more accuracy and precision than the classical TLC method. Its less time-consuming properties make it more preferable than HPTLC and TLC. It is a more accurate procedure to give perfect analytical data. It is also very methodological. Various radio-labelled extracts from plants' origin can also be detected easily by the FFTLC method. ADME studies of these metabolites can be performed perfectly with this technique. FFTLC is also being used as a substitute for solid-phase extraction prior to NMR studies. If we go for a comparison of FFTLC with HPLC and GC, we can see that FFTLC was the only practice that provided all the essential analytical data in the analytical method, where TLC and HPLC failed to distinguish impurities.^61-62

Use of FFTLC in molecular research of bioactive compounds:

Regulatory control of bacterial and fungal poisons and their metabolic by-products in foods is important since they are recognized as causing severe health issues in humans and animals. FFTLC can be used in assay purpose of various metabolites like mycotoxin metabolites in food stuffs, such as rice, wheat, fish. Aflatoxin can be identified by TLC, and HPLC method.^63-64 It can also be performed by fftlc more efficiently which is a high sample throughput and low working cost method. Correspondingly, for certain peptide cyanobacteria toxins such as mirocystins and nodularins, FFTLC methods were also developed.

FFTLC in forensic science:

Its high quality and perfect identification capacity allow it to be used in forensic science. This technique is fruitful for two major reasons: the high peak potential of the columns and the option of two solvent systems with a low reciprocal association between them. In the field of clinical and forensic toxicology, this low-cost, fast-screening procedure is highly recommended. Using FFTLC, other definite assay methods for opiates, alkaloids (poppy) and cannabinoids were also carried out.⁶⁵

CONCLUSION:

The forced-flow supply of the mobile phase in FFTLC provides several advantages. The major feature of this system is that it has a continuous flow velocity of the mobile phase. Kala sz and Nagy demonstrate the existence of an optimum flow velocity versus HETP curves, which Hauck and Jost first confirm. The optimum velocity of the mobile phase depends on the particle size of the stationary phase. The marketing of new, high-efficiency columns has eliminated the need for analysts to arrange their own TLC plates, making the method more available. In FFTLC, the open-heart column offers many distinctive product identification opportunities, including regular on-line systems such as UV, radiation, ELS, ESI-MS, and NMR. The off-line detection technique can also be used here to provide densitometry via a fluorescent indicator. ESI-MS and MALDI-MS can also be applied to the detection method of the FFTLC process. Since this method has all the requisite properties for preparatory isolation, semi-quantitative screening, and product profiling (fingerprinting), this method will now be approved for a few days. This advanced technique allows for the simultaneous, parallel purification of multiple samples using the un-segmented flat columns in a single run. This function gives it the chance for new applications after parallel synthesis in the fields of high throughput sampling, proteomics, combinatorial chemistry, and micro-scale preparatory isolation. One additional advantage of FFTLC is that each impurity that appears at the solvent front or at the point of load can definitely be identified. For the sensitivity purpose of the FFTLC method, we can give an example. Papp et al. tested the technique of authentication and the determination of aflatoxins in wheat. They calculated and also validated specificity, linearity range, accuracy, precision, detection limit, and robustness. Their results were completely compared with those of the requirements. Because of these developments, in modern research laboratories, the forced flow thin layer chromatography technique gets one place.

CONFLICT OF INTEREST:

The authors have no conflicts of interest regarding this investigation.

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Received on 08.05.2022 Modified on 16.07.2022

Asian J. Research Chem. 2022; 15(6):499-505.

DOI: 10.52711/0974-4150.2022.00085