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What are the advantages offered by instrumental Thin-Layer Chromatography?

Instrumental Thin-Layer Chromatography (or Planar Chromatography) is a modern separation technique, established worldwide and distinguished by flexibility, reliability and cost efficiency. Together with HPLC and GC it belongs to the microanalytical methods, which play an important role in research and routine laboratories. In many cases instrumental Thin-Layer Chromatography offers a more suitable solution and often it is used as confirmatory or alternative technique. Due to the off-line principle, Thin-Layer Chromatography offers:

  • Enormous flexibility
  • Parallel separation of many samples with minimal time requirement
  • Unsurpassed clarity and simultaneous visual evaluation of all samples and sample components
  • Simplified sample preparation due to single use of the stationary phase
  • Possibility of multiple evaluations of the plate with different parameters because all fractions of the sample are stored on the plate.

Important fields of application

Clinical applications

  • Lipids
  • Metabolism studies
  • Drug screening
  • Doping control, etc.

Cosmetics

  • Identity of raw material
  • Preservatives, colouring materials, etc.
  • Screening for illegal substances, etc.

Food and Feed stuff

  • Quality control
  • Additives (e. g. vitamins)
  • Pesticides
  • Stability tests (expiration), etc.

Industrial applications

  • Process development and optimization
  • Process monitoring
  • Cleaning validation, etc.

Pharmaceutical applications

  • Quality control
  • Content Uniformity Test (CUT)
  • Identity- and purity checks
  • Stability tests, etc.

Forensics

  • Detection of document forgery
  • Investigation of poisoning
  • Dyestuff analyses, etc.

Herbals

  • Identification
  • Stability tests
  • Detection of adulteration
  • Assay of marker compounds, etc.

Environment

  • Water
  • Soil
  • Residue analysis, etc.

Optional Procedures

In addition to the basic steps of thin-layer chromatography – sample application, chromatogram development, derivatization, chromatogram evaluation – there are optional procedures that complement the technique. Which of these are needed depends on the nature of samples and/or the tasks of the particular laboratory.

These are

  • UV-inspection
  • In-house preparation of TLC plates

UV-INSPECTION

Hardly any TLC laboratory can be without the use of UV light for inspecting chromatograms, not even where only dyes are analyzed, because these may contain colorless but UV active components. Of course, a Reprostar instead of a UV lamp may serve this purpose. Two types of ultraviolet light are required for inspecting thin-layer chromatograms:

Long-wave UV light 366nm
Under 366 nm UV substances with either inherent or reagent induced fluorescence appear as bright spots, often differently colored, on a dark background. The sensitivity of this detection method increases with the intensity of the long-wave UV light and also as more visible light is eliminated. A fluorescent indicator F254 contained in the layer neither contributes to nor interferes with this detection method.

Short-wave UV light 254nm
Under 254 nm UV substances absorbing at that wavelength become visible, provided the TLC layer contains a fluorescent indicator F 254. These substances appear as dark spots on a bright background. For this detection method, UV intensity and complete elimination of visible light are less critical.

IN-HOUSE PREPARATION OF TLC PLATES

To day, in-house preparation of TLC plates is of interest for laboratories which, for economical or logistic reasons, cannot rely on precoated plates exclusively.

The self-coating of TLC plates should also be considered when special layers are required that are not available in the form of precoated plates, e.g. layers containing silver nitrate, buffer substances or other reagents, layers consisting of adsorbent mixtures, or in the exceptional cases where the binder contained in commercial precoated plates might interfere with detection.

Adsorbents for In-House Preparation of TLC Layers

For the self preparation of chromatographic layers, adsorbents in the form of powder are mixed with water or with aqueous solutions of salts or buffering compounds to form a thick slurry which is spread onto glass plates by means of a coating device.

Adding calcium sulfate as a “binder” makes it easier to achieve the correct consistency of the slurry for coating; the calcium sulfate contributes very little to the mechanical strength of the layer.

A fluorescence indicator is required to visualize substances which absorb UV light of a wavelength (254 nm), by which the indicator is excited to emit visible light. Such substances appear as dark spots on a bright background. The fluorescence indicator does not interfere with the chromatographic separation, with any derivatization reactions, nor with densitometric evaluation. Most of these type indicators, however, lose their fluorescence on contact with acids. Fluorescence indicators which are stable against acids are only available in certain precoated layers.

PRECOATED LAYERS

In most cases modern planar chromatography utilizes precoated plates. Not only are they more convenient, their quality is superior to that of layers available for self-coating. Several types of phases, HPTLC layer, etc. are only available in the form of precoated plates.
The following list contains the more common types of precoated plates and sheets manufactured by E. MERCK. Other MERCK precoated plates not listed here may also be ordered through CAMAG.

Test Dye Mixtures

Test dye mixtures are useful for functional checks on individual steps in the TLC procedure and for studying the influence of specific parameters. Dye mixtures are convenient as their fractions are easy to track.

Basic Assemblies

All CAMAG TLC and HPTLC basic kits have been carefully composed so that you can immediately start planar chromatography work. These assemblies are configured to make it possible for you to form a complete system for quantitative TLC analysis by adding items at any time without the fear of duplication or redundancy of tools.
Also the transition from using conventional TLC material to high-performance (HPTLC) layers is straight forward.
Using HPTLC material can offer considerable advantages. It does, however, require the consistent use of appropriate instrumentation.

Sample application in the form of bands or rectangles:

Spraying-on samples as narrow bands allows the application of larger volumes. Narrow bands as starting zones always ensure the highest resolution. Very large sample volumes or samples with high matrix content can be sprayed-on in the form of rectangles which, prior to chromatography, are focused into narrow bands with a solvent of high elution strength.

All types of Thin-Layer Chromatography, whether qualitative, quantitative or preparative, benefit from optimized resolution as a result of the appropriate sample application technique.

Sample application is the first step of instrumental Thin-Layer Chromatography and thus determines the quality of the analysis.

ATS 4 or Linomat 5?

The CAMAG Automatic TLC Sampler (ATS 4) is the ideal instrument for fully automatic application of a large number of samples with highest precision, whether in a research or a routine lab. Connected to winCATS it is perfectly integrated to control and monitor the entire TLC process.

Furthermore the ATS 4 is used for application of sample, in which for safety reasons manual contact is strictly prohibited. Such samples may include

  • extremely toxic solutions (e.g. cytostatics)
  • microbiologically contaminated samples
  • radioactive compounds

In combination with CAMAG‘s FreeMode software application schemes not common to TLC are possible, for instance applications onto nitro cellulose membranes for the production of diagnostic kits.

The CAMAG Linomat 5 is an affordable alternative to the ATS 4 without making any concessions to the precision of volume dosage. In part it is operated manually, and can even be used in stand-alone mode, but when connected to winCATS it is as perfectly integrated to control and monitor the TLC process as the ATS 4.

Chromatogram Development

Thin-layer chromatography differs from all other chromatographic techniques in the fact that in addition to stationary and mobile phase a gas phase is present. This gas phase can significantly influence the result of the separation.

Processes in the Developing Chamber

The »classical« way of developing a chromatogram is to place the plate in a chamber, which contains a sufficient amount of developing solvent. The lower end of the plate should be immersed several millimeters. Driven by capillary action the developing solvent moves up the layer until the desired running distance is reached and chromatography is stopped. The following considerations primarily concern silica gel as stationary phase and developments, which can be described as adsorption chromatography.

Provided the chamber is closed, four partially competing processes occur:

  1. Between the components of the developing solvent and their vapor, an equilibrium will be established eventually (1). This equilibrium is called chamber saturation. Depending on the vapor pressure of the individual components the composition of the gas phase can differ significantly from that of the developing solvent.
  2. While still dry, the stationary phase adsorbs molecules from the gas phase. This process, adsorptive saturation, is also approaching an equilibrium in which the polar components will be withdrawn from the gas phase and loaded onto the surface of the stationary phase (2).
  3. Simultaneously the part of the layer which is already wetted with mobile phase interacts with the gas phase. Thereby especially the less polar components of the liquid are released into in the gas phase (3). Unlike (1) this process is not as much governed by vapor pressure as by adsorption forces.
  4. During migration, the components of the mobile phase can be separated by the stationary phase under certain conditions, causing the formation of secondary fronts.


Definition of plate and chamber formats

These format definitions are used in all CAMAG literature. Note: some plates can be developed in one direction only, e.g. plates with a concentration zone, GLP coded plates, etc. When you order plates make sure you understand the manufacturer‘s size definitions.

In connection with the development process, the following aspects should be considered:

With the exception of single component liquids (neat solvents), developing solvent and mobile phase are, strictly speaking, not the same. Their composition changes with progressing chromatography. Unfortunately the terms »developing solvent« and »mobile phase« are often used as synonyms. In the true sense only the liquid in the chamber should be called developing solvent, while the liquid moving through the layer constitutes the mobile phase. Only the composition of the developing solvent at the time when it is placed into the chamber is positively known. The processes (1) and (2) can be experimentally affected by:

  • Fitting the chamber more or less completely with filter paper soaked with developing solvent.
  • Waiting a certain time between the introduction of developing solvent into the chamber and beginning of chromatography – chamber saturation.
  • Allowing the plate to interact with the gas phase prior to chromatographic development, i.e. without contact to the developing solvent – preconditioning.

An interaction according to (2) and (3) can be effectively prevented by placing a counter plate at a distance of one or a few millimeters to the chromatographic layer. This is called »sandwich configuration«. The further an equilibrium according to (1) and/or (2) has been established and the less different the components of the mobile phase are in respect to their adsorption behavior, the less pronounced is the formation of secondary fronts resulting from (4). In well-saturated chambers and on preconditioned layers secondary fronts are often not observed. In sandwich configuration and particularly in OPLC secondary fronts are very prominent.

During chromatography, components of the developing solvent, which have been loaded onto the dry layer via the gas phase according to (2), are pushed ahead of the true but invisible solvent front. Exceptions are very polar components such as water, methanol, acids, or bases. This results in Rf values being lower in saturated chambers and particularly on pre-conditioned layers, than in unsaturated chambers and sandwich configurations.

Reproducible chromatogram development, here several plates under UV366 nm.

Influence of the activity of the layer (relative humidity) on the separation at equal migration distance. From left: rH 18%, 47% and 75%

The desired activity is set in only a few minutes

Consequences:

Thin-layer Chromatography in most cases proceeds in a non-equilibrium between stationary, mobile, and gas phase. For this reason it is very difficult to correctly describe the conditions in a developing chamber.
Reproducible chromatographic results can only be expected when all parameters are kept as constant as possible. Chamber shape and saturation are playing a predominant role in this regard. Unfortunately this means that the chromatographic result is different in each chamber!
There are neither »good« nor »poor« chambers! However, in some chambers the parameters can be better controlled, i.e. reproduced, than in others.

Choosing a developing chamber

Selection of the »proper« chamber is done during method development and generally follows »practical« considerations such as which chamber is available, which one must be used due to an SOP, or which one has been used in the past if a results comparison is to be made. However, a focus should also be on economical aspects such as time requirement and solvent consumption. A selection of glass chambers can be found here.
The
Horizontal Developing Chambers have proven to be exceptionally economical, flexible and reproducible in operation. Although designed for applications where the plate is developed from two sides, they are also suitable for single-sided developments in unsaturated, saturated and sandwich configuration as well as for preconditioning of HPTLC plates.
The new
Automatic Developing Chamber (ADC 2) is unsurpassed for reproducibility and universal applicability. This instrument does not only eliminate any effects of the operator when introducing the plate into a saturated chamber, but also the activity of the layer prior to start of chromatography can be set and drying of the chromatographed plate is rapid and complete. For development a conventional 20 x 10 cm Twin Trough Chamber is used. This way chamber geometry and chromatographic conditions of already existing analytical procedures can be retained, but environmental and operational effects are standardized.
In case the sample contains polar and non-polar components, which must be separated in the same analysis, the principle of
Automated Multiple Development (AMD) can be employed. Development is performed on the basis of a solvent gradient from polar to non-polar over several steps with intermediate drying.

Postchromatographic Derivatization

It is an inherent advantage of Thin-Layer Chromatography that fractions remain stored on the plate and can be derivatized after chromatography. By derivatization substances that do not respond to visible or UV light can be rendered detectable. In many cases, substances or classes of substances can be identified by specific reagents.

Reasons for choosing derivatization as a step in TLC include:

  1. Changing non-absorbing substances into detectable derivatives
  2. Improving the detectability (lowering detection limits)
  3. Detecting all sample components
  4. Selectively detecting certain substances
  5. Inducing fluorescence.

From a technical point of view only one principal decision must be made: how to transfer the reagent to the plate? Derivatization can be achieved with gas, by liquid spraying or dipping (immersion). In any case the reagent needs to be homogenously transferred to the chromatogram.

By immersing a TLC plate into the derivatizing reagent a very homogenous reagent transfer can be achieved. Dipping and withdrawing has to be performed smoothly in order to avoid tidemarks. Using the Chromatogramm Immersion Device the reproducibility of the derivatization step can be significantly improved compared to spraying. Furthermore, no fumes are generated during this derivatization technique and the exposure to hazardous chemicals is limited.

If the reagent is suitable, dipping should be preferred over spraying.

However, the fact is that spraying is most widely used for reagent transfer onto the TLC plate because it is simple and quick. No expensive equipment is necessary and only small volumes of reagent are needed. In addition spraying is very flexible and indispensable when reagents have to be applied in sequence. Also during method development, when searching for the most suitable reagent, spraying is more frequently mentioned.

Spraying on the other side generates substantial amounts of obnoxious and hazardous fumes, which must be carefully removed using e.g. the TLC Spray Cabinet.

During spraying, particularly for quantitative evaluation, it must be ensured that a homogenous fine spray mist is generated. Reproducibility of derivatization by spraying is greatly dependent on the skill of the operator. Most chemical reactions used in derivatization require heating for completion. The two principal heating devices are ovens and plate heaters. Ovens have two major shortcomings: One, fumes from derivatization agents can be corrosive and two, cross contamination may become an issue.

Derivatization of capsaicin with dichloroquinone chloroimide reagent/
ammonia by spraying (1 g/L, left side) and by dipping (0.25 g/L, right side
)

It is therefore advantageous to use a TLC Plate Heater designed to homogenously heat the TLC plate to the selected temperature.If visualization is not required, derivatization may not be advantageous. For example, progesterone has a chromophor and absorbs UV light at 254 nm, it can thus be analyzed without derivatization. If necessary, the substance can be visualized by derivatization with sulfuric acid. The comparison of the standard deviation for the densitometric evaluation of derivatized and non-derivatized progesterone is given below. The best results are obtained without derivatization, derivatization by spraying yields the least reproducible results across the plate.

Evaluation of progesterone

No
Derivatization

Derivatization
by immersion

Derivatization
by spraying

Densitometry

Abs. 254 nm

Abs. 536 nm

Abs. 536 nm

Progesterone,
CV of 16 samples,
6 mm bands

1.1%

2.4%

2.9%


CAMAG offers a complete range of derivatization here.

thin-layer-chromatography.pdf

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