Why tlc is used

One of the major problems concerned with the manufacture and use of propellants is the measurement of their chemical stability and the prediction of their safe storage life. A solution of nitrocellulose in methanol concentration 0.

Solutions of the same nitrocellulose sensitized by adding ca. In addition they can lead to a loss of calorific value, changes in ballistic properties and cracking in large diameter charges. Small amounts of stabilizing compounds are included in propellant formulations, either singly or as mixtures, in order to react with the degradation products, thus reducing the probability of the adverse effects. For propellants of new formulations, in particular with stabilizers and other relevant components which were not used before in propellant formulations, more elaborate methods, such as the multi-temperature ageing procedure, have to be used.

Until recently, the stabilizers considered a group of aromatic compounds with the ability to react quickly with nitric oxides that occur during the thermal decomposition of nitrocellulose, thereby preventing their autocatalytic effect on further decomposition more recently, this group of stabilizers is treated as "secondary stabilizers" because they reduce the effects of decomposition that is already occurring.

Table 1 List of most common organic stabilizers used in production of ammunition. The gunpowders for classical ammunition, whether single-phase, double or triple, as well as a large number of rocket fuels as the basic component of the composition contain nitrocellulose. Nitrocellulose has a similar aspect to cotton, is white and has a fibrous texture. It is produced from cellulose.

The appearance of the nitrocellulose molecule is shown in Figure 1. The undesirable characteristic of nitrocellulose is chemical degradation with the release of gaseous products. One of simplified mechanisms of nitrocellulose degradation is shown bellow. Long-term storage of nitrate ester compounds, they react with moisture inside or moisture around the packaging, resulting in nitric acid HNO 3 and nitrogen oxides such as NO 2 and NO 3. These reactions of nitric oxide and nitric acid are exothermic reactions that increase the ambient temperature during decomposition: 9.

Effective stabilizer is a defined term which is used to assess the stability in the single temperature test. Daughter stabilizers are substances with stabilizing capabilities that were not included in the propellant formulation but were produced from the initial stabilizers during propellant manufacture or ageing. Most stabilizer depletion products fall into this class. Certain stabilizers can appear both as initial stabilizer incorporated in basic formulation and as daughter stabilizer produced from initial stabilizer , this even in the same propellant.

The content of "effective stabilizer" is calculated from the contents of all initial stabilizers diphenylamine, 2-nitro-diphenylamine, ethyl centralite, methyl centralite, akardite-II, p-nitro-N-methylaniline, resorcinol-except if they are used as surface moderants and the content of N-nitrosodiphenylamineas follows.

For propellants without diphenylamine as well as for propellants with diphenylamine and other stabilizers. Percentage effective stabilizer is the amount of effective stabilizer found, expressed as a percentage by weight of the propellant sample. Initial level is the percentage of effective stabilizer found in the propellant sample prior to ageing.

Thin layer chromatography TLC and high performance thin layer chromatography HPTLC — now also called planar chromatography — are, like all chromatographic techniques, based on a multistage distribution process. This process involves: a suitable adsorbent the stationary phase , solvents or solvent mixtures the mobile phase or eluent , and the sample molecules.

For thin layer chromatography the adsorbent is coated as a thin layer onto a suitable support e. On this layer the substance mixture is separated by elution with a suitable solvent. The principle of TLC is known for more than years now. Specific examples of these applications include: analyzing ceramides and fatty acids, detection of pesticides or insecticides in food and water, analyzing the dye composition of fibers in forensics, assaying the radiochemical purity of radiopharmaceuticals, or identification of medicinal plants and their constituents.

This layer of adsorbent is known as the stationary phase. After the sample has been applied on the plate, a solvent or solvent mixture known as the mobile phase is drawn up the plate via capillary action.

Because different analytes ascend the TLC plate at different rates, separation is achieved. For example, with silica gel, a very polar substance, non-polar mobile phases such as heptane are used.

The mobile phase may be a mixture, allowing chemists to fine-tune the bulk properties of the mobile phase. After the experiment, the spots are visualized. Often this can be done simply by projecting ultraviolet light onto the sheet; the sheets are treated with a phosphor, and dark spots appear on the sheet where compounds absorb the light impinging on a certain area. The plate is shown in Figure Chemical processes can also be used to visualize spots; anisaldehyde, for example, forms colored adducts with many compounds, and sulfuric acid will char most organic compounds, leaving a dark spot on the sheet.

To quantify the results, the distance traveled by the substance being considered is divided by the total distance traveled by the mobile phase. The mobile phase must not be allowed to reach the end of the stationary phase.

This ratio is called the retardation factor Rf. If the solvent front is 6 cm then the R f value for the pigment at 3cm would simply be 0.

Figure 3 Application of a sample on a micro precoated sheet with the aid of a capillary and a TLC spotting guide. In the thin layer chromatography analysis, a glass plate is coated by adsorbent materials such as silica.

Silica is the most commonly used adsorbent material for TLC analysis. Structurally silica gel consists of a matrix of Si-OH groups which can interact with molecules via hydrogen bonding and adsorption. A few micro liters of a dilute solution is put onto the silica surface of the plate using a micro capillary.

The plate is then placed in a jar containing a solvent generally mixtures of ethylacetate and hexanes. As time goes, the solvent gradually rises up the plate due to capillary action carrying the components of the sample with it. Different molecules are carried up the plate to different distances due to variable interactions with the adsorbent material.

For example, when silica is used, polar molecules with groups such as hydroxy OH or amine NH 2 will tend to form hydrogen bonds with the silica matrix Si-OH groups and will therefore not move as fast up the plate. While relatively non-polar molecules will have fewer interactions with the matrix and will tend to be more soluble in the solvent phase and therefore rise faster up with the solvent front. Once the solvent has risen a particular distance sufficient to separate components of the spot, the plate is either visualized directly using ultraviolet light, or it is developed using a stain to check for specific types of molecules.

In general, a substance whose structure resembles the stationary phase will have low R f , while one that has a similar structure to the mobile phase will have high retardation factor. Retardation factors are characteristic, but will change depending on the exact condition of the mobile and stationary phase. For this reason, chemists usually apply a sample of a known compound to the sheet before running the experiment.

The success of thin layer chromatography as a highly efficient micro analytical separation method is based on a large number of advantageous properties: high sample throughput in a short time suitable for screening tests pilot procedure for HPLC after separation the analytical information can be stored for a longer period of time the TLC ready-to-use layer acts as storage medium for data separated substances can be subjected to subsequent analytical procedures e.

IR, MS at a later date rapid and cost-efficient optimisation of the separation due to easy change of mobile and stationary phase. For a chromatographic separation the sample must meet several requirements to obtain good results. It is not possible do go into detail here. However, eventually several steps for sample pretreatment may be necessary. These include sampling, mechanical crushing of a sample, extraction steps, filtration and sometimes enrichment of interesting components or clean-up, i.

In our experiment, we prepared the samples of gunpowders into pieces of diameter up to 2mm and extracted into dichloroethane. The aim of a chromatographic separation determines how the sample should be applied to the TLC plate or sheet. The most frequent technique still is application with a glass capillary as spot or short streak.

Application as streak will yield better results especially for instrumental quantification. For both types of application some manual skill is required to obtain reproducible results. Substance zones which are too large from the beginning will cause poor separations since during chromatography they will become even larger and more diffuse. The mixture to be separated and the reference solution are applied to the micro pre coated sheets as spots by means of glass or plastic capillaries.

Only use each capillary once to avoid contamination of the following samples. The capillaries fill themselves quickly when dipped into organic sample solutions, with aqueous solutions filling will be much slower.

Before emptying the capillary roll the submerged end horizontally on filter paper. Place the capillary on the layer vertically and carefully, vertically so that the capillary empties itself and carefully to avoid damage to the layer.

Damaged layers result in unevenly formed spots. To keep spots as small and compact as possible, it is advisable to apply a solution in several portions with intermediate drying blow with cold or hot air. This is especially important for aqueous sample solutions.

The following figures demonstrate the clean and easy application of samples with the above-mentioned spotting guide. It is recommended to apply 0. The sample zone on the starting line should be mm in diameter, cm apart from the edge of the plate. After application allow the solvent of the samples to evaporate completely about 10minutes or blow with cold or hot air. Development of a chromatogram should never start before the solvent of the applied samples is evaporated completely.

The most frequently used separation technique is ascending TLC in the customary trough chamber standard method, linear development. Usually it is applied as single development.

However, multiple development, with or without change of eluent step technique can improve separation results. For 2-dimensional development only 1 spot of the sample is applied in one edge of a plate. Thus complicated mixtures give 2-dimensional chromatograms taking advantage of the different separating properties of two eluents. A generally applicable standardised optimisation method is described by H Keuker et al. The concept of TLC is simple and samples usually require only minimal pretreatment.

TLC can be used to monitor the progress of a reaction, identify compounds present in a given substance. TLC is also used to separate the identical compounds in a mixture. Many standard methods in industrial chemistry, environmental toxicology, food chemistry, water, inorganic and pesticide analysis, dye purity, cosmetics, plant materials, and herbal analysis rely upon TLC as the preferred approach.

As stated earlier, TLC plates also known as chromatoplates can be prepared in the lab, but are most commonly purchased. Silica gel and alumina are among the most common stationary phases, but others are available as well. Many plates incorporate a compound which fluoresces under short-wave UV nm.

The backing of TLC plates is often composed of glass, aluminum, or plastic. Glass plates are chemically inert and best withstand reactive stains and heat, but are brittle and can be difficult to cut. Aluminum and plastic plates can be cut with scissors, but aluminum may not withstand strongly acidic or oxidizing stains, and plastic does not withstand the high heat required to develop many stains. Aluminum and plastic plates are also flexible, which may result in flaking of the stationary phase.

Never under any circumstances touch the face of a TLC plate with your fingers as contamination from skin oils or residues on gloves can obscure results. Instead, always handle them by the edges, or with forceps. The properties of your sample should be considered when selecting the stationary phase. It is also important to note that silica gel is acidic.

Therefore, silica gel offers poor separation of basic samples and can cause a deterioration of acid-labile molecules. This would be true for alumina plates in acidic solutions as well. It is important to note that there are differences between silica gel and alumina.

Alumina is basic and it will not separate sample sizes as large as silica gel would at a given layer thickness. Also, alumina is more chemically reactive than silica gel and as a result, would require more care of compounds and compound classes. This care would avoid decomposition and rearrangement of the sample. Chromatographic Columns is a good reference to learn more about the different types of columns and stationary phases.

Proper solvent selection is perhaps the most important aspect of TLC, and determining the best solvent may require a degree of trial and error. As with plate selection, keep in mind the chemical properties of the analytes.

A common starting solvent is hexane:ethyl acetate. When performing your experiment, you do not want your values to be 0 or 1 because your components that you are separating have different polarities.

If the value is 0, you need to increase your solvent polarity because the sample is not moving and sticking to the stationary phase. If the value is 1, you need to decrease your solvent polarity because the compound was not able to separate.

If you know that one component of a mixture is insoluble in a given solvent, but another component is freely soluble in it, it often gives good separations. How fast the compounds travel up the plate depends on two things:.

Acids, bases, and strongly polar compounds often produce streaks rather than spots in neutral solvents. Adding a few percent of acetic or formic acid to the solvent can correct streaking with acids.

Similarly for bases, adding a few percent triethylamine can improve results. For polar compounds adding a few percent methanol can also improve results. The volatility of solvents should also be considered when chemical stains are to be used.

Any solvent left on the plate may react with the stain and conceal spots. Many solvents can be removed by allowing them to sit on the bench for a few minutes, but very nonvolatile solvents may require time in a vacuum chamber.

Volatile solvents should only be used once. If the mobile phase is used repeatedly, results will not be consistent or reproducible. Developing a TLC plate requires a developing chamber or vessel. This can be as simple as a wide-mouth jar, but more specialized pieces of glassware to accommodate large plates are available. The chamber should contain enough solvent to just cover the bottom.

It should also contain a piece of filter paper, or other absorbent material to saturate the atmosphere with solvent vapors. Finally, it should have a lid or other covering to minimize evaporation. If fluorescent plates are used, a number of compounds can be seen by illuminating the plate with short-wave UV.

Quenching causes dark spots on the surface of the plate. These dark patches should be circled with a pencil. For compounds which are not UV active, a number of chemical stains can be used. These can be very general, or they can be specific for a particular molecule or functional group.

Iodine is among the most common stains. Plates are placed in a jar containing iodine crystals, or covered in silica gel with iodine dispersed throughout, for approximately one minute. Most organic compounds will be temporarily stained brown. Some popular general use stains are Permanganate, ceric ammonium molybdate CAM , and p-anisaldehyde. These can be kept in jars which plates are dipped into, or in spray bottles. To develop a plate with permanganate, spray or dip the plate and heat it with a heat-gun.

Hold the plate face up 10 to 20 cm above the heat gun until the bulk water evaporates.