Replacing the CFC by petroleum ether a mixture of hydrocarbons like hexanes and heptanes in this formulation is not possible. The stock solution will not or only partially dissolve in the petroleum ether and will give a two-phase solution.
A thin yellow layer of stock solution will be on the bottom, the colorless petroleum ether will float on top. When CFC got banned we developed our own formulation, adding an ether methyl- tert -butylether, MTBE as the main change to the formulation to keep the ninhydrin solution as one phase, uniform solution.
Other formulations are known for example one in which more ethanol is used. This has the disadvantage of increasing the ink-running.
It combines 5 gram of ninhydrin crystals, 3 ml acetic acid, 75 ml ethanol, and 25 ml ethyl acetate for a concentrated solution that is diluted with 1 liter of heptane to obtain the working solution. Compared to the NFN-formulation the amount of ethanol is increased 4 times and the amount of acetic acid slashed by two-thirds, 25 ml of ethyl acetate is added. The performance in number of fingerprints developed on checks was better than the NFN-formulation and the formulation based on HFE published at the same time [5a] by the authors [4,5a].
Due to the flammability and associated risks of explosions for example in ovens when papers where not dry enough, it was not recommended for use by British police forces. Instead the HFEformulation [5a] was recommended. The costs of HFE are appreciably higher than petroleum ether or even heptane though. When papers and especially items like corrugated carton boxes are allowed to dry for a sufficient time before heating them in an oven there is little to no risk of explosion.
Especially for thermal paper cash receipts for example there is a ninhydrin derivative called ThermaNin. This is a hemiketal of ninhydrin and a long-chain alcohol. Applied to paper it will react with the water present and falls apart in ninhydrin and the alcohol, leaving the ninhydrin free to react with the fingerprints. The treatment of papers with the petroleum ether based ninhydrin working solution is not complicated. However, due to the evaporation of the petroleumether which fumes can form an explosive mixture with air there is a need to work in a fume hood.
To treat documents or other papers with ninhydrin a shallow dish can be used in which a thin layer of working solution is poured about cm deep. Either put the paper in the solution or draw it through. It should not be longer in the solution than about 5 seconds, just complete wetting of the paper is sufficient.
For the treatment BVDA has a practical plastic dipping-tray in the program. Because of the round shape of the bottom one can work with small amounts of working solution which results in less unwanted evaporation. Complete wetting of the paper is sufficient for treatment. Use tweezers preferably of a kind that has no grooved surfaces to draw the paper through the solution. Large surfaces like carton boxes for example or paper labels of bottles can also be treated using a soft brush.
Small amounts of used working solution can better be discarded as chemical waste non-halogenated organic solvents. Larger amounts that are not visibly contaminated can be saved for later use. Do not add it back to a container with fresh solution, store it separately in for example a brown glass bottle do not forget to label it, with the date of use.
When the solution is fresh it is light yellow. When due to contamination with water, for example through condensation or from water in the treated paper, a separation of concentrated ninhydrin solution and petroleum ether occurs, this is evident from the presence of the concentrated solution at the bottom and the colorless solution above it.
Before papers especially corrugated cardboard are put in an oven to accelerate the development of the fingerprints the solvents in the working solution must have evaporated completely.
Ninhydrin can be used after papers have been treated with DFO first it is even recommended to do this after DFO treatment, when development of fingerprints and photography of prints with sufficient detail has been done but in principle DFO cannot be used after treatment with ninhydrin.
Reportedly, there have been cases where a fingerprint became visible after treatment with DFO, that had not been visible after prior treatment with ninhydrin. Applying ninhydrin after a paper has been treated with 1,2-IND does not yield any additional prints. A very user-friendly and quick method to treat objects or papers with ninhydrin is the use of ninhydrin in an aerosol can. An excess of polar solvents causes ink-running.
Ninprint does not contain acetic acid, due to its corrosive nature. The Ninprint spray is very "dry" because the solvents evaporate very quickly. The advantage of this is that inks hardly get the chance to run.
It does mean that a piece of paper is not fully wetted with the spray, so both sides of the paper need to be sprayed. The spray mist of the Ninprint is quite irritant for the airways. Probably, this is caused by the presence of small ninhydrin crystals due to evaporation of the solvents. Use it in a fume hood, outside or with a very tight mask take care of eye-protection too, wear safety glasses.
Fingerprints become visible through the reaction of proteins and amino acid present in the sweat with which they were deposited on the surface and ninhydrin.
This can be accomplished for example by storing the papers in a big plastic bag in which a dish with moist paper or cotton balls is put. Direct contact between the wet paper or cotton and the treated papers should be avoided at all costs.
The development of fingerprints can be accelerated by heating the treated papers for some time at a higher temperature and ideally also an elevateded humidity in an oven. The maximum heating period at this temperature is 5 minutes. At lower temperatures this time can be longer. Obviously, the object should be able to stand these temperatures. In case of doubt do a test with a similar article.
The background coloration will be stronger with the application of heat than with development at room temperature.
For maximum contrast development at room temperature or higher temperature using a special climate cabinet with raised humidity like a NINcha chamber is advised. After the heat treatment the object should be stored in the dark, at room temperature and if possible elevated humidity for further development at least 10 days if possible.
Many fingerprints will only become visible after several days. A method used in some laboratories is using a steam iron instead of an oven. The hot iron is moved over the surface of the paper at a distance of about 1. The iron should not touch the paper! On carton or coated paper this method is not advised, since the steam has the tendency to condense on these surfaces. This causes damage to or destruction of the fingerprints. It is known that the Ruhemann's purple can complex with metal salts like nickel nitrate, cadmium and zinc chloride leading to a color change nickel and cadmium give a red color, zinc usually an orange.
Using cadmium is not advisable for safety and environmental reasons, because cadmium salts are highly poisonous. The fluorescence is less intense than that of DFO. The process itself is not very complex. In practice whether zinc treated prints fluoresce or not has been shown to depend strongly on the conditions used for developing the ninhydrin treated prints, the type of paper, and unknown factors.
The prints should not have been subjected to heating at low humidity for accelerating the development and post-processing with zinc should be done fairly soon after ninhydrin treatment. A succesful outcome is thus not assured. See for more information and literature citations Wainwright's et al. By modifying the ninhydrin molecule for example attaching a methoxy or a methylthio substituent [5-MTN] to the aromatic ring it was found that both the reliability and intensity of the fluorescence is significantly better.
Changing the color with a zinc salt also known as zinc toning can be useful in absorption mode also. When viewing or photographing the orange colored prints under blue light a better contrast is obtained than with the purple colored print with white or any other color light. Especially when the paper contains blue or blue-green printed areas this method is very useful. Blue and blue-green printed areas lose contrast with this type of lighting [9].
Zinc chloride treatment of prints on paper, developed with ninhydrin, is a very simple procedure. It is possible to dip the paper in the zinc chloride solution. However, it has the potential to run inks and the concentration of zinc in the solution is higher than needed for dipping. We recommend that a sprayer should be used in a fume hood to apply the zinc chloride. Spray the solution on the article only lightly and allow the solvents to evaporate. For the formation of the fluorescing complex a small amount of water is needed.
Usually, enough water is present in the air for this to happen. The acids that have the greatest solubility in the organic solvent move most rapidly and when the solvent reaches the top of the paper, the paper is removed, dried, and then turned sidewise, and a different solvent allowed to migrate upward. This double migration process gives a better separation of the amino acids than a single migration and results in concentration of the different amino acids in rather well-defined spots.
These spots can be made visible by first drying and then spraying the paper with ninhydrin solution. The final result is as shown in Figure and usually is quite reproducible under a given set of conditions.
The identities of the amino acids that produce the various spots are established by comparison with the behavior of known mixtures. Analysis by thin-layer chromatography see Section B can be carried out in the same way as paper chromatography. The partitioning is now between a solid stationary phase the coating on the plate and the moving solvent front.
The advent of ion-exchange chromatography has revolutionized the separation and analysis of amino acids as well as that of many inorganic substances.
As the name implies, it involves the exchange of ions between a stationary and a moving phase. A common application of this principle is in household water softeners, in which the calcium and magnesium ions in ordinary "hard" water are replaced by sodium ions from the resin Equation This cationic form of the amino acid can be exchanged with the cations associated with the sulfonate groups of the resin:. The process is reversible, and the amino acid cations can in turn be exchanged off the columns.
However, different amino acids have different affinities for the resin, and these are considerably influenced by the pH of the moving phase eluent.
The basic amino acids arginine, lysine , which form cations most readily, are more strongly held by cation-exchange resins than are acidic amino acids aspartic and glutamic acids.
There is a spectrum of affinities of the other amino acid cations for the resin between these extremes. Thus a mixture of amino acids can be separated by ion-exchange chromatography by elution with buffered aqueous solutions. The effluent from the column is mixed with ninhydrin solution and the intensity of the blue color is measured and plotted as a function of time at constant flow rates Figure The identity of an amino acid is determined by the volume of solvent required to elute the amino acid from the column, and the concentration is determined from the intensity of the color developed.
However, these compounds do react with ninhydrin to give yellow compounds, and these colors can be used to identify them satisfactorily. John D. Robert and Marjorie C. Caserio Basic Principles of Organic Chemistry, second edition.
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