11 Mar 2016

Analyzing coloured smoke | Pearl or Quest?

Coloured smoke consists of an aerosol of tiny particles which is combusted or vaporized. The particular pigment or dye of each colour affects the colour of the smoke.

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The mixture usually includes a cool-burning formula based on potassium chlorate oxidiser, dextrin or lactose as fuel. Then one or more dyes are introduced to account for around half of the mixture. A small amount of sodium bicarbonate may also be added as a coolant.

Smoke bombs

The mixture in the smoke grenades you might see at a paintball match is referred to as cool or cold-burning. There is no external flame produced and the temperature of the cartridge is lower than military distress smokes, which burn at a very hot temperature.

The smoke grenade mixture burns without the need for atmospheric oxygen and so should continue to burn underwater.

The colours often include the below composites:

  • Red: Sudan IV (C24H20N4O)
  • Yellow: Aniline Yellow (C6H5N=NC6H4NH2)
  • Blue: Victoria Blue BO (C33H41N3O – hydroxide)
  • Orange: Sudan I (C16H12N2O)
  • Green: Quinizarine Green SS (C28H22N2O2)
  • Violet: Solvent Violet 13 (C21H15NO3)

Aerial acrobatics

The Royal Air Force Aerobatic Team or ‘Red Arrows’, commonly use an oil that does not include harmful chlorinated solvents along with a specifically prepared liquid dye.

Coloured smoke released from an aircraft consists of dye, trichloroethylene or tetrachloroethylene and diesel oil.

The diesel is injected into the hot exhaust from the jet engine. This reaches high temperatures, over 400 degrees centigrade. The mixture vaporises immediately upon release and the colours are of course produced by mixing dye with the vapour, which would otherwise be white. The pilot has buttons in the control column which initiate the smoke release.

If the smoke was produced by the result of combustion and not vaporisation, it would be too dangerous to be used in an aircraft, in case of explosion.

Safety measures

Testing these dyes and oils ahead of performance is important, firstly to ensure that the pilot isn’t at risk of accident from highly combustible elements contaminating the dye or diesel oil. The safety of those watching at 1500ft or more below relies on ensuring dangerous amounts of harmful solvents aren’t present in the mixture. Even the environmental impact of the dyes and oils is an important element of quality control.

The Pearl liquid FTIR spectrometer accessory is great for testing oils, as we’ve discussed in our application note Pearl: New and Used Motor and Hydraulic Oils. But what about liquid dyes?

We know that the Quest ATR spectrometer accessory is a very suitable accessory for the ATR IR spectroscopic analysis of paints along with other liquid and solid samples. But the Pearl is different in that it aids transmission IR spectroscopic analysis, not ATR.

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Figure 1: Castrol LM grease spectrum, Pearl vs Quest

In another application note we compared our Quest and Pearl by testing greases. Overall it became apparent that if you were only interested in testing liquid samples, the Pearl offered the greater spectral resolution, even for samples which weakly absorb in the IR.

The Quest is still a very reliable accessory, but one which has a great appeal because it can test any sample of any consistency, with great accuracy and minimal sample preparation time.

Figure 1 shows an IR spectrum taken from 3500 – 600 cm-1 of Castrol LM grease using both accessories. The resolution of the Pearl is about 10 times stronger and so shows the absorption strength of the sample to a much more detailed degree.

If you’re flying off any time soon to perform some aerial acrobatics, make sure your smoke is safe (and the right colour) with the Specac Pearl liquid FTIR transmission analysis accessory.

Check out #SpectroscopySolutions for more.