I'll second tails's comments; It's important to use a homogeneous sample when testing. It should also be noted that these reagentscan produce similar colours with other compounds.
Mecke for example will give a very dark blue with hydrocodone while Mandelin dosen't react at all and Marguis produces a red-purple colour. Lysergide and opium will give a dark green/black with Mecke, although if lighting is poor it may be hard to distinguish between this and a pure black result.
If your Marquis is bad, it's because the formaldehyde in it has polymerized. It will appear to be various shades of brown depending on the degree of polymerisation. Marquis should be kept at 4 deg C.
Mandelin will last at least 18 months -2 years if manufactured properly and stored correctly (10-35 deg C). While it is possible to still do tests with the precipitate that forms in Mandelin, it does alter sensitivity in the sense that some reactions producing a faint colour can be missed. Properly made and stored at room temperature, precipitation shouldn't occur.
Mecke should be fairly stable at room temps, however, it contains a very nasty toxin - Selenious acid. It should never be stored or used anywhere near food
Simons and and the primary amine formulation (which is Simons formula, but acetylaldehyde is used in place of acetone) should also be kept cool (4 deg C) and away from light. Amine kits containing nitroprusside should never be stored with sulphuric acid based kits. If they mix, a combination of hydrogen cyanide and nitrogen oxides will be produced. Very nasty stuff!
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