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Which germicidal lamps generate the purest 254 nm? (or 254 + 185 )

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radiotech

Sun Aug 07 2011, 05:23PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

"Haven't yet found any quantitative data about atomic-absorption sensitivity in measurement of trace concentrations of pollutants or, uh, tracers."

Prospectors used a portable gadget, that I repaired long ago. It was used to
measure mercury in ore samples because Hg is a pathfinder in assaying.

This gadget had a small cooking 'bomb' and the gasses were passed through
a chamber that used a germacidal lamp and a detector to see if the 254 nM transmission
was reduced.

Bored Chemist

Mon Aug 08 2011, 09:12PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

I'm fairly sure that a simple low pressure mercury lamp will produce fairly pure 254nm (and some 1185nm) together with rather less visible light.
I will look up the details if I remember.
I have seen the demo of the "smoke" from mercury casting a shadow (onto a TLC plate as a screen IIRC). I don't think there was anything special about the lamp.

radiotech

Tue Aug 09 2011, 05:27AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The lamps are tricky and dangerous. it is true they glow with a blue glow
that is fairly dim. But the 254 light is intense and can damage your eyes
because you do not sense it visually. This should be pointed out because the
experiment is rather alluring. The 254 light cant get through normal eyeglasses
we were told.

Older fluorescent lamps sometimes swirled visibly with a dimming in a spiral
path as they first were turned on and was dismissed as normal in the manufacturers
writeups.

Lately they use far less mercury, because they have to. Older lamps had globs you could
see at the extremes. Was this swrling 'mercury shadows?'

I am going to try a setup here using an old experiment in storage.

A 40 watt T12 lamp had both ends removed carefully leaving just a phosphored
glass tube. One of the T8 30 watt germacidal lamps was slid down inside. It was powered
up with wires to the pins. The result was a very well lighted fluorescent
contrivance designed to show how the 254 light was the reason the floursecent lamp
lit up.

If I stand this on end, in a dish of mercury with space for updraft of air, it should act as a chimney andthe interference of the "vapour" shadows should be seen. I have some
mogul tubes to try as well, T16 for more area but they may have vanadium phosphor.

We used to clear cloud chambers with a voltage that swept the ionization tracks away.
I have pushed mercury drops down a glass tube by an electrolytic current from end to end
and made them reverse direction by reversing current. I wonder if the mercury plumes
can be chased about with an electrostatic fields. At any rate I will believe the plumes
when I see one here. Isn't the blue glow in the Torricelli baromener a plume?

klugesmith

Wed Aug 10 2011, 03:16AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

radiotech wrote ...
The lamps are tricky and dangerous. it is true they glow with a blue glow
that is fairly dim. But the 254 light is intense and can damage your eyes
because you do not sense it visually. This should be pointed out because the
experiment is rather alluring. The 254 light cant get through normal eyeglasses
we were told.

254 nm is nasty -- not far from the sharp maximum at 280 nm (biological damage potency in UV radiation safety recommendations, e.g. slide 6 at

)
But it doesn't penetrate the cornea, much less get focused on your retina.
Short-term effect of overexposure is, AFAIK, the same as "welder's flash", sort of a sunburn on the eye's surface.

So it doesn't help not to look at the glowing mercury (lamp, rectifier, etc.).
What counts is whether the source can see your eyes (peek-a-boo!) and for how long, from how far away.

Still waiting for my Hg hollow cathode lamp.

Bored Chemist

Wed Aug 10 2011, 06:54PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Incidentally, since mercury vapour doesn't absorb green light, the experiment in the first post was doomed to fail.

klugesmith

Wed Aug 10 2011, 09:30PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Bored Chemist wrote ...
Incidentally, since mercury vapour doesn't absorb green light, the experiment in the first post was doomed to fail.

Would that be because the 546.1-nm line emission from a Hg lamp comes from electron transitions between two energy states far above ground? So free, un-excited Hg atoms would not be good at absorbing it?
In my limited understanding of AAS, most metals need to be part of a flame or some other energetic process, before they can properly absorb their characteristic radiation. Recently saw some reference which said mercury is an important exception.

Have you any data on real, absolute absorptivity values? (All I've found is technical manuals that say to calibrate each instrument, perhaps even for each session, with accurately diluted standards).

Bored Chemist

Thu Aug 11 2011, 07:14PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

"Would that be because the 546.1-nm line emission from a Hg lamp comes from electron transitions between two energy states far above ground? "
Yes
The usual reason for using a flame is simply to vaporise the metal.
To get an emission spectrum you need to get the atoms into an excited state. Hotter is generally better for that.

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