DIY gas chromatograph - the detector (thermal conductivity detector) - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 21, 2018, 06:40:13 pm
content: So, as a chemist(ry student) I'm on a long quest to be able to design analytical instruments for fun as a hobby. I'm currently musing on gas chromatography, and i've been sorta looking at the little PCB mount gas detectors a lot of places sell, there's especially one, the mq138 that could be adapted for use in detecting general, lower-boiling organic compounds (i think).I'm planning on a very simple setup, with-a packed GC column,-an inlet with rubber septum for sample injection,-an inert (balloon helium or welding argon) carrier gas,-a detector,-and a heater element on inlet, column, and detector.If the MQ138 turns out to be sensitive enough, it'd be great for cost reduction, as most actual GC detectors are either kinda hazardrous or expensive.So the three questions: -how hot does those little heater elements inside get? if my gasseous compounds just condense in the sensor it's no good.- Is it sensitive enough to detect compounds (alchohols, acetone, etc.) in a stream of helium?- How to mount the sensor so that the gas flows across it.The last part might be tricky; i'm thinking i'll have to do away with the protective mesh. There shouldn't be anything that can burn in the system though, so it'll probably be ok.Has anyone adapted this package-type of gas sensors for use with gas flow in tube? The tricky thing with GC is that after the column and before the sensor, you have to maintain a very uniform cross-section size, because if there's a cavity, the gas will slow down, and you lose the chromatographic resolution from the column.Hope someone other than me finds this interesting! Just noticed that a lot of those gas sensors need 0.2% to atmospheric % oxygen, according to the datasheet. I'm uncertain if that's just the range they are tested in, or if oxygen's actually needed for the electrochemical reactions in the sensor. Hm.--Chris

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 21, 2018, 09:25:41 pm
content: So, I'm kindof out of luck with that idea. It seems the standard gas detectors all rely on an oxidized layer that's formed from heating in air, so no good for a airless environment.Idea 2: a thermal conductivity detector! - a wheatstone bridge with resistive wire in a sample line and a reference line with only the carrier gas flowing.This might be easier to achieve, since the two major components are the block and the filaments - the block could be machined from aluminum, the gas ports tapped with 1/4'' npt to accept SWAGE-LOK npt-to-6mm fittings.the filaments could be mounted on little tapped plugs to be screwed in from the top, and the whole block could be wrapped in a heating element and mineral wool for heat-proofing.I'm not that deeply into the machinist side of things, but I can't imagine getting someone who is to drill out the block being budget-breaking.Whatcha think?

author: ikrase
author_rank: Regular Contributor
timestamp: March 22, 2018, 07:30:20 am
content: Maybe you could just use a low-range mass flow controller or mass flow meter? A ready made solution basically.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 22, 2018, 08:36:54 am
content: Quote from: ikrase on March 22, 2018, 07:30:20 amAbsolutely, I do plan to install mass flow controllers for both carrier and reference gas lines. I have a few from an old gc. They're for H2, so I guess I can't really trust the gauges on He, but as long as I see the same flow over both sensors I should be OK.My worry about the design in the image in previous post is the cavities for the filaments- wouldn't the sudden volume cause the gas to slow enough for separated analytes to ram into each other again? I don't know, I think it's worth pursuing. The filaments could be bonded into hollow threaded bolts and screwed in place in the al-block. Maybe water glass is gas-tight and temp resisitant enough.

author: CraigD73
author_rank: Contributor
timestamp: May 01, 2018, 07:41:02 am
content: Way way back in my chemistry days, the GC were very basic and used the thermal conductivity of the carrier gas to detect the presence of the sample. They were most likely heated fine wire platinum resistance elements or possibly just thermistors. They used two of them in a Wheatstone bridge arrangement with vacuum tube type galvanometer that had an output to a chart recorder. Both sensors were in the detector block, one in the gas stream and one not in the gas stream but used to sense the ambient temp. You first had to balance the bridge to a zero output with the carrier gas flowing but no sample. Then you injected the sample with a little air first. The air would be a marker for the time of gas flow travel. As the thermal conductivity of the gas changed the sensor would change resistance and unbalance the bridge providing the output. You attached a burette with a few drops of soapy water in the bottom to the gas output then using a stop watch measure the soap bubble travel time up the burette to calculate the flow rate.Craig

author: kony
author_rank: Regular Contributor
timestamp: May 01, 2018, 09:18:25 am
content: Why not FID - considering it to be too dangerous?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 01, 2018, 11:30:39 am
content: True, tungsten filaments in gas stream is the correct way of doing it. I found a simplified concept of having a diode in a piece of 1/8'' tubing , passing current through the diode, and then comparing the (temperature-dependent) voltage drop over the diode to a reference (battery) voltage, zeroing that out. it works quite well, with an 1N4148 spot-welded in stainless 304 process tubing!-FID might be feasable, but I dont have storage for compressed hydrogen, so it'd have to be a PEM electrolysis cell generating it. This has been done before, i just wanted the simpler solution.

author: LaserSteve
author_rank: Super Contributor
timestamp: May 02, 2018, 05:57:23 am
content: SRI publishes schematics for their simpler detectors. You might want to learn from their TCD design. Using recovered light bulb filaments in a bridge works just fine for a hobbyist short term and would be an order of magnitude more sensitive then your 1n914.In SRI's business model they want their customer to have 100% uptimeSo they openly publish a great deal about their systems. I love their product, it takes a beating in an undergraduate lab I maintain with no issues in four years. . After your done with DIY you might want to look for a used SRI on EBay.Steve

author: ale500
author_rank: Frequent Contributor
timestamp: May 02, 2018, 08:53:03 am
content: I just wonder if it may be, in the beginning, more feasible to use an old Varian (really old ones had different fittings!), HP, Perkin Elmer GC from surplus than to build all from scratch. You get the chassis, the oven!, and probably a working detector, together with some piping.You need a constant flow of gas to move the sample down the column and in many cases a "make-up" gas too. Thermal conductivity seems like a a simpler approach because you don't have to burn anything, and yes without oxygen (almost) nothing burns.Hydrogen can be obtained from... hydrogen generators, but they do not seem to come in cheap despite their simple workings. But there are fuel cells, maybe one could be adapted to get a workable flow, it may be worth exploring.(I'm a chemist too but my field is mass spectrometry, and the home-made MS is also on my todo-list).

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 02, 2018, 09:34:10 am
content: Problem is, I don't have room for a full size lab gc, so I want to fit it in a 3U 19 inch box. I have acquired FID dlow controld modules to get the regulators and sweet sweet swagelok fit tings, but the problem with old gc gear is that if it's older than mid- 80s, I'm worried about asbestos..The correct filament wire (a tungsten alloy) actually goes pretty cheap on ebayour sometimes. I feel the biggest challenge in the detector design is to avoid dead spaces. May be putting the filaments in a short stub of stainless tubing in a T fitting is the way to go.I've settled on balloon helium for carrier, only I hear it ruins capillary columns..Thanks for the interest!

author: LaserSteve
author_rank: Super Contributor
timestamp: May 02, 2018, 11:58:55 am
content: SRI's TCD block is externally heated. That lets them have a block with balanced , symmetrical, flow channels and more like bantam or miniature then trying to do "micro" for the sake of sensitivity. Heating the block would let you use a die cast rf project box as your detector with suitable dividers silver soldered or screwed in.Symmetry is important, it lets you balance the sample and reference carrier flows. You get close to an inherently balanced electrical bridge this way.This way the carrier can be pretty much any inertgas. We have used other gases (AR, N2) in a pinch when the local gas place messed up our order. Again, undergraduate lab, analysing alcohol from our teaching pilot plant.Better Yet, Actually you could order a small cube of Aluminum from McMaster Carr, and make a balanced bridge with just a drill press and cheap metal capillary tubing fittings.Look at the SRI operations / repair docs. I read them every once in a while just to study the design, it is amazingly simple yet the actual engineering is clever and elegant.My day job isSenior Technician, Analytical Instrumentation and Optics.Major UniversitySteve

author: ChristofferB
author_rank: Frequent Contributor
timestamp: May 02, 2018, 12:36:57 pm
content: Ih, right, I have actually looked at SRI's detectors, because they seem to be built into swagelok fittings - their FID's atleast. i couldnt find the schematics, but i'll look again. Thanks for the hint. Another idea for a gas source is those small single-use welding shield gas bottles, usually N2 or Ar.I still don't have a good way of heating my detector externally, maybe some low voltage high current nichrome wire elements in glass wool tubing wrapped around.Thanks for the interest.

author: LaserSteve
author_rank: Super Contributor
timestamp: May 02, 2018, 01:00:26 pm
content: Applicable Links:http://www.srigc.com/cn/downloads/184/CompleteSchematics%20Feb11_2015.compressed.pdfYou want to look at pages AMP-E and AMP-F in above Doc.http://www.srigc.com/cn/downloads/48/TCDman1.pdfhttp://www.srigc.com/cn/downloads/49/TCDman2.pdfhttp://www.srigc.com/cn/downloads/50/SingleFilamentTCDdetector9-2010.pdfBTW, Most TCD filaments in the US are made in one place ... GOW-MAC Inc.http://www.agc-instruments.com/hot-wire-filamentsSteve