Twospoons
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Any soil scientists out there?
I love gardening. I love growing vegetables without nasty pesticides. I want happy healthy plants. So naturally I want to be able to analyse the
nutrient content of my soil. Nitrogen, potassium phosphorus and pH I can do with a kit from the garden store (though the pH colour chart is
appallingly hard to read).
So the question is how do I test (quantitatively) for S, Ca, Mg, Cu, Zn, Mo, B, Fe , given standard lab glassware?
I know the soil labs use fancy hardware like IR and mass spectrometers ( and I suspect thats the only answer) but if anyone has any bright ideas
I'd like to know.
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chemoleo
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Well there are numerous inorganic reactions to do this with a high degree of sensitivity, but qualitatively. It is very hard to determine the amount
of something when it's in the microgram range (such as Mo).
Really, what you need is a good spectrometer, and a variety of organic dyes, that specifically react with this ion or that. The amounts are estimated
by the absorption, relating this to the molar absorption constant.
But if you want qualitivative assays, I am sure most here a capable of helping on this 
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Hermes_Trismegistus
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Invest in a spectrophotometer
This might sound like a trite answer to your question but believe me it's not.
The spectrophotometer has been around (sans electrics) for a LONG time and is the undisputed workhorse of analytic chemistry.
I personally beleive that after a vacuum pump, a spectrophotometer is the next major piece of equipment that an amateur should purchase.
I am currently wondering myself if I should start saving for one now, or maybe take up B+W photography. Tough choice but with about 4-500 bucks for a used spectrophotometer, I might just take up
photography.
The only good news is that they are REALLY common on the used market, because both chemistry and biology use them extensively. LabX, ebay etc.
spectrophotometry basics
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Twospoons
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"The devil farts in my face again ..." *sigh*
As you say, a quick look at Ebay reveals a bewildering range of used gear. Any pointers on what to look for, should I decide to invest?
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denatured
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Until you buy a spectrometer ... don't worry about nutrient contents in your garden...
What is its type ? have you tried growing something and nutrient deficiency signs appeared?
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Tacho
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Why not built one? Seems pretty straightforward to me. A cheap digital multimeter, a photocell, photodiode or both, some high power LEDs of different
collors...
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Twospoons
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I had a look at the principle of the atomic absorbtion spectrometer, and it seems reasonably straightforward. The key to isolating specific elements
appears to be in the light source which is a hollow cathode lamp containing the element of interest. So it only emits light on the wavelengths which
will be absorbed by the element you're looking for. Broader sources would seriously compromise the sensitivity, as other compounds in the sample
would affect the total absorbtion.
The elements I'm looking for would be in the 10-100 ppm range.
Still, the lamps can be had from ebay, for less than the whole machine, and the rest doesn't seem too hard to build.
On the other hand I can get a soil sample tested for $60 by a professional lab so unless I find another compelling reason to get my own analyser, $60
is pretty cheap really.
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Tacho
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When I think of a spectrophotometer analisys I think of adding a few drops of a specific reagent to the solution to be tested and measure the change
in light absorbtion compared to reference samples. Am I wrong?
Can you do that without a specific reagent?
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DrP
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| Quote: |
When I think of a spectrophotometer analisys I think of adding a few drops of a specific reagent to the solution to be tested and measure the change
in light absorbtion compared to reference samples. Am I wrong?
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The light is absorbed at distinct wavelengths which correspond to the frequencies of vibration of the bonds in the sample being tested. (Only light
vibrating at the same frequancy as the atomic or molecular bonds is absorbed) Thus, the absorption specrum is an 'Atomic/molecular
fingerprint' which can be used to identify what types of bonds are in the sample and thus tell you what your unknown sample is (with experience
in the interpretation of spectra)
The idea of a reference sample is usually to compare the spectrum from the solvent to that of your sample, so you can ignore the peaks which come
from the solvent when trying to determine your unknown.
Sorry if you know this already Tacho, but it sounded like you may not from your last post - I may have misread it.
Regards.
[Edited on 22-11-2005 by DrP]
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Tacho
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Let me see...
1- spectrometer - outlines the absorbance spectrum of a sample in a range of electromagnetic frequencies (not necessarily light).
2- spectrophotometer - says how much light was absorbed in a given frequency.
I always saw # 2 as a way to compare the result of a light-absorbance-changing reaction with a set of known samples to test how much reactant was
there in the first place.
Hermes link seems to confirm my beliefs. There is another thread on spectrometers (not spectrophotometers) where I just posted an idea, by the way. I
would like to hear your opinions there.
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DrP
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Hi Tacho,
Sorry for the delay in the reply - been busy. Im pretty sure the names are fairly interchangable. Last year I bought an old second hand Perkin Elmer
841 Infrared spectrophotometer - It gives absorbance spectrum of a sample across the IR range. It too could be used to follow the change in
absorbance of a mixture as a reaction takes place amounst other things.
To get concentrations out, as you said, you need to have a reference. Atomic absorption spectroscopy is usually used for soil analysis - you have
to know what frequency your target metal atoms will absorb at and beam in light with the same.
I'm not to sure about building one for myself - your idea sounds sound as long as you know the frequencies of the light source and that the
detectors are sensitive enough. I would imagine you could use a prism on rotatable disk to scan.
One of our Uni lecturers built a Raman spectrometer in a large black box using different high powered lasers as the radiation source. Fidley to set
up I think but not my expertise.
Regards.
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Niels Bohr
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Why wouldn't you use chromatography ? You could find the particles you are looking for according to solubility, Size,Charge, and binding
affinity.
just a wild idea ?
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DrP
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You couldn't use chromatography because the elements he's looking for will be tied up in complexes.
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