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building a spectrophotometer from off-the-shelf parts

Discussion in 'Electronic Design' started by [email protected], Oct 1, 2007.

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  1. Guest

    In my search for a simple way to calculate ethanol concentrations in
    beer (of all things!) I remembered back to my college days - in one
    biochemistry lab, we used a spectrophotometer to determine enzyme
    reaction rates. Those Shimadzu spectrophotometers cost about $10,000
    each, and had an on-board printer to print the resulting graph of
    transmission vs. wavelength.

    Oh, look, one can be built nowadays for cheap using off-the-shelf
    parts:
    http://www.rsc.org/Education/EiC/issues/2007Sept/BuildYourOwnSpectrophotometer.asp

    I noticed one part is a 3140 op amp. Is this the one?
    http://www.mouser.com/search/ProductDetail.aspx?R=CA3140Mvirtualkey55320000virtualkey837-CA3140M

    Is there an easy way to do away with the requirement for *two* 9V
    batteries and use just, say, +6V (from a wall wart, or 4-pack AA)?

    Any input would be appreciated. Just imagine what could be done for
    high schools, using/making one of these in labs!

    Thanks,

    Michael
     
  2. Carl Ijames

    Carl Ijames Guest

    The problem for you is that that particular spectrometer covers only the
    visible spectrum, 450-700 nm, and ethanol doesn't absorb anything over
    that range. Do some research and see if there is anything useful in the
    near-infrared, where ethanol absorbs and water doesn't, or maybe see if
    you can use refractive index instead. [pause] Okay, I found some NIR
    info for you:
    http://www.axsun.com/media/pdfs/05-02-085a_liquid_phase_quant.pdf and
    http://en.wikipedia.org/wiki/Near_infrared_spectroscopy. Of course
    water also absorbs in the nir, here are two references (you will have to
    handle the wavenumber to wavelength conversion to compare with the
    ethanol spectra):
    http://cfa-www.harvard.edu/hitran/HITRAN_conf06_presentations/Session4/4.2-Daumont.pdf
    and http://www.lsbu.ac.uk/water/vibrat.html Good luck.
     
  3. Guest



    Yes, absolutely, ethanol solutions are colorless (except for the added
    barley and hops). I had thought using the bottom of an AOL CD as a
    prism would also produce some near IR, but I'd better double-check
    that assumption.

    Thanks for the input,

    Michael
     
  4. Allan Adler

    Allan Adler Guest

    My browser can't open the pictures for some reason. I'll try it from a
    friend's computer later this week. The prices are all given in pounds
    sterling but look cheap.
    This page says that Mouser is going to stop selling this one. Also, doesn't
    mouser have a minimum total price per order?

    Anyway, I guess the thing to do is to study the schematics and see exactly
    how it works and then consider how to modify the design so that different
    parts can be used.
     
  5. There is no good reason why this opamp is better than many
    others for this application. I would use half of an LM358
    dual. But I would also probably replace the light dependent
    resistor with a silicon photo diode. This would improve the
    linearity and still give you a spectral range from about 300
    nm to 1000 nm. If you use a gallium arsenide photo diode
    you can extend the IR end down to something like 1700 nm.
     
  6. Bill Penrose

    Bill Penrose Guest

    A less complicated one can be built using filters. For a few bucks you
    can buy a little booklet of transparent plastic filters with known
    characteristics from Edmund Scientific. Use those to filter the
    incident light and avoid the mechanics of prism and slit.
    http://scientificsonline.com

    The op amp is pretty generic. There are several that will substitute,
    like the LM324. If you can't get it from Mouser, try Jameco. Mouser
    and other wholesalers get pissy when you want to buy one op amp, one
    battery clip, etc, from them. Jameco's a one-at-a-time kinda place.
    http://www.jameco.com/

    A photodiode will work better than a LDR (cadmium sulfide cell), whose
    response time will be slow.

    A spectrophotometer can be pretty simple or highly complicated. For
    simple one wavelength measurements, it can be wonderfully cheap, but
    don't expect Shimadzu quality from the measurements.

    Dangerous Bill
     
  7. The Edmund Scientific ones tend to be a little less sharp in cutoffs
    than other ones (GAM, Rosco, Lee) in my experience. Also, in general,
    dye-based filters have gradual cutoffs at the long wavelength end of a
    passband, making a narrowband slight source with dyed filters close to
    impossible.

    Furthermore, all of these filter gels (Edmund, Rosco, GAM, LEE) have a
    high tendency to pass infrared. I once took a Rosco booklet, separated
    the transparent filters from the diffusing filters and the paper pages,
    fired an infrared LED through them at a phototransistor - a fair amount of
    the IR made it through all of them!

    Most dyes are IR-passing. Furthermore, in stage lighting, it is
    desirable to minimize absorption of IR to reduce heating of the filters

    (all too often only from "burn up soon" to "last a reasonable amount of
    time before getting brittle and/or changing color").

    One can easily do better at spectral analysis with a diffractive "clear
    CD" that comes in some spindle packs of recordable optical discs, or with
    LEDs of known peak wavelength. (Bear in mind that some LEDs are rated by
    "dominant wavelength", which is different and usually displaced slightly
    towards green-yellow from the peak wavelength.)
    How about DigiKey? They merely have a $5 surcharge for failing to make
    minimum order, as well as many items with pricing down to 1 unit.
    Keep in mind a couple of things:

    1. Cadmium sulfides ain't that slow - milliseconds to a fraction of a
    second.

    2. LDRs and phototransistors have nonlinearities. Photodiodes tend to be
    linear.

    3. DigiKey helps again here - it helps to have a spectral response of
    your device. It appears to me that nothing easy to get has flat spectral
    response.
    Closest to flat that I noticed so far is in "blue enhanced"
    silicon photodiodes, which have response through the visible spectrum
    being roughly proportional to wavelength (a bit less than that
    aooproaching UV and IR, falling more rapidly in UV as wavelength
    decreases, and gradually flattening as wavelength increases towards their
    peak response in IR around 900-950 nm or so).
    (Makes me maybe glad to not remember what the spectral response of a
    non-blue-enhanced silicon photodiode looks like!)

    - Don Klipstein ()
     
  8. Paul Mathews

    Paul Mathews Guest

    Old spectrophotometers go really cheap on ebay. You'll pay more for
    the shipping than for the unit, unless you can find one nearby.
    Paul Mathews
     
  9. John Larkin

    John Larkin Guest

    At the Tulane chem department, they had an IR absorption spectrometer
    that they used to check for benzene in the lab ethanol supply. If they
    got a safe batch, they'd have a party.

    John
     
  10. Guest


    Hahaha! Straightforward enough to add yeast to sugar water, then use
    the lab glassware to distill.

    Michael
     
  11. Guest


    That's what I was hoping to hear. Soldering SOIC chips is a bit
    beyond my ability.

    Thanks y'all,

    Michael
     
  12. Hey, I've probably got a few CA3140's here in cans if you want:).
    ISTR I used them as I needed the very high input impedance that
    was difficult otherwise to obtain in 1975.

    SOIC's aren't that hard to solder - you mainly need good light and
    magnification.

    Clifford Heath.
     
  13. G

    G Guest

    Thats why they use the 95% stuff.

    It would real nice if we had an updated, beer, alcohol, and calories, test.
    The last documented test was many many years ago, which still circulates
    the internet. I would settle for just the alcoholic content.

    greg
     
  14. Phil Hobbs

    Phil Hobbs Guest

    Keeping the fusel oil (propyl and butyl alcohols mostly) out of the
    distillate is the hard part. Bread yeast makes some *nasty* hooch.

    Cheers,

    Phil Hobbs
     
  15. Marvin

    Marvin Guest

    <snip>
    Years ago, I was the co-inventor of a simple colorimeter.
    The reference is
    Simple Ultraviolet Photometer.
    R. E. Thiers, M. Margoshes, and B. L. Vallee
    Anal. Chem. 31, 1258-61 (1959)

    If you can't get the publication, I can probably send you a
    copy. It was designed specifically to measure the coenzyme,
    NADH. Ralph Thiers developed a list of chemistries for the
    instrument, which was on the market for some time as the
    Coenzometer, along with packaged reaction mixes called
    Determatubes.

    There are many enzyme reactions that can be used for various
    analytes. For ethanol, an assay with yeast alcohol
    dehydrogenase will work. It is an inexpensive enzyme that
    is easy to work with. Your students will be painlessly
    introduced to biochemical analyses.

    Marvin
     
  16. Guest



    Was it patented in the US? US patents are now freely available
    in .pdf format at www.google.com/patents. Do you have the patent
    number? I'll look it up.

    Thanks,

    Michael
     
  17. Marvin

    Marvin Guest

    At the time, Harvard University (where I worked in the
    Medical School) had a policy against patenting inventions in
    the field of human health. There is no patent, but there is
    the reference I gave.
     
  18. Bill Penrose

    Bill Penrose Guest

    Send the man a reprint, Marv! Or at least a scan.

    What was your UV source?

    DB
     
  19. Mark Thorson

    Mark Thorson Guest

    Betcha a nickel it's the biggest one in the solar system. :)
     
  20. default

    default Guest

    Why not just use specific gravity or polarimetry to measure sugar
    content and calculate alcohol from converted sugar?

    That link you show is somewhat idiotic. An LED is not a white light
    source but a mixture of primary colors (which you can see with a
    compact disk used as a diffraction grating).

    And he shows in the diagram the light passing through a diffraction
    grating - that isn't how it is done.

    A SCANNING spectrophotometer or just an adjustable wavelength spectro
    is still a fairly complex instrument and I don't think the average
    DIY device will do it. They use dual beam balanced instruments to
    compensate for spectral response of the source and detector and
    subtract them from the total absorption.

    More goes on in the UV range than the visible range when it comes to
    analyzing chemical compounds.

    Build a liquid chromatograph - spectro with a flow cell and separation
    column added - much more accurate when you can single out the alcohol
    from the rest of the mixture.

    It may be a valid device for explaining the operation of black boxes
    to chemistry students - demystify the instrument - but a few drawings
    on a black board and popping the cover on a real instrument will
    achieve the same thing with better understanding and accuracy.
     
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