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Week of 16 June 2008


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Monday, 16 June 2008
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09:47 - I'm going into high gear with the home forensics book, so updates here are likely to be infrequent and short.



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Tuesday, 17 June 2008
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08:20 - Rats! I spent quite a bit of time yesterday trying to make an experiment work. I finally gave up. I wanted to use it in the home forensics book, under a title something like "Salicylate Assay Using Visual Fluorimetry", but I just couldn't make it work.

If I had a real fluorometer, it'd be easy enough to do, but fluorometers are extremely expensive pieces of lab gear. You can get a cheap one for $3,500, but it doesn't have the range to do what I wanted to do. Good general-purpose spectrofluorometers start at about the price of a new car and go up quickly from there. As usual, I wanted to do things on the (really) cheap; I was using a $13 portable fluorescent black light as the excitation source and my Mark I human eyeball as the receptor.

The problem is, salicylate ions require an excitation wavelength of about 315 nm, which is on the extreme edge of long-wavelength UV (black light). My portable black light lamp is pretty wimpy to start with, and I'm sure the amount of light it emits at 315 nm is just about undetectable. Furthermore, salicylates emit at about 400 nm, which is on the edge between violet visible light and ultraviolet, where the sensitivity of the human eye is quite low. So, I didn't really expect it to work, but I had to give it a try.

What I may do is a visual fluorimetry session using something like luminol, which pumps out lots of visible light. With that, I could at least illustrate the concept of fluorimetry.



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Wednesday, 18 June 2008
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10:11 - Netflix must think I've lost my mind. Last Friday, they shipped me S1D1 of Eureka. I mailed it back to them Monday, and they acknowledged receiving it Tuesday morning. As soon as I got the email from them acknowledging receipt, I added Eureka S1D1 to my queue and moved it to the top. A couple hours later, I got email from them to say that they'd shipped me Eureka S1D1 to arrive today.

So, did we like it so much that we just had to watch it again, and so soon? Nope. We watched the first two episodes of three on the disc, and I shipped it back not realizing that we still had one left to watch. Oh, well.



Thanks to everyone who's made suggestions about visual fluorimetry of salicylate ions. Several of them were quite elegant, but alas all of them would require a substantial time investment. In addition to keeping the lab sessions inexpensive, I need to keep them fairly short, which means no construction of apparatus unless it can be done in a matter of minutes rather than hours.

From a pedagogical standpoint, what I want to illustrate is the use of fluorimetry to determine concentration of specific solutes. In a real forensics lab, most solutes of interest can be detected by fluorimetry at nanomolar concentrations, many at picomolar, and some at femtomolar. But for learning purposes, it's quite sufficient to use visual fluorimetry to detect and quantify concentrations at even molar levels, and that's easily done with a cheap black light tube and a fluorescent dye like fluorescein (excitation 494 nm; emission 521 nm).


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Thursday, 19 June 2008
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08:40 - Still heads-down work on the forensics book. Today, I'm writing up a segment about determining refractive index of glass fragments. Real forensics labs do this by immersing the questioned sample in a series of liquids of known refractive index, and that's exactly how we'll do it. The problem is that real forensics labs typically use RI liquids from Cargille Laboratories. A full set of those costs nearly $2,000. The Cargille liquids are available individually in bottles of a few mL, but even those are $17.50 each and we'd need a lot of them.

My solution is to use various common liquids, from olive oil and castor oil, both of which have RIs on the low end of the range of RIs of glass fragments, through benzene, which has an RI about the middle of the range, through clove oil, which has an RI on the high end. I'll make a simplifying assumption that a mixture of two oils has an RI proportional to the quantities of each.

For example, at 20 C, olive oil has an RI of 1.466 and castor oil an RI of 1.482, so I'll assume that a mixture of equal volumes of each has an RI of (1.466 + 1.482)/2 = 1.474. That's certainly not entirely accurate, but it's close enough for what we're doing, and it beats paying $17.50 each for a few mL of the Cargille liquids. With a few bucks worth of raw liquids, I can put together a set of ten or a dozen RI liquids that spans the range of glass fragment RIs and provides reasonable accuracy.



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Friday, 20 June 2008
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08:45 - I want to use Mandelin Reagent for one of the drug testing lab sessions in the home forensics book. Mandelin Reagent is made by dissolving 0.10 g of ammonium metavanadate in 10.0 mL of concentrated sulfuric acid. The sulfuric acid is cheap and easy to come by, but the ammonium metavanadate is a different story. Fisher Scientific sells it, but the smallest bottle they have is 125 g for $77.

So I'm going to make my own ammonium metavanadate using vanadium pentoxide as a starting material, and write up the synthesis for the homechemlab.com subscriber newsletter. Pottery supply stores sell relatively pure vanadium pentoxide for about $15 a pound, and I suspect I'll be able to buy a one-ounce bottle for a couple bucks. All I'll need other than that is sodium carbonate and ammonium chloride, both of which are cheap and readily available. I'll end up with 25 g of ammonium metavanadate, or something close to that depending on yield, for about three bucks' worth of materials and maybe half an hour of my time.



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Saturday, 21 June 2008
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Sunday, 22 June 2008
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