Week of 16 June 2008
Update: Friday, 20 June 2008 08:45 -0500
- I'm going into high gear with the home forensics book, so updates here are likely to be infrequent and short.
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.
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
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
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.
- 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).
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.
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.
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.
1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008 by Robert