Re: Help with sampling techiques

"Phil Hobbs" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
> Marvin Margoshes wrote:
> > "sbc news" <[EMAIL PROTECTED]> wrote in message
> > news:[EMAIL PROTECTED]
> >
> >>Hi
> >>
> >>My name is Andrew Ratermann.  I am in the 6th grade.  I am doing a
science
> >>project analyzing suntan lotions.
> >>
> >>Basically I plan to run samples of these products through a
> >>Spectrophotometer to see how much UV light is transmitted through
samples
> >
> > of
> >
> >>various suntan lotions.
> >>
> >>I was wondering if you could suggest any sampling techniques.  From what
I
> >>can tell the  thickness of a sample film is important to get consistent
> >>results (Beers Law)? Can you suggest any techniques to accomplish this?
> >
> > I
> >
> >>have heard of experiments where a transparent film was used to support
an
> >>applied film of suntan lotion.  I do not know of any sources for UV
> >>transparent films that might work.  I am again unsure how to get a
> >>consistent reproducible film of lotion on the support film.
> >>
> >>I have access to a scanning UV-visible spectrophotometer at the local
> >>University that uses standard rectangular quartz cuvettes.  I have
> >>considered diluting the samples with a solvent and scanning them as a
> >>mixture.  I would measure transmission through the sample.  I am not
sure
> >>however that this is a valid approach.
> >>
> >>If you can help, refer me literature I can afford and find, or to
someone
> >>that can help I would greatly appreciate it.
> >>
> >>Thank you
> >>
> >>
> >>Andrew Ratermann
> >
> >
> > Very thin liquid cells are used in infrared spectrometry.  Perhaps you
can
> > get one, and replace the salt windows with quartz plates.   The
university
> > staff should be able to help with that.
> >
> >
>
> I think it's easier than this.  Water is pretty transparent down to  200
> nm or below; I found the following data at
> http://omlc.ogi.edu/spectra/water/abs/
>
>
> T. I. Quickenden and J. A. Irvin, "The ultraviolet absorption spectrum
> of liquid water," J. Chem Phys., 72, 4416--4428, (1980).
>
> lambda absorption
>   (nm)    (1/cm)
> 200 0.007042
> 210 0.002565
> 220 0.001578
> 230 0.001141
> 250 0.000706
> 260 0.000575
> 270 0.000429
> 280 0.000412
> 290 0.000291
> 300 0.000212
> 310 0.000194
> 320 0.000174
>
> What this means is that even at the very far UV end, a centimetre of
> water will cause only a 1.4% error in your absorption data, and in the
> 300-400 nm region, the error will be very much smaller.
>
> I'd go ahead and use the standard cuvettes, with a very much diluted
> sunscreen solution (like 99 parts water to 1 part sunscreen).  Getting
> the dilution right may turn out to be the hardest part--sunscreen has
> greasy stuff in it, which is liable to stick to the glassware and make
> it hard to get accurate dilutions.  A *tiny* bit of detergent would
> probably help.
>
> You'll want to measure the UV absorption of the water+detergent mixture
> to make sure that the detergent isn't highly absorbing.
>
> Cheers,
>
> Phil Hobbs
>
Sunscreens also contain material that won't dissolve in water, even with a
detergent.