Re: Help with sampling techiques

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 0.7% 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