CCD/Dewar contamination

[Willie Koorts]

Posted to CCD-world:
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Hi All

We have been haunted by CCD dewar contamination for quite some time now -
getting it right the one time and wrong the next. This was even partly
responsible for me taking a trip to the US, visiting a number of the folks
subscribed here. 

A few months before I left on this trip, we had some contamination forming on
the window of one of our freshly cleaned dewars in the form of a very slight
condensation.  We removed and kept this window and I have recently found
someone who could analyze this small quantity on a mass spectrometer for us. 
The immediate result yielded some answers but also more questions - the good
news was that we knew what it was and how to get rid of it but the bad news
was that we still did not know its origin. 

The substance was identified as a plasticiser - Di-octyl phthalate 
(C24 H38 O4) to be exact! 

To get rid of it, is by vacuum baking.  While running the mass spectrometer
test, a very prominent line at 149 (molecular mass, I suppose) started
appearing at 60 degrees C already but they kept increasing the temperature to
just over 200C towards the end of the test. The vacuum in the system was
about 2 x 10^-6 (mbar?) throughout the test.  If one can thus vacuum bake (at
minimum, with CCD in) 60-80 C but as high as the oven can go, say ~200C, on
the metal parts at 10^-2mbar or better (the pressures at which most vacuum
baking ovens I've seen on my trip, operate at) for at least 1 day, it should
all disappear.  We have already seen this to some extent when we again had
fogging of the window under vacuum, baking at 45-50C after two days, on
another dewar - it disappeared when we raised the temperature (using heating
tape and foam lagging) to 80C (the theoretical limit for the sole Butal
o-ring we could not get a Viton replacement for) for two days. 

The source of the plasticiser was still a mystery.  All the plastics' used
inside the dewar was supposed to be free of plasticizers - this we checked on
before when someone also suggested plasticizers to possibly be our
contaminant. 

>From what I have seen on my US trip, it now also seems clear that the exact
cleaning process (although important) is thus not the most critical part, as
the vacuum baking afterwards - that has probably been the saving grace of
most other groups who do bake - and everybody I visited did some form of
vacuum baking, ranging from baking separate parts in a proper vacuum oven to
completed systems, wrapped in heating tape on a vacuum pump. 

Trying to find info on this plasticizer proved difficult but after finding
the following on the web, I am quite convinced that I have solved our
mystery!  Quoting from:  http://www.nsc.org/ehc/ew/chems/diphthal.htm

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Description:
~~~~~~~~~~~~
Di(2-ethylhexyl) phthalate (C24H38O4) is the single largest volume member of
the dioctyl phthalates, the common name for a group of related phthalate
esters. 
 
Di(2-ethylhexyl) phthalate, (C.A.S. 117-81-7) also commonly called bis
(2-ethylhexyl) phthalate, is a colorless, oily liquid with a slight odor.
Patented in 1933, it is primarily used as one of several plasticizers in
polyvinyl chloride (PVC) resins for fabricating flexible vinyl products.
These PVC resins have been used to manufacture teething rings, pacifiers,
soft squeeze toys, balls, shower curtains, raincoats, adhesives, polymeric
coatings, components of paper and paperboard, defoaming agents, enclosures
for food containers, animal glue, surface lubricants, flexible devices for
administering parenteral solutions, and other products that must stay
flexible and uninjurious for their lifetime. It is also used to manufacture
vinyl gloves used for medical examinations and surgery. 

As a non-plasticizer, di(2-ethylhexyl) phthalate is used as a replacement for
polychlorinated biphenyls (PCBs) in dielectric fluids for electric [should
probably read 'electrolytic'] capacitors. It is also used as a solvent in
erasable ink, an acaricide for use in orchards, an inert ingredient in
pesticides, a component of cosmetic products, and a vacuum pump oil; it is
used to detect leaks in respirators and to test air filtration systems. 

Chemical properties: 
~~~~~~~~~~~~~~~~~~~~
Di(2-ethylhexyl) phthalate is insoluble in water, miscible with mineral 
oil
and hexane, and soluble in most organic solvents. It is easily dissolved in
body fluids such as saliva and plasma.  Di(2-ethylhexyl) phthalate is a
combustible liquid; it may burn, but does not readily ignite. It produces
poisonous gas in a fire. When heated to decomposition, it emits acrid smoke. 

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Colorless, oily liquid - used to manufacture vinyl gloves - vacuum pump oil -
insoluble in water, soluble in most organic solvents, etc., etc.... by
putting two and two together, I got (hopefully) 4 ..... 

A few years ago, before we changed to oil free pumps (for this very reason),
I would have blamed the vacuum pump oil which was probably responsible for
some early cases of contamination. 

I am now convinced that the last type of gloves we bought - vinyl conductive
gloves from an electronic component supplier - was the source of this
plasticizer.  In preparing a previous system where we never had any
contamination and did not vacuum bake either, we were using latex rubber
gloves (non-conductive unfortunately) but vinyl gloves for the last two
systems when disaster struck again!  Our cleaning routine normally ended with
a laboratory soap and water wash followed by a hot distilled water rinse,
yielding a very nice clean result.  I since tried methods of finishing with
neat alcohols (GR grade), as seen on my USA trip but was never happy with the
result.  Even taken fresh from the bottle, it would leave an oily layer
visible on a shiny surface of the first part I had in it.  I now realize that
the problem arises as I take parts in and out of solvents, using these
gloves.  The plasticizer then gets dissolved in the solvent, contaminating it
and the parts that follow. 

I also found out that the boiling point of this substance is 384C at ambient
pressure which will definitely be much lower under vacuum - as we saw during
the mass spectrometry run.  This also explains why vacuum baking is such a
good idea. 

I would like to hear what your experiences/comments are on this - private
e-mails or through the list. 

May I perhaps conduct a quick survey through the list?  (Private e-mail or
otherwise). 

1) Have you had/still have similar CCD/dewar contamination problems before? 
2) What solvents are used in your cleaning procedure?
3) What type of gloves do you use?  
4) Do you know if they are free from plasticizers? 
5) Do you ever get your gloves wet in solvent while handling dewar parts?  
    If not, how do you handle them - stainless steel tongs, pliers & tweezers? 
6) Do you vacuum bake parts/completed systems after cleaning? 
    If yes, in a vacuum oven or heating tape?  
    Typical temperatures of systems, metals, non-metals, o-rings?

Your response (even just to some of the questions) would be much appreciated. 

Thanks
Willie

                       Mr. W.P. Koorts ( wpk at saao.ac.za )    
                     South African Astronomical Observatory 
                   PO Box 9   Observatory  7935  South Africa
                 Tel.(27) (21) 447 0025  Fax.(27) (21) 447 3639
               World   Wide   Web  ( Work ) http://www.saao.ac.za
             World Wide Web ( Personal ) http://www.saao.ac.za/~wpk/
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