Date: 16 Oct 1997
From: Brent Newman zool <bnewman@pan.uzulu.ac.za>
To: crust-l@VIMS.EDU
QUESTION:
I am busy attempting to determine the dry weights of several spp of crab and prawn larvae, but I am experiencing several problems. Whether the larvae are freeze dried or oven dried I have serious problems with the hygroscopic nature of the larvae in this dry state and I am battling to get stable readings on the scale. Even if the larvae are held in a dessicator after removal from either drying 'machine' I still find a variability in the weight after set periods. Can anyone provide some advice as to how they have overcome this problem?
Brent Newman
University of Zululand
South Africa
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COMMENTS 1:
Date: 16 Oct 1997
From: "Shiao Y. Wang" <sywang@WHALE.ST.USM.EDU>
To: Brent Newman zool <bnewman@pan.uzulu.ac.za>
In my opinion consistency is more important than absolute accuracy because I'm usually interested in relative differences. Therefore, I let my samples reach equilibrium in a room where the relative humidity is fairly constant before weighing.
Another option is to use a different measurement that is more absolute if possible. Examples include length, total protein, total DNA, etc.
Shiao Y. Wang
University of Southern Mississippi
sywang@whale.st.usm.edu
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COMMENTS 2:
Date: 16 Oct 1997
From: Alan Harvey <aharvey@AMNH.ORG>
To: CRUST-L@VIMS.EDU
I agree with Shiao that consistency is most important.
However, if you want precise weight measurements of freshly, fully dried crabs, what about the following? Weigh a small covered vial (e.g., microcentrifuge tube) filled with water; dry the crab fully, then
immediately place it in the tube. Reweigh the vial; the difference,
obviously, will be the weight of the crab, which can absorb water, but only from the preweighed vial, to its cuticle's content, without affecting the results. Extra work, but should be accurate, I think.
Alan W. Harvey (aharvey@amnh.org)
Assistant Curator of Invertebrates
American Museum of Natural History
Central Park West at 79th Street
New York, NY 10024
(212) 769-5638; fax (212) 769-5783
http://research.amnh.org/~aharvey
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COMMENTS 3:
Date: 17 Oct 1997
From: Tom McRae <mcraet@DEAKIN.EDU.AU>
To: CRUST-L@VIMS.EDU
Since the problem is absorption of water from the atmosphere, can you circumvent it by intentionally adding a known mass of water to the larvae after drying? I visualise this as simply weighing an empty vial, adding your larvae and re-weighing, freeze drying, adding say 200 microlitres of water, weighing the vial again and subtracting the
200mg weight of water. The success of this would depend on the accuracy of the volume of water and the rate of evaporation of water.
Tom McRae
Aquatic Science and Natural Resources Management
Deakin University, Warrnambool Campus
P O Box 423,
Warrnambool, Vic. 3280
Australia.
Ph: + 61 (0)3 55 633 461
Fax: + 61 (0)3 55 633 462
E-mail: mcraet@deakin.edu.au