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I am a biologist currently doing field work where I need to quickly dry leaf samples using silica gel crystals. I use the orange indicating kind that are 2-4mm in size. The setup is like this. I stuff 1-2 leaves in a manila paper envelop which I drop in a ziplock bag containing ~1/8th cup of silica gel. Then I squeeze out most of the air before sealing the ziplock. The end goal here is to extract DNA from dried leaves when I am back in the lab.

I have had varied results in terms of drying and looking to achieve the maximum efficiency. Recently it occurred to me whether squeezing air out is actually counterproductive to fast drying.

So my question is, whether presence of sufficient air necessary for silica gel to absorb moisture from leaves? Essentially I am creating partial vacuum inside the ziplock. Is that keeping silica from doing its job?

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  • $\begingroup$ Welcome to Biology.SE! I don't think you can create any sort of vacuum by squeezing the air out of a bag — the pressure inside and outside will be identical. What you have done is minimized the volume of air inside the bag. I would guess that the placement of the silica gel relative to the leaf and the porosity of the paper would be the most important factors in how rapidly the moisture would be transferred. $\endgroup$
    – tyersome
    Commented Jul 20, 2019 at 23:44
  • $\begingroup$ @tyersome I agree, the proximity of beads to leaf seems to be pretty important for drying. It was mainly for this reason that I started squeezing air out so the beads wouldn't move as much. I try to get them to cover the envelop. Next, I will try using tea bags which should allow much rapid exchange of vapor. $\endgroup$
    – cryptic0
    Commented Jul 20, 2019 at 23:48
  • $\begingroup$ That sounds like a good strategy — making sure the leaves are covered by silica gel on both sides should also help. The waxiness of the leaf cuticle could also be a significant barrier — could you try crushing the leaves or dipping them in solvent? $\endgroup$
    – tyersome
    Commented Jul 21, 2019 at 0:09
  • $\begingroup$ @tyersome Crushing leaves is quite impractical as I am often collecting upwards of 60 specimen on a given day. Moreover, contamination between samples is an important concern. The less contact I make with leaves, the better. I work with poplar species in the mountain west USA which tend to have sticky balsam on cuticle, which likely is also slowing the drying process down. $\endgroup$
    – cryptic0
    Commented Jul 21, 2019 at 0:30

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