I work on fruit, and I have occasionally had some problems when lyophilising samples which have a high sugar content and/or are very ripe. In such cases, your lyophilised sample is rather gummy instead of perfectly dry.

In these cases, I found that I could improve this simply by grinding my gummy lyophilised sample in liquid nitrogen, using for instance a domestic blender. I always succeeded in obtaining a dry, non-sticky powder.

I hope this helps. Best wishes,

Isabel

You need to prepare samples correctly. Before you put the the samples on freeze drier, samples should be in frozen state in the freezers before they can be freeze-dried. Then samples should be placed in containers which allow as much of the sample to be exposed to the vacuum as possible. Petri dishes are ideal for sediment. Freeze drying time also has influence on getting samples more like powder. For colloid, usually they take 2 days at -40 degree celcius.

Hi Khaled,

immerse your flask into a container of liquid nitrogen first for 5 minutes to make sure your sample is completely freezed and then place it in the freeze dryer over night. you know how it works, No ?

hope this helps 

Hasan

Hi, I just want to add. For freeze-drying of nanoparticles/powder (if what you meant the samples are in nanoparticles or powder form before you do the freeze-drying), the freeze-dried nanoparticles should have certain desirable characteristics, including: the preservation of physical and chemical characteristics of the product, long term stability and an acceptable relative humidity. Or in other words, for obtaining product with high quality, you have to control: the formulation, freeze-drying process and storage condition. For freeze-drying the samples, you need to do the freeze-drying process including: 1. freezing step. You can do the freezing by the slow freezing (like I said before) or fast freezing (using liquid nitrogen,like Hasan said before. But the cost is high for liquid nitrogen, but it is fast, and the main point is that liquid nitrogen must be fully evaporated from samples before freeze-drying (to make sure the safety, because I don't know LN is fully evaporated or not during freeze-dry)). 2. Annealing to optimize freeze-drying process. 3. Primary drying and secondary drying.

For further reading, you can read Freeze-drying of nanoparticles: Formulation, process and storage considerations by Abdelwahed et al.

In addition to previous answers, after drying, leave the sample for 3 days inside dissector over NaOH.

As Isabel said I experienced that with samples with high sugar content (i.e. fruits).

We solve that problem with a previous grinding step of the samples with a criogenic mill we have (but other cheaper solution could be a domestic mill with liq nitrogen).

Then, the freeze-dryed material is perfect.

Hi Khaled,

for PEG-peptide conjugtes we always need to do freeze-drying several times over night and with time, they get more and more powder-like, while in the beginning they stick to the flasks wall as a net-like structure, as you described. So you could freeze-dry several times, re-dissolve with water after each drying step.

We dissolve the conjugates in a round-bottom flask in an appropriate amount of water (100-200 mg in approx. 125 ml, 300-500 mg in approx. 250 ml) and then freeze the sample in liquid Nitrogen by rotating, e.g. with the help of the rotary evaporator (easy-cheezy way) or by hand (you need to be patient...) - this creates a thin layer on the flask wall with a high surface area/surface to volume ratio so that the water can sublimate quickly. You wil hear a rather loud bubbling noise and then silence from the ntrogen bath when the flask has been frozen completely. Do not use flasks with 1 l or higher volume for later freeze-drying steps, the vacuum will we not so sufficiant as for smaller flasks. We use 1 l flasks e.g. only for the first two freeze-drying steps, then disslove and transfer to 500 ml flasks or smaller, depends on the total sample amount. I strongly do not recommend to put the flask simply in a freezer - in that case you create a massive block and it takes ages to dry this (as I know by personal experience).

If you want to play around with different freezing speed (which I personally would not do right from the beginning), you can prepare EtOH/dry ice mixture or other cooling mixtures - I attached a link how to prepare them. Still use rotation to freeze the sample (higher surface area), which will take more time for elevated freezing temperature.

Due to the fact we purify the conjugates by preparative HPLC, where have 0.1 % Trifluoro acetic acid (TFA) in the mobile phase, we add appropriate amounts of HCl to the liquid (1/10th by mole, because HCl is a 10x stronger acid then TFA) to kick out the TFA - it evaporates quickly as acid (CF3-COOH), but practically not as a salt (e.g. CF3-COONa), which would always "melt" the sample. Maybe you have comparable components in your sample, that's why I mention that.

Good luck!

http://en.wikipedia.org/wiki/Cooling_bath

The stickiness may be happening because the compounds are hygroscopic and are absorbing water from the air after removal from the freeze dryer.  I blow nitrogen into the valve as I release the vacuum to prevent this, and then quickly seal the flask.

maybe you could try to add vacuum in order to make solvent evaporation faster and more completely

If your extract is of aqueous, then you can freeze dry and make it powder, if it solvent, try for spray drying (hot condition)

It may be your compounds are hygroscopic and blowing nitrogen instead of air into the flask when the vacuum is released may help.  If you want to use an absorbent, then diatomaceous earth could be used.