Selection of molecular weight (MWCO) and width of dialysis bag

Selection of molecular weight of dialysis bag:

Because the dialysis membrane is composed of spongy cross-linked polymers, the molecular weight cutoff (MWCO) representing the pore size of the membrane is an indirect parameter to measure the membrane separation performance. More precisely, the molecular weight of the membrane can be defined as the molecular weight of the solute with a retention rate of at least 90%.

Since the permeability of solute depends on the molecular shape, hydration degree, ionic charge and polarity, we suggest that the molecular weight of dialysis bag should be less than half of the molecular weight of the intercepted substance, the smaller the better, or more than twice the maximum molecular weight of the substance to be permeated, the larger the better.

Note: The manufacturer's latest research suggests that the molecular weight of the intercepted substance should be 1-2 times of the molecular weight of the selected dialysis bag, so the effect is better; The molecular weight of the selected dialysis bag is 1-2 times of the molecular weight of the material to be removed, and the effect is better.

In addition, the following points should be paid attention to when selecting the molecular weight of dialysis bag:

★ The effective size of most solute molecules will be affected by the pH value and ionic strength of the solution.

★ When selecting * MWCO for various applications, it is necessary to use a variety of membranes for testing.

★ In order to make the dialysis rate as fast as possible, the membrane selected for use should have the largest MWCO without causing a large loss of required substances.

Selection of dialysis bag width:

The flat width of dialysis bag depends on the sample volume and dialysis capacity. The smaller dialysis tube can perform dialysis faster; The larger dialysis tube has a longer diffusion distance and the dialysis is slower. For ease of use, it is recommended to use a dialysis bag with a total length of about 10-15 cm (including the closing clip and the top space). In short, the more dialyzed samples, the wider the dialysis bag to be selected, and vice versa. Refer to the following figure for the relationship between the diameter and width of dialysis bag:

The most common usage of dialysis bag:

When using, one end is fastened with a rubber band or string, or it can be clamped with a specially made dialysis bag clamp, the other end is filled with water, and the other end is slightly pressurized with your finger to check that there is no leakage before it can be loaded into the solution to be dialyzed. Generally, one third to one half of the space should be left to prevent the water outside the bag and buffer solution from excessive entering the bag to burst the bag when the dialysis small molecular weight is large. When protein solution with high salt content is dialyzed overnight, it is normal to increase its volume by 50%. In order to speed up dialysis, in addition to changing the dialysate for many times, magnetic stirring can also be used. The container for dialysis should be larger, and large beakers, large cylinders and plastic buckets can be used. For dialysis with a small volume of solution, put a section of glass rod with two ends burned or glass tube with two ends sealed in the bag to make the dialysis bag sink below the liquid level.