How to prepare 1g G418 powder with 100mg/mL mother liquor and package for storage?

The prepared 100 mg/mL stock solution should be fully dissolved in sterile PBS or ultrapure water, filtered through a 0.22 µm filter, aliquoted into sterile small tubes, and stored at -20°C protected from light. In general, an acceptable decrease in activity may occur within 6–12 months, but repeated freeze-thaw cycles will accelerate loss of potency, so it is recommended to aliquot according to 1–2 weeks of usage, for example, 0.2–1 mL per tube. For routine use, working solutions should not be kept at 4°C for more than 1–2 weeks; discard them if there is obvious darkening of color, precipitation, or drift in the cell killing curve. When preparing the reagent, avoid directly diluting the stock solution with serum-containing medium; prolonged strongly alkaline conditions or repeated light exposure can also affect activity.

Don't compare prices based on the 1 g quote alone; evaluate potency, shipping temperature, aliquot specifications, and COA completeness together. Imported original 1 g products may have long lead times, especially when customs inspection occurs; domestic repackaged or domestic-lot products usually arrive faster, but lot-to-lot potency often fluctuates. If laboratory usage is not large, it is advisable to first purchase a 1 g aliquot for trial, and perform potency titration and cytotoxicity curves during acceptance testing; if screening on a large scale, imported bulk packaging may be more suitable when calculated on an active ingredient basis. Before purchasing, confirm whether cold chain is required, whether a potency test report is available, and whether the sales representative can provide screening references from the same lot.

G418 is a sulfate, and the nominal potency of different brands is often labeled according to the active unit of the active ingredient. The rigorous approach is to convert according to the potency on COA: effective mass=total powder mass x potency/100. For example, with a nominal concentration of 700 µ g/mg, the effective G418 in 1g of powder is about 700 mg. If you want to mix 100 mg/mL of effective mother liquor, you need to dilute about 1.43g of powder to 10mL. If you directly mix 1g/10mL based on the total powder mass, you will get a total powder concentration of 100 mg/mL, and the actual effective concentration is about 70 mg/mL. Many literature do not strictly distinguish the screening concentration. It is recommended to make a killing curve after fixing the brand batch number, and not directly mix different batches.

HEK293 and HeLa do not have an absolutely unified G418 concentration, and can only provide an empirical range: HEK293 commonly maintains a concentration of about 400-800 µ g/mL (based on total powder/manufacturer calibration), while HeLa often starts from 200-600 µ g/mL for lethal curves. The key is to apply different concentration gradients (such as 0, 200, 400, 600, 800, 1000 µ g/mL) after obtaining the new batch number, and observe the concentration that completely kills untransfected cells for 7-14 days. When screening, do not only use the lowest lethal concentration, stable strains can be maintained at 50% to 70% of the screening concentration. After transfection, the cells should be allowed to recover for 24-48 hours before adding medication to avoid misjudgment of transfection damage.