Procedure of making my samples.

i Weighed 1 g of dextran with a molecular weight of 1000Da, add 4 mL of water, and dissolve in a 550C water bath. After dissolution of dextran properly i added 0.15g of NaBr and let it dissolve completely now balanced it's ph to 9-10 by using 20% NaOH and then 2ml of NaCLO was added; keep the reaction pH basically unchanged until the addition of the oxidant is completed, and the additional time is maintained at about 1 h. After all the addition is complete, maintain the temperature of the water bath at 550C, and react for 3.5 h. The oxidized dextran is subjected to a complex reaction at a temperature of 55°C. The mass fraction of 20% FeCl3 solution added slowly therein will reduce the pH (dextran: ferric chloride is 1: 2.5). At the same time, 15% Na2CO3 solution was added dropwise to maintain the pH at 2.8-3.5. Adjust the pH to about (8-11) with NaOH. When the two additions are complete (about 1 h), keep the reaction for 2hours at (70-90)0c. Then carry out alcohol precipitation with absolute ethanol and wash several times. After alcohol precipitation, the obtained product is dried in an oven below 50oC.

Weigh 0.1 g of iron dextran and add 1 mL of Mobile phase water(Weigh 7.1 g of Na2SO4 solid and 0.2 g of NaN3 powder and dissolve them with 1 L of liquid phase water. After the solid powder is completely dissolved, filter it with a sand core filter to obtain the filtrate. Then the filtrate was ultrasonicated for more than 25 min for later use). After dissolving, add 50 ul of HCl, 300C water bath for 15 min. Afterwards, filter with a 0.45 btm filter membrane, and take the subsequent filtrate as the test solution. Referring to the method of the Chinese Pharmacopoeia its chromatographic conditions were made, and first took the dextran series standard 20 microliters of product inject liquid chromatograph, record retention time tT, and to.

P.S I have tried differently concentration, time and ph. and water as mobile phase also but everytime it shows me diffrent results.

what should i do to get desired molecular weight?