1. The necessity of water sample pretreatment In unfiltered samples, changes in the chemical form distribution of heavy metals in the sample may occur due to the interaction between the particles and other substances dissolved in the sample. Researchers have found that the adsorption-desorption equilibrium time of heavy metals in a mixture of sediment and water is very fast, generally no more than three days, and the maximum adsorption occurs at pH=7.5 . After sampling, any change in solution equilibrium, the adsorption sites provided by the particles will provide a path for the migration of the metal form, and under certain conditions, it is possible to desorb the adsorbed metal. Usually for trace element or organic analysis, the particulate matter in the water sample must first be removed by filtration or centrifugation ( if the pollutant component in the particle is determined, this part of the sample is collected ) , then the protective agent is added and the water sample is placed. Store in a non-contaminated container and store at a suitable temperature to prevent loss, degradation or morphological changes in the active ingredient. High bacterial concentrations accompanied by the presence of deposits also result in loss of water soluble metal form. The growth of bacteria and algae, including photo synthesis and oxidation, will change the content of C0 2 in the water sample, which will lead to changes in pH . The change in pH often leads to precipitation, changes in chelation or adsorption behavior, and oxidation of metal ions in solution. Restore effect. Due to the unpredictable nature of bacterial growth and reproduction in stored samples, the earlier the filtration after sampling, the better. If the time is postponed to a few hours, the sample is preferably cryopreserved or acidified to inhibit bacterial growth. 2. Test instrument selection         Using a 0.45 μ m microporous film can be easily dissolved and particulate matter separate areas, by membrane filtration liquid may also contain microorganisms and bacteria micelles 0.1 ~ 0.001 μ m and 0.001 μ m is set smaller than dissolve in water Minute. 0 . 45 μ m membrane filter can filter out all of the vast majority of bacteria and phytoplankton. Continuous filtration can sometimes cause clogging of the membrane, which usually requires replacement of the membrane or pressure filtration.          When using a filter instrument, you should pay attention to the material in contact with the solution, and also consider the type of filter ( vacuum or pressurized ) . Glass filters use rubber stoppers to cause contamination, and vacuum filtration systems using borosilicate glass are generally selected. Prior to filtration, the filter equipment is washed with dilute acid and can usually be soaked in 1 to 3 mol / L hydrochloric acid.         The surface of the untreated filter membrane easily adsorbed cadmium and lead in water, but when used to filter river water, no change in the concentration of the above elements was observed. The use of untreated membranes to filter mercury-containing samples from seawater samples can result in losses of 10 % to 30 %. However, with the treatment of treated glass fibers, the loss of mercury can be reduced to less than 7 %. The general filter is washed with 20 mL of 2 mol / L HNO 3 and then with 50 - 100 mL of distilled water. The beaker or flask must be rinsed with distilled water. And removing the first 10 ~ 20mL filtrate collected. For the deep water filtration of the ocean. The filter membrane is preferably first soaked in dilute nitric acid.         Pressure filtration or vacuum filtration is two methods that are commonly used. The pressure filtration speed is fast, suitable for filtering river water samples containing a large amount of sediment, if using φ Objective To study the application of spiral fixation for central venous catheter in clinical nursing. Central Venous Catheter(CVC) Cvc Catheter,Medical Cvc Catheter,Single Lumen Central Venous Catheter,Triple Lumen Central Venous Catheter Zhejiang Hisern Medical Technology Co., Ltd , https://www.hisernmedical.com
Workers engaged in water quality analysis should be aware that unless the water sample is analyzed immediately, proper pretreatment must be performed before the water sample is stored. The pretreatment is mainly based on the different requirements of the water sample to be tested, and the filtration method is a commonly used pretreatment method.