实验室用水的种类和区别

水是实验室内一个常常被忽视但至关重要的试剂。实验室用水有那些种类？能达到什么级别？不同实验对水的要求有那些？

实验室常见的水的种类：

1、蒸馏水（Distilled Water ）：

实验室最常用的一种纯水，虽设备便宜，但极其耗能和费水且速度慢，应用会逐渐减少。蒸馏水能去除自来水内大部分的污染物，但挥发性的杂质无法去除，如二氧化碳、氨、二氧化硅以及一些有机物。新鲜的蒸馏水是无菌的，但储存后细菌易繁殖；此外，储存的容器也很讲究，若是非惰性的物质，离子和容器的塑形物质会析出造成二次污染。

2、去离子水（Deionized Water ）：

应用离子交换树脂去除水中的阴离子和阳离子，但水中仍然存在可溶性的有机物，可以污染离子交换柱从而降低其功效，去离子水存放后也容易引起细菌的繁殖。

3、反渗水（Reverse osmosis Water）：

其生成的原理是水分子在压力的作用下，通过反渗透膜成为纯水，水中的杂质被反渗透膜截留排出。反渗水克服了蒸馏水和去离子水的许多缺点，利用反渗透技术可以有效的去除水中的溶解盐、胶体，细菌、病毒、细菌内毒素和大部分有机物等杂质，但不同厂家生产的反渗透膜对反渗水的质量影响很大。

4、超纯水（Ultra-pure grade water）：

其标准是水电阻率为18.2M Ω-cm 。但超纯水在TOC 、细菌、内毒素等指标方面并不相同，要根据实验的要求来确定，如细胞培养则对细菌和内毒素有要求，而HPLC 则要求TOC 低。评价水质的常用指标：

1、电阻率（electrical resistivity）：

衡量实验室用水导电性能的指标，单位为M Ω-cm ，随着水内无机离子的减少电阻加大则数值逐渐变大，实验室超纯水的标准:电阻率为18.2M Ω-cm 。

2、总有机碳（Total Organic Carbon ，TOC ）：

水中碳的的浓度，反映水中氧化的有机化合物的含量，单位为ppm 或 ppb。

3、内毒素（Endotoxin ）：

革兰氏阴性细菌的脂多糖细胞壁碎片，又称之为“热原”，单位cuf/ml。

1、自来水（Tapwater ）

Tap water is usually of uncontrolled quality, may have seasonal variations suchas level of suspended sediment depending on the source (municipal reservoir,river, well), may contain other chem-icals purposely added to drinking water(chlorine, ﬂuoride), and is generally unsuitable for use in important experiments. Tapwater is ﬁne for washing glassware but should always be followed by a rinsewith a higher-grade water (distilled, deionized, etc.).

2、蒸馏水（DistilledWater ）

Distillation generally eliminates much of the inorganic

con-taminationandparticularly sediments present in tap water feedstock. Itwill also help reduce the level of some organic con-taminants in the water.Double distilling simply gives a slightly higher grade distilled water, butcannot eliminate either inorganic or organic contaminants. Distilled water is often produced in large stills that serve an

entiredepartment, or building. The quality of the water is dependent on how well theequipment is maintained. A signiﬁcant stir occurred within a large university ’sbiochemistry department when the ﬁrst mention of a problem with the housedistilled water was a memo that came out from the maintenance department thatstated: “We would like to inform you that the repairs have been made to thestill serving the department. There is no longer any radium in the water.” Thenext day, a follow -up memo was issued that stated:“Correction—there is nolonger any sodium in the dis-tilled water.”

3、去离子水（DeionizedWater ）

Deionized water can vary greatly in quality depending on the type and ef ﬁciencyof the deionizing cartridges used. Ion exchange beds used in home sy stems, forinstance, are used primarily to reduce the “hardness” of the water usually dueto high levels of divalent cations such as magnesium and calcium. The resin bedconsists of a cation exchanger, usually in the sodium form, which releasessodium into the water in exchange for removing the diva-lent ions. (Rememberthat when you attempt to reduce your sodium intake!) These beds therefore donot reduce the ionic content of the water but rather exchange one type of ionfor another.

Laboratory deionizing cartridges are usually mixed-bed cartridges designed toeliminate both anions and cations from the water. This is accomplished bypreparing the anion-exchange bed in the hydroxide (OH-) form and thecation-exchange resin in the acid (H+) form. Anions or cations in the water(including monovalent) are exchanged for OH-or H+,

respectively, which combineto form neutral water. Any imbalance in the removal of the ions can result in apH change of the water. Typically water from deion-izingbeds is slightly acidic, often between pH 5.5 to 6.5. The deionizing resins can themselves increase the organiccon-taminant level in the water by leaching of resin contaminants, monomer, andso on, and should always be followed by a bed of activated carbon to eliminatethe organics so introduced.

4、18M Ω水 (ReverseOsmosis/MilliQTM)

The highest grade of water available is generally referred to as 18MW water.This is because when the inorganic ions are completely removed, the ability ofthe water to conduct electric current decreases dramatically, giving aresistance of 18 MW.Com-mercial systems that produce this grade of waterusually apply a multiple-step cleanup process including reverse osmosis,mixed-bed ion exchangers, carbon beds, and ﬁlter disks for particulates. Somemay include ﬁlters that exclude microorganisms,

resulting in a sterile waterstream. High-grade 18 MW water tends to be fairly acidic—near pH 5. Necessary pH adjustments of dilute buffer solutions preparedusing 18 MW water could cause discrep-ancies in the ﬁnal ionic concentration ofthe buffer salts relative to buffers prepared

using other water sources.

5、WhenIs 18MΩ Water Not 18MΩWater?

Suppose that your research requires 18 MW water, and you pur-chased the systemthat produces 500ml/min instead of the 2L/min version. If your resear ch doesn’trequire a constant ﬂow of water, you can connect a 20L carboy to your system tostore your pris-tine water. Bad Move.

18MW is not the most inert solvent; in practice, it is very aggres-sive. Waterprefers the presence of some ions so as your 18 mW water enters the plasticcarboy, it starts leaching anything it can out of the

plastic,contaminatingthequality of the water.The same thing happens if you try to store the water inglass. 18mW water loves to attack glass, leaching silicates and other ionsfrom the con-tainer. If you need the highest purity water, it’s best not tostore large quantities, but rather prepare it fresh.

For the same reason, the tubing used to transfer your high-grade water shouldalways be the most inert available, typically TeﬂonTM or similar materials. Never use highly plasticized ﬂexible plastic tubing.

Absolutely avoid metalssuch as copper or stainless steel, as these almost always guarantee some levelof contaminants in your water.

6、水的初始pH 值是多少？

As mentioned above, the initial pH of typical laboratory-gradedistilled and deionized water is often between 5.5 and 6.5.Check your water supply from time to time, particularly when deionizing bedsare changed to ensure that no major change in pH has occurred because ofseasonal variation or improperly conditioned resin beds.

Although the initial pH of laboratory water may be slightly acidic, the goodnews is deionized water should have little or no buffer capacity, so yournormal pH adjustment procedures should not be affected much. Payparticular attention if your buffer concentrations are very low (

7、水中有哪些有机物质：

The answer to this important question depends on the upstream processing of thewater and the initial water source. Municipal water drawn from lakes or streamscan have a whole host of organics in them to start with, ranging from petroleumproducts to pesticides to humic substances from decaying plant material tochlorinated species like chloroform resulting from the chlorina-tionprocess.Well water may have lower levels of these

contami-nants (since the water hasbeenﬁltered through lots of soil and rock, but even groundwater may containpesticides and chlori-nated species like trichloroethylene depending on landuse near the aquifer.

Municipal processing will remove many organic contaminants from the tap water,but your in-lab water puri ﬁer is responsible for polishing the water to a grade ﬁt for experimental use. Most commercial systems do a good job

of that, but asmentioned pre-viously, care must be taken to not introduce contaminants afterthe water has been polished. Plasticizers from tubing or plastic storage tanks,monomer or resin components from deionizer beds, and surfactants or lubricantson ﬁlters or other system compo-nents are the most common type of organic to befound in a newly installed system. Another common, yet often overlooked source, is microbialcontamination . In one case, a high-grade water puriﬁermounted on a wall near a window suddenly started showing evidence of organicbackground. Changing the carbon cartridge did not help the situation. Closeinspection of the system showed the translu-cent plastic tubing connecting thereverse osmosis holding tank to the deionizer beds, and ultimately the linesthat delivered the polished water to the spigot, had been contaminated bymicrobial growth. It was surmised that the intense sunlight during part of theday was providing a more hospitable environment for microorganisms to gain afoothold in the system. The clear tubing was replaced with opaque tubing andthe problem disappeared.

In a second instance, a facility changed its water source from wells to a riverdraw-off. This drastically changed the stability of the incoming water quality.During periods of heavy rain, silt levels in the incoming water increaseddramatically, quickly destroying expensive reverse osmosis cartridges in thewaterpuri-ﬁer system. The solution was to install two pre-ﬁltersofdecreas-ing porosity in line ahead of the reverse osmosis unit. The ﬁrst ﬁlterneeded replacing monthly, but the second ﬁlter was good for three to six months.The system functioned properly for a while, but then problems reappeared in thereverse osmosis unit.

Inspec-tion showed heavy microbial contamination in thesecond pre-ﬁlter which had a clear housing, admitting sunlight. After cleaningand sterilizing the ﬁlter unit, the outside of the housing was covered withblack electrical tape, and the microbial contamina-tion problem never returned.

As discussed in Chapter 12, dispensing hoses from water reservoirs resting insinks can also lead to microbial contamination.

8、在水的使用中还有哪些问题？

Leaks

Leaks are sometimes one of the most serious problems that can occur with in-labwaterpuri ﬁcation systems. Leaks come in three kinds, typically. Leaks of the ﬁrst kind start as slow drips, and can be spotted and corrected beforedeveloping into big unfriendly leaks.

Leaks of the second kind are generally caused by a catastrophic failure of asystem component (tubing, valve, automatic shutoff switch, or backﬂush drain).Although highly uncommon, they usually occur around midnight on Fridays so asto maximize the amount of water that can escape from the

system, thereforemax-imizing the resulting ﬂooding in the lab. The likelihood of a leak of thesecond kind seems to increase exponentially with the cost of instrumentation inlaboratories on ﬂoors directly below the lab with the water puriﬁer system.

Leaks of the third kind result when a person places a relatively large vesselbeneath the water system, begins ﬁlling, and walks away to tend to a few minortasks or is otherwise distracted. The vessel over ﬂows, ﬂooding the lab with theextent of the ﬂood depending on the duration of the distraction.

Leaks of the third kind are by far the most common type of leak, and are alsothe most preventable. Locating the water puriﬁ-cation system

immediately abovea sink, so that any vessel being ﬁlled can be placed in the sink, usuallyprevents this type of cata-strophe. If placement above a sink is not possible,locating the water puriﬁcation system in a (relatively) high-trafﬁc orwell-used location in the lab can also

minimize or eliminate the possibility ofmajor spills, since someone is likely to notice a spill or leak.

Leaks of the ﬁrst or second type are highly uncommon, but do occur. The bestprevention is to have the system periodically

inspected and maintained by qualiﬁed personnel, and never have major servicingdone on a Friday. Problems seem to be most likely after the system has beenpoked and prodded, so best to do that early in the week. Then the system can beclosly watched for a few days afterward before leaving it unattended.