定性分析和定量分析:它们各自告诉我们什么？
定性分析和定量分析:它们各自告诉我们什么?
The discipline of analytical chemistry consists of qualitative analysis and quantitative analysis. The former deals with the identification of elements, ions, or compounds present in a sample (we may be interested in whether only a given substance is present), while the latter deals with the determination of how much of one or more constituents is present. The sample may be solid, liquid, gas, or a mixture. The presence of gunpowder residue on a hand generally requires only qualitative knowledge, not of how much is there, but the price of coal will be determined by the percent of sulfur impurity present.
分析化学学科包括定性分析和定量分析。前者解决样品中所存在元素、离子和化合物的鉴别问题(我们感兴趣的仅是一种给定的物质是否存在)，而后者要解决测定已存在的一种或多种组分含量是多少的问题。检测样品可能是固体、液体、气体，也可能是混合物。手上的火药残留通常只需要定性的了解即可,并不需要知道残留量是多少，但煤炭的价格则由其中硫杂质的百分含量所决定。
Qualitative tests may be performed by selective chemical reactions or with the use of instrumentation. The formation of a white precipitate when adding a solution of silver nitrate to a dissolved sample indicates the presence of chloride. Certain chemical reactions will produce colors to indicate the presence of classes of organic compounds, for example, ketones. Infrared spectra will give “fingerprints” of organic compounds or their functional groups.
定性鉴别可能通过选择性的化学反应或者使用仪器完成。当把硝酸银溶液滴加到一份溶解样品中，生成白色沉淀就说明了氯离子的存在。特定的化学反应会产生颜色，指示某一类有机化合物的存在，如酮类化合物。红外光谱可以给出有机化合物或官能团的“指纹图谱”。
A clear distinction should be made between the terms selective and specific:
● A selective reaction or test is one that can occur with other substances but exhibits a degree of preference for the substance of interest.
● A specific reaction or test is one that occurs only with the substance of interest.
对选择性和专属性这两个术语应该做以明确的区分：
•选择性反应或鉴别是指可以和其他物质发生反应但对敏感物质显示一定程度的偏好。
•专属性反应或鉴别是指仅与敏感物质发生反应。
Unfortunately, few reactions are specific but many exhibit selectivity. Selectivity may be achieved by a number of strategies. Some examples are:
●Sample preparation (e.g., extractions, precipitation)
●Instrumentation (selective detectors)
●Target analyte derivatization (e.g., derivatize specific functional groups with detecting reagents)
●Chromatography, which provides powerful separation
可惜虽然许多反应呈现出选择性，但极少数反应具有专属性。通过许多途径可以实现选择性，例如：
•样品处理(如萃取和沉淀)
•使用仪器(选择性检测器)
•目标分析物衍生化(如用检测试剂衍生化特定的官能团)
•色谱法，可达到充分分离。
For quantitative analysis, a history of the sample composition will often be known (it is known that blood contains glucose), or else the analyst will have performed a qualitative test prior to performing the more difficult quantitative analysis. Modern chemical measurement systems often exhibit sufficient selectivity that a quantitative measurement can also serve as a qualitative measurement. However, simple qualitative tests are usually more rapid than quantitative procedures. Qualitative analysis is composed of two fields: inorganic and organic. The former is usually covered in introductory chemistry courses, whereas the latter is best left until after the student has had a course in organic chemistry.
对于定量分析，我们通常要知道样品组成的来历(就像我们知道血液中含有葡萄糖)，否则在更具难度的定量分析前，分析工作者先需要进行定性实验。现代化学检测系统往往具有较好的选择性，定量检测方法同样适合于定性检测。然而，简单的定性鉴别常常比定量的过程更为快捷。定型分析由无机和有机两个领域组成，前者通常包含于入门式的化学课程中，而后者最好在学生学完一门有机化学课程后再去涉猎。
In comparing qualitative versus quantitative analysis, consider, for example, the sequence of analytical procedures followed in testing for banned substances at the Olympic Games. The list of prohibited substances includes about 500 different active constituents: stimulants, steroids, beta-blockers, diuretics, narcotics, analgesics, local anesthetics, and sedatives. Some are detectable only as their metabolites. Many athletes must be tested rapidly, and it is not practical to perform a derailed quantitative analysis on each. There are three phases in the analysis: the fast-screening phase, the identification phase, and possible quantification. In the fast-screening phase, urine samples are rapidly tested for the presence of classes of compounds that will differentiate them from “normal” samples. Various techniques include immunoassays, gas chromatography, and liquid chromatography. About 5% of the samples may indicate the presence of unknown compounds that mayor may not be prohibited but need to be identified. Samples showing a suspicious profile during the screening undergo a new preparation cycle (possible hydrolysis, extraction, derivatization), depending on the nature of the compounds that have been detected. The compounds are then identified using the highly selective combination of gas chromatography/mass spectrometry (GC/MS). In this technique, complex mixtures are separated by gas chromatography, and they are then detected by mass spectrometry, which provides molecular structural data on the compounds. The MS data, combined with the time of elution from the gas chromatograph, provide a high probability of the presence of a given detected compound. GC/MS is expensive and time consuming, and so it is used only when necessary. Following the identification phase, some compounds must be precisely quantified since they may normally be present at low levels, for example, from food, pharmaceutical preparations, or endogenous steroids, and elevated levels must be confirmed. This is done using quantitative techniques such as spectrophotometry or gas chromatography.
比较定性分析和定量分析，我们不妨以奥林匹克运动会对违禁药物的检验为例了解分析过程的步骤。禁止药物目录中包含了大约500种不同的活性成分:兴奋剂类、类固醇，β阻断剂、利尿剂、麻醉剂、止痛剂、局部麻醉剂和镇静剂。有些只有以代谢物的形式才能被检测。许多运动员必须快速被检测，对每个人做细致的定量分析是不可行的。分析过程中有三个阶段:快速筛选期、鉴别期以及可能需要的定量期。在快速筛选期，对尿样进行快速检测，找出其中区别于正常样品的各类组分,所使用的技术包括免疫分析、气相色谱及液相色谱。大约5%的样品会检测出未知化合物，不论这些物质被禁止与否，都需要做进一步鉴别。在筛选过程中显示出可疑情况的样品，需要根据已检出化合物的性质进行新一轮的处理过程(可能水解、萃取或衍生化)，然后使用高选择性的气相色谱一质谱联用技术(GC/MS)对该化合物进行鉴定。在该技术中，复杂混合物通过气相色谱得到分离，然后再通过可提供化合物的分子结构信息的质谱进行检测。结合质谱数据和气相色谱的洗脱时间，我们对确认给定的待测化合物的存在就有更大的把握。GC/MS昂贵、费时，因此在必要时才会使用。经过鉴定阶段以后，一些化合物需要准确定量。这些可能 源于如食物、药品或内源性类固醇的化合物，在正常情况下即以很低的浓度存在 的水平也需要确定。这可以通过分光光度法或气相色谱等定量技术来解决。
