定性分析和定量分析:它們各自告訴我們什么？

定性分析和定量分析:它們各自告訴我們什么?

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.

分析化學(xué)學(xué)科包括定性分析和定量分析。前者解決樣品中所存在元素、離子和化合物的鑒別問題(我們感興趣的僅是一種給定的物質(zhì)是否存在)，而后者要解決測定已存在的一種或多種組分含量是多少的問題。檢測樣品可能是固體、液體、氣體，也可能是混合物。手上的火藥殘留通常只需要定性的了解即可,并不需要知道殘留量是多少，但煤炭的價格則由其中硫雜質(zhì)的百分含量所決定。

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.

定性鑒別可能通過選擇性的化學(xué)反應(yīng)或者使用儀器完成。當(dāng)把硝酸銀溶液滴加到一份溶解樣品中，生成白色沉淀就說明了氯離子的存在。特定的化學(xué)反應(yīng)會產(chǎn)生顏色，指示某一類有機(jī)化合物的存在，如酮類化合物。紅外光譜可以給出有機(jī)化合物或官能團(tuán)的“指紋圖譜”。

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.

對選擇性和專屬性這兩個術(shù)語應(yīng)該做以明確的區(qū)分：

•選擇性反應(yīng)或鑒別是指可以和其他物質(zhì)發(fā)生反應(yīng)但對敏感物質(zhì)顯示一定程度的偏好。

•專屬性反應(yīng)或鑒別是指僅與敏感物質(zhì)發(fā)生反應(yīng)。

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

可惜雖然許多反應(yīng)呈現(xiàn)出選擇性，但極少數(shù)反應(yīng)具有專屬性。通過許多途徑可以實現(xiàn)選擇性，例如：

•樣品處理(如萃取和沉淀)

•使用儀器(選擇性檢測器)

•目標(biāo)分析物衍生化(如用檢測試劑衍生化特定的官能團(tuán))

•色譜法，可達(dá)到充分分離。

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.

對于定量分析，我們通常要知道樣品組成的來歷(就像我們知道血液中含有葡萄糖)，否則在更具難度的定量分析前，分析工作者先需要進(jìn)行定性實驗?，F(xiàn)代化學(xué)檢測系統(tǒng)往往具有較好的選擇性，定量檢測方法同樣適合于定性檢測。然而，簡單的定性鑒別常常比定量的過程更為快捷。定型分析由無機(jī)和有機(jī)兩個領(lǐng)域組成，前者通常包含于入門式的化學(xué)課程中，而后者最好在學(xué)生學(xué)完一門有機(jī)化學(xué)課程后再去涉獵。

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.

比較定性分析和定量分析，我們不妨以奧林匹克運(yùn)動會對違禁藥物的檢驗為例了解分析過程的步驟。禁止藥物目錄中包含了大約500種不同的活性成分:興奮劑類、類固醇，β阻斷劑、利尿劑、麻醉劑、止痛劑、局部麻醉劑和鎮(zhèn)靜劑。有些只有以代謝物的形式才能被檢測。許多運(yùn)動員必須快速被檢測，對每個人做細(xì)致的定量分析是不可行的。分析過程中有三個階段:快速篩選期、鑒別期以及可能需要的定量期。在快速篩選期，對尿樣進(jìn)行快速檢測，找出其中區(qū)別于正常樣品的各類組分,所使用的技術(shù)包括免疫分析、氣相色譜及液相色譜。大約5%的樣品會檢測出未知化合物，不論這些物質(zhì)被禁止與否，都需要做進(jìn)一步鑒別。在篩選過程中顯示出可疑情況的樣品，需要根據(jù)已檢出化合物的性質(zhì)進(jìn)行新一輪的處理過程(可能水解、萃取或衍生化)，然后使用高選擇性的氣相色譜一質(zhì)譜聯(lián)用技術(shù)(GC/MS)對該化合物進(jìn)行鑒定。在該技術(shù)中，復(fù)雜混合物通過氣相色譜得到分離，然后再通過可提供化合物的分子結(jié)構(gòu)信息的質(zhì)譜進(jìn)行檢測。結(jié)合質(zhì)譜數(shù)據(jù)和氣相色譜的洗脫時間，我們對確認(rèn)給定的待測化合物的存在就有更大的把握。GC/MS昂貴、費(fèi)時，因此在必要時才會使用。經(jīng)過鑒定階段以后，一些化合物需要準(zhǔn)確定量。這些可能 源于如食物、藥品或內(nèi)源性類固醇的化合物，在正常情況下即以很低的濃度存在 的水平也需要確定。這可以通過分光光度法或氣相色譜等定量技術(shù)來解決。

醫(yī)學(xué)英語