XRF熒光光譜儀制樣方法的原理和依據(jù)

樣品處理是整個(gè)分析檢測(cè)過(guò)程的重要組成部分。其目的是利用各種化學(xué)方法將測(cè)試元素以離子的形式從固（液）態(tài)樣品中定量轉(zhuǎn)移到測(cè)試溶液中。選擇合理的樣品分解方法可以大大簡(jiǎn)化分析程序，大大提高分析方法的適應(yīng)性和準(zhǔn)確性。

設(shè)計(jì)樣品處理的原則是：

①保證樣品中所有被測(cè)元素都定量轉(zhuǎn)移到試液中，即樣品必須*分解；

② 避免在樣品處理過(guò)程中引入干擾元素，同時(shí)有助于去除干擾元素；

③分解方法應(yīng)盡可能簡(jiǎn)單、易操作、經(jīng)濟(jì)、快速、安全，盡量減少對(duì)環(huán)境的污染；

④ 便于批量處理樣品。

要設(shè)計(jì)出滿(mǎn)足這些條件的樣品處理方法，您必須深入了解：

a) 被測(cè)元素及其化合物的理化性質(zhì)；

b) 樣品中被測(cè)元素的含量范圍和出現(xiàn)形式；

c) 樣品基體的組成和性質(zhì)；

d) 使用的最終測(cè)試方法和技術(shù)。

以國(guó)內(nèi)外測(cè)定As、Se、Hg所用樣品的各種處理方法為例

對(duì)于國(guó)內(nèi)外各種樣品中As、Se、Hg的測(cè)定，所采用的樣品處理方法大致分為三類(lèi)：

1) 濕酸/堿分解；

2）密閉系統(tǒng)燃燒法（氧彈燃燒法）；

3）燒結(jié)（半熔）法。

在煤中微量元素的定性和定量檢測(cè)過(guò)程中，樣品前處理往往比檢測(cè)技術(shù)本身更為重要。特別是對(duì)于各種原子光譜技術(shù)，一般需要進(jìn)行液體進(jìn)樣或液體進(jìn)樣，以達(dá)到高精度。為了保證檢測(cè)的準(zhǔn)確性，在充分溶解成分復(fù)雜的煤樣的同時(shí)，要避免被測(cè)元素的揮發(fā)性元素或化合物形式的損失。目前，國(guó)內(nèi)外有以下幾種方法：

(1)直接溶解在密封的坩堝中。

該方法是在密閉容器中用混酸直接分解樣品，在微波爐中加熱消解樣品。具有進(jìn)樣量小、溶解效率高、操作簡(jiǎn)單、安全、易控制、避免揮發(fā)等優(yōu)點(diǎn)；但是直接分解法不可避免地存在分解不*的缺點(diǎn)。該方法常用于檢測(cè)燃燒后煤灰或煤提取物樣品中的微量元素。

(2)低溫灰化法。

樣品采用等離子技術(shù)在 150°C 左右灰化，然后在聚四氟乙烯容器中用混酸分解。該方法是目前*的較好的預(yù)處理技術(shù)，但也存在設(shè)備運(yùn)行成本高、耗時(shí)長(zhǎng)等缺點(diǎn)。

濕酸分解

對(duì)于地質(zhì)樣品的測(cè)定，常使用HNO3-HF-HClO4系統(tǒng)進(jìn)行酸溶。用這種方法分解樣品時(shí)，如果加熱強(qiáng)度稍高，蒸發(fā)過(guò)干，樣品中的部分硒會(huì)因揮發(fā)而流失。原因可能是樣品酸分解產(chǎn)生的Se(ClO4)2可以分解為HCl和SeO2，這兩種化合物可以反應(yīng)生成SeCl2，可以在較低溫度下升華。據(jù)了解，樣品在加工過(guò)程中會(huì)蒸發(fā)至干，樣品中的硒會(huì)損失40%左右。因此，用這種方法分解樣品時(shí)，必須小心控制加熱溫度和時(shí)間，蒸1～2mL透明溶液時(shí)應(yīng)停止加熱。還有利用HNO3-H2SO4體系酸溶解和HNO3-HClO4體系分解。濕法分解操作程序復(fù)雜，酸霧揮發(fā)到大氣中，不利于環(huán)保。另外，對(duì)于煤，由于煤中含有大量有機(jī)物，酸分解不方便。

氧彈燃燒

該方法是將煤樣置于充滿(mǎn)高壓氧的不銹鋼筒中，用電點(diǎn)燃煤樣。煤中的有機(jī)物得到充分燃燒，無(wú)機(jī)礦物也被氧化分解。煤中的硒元素被轉(zhuǎn)化為氧化物。它溶解在吸收液（水或

稀堿）以氣態(tài)形式存在于墨盒中。國(guó)外一些實(shí)驗(yàn)室采用這種方法測(cè)定煤中的硒。優(yōu)點(diǎn)是樣品處理不引入除氧氣外的其他試劑，減少了引入干擾元素的機(jī)會(huì)。缺點(diǎn)是高灰分煤可能分解不*（未*燃燒），導(dǎo)致分析結(jié)果偏低；操作較麻煩，樣品處理效率低；不利于樣品的批量分解。

燒結(jié)（半熔）法

該方法是將與試樣混合均勻，加熱燃燒，使煤中的As和Se氧化成氧化物，再與Na2CO3和MgO反應(yīng)生成鹽和硒酸鹽。然后用HCl溶解燒成物，將As和Se以離子的形式轉(zhuǎn)移到溶液中。以煤為例，反應(yīng)方程式為：

這種方法的優(yōu)點(diǎn)是操作簡(jiǎn)單，易于批量處理和分解樣品。只要正確控制燃燒條件，就可以對(duì)被測(cè)元素進(jìn)行定量轉(zhuǎn)化?？紤]到目前煤中其他元素的分析標(biāo)準(zhǔn)中，采用半熔體分解煤樣的方法很多，分析人員容易接受，工作效率高；國(guó)家標(biāo)準(zhǔn)和美國(guó)標(biāo)準(zhǔn)也采用半熔融法處理樣品。本實(shí)驗(yàn)通過(guò)對(duì)各種處理方法的比較，采用半熔法對(duì)樣品進(jìn)行處理。

為了將被測(cè)元素從燃燒后的樣品中轉(zhuǎn)移到溶液中，燃燒后的物質(zhì)必須用合適的酸溶解。為此，需要對(duì)酸的種類(lèi)和加酸方法進(jìn)行實(shí)驗(yàn)。

酸的種類(lèi)

使用不同的酸處理樣品對(duì)結(jié)果有影響，因此本實(shí)驗(yàn)考察了 HCl、H2SO4 和 HNO3 在燒傷治療中的溶解情況及其對(duì)測(cè)定的影響。

H2SO4：測(cè)試發(fā)現(xiàn)用H2SO4溶解燒焦會(huì)引起CaSO4沉淀。此外，H2SO4 中含有高 As 和 Se，導(dǎo)致空白過(guò)高。如果提前去除，會(huì)增加實(shí)驗(yàn)工作量。因此，不宜選用 H2SO4 作為溶解酸。 .

HNO3：測(cè)試發(fā)現(xiàn)，當(dāng)HNO3溶解樣品時(shí)，其原子吸收信號(hào)低于HCl介質(zhì)。原因可能是HNO3起氧化劑作用，與樣品中的還原性物質(zhì)反應(yīng)生成亞硝酸，而NO2-對(duì)氫化硒的生成有抑制作用。國(guó)外一些實(shí)驗(yàn)室采用HNO3作為測(cè)定氫化硒發(fā)生原子吸收的介質(zhì)，以降低靈敏度為代價(jià)，換取更好的穩(wěn)定性。

HCl：用HCl溶解燒焦是國(guó)內(nèi)外廣泛使用的方法。大多數(shù)氯化物是易溶鹽。 HCl 具有還原性。它是一種還原劑，可將 Se6+ 還原為 Se4+。 HCl中As、Se含量少，試劑空白低，有利于測(cè)定。該方法使用 HCl 來(lái)溶解燃燒的材料。

ilute alkali) in the cartridge in a gaseous form. Some foreign laboratories use this method to measure Se in coal. The advantage is that the sample processing does not introduce other reagents except oxygen, which reduces the chance of introducing interfering elements. The disadvantage is that the high-ash coal may be incompletely decomposed (not completely burned), which can lead to low analysis results; the operation is more troublesome, and the sample processing efficiency is low; it is not conducive to batch decomposition of samples.

Sintering (semi-melting) method

This method uses Aldrin and the sample to mix uniformly, heat and burn, so that the As and Se in the coal are oxidized to oxides, and then react with Na2CO3 and MgO to form arsenate and selenate. Then dissolve the burned material with HCl, and transfer As and Se into the solution in the form of ions. Taking coal as an example, the reaction equation is:

The advantage of this method is that it is easy to operate and easy to process and decompose samples in batches. As long as the burning conditions are correctly controlled, the measured elements can be quantitatively transformed. Considering that in the current analysis standards for other elements in coal, there are many methods that use aldrin semi-melt to decompose coal samples, which are easy for analysts to accept and have high work efficiency; national standards and American standards also use aldrin semi-melt method for processing sample. Through the comparison of various treatment methods, this experiment uses the semi-melting method to treat the samples.

In order to transfer the measured element from the burned sample into the solution, the burned substance must be dissolved with a suitable acid. To this end, experiments should be conducted on the type of acid and the method of adding acid.

Types of acid

The use of different acids to process the samples has an impact on the results, so the experiment investigated the dissolution of HCl, H2SO4 and HNO3 in the treatment of burns and their influence on the determination.

H2SO4: The test found that the use of H2SO4 to dissolve the burnt will cause CaSO4 precipitation. In addition, H2SO4 contains high As and Se, which causes the blank to be too high. If it is removed in advance, it will increase the experimental workload. Therefore, it is not suitable to choose H2SO4 as the dissolving acid. .

HNO3: The test found that when HNO3 dissolves the sample, the atomic absorption signal is lower than that of the HCl medium. The reason may be that HNO3 acts as an oxidant and reacts with the reducing substances in the sample to form nitrous acid, and NO2- has an inhibitory effect on the formation of Se hydride. Some foreign laboratories use HNO3 as the medium for the determination of Se hydride generation atomic absorption, at the cost of reduced sensitivity, in exchange for better stability.

HCl: The use of HCl to dissolve the burnt is a widely used method at home and abroad. Most of the chlorides are easily soluble salts. HCl has reductive properties. It is a reducing agent that reduces Se6+ to Se4+. The content of As and Se in HCl is small, and the reagent blank is low, which is beneficial for the determination. This method uses HCl to dissolve the burnt material.

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