XRF080023 环境检测方法─ EPA 6200 Method 标准土壤底泥检测方法

环境检测方法─ EPA 6200 Method标准土壤底泥检测方法

 

     根据美国环境保护署（美国环境保护局）的EPA 6200 Method，以携带式X - 射线萤光光谱仪（Field portable X - ray fluorescence，以下简称 FPXRF）来分析并推估土壤和污泥中元素浓度。6200 Method法检测土壤和底泥中 26 种金属元素之侦测限值。有些常见的元素没有被列在其中，因为它们是属於”轻（Light）”的元素，无法使用 FPXRF 来分析，这些元素包括锂、铍、钠、镁、铝、矽和磷。一般原子序在 16 以上的元素皆可使用 FPXRF 仪器来检测和定量。

 

l         XRF原理

      X-光萤光分析仪 (X-ray Fluorescence Spectrometer, XRF )系利用X-光束照射试片以激发试片中的元素，当原子自激发态回到基态时，侦测所释放出来的萤光，经由分光仪分析其能量与强度後，可提供试片中组成元素的种类与含量，具有快速、非接触、非破坏性及多元素分析等特点。

    其原理是利用放射源(如Cd109)或高电压激发光管(非辐射放射源)放射出X-ray，激发待测物内层的电子，此时为保持能量平衡，分子外围电子补入内层电子的空缺。因外围电子能量较高，故外为电子进入内层轨域时，放出特定的萤光能量，藉由能量的大小推知待测物的量。

 

一. 设备与材料

(一）FPXRF 光谱仪∶

一般而言 FPXRF 光谱仪应至少包含四个主要组件，以下针对各组件作一概述∶

1.X -射线放射源∶产生 X - 射线的放射源，如以放射性同位素放射源（如 Fe - 55、Cd - 109、Am – 241 或Cm – 244 等）或以 X - 射线管（以高电压加速电子撞击阳极靶材如铜、钼或银等产生电子激发）当作 X - 射线的放射源。

2.样品承接装置∶现场直接检测或以样品采集後再放置在样品承装器中分析。

3.X - 射线侦测器∶包括固态式侦测器或气体填充式比例计数侦测器等。固态式侦测器包括碘化汞（HgI₂）、矽汞和锂矽Si（Li），碘化汞（HgI₂）侦测器可使用低必v热电式冷却装置来控制操作温度在低於室温下，矽汞侦测器也可经由热电式Peltier效应来冷却之，而锂矽侦测器则必需被冷却至最少 － 90 ℃，其冷却方式可使用液态氮或经由热电式 Peltier 效应来冷却之。

4.数据处理系统∶包括测量脉冲振幅之多频路分析仪（MCA），可将讯号处理成 X - 射线能量光谱，然後再计算出样品中元素浓度的分析仪，及分析数据显示和储存系统。

（二）备用电池或充电器。

（三）聚乙烯（Polyethylene，PE）制样品承装器∶直径 31 mm 到 40 mm，有轴环或同级品（适用在 FPXRF 仪器）。

（四）X -射线端视窗上用之薄膜∶Mylar^TM、Kapton^TM、Spectrolene^TM、Polypropylene 或同级品；厚度 2.5 至 6.0 μm。

（五）研钵和碾槌∶玻璃，玛瑙，或氧化铝材质，用来磨碎土壤和底泥样品。

（六）样品瓶∶玻璃或塑胶制品，用来贮存样品。

（七）筛网∶60 筛目（0.25 mm），不锈钢，尼龙或同级品，用来制备土壤和底泥样品。

（八）小铲子∶用来弄平土壤表面及采集土壤样品用。

（九）塑胶袋∶承装土壤及混合土壤使之均匀化。

（十）烘箱∶烘乾土壤和底泥样品用的烘箱或烤箱。

二.   SUMMARY OF METHOD

1.      The FPXRF technologies described in this method use either sealed radioisotope sources or x-ray tubes to irradiate samples with x-rays. When a sample is irradiated with x-rays, the source x-rays may undergo either scattering or absorption by sample atoms. This latter process is known as the photoelectric effect. When an atom absorbs the source x-rays, the incident radiation dislodges electrons from the innermost shells of the atom, creating vacancies. The electron vacancies are filled by electrons cascading in from outer electron shells. Electrons in outer shells have higher energy states than inner shell electrons, and the outer shell electrons give off energy as they cascade down into the inner shell vacancies. This rearrangement of electrons results in emission of x-rays characteristic of the given atom. The emission of x-rays, in this manner, is termed x-ray fluorescence.

Three electron shells are generally involved in emission of x-rays during FPXRF analysis of environmental samples. The three electron shells include the K, L, and M shells. A typical emission pattern, also called an emission spectrum, for a given metal has multiple intensity peaks generated from the emission of K, L, or M shell electrons. The most commonly measured x-ray emissions are from the K and L shells; only metals with an atomic number greater than 57 have measurable M shell emissions.

Each characteristic x-ray line is defined with the letter K, L, or M, which signifies which shell had the original vacancy and by a subscript alpha (α), beta (β), or gamma (γ) etc., which indicates the higher shell from which electrons fell to fill the vacancy and produce the x-ray. For example, a K_α line is produced by a vacancy in the K shell filled by an L shell electron, whereas a K_β line is produced by a vacancy in the K shell filled by an M shell electron. The K_α transition is on average 6 to 7 times more probable than the K_β transition; therefore, the K_α line is approximately 7 times more intense than the K_β line for a given element, making the K_α line the choice for quantitation purposes.

The K lines for a given element are the most energetic lines and are the preferred lines for analysis. For a given atom, the x-rays emitted from L transitions are always less energetic than those emitted from K transitions. Unlike the K lines, the main L emission lines (L_α and L_β) for an element are of nearly equal intensity. The choice of one or the other depends on what interfering element lines might be present. The L emission lines are useful for analyses involving elements of atomic number (Z) 58 (cerium) through 92 (uranium).

An x-ray source can excite characteristic x-rays from an element only if the source energy is greater than the absorption edge energy for the particular line group of the element, that is, the K absorption edge, L absorption edge, or M absorption edge energy. The absorption edge energy is somewhat greater than the corresponding line energy. Actually, the K absorption edge energy is approximately the sum of the K, L, and M line energies of the particular element, and the L absorption edge energy is approximately the sum of the L and M line energies. FPXRF is more sensitive to an element with an absorption edge energy close to but less than the excitation energy of the source. For example, when using a cadmium-109 source, which has an excitation energy of 22.1 kiloelectron volts (keV), FPXRF would exhibit better sensitivity for zirconium which has a K line energy of 15.77 keV than to chromium, which has a K line energy of 5.41 keV.

2.      Under this method, inorganic analytes of interest are identified and quantitated using a field portable energy-dispersive x-ray fluorescence spectrometer. Radiation from one or more radioisotope sources or an electrically excited x-ray tube is used to generate characteristic x-ray emissions from elements in a sample. Up to three sources may be used to irradiate a sample. Each source emits a specific set of primary x-rays that excite a corresponding range of elements in a sample. When more than one source can excite the element of interest, the source is selected according to its excitation efficiency for the element of interest.

For measurement, the sample is positioned in front of the probe window. This can be done in two manners using FPXRF instruments, specifically, in situ or intrusive. If operated in the in situ mode, the probe window is placed in direct contact with the soil surface to be analyzed. When an FPXRF instrument is operated in the intrusive mode, a soil or sediment sample must be collected, prepared, and placed in a sample cup. The sample cup is then placed on top of the window inside a protective cover for analysis.

Sample analysis is then initiated by exposing the sample to primary radiation from the source. Fluorescent and backscattered x-rays from the sample enter through the detector window and are converted into electric pulses in the detector. The detector in FPXRF instruments is usually either a solid-state detector or a gas-filled proportional counter. Within the detector, energies of the characteristic x-rays are converted into a train of electric pulses, the amplitudes of which are linearly proportional to the energy of the x-rays. An electronic multichannel analyzer (MCA) measures the pulse amplitudes, which is the basis of qualitative x-ray analysis. The number of counts at a given energy per unit of time is representative of the element concentration in a sample and is the basis for quantitative analysis. Most FPXRF instruments are menu-driven from software built into the units or from personal computers (PC).

The measurement time of each source is user-selectable. Shorter source measurement times (30 seconds) are generally used for initial screening and hot spot delineation, and longer measurement times (up to 300 seconds) are typically used to meet higher precision and accuracy requirements.

FPXRF instruments can be calibrated using the following methods: internally using fundamental parameters determined by the manufacturer, empirically based on site-specific calibration standards (SSCS), or based on Compton peak ratios. The Compton peak is produced by backscattering of the source radiation. Some FPXRF instruments can be calibrated using multiple methods.

 

资料来源
美国环境保护署

 

    XRF 在钗h研究报告中已被证实为有效的「筛选工具」，已渐渐的替代传统化学分析方式，而成为主要「分析工具」之一。科迈斯集团对於XRF 制造商也积极对软硬体技术、标准样品进行研发新的设备，针对「取样点的选择」、「取样数量」及「数据可信度」做更广泛的研究与讨论定义新的测试方法。