What is a sample preparation process?

Sample preparation refers to the series of actions taken to transform a sample from its original state to a form that is suitable for analysis in a laboratory setting. 具体操作流程会因样品类型和分析方法而存在显著差异。然而，样品制备的核心目标在于：从样品基质（样品主体）中分离出目标分析物（感兴趣的组分），去除任何污染物或干扰物质，并将分析物转化为与所用分析技术兼容的形式。

Generally, different steps are involved in a sample preparation:

Sampling / sample division: The first step involves collecting a representative portion of the material or substance to be analyzed. This is crucial for ensuring that the results are reflective of the whole sample.
Drying / embrittlement: Size reduction and homogenization of sticky, moist, elastic or tough samples can be challenging. Drying makes the sample more brittle and facilitates the grinding process. Embrittling elastic or tough samples, which cannot be pulverized at room temperature, has the same effect.
清洁/筛分 This step is necessary to remove any substances that might interfere with the analysis. Techniques like chromatography or the use of selective adsorbents can be employed to purify the sample.
Size Reduction / homogenization: For solid samples, this might involve grinding or crushing to reduce the particle size, making the analytes more accessible for further processing.
Extraction: This step involves separating the analytes from the sample matrix. Various extraction methods can be used, including solvent extraction, solid-phase extraction (SPE), and solid-phase microextraction (SPME), depending on the nature of the sample and analytes.
浓缩/压实 通常，分析物需要被浓缩才能被分析仪器检测到。这可能涉及溶剂蒸发、离心或过滤等步骤，以减少样品体积并提高分析物的浓度。
Derivatization: In some cases, especially for gas chromatography (GC) or certain types of spectroscopy, the analytes may need to be chemically modified to make them more volatile, detectable, or analytically stable. This process is known as derivatization.
Transfer to Analysis Vessel: 最后，将制备好的样品转移至适合分析仪器的容器中，例如色谱瓶、自动进样器容器或用于光谱分析的样品杯。

样品制备所用的具体技术和设备取决于样品类型（如固体、液体、气态）、基质复杂度、所需分析物以及分析技术（如色谱、光谱、质谱）。正确的样品准备对于获得准确、可靠和可重复的分析结果至关重要。Retsch提供用于样品制备不同步骤的仪器：

采样与样品分割、干燥或脆化、清理与筛分、尺寸减小与均质化、提取以及浓缩和压实。样品制备过程的每个环节将在后续章节中介绍。

1. Sampling and sample division

To ensure reproducibility in sample preparation, it is essential to extract a sub-sample that truly represents the bulk material. This means the sub-sample must share the same properties as the bulk. Given that most samples are inhomogeneous mixtures, care must be taken to avoid segregation due to varying particle sizes, especially during transport. If the entire sample isn't processed, a representative portion must be taken. The quantity of the sample is crucial; it must be sufficient for analysis and proportionate to the total sample size and grain size. These factors dictate the minimum quantity needed for the sub-sample to accurately reflect the bulk.

Some important questions should be clarified in advance:

a. Which quantity is required to ensure representativeness of the original sample?
b. From which part of the original material should the sample be taken?

分样仪

A. 1. Which quantity is required?

某些行业特定标准为正确的采样过程提供了指导方针，例如煤炭行业的DIN 51701-2:1985标准。该标准包含如下公式：

G [千克] = 0.07 [kg/mm] × z [mm]

该公式表明从最大粒径为“z”的散装样品中需提取多少样品量“G”才能获得具有代表性的样本。以最大粒径为5厘米的煤样为例，计算如下：

G [kg] = 0.07 kg/mm × 50 mm

G = 3.5 kg

因此，最大粒径为50毫米的样品需提取至少3.5千克才能确保代表性。对于煤炭以外的其他物料，应考虑不同的密度系数。

B. 2. From which part is the sample taken?

从原始样品中提取的子样品必须准确代表整个堆料，这需要仔细考虑采样位置（堆料的中部、顶部、底部、内部或外部）。堆体内部水分含量等因素可能存在差异，从而影响分析结果。例如内部区域可能比外部区域更湿润。此外，由于分层现象，顶部与底部颗粒成分可能不同。对船运或火车运输等大体积物料取样存在挑战。为获得具有代表性的子样本，需从多个位置采集样本并混合。另一种方法是从生产单元的物料流中直接取样。

1.1. 分样仪

Once the sample is in the laboratory, the sample amount may be too large for the subsequent steps in the sample preparation process. Therefore, further sample division may be required. The selection of the division method and the instrument depends on the sample material and quantity. Dry, free flowing samples can be fed via vibratory feeders to rotary tube dividers and sample dividers with a rotating dividing head.

分样仪 PT 100

旋转管式分样仪 PT 200

旋转分样仪 PT 300

1.2. 槽式分样仪

Sample splitters are used for materials with a low flowability. Manual random sampling is only acceptable if the sample is homogeneous with regards to material and grain size. However, without preliminary examination, this is difficult to ascertain.

槽式分样仪 RT 6.5 - RT 75

槽式分样仪 RT 100

The importance of sampling is demonstrated in the figure: Even if the analysis is carried out correctly, random sampling (e.g. with a scoop) leads to varying results which are not reproducible although the samples come from the same initial material. As shown, three different samples taken from the same initial material show variations of up to 20 % for the fraction below 2 mm. Therefore, it is essential that sampling is carried out with utmost care.

Random sampling with scoop; three correct sieve analyses lead to three different results

Professional sample dividers with a marginal standard deviation should be used for the extraction of representative sub-samples. The figure shows the qualitative sampling errors of the different methods. It can clearly be seen that rotary tube sample dividers produce the smallest qualitative variation (A). They achieve the highest degree of reproducibility and are clearly superior to all other methods.

Qualitative sampling errors (standards deviations) of the different sampling methods

通过使用分样器进行正确的样品分样，可大幅降低标准偏差——例如在分析塑料样品的含水量时。

2. Why does drying or embrittlement help in sample preparation?

Grinding moist or wet sample materials can lead to undesirable side effects. Such materials are prone to clogging the mill's ring and bottom sieves, potentially causing a machine blockage. This results in material loss during the sample preparation process and requires significant time and effort for cleaning. There are exceptions, however, such as colloidal grindings in ball mills that necessitate adding a liquid. Fresh fruits and vegetables can be processed in knife mills without losing material. Typically, moist samples must be dried (like leaves) before size reduction. The chosen drying method and temperature must not alter the sample's properties, particularly volatile components. These samples are usually air-dried at room temperature. RETSCH's TG 200 is designed for rapid and gentle drying, employing fluidized bed drying, akin to industrial dryers, and can dry many products in just 5 to 20 minutes.

快速干燥仪 TG 200

Drying ovens from Carbolite Gero

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Embrittlement (with liquid nitrogen or dry ice) helps to improve the sample´s breaking behavior, which is beneficial for sample preparation. Therefore, temperature-sensitive materials, such as some types of plastics, have to be cooled directly before they can be subjected to preliminary or fine size reduction. One way is to embrittle the sample in liquid nitrogen before grinding. At a temperature of -196°C even soft rubber becomes so hard and brittle that it can be ground without problems. Another possibility is to mix the sample with dry ice. However, materials which must not become moist should not directly be treated with cooling agents. The reason for this is that the steam in the air is frozen and is precipitated as water when it unfreezes. Cooling agents should not be used in closed grinding tools as evaporation causes overpressure.

Cooling a jar in liquid nitrogen

3. 金属的分离与筛分

工业废料、可回收废料和二次燃料等样品通常含有无法用实验室碾磨粉碎的金属成分。相反，钢钉或铁螺丝等金属物体会损坏研磨工具，导致磨仪性能大幅下降。因此，在进入样品准备的下一阶段：研磨前，必须先分离金属成分。如有需要，组分需分别评估。如果不想要的颗粒如金属部件大小与需要分析的颗粒不同，也可以通过筛分去除。在这里，Retsch筛分机是首选设备。RETSCH筛筛和筛分机不仅覆盖了广泛的测量范围。由于筛分动作和筛子尺寸不同，RETSCH 筛系列适用于几乎所有散装物料。Retsch分析筛和筛分仪能够产生精确且可复现的结果，并符合DIN EN ISO 9000及以下测试材料监测的要求。

分析筛分仪

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作用原理

4. Size reduction and homogenization

实验室研磨仪专为粉碎或研磨小型材料样品而设计，适用于成分分析、物理特性分析或其他测试目的。该设备能将样品制备成均匀的粒度或质地，确保精确评估的一致性。市面存在多种样品研磨仪型号，每种均针对特定材料和特殊用途定制。选择合适的研磨仪需依据材料特性及样品处理的预期结果。对于原子吸收光谱（AAS）、近红外光谱（NIR）、电感耦合等离子体光谱（ICP）或X射线荧光光谱（XRF）等分析技术，必须将样品充分均质化至适宜的分析细度。唯有通过一致的样品制备流程，才能确保分析结果的可靠性与准确性。RETSCH提供种类齐全的样品研磨仪与粉碎仪，可对几乎所有物质进行粗碎、细磨及超细粉碎处理。我们种类繁多的研磨仪器及配件，确保设备能在实验室测试前实现无污染、可靠的样品制备。

RETSCH Grinding Mills

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5. Retsch benefit for the extraction step

从研磨样品中提取化学分析物是分析化学、环境科学和生物化学中至关重要的样品制备步骤，旨在分离特定化合物以进行详细分析。该过程对于监测环境污染（如农药污染）、评估土壤养分水平、检测有害物质以及开展各类科学研究具有重要意义。

萃取溶剂与技术的选择至关重要，需根据目标分析物的性质及土壤样品基质特性确定。常用技术包括固相萃取（SPE）、液-液萃取（LLE）、固相微萃取（SPME）及超临界流体萃取（SFE）等。

提取过程通常涉及将目标分析物溶解于适宜溶剂中，使其从固体基质及其他非目标组分中分离。提取后，分析物可能经过进一步纯化和浓缩步骤，随后采用气相色谱（GC）、液相色谱（LC）或质谱（MS）等技术进行定量与鉴定，从而识别目标化合物。

冷冻混合球磨仪 MM 400

QuEChERS法（意为“快速、简便、廉价、高效、稳健且安全”）简化了农药残留分析的样品制备流程。测试表明，该方法所得结果与其他方法相当。均质化过程中需避免样品过热，因部分农药具有挥发性。冷却样品不仅能防止挥发，还能增强材料的破碎性能，从而获得更佳的细度与均匀度。提取过程需取10克食品或土壤样品，用10毫升乙腈进行萃取。为消除色谱图中的鬼峰，萃取时需添加盐混合物（通常为氯化钠与硫酸镁按1:2比例配制）。为使农药转移至有机相，需将样品与乙腈盐溶液振荡1至3分钟。可选用实验室研磨机（如RETSCH MM 400）在50毫升Falcon离心管中以最高30赫兹频率搅拌混合物，确保充分且可重复的混合效果，这对萃取过程至关重要。

6. 浓缩与压实

在样品制备的关键步骤中，Retsch的Pellett Press PP40压片机可提供有力支持。该设备能将粉末压制成表面光滑的片剂，这正是X射线荧光（XRF）分析常需的样品形态。XRF分析的精确度取决于X射线束与样品表面的相互作用，而与表面状态相关的多个因素会影响数据质量：

光滑表面能确保X射线与样品均匀交互。粗糙或不规则表面会导致X射线发生不均匀散射与吸收，从而产生不准确或不一致的结果。

光滑表面有助于实现多次测量或样品间的可重复性。表面纹理差异会引入测量不一致性，导致结果难以比对。在块状样品中，光滑且精细抛光的表面能最大限度地减少颗粒尺寸变化的影响。粗糙表面上的较大颗粒可能无法被X射线束完全穿透，导致其元素组成在分析中被低估。

对于固体样品，样品与探测器窗口之间的良好表面接触至关重要。光滑表面能确保更紧密的接触，这对发射低能X射线的光元素尤为关键——因空气间隙或接触不均易吸收此类辐射。

粗糙表面上的凸起点会形成阴影区域，导致该区域未能充分暴露于X射线束下，从而造成样品元素组成的分析不完整。

压片机 PP 40

RETSCH的仪器在所有的工业领域中均有应用。在生产控制和质量监控的框架下，RETSCH的仪器广泛用于各种分析程序中的样品前处理和颗粒粒径分析。
鉴于这些领域内对仪器设备的要求日益增高，RETSCH有其明确的产品哲学。借用古希腊大哲学家亚历士多德的一句话说，就是：

产品选择

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Sample Preparation Process - 常见问题

Why is sample preparation important?

在使用先进的科学设备和仪器分析样品之前，必须对样品进行适当的处理和制备。这一初步步骤是整个分析过程中的重要环节，它有助于防止污染、提高准确性并最大限度地降低结果失真的风险。

What is the first step in sample preparation?

样品制备的第一步是采集。简而言之，就是从较大样本源中提取具有代表性的样本。采集对象可以是细胞悬浮液样本，也可以是食品样本。关键要点在于最大限度减少样本损失、避免污染并确保样本一致性。