[Reproduced] ICP-MS: Key Steps to Control Contamination and Achieve Low Detection Limits

Ⅰ. Select a suitable laboratory analysis environment
Laboratories performing trace element analysis in industries such as environmental, food, pharmaceutical and clinical often do not require special clean laboratory environments to achieve the required method detection limits (MDL). But even a common trace element analysis laboratory should be designed and constructed in such a way as to minimize sources of elemental contamination.
In integrated circuit manufacturing and the high-purity chemicals industry, elements are often measured at ppt and sub-ppt levels, so semiconductor laboratories often have dedicated cleanrooms installed. Cleanrooms are classified according to the number of particles of various sizes between 0.1 ~ 5 microns per unit of laboratory air volume. For example, an International Organization for Standardization (ISO) Type 3 laboratory (equivalent to the earlier US Federal Standard 209E [FS209E] Type 1) can contain up to 8 1-micron particles and 1000 0.1-micron particles per cubic meter of air. ISO Class 7 laboratory (FS209E Class 10,000) contains up to 83,200 particles of 1 micron per cubic meter.

ISO Class 1~4 cleanrooms are expensive to build and maintain, and their rated classification can only be maintained through strict access controls and work practices. A lower cost alternative is to install the ICP-MS in a Class 10,000 cubicle located in a Class 10 laboratory. Another option is to place the autosampler in a clean protective enclosure, such as a high-efficiency particulate air (HEPA) filtered laminar flow hood. Preparation of samples and standards can be performed in the hood, avoiding contamination of vials and solutions when opened on the bench.

Ⅱ. Limit particle pollution sources
In a typical laboratory, one of the important sources of sample contamination is particulate material in the air. Therefore, when looking for possible sources of sample contamination, it is necessary to eliminate the source of particles in the laboratory as much as possible. Common sources of particles in laboratories include air conditioning units (especially those with overhead vents), corroded bare metal surfaces, printers, personal computers (PCs), and circulating water chillers, especially air-cooled heat exchangers. Dirt and dust from shoes, clothing, and personal items entering the lab can also contribute to contamination.

The contribution of these sources to pollution can be eliminated or reduced through simple steps. Examples include placing water recirculators in cubicles; printing with remote equipment; and sticky mats next to entry doors that can significantly reduce the amount of dust people bring into the lab on their shoes. Using appropriate materials in laboratory activities can also help reduce contamination. For safety reasons, gloves are often used when handling corrosive acids, where powder-free nitrile gloves will help minimize particle contamination. In conclusion, with a little thought and attention to how laboratory services, equipment and personnel are managed, particle contamination in the laboratory can be minimized and the high cost of cleanroom installation and operation can be reduced.

Ⅲ. Avoid glass! Minimize the background of elements in plastic containers

The following recommendations apply to laboratories that typically analyze samples for ICP-MS prepared and stabilized in dilute acid solutions. If other solvents are used, the details of the cleaning and rinsing regimen will need to be adjusted appropriately.
Vials, pipette tips, and other labware that come into contact with sample solutions can be significant sources of contamination. Acidic solutions should not be prepared or stored in glassware, even if it has been pre-cleaned, because the acid will extract metals from the glass. Basic solvents such as ammonium hydroxide (NH4OH) and tetramethylammonium hydroxide (TMAH) will also extract metals from glass containers. Although plastic utensils are much cleaner than glass, products with metallic additive pigments should still be avoided. Clear plastic vessels made of materials such as polypropylene (PP), low-density polyethylene (LDPE), polyethylene terephthalate (PET), or fluoropolymers (PTFE, FEP, and PFA) are recommended, For optimum chemical resistance and lowest contamination levels. New labware should be pickled before use to remove surface contamination and manufacturing residue.

Nanjing Binglab PFA vials and graduated cylinders, volumetric flasks, reagent bottles are inexpensive, essentially free of metal contamination, and are Class A graduated for sample preparation. Many laboratories prepare standards and dilute samples gravimetrically. The conical bottom centrifuge tubes can stand upright on a balance or bench without being placed in a sample rack due to the surrounding skirt.

Ⅳ.  Implement effective laboratory utensil cleaning, storage and use procedures

If laboratory workflow and sample throughput permit, it is best to soak vials and tubes in a covered clear plastic box filled with dilute acid (eg 0.1% HNO3) before use. These plastic containers are available at relatively low cost from grocery stores. This pre-cleaning method helps remove manufacturing residues, such as mold release agents, which may contain metals, including Al and Zn. More aggressive cleaning may of course be required to achieve the lowest background levels for ultratrace analysis. Containers such as sample vials should be rinsed three times in ultrapure water before use.

The same pre-cleaning method can also be used for pipette tips, although the clear tips provided in the tip box are dust-free and pre-cleaning is usually not required unless ultra-low level analysis is required. Acid tanks should not be heavily contaminated and can usually last a year or more before needing to be replaced with ultrapure water or acid.

Similar clear, airtight plastic boxes can be used for cleaning and storing ICP-MS sampling system components, ideally separate containers for different types of sampling system components, such as separate containers for standard and inert sampling systems. Sampling systems, such as quartz spray chambers and torches, can be soaked in acid and then dried before use. Interface cones can be sonicated in ultrapure water or a diluted cleaning agent such as Citranox, then dried and stored in an airtight container. For heavily contaminated cones, fine abrasive polishing powder and a pointed cotton swab (Q-tip) can be used for treatment, followed by sonication in ultrapure water.

Ⅴ. Use appropriate quality reagents for analysis

High-quality deionized water (18 MΩ.cm) is essential to maintain the low elemental background levels required for trace analysis, especially for common contaminants such as Na, Al, and Fe. Elements B and Si are more difficult to remove for ion exchange systems, so special attention should be paid to background levels of these elements. An increase in B or Si background may indicate that the ion exchange column of the ultrapure water system needs to be replaced.

The quality of reagents such as acids needs to be adapted to the level of trace elements being measured, but in general, using a better quality acid will reduce the time spent dealing with contamination. A viable alternative to obtaining high-quality acids is to purchase common reagent grade acids and purify them in the laboratory using subboiling distillation.

When using high-quality concentrated acids to prepare samples, calibration standards, and quality control standards (QCS), a small amount of concentrated acid should be poured into a perfluoroalkoxyalkane (PFA) bottle before pipetting into a sample or standard bottle . This will help avoid contaminating the bottle with acid. Once the high-quality concentrated acid has been used up, its PFA bottle can be rinsed and dried, and then makes an excellent metal-free container for subsequent use. These bottles can be used to prepare and store diluents, and can even be used for standard addition analysis at low levels, ie, directly into reusable vials.

Ⅵ. Summary

These practical tips from experienced ICP-MS analysts in the semiconductor industry have been shown to reduce errors caused by contamination. By implementing a few simple procedures and ensuring that reagents and consumables are suitable for the intended analysis, ICP-MS users can minimize the possibility of trace element contamination affecting data quality.