Reliable Measurements   • Introduction / Contents • About the Author  Foundations   • 1. Laying the Foundation  Sample Collection   • 2. Planning the Project • 3. Sampling and Sub-sampling  Sample Preparation   • 4. An Introduction to Sample Preparation • 5. Container Material Properties • 6. Container Transpiration • 7. Stability of Elements at ppb Concentration Levels  Contamination   8. Environmental Contamination • 9. Contamination From Reagents • 10. Contamination From the Analyst and Aparatus  Preparation Techniques   • 11. Acid Digestions of Inorganic Samples • 12. Acid Digestions of Organic Samples • 13. Sample Preparation by Fusion • 14. Ashing  Sample Measurement   • 15. ICP-OES Measurement • 16. ICP-MS Measurement  Conclusions   • 17. Method Validation

During the course of a sample preparation, contamination can occur from the environment, the reagents, the apparatus, and even the analyst. In many cases the blank determines the lower limit of detection. The next several parts of this guide deal with contamination.

Environmental contamination is caused by particulate and/or gaseous matter in the air. It has been reported that air in an analytical laboratory can contain up to 200 �g/m³ of particulate matter containing Ca, Si, Fe, Na, Mg, K, Tl, Cu, Mn, and lesser amounts of other elements¹. In addition, this reference states that normal rural area airborne particle counts have been reported to be 1,400,000/m³ for particles greater than 0.5 �m and that normal metropolitan area particle counts have been reported to be 53,000,000/m³ for particles greater than 0.5 �m.

Use the chemical blank as the performance criteria. Dealing with environmental contamination is expensive. Therefore, reduce the blank to the lowest possible value by eliminating contamination from the sample container, the chemist, and sample preparation apparatus. Achieve this by doing the following:

• Separate the sample preparation and handling area from all other areas.
• Eliminate as many foreign objects as possible from the preparation area (i.e. - Motorized objects such as stirring plates).
• Use all-plastic hoods.
• Keep the sample out of the hood area as much as possible.

The effect of the laboratory atmosphere on lead blank levels is shown in Table 8.1.

Hoods, which enhance atmospheric exposure and hence contamination, are necessary for the safety of the analyst. Atmospheric contamination has been reduced significantly through the use of "clean rooms" that use HEPA filtered air. HEPA (High Efficiency Particulate Air) filters are 99.99 % efficient in removing particulates down to 0.3 �m.

A Positive Pressure Filtered Air "Clean" Room is shown in the following architectural diagrams.

The effectiveness of clean rooms in eliminating environmental contamination is illustrated in Tables 8.2 and 8.3 which show a significant reduction in the blank.

For budgets that cannot handle the cost associated with clean room facilities there have been several ingenious designs that are effective in dealing with environmental contamination. Diagram 4.4 illustrates some of these designs using equipment that is relatively inexpensive and easily obtained.

1. Teflon coat or epoxy paint the (hopefully few) metal objects in the laboratory. Epoxy paint the walls. Walls should be washed down weekly and repainted annually.
2. Use special adhesive floor pads for all openings to the laboratory.
3. Use disposable plastic gloves and disposable paper laboratory coats.
4. Vinyl covered floors should be mopped daily. Floors with drains allow for easier daily washing. 

 Part 7 • Part 9