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Environmental Contamination

Overview

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 µg/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 µm and that normal metropolitan area particle counts have been reported to be 53,000,000/m3 for particles greater than 0.5 µm.

Pb found in 1 mL of Acid
Acid Blank 2
Covered for 2 hrs on bench 1, 1.5
Uncovered for 2 hrs on bench 5.5, 6.2
Covered in hood for 3 hrs 4.5
Uncovered in hood for 3 hrs 25

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 µm.

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

Diagram 8.1: Clean Room Layout (Top View)

Diagram 8.1: Clean Room Layout (Top View)

Diagram 8.2: Clean Room Layout (Front View)

Diagram 8.2: Clean Room Layout (Front View)

Diagram 8.3: Clean Room Layout (Side View)

Diagram 8.3: Clean Room Layout (Side View)

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.


Table 8.2: Analytical Results for Trace Impurities in Ultra-Pure Nitric Acids2
(Concentrated in Conventional Chemical and Clean Rooms)


NaCaFeZnPb
A) Conventional chemical room Found value (ng/mL) 5.4 2.2 2.4 0.45 0.29
RSD(%) from 9 samples A) 13 14 63 20 38
RSD(%) in 5 days B) 11 13 13 10 12
(B) Clean room Found value (ng/mL) 0.84 0.8 1.3 0.051 0.038
RSD(%) from 9 samples 40 41 31 61 63
Reference value (ng/mL) C) 0.39 0.2 0.56 <0.05 <0.01
Difference of the value between (A) and (B) 4.6 1.4 1.1 0.4 0.25

Table 8.3: Analytical Results for Trace Impurities in Ultra-Pure Water Samples2
(Concentrated in Conventional Chemical and Clean Rooms)

RSD(%) in 5 days B)1113131012

NaCaFeZnPb
A) Conventional chemical room Found value (ng/mL) 3.2 1.9 2.5 0.2 0.14
RSD(%) from 9 samples A) 11 20 48 60 45
(B) Clean room Found value (ng/mL) 0.45 0.27 0.17 0.025 0.013
RSD(%) from 9 samples 110 120 100 60 240
Reference value (ng/mL) B) <0.005 <0.005 <0.01 <0.005 <0.01
Difference of the value between (A) and (B) 2.8 1.6 2.3 0.18 0.13

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

Diagram 8.4: Evaporation Chamber Designs1

Diagram 8.4: Evaporation Chamber Designs1