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• Negative Contamination — The potential for the measured analyte concentration to be <br />artificially low because of losses that are due to precipitation, volatilization, or absorption. <br />• Positive Contamination — The potential for the measured analyte to be artificially high <br />because of leaching or introduction of foreign matter into the sample by particle fallout or <br />gaseous air contaminants. <br />• Cross -contamination — The introduction of part of one sample into a second sample. <br />B.6.5. Equipment Selection Considerations <br />Because each sampling situation is unique, the sampling technician may have to modify <br />equipment and applications to ensure that a representative sample is collected and to maintain <br />the sample's physical and chemical integrity. No one particular device will fit every situation. <br />The sampling technician shall understand the various sampling devices and their advantages <br />and disadvantages to effectively use them to collect samples. The following items shall be <br />considered when selecting a sampling device: <br />• Potential impact of the device on sample integrity <br />• Method of sample delivery <br />• Flow controllability <br />• Depth of sampling interval <br />• Ease of operation, cleaning, and maintenance <br />• Reliability and durability <br />• Portability of the device <br />• Initial cost and operational cost <br />There are four basic types of sampling devices: <br />• Grab mechanisms <br />• Suction -lift mechanisms (not currently used by NS) <br />• Positive -displacement mechanisms <br />• Submersible pumps <br />A discussion of the uses, advantages, and disadvantages of these devices follows. <br />B.6.6. Grab Mechanisms <br />Grab mechanisms consist of bailers and dip -type samplers. These devices are the oldest and <br />simplest for collection of liquid samples. They can be made of various materials and can be <br />used to collect liquid samples from almost any source. Table 5 presents the advantages and <br />disadvantages of a grab -type mechanism. <br />Daub & Associates, Inc. <br />Page B-14 Natural Soda LLC 2015 S A P <br />