3.2 Laboratory Preparation Prior to Sample Analysis
83
Rinse water should meet the specifications of the American Society for
Testing and Materials for type III water (http://www.astm.org/Standard/
standards-and-publications.html). A typical water deionization system that
produces type III water includes, in sequence, a carbon removal tank, 1-μm filtration, and two mixed-bed cation-anion removal tanks. To ensure consistent
water quality, specific conductance (or resistance) of the water should be monitored between the two primary treatment modules, in this case the mixedbed tanks. If the specific conductance exceeds the preset limit of 1.0 μS/cm,
an indicator light (which is normally on) is deactivated. The system is checked
daily, and if the indicator light is off, the first tank is removed and replaced by
the second tank. A new tank is then placed in the second position. By monitoring between the two tanks, the initial tank serves as a first-level treatment
and the second tank serves as a polisher. By switching the polisher to the first
position when the first-level tank no longer meets the standard, you extend
the life span of the tanks and, most importantly, prevent substandard water
from being used. These tanks degrade gradually, so the two-stage approach
is needed to maintain a consistent level of water quality. The filter is replaced
with every tank change, and the carbon tank is replaced depending on the
volume of water treated and the organic carbon concentration of the influent.
Once every 6 months is a typical replacement frequency. Note that appropriate
methods for tank replacement are to some degree equipment specific. The procedure described here is one possible approach to producing thoroughly rinsed
bottles. Other procedures may also be acceptable.
To ensure that the wash acid does not itself become a contaminant,
repeated rinsing is followed by leaching the bottle with DIW. This is done by
filling the rinsed bottle with DIW and storing it for 24 h or longer (Table 3.1).
This step is necessary because acid and other contaminants (such as those
from a previous sample) can migrate into the plastic matrix of the bottle wall.
Over time, the contaminants can slowly leach out and change the sample
concentration. The level of contamination caused by this process is usually
low but can be sufficient to cause measurable increases in low-nutrient, lowionic-strength samples. Simple rinsing does not necessarily eliminate this
type of contamination.
Finally, each bottle is filled with DIW before shipping the bottles to the project location. Measurement of specific conductance of DIW stored in sample
bottles, with an acceptance criterion of 1.2 μS/cm or less, provides QA for the
bottle-cleaning procedure. We recommend using an acceptance criterion of
1.2 μS/cm or less, but we recognize that some laboratories use a somewhat
higher criterion, as high as about 2.0 μS/cm. Specific conductance testing can
be done on selected sample bottles that are treated as sample blanks.
Aliquot bottles that have had acid added for sample preservation are
more likely to have contamination from bottle leaching than those that are
83
Rinse water should meet the specifications of the American Society for
Testing and Materials for type III water (http://www.astm.org/Standard/
standards-and-publications.html). A typical water deionization system that
produces type III water includes, in sequence, a carbon removal tank, 1-μm filtration, and two mixed-bed cation-anion removal tanks. To ensure consistent
water quality, specific conductance (or resistance) of the water should be monitored between the two primary treatment modules, in this case the mixedbed tanks. If the specific conductance exceeds the preset limit of 1.0 μS/cm,
an indicator light (which is normally on) is deactivated. The system is checked
daily, and if the indicator light is off, the first tank is removed and replaced by
the second tank. A new tank is then placed in the second position. By monitoring between the two tanks, the initial tank serves as a first-level treatment
and the second tank serves as a polisher. By switching the polisher to the first
position when the first-level tank no longer meets the standard, you extend
the life span of the tanks and, most importantly, prevent substandard water
from being used. These tanks degrade gradually, so the two-stage approach
is needed to maintain a consistent level of water quality. The filter is replaced
with every tank change, and the carbon tank is replaced depending on the
volume of water treated and the organic carbon concentration of the influent.
Once every 6 months is a typical replacement frequency. Note that appropriate
methods for tank replacement are to some degree equipment specific. The procedure described here is one possible approach to producing thoroughly rinsed
bottles. Other procedures may also be acceptable.
To ensure that the wash acid does not itself become a contaminant,
repeated rinsing is followed by leaching the bottle with DIW. This is done by
filling the rinsed bottle with DIW and storing it for 24 h or longer (Table 3.1).
This step is necessary because acid and other contaminants (such as those
from a previous sample) can migrate into the plastic matrix of the bottle wall.
Over time, the contaminants can slowly leach out and change the sample
concentration. The level of contamination caused by this process is usually
low but can be sufficient to cause measurable increases in low-nutrient, lowionic-strength samples. Simple rinsing does not necessarily eliminate this
type of contamination.
Finally, each bottle is filled with DIW before shipping the bottles to the project location. Measurement of specific conductance of DIW stored in sample
bottles, with an acceptance criterion of 1.2 μS/cm or less, provides QA for the
bottle-cleaning procedure. We recommend using an acceptance criterion of
1.2 μS/cm or less, but we recognize that some laboratories use a somewhat
higher criterion, as high as about 2.0 μS/cm. Specific conductance testing can
be done on selected sample bottles that are treated as sample blanks.
Aliquot bottles that have had acid added for sample preservation are
more likely to have contamination from bottle leaching than those that are