130
Exercise 9
the dull side of the foil against the filters. V se clean forceps in handling of the filters. The
amount of water filtered varies with the concentration of the particulate organic matter.
In lakes of moderate productivity, between 0.5 and 1.0 I of water can be filtered onto 47mm diameter filters to give an adequate POC sample. Vacuum differential applied to
the filtration apparatus should not exceed 0.3 atm.
The filters for POC analysis should be placed onto labeled, clean aluminum foil (dull
side) and carefully stored under desiccation immediately. The filtrate should be
processed immediately for DOC content.
DISSOLVED ORGANIC CARBON
A sample of the filtrate is analyzed for DOC by first acidifying the water to pH ~ 3 to
convert all inorganic carbon to CO 2 . The sample is then bubbled vigorously but briefly,
to minimize evaporation, with an inert gas (Oz, N 2 , or He) to sparge all inorganic CO2
from the sample. The remaining dissolved organic carbon of the acidified sample is then
treated with a strong oxidant such as potassium persulfate at elevated temperature and
pressure. Organic carbon compounds are oxidized to CO 2 in sealed ampoules. The
amount of CO 2 generated from the DOC is then measured by an infrared CO2 analyzer
and is a measure of the DOC of the sample.
Procedures
1. Obtain enough ampoules of 1-ml size for triplicate analyses of each water sample.
Precombust for 8 h at 525°C in a muffle furnace. Store in a clean, closed container.
2. Place a known amount of potassium persulfate into each ampoule by means of a
quantitative "microscoop" (which can be made from the tapered end of a plastic
semimicro centrifuge tube). Add approximately 200 mg K 2 S 2 0 S to 1-ml ampoules.
3. Prepare a series of organic carbon standards, using glucose, to extend over the
range of DOC in samples to be analyzed (see "Apparatus and Supplies," p. 136).
4. Place a precise amount of sample (200 to 500,u1) into the ampoules using an
accurate pi petting system (e.g., Eppendorf).
5. Add 3% phosphoric acid (see p. 136) to each sample at a ratio of200,u1 per 0.5 ml of
sample.
6. Sparge the sample with purified oxygen for 10 mins, bubbling at a flow rate of about
200 mljmin. The gas should pass a V-trap of ascarite (a CO 2 absorbing compound)
before entering a flow meter and stripping manifold consisting of Tygon tubing
connected to 20-gauge, 15-cm stainless steel needles or cannulae.
7. With the oxygen gas still flowing into the ampoule, insert a second cannula, with
flowing oxygen, affixed to a modified electrician's pliers (Fig. 9.1). Remove the
initial sparging cannula. Holding the lower portion of the ampoule with a free
hand, place the neck of the ampoule into a propane- or methane-oxygen
microburner (Fig. 9.1) and immediately rotate while pulling downward to seal the
ampoule. In this manner, the flow of oxygen continues uninterrupted throughout
the sealing procedure (McDowell et aI., 1987) and minimizes the entry of CO or
CO 2 from the flame.
8. Autoclave the samples at 121°C for 45 min after the autoclave has come to full
temperature and pressure. Caution: Reduction of pressure and temperature at the
end of autoclaving must be done by slow exhaust. The samples may be stored
indefinitely after autoclaving.
Exercise 9
the dull side of the foil against the filters. V se clean forceps in handling of the filters. The
amount of water filtered varies with the concentration of the particulate organic matter.
In lakes of moderate productivity, between 0.5 and 1.0 I of water can be filtered onto 47mm diameter filters to give an adequate POC sample. Vacuum differential applied to
the filtration apparatus should not exceed 0.3 atm.
The filters for POC analysis should be placed onto labeled, clean aluminum foil (dull
side) and carefully stored under desiccation immediately. The filtrate should be
processed immediately for DOC content.
DISSOLVED ORGANIC CARBON
A sample of the filtrate is analyzed for DOC by first acidifying the water to pH ~ 3 to
convert all inorganic carbon to CO 2 . The sample is then bubbled vigorously but briefly,
to minimize evaporation, with an inert gas (Oz, N 2 , or He) to sparge all inorganic CO2
from the sample. The remaining dissolved organic carbon of the acidified sample is then
treated with a strong oxidant such as potassium persulfate at elevated temperature and
pressure. Organic carbon compounds are oxidized to CO 2 in sealed ampoules. The
amount of CO 2 generated from the DOC is then measured by an infrared CO2 analyzer
and is a measure of the DOC of the sample.
Procedures
1. Obtain enough ampoules of 1-ml size for triplicate analyses of each water sample.
Precombust for 8 h at 525°C in a muffle furnace. Store in a clean, closed container.
2. Place a known amount of potassium persulfate into each ampoule by means of a
quantitative "microscoop" (which can be made from the tapered end of a plastic
semimicro centrifuge tube). Add approximately 200 mg K 2 S 2 0 S to 1-ml ampoules.
3. Prepare a series of organic carbon standards, using glucose, to extend over the
range of DOC in samples to be analyzed (see "Apparatus and Supplies," p. 136).
4. Place a precise amount of sample (200 to 500,u1) into the ampoules using an
accurate pi petting system (e.g., Eppendorf).
5. Add 3% phosphoric acid (see p. 136) to each sample at a ratio of200,u1 per 0.5 ml of
sample.
6. Sparge the sample with purified oxygen for 10 mins, bubbling at a flow rate of about
200 mljmin. The gas should pass a V-trap of ascarite (a CO 2 absorbing compound)
before entering a flow meter and stripping manifold consisting of Tygon tubing
connected to 20-gauge, 15-cm stainless steel needles or cannulae.
7. With the oxygen gas still flowing into the ampoule, insert a second cannula, with
flowing oxygen, affixed to a modified electrician's pliers (Fig. 9.1). Remove the
initial sparging cannula. Holding the lower portion of the ampoule with a free
hand, place the neck of the ampoule into a propane- or methane-oxygen
microburner (Fig. 9.1) and immediately rotate while pulling downward to seal the
ampoule. In this manner, the flow of oxygen continues uninterrupted throughout
the sealing procedure (McDowell et aI., 1987) and minimizes the entry of CO or
CO 2 from the flame.
8. Autoclave the samples at 121°C for 45 min after the autoclave has come to full
temperature and pressure. Caution: Reduction of pressure and temperature at the
end of autoclaving must be done by slow exhaust. The samples may be stored
indefinitely after autoclaving.