Marine Chemistry 81 (2003) 89 – 104
www.elsevier.com/locate/marchem
Dissolved organic carbon measurement using a modified
high-temperature combustion analyzer
Michael L. Peterson a,*, Susan Q. Lang a, Anthony K. Aufdenkampe a,b, John I. Hedges a
a
College of Ocean and Fisheries Sciences, University of Washington, Box 355351, Seattle, WA 98195-5351, USA
b
Stroud Water Research Center, 970 Spencer Road, Avondale, PA 19311, USA
Received 1 May 2002; received in revised form 7 January 2003; accepted 14 January 2003
Abstract
The performance of a dissolved organic carbon (DOC) analyzer operating on the principle of high-temperature combustion
(HTC) is subject to numerous design and operation characteristics. Here we describe modifications and performance tests of a
commercial HTC analyzer (MQ Scientific, model MQ-1001), many of which are applicable to other HTC instruments. Design
improvements include a new combustion column, auto-sampler needle assembly, and a smaller sparger/water trap that
automatically maintains constant water level and pH. Techniques for monitoring and compensating for carrier gas flow rate
fluctuations, as well as electronic improvements to the auto-sampler advance control and the high-pressure injection gas pulse,
are also described. A new model LICOR 7000, nondispersive IR (NDIR) detector is shown to provide a 50-fold increase in
sensitivity over the previous LICOR 6252 model.
The total blank for the modified instrument is initially f 27 ng C and declines during use to f 9 ng C as the combustion
column ages. For an instrument with a well-conditioned combustion column, approximately half of this background is
resolvable into a reagent component coming largely from the deionized, UV-irradiated (DUV) water used to rinse the sample
onto the combustion column: the other half is intrinsic to the instrument and appears associated with the quartz bead packing.
Injection of varying volumes of DUV water with and without an added constant C background, indicates that our DUV water
contains between 2 and 9 AM DOC, depending on the variable performance of the water purifier. Similar experiments indicate
that the intrinsic instrument blank is variable over time and depends complexly on both the wait time between individual water
injections and the overall time that the combustion column has been conditioned. The modified instrument fitted with the new
LICOR 7000 detector measures 46.1 F 1.3 AM DOC in Sargasso Sea water reference material (44 – 45 AM) against a total
instrument blank equivalent to about a fourth of this value. Overall, the modified MQ-1001 analyzer is capable of dependable,
automated analysis of relatively challenging deep seawater samples with an average accuracy of about F 3.8% of the
consensus value.
D 2003 Elsevier Science B.V. All rights reserved.
Keywords: Dissolved organic carbon (DOC); Analytical techniques; Instruments; Instrument blank; Reagent blank; HTC
* Corresponding author. Tel.: +1-2065436670; fax: +1-2066853351.
E-mail address: [email protected] (M.L. Peterson).
0304-4203/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0304-4203(03)00011-2
90
M.L. Peterson et al. / Marine Chemistry 81 (2003) 89–104
1. Introduction
Over the past several decades there has been steady
progress in the analysis and characterization dissolved
organic carbon (DOC) in environmental waters (Hansell and Carlson, 2002). Although DOC measurements involving direct injection of liquid samples
and high-temperature combustion (HTC) to CO2 were
first performed in the 1960s and the 1970s (Van Hall
et al., 1963; Sharp, 1973), it has not been until the last
decade that HTC has emerged as the preferred analytical technique for seawater DOC measurements
(Wangersky, 1993; Spyres et al., 2000). HTC analyzers are advantageous because of their small sample
volume requirement, fast throughput, and high oxidation efficiency (Qian and Mopper, 1996; Skoog et al.,
1997; Chen et al., 2002). Most modern HTC instruments are now fully automated, incorporating sensitive CO 2 -specific detectors, auto-samplers, and
computer-based instrument control, signal acquisition,
and data processing. New-generation HTC analyzers,
however, require that multiple-system components be
precisely coordinated for dependable automatic operation, the combustion/detection modules be able to
accommodate large-pressure pulses when injected
water is vaporized to steam (Perdue and Mantoura,
1993), and careful attention be given to significant
instrument-derived blanks that are typical of these
instruments (Benner and Strom, 1993; Cauwet, 1994).
This article describes modifications to and performance characteristics of a model MQ-1001 HTC analyzer (MQ Scientific, Qian and Mopper, 1996).
Modifications to the analyzer were aimed at improving
mechanical reliability as well as analytical sensitivity
and precision. Improvements primarily involved
changes to the combustion column, the water trap
located immediately downstream of this column, the
auto-sampler needle design, the system dead volumes,
and the detector. Although these modifications were
made using a particular HTC analyzer, many of the
concepts and modifications should prove generally
useful for other HTC systems. Moreover, generally
applicable strategies for maximizing analytical precision and evaluating reagent and instrument blanks are
discussed. Finally, preliminary performance characteristics are described for the recently introduced LICOR
model 7000 nondispersive IR (NDIR) detector, which
offers approximately 50 times greater sensitivity than
the LICOR model 6252 detector with which the instrument was initially distributed. Overall, these modifications have produced an analyzer capable of ship-based
operation that is dependable, fully automated, and
highly precise in its measurements of DOC.
2. Modified analyzer
The MQ-1001 instrument is typical of many newer
HTC analyzers in that a small volume ( f 100 Al) of
sample water is automatically injected onto the head
of a heated (750 jC) combustion column that is
purged by a continuous flow of high purity O2 gas.
The MQ-1001 incorporated features from previous
HTC instruments such as: (1) a highly sensitive nondispersive infrared detection of CO2 (Sharp, 1973),
(2) a vertical combustion column incorporating CuO
and Sulfix to facilitate the complete combustion of
injected organic compounds to CO2 and to remove
halogen- and sulfur-containing gases (Sugimura and
Suzuki, 1988; Suzuki et al., 1992), and (3) a two-stage
oven with a hotter first section and a cooler second
section containing the CuO and Sulfix, thereby allowing higher combustion temperatures (Peltzer and
Brewer, 1993).
Novel features of the MQ-1001 included replacing
needle and septum style sample injection with a
closed system, continuous flow loop-type injection
valve, and replacing expensive Pt-based ‘catalysts’
with inexpensive quartz beads that serve as an inert
heat exchanger (for specifics, see Qian and Mopper,
1996). The improved injection design facilitated the
coupling of the instrument to an auto-sampler, thereby
allowing completely automated analyses. Water vapor
explosively formed from injection of 100 – 200 Al of
water is sequentially removed from the carrier gas
stream by condensation in a sparger tube, by perevaporation through a selectively permeable membrane,
and by absorption onto Mg(ClO4)2. The dried gas is
then passed to a LICOR NDIR detector that measures
the CO2 component and forwards the data to a
chromatography data system for processing. Newer
model MQ Scientific analyzers have also made
improvements in the detector, the injection gas pulse,
the sparge/condensation trap, and auto-sampler control, but through different design changes than
described below.
M.L. Peterson et al. / Marine Chemistry 81 (2003) 89–104
2.1. Configuration modifications
As originally configured, reliability issues with our
HTC system included a short combustion column life
due to clogging, a dual injection/purge needle that was
easily bent and attacked by acid, and an erratic control
of the auto-sampler advance function. Analytical
precision issues were, in part, related to the large
water trap/sparger located just downstream of the
combustion column, fluctuations in carrier gas flow
during peak detection, and imprecise control of the
carrier gas high-pressure pulse during water injection.
Additional modifications to the instrument included
adding a sparge line for the large reservoir of deion-
91
ized, UV-irradiated (DUV) water, as well as incorporating a vapor expansion compensation tube, a more
sensitive detector, and provisions to continuously
monitor carrier gas flow rate and detector temperature.
Table 1 lists the redesigned components of our HTC
analyzer, the reason for the change, and the resultant
outcome. Detailed information describing these modifications to the MQ-1001 (including circuitry and
software control files) are available from the corresponding author upon request.
A schematic diagram of the modified fluid, O2 and
N2 flow paths is illustrated in Fig. 1, following a
format similar to that shown in Qian and Mopper
(1996, Fig. 1). No electronic circuitry is shown.
Table 1
Modifications made to existing MQ-1001 TOC analyzer
Component
Performance issues
Modifications
Outcome
Combustion column
Short column life; expensive
Redesigned the inlet capillary and
sample line adapter (see Fig. 2A);
reagent cleanup procedure
Increased life span; less
expensive in house fabrication
Sparger/water trap
pH neutral; variable water level
Automate 4-way valve new plumbing
to acidify and drain water trap
(see Fig. 2B)
Improved precision; improved
peak shape; no trap overflow
Dual-sample/sparge
injection needle
Easily bent; solder
attacked by acid
Single/dual-function needle made from
commercially available fittings
(no welding, see Fig. 2C)
Extended needle life;
simple construction
Auto-sampler
Erratic advancement
Precise control of advance
signal via new circuitry
Reliably advances once (and
only once) for each advance
command sent from PC
DUV water reservoir
No sparge line
Added sparge line
Improved CO2 purging of
DUV water reservoir
Teflon fluid lines
CO2 permeable; CO2 released
during trap acidification
Replaced with PEEK tubing
No CO2 response during trap
acidification
NafionR perevaporation
drying tube
Incomplete water removal
Lengthened tube and increased
N2 flow rate
Very dry gas stream; increased
life of Mg(ClO4)2 drying tube
Gas expansion loop
Insufficient flow stabilization
before CO2 peak elution
Added 47 m of 2.3-mm ID tubing
immediately before the detector
Increased peak elution time to
allow for carrier flow stabilization
Electronic flow meter
Monitoring carrier gas flow rate
at the time of CO2 detection
Record analog signal from flow
meter using chromatography software
Confirmation that all samples were
measured at the same flow rate
LICOR 6252 NDIR
detector
Low sensitivity
Replaced with LICOR 7000
Sensitivity improved by a factor of
50 (see Table 2)
Schematic of components shown in Fig. 1.
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M.L. Peterson et al. / Marine Chemistry 81 (2003) 89–104
Fig. 1. Schematic of fluid, O2 and N2 flow paths for the modified MQ-1001 analyzer. Capital letters correspond to the software controlled relay
for the labeled component: (A) Sparge gas solenoid and four-port valve control (ON=sparging and with the pump on, drawing fluid from the
water trap; (B) Carrier gas regulator bypass (ON=high pressure carrier gas pulse); (C) Pump on/off control; (D) On/off solenoid control for DUV
water flow into the DUV water loop and water trap (valve A=OFF); (E) 10-port valve control (OFF-valve in the carrier gas bypass position); (F)
Raise needle; (G) Advance auto-sampler; (H) Lower needle. Bold flow path line follows the O2 carrier gas during a sample injection.
Nitrogen pressurizes the pneumatic controllers for
multiport valves A and E and serves as the counterflow drying gas for the NafionR perevaporation tube
downstream of the sparger. Inflowing O2 gas (grade
4.4, 400 kPa) is first scrubbed of CO2 and water, and
then split four ways into: (1) carrier gas, (2) sample
sparge gas, (3) DUV water sparge gas, and (4)
pressurization gas for the DUV water reservoir. Oxygen carrier gas ( f 100 kPa) flows to the 10-port
valve, E, which is maintained in the ‘off’ (bypass)
position except for a few seconds during sample
injection into the combustion column. At the time of
injection, valve E is rotated to divert carrier gas
through the DUV rinse water loop, the sample loop,
and then the combustion column. During sample
injection, carrier gas flow rate is momentarily
increased by activating solenoid B and bypassing
the carrier gas pressure regulator. The fluid flow path
for loading the sample loop is the same as in the
original unit in that the sample is pulled by a pump
from the auto-sampler through a four-port valve (A)
and into the sample loop attached to the 10-port valve
(E). Filling the acidified DUV rinse water loop is
similar to the original unit except that the overflow
from the loop is redirected into the water trap via
valve A.
Operation of the MQ-1001 is controlled by PCbased Peak SimpleR software that allows programmed control of eight relays (A through E) by
way of a relay circuit board. These relays control
M.L. Peterson et al. / Marine Chemistry 81 (2003) 89–104
valve movements (A and E), auto-sampler functions
(F, G and H), three solenoids (A, B and D), and a fluid
pump (C) (Fig. 1). Like most commercial chomatographic software, Peak SimpleR is capable of simultaneously acquiring multiple channels of data. We
used this feature to record not only the CO2 response
but also the carrier gas flow rate and the detector flow
cell temperature during each sample run. Both the
digital flow meter (Omega FMA 1810) and the
LICOR detector cell temperature sensor have analog
outputs that can be accessed for this purpose. Continuous monitoring of carrier gas flow has proven to
be valuable in assuring flow reproducibility and
evaluating system performance during unattended
operation. In our laboratory, variations in the flow
cell temperature never exceeded 3 jC and had no
discernable effect on measured CO 2 response.
Although both the LICOR models 6252 and 7000
NDIR detectors feature automatic internal temperature
compensation functions that apparently were effective
within this temperature range, this may not be the case
for larger temperature fluctuations or other NDIR
detectors.
Other minor configuration changes to the original
unit include: (1) an additional electronic circuitry to
precisely control the high-pressure sample injection
gas pulse and the advance signal to the auto-injector;
(2) adding a O2 gas sparge line for the DUV water
reservoir; (3) a longer NafionR tube (3.7 m) with an
increased N2 drying gas flow which resulted in a
relatively dry gas stream entering the Mg(ClO4)2 trap,
substantially extending its lifetime; and (4) a small
tube filled with gold shavings added downstream of
the Mg(ClO4)2 trap to remove impurities (such as
elemental mercury from poisoned samples) that might
otherwise react with the gold-lined flow cells of many
NDIR detectors.
2.2. Redesigned components
The combustion column (Fig. 2A) was redesigned
in order to increase its life span and simplify construction. MQ Scientific columns generally lasted less
than 1000 seawater injections due to salt blockage
either within the narrow platinum inlet tube or where
the tube contacted the quartz beads inside the column
body. The overall design is similar to the commercial
column except for the entrance capillary tube design,
93
a slight change in the overall length, and a 2-Am-porediameter stainless steel frit inserted into the fitting at
the downstream end of the column. Packing materials
remained the same in type and order (quartz beads,
CuO, and Sulfix) as for the original commercial
column. A 5.5 cm length of both the Sulfix (8 –20
mesh, Wako) and the CuO (Wako, 14 – 24 mesh)
packing was used. The remaining column length
was filled with 3 3 mm clear fused quartz beads
(Quartz Scientific). These modifications lowered
the replacement cost of a combustion tube from
f US$800 to f US$300 (including packing materials) and allowed columns to be made rapidly as
needed by local shops.
A cleanup procedure for the packing materials was
employed which significantly reduced the blank from
a new combustion column. This treatment consisted
of placing the CuO and the Sulfix (separately) on a
sieve (25 mesh, f 0.8 mm) and thoroughly rinsing
them with a stream of DUV water to remove fine
particles. All quartz and chemical column components
were then muffled in air at 800 jC for 5 h. The
combustion tube was held inverted in a ring stand and
loosely packed through the bottom 12 mm opening
after which it was turned upright and gently tapped to
form a f 1 cm gap, where the capillary tube joins the
column body. This void space at the top of the column
is essential to prevent salt deposits from quickly
blocking the capillary inlet tube.
Assembled columns were conditioned with
repeated injections of acidified (HCl, pH = 1 – 2)
DUV water (100 Al sample loop plus 75 Al DUV
water loop) for f 20 h ( f 300 injections) to give a
low, relatively stable instrument blank (see Section
3.3 below). Direct comparisons of 0.1 Ag C (100 Al of
83.3 AM C solution made by dissolving potassium
biphthalate in DUV water acidified with HCl to
pH = 1– 2) injections showed no significant difference
in the performance between our modified columns
and the ones supplied by MQ Scientific. Alternate
injections of 0.1 Ag DOC and DUV water showed that
both columns had little, if any, carryover between
samples at naturally occurring DOC concentrations.
The redesigned columns typically accommodate
2000 – 3000 seawater injections before becoming
clogged with salt deposits, and the injection limit for
freshwater samples is many times that of saltwater
samples.
94
M.L. Peterson et al. / Marine Chemistry 81 (2003) 89–104
Fig. 2. Redesigned components for the MQ-1001 analyzer. (A) Redesigned combustion column specifications and packing material diagram.
(B) Low-volume sparger (water trap) plumbed for automatic acidification and water removal. (C) Dual-function auto-injector needle. See text
for details.
In our experience with the original instrument,
analytical response would slowly decrease over a
series of analyses due to the increasing water level
in the sparger/water condensation trap located immediately downstream of the combustion column (Fig.
1). As this trap filled with water of nearly neutral pH,
it would retain increasing amounts of dissolved
inorganic carbon. Experiments showed a 10% to
15% drop in instrument response as the trap filled
over the course of about 100 injections. The response
would substantially rebound after the water trap was
drained, but then slowly decreased as the trap filled
once again.
The decreasing response problem was resolved by
draining the water trap to a constant level and by
acidifying the remaining water before each injection.
With the 10-port valve (E) in the ‘off’ position,
rotating the four-port valve (A) to the ‘on’ position
and turning on the sample pump (C) draws water from
the water trap down to the level of the tube inside the
trap. As the trap is drained, the water flows through
the sample loop, thereby rinsing the line between the
valve and the sample loop. Solenoid D must be kept
‘off’ whenever A is ‘on’ to prevent flow of DUV
water into the sample vial. Acidification of the water
trap is accomplished while filling the DUV water
loop. Diverting the acidified DUV water overflow
back into the water trap (E and A ‘off’, D ‘on’)
effectively maintains a pH of < 3 in the trap. Initially,
acidification of the water trap produced a slight
M.L. Peterson et al. / Marine Chemistry 81 (2003) 89–104
response by the detector (Fig. 3) that could potentially
interfere with the sample CO2 peak if not performed at
the correct time. Subsequently, it was discovered that
the source of the CO2 was diffusion of atmospheric
CO2 through the walls of the FEP TeflonR tubing
used for sample loops and fluid transfer lines. Replacing all TeflonR tubing with PEEKR tubing of equivalent dimensions eliminated this small CO2 peak,
provided the DUV reservoir was completely sparged
of CO2.
Automating the water trap drain function made it
possible to replace the original water trap with a
smaller, custom-designed version (Fig. 2B). The fluid
line from the four-port valve (A, Fig. 1) enters the trap
through a short-side stem, eliminating the fragile glass
‘J’ tube of the original design. This fluid line is used
to both acidify and drain the water trap. The reduced
dead volume (10 vs. 40 ml) of the new condenser
significantly improved the shape of the CO2 peak. For
a 0.1 Ag DOC injection, the full peak width at onetenth the maximum height was reduced 33%, and the
peak height was increased by 25% at a carrier gas
flow rate of 150 ml min 1.
95
The dual function injection/sparge needle supplied
with the MQ-1001 tended to easily bend, and the
solder holding it together was prone to failure upon
repeated exposure to acidified water samples. Fig. 2C
shows the redesigned sampling/sparging needle that
can be mounted onto the ISCO auto-sampler in the
same manner as the original needle. The design
employs a sharpened #13 10.1 cm stainless steel
pipetting needle connected to a modified PEEKR tee
fitted with a Luer adapter. The straight through path of
the tee and the Luer adapter are bored out to allow the
sample intake tubing to pass through the tee fitting to
the tip of the needle. The sample contacts only the
PEEK tubing as it is drawn into the needle. The right
angle branch of the tee is connected to the sparge gas
line to allow the gas to flow between the outer wall of
the sample tube and the inner wall of the needle. No
soldering or tube bending is required, resulting in an
easily constructed, more durable needle assembly. The
needle can be readily disconnected from the tubing at
the Luer connector so that the sample and sparge lines
can be moved without changing the alignment of the
needle in the auto-sampler.
Fig. 3. CO2 response and carrier gas flow (without dead volume compensation) for a 0.1 Ag DOC injection (175 Al). The relative elution times
for the CO2 peak with and without the 190 ml dead volume compensation tube (Fig. 1) in line is shown. Response data were acquired at a rate of
5 Hz using a LICOR 6252 NDIR detector in absolute mode, a 0 – 100 mV D/A conversion of 0 – 300 ppm CO2, and the signal averaging time
function set to zero (no averaging).
96
M.L. Peterson et al. / Marine Chemistry 81 (2003) 89–104
3. System performance
3.1. Carrier gas flow rates
Explosive vaporization of injected liquid water
samples is a major consideration in the design and
the operation of essentially all HTC analyzers (Perdue
and Mantoura, 1993). In the specific case of the
unmodified MQ-1001, the total dead volume of the
flow path was f 75 ml, of which f 30 ml was in the
combustion column, f 40 ml in the water trap, and
< 5 ml in flow lines. In this configuration, a typical
injection of 100 Al of sample plus 75 Al of ‘backing’
DUV water generated well over 700 ml of gas in less
than a second. Fig. 3 shows a typical gas flow
fluctuation for a 175 Al injection, along with the
simultaneous CO2 response. The water vaporization
pulse immediately following sample injection
increases the carrier flow rate from f 150 to >250
ml min 1, after which flow abruptly drops to < 100
ml min 1 before returning to the initial value. This
period of rapidly changing gas flow lasts about 30 s
following sample injection. During an injection, the
gas flow rate (and, hence, residence time in the
combustion column) is primarily determined by the
kinetics of water vaporization in the combustion tube
and not by the carrier gas flow rate setting. It is not
surprising, therefore, that column properties, such as
combustion efficiency, are little affected by relatively
small changes in the carrier gas flow rate (Qian and
Mopper, 1996).
Detector response, on the other hand, is highly
sensitive to changes in carrier gas flow rate as CO2
passes through the detector. The response as measured by peak area is not only a function of the CO2
concentration in the gas stream but also is inversely
related to the rate at which the gas stream passes
through the flow cell. This negative relationship
occurs because higher carrier gas flows push a given
amount of CO2 through the detector faster, thereby
reducing the width (but not necessarily the height) of
the generated peak. Fig. 4 shows the effect of
varying flow rate on peak area and height for a
0.1 Ag C sample. The CO2 response as measured by
peak area decreases by more than a factor of two as
the flow rate changes from 120 to 180 ml min 1
with a least-square regression slope of 0.26 F 0.01
(r2 = 0.97, n = 30). This drop corresponds to a sensitivity loss (i.e., the slope of the standard curve
decreases) of 0.84% for every 1 ml min 1
Fig. 4. Peak area and height vs. carrier gas flow rate for 0.1 Ag C injections. Data were acquired using a LICOR 6252 NDIR configured as
described in Fig. 3.
M.L. Peterson et al. / Marine Chemistry 81 (2003) 89–104
increase in flow rate. Similar data plotted in Qian
and Mopper (1996, Fig. 5) show a somewhat lower
sensitivity decrease of 0.53% for every 1 ml
min 1 increase in flow rate.
The volume of the original LICOR 6252 flow cell
is f 12 ml, giving residence times of only f 6 and 4
s for carrier gas flow rates of 120 and 180 ml min 1,
respectively. These residence times are much shorter
than the duration of CO2 peak elution (Fig. 3) yet,
peak heights show little variation with carrier flow
rate (Fig. 4). A nearly constant peak height suggests
that the maximal amount of CO2 in the flow cell as the
peak elutes is not changing significantly with carrier
gas flow rate. This insensitivity to carrier flow rate is
consistent with the observation that at the time of CO2
plug formation, total gas flow is driven primarily by
steam generation and condensation (Fig. 3).
In the original MQ-1001 instrument, the CO2 peak
passes through the detector about 30 s after the injection, at which time the carrier gas flow system may not
have completely recovered from the vaporization/condensation cycle (see Fig. 3 for an example of barely
adequate stabilization). In our experience, this recovery
time increases as the injection volume increases, but
this effect was not quantified. Because reproducible
results depend on a constant carrier gas flow rate (Fig.
4), it is important to ensure for all operating conditions
Fig. 5. Peak area response vs. volume of water added with and
without a constant carbon background of 25 ng added to each
injection. Data were acquired using a LICOR 7000 NDIR detector
in absolute mode.
97
and water injection volumes that gas flow has completely stabilized before the CO2 peak elutes. A simple
solution to this problem is to insert a long length of
narrow diameter nylon tubing (47 m 2.3 mm
I.D. 3.2 mm O.D.) immediately before the detector
(Fig. 1), which enlarges the dead volume of the system
by f 190 ml without significantly affecting the shape
of the eluted CO2 peak. The added dead volume
increases the time between injection and detection to
f 90 s, allowing the carrier flow ample time under
almost all operating conditions to stabilize before the
CO2 peak elutes through the detector (Fig. 3). Due to
the sensitivity of CO2 response to the elution rate for
NDIR detectors, carrier gas flow rate should be routinely monitored if at all possible for all NDIR-based
HTC analyzers, many of which should be amenable to a
similar modification to minimize gas flow instabilities
during CO2 elution. Knowledge that up to f 50 m of
2.3 mm I.D. tubing can be inserted between the last
water trap and the NDIR detector without undue CO2
peak broadening might also be strategically useful in
adding other in-line detectors or gas-trapping devices
to the analytical system.
3.2. Response and temporal stability
All data presented to this point were acquired using
a LICOR 6252 NDIR detector. Table 2 compares
several standard curves and associated values for
certified Sargasso Sea water reference material (Hansell, 2001; 44– 45 AM DOC) determined using either
the older LICOR 6252 or the new model LICOR 7000
detectors. Data for the LICOR 7000 was collected
both in the laboratory and at sea. Response factors
(slopes) for the new LICOR detector averaged 50
times greater than that for the older model. Comparing
laboratory data, response factors for standard calibration curves (0 –500 ng C) varied by 7.5% and 10%
(mean deviations) for the new and old detectors,
respectively. The intercepts were more variable at
15% and 20%, respectively. Variation in the slope of
the response curve on the order of 5 – 10% could
simply be the result of carrier gas flow variations of
only 6 –12% (Fig. 4), while variability in the intercept
is more likely related to the overall system blank
which can be substantial (see below). In the laboratory, the signal-to-noise ratio of the new detector is
about twice that of the LICOR 6252 (60 and 28,
98
M.L. Peterson et al. / Marine Chemistry 81 (2003) 89–104
Table 2
Temporal variability in standard curve responses (slopes) and intercepts, blank peak areas, average deep seawater determinations, and sample
peak height-to-noise ratios for the modified MQ-1001 TOC analyzer using newer (7000) and older (6252) model LICOR NDIR detectors
Sample ID
Standard curve
Response
(mV min ng 1)
Intercept
(mV min)
Total blank
Area
(mV min)
Sargasso Sea deep water (Hansell, 2001)
As C
mass (ng)
Average
[C] (AM)
a
F 1 MD
N
Percent
deviation
Signal
C mass
(ng)
Noise
LICOR 7000 in the lab—Data from a single, relatively new column
September 3, 2001
14.9
330
253
16.9
September 11, 2001
12.4
252
198
16.1
September 19, 2001
11.9
221
132
11.1
September 20, 2001
14.2
217
178
12.5
October 8, 2001
13.3
207
161
12.1
Average
13.4
245
185
13.7
F 1 MD
1.0
36
33
2.2
46.3
47.4
43.4
45.5
47.7
46.1
1.3
0.2
2.9
0.3
1.8
2.3
2
5
3
5
4
0.5
6.0
0.7
3.9
4.8
55.6
56.9
52.1
54.6
57.4
55.3
1.6
68
54
54
60
66
60
5
LICOR 7000 at sea R/V Atlantis—Data from a single, relatively new column
September 5, 2002
13.1
293
295
22.5
September 7, 2002
13.3
231
254
19.1
September 11, 2002
13.2
196
250
18.9
September 12, 2002
13.3
136
174
13.1
September 15, 2002
13.7
146
168
12.3
September 17, 2002
14.3
125
145
10.1
September 21, 2002
14.8
213
131
8.9
Average
13.7
191
202
15.0
F 1 MD
0.5
48
55
4.4
48.3
45.4
49.2
48.5
44.6
42.9
45.5
46.3
2.0
1.7
2.0
1.8
1.5
1.9
1.7
2.2
4
6
5
2
6
3
3
3.6
4.6
3.8
3.2
4.2
4.0
4.9
58.0
54.5
59.0
58.2
53.5
51.5
54.6
55.6
2.4
30
26
29
26
25
22
35
28
3
LICOR 6252 in the lab—February – March
February 9, 2001
0.202
February 21, 2001
0.214
March 15, 2001
0.269
March 17, 2001
0.296
March 18, 2001
0.270
March 19, 2001
0.289
March 21, 2001
0.235
March 25, 2001
0.219
March 28, 2001
0.235
March 29, 2001
0.237
June 6, 2001
0.221
June 7, 2001
0.250
Average
0.245
F 1 MD
0.025
data from multiple well-conditioned columns; June data from
3.61
3.07
15.2
50.0
1.1
4.76
4.04
18.9
48.3
1.6
3.54
3.33
12.4
51.1
2.1
2.84
3.39
11.4
45.8
0.7
2.34
3.06
11.3
43.6
0.8
4.70
3.35
11.6
44.7
0.9
4.38
3.75
16.0
47.3
1.7
3.73
4.10
18.7
51.3
0.3
3.31
3.95
16.8
47.9
0.8
5.18
3.89
16.4
44.1
2.4
5.23
6.07
27.4
46.8
3.2
3.37
5.51
22.0
45.7
1.4
3.92
3.96
16.5
47.2
0.78
0.65
3.6
2.1
a relatively new
2
2.1
2
3.4
1
4.1
6
1.5
3
1.8
4
2.0
5
3.6
2
0.6
1
1.7
1
5.5
7
6.8
6
3.1
column
60.0
58.0
61.4
55.0
52.4
53.6
56.8
61.5
57.5
52.9
56.1
54.9
56.7
2.5
6.1
4.9
6.6
5.3
5.1
5.4
5.0
5.8
5.4
6.1
6.2
6.8
5.7
0.5
Variability is computed as mean deviations (MD).
a
Where n = 1, MD is based on replicate injections of a single sample; where n>1, MD is based in replicate samples.
respectively) due entirely to the increased noise levels
aboard the ship. With the exception of the signal-tonoise ratio, there was no significant difference in the
performance of the LICOR 7000 while at sea or in the
laboratory. The greater sensitivity of the new detector
provided slightly better precision for the standard
seawater analyses with values averaging 46.1 F 1.3
AM C ( F 2.8%) for the LICOR 7000 and 47.2 F 2.1
AM C ( F 4.5%) for the LICOR 6252. Both sets of
data are within the 2.0– 6.6% variability for replicate
analyses run on different days as reported in a
communitywide DOC intercalibration study (Sharp
et al., 2002). Our average DOC concentration in the
reference seawater is slightly higher than the consensus value of 44 – 45 AM C (Hansell, 2001), but is
similar to the average value of 47.1 F 3.0 AM for deep
Sargasso Sea water determined by 14 different analysts (Sharp et al., 2002, Table 6).
M.L. Peterson et al. / Marine Chemistry 81 (2003) 89–104
3.3. System blank
3.3.1. Reagent blank
The total system blank typically has two potential
sources of contaminant C; a reagent blank and an
intrinsic instrument blank. The reagent blank originates from materials added to the sample being
analyzed and characteristically produces a linear
response as known amounts of each of the added
components (including rinse water) are systematically
varied (e.g., Benner and Strom, 1993; Cauwet, 1994).
In the case of the MQ-1001 analyzer, the reagent
blank includes the DOC contained in the HCl used to
acidify the sample as well as the DUV water used to
‘‘chase’’ the sample through the injector and onto the
combustion column.
A representative trend for response (LICOR 7000)
vs. volume of acidified DUV water injected into a
well-conditioned combustion column is shown in Fig.
5. In this experiment, sample and rinse water loops of
different lengths were combined to control the total
volume of injected DUV water between 32 and 300
Al (loop volumes were calibrated based on the mass
of contained water). This test produced a straight line
of positive slope that extrapolated to a detector
response of about 33 mV min at zero DUV volume.
Based on the response factors listed in Table 2, the
intercept corresponds to 2.5 ng C and the slope of the
DUV line corresponds to 9.3 AM DOC in the injected
DUV water. A second test was then carried out to see
whether concordant data are obtained when the same
experiment is repeated, but with a constant amount of
sample DOC injected along with varying volumes of
the same DUV water. These experiments were performed by combining a 25 Al sample loop containing
25 ng C (potassium biphthalate) with DUV rinse
water loops of varying volumes. A single straight
line was again obtained (Fig. 5), but with (as
expected) a much higher intercept of 634 mV min.
Based on the average response for the LICOR 7000
(Table 2), the slope of this trend line corresponds to
that expected for injections of DUV water having a
DOC concentration of 3.6 AM. This estimate
decreases to 2.0 AM DOC in the injected DUV water,
if the difference in the two intercepts in Fig. 5 is used
to determine an ‘‘internal’’ response factor derived
from the 25 ng C injected with every sample in the
second experiment.
99
A DOC concentration of 9.3 AM for DUV water is
somewhat higher than reported values or the specifications indicated by the manufacturer of the distilled
water purifier. Concentrations of 3.6 and 2.0 AM
DOC, however, are in agreement with values reported
by others researchers who prepare low-DOC distilled
water (Benner and Strom, 1993; Cauwet, 1994; Peltzer et al., 1996; Sharp et al., 2002) and represent 7.6
and 4.2 Ag C, respectively, for a 175 Al injection. The
fact that these experiments were carried out over
several weeks using different batches of DUV water
may account for at least part of the variability in the
measured DOC concentration of the DUV water.
Despite the variability in these estimates of DUV
water DOC concentration, these results demonstrate
that nanogram levels of C, similar to amounts injected
when analyzing natural waters, can be reproducibly
measured by the modified MQ-1001 analyzer over a
range of injection volumes considerably larger than
typically employed. In addition, extremely small
amounts of DOC injected in varying volumes of
DUV (1– 6 ng) can be precisely measured down to
equivalent concentrations of f 5 AM (6 ng (100
Al) 1), about an order of magnitude less than deep
seawater concentrations.
3.3.2. Instrument blank
Instrument blanks are typically more difficult to
quantify because they are intrinsic to the instrument
itself and are derived, in part, from heterogeneous
catalysts and other column packing materials (Bauer
et al., 1993; Benner and Strom, 1993; Perdue and
Mantoura, 1993; Skoog et al., 1997) whose relative
amounts are not readily changed without introducing
system performance artifacts. Instrument blanks for
various HTC analyzers have been shown to be both
volume-dependent (varying with the total amount of
water injected; Tanoue, 1992; Benner and Strom,
1993; Cauwet, 1994) and time-dependent (varying
with the time between injections; Tanoue, 1992; Qian
and Mopper, 1996). It is also probable that there is a
constant intrinsic blank component that is independent
of the injection process, which may be part of a
constant analytical background (see below).
In order to investigate the effect of the time period
before an injection, the ‘wait time’ preceding a series
of closely spaced injections was varied between 8 and
248 min (Fig. 6A) in an experiment similar to Qian
100
M.L. Peterson et al. / Marine Chemistry 81 (2003) 89–104
Fig. 6. Peak area response for various column configurations and wait times. Each group of four bars represents a sequence of four, 175 Al DUV
water injections spaced 5 min apart. Wait times before the first injection (open bar) of each quadruplicate are given on the abscissa for panels A
and B and above each set of bars for panels C and D. (A) Acidified DUV water injected onto a normal column with quartz beads, CuO, and
Sulfix packing. (B) Exponential fit to the peak area of first peak (open bar, panel A) in each group vs. wait time. (C and D) Neutral (C) and
acidic (D) DUV water injected onto a column either filled with only quartz beads or completely empty. LICOR data acquisition parameters are
as described in Fig. 3.
and Mopper (1996, Fig. 6). The following experiments were all carried out with a LICOR 6252
detector. The injection series consisted of four successive injections (one every 3 min) of 175 Al of DUV
water at a carrier gas flow rate of 150 ml min 1. As
wait times increased by a factor of about f 30, the
area of the response for the initial injection within
each group of four increased by a factor of f 5 (from
M.L. Peterson et al. / Marine Chemistry 81 (2003) 89–104
7 to 35 mV min). The response increases for the initial
injections are best fit with an exponential function
(Fig. 6B, r2 = 0.97) which can be expected to reach an
asymptotic value of f 45 mV min for the conditions
used in this experiment. Using the average response
factor for the LICOR 6252 (Table 2), the slope of the
initial portion of the curve (23 –68 min) suggests that
the rate of C buildup in the combustion column is
f 0.6 ng C min 1 and the asymptote represents 184
ng C. Tanoue (1992) estimated that the rate of carrier
gas contaminant CO2 (derived from hydrocarbon
impurities) adsorption onto alumina could theoretically be as high as 24 ng C min 1 and that this CO2
would desorb when replaced by water during an
injection. His attempts to measure this effect, however, suggested that the adsorption rate (if real) was
much less than 24 ng C min 1. Results reported in
Qian and Mopper (1996) suggest that carbon accumulated at about one-tenth the rate calculated here
(0.041 AM C min 1 for a f 100 Al injection volume). Variability in these estimates of carbon build up
are most likely due to differences in column packing
materials and conditioning state. As the columns age
and accumulate salt, the surface characteristics of
column packing materials may change (e.g., during
the devitrification of the quartz beads and column
walls), which may in turn change the intrinsic instrument blank.
The response pattern shown in Fig. 6A and B
might also be expected if hydroxide radicals generated
directly from the injected water are the effective
oxidizing agent in HTC analyzers (Chen et al.,
2002) and if oxidation-resistant carbonaceous materials (derived from column packing materials) accumulate within the combustion column between injections
(even in the presence of O2). Whatever the mechanism
of the wait-time effect, it is critical when performing
DOC analyses with the MQ-1001 analyzer (and likely
most HTC units) that all injections of samples and
standards be preceded by identical wait times (Qian
and Mopper, 1996). For a typical wait time between
injections of 5 min, our data suggest that f 3 ng of C
accumulates in the column, which if released upon
water injection would represent f 20% of the average total blank masses shown in Table 2. Reportedly,
wait-time effects can be minimized by humidifying
the O2 carrier gas stream before it passes through the
combustion column (Chen et al., 2002) so that small
101
amounts of hydroxide radical are continuously produced.
In an attempt to determine the internal source of
the observed wait time-induced response, a column
containing only quartz beads and a completely empty
combustion tube were comparatively tested (Fig. 6C
and D). The column packed with nothing but quartz
beads produced a large response regardless of wait
time with values ranging from f 100 to 250 mV min.
The empty combustion tube, on the other hand,
exhibited a response similar to the total instrument
blank (Table 2) and showed little correlation with wait
time. This result was the same whether or not acid was
present in the DUV water, ruling out possible artifacts
related to the absence of CuO and Sulfix that would
remove any HCl vapor in the gas stream. Sequential
injections of DUV water onto the column filled with
only quartz beads showed a wait-time response similar to that seen for a typical combustion column
containing quartz beads, CuO, and Sulfix. Placing a
soda lime trap in line after the Mg(ClO4)2 trap
eliminates any detector response in these experiments,
indicating that the detector response was most likely
due to CO2 or some other acidic gas that absorbs
strongly in the infrared at 4.26 Am.
3.3.3. Total system blank
Typically newly installed HTC columns regardless
of packing materials show a large blank response that
rapidly decreases with increasing conditioning time
and number of injections (i.e., Benner and Strom,
1993). For the modified MQ-1001 analyzer, the total
system blank (Table 2) ranges between 9 and 27 ng
for a 175 Al injection volume, depending on the
conditioning state of the column. These values are
similar to other reported HTC system blanks of 24 –36
ng (Tanoue, 1992), 24 ng (Benner and Strom, 1993),
and 12 –24 ng (Skoog et al., 1997) although these
units used different column packing materials (platinized alumina vs. quartz beads, CuO, and Sulfix) and
smaller injection volumes (100 Al). The variability of
the system blank was not significantly decreased by
using the more sensitive LICOR 7000 detector.
The previous observations described above concerning the wait time-dependent blank response point
toward quartz beads as a major continuous source of
the intrinsic blank in our modified DOC analyzer.
Fig. 7 shows column-conditioning profiles for two
102
M.L. Peterson et al. / Marine Chemistry 81 (2003) 89–104
Fig. 7. Peak area response vs. injection number during conditioning
of two new HTC columns and the best-fit hyperbolic curves (Eq.
(1)). Data were acquired using either (A) the LICOR 6252 or (B) the
newer LICOR 7000 NDIR detector. LICOR data acquisition
parameters are as described in Figs. 3 and 5.
quartz – CuO – Sulfix columns using the old and the
new LICOR detectors. Repeated 175 Al injections of
DUV water were made at 3-min intervals to condition the column. Under these conditions, the
amount of C that is released by the column materials
during any one injection, n, can be estimated by
fitting the profile to a modified hyperbolic function
of the form
y¼aþb
1
ð1 þ cnÞ1=d
¼ a þ bk
ð1Þ
where a, b, c, and d are constants. The constant a
may be interpreted as the theoretical lower response
limit reached when all sources of intrinsic blank
associated with DUV water injections have been
exhausted and only the reagent blank and any noninjection-related intrinsic blank remain (n ! l). The
constant b represents the theoretical total injectionrelated response (n = 0). The variable third term,
k = f(n), is the fraction of the total C released by
each successive injection. The fitted curves shown in
Fig. 7 are constrained by setting b equal to the
measured total integrated response and then varying
a, c, and d to minimize the sum of the relative
absolute differences between the column-conditioning response generated by Eq. (1) and the data.
The total C mass released during column conditioning is obtained by multiplying the total integrated
response (minus that due to DUV water C) by the
appropriate response factor given in Table 2 and is 111
and 18.1 Ag C for the data shown in Fig. 7A and B,
respectively. The total C mass released during conditioning is a function of how the injections are
performed and is certainly less than the total C
contained in the column materials. For example,
increasing the wait time between injections during
conditioning would shift the entire profile to higher
values. Also, judging by the large difference between
the integrated C values for the two columns shown in
Fig. 7, the purity of packing materials and the efficiency of the clean up procedure probably play major
roles in determining the mass of C released during the
initial injections of conditioning process.
Table 3 shows the decreasing total C mass released
for specific injection numbers as predicted by Eq. (1)
and how it is distributed among C contained in the
injected DUV water, C derived from other injectionrelated sources (i.e., steam-induced desorption from
quartz surfaces), and an intrinsic constant C bleed
(i.e., not injection-related). Although the initial portions (n < f 300) of the two conditioning profiles
(Fig. 7) are quite different, for n>400, the total C
released per injection is about the same for the two
columns reaching values between 19 and 11 ng C for
the 1000 to 2000 injection range. These C masses are
similar to the actual average total blank measurements
given in Table 2 of 14– 17 ng C. For either column,
about 40 – 50% of the total blank is derived from the
DUV water (assuming a 2.8 AM DOC concentration).
The remainder of total blank is instrument-derived,
but is partitioned by the model differently between
M.L. Peterson et al. / Marine Chemistry 81 (2003) 89–104
Table 3
Blank carbon masses (ng) from injected DUV water and other
injection- and noninjection-related sources as predicted from the
hyperbolic fit (Eq. (1)) for two-column conditioning data sets
obtained using new (7000) and old (6252) models LICOR NDIR
detectors (Fig. 7)
Carbon source
Injection number (n)
100
400
1000
2000
5.9
5.9
5.9
5.9
LICOR 7000
Injection-related (minus DUV)
Noninjection-related
Total blank
48
1.6
55
20
1.6
28
11
1.6
19
6.9
1.6
14
LICOR 6252
Injection-related (minus DUV)
Noninjection-related
Total blank
75
3.9
85
12
3.9
22
3.6
3.9
13
1.4
3.9
11
DUV water (2.8 AM DOC)
a
a
Average of 3.8 and 2.0 AM values discussed in Section 3.3.1.
injection-related and constant background sources for
the two columns. The total column blank decreases by
a factor of about 2 over the useful life of a combustion
column (i.e., n>400) and can vary between 20% to
40% of the DOC mass in a typical 100 Al deep
seawater sample (see Table 2, column 5 LICOR
7000 data).
An implication of these observations is that the
quartz– CuO – Sulfix-based combustion columns (and
likely those containing of other materials) continuously generate measurably large and variable blanks
throughout their service lives, only a fraction of which
is evident when water is injected. This phenomenon,
however, does not preclude the possibility that a
significant fraction of the blank is continuously converted to IR-measurable products in the absence of
injected water as, in fact, would be expected for a
steady-state production process. If a component of the
intrinsic blank is continuously generated and, in fact,
occurs as CO2, then this (and other) HTC analyzers
may have a relatively large bleed of this gas that is not
separable from the analytical baseline. This background would not necessarily affect routine DOC
analyses, but could interfere substantially if subsequent characterization of the eluted CO2 were attempted by an independent method such as stable C isotope
analysis.
103
4. Overview
Modifications to the MQ-1001 HTC TOC analyzer, including a redesigned combustion column
and water trap, auto-sample improvements, and a
more sensitive detector, have substantially increased
the commercial unit’s reliability and sensitivity. As
modified, this instrument is suitable for DOC analysis
of a variety of natural waters including deep ocean
samples. These modifications, however, have resulted
in only minor improvement to overall analytical
precision due primarily to persistent intrinsic instrument blanks and variability in the carrier gas flow. As
a result, long-term variability over weeks to months is
comparable to variability during a single analytical
run. Given the greatly increased sensitivity and the
higher signal-to-noise characteristics of new NDIR
detectors such as the LICOR 7000, attempts to
improve the precision of seawater DOC analyses
should be focused on minimizing both the amount
of ‘‘inert’’ support in combustion columns and the
amount of water injected, as well as better moderation
of (or compensation for) flow rate fluctuations.
Acknowledgements
John Hedges unexpectedly passed away in July
2002 while this manuscript was in review. I (M.P.)
was truly privileged to be John’s fellow researcher and
laboratory manager for the past 16 years: a better
friend, mentor, colleague, and employer, I could not
have had. We thank Dennis Hansell for providing the
Sargasso Sea deep water reference material and the
UW Marine Organic Geochemistry (MOG) reading
group for in-house reviews. This research was
supported by NSF grants OCE-9310684, OPP9531763AM02, and OCE-0085615 to J.H. and a
National Defense Science and Engineering Graduate
Fellowship from DoD to S.L.
Associate editor: Dr. Edward Peltzer.
References
Bauer, J.E., Occelli, M.L., Williams, P.M., McCaslin, P.C., 1993.
Heterogeneous catalyst structure and function: review and implications for the analysis of dissolved organic carbon and nitrogen in natural waters. Mar. Chem. 41, 75 – 89.
104
M.L. Peterson et al. / Marine Chemistry 81 (2003) 89–104
Benner, R., Strom, M., 1993. A critical evaluation of the analytical
blank associated with DOC measurements by high-temperature
catalytic oxidation. Mar. Chem. 41, 153 – 160.
Cauwet, G., 1994. HTCO method for dissolved organic carbon
analysis in seawater: influence of catalyst on blank estimation.
Mar. Chem. 47, 55 – 64.
Chen, W., Zhao, Z., Koprivnjak, E., Perdue, M., 2002. A mechanistic study of the high temperature oxidation of organic matter
in a carbon analyzer. Mar. Chem. 78, 185 – 196.
Hansell, D.A., 2001. The good, the bad and the ugly: how good are
my DOC results? U.S. JGOFS Newslett. 11 (2), 14 – 15.
Hansell, D.A., Carlson, C.A., 2002. Biogeochemistry of Marine
Dissolved Organic Matter. Elsevier, Amsterdam.
Peltzer, E., Brewer, P.G., 1993. Some practical aspects of measuring
DOC-sampling artifacts and analytical problems with marine
samples. In: Hedges, J.I., Lee, C. (Eds.), Measurements of Dissolved Organic Carbon and Nitrogen in Natural Waters. Mar.
Chem, vol. 41, pp. 243 – 252.
Peltzer, E.T., Fry, B., Doering, P.H., McKenna, J.H., Norrman, B.,
Zweifel, U.L., 1996. A comparison of methods for the measurement of dissolved organic carbon in natural waters. Mar. Chem.
54, 85 – 96.
Perdue, E.M., Mantoura, F., 1993. Mechanisms subgroup report.
Mar. Chem. 41, 51 – 60.
Qian, J., Mopper, K., 1996. An automated, high performance, high
temperature combustion dissolved organic carbon analyzer.
Anal. Chem. 68 (18), 3090 – 3097.
Sharp, J.H., 1973. Total organic carbon in seawater—comparison of
measurements using persulfate oxidation, and high temperature
combustion. Mar. Chem. 1, 211 – 229.
Sharp, J.H., Carlson, C.A., Peltzer, E.T., Castle-Ward, D.M., Savidge, K.B., Rinker, K.R., 2002. Final dissolved organic carbon
broad community intercalibration and preliminary use of DOC
reference materials. Mar. Chem. 77, 239 – 253.
Skoog, A., Thomas, D., Lara, R., Richter, K., 1997. Methodological
investigations on DOC determinations by the HTCO method.
Mar. Chem. 56, 39 – 44.
Spyres, G., Nimmo, P.J., Worsfold, E.P., Achterberg, E.P., Miller,
A.E.J., 2000. Determination of dissolved organic carbon in seawater using high temperature catalytic oxidation techniques.
Trends Anal. Chem. 19, 498 – 506.
Sugimura, Y., Suzuki, Y., 1988. A high temperature catalytic oxidation method for the determination of non-volatile dissolved
organic carbon in seawater by direct injection of a liquid sample.
Mar. Chem. 24, 105 – 131.
Suzuki, Y., Tanoue, E., Ito, H., 1992. A high-temperature catalytic
oxidation method for the determination of dissolved organic
carbon in seawater: analysis and improvement. Deep-Sea Res.
39, 185 – 198.
Tanoue, E., 1992. Vertical distribution of dissolved organic carbon in the North Pacific as determined by the high temperature catalytic oxidation method. Earth Planet. Sci. Lett. 111,
201 – 216.
Van Hall, C.E., Safranko, J., Stenger, V.A., 1963. Rapid combustion
method for the determination of organic substances in aqueous
solutions. Anal. Chem. 35, 315 – 319.
Wangersky, P.J., 1993. Dissolved organic carbon methods: a critical
review. In: Hedges, J.I., Lee, C. (Eds.), Measurements of Dissolved Organic Carbon and Nitrogen in Natural Waters. Mar.
Chem, vol. 41, pp. 61 – 74.