Dissolved organic carbon measurement using a modified high-temperature combustion analyzer

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

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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.

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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-

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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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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

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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,

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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.

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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

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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.

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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).

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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.

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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.

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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,

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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).

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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.

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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

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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

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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

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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.

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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.

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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.