DOC/DON Reference Materials

Transcription

1 DOC/DON Reference Materials Presentation at 2009 INSS International Workshop, Paris Jonathan H. Sharp College of Marine and Earth Studies University of Delaware Lewes, DE USA with assistance from Kelly Peeler Susan Curless (now at U. Hawaii)

2 Dissolved organic carbon (DOC) measured irregularly since 1910; periodic interest for trophic transfer role. DOC not considered of major importance until linkage to global ocean oxygen cycle in 1960s; but not measured much. With controversy on DOC measurements in late 1980s (Suzuki AOU correlation), recognition of importance of DOC to global carbon cycle.

3 Timeline Early 1990s need for more accurate DOC highlighted at Seattle and WHOI workshops. Agreement from about 30 labs at level of ± 100%. Sharp started DOC intercalibration to improve numbers before JGOFS EQPAC sampling; expanded as general international effort ( ). Preliminary DOC reference materials in late 1990s; Hansell s DOC references outgrowth of this effort.

4 100 DOC (µm C) Analysis Number Results of measurements by over 50 analysts of 5 samples from depth profile in Pacific Ocean. Oriented for 2500 m sample in order of decreasing concentration; also showing sfc, 150 m, and 4000 m samples. Agreement with uncontrolled methods at a level of better than ± 10% (CV of group = (SD/mean)*100.

5 Comparison of Performances with Daily Use of Reference Materials Deep Ocean Milli-Q TOC + Blank Number n Range Mean SD Range Mean SD Expected to to to to to to to to to to to to * 40.0 to Subtracted to to to to to to * 38.3 to Subtracted to to to Not reported to to Report on early use of references from 14 labs. Expected values from careful calibration in 3 labs.

6 From Four Similar Instruments With and Without Adjustment to Reference (DOC data in µm C) Raw Data Adjusted to DSW Sample Mean SD %CV Mean SD %CV (10) (45) (100) (250) (500) (800) (1900) Average %CV 5.89 Average %CV 3.66 Results using same instrument, data adjusted with deep ocean reference value. Do not adjust, reference use can guide in decision of whether or not to run.

7 For many oceanic analytes, need to think of goals. Accuracy Almost impossible to actually determine; often must be relational. Precision Can be accurate with different levels of precision. Reliability Much of prior problems have been in method (instrument) reliability, not in accuracy. Reference use needs strict adherence and documentation.

8 Success of international dissolved inorganic carbon (TCO 2 ) comparability because of use of single instrument (SOMMA analyzer) and strict use of Dickson s CRM. Replicate precision on order of ± 0.03%; about (0.8 µg at kg -1 ). Few other analytes have as tight agreement and as strict controls on analytical methods. With DOC, now can have agreement on the order of ±1% (1-2 µg at kg -1 ); but only with strict reference use.

9 With accurate DOC measurements, can now see carbon flux in secondary biological zone ( m) and geographic deep ocean trend. Dissolved organic nitrogen (DON) important for control by N of oceanic biology. Also for global N-flux with flux from surface to deep ocean and with denitrification. With global N enrichment of waters, need for better understanding of global N cycle.

10 DON Measurement DOC measured almost directly remove TCO 2 and then conversion to CO 2 of all remaining C as DOC. Removal consistent and complete. Most DOC measurements use very similar instrumentation. No good direct DON measurement must measure TDN (total dissolved nitrogen) and then subtract measured DIN (total inorganic = nitrate, nitrite, and ammonium). Variety of TDN measurements.

11 For DON, need precise, accurate, and reliable measurements of TDN and DIN. Since late 1990s, intercalibration efforts on TDN and DON. Problems with DIN measurements: Reliability and accuracy of nitrate plus nitrite analysis Occurrence and accuracy of ammonium analysis very low in oceanic waters and assumed to be zero TDN analysis very susceptible to unrecognized ammonium contamination

12 Results of intercalibration exercise with about 30 analysts. Samples from lower Delaware Estuary and depth profile in North Atlantic Ocean. Nitrite (NO 2 ) Estuary Oceanic (Depth Profile) sta 24 sta 26 5 m 50 m 200 m 1000 m 2000 m Mean SD Five analysts returned results for NO 2 excellent agreement.

13 Nitrate plus nitrite (N&N) Estuary Oceanic (Depth Profile) sta 24 sta 26 5 m 50 m 200 m 1000 m 2000 m All Analysts Mean SD %CV Best Five Analysts Mean SD %CV Fifteen analysts returned results for NO 3 & NO 2 : average agreement for the five high concentration samples was ±7.3%. Agreement of five best (arbitrary) analysts was ±1.7 %. Can get much better continuity with selection.

14 Ammonium (NH 4 ) Estuary Oceanic (Depth Profile) sta 24 sta 26 5 m 50 m 200 m 1000 m 2000 m All Analysts Mean SD %CV Best Four Analysts Mean SD %CV analysts returned NH 4 results poor agreement, but not routine for some of the analysts. With best four analysts, improved consistency; average SD for low concentration samples of ±0.08 µm N. May be necessary to have routine NH 4 analysis for accurate DON measurement.

15 Dissolved organic carbon (DOC) Estuary Oceanic (Depth Profile) sta 24 sta 26 5 m 50 m 200 m 1000 m 2000 m CRM All Analysts Mean SD %CV Analysts Using CRM Mean SD %CV DOC results from 12 analysts uncontrolled ; results from 6 analysts who used DOC references to qualify their results. Those 6 analysts showed an average of 3.3% agreement on their runs of the DOC references and about 4% for the seven samples.

16 Total Dissolved Nitrogen (TDN) Estuary Oceanic (Depth Profile) sta 24 sta 26 5 m 50 m 200 m 1000 m 2000 m All Analysts Mean SD %CV CRM 27 analysts, using high temperature combustion (HTC), UV, and persulfate oxidation methods for TDN. Overall agreement of ±12.3%. HTC not necessarily better than wet chemical methods.

17 Total Dissolved Nitrogen (TDN) Estuary Oceanic (Depth Profile) CRM sta 24 sta 26 5 m 50 m 200 m 1000 m 2000 m All Analysts Mean SD %CV Analysts Using CRM Mean SD %CV Nine analysts performed TDN analysis on DOC references with agreement for references of ±4.5%; BUT average agreement for the 7 samples was only ±14.5%. Better reference or reference use needed.

18 Total Dissolved Nitrogen (TDN) Estuary Oceanic (Depth Profile) sta 24 sta 26 5 m 50 m 200 m 1000 m 2000 m Best Nine Analysts Mean SD %CV CRM Arbitrarily chosen best 9 analysts with agreement of ±4.5%. Need to find a way to institute an effective reference to insure that best is really accurate.

19 TDN NO 3 & NO 2 NO 2 NH 4 Calibration Sfc 5.7 ± HTC Sfc 5.9 ± 0.4 Wet Chem Sfc 5.7 ± ± 0.16 Calibration Deep 20.9 ± ± HTC Deep 20.6 ± 0.5 Wet Chem Deep 20.9 ± ± ± 0.17 Preliminary experiment (2007) with DON references. Surface and deep ocean references calibrated in 5 laboratories. Results of analyses from 8 laboratories (10 to 30 analyses for each analyte).

20 Ongoing (Final) DON Reference Program Three samples collected (2008): sfc and deep ocean and estuarine Calibrated in four laboratories About 3000 ampoules prepared (15 ml and 50 ml size ) and sent to 16 labs for routine daily reference use Awaiting results

21 Calibration of DON References DOC (µm C) TDN (µm N) DON (µm N) Sample Mean SD %CV Mean SD %CV Sfc % Deep % Est % nitrate plus nitrite (µm N) nitrite (µm N) ammonium (µm N) DIN Sample Mean SD %CV Mean SD Mean SD %CV Sfc Deep % Est % % phosphate (µm P) Sample Mean SD %CV Sfc Deep % Est

22 Conclusions We have shown that we can improve quality of DOC analyses through reference use need a better buy-in to assure continuity. Preliminary DON reference demonstration need improved TDN, DIN; cannot assume NH 4 is insignificantly low. Need for very careful sample collection and preservation to avoid NH 4 and DOC contamination.

23 DOC and DON on INSS References? Preliminary check on DOC, TDN, and NH 4 in INSS references (BF, BG). DOC looks good. TDN shows high correction (DON should only be about 3-4 µm ); some NH 4 contamination suspected. Careful preparation of references to avoid DOC and NH 4 contamination. For accurate DON, will need nutrients to subtract from TDN. Good potential for multiuse reference.

24 Acknowledgements Support through several grants from the Chemical Oceanography Program of the US National Science Foundation. A number of analysts from around the world have voluntarily helped with this effort over past several years. Especially helpful in calibrations: Hiroshi Ogawa (U. Tokyo); Craig Carlson (U California, Santa Barbara); HOT Program of U. Hawaii; Debbie Bronk (Virginia Institute of Marine Sciences).