San

Francisco

Sacramento

U.S. Geological Survey

U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 03-366

Method of Analysis by the U.S. Geological Survey California

District Sacramento Laboratory—Determination of Dissolved

Organic Carbon in Water by High Temperature Catalytic

Oxidation, Method Validation, and Quality-Control Practices

Los Angeles
Method of Analysis by the U.S. Geological 

Survey California District Sacramento 

Laboratory—Determination of Dissolved 

Organic Carbon in Water by High Temperature 

Catalytic Oxidation, Method Validation, and 

Quality-Control Practices

By Susan M. Bird, Miranda S. Fram, and Kathryn L. Crepeau

Sacramento, California

2003

Open-File Report 03-366

U.S. GEOLOGICAL SURVEY 8006-51
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does 

not imply endorsement by the U.S. Government.

For additional information write to:

District Chief

U.S. Geological Survey

Placer Hall – Suite 2012

6000 J Street

Sacramento, California 95819-6129

http://ca.water.usgs.gov

Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Building 810

Box 25286, Federal Center

Denver, CO 80225-0286
Contents iii

CONTENTS

Abstract ................................................................................................................................................................ 1

Introduction .......................................................................................................................................................... 1

Method of Analysis of Dissolved Organic Carbon in Water (Parameter Code 00681) by High Temperature 

Catalytic Oxidation (U.S. Geological Survey Method Number O-2201-04)....................................................... 2

Scope and Application ................................................................................................................................ 2

Summary of Method ................................................................................................................................... 2

Equipment and Materials ............................................................................................................................ 2

Sample Collection, Filtration, and Storage ................................................................................................. 3

Standards..................................................................................................................................................... 4

Carbon Analyzer Performance.................................................................................................................... 4

Data Processing and Archiving................................................................................................................... 5

Difficult Samples ........................................................................................................................................ 7

Method Validation................................................................................................................................................ 8

Precision...................................................................................................................................................... 8

Method Detection Limit and Laboratory Reporting Level ......................................................................... 9

Method Performance Evaluation................................................................................................................. 10

Quality-Control Practices..................................................................................................................................... 11

Blank Samples............................................................................................................................................. 11

Duplicate Samples....................................................................................................................................... 12

Calibration Verification .............................................................................................................................. 12

Standard Deviation and Coefficient of Variation........................................................................................ 12

Maintenance and Corrective Actions.......................................................................................................... 13

Summary .............................................................................................................................................................. 14

References Cited .................................................................................................................................................. 14
iv Figures

FIGURES

Figure 1. Typical sample run documentation sheet........................................................................................... 6

Figure 2. Graph showing variation in measured total dissolved carbon concentration as a function of pH 

for a sample containing 3.6 milligrams per liter (mg/L) dissolved organic carbon and 30 mg/L 

dissolved inorganic carbon ................................................................................................................ 8

Figure 3. Graph showing measured and calculated dissolved organic carbon (DOC) concentrations in 

samples of potassium hydrogen phthalate (KHP), caffeine, and five natural fulvic acid 

(FA) isolates....................................................................................................................................... 11
Tables v

TABLES

Table 1. Laboratory equipment and reagents used for analysis of dissolved organic carbon.......................... 3

Table 2. Analytical precision for dissolved organic carbon analyses of standard solutions and representative 

water samples..................................................................................................................................... 9

Table 3. Maintenance program for the Shimadzu TOC-5000A Total Organic Carbon Analyzer................... 13
vi Conversion Factors, Abbreviations, and Acronyms

CONVERSION FACTORS, ABBREVIATIONS, AND ACRONYMS

CONVERSION FACTORS

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F = (1.8 × °C) + 32

ABBREVIATIONS 

mg/L milligram per liter

μm micrometer

μS/cm microsiemens per centimeter

mm millimeter 

mL milliliter 

mS/m millisiemens per meter

psi pounds per square inch

μL microliter

mL/min milliliter per minute

CO2 carbon dioxide

CO carbon monoxide

NaHCO3 sodium bicarbonate

NaCl sodium chloride

HCl hydrochloric acid

ppm parts per million

ACRONYMS

CV coefficient of variation

DIC dissolved inorganic carbon

d duplicate

Dil dilution

DOC dissolved organic carbon

EPA U.S. Environmental Protection Agency

GR# LIMS number

HC hydrocarbon

KHP potassium hydrogen phthalate, C8H5KO4

LIMS laboratory information management system

Multiply By To obtain

gram (g) 0.03527 ounce, avoirdupois

millimeter (mm) 0.03937 inch
Conversion Factors, Abbreviations, and Acronyms vii

LRL laboratory reporting level

LT-MDL long-term method detection limit

MDL method detection limit

N normal

NPOC non-purgable organic carbon

NWIS U.S. Geological Survey National Water Information System database

OWQRL U.S. Geological Survey Ocala Water-Quality and Research Laboratory

SD standard deviation

THMFP trihalomethane formation potential

TOC total organic carbon

USGS U.S. Geological Survey
Introduction 1

Method of Analysis by the U.S. Geological Survey 

California District Sacramento Laboratory— 

Determination of Dissolved Organic Carbon in Water by 

High Temperature Catalytic Oxidation, Method 

Validation, and Quality-Control Practices

By Susan M. Bird, Miranda S. Fram, and Kathryn L. Crepeau

ABSTRACT 

An analytical method has been developed 

for the determination of dissolved organic carbon 

concentration in water samples. This method 

includes the results of the tests used to validate the 

method and the quality-control practices used for 

dissolved organic carbon analysis. Prior to 

analysis, water samples are filtered to remove 

suspended particulate matter. A Shimadzu TOC5000A Total Organic Carbon Analyzer in the 

nonpurgeable organic carbon mode is used to 

analyze the samples by high temperature catalytic 

oxidation. The analysis usually is completed 

within 48 hours of sample collection. The 

laboratory reporting level is 0.22 milligrams per 

liter.

INTRODUCTION

Accurate, precise, and timely measurements of 

dissolved organic carbon (DOC) concentrations are 

essential for studies of the roles of natural organic 

matter in environmental, ecological, and geochemical 

processes. The capability to analyze DOC was 

established at the U.S. Geological Survey (USGS) 

California District research laboratory in Sacramento 

to provide analytical support for research projects 

concerning natural organic matter in the SacramentoSan Joaquin Delta in central California. 

Research projects in the California District range 

from water-quality investigations to subsidence 

mitigation studies. Oxidation of peat islands in 

agricultural production is a primary cause of land 

subsidence in the Delta (Miller and others, 2000). The 

subsidence mitigation research projects involve 

construction of wetland systems on Twitchell Island in 

the Delta and detailed investigation of carbon cycling 

and soil accretion in these wetlands. DOC is an 

important component of the carbon mass balance in the 

wetlands. 

The Delta is the source of drinking water for 

more than 22 million Californians as well as the 

primary source of the organic carbon that feeds the 

base of the food web in the San Francisco Bay Estuary. 

DOC enters the Delta channels mainly from wetlands, 

agricultural lands, and urban runoff. The quantity 

(concentration and load) and quality (compositional 

characteristics) of the DOC in the Delta is critically 

important to the drinking water and ecological 

functions of the water. Water-quality research projects 

on DOC include chemical characterization and 

quantification of loads from different sources and landuse practices. Chemical characterization has focused 

on disinfection by-product formation potentials, optical 
2 Method of Analysis by the USGS CA District Laboratory— Determination of Dissolved Organic Carbon in Water by High Temperature Catalytic Oxidation

properties, microbial utilization, and stable isotopic 

values. DOC reacts with disinfectants added during the 

drinking water-treatment process to produce an array of 

potentially carcinogenic disinfection by-products, 

including trihalomethanes and haloacetic acids. 

Analyses of disinfection by-product formation 

potentials and optical properties require DOC 

concentrations and must be performed as rapidly as 

possible after sample collection because the 

concentrations may change as the DOC in the samples 

ages (K.L. Crepeau, U.S. Geological Survey, written 

commun., 2003; E. Kalve, U.S. Geological Survey, 

written commun., 2003). Standard procedures for 

sample preservation are not used in this method 

because they may change the quality (not the quantity) 

of the DOC.

DOC analyses must be made with great care 

because there are many potential pitfalls, including 

contaminated blank water, insufficient removal of 

inorganic carbon, inefficient oxidation of all the 

organic carbon, and treatment of difficult samples 

(Hedges and others, 1993; Urbansky, 2001; Aiken and 

others, 2002). A method was not developed for total 

organic carbon (TOC) because of the known problems 

with analysis of particulates by instrumentation used 

for DOC analyses (Aiken and others, 2002).

METHOD OF ANALYSIS OF DISSOLVED 

ORGANIC CARBON IN WATER 

(PARAMETER CODE 00681) BY HIGH 

TEMPERATURE CATALYTIC OXIDATION

(U.S. GEOLOGICAL SURVEY METHOD 

NUMBER O-2201-04)

Scope and Application

The analytical method used by the USGS 

California District research laboratory in Sacramento is 

suitable for the analysis of filtered water samples 

containing at least 0.2 milligrams per liter (mg/L) of 

DOC. The calibration range for the method is 0 to 50 

mg/L. Water samples with DOC concentrations greater 

than 50 mg/L are diluted and reanalyzed.

Summary of Method

Water samples are collected and filtered to 

remove suspended particulate material. A number of 

different collection and filtration methods are used, 

depending on the source and size of the water sample. 

Filtered water samples are stored at 4oC either with no 

added preservative or with concentrated hydrochloric 

acid (HCl) added to pH of 2 until analysis if stored for 

more than 3 weeks. All samples are logged into the 

laboratory’s information management system (LIMS; 

Labworks, Analytical Automation Specialists, Inc.) 

and assigned a unique laboratory identification number. 

Analysis of the sample generally is performed within 

48 hours of arrival at the lab.

DOC analyses are made using a Shimadzu Total 

Organic Carbon Analyzer (Shimadzu TOC-5000A) 

with a Shimadzu Autosampler (Shimadzu ASI-5000A). 

Inorganic carbon is removed from the samples by 

automated acidification with HCl to a pH of 

approximately 2 and purging of the resulting carbon 

dioxide (CO2) with high purity air. High purity air is a 

composite air made of pure nitrogen and pure oxygen. 

The air does not contain more than 1 part per million 

(ppm) each of carbon dioxide (CO2), carbon monoxide 

(CO), and hydrocarbon (HC), respectively (Shimadzu 

Corporation, 1997). Organic carbon is oxidized to CO2

by high-temperature catalytic oxidation and the CO2 is 

measured by a nondispersive infrared detector. 

Detector response is linear and calibrated (for each run) 

with standards containing known concentrations of 

potassium hydrogen phthalate (KHP).

Equipment and Materials

The equipment and materials used for analysis of 

DOC are listed in table 1. DOC analysis procedure is 

performed in a laboratory that contains no organic 

solvents to eliminate vapor-phase contamination of the 

water. Organic-free water is produced onsite with a 

recirculating Picotech water system (Hydro Service 

and Supplies, Inc.). Inlet water for the Picotech system 

is deionized and produced onsite with an Ionics Ultra 

Pure water system. Scheduled routine maintenance and 

replacement of cartridges is done on the Picotech and 

Ionics Ultra Pure water systems.
Method of Analysis of Dissolved Organic Carbon in Water (Parameter Code 00681) by High Temperature Catalytic Oxidation (U.S. Geological Survey Method 

Number O-2201-04) 3

The organic-free water is tested frequently by 

direct analysis of DOC (see section in this report on 

Quality-Control Practices, instrument blanks) and by 

analysis of trihalomethane formation potential 

(THMFP). THMFP is determined by chlorinating the 

water sample and then measuring the amount of 

trihalomethane produced after a 7-day incubation 

period. The organic-free water generally has a THMFP 

of 1 to 2 micrograms per liter (K.L. Crepeau, U.S. 

Geological Survey, written commun., 2003). Most 

reported specific THMFP values for natural DOC in 

water samples are between 1 and 15 carbon atoms per 

1,000 carbon atoms in the DOC (Oliver and Thurman, 

1983; Reckhow and others, 1990; and Fram and others, 

1999). Assuming that the contaminant is natural DOC, 

a THMFP of 1 to 2 μg/L translates into a DOC 

concentration of less than 0.005 mg/L in the organicfree water.

Vials and other glassware items used for DOC 

analysis are cleaned rigorously. Glassware is washed 

with dilute Liqui-Nox soap and rinsed with copious 

amounts of organic-free water. Openings are covered 

with aluminum foil and then the glassware is baked for 

4 hours at 450oC in a muffle furnace. Baked glassware 

is stored in closed drawers or cabinets until use. Glassfiber filters are baked under the same conditions. 

Samples are stored in baked, amber glass bottles with 

Teflon-lined caps supplied by the USGS Ocala WaterQuality Laboratory (OWQRL). These bottles, made of 

virgin glass, were baked at 500°C for 1.5 hours and, 

thus, are used without further cleaning. The bottles can 

be reused if necessary and will go through the cleaning 

process mentioned above. 

Sample Collection, Filtration, and Storage

The specific methods used for sample collection 

and filtration varies with the field setting and the 

volume of the sample being collected. However, all 

methods used have several important features in 

common: only clean glass, steel, or Teflon comes in 

contact with the sample; no organic solvents are used to 

clean the sampling and filtration apparatuses; and 

samples are filtered in the field or in the laboratory 

within 24 hours of collection. Many samples are 

filtered in 47-mm diameter Teflon filtration towers 

(Savillex). The samples drip through 0.3 or 0.7-micron 

pore size baked glass fiber filters by gravity. A stack of 

baked glass fiber filters consisting of 2.7-, 1.6-, and 

0.3-micron pore size filters is often used to slow 

clogging of the 0.3-micron pore size final filter. The 

filtration methods described in the USGS National 

Field Manual section 5.2.2.C (Radtke and others, 2002) 

also are commonly used.

Table 1. Laboratory equipment and reagents used for analysis of dissolved organic carbon

Equipment

- Shimadzu TOC-5000A TOC Analyzer

- Shimadzu ASI-5000A Autosampler

- Clean, amber glass bottles with Teflon-lined lids from the U.S. Geological Survey Ocala Water-Quality

Laboratory in Ocala, Florida 

- Computer with Shimadzu Total Organic Carbon Control software (version 1.05.01)

- Clean autosampler vials, 45- and 6-milliliter sizes (Shimadzu Corporation)

- Clean, class A volumetric flasks

- Analytical balance, certified accuracy of 0.050 gram ± 0.0001

- 250 microliter syringe (Hamilton Company)

- Motorized microliter pipettes (Rainin Instrument Company, Inc.) with disposable tips

Reagents

- Ultra-high purity dry, carbon-free air (Airgas)

- Hydrochloric acid, concentrated, American Chemical Society grade, certified organic carbon content less 

than 0.5 percent 

- Potassium hydrogen phthalate, minimum 99.95-percent purity

- Ultra-pure, organic carbon-free, deionized water (Picosystem Plus, Hydro Service and Supplies, Inc.)

- Sodium hydroxide, certified American Chemical Society grade
4 Method of Analysis by the USGS CA District Laboratory— Determination of Dissolved Organic Carbon in Water by High Temperature Catalytic Oxidation

Filtered samples are stored at 4oC until analysis. 

If samples are to be stored for longer than 3 weeks, 

they are preserved by addition of reagent-grade, 

concentrated HCl. Sufficient HCl is added to lower the 

pH of the sample to 2. Although acidification does not 

change the DOC concentration, it can change the DOC 

quality (E. Kalve, U.S. Geological Survey, written 

commun., 2003).

Standards

Solutions of potassium hydrogen phthalate 

(KHP, C8H5KO4) are used as analytical standards. The 

1,000-milligrams per liter DOC stock solution is 

prepared by weighing 1.0625 grams of KHP with an 

analytical balance and dissolving it in organic-free 

water in a 500-milliliter volumetric flask. This stock 

solution can be stored in a baked amber glass bottle for 

as long as 2 months at 4°C (Shimadzu Corporation, 

1997). Calibration standard solutions are prepared from 

the stock solutions using volumetric glassware and 

pipettes with concentrations ranging from 0.5 to 50.0 

mg/L DOC. These solutions can be stored for as long 

as 1 week at 4°C. 

Standard curves are constructed using four 

appropriate standard concentrations per curve. Samples 

with DOC concentrations less than 2 mg/L are 

analyzed with a standard curve comprising 0-, 0.5-, 

1.0-, and 2.0-milligrams per liter calibration standards. 

Samples with DOC concentrations between 2 and 10 

mg/L are analyzed with a standard curve comprising 

0-, 2.5-, 5.0-, and 10-milligrams per liter calibration 

standards, and samples with DOC concentrations 

between 10 and 50 mg/L are analyzed with a standard 

curve comprising 0-, 10.0-, and 20.0-, and 50-

milligrams per liter calibration standards.

Carbon Analyzer Performance

The procedures for operating the Shimadzu 

TOC-5000A and ASI-5000 are adapted from the 

Shimadzu operation manual (Shimadzu Corporation, 

1997) and Standard Method 5310B (American Public 

Health Association and others, 1995). The operational 

parameters for the Shimadzu TOC-5000A are as 

follows:

Gas: Ultra high purity dry CO2-free air, 80 

pounds per square inch (psi)

Carrier gas flow rate: 150 milliliters per minute 

(mL/min)

Purge gas flow rate: 50 mL/min (adjust such that 

samples do not bubble out of the vials during purging)

Combustion temperature: 680oC

Combustion tube packing (from base): two 

sheets platinum net, 5-millimeter quartz wool (regular 

sensitivity only), 110-millimeter catalyst

Analysis mode: non-purgeable organic carbon 

(NPOC)

Rinses: two with water and two with sample

Injection size: 100 microliters (μL) for high 

sensitivity and 53 or 55 μL for regular sensitivity

Injection number: three, maximum five

The Shimadzu TOC-5000A can be configured 

with a regular or high sensitivity combustion catalyst. 

The regular sensitivity catalyst consists of platinumcoated alumina beads (Shimadzu part number: 017-

42801-01) and is used for analysis of samples with 

DOC concentrations greater than 2 mg/L. The catalyst 

accumulates salts from the samples with usage and 

some of the beads crack. The catalyst can be removed 

for rinsing with water to remove salts and sieving to 

remove broken beads. The high sensitivity catalyst 

consists of platinum-coated quartz wool (Shimadzu 

part number: 630-00996-00) and is used for analysis of 

samples with DOC concentrations less than 2 mg/L. It 

is discarded after the quality-control numbers show that 

its performance has deteriorated (see Quality-Control 

Practices section in this report).

Before any standards or samples are analyzed, 

organic-free water is analyzed until the reported peak 

area reaches an acceptably low and stable value. Peak 

areas of 1,000-area counts for a 100-microliter 

injection of organic-free water in high sensitivity mode, 

and 2,250 area counts for a 53-microliter injection in 

regular sensitivity mode are acceptable. It can take 

more than 100 injections to reach an acceptably low 

peak area value. Failure of the peak area to reach an 

acceptably low and stable value indicates that 

replacement of the combustion catalyst is needed.

Samples, standards, and blanks are acidified with 

HCl and purged for 3 minutes with high purity air 

before analysis. This procedure removes dissolved 

inorganic carbon from the sample. Purging is 

automated with the Shimadzu ASI-5000A and acid 

addition may be done manually or by the autosampler. 

A subset of samples are pretested to determine the 
Method of Analysis of Dissolved Organic Carbon in Water (Parameter Code 00681) by High Temperature Catalytic Oxidation (U.S. Geological Survey Method 

Number O-2201-04) 5

amount of acid required to reduce sample pH to less 

than 2.5. Amounts generally range from 15 to 40 μL of 

0.5–2.0 N HCl for 5 milliliters of sample. 

Initial calibration curves are generated before 

any samples are analyzed. The computer software 

generates a linear regression equation for detector 

response (peak area) over the concentration range of 

the calibration curve. The detector response for each 

standard is the mean of three injections from the same 

vial. The computer software calculates the coefficient 

of variation (CV) and standard deviation (SD) of the 

three injections using the following equations:

(1)

(2)

(3)

where

If the CV is greater than 2 percent and the SD is 

greater than 200 area counts, as many as two additional 

injections can be made and the combination of three 

injections that yields the lowest coefficient of variation 

and standard deviation is chosen. If the correlation 

coefficient of the regression is greater than 0.999 and 

the CV of each standard is 2 percent or less or the SD is 

200 area counts or less, the calibration is accepted. For 

regular sensitivity runs, two separate standard curves 

are generated—the first with calibration standards of 0, 

2.5, 5.0, and 10.0 mg/L and the second with calibration 

standards of 0, 10.0, 20.0, and 50.0 mg/L. If the 

samples are known to be less than 10 mg/L, then only 

the calibration standards of 0, 2.5, 5.0, and 10.0 mg/L 

are generated. Both standard curves encompass the 

same range in peak area counts because the instrument 

automatically adjusts the gain for the detector response 

such that the response for the 0-50 mg/L standard curve 

is one-fifth of the response of the 0–10-milligram per 

liter standard curve. The computer software selects 

injection volumes such that the highest concentration 

calibration standard in each standard curve yields a 

peak area of approximately 36,000 counts. 

When a sample is analyzed, the first injection is 

compared to the first standard curve. If the peak area 

exceeds the range of the first standard curve, the 

computer software automatically adjusts the injection 

volume to match that of the second standard curve for 

the remaining injections. The measured peak areas then 

are compared to the second standard curve.

In addition to the standard curve, an analytical 

sample run includes analysis of standards as calibration 

verification samples, duplicates, and blanks. A typical 

sample run documentation sheet is shown in figure 1. 

Data Processing and Archiving

The computer software calculates the mean DOC 

concentration, SD, and CV for each sample in the 

analytical run. The instrument operator transfers the 

mean DOC results to an Excel spreadsheet and 

scrutinizes the results after each run. Acceptance 

criteria for results of analyses of standards run as 

unknowns, duplicates, and blanks are discussed in the 

section “Quality-Control Practices” of this report. If 

quality-control criteria are not met, the samples are 

rerun. The Excel spreadsheets for each analytical run 

are stored electronically on a local computer and are 

backed up weekly on the USGS California District’s 

computer back-up system. The spreadsheets also are 

printed and stored by the instrument operator. 

= DOC concentration in the i

th injection

= number of injections

= average DOC concentration.

C

Ci

i = 1

n

∑

n = --------------

SD

( ) Ci – C 2

i = 1

n

∑

n – 1 = -------------------------------

CV SD

C = ------- × 100%

Ci

n

C
6 Method of Analysis by the USGS CA District Laboratory— Determination of Dissolved Organic Carbon in Water by High Temperature Catalytic Oxidation

d081902.xls

DOC Run Sheet

mean

Standards area Run Date: 8-19-02

blank 1589 Analyst: SB

2.5 ppm 11042 Sensitivity: regular

5 ppm 19649 Max. samples: 44

10 ppm 38251

r2 = .999799

d = duplicate Circled #'s have a standard deviation > 200 and a CV 

v = verification samples > 2%. For rerun.

ppm ppm

Final Final

# Samples GR# Dil DOC DOC % # Samples GR# Dil DOC DOC %

1 blank 0.037 40

2 2.5 v 2.517 100.7 41

3 5 v 4.986 99.7 42

4 10 v 10.08 100.8 43

5 blank 0.019 44

6 blank 0.018 45

7 Tw-0503 3.751 46

8 Tw-0504 3.736 47

9 Tw-0501 3.461 48

10 Tw-0502 1.654 49

11 Tw-0503 d 3.720 99.2 50

12 Tw-0504 d 3.651 97.7 51

13 blank 0.161 52

14 2.5 v 2.557 102.3 53

15 5 v 4.976 99.5 54

16 10 v 10.04 100.4 55

17 blank 0.058 56

18 Tw-0503 3.719 57

19 Tw-0504 3.615 58

20 Tw-0501 3.393 59

21 Tw-0502 1.683 60

22 Tw-0503 d 3.558 95.7 61

23 Tw-0504 d 3.539 97.9 62

24 blank 0.032 63

25 2.5 v 2.517 100.7 64

26 5 v 4.942 98.8 65

27 10 v 9.936 99.4 66

28 blank 0.062 67

29 blank -0.010 68

30 69

31 70

32 71

33 72

34 73

35 74

36 75

37 76

38 77

39 78

Figure 1. Typical sample run documentation sheet.

(DOC, dissolved organic carbon; ppm, parts per million; SB, Susan Bird; r2, coefficient of determination of the slope of the standard curve; d, duplicate; #, 

number; GR#, Laboratory Information Management System number; dil, dilution; %, percent; CV, coefficient of variation; >, greater than; Max., maximum; v, 

verification samples)
Method of Analysis of Dissolved Organic Carbon in Water (Parameter Code 00681) by High Temperature Catalytic Oxidation (U.S. Geological Survey Method 

Number O-2201-04) 7

After quality-control criteria are met 

satisfactorily, the instrument operator transfers the final 

DOC concentrations to the laboratory information 

management system (LIMS) into the appropriate 

location. Because all samples are logged into the LIMS 

and assigned a unique identification number, DOC data 

are archived easily. The LIMS entries are checked by 

the instrument operator and by the chief or designate of 

the research project for which the samples were 

analyzed, and then flagged as “verified”. The entire 

LIMS database is mirrored daily to a second hard-drive 

and backed up weekly on the USGS California 

District’s external, secure data back-up system. 

Procedures are being developed to export DOC 

concentrations from the LIMS database to the USGS 

National Water Information System (NWIS) database.

Difficult Samples

 Many of the research projects require analysis of 

soil pore water extracts or ground-water samples. 

These water types can be difficult to analyze because 

they may flocculate under laboratory or analysis 

conditions. For example, samples collected from 

piezometers installed in the deep ground-water system 

beneath the peat layer on Twitchell Island often contain 

high concentrations of dissolved iron. Upon exposure 

to the atmosphere, the iron precipitates – and DOC can 

be sorbed to the iron precipitate. Samples collected 

from piezometers installed in the peat soil or aqueous 

extracts of the peat soils produced in the laboratory 

often contain very high concentrations of DOC and 

sometimes flocculate when acidified. The fraction of 

DOC that precipitates at a pH of 2 is defined as the 

humic acid fraction (Thurman, 1985). Flocculation of 

DOC and formation of precipitates may interfere with 

DOC analysis because particles may not be sampled 

reliably by the narrow-bore needle, syringe, and tubing 

in the carbon analyzer and because particles may not be 

efficiently oxidized to CO2 during combustion of the 

sample (Aiken and others, 2002). 

Three strategies are used to minimize the 

flocculation of DOC from peat soil water samples. 

First, samples that flocculate can be diluted with 

organic-free water using volumetric pipettes and 

reanalyzed. 

Second, samples that flocculate are acidified 

with the minimum amount of acid necessary to assure 

removal of dissolved inorganic carbon (DIC). A test 

was done to determine the pH value required to assure 

removal of DIC. The sample pH value required to 

assure removal of DIC was determined using a sample 

(that was known to flocculate) containing 3.6 mg/L 

DOC and 30 mg/L DIC [added as sodium bicarbonate 

(NaHCO3)]. The sample was acidified to pH values 

ranging from 8.3 to 1.9. The DOC concentration 

decreased as pH decreased from 8.3 to about 2.7 and 

then remained roughly constant as pH decreased 

further (fig. 2). The results indicated that acidification 

to pH < 2.7 is necessary for removal of DIC (fig. 2). 

For samples that flocculate, acidification to lower pH 

values increases the amount of flocculation. (The 

standard procedure for samples that do not flocculate 

involves acidification to a pH of 2 because adding 

excess acid insures against incomplete removal of DIC 

from the occasional sample with a very high 

concentration of DIC.) The analyst must test the 

sample to ensure that the correct amount of acid is 

added to reach a pH of less than or equal to 2.7. 

Third, samples that flocculate are analyzed as 

soon as possible after acidification. Flocculation 

generally occurs during a period of hours. The 

Shimadzu ASI-5000A acidifies all of the samples prior 

to beginning analysis of the first sample. Thus, samples 

placed near the beginning of the run will be analyzed 

sooner after acidification. Alternatively, samples can be 

acidified manually just prior to analysis. In most cases, 

these three strategies are sufficient to permit analysis of 

samples that have a tendency to flocculate.
8 Method of Analysis by the USGS CA District Laboratory— Determination of Dissolved Organic Carbon in Water by High Temperature Catalytic Oxidation

METHOD VALIDATION

Several tests were done to validate the analytical 

method. Precision was assessed by replicate analyses 

of several samples that represent the types of samples 

collected during research projects as well as by 

replicate analyses of standards analyzed as calibration 

verification samples (fig. 2). Overall method 

performance was evaluated by analyzing test samples 

similar to those used by Aiken and others (2002). The 

method detection limit (MDL) and laboratory reporting 

level (LRL) were determined from replicate analyses of 

calibration verification standards and instrument 

blanks.

Precision

Analytical precision is expressed in terms of the 

coefficient of variation of replicate analyses of a water 

sample (eq. 3). Three standard solutions and five 

representative water samples were analyzed 10–12 

times (table 2). The 1.0-milligram per liter DOC 

standard and sample Twa-0450 were analyzed using 

the high sensitivity combustion catalyst; whereas, the 

5.0-and 20.0-milligram per liter DOC standards and 

samples Swa-0439, Twa-0160, and Cfa-Y0056 were 

analyzed with the regular sensitivity catalyst. Swa0439 was a surface-water sample with a specific 

conductance of 900 microsiemens per centimeter 

(μS/cm) at 25°C. Twa-0450 was 201 μS/cm. Twa-0160 

was 368 μS/cm. Cfa-Y0056 was a column eluate 

obtained during fractionation of DOC in a surface 

water sample by XAD-4 resin column chromatography. 

The standard solutions yielded estimates of method 

precision ranging from 0.7 to 1.6 percent; whereas, the 

water samples yielded estimates ranging from 0.3 to 

2.9 percent (table 2). The lower precision estimated 

from replicate analyses of natural water samples may 

reflect slight heterogeneities present in natural water 

samples. Estimates of method precision were similar 

for the high sensitivity and regular sensitivity catalysts 

(table 2). Based on these results, the reported precision 

for DOC analyses is estimated conservatively as 3 

percent for DOC concentrations between 1 and 25 

mg/L and 10 percent for DOC concentrations less than 

1 mg/L. For comparison, the USGS National WaterQuality Laboratory determined method precision 

ranging from 8.4 percent at a DOC concentration of 0.3 

mg/L to 1.3 percent at a DOC concentration of 17.9 

mg/L (Brenton and Arnett, 1993). 

40

35

30

25

20

15

10

5

0

98 7 6 54 3 2 1

pH

4.40

4.25

4.00

3.80

3.60

3.40

5432

TOTAL DISSOLVED

CARBON, IN MILLIGRAMS PER LITER

Figure 2. Variation in measured total dissolved carbon concentration as a function of pH for a sample containing 3.6 milligrams per liter (mg/L) dissolved 

organic carbon and 30 mg/L dissolved inorganic carbon. Inset shows data at low pH with an expanded scale on the total dissolved carbon axis.
Method Validation 9

A precision of 3 percent indicates that for DOC 

concentrations less than 10 mg/L, the digit in the 

hundredths place is significant, and the digit in the 

tenths place is significant for DOC concentrations 

greater than 10 mg/L (American Society for Testing 

and Materials, 1993). DOC concentrations are reported 

with one digit beyond the last significant digit.

Method Detection Limit and Laboratory Reporting 

Level

The U.S. Environmental Protection Agency 

(EPA) defined the method detection limit (MDL) as the 

minimum concentration that can be measured and 

reported as greater than zero at the 99-percent 

confidence level (U.S. Environmental Protection 

Agency, 1997). Two estimates were made of the MDL 

for the DOC analytical method. The first estimate was 

calculated according to the procedure specified by the 

U.S. Environmental Protection Agency (1997).

The MDL was calculated using the formula:

(4)

where

The MDL is determined from replicate analyses 

of a standard with a concentration of no greater than 

five times the MDL. Eleven replicate analyses of a 

standard with a DOC concentration of 0.33 mg/L 

yielded an MDL of 0.08 mg/L DOC (table 2).

The second estimate of the MDL was based on 

the procedure for determining the long-term MDL (LTMDL) specified in Oblinger Childress and others 

(1999). The LT-MDL is calculated using the same 

formula as the MDL (eq. 4), but the replicate analyses 

of the low-concentration spike sample are made during 

an extended period of time, typically 6–12 months. 

Both procedures assume that the SD of replicate 

analyses is constant at low concentrations and, thus, the 

SD of replicate analyses of a low-concentration spike 

sample equals the SD of replicate analyses of a 

hypothetical sample containing a concentration of zero. 

We used the measured DOC concentration in replicate 

analyses of instrument blanks to estimate the LT-MDL. 

The high sensitivity catalyst was used to analyze 336 

blanks in 38 separate runs between May 2002 and May 

2003 (table 2). These analyses yielded an estimate of 

the LT-MDL of 0.11 mg/L, which is similar to the 

MDL estimated using the first procedure.

= method detection limit,

= standard deviation of replicate 

analyses at the lowest concentration 

(in milligrams per liter),

MDL SD t(, ) n – 1 α=0.01 = ×

MDL

SD

= number of replicate analyses, and

= Student’s t-test value for the 99-

percent (1-α) confidence level with 

-1 degrees of freedom. (Student’s 

t-test addresses the problems 

associated with inference based on 

small samples).

n

t(, ) n – 1 α=0.01

n

Table 2. Analytical precision for dissolved organic carbon analyses of standard solutions and representative water samples

[mg/L, milligram per liter; H, high sensitivity catalyst; R, regular sensitivity catalyst; NA, not applicable]

Sample Catalyst Number of 

replicates

Mean

concentration 

(mg/L)

Range of 

concentrations

(mg/L)

Standard 

deviation of 

replicates 

(mg/L)

Coefficient of

variation of

replicates

(percent)

Instrument blanks H 336 0.021 -0.125–0.147 0.047 NA

0.33-milligram per liter standard H 11 0.340 0.292–0.403 0.033 9.8

1.0-milligram per liter standard H 11 1.000 0.982–1.022 0.014 1.4

Twa-0450 H 11 1.680 1.618–1.747 0.045 2.7

5.0-milligram per liter standard R 10 5.053 4.976–5.218 0.080 1.6

Swa-0439 R 12 5.256 4.997–5.578 0.155 2.9

20.0-milligram per liter standard R 10 20.23 20.10–20.56 0.13 0.7

Twa-0160 R 10 19.91 19.11–20.68 0.56 2.8

Cfa-Y0056 R 10 22.39 22.23–22.47 0.08 0.3

Tw-0653 (diluted sample) R 11 18.56 17.79–19.06 0.37 2.0
10 Method of Analysis by the USGS CA District Laboratory— Determination of Dissolved Organic Carbon in Water by High Temperature Catalytic Oxidation

The laboratory reporting level (LRL) is defined 

as the minimum concentration that can be measured 

and reported as greater than the MDL at the 99-percent 

confidence level and is equal to twice the MDL 

(Oblinger Childress and others, 1999). The LRL for the 

DOC method is estimated to be 0.22 mg/L. 

Concentrations between the MDL and the LRL are 

reported, but are considered only semi-quantitative.

Method Performance Evaluation

Overall method performance was evaluated by 

analyzing test samples similar to those used by Aiken 

and others (2002) in an interlaboratory comparison 

study designed to assess the relative accuracy of 

determination of DOC and TOC by different 

laboratories, instruments, and analysts. A set of 

standards was prepared and sent to 31 laboratories for 

analysis of TOC and DOC. To evaluate the 

performance of the DOC analysis method used in the 

USGS California District Laboratory, a set of standards 

similar to the set used by Aiken and others (2002) was 

prepared and analyzed. Because the method is not 

applicable to TOC, only DOC standards were prepared. 

Standards were prepared from reagent-grade KHP, 

technical-grade caffeine, and five different fulvic acid 

isolates. The organic carbon contents of the fulvic acid 

isolates were determined by elemental analysis, and the 

KHP and caffeine were assumed to have stoichiometric 

compositions. Solutions of each standard were 

prepared using clean volumetric glassware and 

organic-free water. The DOC concentration range for 

the dissolved standards was 3 to 5 mg/L. KHP is 

oxidized easily and, thus, easy to analyze. Caffeine was 

chosen because it is more difficult to oxidize than is 

KHP and, therefore, may be more difficult to analyze. 

Fulvic acid is naturally occurring DOC and constitutes 

a large proportion of the DOC in most water samples. 

Fulvic acid contains refractory organic material and, 

thus, may be more difficult to oxidize than KHP.

The effect of sample matrix on analytical 

accuracy was examined by amending solutions with 

DIC or sodium chloride (NaCl). Each standard was 

divided into three aliquots. One aliquot was amended 

with reagent-grade sodium bicarbonate (NaHCO3) to 

yield a solution with a DIC concentration of 30 mg/L. 

A second aliquot was amended with reagent-grade 

NaCl to yield a solution with a specific conductance of 

630 μS/cm at 25°C (a chloride concentration of 182 

mg/L). The standards were prepared and labeled by a 

laboratory chemist and submitted to the DOC 

instrument operator with no information other than a 

LIMS number. 

Analytical results for the standard solutions are 

compared to the calculated DOC concentrations in 

figure 3. No systematic differences were seen between 

DOC concentrations in sample aliquots with the three 

different matrices (no additions, added DIC, and added 

NaCl). This indicates that the analytical method is 

successful in analyzing DOC in the presence of 

moderate amounts of potential interferences from DIC 

and chloride. For KHP, caffeine, and three of the fulvic 

acid samples (FA3–5) the measured concentrations 

were within ±3 percent of the calculated concentration 

(fig. 3). A relative accuracy of ±3 percent for analysis 

of DOC concentrations is significantly better than the 

±20 percent required for EPA-certified laboratories and 

better-than-average relative accuracies reported in 

Aiken and others (2002) interlaboratory comparison. 

The other two fulvic acid samples had measured DOC 

concentrations that were 14 to 22 percent lower than 

the calculated concentrations (fig. 3). The poor 

accuracy for these two samples may be due to 

incomplete dissolution of the fulvic acid solids during 

preparation of the samples.
Quality-Control Practices 11

QUALITY-CONTROL PRACTICES

Several different types of quality-control samples 

are analyzed regularly. These include field blanks, 

instrument blanks, duplicate field samples, duplicate 

analysis samples, and standards analyzed as unknown 

samples. Acceptance criteria for results of analyses of 

quality-control samples and corrective actions 

prescribed in response to unacceptable quality-control 

results have been established in this study. 

Blank Samples

Field blanks are samples of organic-free water 

filtered under the same conditions as the environmental 

samples. The purpose of a field blank is to verify that 

there is negligible contamination of samples with DOC 

during the entire sample collection and filtration 

protocol. For sample collection and filtration protocols 

that involve filtration at the collection site, the field 

blank is collected from a carboy of organic-free water 

that is brought to the collection site. Field blanks are 

collected at the inception of a new sample collection 

and filtration protocol, when a protocol is modified, at 

the beginning of a new research project, and at intervals 

of approximately 6 months for ongoing research 

projects. Field blanks are entered into the LIMS and 

analyzed as samples. Because field blanks contain low 

concentrations of DOC, they are analyzed using the 

high sensitivity combustion catalyst. A measured DOC 

concentration in a field blank above the MDL 

necessitates a thorough examination of each step of the 

collection and filtration protocol to determine the 

source of the contaminant and eliminate it. Such 

occurrences are rare for established protocols.

DISSOLVED ORGANIC CARBON,

IN MILLIGRAMS PER LITER

6

5

4

3

2

SAMPLE

Calculated dissolved organic carbon

Measured dissolved organic carbon

Sample

Sample + NaHCO3

Sample + NaCl

KHP FA1 FA2 FA3 FA4 FA5 Caffeine

EXPLANATION

Figure 3. Measured and calculated dissolved organic carbon (DOC) concentrations in samples of potassium hydrogen phthalate (KHP), caffeine, and five 

natural fulvic acid (FA) isolates. (NaHCO3, sodium bicarbonate; NaCl, sodium chloride)
12 Method of Analysis by the USGS CA District Laboratory— Determination of Dissolved Organic Carbon in Water by High Temperature Catalytic Oxidation

Instrument blanks are samples of organic-free 

water that are put directly into the autosampler vials. 

Instrument blanks serve several purposes. Blanks 

placed directly after samples or standards with high 

DOC concentrations are used to assess the amount of 

carry-over between samples. Blanks analyzed at 

intervals during the run (fig. 1) are used to track the 

drift of the detector baseline. Baseline drift of greater 

than 0.05 mg/L correlates with unacceptable results for 

the calibration verification samples. Baseline drift is 

minimized by running a sufficient number of blank 

injections before the calibration curve is generated. 

Duplicate Samples

A minimum of 10 percent of samples are 

collected in duplicate for research projects involving 

collection of water samples at field sites. Research 

projects involving generation of water samples in the 

laboratory have sample duplication rates ranging from 

10 to 100 percent, depending on the design of the 

project. Duplicate samples are entered into the LIMS 

and analyzed as distinct samples. The purpose of 

collecting duplicate water samples from field sites is to 

verify that the sample-collection and filtration 

protocols, the laboratory sample handling procedures, 

and the analytical methods all are reproducible. 

Duplicate samples also are used to test the 

reproducibility of laboratory experiments.

Instrument duplicates are aliquots of one sample 

that are placed in separate autosampler vials. Two 

samples from every set of 10 samples are selected 

randomly to be analyzed in duplicate. The duplicate 

vials are placed at the end of the set of 10 samples 

(fig. 1). The reproducibility of instrument duplicates 

(Rdup) is reported as the percent relative agreement 

between the two analyses and is calculated as follows:

(5)

where and are the DOC concentrations 

from first and second analyses, respectively.

Rdup values between 97 and 103 percent are 

acceptable. Rdup values between 95 and 97 percent or 

between 103 and 105 percent are in the cautionary 

range. Rdup values less than 95 percent or greater than 

105 percent are unacceptable. If Rdup values for both 

instrument duplicates analyzed in a set of 10 samples 

are unacceptable, then the entire set of samples must be 

reanalyzed. If only one Rdup value is unacceptable, 

then that sample is reanalyzed and the remaining 

samples in the set also may be reanalyzed if the 

calibration verification samples for that set of samples 

are also unacceptable.

Calibration Verification

Calibration verification samples are aliquots of 

the calibration standards that are analyzed as unknown 

samples. A full complement of calibration verification 

samples are analyzed at the beginning and end of each 

run, and two additional calibration verification samples 

are analyzed between each set of 10 environmental 

samples (fig. 1). The degree to which the measured 

concentrations reproduce the known concentrations of 

the standards (Rstd) is reported as the percent relative 

agreement and is calculated as follows:

(6)

where and are the measured and known 

concentrations, respectively.

The acceptance criteria for values are the 

same as the criteria for values.

Standard Deviation and Coefficient of Variation

The DOC concentration for each sample 

represents the mean value obtained for three injections 

of sample from the same vial. The computer software 

calculates the average, SD, and CV for the peak areas 

obtained for the first three injections (eq. 1–3). If SD is 

greater than 200 area counts and CV is greater than 2 

percent, the instrument will analyze as many as two 

additional injections of sample from the same vial. The 

computer software then selects the three injections that 

yield the lowest SD and CV. The software reports the 

peak areas and DOC concentrations for all of the 

individual injections and the average DOC 

concentration, SD, and CV for the three selected 

injections. The analyst examines these data. Samples 

with SD greater than 200 area counts and CV greater 

than 2 percent are reanalyzed.

Rdup 100

C1 C2 –

C1 C2 ( ) + ⁄ 2 ------------------------------ ⎝ ⎠ ⎛ ⎞ = + × 100

C1 C2

Rstd

C

Cstd

= --------- × 100

C Cstd

Rstd

Rdup
Quality-Control Practices 13

Maintenance and Corrective Actions

Production of consistently precise and accurate 

DOC analyses requires adherence to a maintenance 

program for the carbon analyzer. The Shimadzu TOC5000A contains a number of parts that are replaced on 

a regular schedule (table 3). The regular sensitivity 

combustion catalyst is cleaned after approximately four 

analytical runs. Cleaning consists of rinsing with 

organic-free water to remove accumulated salts and 

sieving to remove cracked alumina beads. The regular 

sensitivity catalyst is replaced if cleaning does not 

improve the instrument’s performance. The high 

sensitivity catalyst is replaced when the instrument’s 

performance deteriorates. Deteriorating catalyst 

performance is identified first by inability to reach a 

low (1,500 area counts of 100 μL injection on high 

sensitivity catalyst and 2,500 area counts of 53 μL 

injection on a regular sensitivity catalyst) and stable 

peak area after repeated injections of organic-free 

water or high standard deviations (>200 area counts) 

and high coefficient of variations (>2 percent) for 

multiple sample injections from a single vial.

Table 3. Maintenance program for the Shimadzu TOC-5000A 

Total Organic Carbon Analyzer

Parts replaced on a regular schedule

Part Replacement 

interval

Carbon dioxide scrubber Biannually

Halogen scrubber Annually

Catalyst As needed

Combustion tube As needed

Syringe tip As needed

Humidifier solution Monthly

Items checked prior to each run

Injection and purge needles

Water level in humidifier container 

Water level in rinse water container

Operation of syringe

Remove any air bubbles in syringe

Syringe pump zero point detection (checked 

approximately monthly)

Gas flow rates

Temperature and stability of combustion furnace
14 Method of Analysis by the USGS CA District Laboratory— Determination of Dissolved Organic Carbon in Water by High Temperature Catalytic Oxidation

SUMMARY

 This report describes the method for routine 

analysis of dissolved organic carbon concentration in 

water samples. The Shimadzu TOC-5000A Total 

Organic Carbon Analyzer uses high temperature 

catalytic oxidation to oxidize the organic carbon to 

CO2. A nondispersive infrared detector measures the 

CO2. Keeping the instrument clean and maintained by 

replacing parts and solutions when needed is necessary 

for ensuring accurate DOC results. The method is 

validated by testing the precision of several types of 

samples, assessing the relative accuracy, and 

determining the laboratory reporting level (the 

laboratory reporting level is 0.22 mg/L). Qualitycontrol samples such as instrument blanks, duplicate 

analysis samples, and calibration verification samples 

are used to determine the acceptability of the analytical 

results. 

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