T E C H N I C A L
B R I E F
Measurement
of Total Organic Carbon in High-Purity Water
TO ENSURE SUCCESS OF SENSITIVE RESEARCH METHODS
Types of Organic
Contaminants
• Particulate Organic Carbon (POC) –
Organic matter (TOC) retained by
a 0.45-µm filter
The total carbon content in water is
expressed as the sum of the carbon
contained in different organic fractions.1 Because each organic fraction
differs in physical and chemical properties, an appropriate analytical
method must be used to effectively
measure specific families of organic
compounds. The major organic fractions are defined as follows:
• Dissolved Organic Carbon (DOC) –
Organic matter (TOC) which
passes through a 0.45-µm filter
• Total Carbon (TC) – The total
amount of elemental carbon in a
substance or solution
• Total Inorganic Carbon (TIC) –
Bicarbonate, carbonate and dissolved CO2 in aqueous solution
• Total Organic Carbon (TOC) – The
amount of carbon covalently bonded in organic molecules
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• Volatile Organic Carbon (VOC) –
Organic matter (TOC) removed
from an aqueous solution via vapor
transfer or by displacement using a
purge gas under specific conditions
TOC measurements in high-purity
water detect primarily DOC and a
portion of VOC. The amount of VOC
detected may vary and is a function
of the sampling conditions (e.g. temperature, pressure) and the volatile
nature (vapor pressure) of the organic
compounds. TIC is not included in
TOC measurement, and high levels of
ions (conductivity) may cause interference in some test methods. Overall,
TOC measurement is an effective way
to account for and represent the
organic species in water.
The Impact of
Organics in Water
As a result of the dramatic increase in
the sensitivity of analytical instrumentation and experimental methods,
organic contamination in reagent
water is a major concern in the laboratory. High concentrations of organic
substances in water may result in
problems such as:
Impact of Ultrapure Water TOC Content on Protein Analysis
Analysis of neonatal hemoglobins via ion-exchange chromatography: after
approximately 100 sample runs
0.050
Column: MemSep 500 CM
Gradient: 0-50% NaCI, 20 mM NaPO4
20 ul injection at pH 7.0
Detector: 415 nm
Run Time: 3 minutes
0.030
AU
0.020
0.010
• Poor detection limits
0.000
0.0
• Poor reproducibility (see figure 1)
• Elevated blank background (see
figure 2)
buffers using H20 at 5 ppb TOC
buffers using H20 at ~100 ppb TOC
0.040
0.5
1.0
1.5
2.0
2.5
3.0
Retention Time (Minutes)
Figure 1
• Coating of reactive surfaces
• Chemical interference
Impact of Ultrapure Water TOC Content on LC-MS Baseline
Total ion counts for trace-enriched blank water samples
• Dispersive or non-dispersive effects
1.0E+4
100
• Fouling of separation or purification
media
System: PE BioSystems Mariner LC-MS
Column: Phenomenex Luna C18 2.0 x 50 mm
Solvent A = 98% H2O: 2% ACN: 0.1% formic acid
Solvent B = 98% ACN: 2% H2O: 0.1% formic acid
Gradient: Linear 0 –100% A to B
50 mL trace enrichment of sample
Flow Rate: 0.20 mL/min
90
80
• Promotion of microbial growth
70
• Toxicity
The reduction of organics from
water can be accomplished using a
combination of purification technologies (e.g. activated carbon, ultraviolet
(UV) oxidation and ion-exchange) that
are properly configured into a water
purification system. Selection of a system that produces water of suitable
organic purity is critical to obtaining
optimal performance from analytical or
preparative techniques.
Equally important is the need to
accurately monitor the organic purity
of the water. The ionic purity of water
is commonly measured using on-line
sensors that produce a continuous display of resistivity (in MΩ-cm) or conductivity (in µS/cm). However, since
many organics undergo little or no ionization in water, resistivity measurement cannot give an accurate indication of organic levels. Additionally,
many off-line organic analysis methods
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% Intensity
60
50
40
30
20
H20 at 12 ppb TOC via internal monitor
H20 at 4 ppb TOC via internal monitor
10
0
0
3.4
6.8
10.2
Retention Time (Minutes)
Figure 2
have poor detection limits, require
long analysis times, or are prone to
sample contamination.2 On-line organic detection provides an accurate,
sensitive and rapid measurement of
the dissolved organics in water.
Incorporating an on-line TOC monitor
within a benchtop laboratory water
purification system is an ideal solution
for monitoring and maintaining the
organic levels in high-purity water.
13.6
17.0
TOC Measurement via
UV Oxidation
TOC may be measured in high-purity
water using a range of methods
including combustion, wet oxidation,
UV photo-oxidation and persulfate-UV
oxidation.3
Ultraviolet (UV) photo-oxidation has
proven to be an effective method to
measure low TOC levels found in
ultrapure water. All matter is composed of molecules held together by
chemical bonds. When certain molecules absorb energy from UV radiation, the chemical bonds break to produce molecules called free radicals.
Free radicals are short-lived, highly
reactive species which can oxidize
organic molecules. One of the most
powerful oxidizers is the hydroxyl radical (OH•). Free radicals ionize organics and subsequently create a change
in conductivity of the water (figure 3).
The conductivity changes can be used
to calculate the respective TOC concentration in the water.
A TOC measurement device consists of a UV oxidation lamp, conductivity sensor and a temperature sensor
enclosed in a flow-through cell (figure
4). A measured volume of purified
water is captured within the cell. The
UV lamp ignites and oxidizes the
organic material in the water sample,
and the change in conductivity during
the oxidation is recorded. The cell is
purged, collects another water sample, and repeats the oxidation process.
Data collected are processed through
a mathematical equation that accurately relates the oxidation and conductivity conditions to TOC concentra-
Applications and
Benefits of TOC
Monitoring
The on-line monitoring of TOC within
a water system provides some unique
benefits in laboratory analysis and
quality control. Often, organic impurities will break through the purification
media before the ionic impurities (figure 6). Resistivity alone cannot detect
increases or fluctuations of organic
levels resulting from organic breakthrough.
The monitoring of TOC can provide
an early warning of contaminant
tion that is continuously displayed on a
digital monitor.
TOC in water is composed of a
complex mixture of organic molecules.
In order to accurately predict the TOC
values, numerous data must be collected and processed to account for the
differing oxidation rates of various
organics in the water (figure 5). A
simple measurement of conductivity
change in the water stream without
consideration for the various organic
oxidation rates will yield an incorrect
TOC measurement.
Organic Breakthrough
Feed Water
TOC Measurement
Device in Ultrapure Water
System
Carbon
Mixed Bed Ion
Exchange Resin
Outlet
Sampling Valve
Organic Scavenger
H 2O
Conductivity
Temperature
Sensors
UV Lamp @
185+254 nm
Inorganic
ions
Order
of Breakthrough
Lamp Power
Supply
Silica
Dissolved
organics
Microorganisms
Nitrogen
Product Water
Microorganisms
Dissolved organics
Quartz Window
H2O
Inlet
Silica
Inorganic ions
Figure 4
Chemical Reactions Occurring during the
TOC Measurement Cycle
Monitor Function
Chemical Reaction
Fill
CxHxOx + H2O
Oxidize
Measure
CxHxOx
UV185-nm
CO2 + H2O
H2CO3
Conductivity versus Photo-oxidation Time for
Organics Analyzed via TOC Monitor
3 high-purity water samples each containing 9 ppb TOC
Sample B
0.5
CO2 + H2O
H2CO3HCO3- + H+
Conductivity
µS/c m
Sample A
Sample C
Oxidation Time
Figure 3
0.055
Toxa
Toxb
Complete Oxidation
Figure 5
Toxc
For Further Information
Resistivity and TOC versus Time During Organic Challenge
(Sucrose) to an Ultrapure Water System
20
1200
18
1000
16
800
14
600
12
400
TOC (ppb)
Resistivity (M Ω-cm)
Resistivity
In the U.S. and Canada, call toll free
1-800-MILLIPORE (1-800-645-5476).
In the U.S., Canada and Puerto Rico, fax
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On the Internet go to
www.millipore.com/H2O
TOC
200
10
0
0
30
60
90
120
150
180
210
0
240
Time (Minutes)
Figure 6
breakthrough and prevent the use of product water with unacceptable levels of
organic contamination. Other benefits include:
• Compliance with regulatory standards (USP, BP, GLP)
• Quality control of reagents and process fluids
• Optimization of analytical methods (detection limits, retention times, column life)
• Troubleshooting of analytical methods
The on-line measurement of TOC provides an essential tool for monitoring and
maintaining an ultrapure water system. The use of TOC measurement technology
allows greater control over laboratory results when performing organic-sensitive
applications.4
REFERENCES
1. “Standard Methods for the Examination of Water and Wastewater, 19th edition.” 5310
Total Organic Carbon, pp. 5-16, 5-17, American Public Health Association,
Washington, DC, 1995.
2. “Standard Methods for the Examination of Water and Wastewater, 19th edition.” 5310
Total Organic Carbon, pp. 5-17, 5-21, American Public Health Association,
Washington, DC, 1995.
3. Egozy, Y., J. Denoncourt, “Trace Level Analysis of High-purity Water Part 2: Total
Organic Carbon,” Ultrapure Water, November/December 1986, Tall Oaks Publishing.
4. Clark, K., M. Retzik, D. Darbouret, “Measuring TOC to Maintain High-purity Water,”
Ultrapure Water, February 1997, Tall Oaks Publishing.
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