CARBON ALGORITHM SELECTION BASED ON STANDARD CONDUCTIVITY

FIELD OF THE INVENTION

This invention relates generally to methods and apparatus, and specifically to the selection of the most appropriate carbon computation algorithm, in connection with making highly accurate and consistent determinations of the total carbon (TC), inorganic carbon (IC), and/or organic carbon (OC) concentrations in an aqueous sample or sample stream using a total organic carbon (TOC) analyzer together with one or more standard solutions for periodically calibrating the analyzer.

BACKGROUND OF THE INVENTION

It is well known in the art to employ total organic carbon (TOC) analyzers to determine or monitor carbon concentrations present at very low levels in aqueous samples or sample streams. Some recent patents representing the state of the art in this field include U.S. Patent Nos. 5,132,094 (Godec et al.); 5,443,991 (Godec et al.); 5,820,823 (Godec et al.); 5,902,751 (Godec et al.); 5,798,271 (Godec et al.); 6,183,695 (Godec et al.); and 6,228,325 (Godec et al), each of which is incorporated herein by reference.

In a typical TOC analyzer application as described in the prior art, a derivative aqueous solution is formed by contacting deionized water (or another aqueous solution) with one side of a Cθ ₂ -permeable membrane at the same time that the sample of interest is contacted with the opposite side of the membrane such that at least some CO ₂ diffuses through the membrane from the sample of interest into the derivative aqueous solution. Some of the CO ₂ dissolves in the derivative aqueous solution and forms bicarbonate

(HCO ₃ ^'1 ), thereby increasing the conductivity of the derivative solution. Thereafter, temperature and conductivity measurements are made on the derivative aqueous solution. A sophisticated and carefully designed algorithm or computational formula can then be used to correlate the measured conductivity or conductivity increase of the derivative aqueous solution with the carbon concentration in the sample of interest.

It is known in the art to acidify a sample which may contain inorganic carbon prior to the membrane contacting step to convert such inorganic carbon to carbon dioxide (CO ₂ ). It is also known in the art to decompose organic carbon that may be present in a sample, for example by using oxygen and/or exposure to UV light, prior to the membrane contacting step to convert such organic carbon, at least in part, to carbon dioxide and bicarbonate ions.

To assure the reliability of the analyzer unit, it is common practice to periodically calibrate the analyzer against one or more known standards. Calibration standards for testing TOC analyzer accuracy may be packaged in either glass or plastic containers or vials, and those standards may either be acidified or not acidified. There is the potential, however, for the standards themselves to become contaminated from contact with the insides of the containers in which they are stored prior to use.

For example, standards stored in plastic containers typically maintain a stable pH and do not experience any increase over time in the level of inorganic carbon. Standards stored in plastic containers, however, whether or not they are acidified, may experience leaching from the container walls which will contaminate the standards by increasing the organic carbon content. Such contamination effect may be increased in the case of an acidified standard stored in a plastic container. Thus, this contamination effect can create

a dilemma in that acidifying the standard for storage can be useful in inhibiting biological growth, which is another potential contamination source.

Standards stored in glass containers, on the other hand, do not experience any increase over time in the level of organic carbon because no organic matter can leach out of the glass container walls. But, glass containers present their own set of potential contamination and/or measurement problems. In the case of non-acidified standards stored in glass containers, inorganic carbon levels and/or the ionic content of the standards may increase over time due to leaching from the glass walls and/or absorption of atmospheric carbon dioxide. As sodium ions leach out of the glass, the pH of a standard, originally at or near 7.0, may increase significantly. As the pH of the standard increases above 7.0, there is a growing tendency for the standard to absorb atmospheric CO ₂ thereby adding to the IC concentration and introducing measurement errors.

Although such standard contamination effects may occur only at very low levels, these effects can significantly adversely impact efforts to accurately determine TOC and/or IC in samples at very low levels such as at a few parts per billion. For some modern industrial and laboratory applications, it is desirable to determine TOC and/or IC at even a few parts per trillion level.

Utilizing acidified standards stored in glass containers can avoid most if not all of these standards contamination problems. Organic carbon cannot leach into such standards. The standards' pH remains stable, and inorganic carbon levels in such standards also remain generally stable over time. Furthermore, biological growth in the container is inhibited.

It has been found, however, that use of an acidified standard alters the usual mathematical correlations between measured conductivity and TOC determination. This

can lead to inaccurate analyzer calibration results if the usual algorithm or computational formula is applied to the conductivity measurement based on testing using the acidified standard. On the other hand, an algorithm or computational formula that provides the correct TOC results based on an acidified standard would lead to inaccurate results if it was also applied to a non-acidified standard. These and other problems with and limitations of earlier approaches to calibrating TOC analyzers in such applications are addressed in whole or at least in part by the methods and apparatus of this invention.

OBJECTS OF THE INVENTION

Accordingly, a general object of the present invention is to provide methods and related apparatus for accurately determining the TOC concentration of a sample and, alternatively, for calibrating and testing the accuracy of a TOC analyzer.

A principal object of the present invention is to provide alternative algorithms for computing TOC concentrations, and also methods and related apparatus for selecting among the alternative algorithms depending on a physical and/or a chemical characteristic of a solution being sent to a TOC analyzer for analysis.

A specific object of the present invention is to provide methods and apparatus for determining whether or not a standard being sent to a TOC analyzer for analysis as part of a calibration procedure is acidified, and for selecting among alternative correlation algorithms according to whether or not the standard is acidified.

Another object of this invention is to provide an algorithm database containing a plurality of algorithms from among which a suitable algorithm can be selected to determine carbon concentration of a standard based on temperature/ conductivity data of derivative solution(s) in accordance with whether or not the standard is acidified.

Still another object of this invention is to provide a system for automating the operations of determining a carbon concentration of a standard including the step of selecting an appropriate algorithm from among a plurality of algorithms to apply to temperature/conductivity data based on whether or not the standard is acidified.

Yet another object of this invention is to provide a computer-controlled automated system for carbon concentration determinations of a standard including selection of an appropriate algorithm from an algorithm database to apply to temperature/conductivity data.

These and other objects and advantages of this invention will be apparent from the following detailed description with reference to the attached drawings.

SUMMARY QF THE INVENTION

Methods and apparatus are disclosed for TOC analysis wherein a plurality of algorithms are provided for computing a carbon concentration in a standard based on a temperature/conductivity measurement of a derivative solution, and further wherein the selection of a particular algorithm from among those provided is based on a physical characteristic (such as conductivity) and/or a chemical characteristic (such as the pH) of the standard.

In accordance with this invention, the total carbon (TC) and inorganic carbon (IC) concentrations of a standard are calculated by a TOC analyzer using one of two alternative algorithms, wherein selecting between the two algorithms is based on whether the standard is determined to be unacidified or acidified. In a representative embodiment of this invention, a calibration standard conductivity cell measures the conductivity of a calibration standard and, if the conductivity exceeds a threshold value, the calibration

standard is determined to be "acidified." In this event, the algorithm for acidified standards is applied to the data. Alternatively, if the conductivity is at or below a threshold value, the calibration standard is determined to be "unacidified," and instead the algorithm for unacidified standards is applied to the data. The differences in these two algorithms take into account the amount of CO ₂ available for transfer across the membrane. Based on the teachings provided herein, the known prior art in this field, and routine experimental work, one of ordinary skill in this art would be able to compute/design appropriate "acidified" and "unacidified" algorithms for a specific TOC system without undue experimentation.

The reason that two algorithms are needed is that when CO ₂ dissolves in water it partially forms bicarbonate (HCO ₃ ^"1 ) and comes to equilibrium with the dissolved CO ₂ . Thus, with non-acidified calibration standards, the amount of CO ₂ available for transfer is less than it is with acidified calibration standards because with acidified calibration standards the pH shifts the equilibrium towards the formation of CO ₂ and away from bicarbonate. ha a preferred invention embodiment, calibration standard conductivity cell measurements are used to determine if the calibration standard is unacidified or acidified, which in turn is used to select the correct algorithm. In a representative invention embodiment, a calibration standard is periodically introduced into a TOC analyzer system. The conductivity of the calibration standard is measured by the calibration standard conductivity cell. The TOC analyzer calculates the TC and IC concentrations of the calibration standard using one of two algorithms depending on whether the calibration standard is determined to be "acidified" or "not acidified." Alternatively, the pH of the calibration standard can be used to determine whether the standard is "acidified" or "not

acidified," which can also be based on conductivity measurements. This method is employed in_each_individual analysis/calibration cycle.

For example, a TOC system according to this invention can be set up such that the calibration standard conductivity cell assumes that any calibration standards run during one of the calibration or validation protocols is "acidified" if the conductivity of the calibration standard is above a certain level, for example above 60 μS/cm. Calibration standards prepared by a user can be acidified to know for certain that an increase in conductivity is due to acidification. If a user prepares "non-acidified" calibration standards in plastic vials, the conductivity will increase to approximately 1.2 μS/cm due to the dissolution of CO ₂ . 1.2 μS/cm is clearly much less than the 60 μS/cm conductivity threshold, so the correct algorithm will be selected and applied. If a user prepares "non- acidified" calibration standards in glass vials, the expected increase in alkalinity (from the glass) will also increase the conductivity. However, such a conductivity increase will not be enough to exceed the 60 μS/cm limit. Thus, here again, the correct algorithm will be applied.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic view of an illustrative TOC analyzer apparatus designed and arranged to facilitate the practice of this invention.

Figure 2 is a decision-tree flow chart illustrating a method according to this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The TOC analyzer apparatus 10 as illustrated in Fig. 1 comprises the following subsystems in combination:

(a) a sample/standards supply system for providing either a sample to be analyzed or, alternatively, a calibration standard solution to the analyzer system;

(b) an analyzer system comprising two fluid flow paths, one of which includes a calibration standard conductivity cell together with two CO ₂ transfer modules, one in each of the two fluid flow paths, each CO ₂ transfer module being associated with a downstream temperature and conductivity cell; and,

(c) a deionized (DI) water system for supplying deionized water to a CO ₂ - receiving side of each of the CO ₂ transfer modules.

The sample/standards supply system as illustrated in Fig. 1 may comprise an inlet line 20, an inline filter 21, a flow sensor 23, a waste line 24, and a sample/ standard supply line 25 connecting to a T-junction or comparable 3-way fluid connector 26.

As shown in Fig. 1, the TOC analyzer system comprises two measurement modules 36 and 38, each associated with a separate sample/standard flow path. Each measurement module 36 and 38 comprises a CO ₂ transfer module, comprising fluid chambers on either side of a CO ₂ -permeable membrane, and a temperature-conductivity cell to obtain temperature/conductivity data on a derivative solution coming from a deionized water side of the CO ₂ transfer module.

A sample or calibration standard introduced into the analyzer system is split into two portions at the 3-way connector 26, which may in some embodiments comprise a flow control valve. A first portion of the split sample or standard is directed along a first flow path via fluid line 27 of the analyzer system through an oxidation reactor 31 to decompose

at least a part of any organic carbon materials which might be present in this first portion of the split sample or standard into CO ₂ and/or bicarbonate before passing the oxidation reactor-treated sample or standard portion via flow path 33 into and through a first CO ₂ transfer module 14. The first CO ₂ transfer module 14 comprises fluid chambers 40 and 41 on opposite sides of a CO ₂ -permeable membrane 15. While the oxidation reactor-treated sample or standard portion is contacted with one side of the membrane 15 in fluid chamber 40, a first deionized (DI) water portion supplied by line 67 is contacted with the opposite side of the membrane 15 in fluid chamber 41. Thereafter, the first DI water portion, now containing CO ₂ that diffused through the membrane 15 from the reactor-treated sample or standard portion, is passed by line 42 to a first temperature/conductivity cell 43.

A second portion of the split sample or standard is directed along a second flow path 28 of the analyzer system through a calibration standard conductivity cell 29 (to measure the conductivity of the second portion) and a delay coil 32 before passing into a second CO ₂ transfer module 16. Although the thrust of this invention concerns using cell 29 to obtain a conductivity measurement of the second portion of a split standard stream as part of the method for selecting the proper algorithm to compute the carbon concentrations of the standards, it will be understood that cell 29 can be and is typically also used to measure the conductivity of the second portion of a split sample stream when the system is in its normal (non-calibration) mode of operation.

The second CO ₂ transfer module 16 also comprises two fluid chambers 50 and 51 on opposite sides of a Cθ ₂ -permeable membrane 17. While the second portion of the split sample or standard stream is contacted with one side of the membrane 17 in fluid chamber 50, a second DI water portion supplied by line 66 is contacted with the opposite side of the membrane 17 in fluid chamber 51. Thereafter, the second DLwater portion, now

containing CO ₂ that diffused through the membrane 17 from the second portion of the split sample or standard is passed by line 52 to a second temperature/ conductivity cell 53.

Before applying an algorithm to a temperature/conductivity cell conductivity measurement in order to compute a carbon concentration in a standard portion during a periodic calibration or validation procedure, the calibration standard conductivity cell measurement from cell 29 is used to make a preliminary determination about whether the calibration standard portion qualifies as "acidified" according to a predetermined "threshold" pH. Because the conductivity of the standard portion is normally correlated with the degree of acidification, a conclusion about the pH Qf the standard portion can be reached from this calibration standard conductivity data.

The deionized (DI) water system as illustrated in Fig. 1 may comprise a DI water pump 61, a canister of ion exchange resin 63, and a set of related fluid lines to circulate the water. Thus, a water portion containing CO ₂ from CO ₂ transfer module 14 is passed through temperature/conductivity cell 43, then via line 44 to a solenoid valve 45, then via line 46 to a flow restrictor 47, then via line 48 to return 60 going to the DI water pump 61. Similarly, a water portion containing CO ₂ from CO ₂ transfer module 16 is passed through temperature/conductivity cell 53, then via line 54 to a solenoid valve 55, then via line 56 to a flow restrictor 57, then via line 58 to return 60 going to the DI water pump 61.

From DI water pump 61, the fluid returned to the DI water system is sent by line 62 to the ion exchange unit 63 which removes CO ₂ , bicarbonate and/or carbonate from the returned fluid. Fresh DI water coming from unit 63 is passed via line 64 to a four-way connector 65. Line 67 coming from connector 65 passes DI water into chamber 41 of CO ₂ transfer module 14. Line 66 coming from connector 65 passes DI water into chamber 51

OfCO ₂ transfer module 16. Line 68 from connector 65 recirculates extra DI water to pump 61 for mixing with returned fluid.

As also shown in Fig. 1, sample streams coming from chamber 40 of module 14 and from chamber 50 of module 16 are passed respectively by lines 70 and 71 to a sample pump 72, and from there via lines 73 and 74 to a waste line 75.

Fig.2 is a decision tree-type flow chart to further illustrate the methods of this invention. At step 202, the operator or the automated system initiates an instrument protocol, which may or may not involve the use of standards. If the protocol involves the use of standards, one or more of those standards may be acidified, and one or more of the standards may not be acidified. Step 204 asks whether the analysis/testing protocol requires the use of calibration standards. IfNo," a first, "non-acidified" algorithm is applied to compute carbon concentrations (step 210) and the procedure is ended.

If "Yes" in step 204, the method moves on to step 206, this time looking at whether the measured conductivity of the calibration standard is greater than a pre- established threshold level, for example greater than 60 μS/cm. If "No," once more the first, "non-acidified" algorithm is applied to compute carbon concentrations (step 210), and the procedure is ended. If "Yes" in step 206, however, the method moves on to step 208 in which a second, "acidified" algorithm is applied to the conductivity measurements of the DI water portions coming out of the CO ₂ transfer modules (see Fig. 1) in order to compute carbon concentrations in the calibration standard. Following step 208, the procedure is ended.

In a preferred embodiment of this invention, a computerized control system, including providing an algorithm database containing a plurality of algorithms, can be arranged and programmed to carry out the steps of the method as above described. Based

on the teachings provided herein and the known prior art in the computer programming arts, one of ordinary skill in these areas could develop new software and/or adapt existing commercially-available software to carry out an automated, computer-controlled TOC analysis procedure in accordance with this invention without undue experimentation.

The apparatus and methods of this invention are novel in that they allow the TOC analyzer system (with no pH control) to sample TOC standards contained in glass vials as well as those contained in plastic vials and to test appropriately for the correct algorithm to be applied. This is advantageous because plastic vials are known to contaminate TOC standards by leeching TOC into the standards. This invention also solves the problem of increased pH and IC associated with standards stored in glass vials as these standards can be acidified to control the pH and IC. This invention can readily be incorporated into existing TOC analyzer product lines, and also has the potential for use in TOC products still under development.

This invention also solves the pH problem attributed to using glass vials with unacidified standards. As previously discussed, the pH of unacidified standards in glass vials increases due to sodium leeching from the glass into the standard, and this effect is variable from vial to vial. Such a pH increase causes measurement errors in TOC analyzers where the pH is not controlled. With acidified standards, the pH is typically controlled to a pH of 3 +/- 0.5.

This invention also limits the increase in IC concentration in the standards. The increase in pH in unacidified standards in glass allows for more atmospheric CO ₂ to dissolve in the standard thereby significantly increasing the level of IC in the standards (e.g., by as much as 180 ppb C to 800 ppb C). This increase in IC is variable from vial to vial and can significantly affect the accuracy of the TOC measurement on.sub-500 ppb C

TOC standards since a relatively small TOC concentration must be resolved from large TC and IC measurements. Acidification of the standards keeps the IC level at a manageable level, such as at or below 250-300 ppb C.

The present invention has been described in detail with reference to preferred embodiments thereof, and although specific terms are employed in describing this invention, they are used and are to be interpreted in a generic and a descriptive sense only and not for purpose of limitation. Accordingly, it will be understood to those of ordinary skill in the art that various changes, substitutions and alterations in form and details may be made without departing from the spirit and scope of the present invention.

Having described the invention, what is claimed is: