GB1561571A - Method and apparatus for chromatographically analysing a liquidsample - Google Patents

Description

(54) METHOD AND APPARATUS FOR CHROMATOGRAPHICALLY ANALYZING A LIQUID SAMPLE (71) We, THERMO ELECTRON CORPORATION of 101 First Avenue, Waltham, Massachusetts, United States of America, a corporation organized and existing under the laws of the State of Delaware, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: Chromatography is an analytical science in which a complex mixture is separated into its individual constituents so that the constituents can be identified and quantified. Since its advent in the nineteen sixties, gas chromatography has caused a major revolution in organic chemistry.More recently, there has been developed apparatus for high pressure liquid chromatography in which a liquid sample is introduced into a chromatograph column and then, instead of being vaporized, is kept in the liquid phase during the separation process.

Liquid chromatography offers numerous advantages over gas chromatography. Among them, a liquid chromatograph column can be operated at ambient temperature. Also, separation of the components of the sample mixture in the liquid phase facilitates a degree of control not easily available with gas chromatograph columns. Some compounds which tend to be broken down or suffer unwanted molecular reorganization in a gas chromatograph column can be separated quite readly in a liquid chromatograph column.

For example, many organic compounds, because of their polarity, high molecular weight or thermal instability, are not amenable to gas chromatographic techniques but are well suited to high pressure liquid chromatographic analysis. On the other hand, existing detectors interface quite readly with gas chromatographs, but liquid chromatography tends to be limited by poor sensitivity of the compatible detectors. Substantial effort has been devoted to improvement of the most widely used detectors, namely refractive index and ultra violet absorbance detectors. Other techniques have also been explored, including micro absorption, polarographic, and conductivity detectors.

The early Nobel prize winning work of A.J.P. Martin and R.L.M. Synge Biochem. J. 35, 91, 1358 (1941) set forth the basic liquid chromatographic techniques used in systems today.

However, practical application for these techniques has been severely limited by available detectors. As a result, liquid chromatographic analysis is generally a length procedure often taking hours and even days. The availability of high pressure pumps in excess of 5,000 psi) permits the use of long, narrow bore (e.g. imm columns having small diameter packing particles, and use of such small diameter columns minimizes the time required for quid chromatographic analysis, but the level of sensitivity is still below that associated with gas chromatographic systems.

In our patent No. 1513007 we describe and claim a method and apparatus for the high speed liquid chromatographic analysis of samples in liquid form to detect the presence of a predetermined compound or compounds, bemg particularly concerned with the detection of nitrogen and/or sulphur contaning organic compounds, and the present invention relates to a similar method and apparatus.

According to the invention, a method of analysing a sample for the presence of a predetermined compound comprises passing the sample in liquid form through a liquid chromatograph so that different compounds in the sample are separated from each other and are discharged from the chromatograph at different times, injecting the discharge from the chromatograph into a converter, also supplying to the converter a gaseous reactant, mixing and atomizing the discharge and the gaseous reactant in the converter, maintaining in the converter a temperature such that the mixed and atomized reactant and the predetermined compound react in the converter to produce a specific gas, and passing the gaseous products of the converter to a detector which is sensitive to the specific gas whereby detection of the specific gas is indicative of the presence of the predetermined compound in the sample.

The invention basically involves the use of a liquid chromatograph in conjunction with a converter in which the effluent from the chromatograph is atomised and reacted to form a specific gas from a predetermined compound in the effluent, and monitoring the products of the converter with a specific gas detector to yield information about the chromatographic effluent and hence the initial sample. In this way the advantages of both liquid and gas chromatographic techniques may be obtained.

In a preferred embodiment of the invention in which a sample is analysed for the presence of organic compounds containing either nitrogen or sulphur, the sample in a liquid phase solvent is passed through a high pressure liquid chromatograph to separate the components of the sample in a timewise fashion. Typically, the high pressure liquid chromatograph includes a high pressure pump, a sample injector, a chromatograph column, and an output port from the column. The effluent from the chromatograph column is received by an atomizing nozzle for injecting and atomizing the effluent into an oxidizing converter. The nozzle receives the chromatograph effluent along one path and, along anther path, a supply of oxygen which is sufficient to fully oxidise the nitrogen or sulphur contaning compounds in the effluent from the chromatograph.An excess of oxygen is preferred to assure that complete oxidation will occur in the converter. Optionally, an inert gas may be supplied to the converter, either mixed with the oxygen or along a separate path, to assist in optimizing the temperature profile within and along the oxidizing converter.

The conditions of oxygen-rich oxidation taking place in the converter may vary in some respects depending on the constituents injected therein. For example, the solvent used in the liquid chromatograph may be a flammable solvent, such as acetone, or an inflammable solvent such as water. It will be understood that an appropriate solvent may be selected to dissolve the sample so that none of the elements which are to be detected are present in the solvent. As a result, trace compounds may be detected without being masked by the solvent and without requiring removal of the solvent prior to detection.In the case of nitrogen containing compounds, oxidation occurs mainly in the temperature range from 600"C to 1800"C. Between 17500C and 18500c elemental nitrogen begins to oxidize to produce NO.

Above these temperatures such production of NO tends to constitute interference which deteriorates the accuracy of the analysis. Converter temperatues somewhat below 600"C may be effective to oxidize combined nitrogen, the exact operating temperature being variable depending on sensitivity and selectivity desired. A preferred temperature range for most applications is 900"C to 1150"C. The solvent, oxygen, and chromatographically separated sample are thoroughly mixed in the converter to produce conversion to nitrogen oxide of the bound nitrogen in organic nitrogen containing compounds of the sample.

Mixing, and thus oxidation, can be facilitated by packing at least a portion of the converter with inert particles, such as ceramic particles. An inert particulate packing within at least a portion of the converter is particularly useful to enhance complete oxidation when inflammable solvents are used.

External heaters may be used to maintain the temperature of the converter at a preset level in the range mentioned above. If flammable solvents are used, the solvent-sampleoxygen mixture may burst into flame in the converter, in which case external heaters are used to a lesser extent or not at all, as required to maintain the desired temperature. If the solvent is highly flammable, it may tend to combust at or very near the nozzle tip. This can produce overheating of the converter, and of the nozzle in some cases. To avoid this, it may be desirable to introduce an inert gas with the oxygen to slow the reaction and produce oxidation further downstream in the converter.

When the organic compounds within the sample contain nitrogen and the system is operated to detect such nitrogen-containing compounds, effluent from the converter is directed to a nitric oxide analyser. The reaction taking place in the converter converts the nitrogen to nitric oxide according to the following general reaction:

All organic nitrogen containing compounds which are fully oxidized will combine, as indicated above, to produce carbon dioxide, water and nitric oxide. Accordingly, the reading from the nitric oxide detector provides a measure of nitrogen containing organic compounds in the original sample. When necessary, a cold trap or other type of trap may be introduced between the converter and the gas detector for removing an excess volume of water or other interfering materials.If it is desirable to remove solvents prior to their detection, the cold trap is an effective means by which this can be accomplished. One circumstance which could dictate the necessity of a cold trap might be that where, for some reason, it is desirable to use a solvent which tends to mask the element which is to be detected by the gas detection device. The use of a cold trap in a liquid chromatographic analysis system is described in more detail in our Specification No. 1,512,123. It should be understood, however, that water, carbon dioxide and most other contaminants do not influence the reading if a high selective nitric oxide detector, such as a chemiluminescent analyzer, is used.

When the sample subjected to liquid chromatographic analysis includes sulphurcontaining compounds and it is desired to detect such compounds, the system operates in a similar fashion to that described above in connection with nitrogen compounds, a sulphur dioxide analyzer being substituted for a nitric oxide analyzer. The reaction taking place in the converter is according to the following general formula:

As can be seen from the above general reaction, sulphur in the organic sulphur containing compound is converted to SO2 whereas carbon and hydrogen are oxidized to carbon dioxide and water, respectively. The gas detector selective to sulphur dioxide will provide a reading functionally related to the sulphur-containing organic compounds in the sample.One suitable sulphur dioxide detector is the Model 43 fluorescent SO2 detector manufactured and sold by Thermo Electron Corporation of Waltham, Massachusetts, U.S.A. or as described in U.S. Patent Specification No. 3,845,309. Other examples of compatible SO2 analyzers are detectors based on a flame photometric principle, an ultraviolet absorption principle or a coulometric principle.

The ultimate sensitivity of a system with which the method in accordance with the invention is carried out is dependent, among other things, on the sensitivity of the specific gas detector. Chemiluminescent detectors for detecting nitric oxide in a mixed gas sample are sensitive in the range of 1 part in 109. Detectors sensitive to sulphur dioxide such as the Model 43 fluorescent detector mentioned above, are sensitive in the range of 1 part in 106.

These instruments can be compatibly coupled to the output from a high pressure liquid chromatograph through the converter described above. Important to the operation of the converter is its facility to mix the constituents from the liquid chromatograph with the oxygen and, in turn, the way in which the liquid chromatograph effluent is introduced into the converter is important if optimum performance is to be obtained.

As mentioned earlier, the effluent from the liquid chromatograph is atomised in the converter, this optimizing performance, particularly when analysing samples including non-volatile constituents. While volatile constituents tend to oxidize more readily, it is particularly difficult to produce total molecular contact between the non-volatile constituents and oxygen to assure that complete oxidation of the constituents in the sample to be measured occurs.

According to a further aspect of the invention, apparatus for carrying out the method in accordance with the invention comprises a liquid chromatograph for separating from each other the different compounds in a sample which is passed in liquid form through the chromatograph and discharging the separated compounds at different times, a converter for reacting the liquid discharge from the chromatograph with a gaseous reactant substantially in the temperature range of from 600"C to 18000C, a nozzle for injecting the gaseous reactant and the discharge from the chromatograph into the converter whereby the discharge is atomised in the converter, the nozzle comprising liquid passage means extending from the chromatograph into the converter for carrying and injecting into the converter the chromatograph discharge, the temporal separation of the compounds in the injected discharge being substantially unchanged from that established in the chromatograph, and gaseous reactant passage means extending into the converter and surrounding the liquid passage means so that the gaseous reactant is injected into the converter in a pattern around the injected liquid discharge, and a specific gas detector for receiving the effluent from the converter.

Suitable nozzle configurations may vary. For example, fluid mixing nozzles, preferred examples of which will be described later, and ultrasonic atomization nozzles may be used.

Excellent results have been obtained with liquid nozzles which introduce materials into the converter along paths which are concentric one with the other. In the preferred embodiment, effluent from the liquid chromatograph is introduced into the converter in the form of a thin jet, through an elongated tube. Surrounding this tube is a second tube through which oxygen or other gases are admitted. The gases then form an output surrounding or approximately surrounding, the jet of chromatograph effluent to break it up and disperse it into small particles, thereby atomizing it. A dual tube arrantement of the type referred to in the previous sentence may be used, or if more numerous inputs to the converter are desired, additional concentric tubes may be used to establish additional concentrically arranged flow paths.

Various examples of methods and apparatus in accordance with the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram illustrating a preferred apparatus in accordance with the invention; Figure 2 is a block diagram illustrating the embodiment of Figure 1 and other similar arrangements; Figure 3 is a sectional view showing a portion of the apparatus illustrated in Figure 1; Figure 4 is a cross-section taken along the line 4-4 in Figure 3; Figure 5 is a view similar to that of Figure 4 illustrating an alternative embodiment of the invention; Figure 6 is a block diagram illustrating a specific example of the invention; and, Figures 7 and 8 show chromatograms resulting from two different operations using apparatus of the type illustrated in Figure 6.

Figure 1 illustrates a preferred embodiment of the present invention, showing three fundamental components, namely a liquid chromatograph 10, an oxidizing converter 12, and a specific gas detector 14 which in this case is a nitric oxide detector. The liquid chromatograph 10 separates constituents in a liquid mixture and provides an effluent in which the constituents are separated from each other in time. The effluent is injected into the converter 12 and oxidized. The oxidation products from the converter are then introduced to the nitric oxide detector 14, and the nitric oxide detected by the detector 14 provides a reading of the nitrogen containing organic compounds contained in the effluent from the chromatograph 10. The apparatus and its method of operation is more fully described below.

The chromatograph 10 includes a chromatographic column 16 used in association with a high pressure pump (not shown in Figure 1) for forcing liquids through the column 16. A sample suspected of including nitrogen-containing organic compounds is presented in a liquid form, possibly dissolved in a solvent, to the liquid chromatograph 10. As the sample is forced by the high pressure pump through the chromatographic column 16, various constituents in the sample are separated from each other and discharged from the column 16 individually in timewise distribution. Each constituent can be identified by its time of discharge from the column, and it is important in passing effluent from the chromatographic column through the converter 12 and the nitric oxide detector 14 that the timewise distribution of the constituents is maintained.In this manner, the output of the detector 14 is properly associated with the various constituents.

To maintain the timewise distribution, the effluent from the chromatograph is conducted to a nozzle 18, which will be described in detail subsequently. The nozzle 18 also receives oxygen from an oxygen supply 20 and, optionally, may receive an inert gas through an inert gas supply 22. The nozzle 18 atomizes effluent from the column 12 and introduces it into an oxidizing chamber 24. This chamber is associated with an electrical heater 82 for maintaining the chamber temperature at a selected level.

The oxidizing chamber 24 is of a construction which will assist intimate mixing of the effluent from the chromatographic column with the oxygen so that all the combined nitrogen in organic compounds of the sample is oxidized. Oxidation may be with or without combustion. The chamber described below is one which provides successful operation for all types of liquid chromatographic column effluent and solvents, including volatile and non-volatile effluent sample constituents and including flammable and inflammable solvents. Fundamentally, the chamber 24 comprises an elongated tubular section 26 in which the oxidizing reactions take place. To promote mixing of the reactants, the section 26 may be filled with an inert particulate material 28, such as ceramic balls. The inert particulate packing material 28 is particularly desirable for reactants which include either a non-volatile constituent or an inflammable solvent. The particulate material is held in place by perforated or porous members 30. The oxidizing chamber 24 also includes an enlarged fore-section 32 which receives the discharge from the nozzle 18. This section 32 provides preliminary mixing of the materials before they enter the section 26 containing the packed bed of inert particulate material 28. Also, the enlarged fore-section 32 may serve as a combustion chamber where highly flammable constituents experience at least the initial phases of combustion prior to entering the particulate bed 28.In situations where the fore-section 32 might tend to overheat, inert gas supplied through the inert gas supply 22 will serve to slow the reaction and maintain an acceptable temperature within the fore-section 32. Downstream from the tubular section 26 there is an aft-section 34 of reduced diameter. This section 34 physically conducts the combustion products from the chamber 24 to the gas detector 14, and is of reduced diameter to increase flow rate and reduce the time required for materials to travel its length. It is desirable to minize transit time through the aft-section 34 to avoid a loss therein of nitric oxide, which tends to oxidise to nitrogen dioxide under certan conditions, the reaction being temperature and time dependent. The nitric oxide passing through the aft-section 34 is in the presence of the remaining oxygen introduced into the chamber from the nozzle 18.At higher temperatures, the nitric oxide tends to be stable and, as the temperature is lowered, nitrogen dioxide tends to be produced. For further information relating to the stability of nitric oxide see "Principle of Operation of the Thermal Energy Analyzer for Trace Analyzing Volatile and Non-volatile N-Nitroso Compounds" by D.H. Fine, D. Lieb and F. Rufeh; Journal of Chromatography, 107 (1975) 351 - 357.

The oxidizing converter 12 may also include a trap 36, which may be a cold trap, to remove unwanted constituents from the gas stream. Additionally a throttle 38 may be used to produce a pressure drop if reaction in the nitric oxide detector 14 is to be carried out at sub-atmospheric pressure.

From the aft-section 34 the oxidized effluent from the oxidizing reaction chamber 24 enters the gas detector 14. The gas detector 14 may be any suitable instrument for measuring oxidized nitrogen. For example, the detector may measure NO directly or convert NO to NO2 and measure NO2. An instrument of the chemiluminescent type is illustrated schematically. A chemiluminescent reaction chamber 40 is associated with a photo-multiplier tube assembly 42. A shutter 44 driven by a motor 46 is interposed between the photo-multiplier tube and the reaction chamber to increase sensitivity. By use of the shutter, the tube assembly 42 alternately samples the desired signal and a reference signal, in order to achieve a better signal to noise ratio. The chemiluminescent reaction chamber 40 is connected to an ozone (03) supply 48.Reaction in the chamber 40 between the ozone and nitric oxide in the discharge from the chamber 24 produces a chemiluminescent reaction in which electrically excited nitrogen dioxide is formed. The nitrogen dioxide decays back to its ground state with the emission of light which is detected by the photo-multiplier tube assembly 42. The intensity of light emitted in the chemiluminescent reaction is directly related to the number of moles of nitrogen in the sample discharged through the nozzle 18.

The chemiluminescent reaction occurs in accordance with the following reaction: NO + 03 = NO2* + O2 NO2* = NO2 + hv The photo-multiplier tube is associated with a signal processor 50 which provides an output 52 for a recorder and a visual output indicator 54. If it is desired to operate the reaction chamber 40 below atmospheric pressure, a vacuum pump 56 is communicated with the chamber 40 through its exhaust means 57. During operation the vacuum pump 56 and the flow throttle 38 co-operate to reduce the pressure in the reaction chamber 40 to a sub-atmospheric level. If operation is to be at atmospheric pressure, both the pump and the flow throttle 38 are unnecessary. Typical chemiluminescent analyzers are disclosed in U.S.

Patent Specifications Nos. 3,746,513 and 3,763,877.

The apparatus described above for detecting organic nitrogen containing compounds can be controlled by any suitable control system. The system can be relatively sophisticated or relatively simple. A control ystem 60 is illustrated schematically to identify various functional parameters which can be detected or controlled in an operating apparatus. A central electrical control 62 may detect operating conditions in various parts of the system and, in response thereto, determine operating conditions in various other parts of the system. For example, in response to detected conditions flow, temperature and pressure may be regulated as desired. The control system 60 will be described from the output of the chromatograph 16 through to the nitric oxide detector 14.

Chromatograph effluent is fed to the atomizing nozzle 18 of the oxidizing converter 12.

As mentioned earlier, one function of the atomizing nozzle 18 is to enhance mixing of nitrogen-containing organic compounds from the chromatograph with sufficient oxygen from the oxygen supply 20 so that oxidation of the nitrogen in the organic nitrogencontaining compounds will be complete. In the preferred embodiment, oxygen is provided so that a combination of oxygen supply in excess of any anticipated stoichlometric mixture and very thorough mixing produces oxidation which is, for all practical purposes, complete.

Operation can be enhanced by automatic control of the feed of materials to the nozzle 18.

For example, the electric control 62, as indicated by elements 64, 66 and 68, controls the mixture of chromatograph effluent, oxygen and inert gas by establishing predetermined flow rates for each such constituent. On the other hand, the element 64 may be a sensing device which determines the flow of chromatograph effluent to the nozzle 18 and provides a signal, in response to which the control 62 operates the elements 68 and 66 to control the flow of oxygen and inert gas to the nozzle. An element 70 may be used in conjunction with the element 64 to bleed off a fractional portion of chromatograph effluent if the supply is larger in volume than that which is desirable to pass through the oxidizing converter 12 or the nitric oxide analyzer 14.It will of course be understood that flow control could be manual such as by observation of flow indicating devices 72 and 74 and manual operation of valves 76 and 78.

As will be explained later, the nozzle 18 may include, as an optional feature, an internal heating unit. If included this unit can be supplied with power from the electrical control 62, as indicated at 80.

The heater 82 may provide a single heating zone for maintaining uniform temperature or it may be provided with multiple zones 84, 86 and 88, as shown, for maintaning multiple temperature zones within the oxidizing reaction chamber 24. In either event, the temperature established by the heater 82 can be determined by the electrical control 62 in response to temperature sensors located at various points within the oxidizing chamber 24.

If the heater 82 is constructed to provide a single heating zone and it surrounds at least a portion of the oxidizing reaction chamber 24, as shown in Figure 1, the portion of the chamber 24 surrounded by the heater 82 will tend to be at a uniform temperature except for such cooling as may occur as a result of relatively cool flow entering the chamber 24 through the nozzle 18. On the other hand, if independent heating zones 84, 86 and 88 are established, clearly differential temperatures can be established in the fore-section 32, the tubular section 26 and the aft-section 34. For example, thermocouples 90, 92 and 94 can be provided, as shown, for sensing the temperature, respectively, in the enlarged fore-section 32, the tubular section 26 and the aft-section 34.Signals from the thermocouples are transmitted to the electrical control 62 which variably governs power supply to the heater 82 in response thereto, the power supply being furnished through means 96.

Figure 2 is a block diagram illustrating the present invention generally and consistent with the apparatus described with reference to figure 1. Like numerals are used to designate like parts. A pump 15 supplies the driving pressure for the liquid chromatograph column 13/4.

Flow rates in the range 0.1 to 0.8 ml/min are used in the preferred apparatus. Effluent from the column 16 is fed to the oxidizing converter 24, as described above in connection with Figure 1. Thebxidizing reaction taking place in the converter 24 is such that, in addition to nitrogen-containing organic compounds, sulphur-containing organic compounds react.

On the one hand the sulphur and on the other hand the nitrogen is oxidized. In compounds containing both sulphur and nitrogen, both oxidation reactions occur substantially simultaneously. When it is desired to measure the content of organic sulphur-containing compounds in a sample, the gas detector 14a is a suitable sulphur dioxide analyzer associated with an appropriate signal processor 50a. Otherwise, the method and apparatus of the present invention is substantially the same for both sulphur-containing and nitrogen-containing organic compounds.

Figure 3 is a detailed cross-sectional view showing the nozzle 18 extending into the upstream end of the chamber 24. The nozzle 18 comprises a tubular body 104 having at its leading end an outwardly extending collar 98 which abuts the inner surface of the chamber 24. Within the body 104 the nozzle has a cylindrical chamber 102 with means 100 at the leading end forming an opening or ports leading into the reaction chamber 24. An inlet 106 to the cylindrical chamber 102 communicates with the oxygen supply means 20 and, if used, the inert gas supply means 22. Extending coaxially through the cylindrical chamber 102 is an effluent feed tube 108 from the liquid chromatograph 16. Effluent from the chromatograph is fed through the tube 108 to a discharge port 110 at its terminal end, and is discharged into the reaction chamber 24.

Surrounding the body 104 of the nozzle 18 is electrical heating means 80 comprising a thermocouple 83 and a resistance heater 105. The thermocouple senses the temperature of the nozzle 18 adjacent the ports 100 and 110 and provides a signal to the electrical control system 62. The electrical control system, in response to this signal, energises the resistance heater 105 so as to maintain the nozzle 18 at a predetermined temperature. Thus, the nozzle 18 is able to preheat the materials in the feed tube 108 and the chamber 102 prior to their ejection from the nozzle into the reaction chamber 24. Alternatively, if independent temperature control is not required, the heater 82 may extend to the proximity of the discharge end of the nozzle 18, as shown in Figure 3, so that heat therefrom will maintain the discharge end at a temperature dependent on the reaction chamber temperature. If desired, both heating techniques may be used in conjunction with each other.

The ports 100 and 110 may be of various configurations. It is essential however that they are constructed so that the effluent from the liquid chromatograph is atomized as it is discharged into the chamber 24. Atomization is beneficial in all aplications and it is of particular benefit when non-volatile effluent constituents from the liquid chromatograph are to be oxidized, for the reasons described earlier. In one embodiment, as illustrated in Figure 4, the effluent feed tube 108 and its associated port 110 is arranged centrally with respect to the port means 100 which comprise four separate outlets arranged at equal angular intervals about the port 110. Between each of the individual outlets is a portion 112 of the body 104 which extends inwards into contact with the outer surface of the feed tube 108.This construction serves to securely position the feed tube and to restrict the outlet for oxygen.

Figure 5 illustrates an alternative arrangement of the nozzle 18, where three constituents are fed individually into the reaction chamber 24 through concentrically arranged ports 100, 114 and 116. Effluent from the liquid chromatograph is fed through the central part 100 and the other constituents are fed through the ports 114 and 116 which are formed by concentric tubular members 118 and 120, respectively. In this embodiment, the oxygen can be fed through the port 114 and an inert gas may be maintained separate from it and fed through the port 116. One advantage of the construction of Figure 5 is that, under circumstances when the reaction chamber 24 tends to overheat in the vicinity of the nozzle 18, the inert gas fed through the outermost concentric port 116 slows the oxidizing reaction and has a cooling effect on the walls of the chamber 24 in the region adjacent the nozzle 18.It will be appreciated that additional concentric tubes may be provided for individually supplying other constituents to the reaction chamber as desired.

Specific examples of systems in accordance with the present invention and their operation are described as follows: EXAMPLE I This example is described with reference to Figures 6 and 7. A Waters Associates (Milford, Massachusetts, U.S.A.) Model 6000A high pressure liquid chromatograph pump 130 was connected in series to a Waters Associates Model U6K Injector 132, a Waters Associates u-Bondapak-NH2 liquid chromatographic column 134, a Waters Associates Model 440 ultra-violet absorbance detector 136 monitoring absorbance at 254 nm, and a Thermo Electron Model 512 nitrogen specific detector 138 operating with an oxidizing reaction chamber temperature of 1085"C. (The Model 512 detector consists of the oxidizing converter 12 and the detector 14 shown in Figure 1).The ultraviolet absorbance detector 136 does not demonstrate the selectivity or sensitivity of the nitrogen specific detector 138 and is not an essential part of the system of this invention. However, it was used in the examples herein described so that its results could be compared with the results ultimately achieved from the nitrogen specific detector 138. The ultraviolet detector 136 operates in the liquid phase and is non-destructive of its sample input. Its output is delivered to the nitrogen specific detector 138 in the same condition as if effluent had been fed directly from the chromatographic column 134 to the detector 138.

The separation of hydantoin, ethylene urea and ethylene thiourea in a sample was achieved using hexane: methanol. 2-propanol (10:2:1) as a carrier solvent at a flow of 0.5 milliliters per minute. The injector and the pump are used, respectively, to introduce the sample into the column and to deliver carrier solvent isocratically.

In Figure 7, the separation of ethylene urea and ethylene thiourea and hydantoin as detected by the nitrogen specific detector 138 is illustrated, the three being represented, respectively, by peaks 140, 142 and 144 in the chromatogram of Figure 7a. Because of low molar absorbtivity at 254 nm, ethylene urea and hydantoin are not detected by the ultraviolet absorbent detector 136, but ethylene thiourea is detected and is represented by the peak 146 in the chromatogram of Figure 7b. The peak 148 is a solvent front.

EXAMPLE 2 This example will also be described in connection with Figure 6, and the chromatograms associated therewith being shown in Figure 8.

A Spectra Physics (Santa Clara, California, U.S.A. 95051) Model 740B pump 130 was connected in series to a Waters Associates Model U6K injector 132, a Waters Associates u-Cl8 Bondapak Liquid chromatographic column 134, a Waters Associates Model 440 ultraviolet absorbance detector 136 monitoring absorbants at 254 nm, and a Thermo Electron Model 512 nitrogen specific detector 138 operating with an oxidizing reaction chamber temperature of 1070"C.

A measure of caffein present in a sample was achieved using 6.7% 2-propanol in 1% acetic acid in water as a carrier solvent, with a flow rate of 0.5 ml per minute. The sample injector and solvent pump supply the constituents to the chromatographic column 134 and thereafter to the detectors 136 and 138.

In Figure 8a, the chromatograph resulting from the nitrogen specific detector 138 is illustrated, the peak 150 representing the caffein of the sample. In Figure 8b, caffein is detected and is represented by the peak 152 in the chromatogram of the ultraviolet absorbant detector 136. The peak 154 shown in Figure 8b represents the solvent front.

The systems described in these examples are uniquely selective to nitrogen containing organics, there being no known interference. The response is molar, being proportonal to the number of nitrogen atoms present in the molecule. The sensitivity of the systems appeared limited by the background level of the nitrogen containing organic compounds present in the best "distilled in glass' high pressure liquid chromatograph solvents being used. Solvents apparently tend to contain a background impurity level involving nitrogen containing organics but in the best "distilled in glass' solvents these appear minimal.

Obviously, solvents should be selected which do not otherwise contain organic nitrogen containing compounds. Approximately 100 pg (1010g) of nitrogen containing compounds are required for detection in the apparatus of the above-described examples.

Solvents which contain halogens should be converted to their corresponding acid halides and need specific handling to avoid deterioration of the equipment. Otherwise, such solvents do not appear to effect the accuracy of the system. Also, it was found that solvents containing inorganic buffers tended to produce precipitates in the nozzle.

In the above examples, the tubular section 26 of the reaction chamber was of an inert ceramic material such as alumina, although other materials such as glass or quartz could have been used. The tubular member was 9 inches long, 3/8 inch in diameter, and was packed with substantially spherical inert ceramic particles 0.030 to 0.060 inches in diameter.

The fore-section 32 was open and also 9 inches long and 3/8 inch in diameter. Of the overall 18 inches length, the central 12 inches was surrounded by heating means. Generally, the tubular section 26 may be short (to approximately 5cm.) or long (to approximately 50 cm.) and may range in diameter from 0.5 cm to 2 cm. The range for the diameter of the fore-section is approximately the same as that for the tubular section, although the latter will frequently be in the lower part of the range while fore-section will frequently be in the upper part of the range. The length of the fore-section will typically be between 10 cm. and 25 cm.

The invention may be embodied in other forms without departing from its essential characteristics. The present embodiments are therefore to be considered illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description.

WHAT WE CLAIM IS: 1. A method of analysing a sample for the presence of a predetermined compound, comprising passing the sample in liquid form through a liquid chromatograph so that different compounds in the sample are separated from each other and are discharged from the chromatograph at different times, injecting the discharge from the chromatograph into a converter, also supplying to the converter a gaseous reactant, mixing and atomizing the discharge and the gaseous reactant in the converter, maintaining in the converter a temperature such that the mixed and atomized reactant and the predetermined compound react in the converter to produce a specific gas, and passing the gaseous products of the converter to a detector which is sensitive to the specific gas whereby detection of the specific gas is indicative of the presence of the predetermined compound in the sample.

2. A method of analysing a sample for the selective detection of organic nitrogen containing compounds, the method comprising passing the sample in liquid form through a liquid chromatograph so that different organic nitrogen containing compounds are separated from each other and from other compounds in the sample and the separated compounds are discharged from the chromatograph at different times, injecting into a converter at least the portion of the discharge from the chromatograph containing the organic nitrogen containing compounds, also introducing into the converter oxygen, as a gaseous reactant, in an amount sufficient to fully oxidise the combined nitrogen in the organic compounds contained in the discharge from the chromatograph which is injected into the converter, mixing and atomizing the injected discharge and the oxygen in the converter, maintaining in the converter a temperature not substantially less than 600"C or substantially greater than 1800"C whereby the organic nitrogen containing compounds injected into the converter from the chromatograph are oxidised to form nitric oxide from the nitrogen in the compounds, and transferring effluent from the converter to a nitric oxide detector wherein the amount of nitric oxide in the effluent is measured, the amount of nitric oxide detected at a particular time being directly related to the amount of a particular nitrogen containing compound in the sample.

3. A method according to claim 1 or claim 2, in which inert gas is mixed with the gaseous reactant prior to the introduction of the gaseous reactant into the converter.

4. A method according to claim 1 or claim 2, in which inert gas is introduced into the converter simultaneously with the gaseous reactant, but separately therefrom.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (27)

**WARNING** start of CLMS field may overlap end of DESC **. illustrated, the peak 150 representing the caffein of the sample. In Figure 8b, caffein is detected and is represented by the peak 152 in the chromatogram of the ultraviolet absorbant detector 136. The peak 154 shown in Figure 8b represents the solvent front. The systems described in these examples are uniquely selective to nitrogen containing organics, there being no known interference. The response is molar, being proportonal to the number of nitrogen atoms present in the molecule. The sensitivity of the systems appeared limited by the background level of the nitrogen containing organic compounds present in the best "distilled in glass' high pressure liquid chromatograph solvents being used. Solvents apparently tend to contain a background impurity level involving nitrogen containing organics but in the best "distilled in glass' solvents these appear minimal. Obviously, solvents should be selected which do not otherwise contain organic nitrogen containing compounds. Approximately 100 pg (1010g) of nitrogen containing compounds are required for detection in the apparatus of the above-described examples. Solvents which contain halogens should be converted to their corresponding acid halides and need specific handling to avoid deterioration of the equipment. Otherwise, such solvents do not appear to effect the accuracy of the system. Also, it was found that solvents containing inorganic buffers tended to produce precipitates in the nozzle. In the above examples, the tubular section 26 of the reaction chamber was of an inert ceramic material such as alumina, although other materials such as glass or quartz could have been used. The tubular member was 9 inches long, 3/8 inch in diameter, and was packed with substantially spherical inert ceramic particles 0.030 to 0.060 inches in diameter. The fore-section 32 was open and also 9 inches long and 3/8 inch in diameter. Of the overall 18 inches length, the central 12 inches was surrounded by heating means. Generally, the tubular section 26 may be short (to approximately 5cm.) or long (to approximately 50 cm.) and may range in diameter from 0.5 cm to 2 cm. The range for the diameter of the fore-section is approximately the same as that for the tubular section, although the latter will frequently be in the lower part of the range while fore-section will frequently be in the upper part of the range. The length of the fore-section will typically be between 10 cm. and 25 cm. The invention may be embodied in other forms without departing from its essential characteristics. The present embodiments are therefore to be considered illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description. WHAT WE CLAIM IS:

1. A method of analysing a sample for the presence of a predetermined compound, comprising passing the sample in liquid form through a liquid chromatograph so that different compounds in the sample are separated from each other and are discharged from the chromatograph at different times, injecting the discharge from the chromatograph into a converter, also supplying to the converter a gaseous reactant, mixing and atomizing the discharge and the gaseous reactant in the converter, maintaining in the converter a temperature such that the mixed and atomized reactant and the predetermined compound react in the converter to produce a specific gas, and passing the gaseous products of the converter to a detector which is sensitive to the specific gas whereby detection of the specific gas is indicative of the presence of the predetermined compound in the sample.

2. A method of analysing a sample for the selective detection of organic nitrogen containing compounds, the method comprising passing the sample in liquid form through a liquid chromatograph so that different organic nitrogen containing compounds are separated from each other and from other compounds in the sample and the separated compounds are discharged from the chromatograph at different times, injecting into a converter at least the portion of the discharge from the chromatograph containing the organic nitrogen containing compounds, also introducing into the converter oxygen, as a gaseous reactant, in an amount sufficient to fully oxidise the combined nitrogen in the organic compounds contained in the discharge from the chromatograph which is injected into the converter, mixing and atomizing the injected discharge and the oxygen in the converter, maintaining in the converter a temperature not substantially less than 600"C or substantially greater than 1800"C whereby the organic nitrogen containing compounds injected into the converter from the chromatograph are oxidised to form nitric oxide from the nitrogen in the compounds, and transferring effluent from the converter to a nitric oxide detector wherein the amount of nitric oxide in the effluent is measured, the amount of nitric oxide detected at a particular time being directly related to the amount of a particular nitrogen containing compound in the sample.

3. A method according to claim 1 or claim 2, in which inert gas is mixed with the gaseous reactant prior to the introduction of the gaseous reactant into the converter.

4. A method according to claim 1 or claim 2, in which inert gas is introduced into the converter simultaneously with the gaseous reactant, but separately therefrom.

5. A method of analysing a sample for the selective detection of organic nitrogen

containing compounds, the method comprising passing the sample in liquid form through a liquid chromatograph so that different organic nitrogen containing compounds are separated from each other and from other compounds in the sample and the separated compounds are discharged from the chromatograph at different times, injecting into a converter at least the portion of the discharge from the chromatograph containing the organic nitrogen containing compounds, also introducing into the converter oxygen, as a gaseous reactant, in an amount sufficient to fully oxidize the combined nitrogen in the organic compounds contained in the discharge from the chromatograph which is injected into the converter, mixing and atomizing the injected discharge and oxygen in the converter, maintaining in the converter a temperature whereby oxides of nitrogen are formed by oxidation of the organic nitrogen containing compounds injected into the converter from the chromatograph but not by oxidation of free nitrogen in any substantial amount, and transferring effluent from the converter to an oxides of nitrogen detector wherein the amount of oxides of nitrogen in the effluent is measured and is indicative of the nitrogen containing compounds in the sample.

6. A method according to claim 5, in which the oxidation of the organic nitrogen containing compounds injected into the converter is effected at a temperature not substantially less than 600"C or substantially in excess of 1800"C.

7. A method according to claim 6, in which the oxidation is effected at a temperature in the range of from 900"C to 11500C.

8. A method of analysing a sample for the selective detection of organic sulphur containing compounds, the method comprising passing the sample in liquid form through a liquid chromatograph so that different organic sulphur containing compounds are separated from each other and from other compounds in the sample and the separated compounds are discharged from the chromatograph at different times, injecting into a converter at least the portion of the discharge from the chromatograph containing the organic sulphur containing compounds, also introducing into the converter oxygen, as a gaseous reactant, in an amount sufficient to fully oxidize the combined sulphur in the organic compounds contained in the discharge from the chromatograph which is injected into the converter, mixing and atomizing the injected discharge and oxygen in the converter, maintaining in the converter a temperature whereby oxides of sulphur are formed by oxidation of the organic sulphur containing compounds injected into the converter from the chromatograph but not by oxidation of free sulphur in any substantial amount, and transferring effluent from the converter to an oxides of sulphur detector wherein the amount of oxides of sulphur in the effluent is measured and is indicative of the sulphur containing compounds in the sample.

9. A method of analysing a sample for the presence of nitrogen containing organic compounds comprising passing the sample in liquid form through a liquid chromatograph so that different compounds in the sample are separated from each other and are discharged from the chromatograph at different times, injecting the discharge from the chromatograph into a converter, also introducing into the converter one or more streams of oxygen surrounding the discharge whereby the discharge from the chromatograph is atomized and is mixed with the oxygen in the converter, reacting in the converter the atomised discharge from the chromatograph with the oxygen at a temperature in the range 600"C to 18000C to produce nitric oxide from organic nitrogen containing compounds in the discharge, and detecting the nitric oxide produced to indicate the presence of any nitrogen containing compounds in the sample.

10. Apparatus for detecting the presence of organic nitrogen containing compounds in a sample, comprising a liquid chromatograph for separating from each other the different compounds in a sample which is passed in liquid form through the chromatograph and discharging the separated compounds at different times, a converter for oxidizing the discharge from the chromatograh to convert the nitrogen in organic nitrogen containing compounds to nitric oxide, means for introducing oxygen into the converter, means for injecting the discharge from the chromatograph into the converter so that the discharge is atomized in the converter, means for maintaining a predetermined temperature in the converter in the range from 600"C to 1800"C, and means for detecting nitric oxide in the effluent from the converter.

11. Apparatus for detecting the presence of organic nitrogen containing compounds in a sample, comprising a liquid chromatograph for separating from each other the different compounds in a sample which is passed in liquid form through the chromatograph and discharging the separated compounds at different times, a converter for oxidizing the discharge from the chromatograph to convert the nitrogen in organic nitrogen containing compounds to oxides of nitrogen, means for introducing oxygen into the converter, means for injecting the discharge from the chromatograph into the converter so that the discharge is atomised in the converter, means for maintaining a predetermined temperature in the converter in a temperature range effective to convert combined nitrogen in organic nitrogen containing compounds to oxides of nitrogen and ineffective to oxidize free nitrogen in substantial amounts, and means for detecting oxides of nitrogen in the products from the converter.

12. Apparatus for detecting the presence of organic sulphur containing compounds in a sample, comprising a liquid chromatograph for separating from each other the different compounds in a sample which is passed in liquid form through the chromatograph and discharging the separated compounds at different times, a converter for oxidizing the discharge from the chromatograph to convert the sulphur in organic sulphur containing compounds to oxides of sulphur, means for introducing oxygen into the converter, means for injecting the discharge from the chromatograph into the converter so that the discharge is atomised in the converter, means for maintaining a predetermined temperature in the converter in a temperature range effective to convert combined sulphur in organic sulphur containing compounds to oxides of sulphur and ineffective to oxidise free sulphur in substantial amounts, and means for detecting oxides of sulphur in the products from the converter.

13. Apparatus for carrying out the method according to claim 1, comprising a liquid chromatograph for separating from each other the different compounds in a sample which is passed in liquid form through the chromatograph and discharging the separated compounds at different times, a converter for reacting the liquid discharge from the chromatograph with a gaseous reactant substantially in the temperature range of from 600"C to 18000C, a nozzle for injecting the gaseous reactant and the discharge from the chromatograph into the converter whereby the discharge is atomised in the converter, the nozzle comprising liquid passage means extending from the chromatograph into the converter for carrying and injecting into the converter the chromatograph discharge, the temporal separation of the compounds in the injected discharge being substantially unchanged from that established in the chromatograph, and gaseous reactant passage means extending into the converter and surrounding the liquid passage means so that the gaseous reactant is injected into the converter in a pattern around the injected liquid discharge, and a specific gas detector for receiving the effluent from the converter.

14. Apparatus according to claim 13, further comprising means for heating the nozzle.

15. Apparatus according to claim 14, in which the heating means comprises an electrical heating element within the converter and adjacent the nozzle.

16. Apparatus according to claim 14, in which the heating means comprises the converter and a heating element external of the converter.

17. Apparatus according to any one of claims 14 to 16, in which at least a portion of the converter is packed with inert particulate material.

18. Apparatus according to claim 17, in which the converter is elongated and comprises an unpacked portion between the packed portion and the nozzle.

19. Apparatus according to claim 18, in which the unpacked portion has a crosssectional area normal to the longitudinal axis of the converter which is larger than such cross-sectional area of the packed portion.

20. Apparatus according to claim 19, in which the converter comprises an unpacked portion which is between the packed portion and the specific gas detector and which has a cross-sectional area normal to the longitudinal axis of the converter smaller than such cross-sectional area of the packed portion.

21. Apparatus according to any one of claims 13 to 20, in which the converter comprises an elongated conduit of inert ceramic material.

22. A chromatographic analysis system including a liquid chromatograph, an elongated conduit forming a heated reaction chamber, and a nozzle for injecting reactants into the conduit whereby the reactants are mixed and atomized, the nozzle comprising an elongated tube extending from the chromatograph into the conduit for carrying and injecting into the conduit constituents separated in the chromatograph, the constituents injected into the conduit being in the timewise distribution established in the chromatograph, gaseous reactant passage means extending into the conduit and surrounding the tube so that gaseous reactant is injected into the conduit in a pattern around the tube, and means for preventing escape of reactant from between the tube, the gaseous reactant passage means, and the conduit.

23. A system according to claim 22, in which heating means surrounds the gaseous reactant passage means.

24. A system according to claim 22, in which means interposed between the tube and the gaseous reactant passage means forms a plurality of gaseous discharge openings surrounding the tube.

25. A system according to claim 22, in which the gaseous reactant passage means comprises a plurality of concentric tubes surrounding the first said tube to form a plurality of gas passages.

26. A method according to claim 1, substantially as described with reference to the accompanying drawings.

27. Apparatus according to claim 13, substantially as described with reference to figures 1 to 4, or to figures 1 to 4 modified as shown in Figure 5, of the accompanying drawings.