Method and Apparatus for Analysis of a Sample for Nitrogen

"Method and Apparatus for Oxidative Decomposition and Analysis of a Sample", A. R Forster and G. J. Kamla, Ser. No. 07/253,549.

"Method and Apparatus for Reductive Decomposition and Analysis of a Sample", A. R. Forster and G. J. Kamla, Ser. No. 07/253,550.

"Method and Apparatus for Analysis of a Sample for Sulfur", A. R. Forster and G. J. Kamla, Ser. No. 07/249,255.

This invention relates to analysis of materials, and more particularly, relates to method and apparatus for the decomposition and quantitative determination of the amount of nitrogen in a sample.

A common form of sample preparation for elemental analysis involves the combustion of a sample followed by the use of the combustion gases from this sample for the detection of the desired constituent(s) or analyte(s). Examples of this include halogen and sulfur determination using a microcouloumetry, nitrogen determination by chemiluminescence of excited state nitrogen dioxide, sulfur determination using SO.sub.2 fluorescence, and carbon and hydrogen determination by gravimetric or Pregl-Dumas techniques. With the exception of carbon and hydrogen determinations, there is a problem associated with the combustion process which can cause unreliable analytical results and this problem centers on the sample introduction step.

If the sample is introduced using a syringe needle the needle must be placed directly into or very close to the hot zone of a combustion furnace to ensure the sample is transferred into the combustion zone and combusted therein. Unfortunately, heavy organic fractions or salts can remain within the needle and possibly clog it temporarily, or permanently, as well as the hostile environment damaging the needle. One approach taken to overcome this problem is to introduce the sample into the hot portion of the furnace using a small boat which has been loaded with the sample when the boat was positioned in a relatively cool portion of the furnace tube. In either case, however, the sample introduction and subsequent combustion is a transient process. Therefore, the oxygen concentration in the combination tube changes over time during this process. This can be detrimental in cases where equilibria involving oxygen are important to the instrumental stability, sensitivity, or detection limits.

Accordingly, there is a need for a sample introduction scheme to allow for longer integration times in the detection phase which would then improve detection limits and also allow for use as an on-line monitor in process control, or as a chromatographic detector.

These and other limitations and disadvantages of the prior art are overcome by the present invention, however, and an improved method and apparatus are provided for analysis of samples for nitrogen.

In a preferred embodiment of the present invention, methods and apparatus are provided for the quantitative determination of the amount of nitrogen in a sample. In the presently preferred method for the analysis of a sample, the sample is first prepared for analysis by mixing a preselected quantity of the sample with a preselected quantity of a preselected material; the preselected material may serve to dilute and dissolve the sample, although the sample itself may also serve as this material. When the sample is this material, the analysis may be conducted substantially continuously. The sample and/or material may then be nebulized; the sample and/or material are nebulized in a nebulizer zone where an inert gas, such as argon, is used to disperse the sample and/or material into fine droplets which form an aerosol with the argon gas. The aerosol is then transported to an oxygen-rich combustion zone where the nebulized sample is completely burned and decomposed. The decomposed combustion products of the sample are then transported to an appropriate detection zone. In the detection zone the decomposed constituents of the sample are analyzed for oxides of nitrogen.

The presently preferred apparatus of the present invention is a nebulizer device positioned to delivery a liquid sample and/or material into the hot region of a combustion tube. A continuous aerosol stream of an appropriate preselected material and/or sample is injected into the combustion tube through the nebulizer with an argon carrier gas; the carrier gas serves to convert the preselected material and/or sample in the nebulizer into an aerosol form which becomes fully vaporized before it enters the combustion region of the combustion tube, where it is completely combusted. A small portion of sample may be substantially continuously injected, with or without the preselected material into the nebulizer by an appropriate pump, thereby providing for a substantially continuous portion of the sample to be combusted in the combustion tube. The sample may be dissolved and/or diluted in the material, or for appropriate samples only the sample may be injected into the combustion tube, via the nebulizer. The combustion tube is supplied with oxygen to ensure complete combustion of the preselected material and/or sample. The combustion products from the combustion tube are exhausted through an appropriate discharge opening and may then be optionally dried and/or filtered prior to passage to an appropriate detector for nitrogen, such as for example, but not limited to an NO analyzer. The output of the detector may in turn be connected to an appropriate recorder or controller.

It is an object of the present invention to provide an apparatus for quantitative analysis of the amount of nitrogen in a sample.

It is also an object of the present invention to provide a method for quantitative analysis of the amount of nitrogen in a sample.

It is a specific object of the present invention to provide a method for analyzing a sample for nitrogen, comprising, nebulizing said sample, transporting said nebulized sample to a decomposition zone, decomposing said sample in an oxygen-rich atmosphere of oxygen and an inert gas at a temperature sufficient to ensure complete combustion of said sample, transporting said decomposed sample to a detection zone, and analyzing said decomposed sample for nitrogen.

These and other advantages and objects of the present invention will become apparent from the following detailed description wherein reference is made to the Figures in the accompanying drawings.

FIG. 1 is a simplified functional diagram depicting the general arrangement of a nebulizer and combustion tube for use in the apparatus or methods of the present invention.

FIG. 2 is a simplified cross-sectional diagram depicting the arrangement of a combustion tube for use in the apparatus of the present invention.

FIG. 3 is a simplified functional diagram of one embodiment of the apparatus of the present invention.

FIG. 4 is a simplified representation of the data collected by the preferred embodiment of the present invention illustrating the presence of the amounts of nitrogen in samples.

FIG. 5 is a simplified flow chart of the basic steps of the preferred method of the preferred invention.

The present invention provides method and apparatus for quantitative determinations of the amount of nitrogen in a sample. Referring now to FIG. 1, there may be seen a simplified functional diagram depicting the general arrangement of a nebulizer 10 and combustion tube 20 for use in the apparatus or methods of the present invention. More specifically, it may be seen that the present invention employs a nebulizer 10 operatively connected to a combustion tube 20. In particular, it may be seen that the sample and/or material 30 is injected into the nebulizer 10 via a stream of preselected material, which may be a liquid solvent stream. A stream of inert gas 12, such as for example, but not limited to argon, is also supplied to the nebulizer 10 via inlet 13 to turn the sample and/or material 30 stream into an aerosol 14 which then is transported by the argon into the combustion tube 20. The combustion tube 20 is supplied with oxygen 40 via inlet 22 and may be externally heated by furnace 50 to maintain an appropriate hot zone for complete combustion. The sample and/or material 30 are vaporized in the furnace sleeve 21 (hereinafter "sample carrier sleeve" or "carrier tube") and the vapors exit this sleeve 21 to reach the proper combustion temperature. Upon reaching the hot zone, the material and/or sample 20 combust in the oxygen 40 atmosphere. Under appropriate circumstances, as noted later herein, the sample may also be the preselected material.

Since the nebulizer 10 is positioned in a relatively cool region at the front end of the combustion tube 20, the chance of the sample transport system becoming damaged or clogged, as in the prior art are eliminated. Thus, a sample introduction scheme, which may be substantially continuous, may be easily maintained.

There are several important aspects of this invention which must be properly followed. Careful placement of the nebulizer 10 relative to the hot portion of the furnace tube 20 and the inlet 22 for oxygen gas is presently considered very important. It is necessary to position the nebulizer 10 relatively close to the hot zone so that the sample and/or material 30 does not contact a cool furnace wall 20. The nebulizer 10 is held in place within the cool end of the combustion tube 20 using a septum 26 constructed of an appropriate material, such as, for example, but not limited to silicone.

The oxygen 40 must also be added close to the beginning of the hot region, although, as noted later herein, the exact location is not critical. The oxygen supply must be free of nitrogen or an appropriate adjustment to the data based upon the actual oxygen supply employed must be determined before or after a sample is analyzed for nitrogen. The argon flow rate must be sufficient to effectively nebulize the sample and/or material stream and to transport this stream to the hot zone and yet remain low enough so that it does not dilute the vaporized sample and/or material gases to point to inhibiting complete combustion of the sample. For high inert gas flow rates the inert gas may dilute the sample and may also limit the ultimate sensitivity of the instrumental detection schemes that may be employed to analyze the combustion products. The sample and/or material flow rate must be low enough so that complete combustion can occur without soot formation.

Merely increasing the gas flows to compensate for excess sample and/or material input may create excessive heat within the quartz furnace tube and may induce subsequent failure. Conversely, too little flow of sample and/or material makes it very difficult to support continuous combustion. A pump, preferably with a precision flow output, such as for example, but not limited to an HPLC pump, is necessary for material and/or sample delivery to the nebulizer 10. A typical sample flow rate (depending upon the material type) is approximately 100 micro liters per minute (.mu.l/min). Sample introduction may be achieved using an HPLC injection valve and sample loop, with the pump pushing the sample in the sample injection loop (not shown) with an appropriate deaerated material from a material supply reservoir. (See FIG. 3) Alternatively, the sample may be the material, or the sample may be dissolved and/or diluted by the material by mixing a preselected quantity of sample in a preselected quantity of material to provide a known concentration of sample.

The supply of oxygen and argon into the combustion tube provides a positive pressure in the combustion tube, to exclude any undesirable gases. The discharge end of the combustion tube is also at a pressure slightly higher than atmospheric pressure. (See FIG. 2) This allows the combusted sample products to be positively swept from the combustion tube into subsequent sample discharge tubing. Thus, this configuration of the nebulizer 10 and combustion tube 20 provides for sample decomposition. The sample discharge tubing may then be connected to an appropriate nitrogen detection apparatus after drying and filtering, if necessary. (See FIG. 3)

The connection between the combustion tube and the detector may include a membrane dryer to remove any water produced during combustion and a filter to appropriate pore size (preferably about 5 .mu.m pore size) to prevent contamination of the detector in the event of soot formation during combustion. (See FIG. 3) A tee-fitting with one leg open, may be used at the input of such a detector to ensure atmospheric pressure at the detector input, if the detector requires samples at atmospheric pressure. Thus, this configuration of equipment provides method and apparatus for analyzing a sample for nitrogen.

Referring now to FIG. 2 there may be seen a simplified depiction of the arrangement of a combustion tube 20 for use in the apparatus of the present invention. More particularly, the combustion tube 20 is seen to consists of an outer envelope 23 of preferably quartz with an inlet 22 for injecting oxygen into the tube and with an outlet neck 24 and joint 25 at the opposite end for removal of any combustion gases. This outlet joint 25 is preferably a quartz to glass connection joint.

Also shown is the furnace sample "carrier" sleeve 21 which allows for the introduction of a sample into the combustion tube 20. For the present invention an appropriate septum 26 and nebulizer 10 are inserted into the exterior opening of this sleeve 21 (as shown functionally in FIG. 1).

The length of the furnace tube 20 is sized to allow for its use in an external furnace 50, such as for example, but not limited to a Dohrmann Model S-300 pyrolysis furnace; Dohrmann sells several commercial models of such furnaces. Once the length is thus roughly selected, the volume of the furnace tube is maximized to allow for larger sample and/or material injection rates while still achieving complete combustion of such materials and/or samples; this is generally accomplished by maximizing the outside diameter of the tube so that it narrowly fits inside the opening for a furnace tube in the external furnace 50.

The sample carrier sleeve 21 also has as large a diameter as possible to allow for a larger sample and/or material injection rates and has a length to ensure substantially complete vaporization of the sample and/or material stream before the stream exits the sleeve 21 into the oxygen 40 atmosphere of the furnace tube 20. A sleeve 21 length of about twenty percent of the furnace tube 20 length has been found to be satisfactory from experimental determination. A small amount of oxygen may also be introduced with the stream to avoid the formation of coke on this sleeve 21 during vaporization; this is most easily accomplished by injecting a small flow rate of oxygen into the nebulizer 10 via inlet 13 with the inert gas 12.

The combustion oxygen supply is preferably injected at the cool end of the tube 20, which is located physically outside the external furnace 50. This arrangement allows for a continuous, maximum outside diameter tube of maximum volume to be contained in the external furnace 50. The tube 20 may contain baffles, constrictive necks, and/or quartz chips to provide positive mixing of gases and vapors to ensure complete combustion. The exact point of injection of oxygen has been found to not be a critical aspect of the invention and may be located in the middle of the tube, or even at or near the discharge end of the tube. Preferably, however, this inlet 22 is adjacent the cool end as depicted.

The volume of the furnace tube 20 and the volume of the sample carrier sleeve 21 must be balanced versus the flow rates of oxygen and sample/material/inert gas aerosol 14 to ensure substantially complete vaporization before leaving the sleeve 21 (without coking) and to ensure complete combustion in flame 15 (without soot formation) before leaving the furnace tube 20 (See FIG. 1). However, the inert gas flow rate is principally determined by the selection of the nebulizer 10, since the nebulizer 10 determines the minimum inert gas flow rate capable of nebulizing the sample and/or material.

For the Dohrmann Model S-300 pryolysis furnace the quartz furnace tube 20 has: an overall length of 575 mm, an outside diameter of 22 mm, a 6 mm outside diameter oxygen injection line, a 12 mm outside diameter discharge outlet, a discharge outlet length of 25 mm, a carrier tube outside diameter of 12 mm, an overall carrier tube length of 76 mm, and an exterior carrier length of 25 mm.

Referring now to FIG. 3, there may be seen a simplified functional diagram of one embodiment of the apparatus 60 of the present invention. More particularly, there may be seen a depiction of the nebulizer 10 and combustion tube 20, as previously shown and described for FIG. 1, as well as other items. There may be seen pump 31 (as described hereinbefore), interconnected with material supply 32, for delivering at a controlled rate, material to sample injection valve 33. The sample injection valve 33 may be operated to inject a fixed portion of the sample (depending upon the length of the sample loop - not shown), at a controlled rate, into the nebulizer 10. For each such separate injection of sample, the sample stream replaces the material stream; accordingly, the sample must be introduced at a fixed rate over a long enough time to ensure a steady-state combustion or the differences between chemical and combustion characteristics of the sample and material minimized, so that the sample portion does not significantly perturb the flame 15. Alternatively, the sample may be dissolved and/or diluted in the material, and this mixture of sample and material injected, at a controlled rate, by the pump 31 into the nebulizer 10.

The nebulizer 10, as described hereinbefore, nebulizes the sample and/or material stream which is then completely combusted in the combustion tube 20. The tube's discharge gases 34 are preferably dried by drying tube 35 to remove any water vapor and filtered by filter 38 to remove any soot before passing the combustion gases into an appropriate nitrogen detector 39. The detector analyses these gases for nitrogen. The detector output may be recorded on an appropriate recorder 41 or used as an input to an appropriate controller (not shown).

The sample and/or material 30 may be totally sample, if the sample is a liquid at room temperature and capable of being nebulized. Any preselected material employed must dissolve the sample (solid or liquid), combust with no soot (if enough oxygen is present), be liquid at room temperature and be extremely pure, i.e. an HPLC grade solvent. Examples of such materials are iso-octane, toluene, and declain; although iso-octane may not be suitable for dissolving solid samples. For aromatic samples and/or materials, the injection rate must usually be lowered to prevent sooting of the furnace tube. A sample and/or material flow rate of about 100 .mu.l/min has been found to be satisfactory for the hardware configuration described later herein with respect to FIGS. 3 and 4. Typically, the sample and/or material flow rate is about 100 .mu.l/min for iso-octane and about 30 .mu.l/min for toluene, since toluene is more aromatic than iso-octane. Solid and/or liquid samples must dissolve in the material, although liquids may be used as the material. For high melting point solids it may be desirable to use a heavier solvent. The sample and/or material flow rate may be controlled by a liquid flow control device (not shown).

The minimum inert gas flow rate is chiefly determined by the size of the nebulizer; this flow rate must be sufficient to ensure the nebulizer nebulizes the sample and/or material in an efficient and continuous manner. Higher inert gas rates tend to dilute the sample but may be acceptable from a detection limit point of view. The inert gas flow rate may be controlled by a gas flow control device (not shown)

The oxygen flow rate into the furnace tube is, as a minimum, high enough to ensure complete combustion; although at very high rates, excessive sample dilution may occur and certain detectors may be quenched. However, the flow rate must also be high enough to prevent sooting. In addition, a small amount of oxygen is preferably injected into the nebulizer 10 along with the inert gas 12 to prevent the sample and/or material 30 from coking up the sleeve 21; this oxygen flow rate should be about 5 to 10 percent of the inert gas flow rate going into the nebulizer 10. Further, as noted hereinbefore, the oxygen supply is preferably free of nitrogen. The oxygen flow rate(s) may be controlled by a gas flow control device(s) (not shown).

As indicated hereinbefore the volume and length of the furnace tube 20 is chiefly determined by the external furnace 50 to be employed. Once the furnace tube 20 size is determined the carrier tube 21 size is then determined. Again, a maximum diameter is desired to maximize the possible range of sample injection rates. The length of the carrier tube 21 is about 20 percent of the furnace tube 20 length, although shorter and longer lengths have been found to work satisfactorily. If the carrier tube 21 length is too short, the combustion will flicker and be sporatic due to incomplete vaporization. If the carrier tube 21 length is too long, the walls of the carrier tube 21 may eventually become coated with sample and/or material films. The carrier tube 21 also keeps the sample from the oxygen to prevent flash backs to the nebulizer 10. Although the carrier tube 21 serves to protect the nebulizer 10 to provide for long-term sample injection stability, it may be completely removed and still be within the scope of the present invention.

The nebulizer 10 size is in turn chiefly determined by the physical dimensions of the carrier tube 21, and accordingly the furnace tube 20. That is, the nebulizer 10 must be physically small enough to at least partially fit inside the inner diameter of the carrier tube 21 (as depicted in FIG. 1).

As also noted hereinbefore, the flow rate of inert gas into the nebulizer is chiefly determined by the size and type of nebulizer. The minimum flow rate is that which still effectively nebulizes the sample and/or material and the maximum flow rate is determined by the sample residence time in the hot zone; at too high a flow rate the sample passes through the hot zone before combustion occurs or is completed. As noted hereinbefore, any oxygen injected into the nebulizer is at a rate about 5-10 percent of the inert gas injection rate.

As previously noted herein, the oxygen flow rate must be sufficient to completely combust the sample. Thus, the minimum oxygen flow rate is determined by the sample injection rate, which is in turn related to the nebulizer size and type, and the inert gas flow rate. The oxygen flow rate is at least equal to, but preferably greater than, the inert gas flow rate; as long as this flow rate is still a sufficient flow rate to provide enough oxygen to completely combust the sample.

The sample flow rate is thus chiefly determined by the nebulizer size and type, and the inert gas flow rate, i.e. these rates/sizes must be sufficient to efficiently nebulize the sample.

In general, the volume of the furnace tube (in liters) divided by the sum of the inert gas and oxygen flow rates (in liters/min) may be in the range from 0.05 to 1 or larger. The low (0.05) end of the range is for oversized nebulizers on small volume furnace tubes and the high (1) end of the range is for small nebulizers and large furnace tubes. However, the high end of the range may result in decreased sensitivity of detection for nitrogen.

When samples containing nitrogen compounds are combusted, the nitrogen is converted to nitrogen oxides (NO or NO.sub.2). The nitrogen oxides in the combustion gases can be detected using chemiluminescence detectors, laser-excited fluroescence detector, and/or mass spectrometer. The preferred approach involves the use of chemiluminescence. There are two varieties of chemiluminescence detectors. The presently preferred type involves the gas-phase chemiluminescent reaction of ozone with nitric oxide (NO). The other is a gas/liquid-phase chemiluminescent reaction of nitrogen dioxide and luminol. In mass spectrometry, the combustion gases are ionized, the ions are separated as a function of mass, and the ions of a predetermined mass or mass range are then detected.

A specific example of items for the detection of nitrogen is as follows:

Thermo Electron Instruments Model 14B/E Chemiluminescent NO--NO.sub.2 --NO.sub.x analyzer (Thermo Electron Instruments is now named Thermo Environmental Instruments)

Beckman 100A HPLC pump, with an appropriate pulse dampener

Dohrmann Model S-300 pyrolysis furnace

VERIFLO flow controllers as flow control devices

two - Model SC423BXFT

one - Model SC423BFT

J. E. Meinhard Assoc. nebulizer Model TR-30-Al

Quartz pyrolysis furnace tube with glass elbow adapter and with silicone septa

Perma-Pure Dryer Model PD-6-12p membrane dryer

Cole-Parmer Company Model TV-6623-20 In-Line TFE

Filter holder and model TV-6623-51 47mm diam TFE filters

Various valves, piping, tubing, columns and connectors.

Referring now to FIG. 4, there may be seen the results from the hereinbefore described items connected together, as noted in FIG. 3, to detect nitrogen. These results are for different concentrations of acridine dissolved in decalin. A calibration curve composed of multiple samples of standard solutions with the noted nitrogen concentrations were run as follows: a (0.1 ppmw), b (0.3 ppmw), c (0.6 ppmw), d (1.0 ppmw), and e (3.0 ppmw). These multiple samples (two for each concentration -except d, which had four) demonstrate the reproducibility and linearity of the apparatus and methods of the present invention for determining the amount of nitrogen in a sample. For these data the following flow rates were employed: argon - 300 ml/min; sample/material - 30 .mu.l/min; oxygen - 430 ml/min; and oxygen into the nebulizer - 30 ml/min. However, the flow rates for this combination of equipment may be scaled up or down and still have approximately the same nitrogen sensitivity over the following ranges: argon - 0.1 to 1-2 l/min; sample/material - 0.01 to 0.5 ml/min; and oxygen - 0.2 to 2 l/min. Use of such an apparatus has resulted in a limit of detection for nitrogen which has an appropriate threshold of about 50 parts per billion weight (ppbw).

With respect to nitrogen determinations, the detection apparatus presently preferred is a Thermal Electron Model 14B/E analyzer. The Model 14B/E NO--NO.sub.2 --NO.sub.x chemiluminescence analyzer determines the NO concentration in a sample gas by measuring the light emitted from the gas-phase reaction of nitrogen oxide with ozone. This analyzer consists of an ozone generator, a gas reaction cell, a photomultiplier tube detector, and signal processing electronics. Operation of this analyzer with the combustion system of the present invention is as follows. The outlet of the combustion furnace is connected through a membrane dryer and TFE filter to a tee at the sample inlet of the Model 14B/E analyzer. One end of the tee is left open to maintain atmospheric pressure at the input of the analyzer. A pump is connected to the exhaust port of the reaction cell to draw gases from the sample inlet and ozone generator into the reaction cell. The proper flow rates for sample and ozonator gases are maintained by placement of glass capillaries of appropriate diameter in each gas line. The ozone generator is supplied by air which has first passed through an absorption column to remove the water present. Between the sample inlet and the reaction cell a heated molybdenum catalyst can be switched into the gas line which converts any NO.sub.2 present in the sample gas to NO. The gas-phase reaction of NO with O.sub.3 within the reaction cell results in the formation of excited NO.sub.2 molecules. The excited NO.sub.2 molecules emit light which is detected by the photomultiplier tube. The current signal from the photomultiplier tube is amplified and displayed by the signal processing electronics. A filter mounted between the reaction cell and the photomultiplier tube limits the light which reaches the detector to the wavelengths of NO.sub.2 chemiluminescent emission. The detector background noise level is minimized by cooling the photomultiplier tube.

Thus, it may be seen that the present invention provides a method for analyzing a sample for nitrogen. FIG. 5 depicts the basic steps of the method of the present invention. In particular, the method of the present invention nebulizes the sample 101, combusts this nebulized sample 102, and then analyzes the gases from the combusted sample for nitrogen 103.

For nitrogen detection, the methods and apparatus of the present invention lend themselves well to automated nitrogen analysis in process control or blending applications.

Many other variations and modifications may be made in the apparatus and techniques hereinbefore described, by those having experience in this technology without departing from the concepts of the present invention. Accordingly, it should be clearly understood that the apparatus and methods depicted in the accompanying drawings and referred to in the foregoing description, are illustrative only and re not intended as limitations on the scope of the invention.