I would like to ask what methods can be used to measure NOx at the ppb level

Your question can be addressed using a nitrogen-oxygen analysis instrument equipped with a dedicated infrared detector. You can find information on it by searching on Baidu: http://www.jykeyuan.net/. Instructions for use: I. Main purposes and features: This instrument utilizes the classic Kjeldahl method to determine the nitrogen content in various substances. It is suitable for use in fields such as agriculture, forestry, environmental protection, medicine and health, food processing, feed production, mining, and the chemical industry, where it can be used to measure the nitrogen content in soil, fertilizers, plants, drugs, feeds, and other materials. It can also be used to determine the protein content in grains, foods, feeds, and other substances. This instrument combines the advantages of various nitrogen determination devices from home and abroad, featuring simple operation, fast analysis speed, high accuracy, good repeatability, and easy maintenance. It has good versatility and practicality. II. Main technical specifications 1. Sample quantity: no more than 5 grams for solids, and no more than 15 milliliters for liquids. 2. Measurement range: 2–150 milligrams of nitrogen. Equivalent to 15–1000 milligrams of protein. 3. Maximum single distillation time: 0–99 minutes and 60 seconds. 4. Repeatability: Relative error of the average value ≤ 1%. 5. Recovery rate: ≥99%. 6. Digestive tract (test tube) size: ф40×300mm. 7. Power supply voltage: 220V 50HZ. 8. Power: 800W. 9. Dimensions of the instrument: 250mm×400mm×600mm. 10. Weight: Not more than 20 KG. III. Methods for determining nitrogen: The K-method for nitrogen determination involves three steps: digestion, distillation, and titration. The principle behind this method is that nitrogen-containing organic compounds are decomposed at high temperatures using concentrated sulfuric acid, in the presence of a digesting agent; the nitrogen contained within these compounds is converted into ammonium. Ammonia then reacts with sulfuric acid to form ammonium sulfate. Subsequently, alkali is added and distillation is carried out, causing the ammonia to be absorbed into a boric acid solution. Finally, a standard acid is used for titration, and the nitrogen content in the substance is calculated based on the number of milliliters of standard acid consumed in this titration process. 1. Cooking: The cooking process takes place in a cooking furnace. Our company is equipped with the LWY-84B type temperature-controlled far-infrared cooking furnace; its usage method is described in detail in the accompanying manual, and users can purchase it according to their needs. When nitrogen-containing organic compounds are added to concentrated sulfuric acid, at high temperatures – where sulfuric acid acts as a strong oxidizing agent – these compounds are ultimately converted into (NH4)2SO4, which remains in the digestion solution. 2. Distillation: The nitrogen analyzer is essentially an automated distiller. A test tube containing the digestion solution (NH4)2SO4 is placed under the rubber stopper of the analyzer’s damper. As needed, the “Add alkali/Time confirmation” switch on the control panel is pressed to add an appropriate amount of NaOH solution; steam is then introduced to cause ammonia to escape rapidly. The ammonia gas is cooled in a condenser to form ammonia water, which is collected in a triangular receiving flask containing boric acid. The boric acid absorbs the ammonia gas to form ammonium borate, NH4H2BO3. Once the distillation process is complete after a set time, the device shuts down automatically. 3. Titration (the titration apparatus must be prepared by the user!) ) It is carried out using a common glass burette, with 0.02 mol of HCl used for titration; the nitrogen content in the substance can be calculated using the following formula. N (V – V1) × 0.014 N (%) = ————————————————— × 100% W Where: N —— the equivalent concentration of the standard acid. V —— The number of milliliters of standard acid used in the titration. V1 —— The number of milliliters of standard acid used in the blank sample. 0.014 —— Weight of equivalent ammonia per milliliter (grams). W —— the weight of the sample after drying and weighing. IV. Structure and Installation of the Instrument 1. Overall Structure It is mainly composed of a casing, a steam generator, a damper (distillation head), a condenser, a water level controller (water level gauge), an operation panel, a control circuit, an air pump, and an alkali tank. 2. The ammeter on the control panel shows the operating current of the steam generator; under normal operation, it indicates around 3A ； A four-digit display shows the set or current time value, with the first two digits representing minutes and the last two digits representing seconds ； The three middle indicator lights, from left to right, represent “Distillation”, “Anhydrous”, and “Alkali Addition”; they indicate the status of distillation, the absence of water in the steam generator, and whether alkali addition is in progress ； The following four touch switches, from left to right, are “Standby/Time Setting”, “Run/Decrease Time”, “Stop/Increase Time”, and “Add Alkali/Confirm Time”. Each switch has two modes: upper and lower. The “Run” and “Stop” functions for the machine’s distillation process correspond to the upper mode ； The setting of the distillation time follows the lower-level function. 3. Water pipeline installation: Use rubber hoses to connect tap water to the \"condenser inlet\" and \"condenser outlet\" ports among the four connectors located at the lower right side of the instrument. Place a bucket filled with tap water or distilled water on top of the instrument, and use rubber hoses to connect the outlet of this bucket to the \"evaporator inlet\" port of the instrument. Connect the \"evaporator outlet\" port to the sewer using a rubber hose, and attach a stopcock to that hose as well. 4. Addition of alkali solution: First, fix the tray holding the alkali tank on the right side of the machine. Fill the tank with two-thirds of the alkali solution, then close its lid tightly. Place the tank on the tray, and insert the white rubber plugs for the \"air\" hose and the \"alkali\" hose, which are located on the right-middle side of the machine’s casing, firmly into the openings of the alkali tank, thereby securing the tank lid in place. Once the preparation of the instrument is complete, press the “Add Alkali/Time Confirmation” switch to proceed with the alkali addition process ； The amount of alkali to add can be determined by the operator based on the markings on the test tube and the duration for which the “Add Alkali//Time Confirmation” switch is pressed. V. Usage and Operation Methods 1. Preparation: Take a test tube, insert the vapor silica gel tube of the distillation head into the test tube such that its opening is aligned with the stopper opening. Press down on the handle of the test tube holder to support the bottom of the test tube, then turn the test tube so that its upper end makes good contact. Place a Erlenmeyer flask under the rubber tube at the sample collection point. Turn on the faucet slowly to allow water to circulate within the condenser, ensuring an appropriate flow rate at the drain outlet. Set the power switch on the left side of the instrument to position “1”; the instrument will then be powered on, and the digits on the panel will display a 2-minute countdown. During the 2-minute preparation time, except for the “Stop/Time +1” switch, the machine does not respond to the operations of the other switches on the panel. After about 20 seconds of power-up, a \"click\" sound can be heard; at the same time, the solenoid valve for adding water opens, allowing water to flow into the steam generator and the water level controller. Once the water level reaches the desired height (when the water touches the upper level electrode), another \"click\" sound is heard, the solenoid valve shuts off automatically, water addition stops, and the \"no water\" light goes out. When 2 minutes have passed, the four-digit display shows the previously set distillation time in the instrument. When filling water for the first time after powering on, it is necessary to wait until the water reaches the level of the electrodes at the highest point, and until the \"No Water\" light on the panel goes out, before starting the distillation process ； When the water level does not reach the standard height, distillation cannot be carried out even if the “Run/Time Shift Left” switch is pressed. 2. Setting the distillation time: After 2 minutes of preparation time since startup, or within 2 minutes after pressing the “Stop/Time +1” button, it is possible to set the distillation time for the sample. The four touch switches are used to adjust the settings, with a range of 0–99 minutes and 59 seconds. By pressing the “Standby/Time Setting” button, the time-setting mode is entered, and the rightmost digit of the four-digit display starts to flash. At this point, pressing the “Stop/Time +1” button increments the value of that flashing digit by one, in a cycle from 0 to 9. Once that digit has been set, pressing the “Run/Time Shift Left” button moves the focus to the next digit; pressing “Stop/Time +1” again sets that digit as well. In this way, it is possible to move forward through the digits one by one and set the desired distillation time ； After setting the time, press the “Add Alkali/Time Confirmation” button; the current value will flash three times, indicating that the set value has been confirmed and stored within the machine. Going forward, after a 2-minute preparation time each time the machine is turned on, the preset distillation time will be displayed automatically. 3. Timed distillation of the sample: Once all preparatory tasks are completed, press the “Add alkali/Time confirmation” button to activate the function of adding alkali in stages; an appropriate amount of NaOH solution is then added to the distillation head. The “Add alkali” light turns on while alkali is being added. Press the “Run/Time Shift Left” switch again; when “Run” is selected, the distillation process begins. The distillation light turns on, and the instrument starts counting down according to the set time for timed distillation, displaying the remaining time dynamically. The ammeter shows a current of around 3A, until the set time is reached at which point the digital display reads “00:00”, and the ammeter shows 0 ； Distillation of this sample is complete. Next time, if there is no need to change the time, simply replace the sample and repeat the distillation process (waiting until the “anhydrous” light goes out as well) ； To change the time, reset it first and then repeat the distillation process mentioned above. If it is necessary to stop the operation midway during distillation, press the “Stop/Time +1” switch; the digital display shows the remaining time, while the ammeter indicates 0 ； “The “distillation” light goes out, and the operation stops. If the “Run/Time Shift Left” switch is pressed again, the instrument begins distillation once more from its initial setting. 4. The steam generator must be free of water. Before starting operation for the first time, add water until it reaches the upper water level electrode (the topmost electrode in the water level gauge). Only then will the “no water” indicator light on the panel go out, allowing distillation to proceed. During normal distillation, as the amount of water decreases, the machine automatically adds more water; meanwhile, it is possible to observe the changes in the water level in the gauge and hear the sound of the water addition solenoid valve opening and closing alternately. When the water level is below the lower water level electrode (the middle one in the water level gauge), the electric heater inside the evaporator will be exposed above the water surface, posing a risk of dry burning ； At this point, the machine automatically stops the distillation process; the ammeter shows “0”, and the “no water” indicator light on the panel lights up. “When the “Anhydrous” light is on, it does not respond to the “Run/Time Shift Left” command; thus, distillation cannot be carried out ； The power switch on the left side of the enclosure should be turned off, and the device should be restarted to allow 2 minutes of preparation time or to check the water supply pipeline. 5. Completion of work: After the work is finished, set the power switch on the left side of the enclosure to the “0” position, turn off the condensate water supply, open the stopcock on the hose used for draining water from the vapor generator to release the water inside, and then unplug the power cord. VI. Precautions and Maintenance 1. The tap water flow should not be set too high, to avoid excessive pressure that could cause the connections to loosen and water to flow into the machine, resulting in damage to its components. 2. This machine is equipped with a check valve to prevent backflow. If, during distillation, the gas produced by the vapor generator cannot reach the test tube through the damper (distillation head), it is necessary to check whether the one-way valve between the evaporator and the damper is blocked, in order to prevent excessive pressure from building up inside the container ； If liquid is drawn back in after distillation is complete, it is also necessary to check whether the check valve is clogged or damaged. 3. If the alkali cannot be added, check whether there is too little alkali in the tank, whether the alkali addition solenoid valve is clogged, and whether there are leaks in the air pipes. 4. If the alkalinity level of tap water is too high, it will cause an excessive amount of bubbles in the steam generator, which may prevent it from functioning properly in severe cases; it is best to use distilled water. 5. When using the instrument, the power plug must have its L terminal connected to the live wire, its N terminal connected to the neutral wire, and its ground terminal properly grounded to ensure electrical safety; otherwise, the device’s casing may become electrified. VII. Maintenance 1. The water inside the gas generator should be replaced frequently to prevent scale formation from affecting performance and testing accuracy. After each use, all the water in the generator and condenser should be drained. If there is excessive scale, a mild acetic acid solution should be periodically poured into the generator through the drainage port for cleaning. 2. There are two solenoid valves inside the machine; it should be noted that the crystallization of alkaline solution can affect the opening and closing of the alkali addition valve, while excessive scale can interfere with the proper operation of the water addition valve. 3. Opening the back cover during use helps with heat dissipation and allows for monitoring of the internal operation of the device. VIII. Completeness of the product set 1. 1 KDY–21D nitrogen analyzer 2. 1 temperature-controlled digestion furnace (the user must purchase this separately) 3. 1 2-liter plastic tank (for alkali) 4. Digestion tubes (test tubes) with dimensions of Φ40×300 mm (to be purchased by the user; quantity to be determined by the user) 5. 1 acid burette (to be provided by the user) 6. 1 250-milliliter Erlenmeyer flask (to be provided by the user) 7. 1 set of instruction manuals 8. 1 certificate of conformity This post was last edited by lifanwang on 2009-3-3 at 12:22

Emerson has a chemical fluorescence method; I’ve used the % grade before. You can consult the manufacturer for details!

Gas chromatography with NCD detector

On chromatography, I know that there are HID (helium ion) detectors capable of achieving precision at the PPB level; it mainly depends on the separation of N2, N2O, and NO

Following the methods specified in national standards for measuring nitrogen oxides in air, colorimetric or chemical methods can be used. GB/T 8969-1988 and GB/T 15436-1995 can also be referred to; their detection limits reach the ug level.