Determining Nitrous Oxide Content in Gases - Essay Example

Determination of Nitrous oxide in gases Bhuvana Jaiganesh Determination of Nitrous oxide in gases. 2/04/06 Validation-Determining Nitrous Oxide content in gases Part 1 Nitrous oxide, popularly known as the laughing gas has been used in medicine as an anaesthetic since the late 18th century and also misused for recreational purposes. The gas has many industrial uses and is also naturally emitted during the nitrogen cycle. The use of fertilizers in agriculture, pollution and other human interventions has resulted in increased concentrations of the gas in the atmosphere and work places that has instilled fear for environmental /health safety. In this context, monitoring Nitrous oxide concentrations becomes pertinent. The paper is a comparative review of a few of the methods used in determination of nitrous oxide in gases. It has been concluded after a time, quality and cost-benefit analysis, that the use of passive diffusion sampling methods followed by thermal desorption and analysis with an EC-GC may be the optimal choice. Sampling Methods Sorbant material used for sampling: In all the methods described below, activated molecular sieves of 5A pore size are being used. Ambient air bag sampling- Personnel are fitted with air bags that passively collect fumes of nitrous oxide. The gas is absorbed in an appropriate sorbant material. An active sampling involves filling a sampling bag by means of a personal pump provided with a driving device or another equivalent method. The major disadvantage of the method being that the bags can get punctured and cannot be airlifted unless they are stored in pressurized cabins. Besides the potential for leakage is higher and leak checks using the soap bubble technique have to be performed. (US Govt. NIOSH documents-http://www.cdc.gov/niosh ) Passive dosimeter badges- Workers exposed to Nitrous oxide wear the badges and these absorb the vapour by diffusion principles. The minimum time for exposure is one hour and the maximum is 40 hours (one work day). Sampling capacity 560ppm-hrs, detection limit- 2ppm-hrs, sampling rate- 0.75ml/min, functions between 10%-80% relative humidity, accuracy (Max. Total Error) 25%, functional temperature limits are between 0C-50C (affect result by 5%). The major advantages are that, "no sampling pump is required," and badges are more accurate than diffusion tubes. The major disadvantage is that badges have to be analysed within one week after the sample period for accuracy. Badges must be used before the expiration date and are usually more expensive than diffusion tubes (http://www.assaytech.com; http://www.labsafety.com) Passive diffusion tubes- Steel tubes packed with molecular sieves and caped with diffusive caps. It has been found that the method is valid for detecting Nitrous oxide between 25 and 1000ppm. The quantitative detection limit is at 2 g. Passive monitors work accurate between 60-90% RH at 25C and can measure concentrations as high as 500ppm for 8 hours. The NIOSH declares the method as, "adequate," for measuring Nitrous Oxide in air as the method is not bulky, does not involve liquids and does not require pre and post calibration. Some types of tubes can be stored in temperatures between 20C-25C for a period of 30 days. Sample recoveries in 30 days were found to be 10% compared to results in day 2. Recovery rates were found to be 98%-100% is varying concentrations. The one disadvantage is because of reliance on samples being tested at the vendor site (http://www.cdc.gov/niosh) Active diffusion tubes- The sample is collected filling a sampling bag by means of a personal pump provided with a driving device or another equivalent method. They consist of a pump that pulls a constant amount of air (usually in l/min), the molecular sieve and a clip for attaching the device onto the breathing zone. No change in results were noted under a Relative Humidity (RH) of 90% under 25 C. Recovery rates were found to be between 99%-100% in varying concentration. The advantage of the method is that it is usually more accurate than other methods and the disadvantage is that it is bulky and not a preferred choice by people (http://www.osha.govt.com; http://www.labsafety.com, http://www.cdc.gov/niosh) Comparative Studies A study by Cox and Brown on the comparative merits between active versus passive sampling proved that both are equally effective. They concluded that passive methods are better because of their "convenience and simplicity," (COX & Brown 1984). In another study by Mehta, Burton and Simms to analysed types of sampling methods. They compared the multi spot sampling, the integrated personal sampling and end-tidal sampling methods. They found the integrated personal sampling method to be the best as it saved time and less cumbersome (Mehta, Burton & Simms 1978) Analysis Analysis of the collected samples is usually effected by Thermal desorption of the molecular sieves followed by qualitative and quantitative evaluation of nitrous oxide. This is done either by using Gas Chromatography or through Infra Red spectroscopy. Gas Chromatography In this method, the components in the gaseous mixture are identified by their retention time on the column and quantified subsequent, by a detector. The detector used for Nitrous Oxide analysis is the Electron Capture Detector (ECD). The sensitivity of this system is 10ppb (http://www.jti.slu.se/publikat/rapporter/l&i/r-274jg.pdf#18). Infrared Spectrophotometry The IR spectrophotometer takes advantage of the differential absorption of IR radiation by different elements and compounds. The wavelength of IR radiation absorbed by Nitrous Oxide is 4.5 m. The method has been found to be highly precise during gradient measurements and is very sensitive and detects as low as 1ppb of Nitrous oxide. The chief disadvantage is the costs involved in the system and the reliance on wind direction in the case of instruments being operating in open-air conditions. The method can also be affected by interferences by Carbon-di-oxide whose wavelength for IR absorption is 4.3m. High humidity can also affect the readings (2000, http://www.jti.slu.se/publikat/rapporter/l&i/r-274jg.pdf#18). Choice of sampling and analytical methods After careful analysis it has been concluded that the use of the passive diffusion tubes could be for sampling. This is because it has been found to be as sensitive as the active sampling system but definitely more convenient and simple. The badges though easy to use, are more expensive and need to reach the analytical laboratory within a week. The bags have dangers of leakage. The IR analysis is routinely being recommended by NIOSH. This system takes less than 2 minutes and is very sensitive. However, there are interferences and the costs are prohibitive. Therefore it is recommended that the analysis be done using an Electron Capture Gas Chromatography. Part 2 Sampling methods: The passive diffusion method involves the following steps: Caps should be opened only when collection of sample is about to start and capped again immediately after collection. Record sampling start and stop times Sampling tubes should be given a sample number Blanks or dosimeter should accompany every sample Leak checks should be made with direct reading IR or other suitable instruments. Guidelines for tube calibration: Conditioning of tubes: Tubes exposed to 50ml/min dry 5.0 grade nitrogen or helium at 350C for 2 hours or 300C overnight. Sealing: -inch, brass swagelok-type caps and combined PTFE Ferrules are used for sealing. Sealing completed by finger tight plus a further half turn with a Cap-LokTM tool. Removed caps just before sample collection and replaced immediately afterwards. Blanks: Caps are removed from blank tubes at the sampling location and immediately replaced. Storage and transport: Clean, airtight containers to be used for storage and transport. Analysis: Thermal desorption and Gas Chromatography Analysis The method is identical for analyzing samples collected diffusively or from breath. Tubes containing samples of nitrous oxide should be thermally desorbed using a carrier gas in forward flow direction (flowing through from the sample). If the conventional Zeolite molecular sieves are used, dry purging would be ineffective as the zeolite is highly hydrophilic. The water (depending on the design of the desorber) will affect the degree of splitting and the dimensions of the column. (http://www.iqa.com) The desorption temperature is kept at a low 160C- 170C. This disallows desorption of water molecules that get trapped in the column during sample collection while allowing complete desorption of Nitrous oxide. Suggested analytical conditions are as follows: Tube desorption: 3-5 minutes at 165C Desorption flows: Read More