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N-Nitrosodimethylamine, C2H6N2O , is a semi-volatile organic chemical of the nitrosamine group that is highly toxic and is a suspected human carcinogen. It may present in a variety of foods, potable water and wastewater (Woosuk Cha 2006). NDMA is one of the emerging chemicals in treated sewage that is extremely soluble in water and is an industrial by-product that may occur during chlorination and chloramination. Wastewater that contains organic nitrogen is able to stimulate the by-product formation process. NDMA can also form or be leached during treatment of water by ion exchange resins (ALS). The unit risk is the quantitative estimate in terms of either risk per ug/L drinking water or risk per ug/cu.m air breathed (http://www.epa.gov/ncea/iris/subst/0045.htm)
1.2 Concentrations of concern in drinking water and wastewater
The concentration of NDMA occurs in drinking water is of great concern as its carcinogenic property is extremely related to human health, an acceptable concentration of 9ng/L is issued in 2003 by Ontario in drinking water quality (www.ene.gov.on.ca/envision/gp/4449e.pdf). However, current techniques are of great difficulty in detecting the chemical in such a low concentration, thus make it much more difficult to get removed from drinking water. In addition, although there is little information regarding the potential for biological removal of NDMA, NDMA is somewhat resistant to biodegradation and is difficult to remove by air stripping and ozonation (J. Chung et al. 2009).
As it is for wastewater, recycled water that is for irrigation use or other recreative water use should also be taken into account when doing wastewater quality evaluation. People may ingest a small amount of water directly or indirectly when they are using recreation activities, such as oral exposure or inhalation exposure. Thus it is also requisite to take NDMA analysis is wastewater. NDMA concentration in secondary wastewater effluents is typically exceed 100ng/L (W.A. Mitch et al. 2003). In 1994, a regulatory level of 200ng/L in effluents was established for NDMA by the Ministry of Environment and Energy (V.Y. Taguchi et al. 1994).
1.3 Toxic property regarding to NDMA
The toxic effects on humans is still not clear yet as experiments only taken on animals and the outcomes are remain unpublished. Symptoms related to NDMA toxic include headache, fever, vomiting, abdominal pain, scattered intradermal hemorrhage, lethargy, nausea, and diarrhea (http://en.wikipedia.org/wiki/N-Nitrosodimethylamine). Toxicity and carcinogenicity associated with NDMA has been taken experiments on rats , and that can cause live tumours at a relatively low doses (ALS).
Monitoring program outline
In this paper, we are going to take out a monitoring program at the Hydrophobia Water Quality Laboratory in Western Sydney. The chemical we focus on is NDMA, which is detected as a chlorination by-product in water and wastewater treatment processes.
To analysis the chemical in water and wastewater, sample collection including sampling frequency, sampling location is described in the first part, following sample extraction methods and associated techniques. Advantages and disadvantages of each technique are also described in the following part.
Instrumentation for sample analysis including HLPC, GC, GC-MS/MS and LC-MS/MS are introduced briefly as well as pros and cons of each. A suitable approach is to be taken for accurate quantitation of the analyte. Quality control procedures such as analytical blanks is to be analyzed in details. As for the final part, typical analytical detection limits including LODs and LOQs that can be achieved will be briefly discussed.
Water and wastewater sample collection
2.1 Water sample collection location and frequency
Chart 2.1 Process schematic in Drinking Water Treatment Plant
Dilution with groundwater
- Sampling points
According to the DWTP process flow chart, 8 sampling points are indicated, thus samples should be collected pre-chlorination, pre-flocculation, pre-sane filtration, pre-dilution, pre-ozonation, pre-GAC filtration, post-GAC filtration and post-final chlorination, respectively. Due to the high relationship between NDMA concentration and human health, sampling prior to and after each process is required for NDMA concentration analysis, and for further removal analysis.
Also, the collection comes up once per month and 2 samples of each point are required when analyzing.
n = 8*2 = 16 samples/month (C. Planas et al. 2008)
2.2 Wastewater sample collection location and frequency
Chart 2.2 Process schematic in Wastewater Treatment Plant (Megan H. Plumlee et al.)
- Sampling points
The chart above is the process flow chart of the Interim Water Purification Facility (IWPF), a now-decommissioned advanced wastewater treatment plant operated by the OGWD in Southern California. This system comprises dose of sodium hypochlorite prior to micro filtration, thus chloramines were formed due to the high ammonia concentration. NDMA is likely to form during disinfection process and make this example typical for analysis.
6 point samples are to be collected once per month, and the points are effluent, chloraminated effluent, pre-RO, pre-UV, post-UV and post-blending, respectively. Raw water and treated water require NDMA sampling as they are important datas for treatment process and human health. Sampling after sodium hypochlorite dose is to analysis the influence of hypochlorite dose to NDMA formation. Post-microfiltration sampling is required to analysis MF to NDMA removal. As for UV process, it is extremely significant for NDMA removal, thus sampling prior to and after UV process are essential.
2 samples of each point each time are required when taking samples.
n = 6*2 = 12 samples/month
2.3 Sample transportation
Samples collected at both plants are to be transported to laboratories to do further analysis thus should be well kept after sampling. Collection of all samples are using 1L amber glass bottles as the organic compounds are sensitive to light, a quenching agent - 100 mg of sodium thiosulfate is added into bottles as soon as sampling to quench chlorine.
Samples should be kept in cool and dark condition and transported to laboratory immediately after collection. All the samples will be stored at 4Â°C prior to analysis. Following step of extraction should be taken within 7 days (Megan H. Plumlee et al. 2008).
Available techniques for sample extraction
Extraction of NDMA from aqueous samples is necessary for sensitive detection and is known to be difficult because of the hydrophilic and non-adsorptive characteristics of NDMA (Woosuk Cha 2006). Methods associated with NDMA extraction include liquid-liquid extraction(LLE), solid-phase extraction(SPE) and solid-phase microextraction(SPME).
3.1 Liquid-liquid extraction (LLE)
Liquid-liquid extraction, also known as solvent extraction and partitioning, is a method to separate compounds based on their relative solubilities in two different immiscible liquids, usually water and an organic solvent. It is an extraction of a substance from one liquid phase into another liquid phase (Wikipedia).
To cope with LLE, a separatory funnel is used on a small scale in laboratory analysis., it is normally done on the industrial scale using machines that bring the two liquid phases into contact with each other. Such machines include centrifugal contactors, thin layer extractors, spray columns, pulsed columns, and mixer-settlers (Wikipedia).
Dichloromethane is used to improve extraction efficiency in LLE, scholars adopted 100 to 300mL of dichloromethane to LLE with a extraction time of 6 to 18 hours, and then use rotary evaporator or nitrogen stripping, dichloromethane extracts to 1mL or less. Though it has improved extraction efficiency, it still cannot reach the ng/L level in drinking water (Raksit A. & Johri S. 2001). LLE is typically coupled with GC for NDMA analysis.
3.2 Solid-phase extraction (SPE)
In recently years, SPE has been developed rapidly due to its apparent advantages. Thus has been adopted as the main technique for extraction. Solid-phase extraction is a separation process by which compounds that are dissolved or suspended in a liquid mixture are separated from other compounds in the mixture according to their physical and chemical properties. Analytical laboratories use solid phase extraction to concentrate and purify samples for analysis (Wikipedia).
SPE can be conducted in either batch or continues mode(CSPE). In batch SPE, suspended extraction medium in sample solution is required for a specified time to contact with NDMA. In continues SPE (CSPE), water samples flow through a fixed medium at a relatively low rate. Hydrophilic material such as ambersorb 572 is used to extract NDMA selectively from aqueous solution. Normally, ambersorb 572 and CSPE are combined to improve recovery efficiency of NDMA from water samples (Woosuk Cha et al. 2006).
Two methods based on SPE techniques can be used for extaction: manual SPE and automated SPE (C. Planas et al. 2008).
3.2.1 Sorbent Ambersorb 572
This method is based on manual SPE with a hydrophilic material sorbent ambersorb 572. Add 2mL of methanol into 50Î¼L of the internal standard solution, add 300mg of Ambersorb 572 into 1L of sample that is contained in an amber glass bottle. After stirred with a magnet at 50rpm for 1 hour and filtered through a filter paper, Ambersorb 572 is collected through this way. The bottle will be rinsed with high-purity water and the suspension will be filtered to ensure a complete transfer of the Ambersorb 572 to the filter. Next, air dry the filter paper with Ambersorb 572 for 60 minutes and the dried Ambersorb 572 will be transferred to an amber vial, and 950Î¼L of dichloromethane along with 50Î¼L of the recovery standard solution (toluene-d8 at 1 ng/Î¼L) will be added (C. Planas et al. 2008).
3.2.2 Coconut charcoal EPA 521 cartridges
25Î¼L of internal standard solution and 2mL of methanol spiked is added into 500mL of sample. Then the sample will be homogenised by sonication and extracted using the automated SPE system Power-Prep/SPE. 20mL of dichloromethane, methanol and Milli-Qwaterwere will pass through an SPE coconut charcoal EPA521 cartridge at a flow rate of 10 mL/min. Then the sample pass through the cartridge at a flow rate of 10mL/min, and the sorbent will be dried under vacuum for 10 min. Elution was performed with 2Ã- 6mL of dichloromethane and the eluate was passed through an SPE cartridge Isolute Na2SO4. The extract was concentrated to a volume of 500Î¼L on a Turbo Vap II evaporator (Zymark, Hopkinton, MA, USA), using a water bath near room temperature (20-25 â-¦C) and a gentle stream of nitrogen, and transferred to an amber glass vial. Finally, 25Î¼L of the recovery standard solution were added (C. Planas et al. 2008).
3.2.3 Comparison between two methods
According to Planas C.2008, NDMA extraction using Coconut charcoal EPA 521 cartridges are likely to get a better recovery than the other method. Eight analysed nitrosamines out of nine shows that above 70% recovery can be achieved by Coconut charcoal EPA 521 cartridges method, and seven of them exceed a recovery rate of 80%. Compared with Sorbent Ambersorb 572, this method is much more likely to achieve a better recovery efficiency. It is also automatic enough to reduce the operation cost, especially labor cost (C. Planas et al. 2008).
3.3 Solid-phase microextraction (SPME)
A solid-phase microextraction method has been developed recently by Grebel et al. for NDMA and other nitrosamines. The advantage for this method include a short analysis time require. However, using this method cannot achieve detection limits in the 1-10 ng/l range required for drinking water (Grebel et al. 2006).
Solid-phase microextraction (SPME), is a sample preparation technique used both in the laboratory and on-site. it is a simple and inexpensive technique where the use of solvents is not necessary. Solid-phase microextraction involves the use of a fibre coated with an extracting liquid phase when doing NDMA analysis (Wikipedia).
SPME fibers were first conditioned to remove any contaminants. Each fiber was conditioned prior to use in a GC split-splitless inlet, at the temperature and length of time recommended by the manufacturer for each fiber type. 15mL amber open top vials with septa were used for extraction. Sodium chloride and a magnetic stir bar were added to the sample vial, followed by the aqueous sample. The sample was placed on a heater/magnetic stirrer and the septa pierced by the SPME holder. The SPME fiber was immediately exposed to the sample headspace. During extraction only a small fraction of analyte mass is sorbed into the fiber phase. After a set extraction time, the fiber was retracted and removed (Janel E. Grebel et al. 2006).
3.4 Advantages and disadvantages of each extraction method
Table 3.1 Advantages and disadvantages of each extraction method
SPE uses a small amount of solvent for NDMA extraction, which can reduce the amount of time for concentration while processing large-volumes of samples simultaneously.This methold is able to processing large amount of water samples.
The attraction of SPME is that the extraction is fast and simple and can be done without solvents, thus reducing costs and generating less waste. Detection limits can reach parts per trillion (ppt) levels for certain compounds. SPME also has great potential for field applications; on-site sampling can be done even by nonscientists without the need to have gas chromatography-mass spectroscopy equipment at each location. When properly stored, samples can be analyzed days later in the laboratory without significant loss of volatiles.
LLE consumes relatively large amount of solvent and is laborious for large-volume samples. It is also labor intensive and achieve low recoveries (Megan H. Plumlee et al. 2008 ).
The process is complex, the present technique require a long time for extraction. Low recovery rate using Ambersorb 572 as carbonaceous adsorbents (C. Planas et al. 2008).
Samples should be analyzed as soon as possible following extraction.ã€€It has greater standard deviation for replicate analyses (Janel E. Grebel et al. 2006).
Current techniques for sample analysis
Further analysis on NDMA can be carried out using HLPC, GC, GC-MS/MS and LC-MS/MS, GC/NPD.
According from a fact sheet of United States,
For drinking water, EPA Method 521 uses solid phase extraction (SPE) and capillary column gas chromatography (GC) with large-volume injection and chemical ionization tandem mass spectroscopy (MS) (Munch and Bassett 2004).
For wastewater, EPA Method 607 uses methylene chloride extraction, GC, and a nitrogen-phosphorus detector (EPA 2002).
For wastewater, EPA Method 1625 uses isotope dilution, GC and mass spectrometry (MS) (EPA 2002).
An analytical method has also been developed specifically for NDMA precursors such as alkylamines in waste or wastewater (Mitch, Gerecke, and Sedlak 2003).
A recently developed method using liquid chromatography tandem MS (LC/MS/MS) detects both thermally stable and unstable nitrosamines (Zhao et al. 2006).
4.1 High performance liquid chromatography (HPLC)
High performance liquid chromatography is a form of column chromatography used frequently in biochemistry and analytical chemistry to separate, identify, and quantify compounds based on their idiosyncratic polarities and interactions with the column's stationary phase (Wikipedia).
4.1.1 Reagent dose
For the purpose of improve the selectivity and sensitivity of HPLC analytical methods, fluorescence derivatization has been used for NDMA detection. Dansyl chloride is generally used as derivatizing reagent to react with amino acid and peptides to form a highly fluorescent derivative (Lawrence J. & Frei R. 1976). NDMA must be denitrosated to produce dimethylamine (DMA) -a secondary amine to react with dansyl chloride to form a fluorescent dansyl amine (R. Frei & J. Lawrence 1982).
4.1.2 Instrumental apparatus
Derivatized samples were measured with a Waters (Milford, MA, USA) high-performance liquid chromatography (HPLC) unit, which consisted of a Waters 600 Multisolvent delivery system, Waters 600E system controller, Waters 2475 Multi Î» Fluorescence Detector, and Waters 717Plus Autosampler. A NovaPakÂ® C18 column (3.9mmÃ-150 mm, particle size of 4Î¼m) was supplied by Waters and used as an analytical column. A sentry guard column (Waters) was used in conjunction with the analytical column. Data acquisition and processing was done with Millenium32 (Waters) software. An acetonitrile-water mixture (2:1, in volume) was used as a mobile phase (eluent) after degassing for at least 20 min with an ultrahigh-pure grade helium gas. Degassing was continued during analysis at a gas flow rate of 20 ml minâˆ’1. The flow rate of the eluent was 1 ml minâˆ’1 (isocratic mode). The optimum excitation and emission wave lengths for fluorescence detection were determined using standard solutions and varying the wavelengths. The injection volume to the HPLC was 10-20 Î¼l (Woosuk Cha et al. 2006).
Figure 4.2 HPLC apparatus (http://en.wikipedia.org/wiki/File:HPLC_apparatus.svg)
(1) Solvent reservoirs, (2) Solvent degasser, (3) Gradient valve, (4) Mixing vessel for delivery of the mobile phase, (5) High-pressure pump, (6) Switching valve in "inject position", (6') Switching valve in "load position", (7) Sample injection loop, (8) Pre-column (guard column), (9) Analytical column, (10) Detector (i.e. IR, UV), (11) Data acquisition, (12) Waste or fraction collector (http://en.wikipedia.org/wiki/File:HPLC_apparatus.svg).
As LC demand a dry mobile phase added to the complexity and expense of the chromatographic technique, LC is typically coupled with MS for aquatic sample analysis (Water and wastewater analysis and quality requirements)
A liquid chromatograph from Shimadzu (LC-10AD VP) with a Shimadzu SIL-10AD VP autosampler (Columbia, MD, USA) connected to a triple quadrupole mass spectrometer (API3000) from Applied Biosystems (Foster City, CA, USA) was used for analysis of the nitrosamines. A volume of 50 ml of sample was injected at a flow rate of 0.15 ml/min onto a 50mm*2.1mm Targa Sprite C18 column (5 mm pore size, Higgins Analytical, Mountain View, CA, USA) equipped with a C18 Guard Column (Higgins Analytical). The mass spectrometer was operated in multiple reaction-monitoring transition mode at an optimized voltage for each transition in positive-ion mode. NDMA-d6 at a concentration of 110mg/L is added as an internal standard to sample extracts and nitrosamine standards at a ratio of 0.3 internal standard volume (Megan H. Plumlee et al. 2008).
In gas chromatography, the injected sample is volatilized in the injection port after which the gaseous mobile phase introduces the analyte species to the stationary phase. In aquatic water analysis, GC is typically coupled to MS to improve analysis efficiency. Gas chromatography coupled with mass spectrometry detection is a very powerful technique for both qualitative and quantitative analysis of many classes of organic compounds
4.4 Comparison among different detection techniques
Table 4.1 Comparison among different detection techniques
Low-cost and simple to operate, generates a distinct peak for NDMA without interference even in the complex matrix of wastewater effluents. The HPLC with fluorescence derivatization method may be applicable for determining NDMA in water and wastewater samples for various research purposes and for screening environmental samples.
Require the addition of volatile acids/bases or ion-pairing reagents.
High sensitivity to detect NDMA concentration as low as 1 ng/L. Useful when product analysis is required.
GC/MS/MS is not able to do research on unstable nitrosamines under thermal condition.
Used in analysis of a large number of polar contamintants in water samples, it is a extremely useful quantitative tool.
Requires a high-cost instrument and relatively high operational skill. Insufficient sensitivity, selectivity, cannot detect ng/L level of NDMA in aquatic samples.
Able to do research on thermal instability nitrosamines.
The cost related to the method is relatively high on aquatic NDMA analysis (CHU Wenhai et al. 2008).
Quantitation of the analyte
5.1 Quantitation methods
In sample preparation process, it is impossible to avoid analyte losses using preparation techniques such as SPE and SPME as mentioned in Part 3. Thus, quantitation of the target analyte is required. Typical quantitation methods include internal standard, external standard, standard addition and calibration.
Only two quantitation procedures provide a compensation to target compounds: Internal standard and standard addition. Standard compounds are often added to samples prior to analysis to either compare relative retention times or to compensate for analytical variations in sample dilution, sample delivery and sample detection (Lecture Notes 2010, Stuart Khan). In this paper, a traditional single internal standard procedure is introduced for NDMA quantitation.
5.2 Internal standard introduction
An internal standard in analytical chemistry is a chemical substance that is added in a constant amount to samples, the blank and calibration standards in a chemical analysis. This substance can then be used for calibration by plotting the ratio of the analyte signal to the internal standard signal as a function of the analyte concentration of the standards. This is done to correct for the loss of analyte during sample preparation or sample inlet (Wikipedia).
5.3 Internal standard quantitation
Take GC-MS technique for instance, isotope dilution is used with this instrumentation, NDMA-6 is used as quantitation standard. Their ratios in prepared probes directly correspond to their ratios in initial samples; there is no dependence on compound losses during sample preparation. Concentration ratios can be measured in single ion monitoring (SIM) mode with MS detection. Depending on the chemical origin of analytes this method provides the relative standard deviation (RSD) of results in the range 0.5-20% (5-15% in most practical cases) which is considered as appropriate error at low contents of analytes (Igor G. Zenkevich & Evgeny D. Makarov 2007).
Equation associated with the method is outlined as below (Igor G. Zenkevich & Evgeny D. Makarov 2007):
qx = fx,stand Px qstand/Pstand
qx - concentrations of NDMA in initial sample
qstand - concentrations of NDMA-6 in initial sample
Px - parameters of chromatographic peaks ï¼ˆareaï¼‰of NDMA
Pstand - parameters of chromatographic peaks ï¼ˆareaï¼‰of NDMA-6
fx,stand- calibration coefficient
Early methods used to quantify area include plot on graph paper and count squares, cut out and weigh. Now we use computers to integrate peak, Through compare unknowns to standard calibration curve, we are able to calculate relative response of sample and internal standard (Lecture Notes 2010, Stuart Khan).
Quality control procedures