Determination of organophosphorus pesticides in th

2022-08-09
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Water quality determination of organophosphorus pesticides nbsp; This standard is applicable to the determination of medium base parathion, parathion, malathion, dimethoate, dichlorvos and trichlorfon in surface water, groundwater and industrial wastewater

in this method, the above pesticides in water are extracted with chloroform and determined by gas chromatography with flame photometric detector. When determining trichlorfon. Due to its high polarity and strong water solubility, the extraction rate is zero when extracted with chloroform, so the indirect determination method of converting trichlorfon into dichlorvos is adopted

the detection limit of methyl parathion, parathion, malathion, dimethoate, dichlorvos and trichlorfon in this method is 10-9 ~ 10-10g, and the lower limit of determination is usually 5 × 10-4~10-5mg/L。 When the instruments used are different, the detection range of the method is different

2 reagents and materials

2.1 carrier gas and auxiliary gas

a. carrier gas: nitrogen, purity 99.9%, purified with 5A molecular sieve purification pipe

b. combustion gas: hydrogen, 99.9% purity, purified with 5A molecular sieve purification pipe

c. auxiliary gas: air, purified with 5A molecular sieve purification pipe

2.2 reagents and materials used in preparing standard samples and sample pretreatment

2.2.1 chromatographic standards: methyl parathion, parathion, malathion, dimethoate, dichlorvos, trichlorfon The purity is 95% - 99%

2.2.2 chloroform (CHCl3): analytically pure

2.2.3 anhydrous sodium sulfate (Na2SO4) impact: analytically pure

2.2.4 sodium hydroxide (NaOH): analytically pure

2.2.5 hydrochloric acid (HCl): analytically pure, P = 1.19g/ml

2.3 reagents and materials used in the preparation of chromatographic columns

2.3.1 refer to Article 3.4 for chromatographic columns and fillers

2.3.2 solvent used for coating fixed solution: chloroform (2.2.2)

3 instrument

3.1 instrument model: gas chromatograph equipped with flame photometric detector

3.2 recorder: a recorder matching the instrument

3.3 detector

3.3.1 type; Flame photometric detector

3.3.2 device characteristics: the flame photometric detector adopts phosphorus filter, and the high voltage and high resistance of the photomultiplier tube used are usually 675v and 1070 respectively. 3.4 chromatographic column 3.4.1 chromatographic column type and characteristics: hard glass filled column, 2m long and 4mm inner diameter. 3.4.2 pretreatment of chromatographic column

after washing with water, fill the glass column tube with washing liquid and soak it for 2h (if necessary, the washing liquid can be warmed better), then wash it with tap water to neutral, wash it with distilled water and dry it before silanization: fill the glass column tube with 6% - 10% Dichlorodimethylsilane methanol solution and soak it for 2h, then wash it with methanol to neutral, and dry it for standby

3.4.3 filler

3.4.4.1 carrier

white acid pickling silylated diatomite support (0.15 ~ 0.20m/M)

3.4.4.2 fixed liquid

a. name and chemical properties: dimethyl silicone oil (dc-200), polyfluoroalkyl siloxane (qf-1), maximum service temperature 250 ℃

b. liquid phase loading: dc-200 is 5%, qf-1 is 7.5%

c. method of coating fixed solution: weigh a certain amount of fixed solution according to the weight of the support. Dissolve it in chloroform. After it is completely dissolved, pour it into a beaker with a support, and then add chloroform to it until the liquid level is 1 ~ 20m higher. Shake it well and soak it for 2h. Then volatilize the solvent in the fume hood with an infrared lamp (when volatilizing, shake the container constantly to make the fixed liquid coat evenly), and then place it in a 120 ℃ oven for 4H before standby

3.4.4.3 filling method of chromatographic column

plug the tail end of the chromatographic column (connected to one end of the detector) with silanized glass wool, connect the vacuum pump, and connect the other end to a funnel through a hose. After starting the vacuum pump, slowly pour the stationary phase into the chromatographic column, and gently tap the chromatographic column to make the stationary phase fill tightly in the chromatographic column until the stationary phase is no longer pumped into the column. After filling, plug the other end of the chromatographic column with silanized glass wool

3.4.4.4 aging of chromatographic column

install the filled chromatographic column with nitrogen, and take 100 ℃ as the starting point, rise 20 ℃ every 2h to 230 ℃ for continuous aging for 24h. (when aging, the chromatographic column should be disconnected from the detector to avoid polluting the detector)

3.4.5 column efficiency and resolution

total separation efficiency index:

parathion, malathion K1 = 28.08

malathion, methyl parathion K2 = 10.52

methyl parathion, dimethoate K3 = 28.60

dimethoate, dichlorvos K4 = 57.75

peak height resolution of malathion and methyl parathion is 0.9

3.5 instrument used in sample pretreatment

3.5.1 sample bottle: 1L glass ground bottle

3.5.2 evaporation concentrator: k-d type

3.5.3 separating funnel: 250ml, 500ml

3.5.4 water bath

3.5.5 microinjector: 10? L。

3.5.6 relay and contact thermometer

3.5.7ph meter

4 samples

4.1 sample properties

water samples, organophosphorus pesticides in water are not very stable and easy to degrade

4.2 water sample collection and storage methods

collect samples with a glass ground bottle (3.5.1), and rinse the sampling bottle with water sample for 2-3 times before sampling. The water sample should be kept in a weak acid state. Because dichlorvos and trichlorfon are easy to degrade, it should be analyzed as soon as possible. The water samples of the other four organophosphorus pesticides can be stored in a 4 ℃ refrigerator for three days

4.3 pretreatment of samples

4.3.1 determination of methyl parathion, parathion, malathion, dimethoate and dichlorvos

shake the samples well and remove mechanical impurities through filtration. Take 100ml of samples (or depending on the water quality) in a 250ml beaker. Adjust the pH to 6.5, then transfer the sample to a 250ml separating funnel and extract it with chloroform for three times. The dosage of chloroform each time is 5ml (compared with 1:20), shake for 5min, and stand for layering. Combine chloroform and collect the water layer. The combined chloroform is dehydrated by anhydrous sodium sulfate. For determination. Anhydrous sodium sulfate dehydration column has an inner diameter of 1cm, a length of 15cm, and an anhydrous sodium sulfate section of 8cm

if the content of organophosphorus pesticides in the alkyl layer of trichloride is too low and below the minimum detection amount, it needs to be concentrated to the required volume by k-d concentrator before determination. If the content of organophosphorus pesticides in the chloroform layer is too high, it is necessary to take less samples or dilute the samples before extraction

4.3.2 determination of trichlorfon

adjust the pH of the water layer collected in 4.3.1 to 9.6, pour it into a 250ml conical flask, cover the cork, put it in a 50 ℃ water bath for alkaline hydrolysis, and constantly shake the conical flask. After 15min, take out the conical flask, cool it to room temperature, adjust the pH to 6.5, and transfer the solution to a 250ml separatory funnel. The following operations are the same as 4.3.1

note: during sample pretreatment, only use pH meter to adjust, not pH test paper

5 operation steps

5.1 adjustment of instrument

5.1.1 vaporization chamber temperature; 240℃。

5.1.2 column box temperature: 170 ℃

5.1.3 detector temperature: 230 ℃

5.1.4 carrier gas flow rate: 60ml/min

5.1.5 hydrogen flow rate: 160ml/min

5.1.6 recorder paper speed: 4mm/min

5.1.7 attenuation: adjust the decrease of the recorder according to the content of the tested component in the sample

5.2 calibration

5.2.1 quantitative method

external standard method

5.2.2 standard samples

5.2.2.1 times of use: use standard samples for periodic repeated calibration. The length of the cycle depends on the stability of the instrument. Generally, it can be calibrated once after measuring three samples

5.2.2.2 preparation of standard samples

a. stock solution: take chloroform (2.2.2) as solvent, accurately weigh a certain amount of chromatographic pure standard sample (2.2.1), accurate to 0.2mg, and prepare stock solutions of methyl parathion, parathion and malathion with a concentration of 2.5mg/ml respectively; DDVP stock solution with concentration of 0.75mg/ml; Dimethoate stock solution with a concentration of 5.0mg/ml. Dichlorvos stock solution can be stored for two months at 4 ℃. The rest can be stored for half a year

b. preparation of intermediate solution: take a certain amount of stock solution (a) and use chloroform as diluent to prepare intermediate solutions of methyl parathion, parathion and malathion with a concentration of 50 g/ml; DDVP intermediate solution with a concentration of 7.5 g/ml; Dimethoate intermediate solution with a concentration of 100 g/ml

c. preparation of standard working solution: according to the sensitivity of the detector and the measured water concentration Take the intermediate solution (b) by equal volume respectively, and put it in the same volumetric flask. Use chloroform as diluent to prepare a standard working solution that is not only the concentration required for the improvement of rubber materials. It can be stored at 4 ℃ for half a month

5.2.2.3 conditions for using standard samples in gas chromatography a. the injection volume of standard samples is the same as that of samples, and the response value of standard samples should be close to that of samples; b. Adjust the repeatability condition of the instrument: a sample is injected twice continuously, and the relative deviation of its peak height is not more than 5%, that is, the instrument is considered to be in a stable state; c. The standard sample and sample shall be injected and analyzed at the same time as possible. 5.2.3 expression of calibration data the components in the sample are calibrated according to formula (1):

where: Xi - the content of component I in the sample, mg/L

ai - peak height of component I in the sample, cm

ae -- peak height of component I in standard solution, cm

ei -- content of component I in standard sample, mg/L

5.3 test

5.3.1 injection

a. injection method: syringe injection

b. injection volume: general injection volume 5? 50. Maximum injection volume 10? L。

c. operation: before cleaning and start-up, more agreements should be reached between the machine head and the granulator method, as well as in transportation, logistics, port, customs clearance, etc. the blue bolt should be tightened again for one time. After the syringe (3.5.5) is pumped several times in the sample to be tested, the required injection volume should be extracted, the sample in the syringe should be injected into the chromatograph quickly, and the syringe should be pulled out immediately

5.4 investigation of chromatogram

5.4.1 standard chromatogram (see the following figure) (omitted)

5.4.2 qualitative analysis

a. peak sequence of components: dichlorvos (trichlorfon), dimethoate, methyl parathion, malathion, parathion

b. retention time:

dichlorvos 1min

dimethoate 10min

methyl parathion 13min8s

malaryphos 20min

parathion 24min15s

5.4.3.3 quantitative analysis

5.4.3.1 measurement of chromatographic peak

take the starting and ending line of the peak as the peak bottom, and take the time axis as the vertical line from the peak height maximum, from the peak top to the peak bottom.The line segment is the peak height

5.4.3.2 calculation

5.4.3.2.1 calculation of the contents of methyl parathion, parathion, malathion, dimethoate and dichlorvos:

in the formula: CI - pesticide content in the sample, mg/L

ci standard -- pesticide content in standard sample, mg/L

hi -- peak height of pesticides in the sample, cm

hi standard - peak height of pesticides in standard sample, cm

v1 -- standard sample injection volume, l

v2 -- volume of extract, ml

v3 - sample injection volume, l

v6 - volume of water sample to be extracted, ml

k - sample dilution factor

5.4.3.2.2

calculation of trichlorfon content

in the formula:

c - trichlorfon content in the sample, mg/L

c1 - dichlorvos content generated by trichlorfon transformation in the sample, mg/L

0.86 - ratio of molecular weight of dichlorvos and trichlorfon

6 expression of results

6.1 qualitative results

according to the components of the standard chromatogram, in which a and B are the retention time of the loading port, determine the component number and component name in the tested sample

6.2 quantitative results

6.2.1 expression method of content

calculate the content of the existing component according to the calculation formula, and the result is expressed in two significant figures