Steps to detect acetonitrile content in surface water

  Acetonitrile in surface water, groundwater, domestic sewage and other water bodies can be determined by gas chromatography. The principle of this method is as follows.

  Direct injection method: The acetonitrile sample is directly injected into the gas chromatograph after filtration, separated by capillary column, detected by NPD detector, qualitative by relative retention time, and quantified by chromatographic peak area (peak height).
  Purge and trap method: The sample is purged with nitrogen, the purged target is enriched through the trap tube, and then the trap tube is rapidly heated to desorb the acetonitrile adsorbed in the trap tube and enter into the gas chromatography Instrument, separated by capillary column, detected by FID or NPD detector, qualitative by relative retention time, quantitative by chromatographic peak area (peak height).

Reagents required for testing

1. Acetonitrile standard stock solution: 1.00×104 mg/L.
  Commercially available certified standard products, generally using water as the solvent; or using chromatographically pure standard products to prepare, first weigh a 10ml volumetric flask containing some pure water, and then drop a few drops of chromatographically pure acetonitrile 100mg (accurate to 0.1mg) , weighed. Make up to volume and mix with pure water, the concentration is about 1.00×104mg/L (accurate to 10mg/L), as the standard stock solution of acetonitrile. Stock solutions are stored in vials with Teflon screw caps and are stable for 3 months at 20-25°C.
2. Acetonitrile standard solution: 100mg/L.
  Pipette a certain volume of acetonitrile standard stock solution into a 100ml volumetric flask containing some pure water, dilute to volume with pure water, and mix to obtain 100mg/L acetonitrile standard solution. The standard use liquid is ready to use.
3. High-purity nitrogen: 99.999% purity.
4. Hydrogen: purity 99.99%.

Instruments required for testing

1. Purge and trap device: The purge device can be directly connected to the chromatographic part and should have a 5ml purge tube. The trap tube generally uses 1/3 Tenax, 1/3 silica gel, 1/3 activated carbon mixed adsorbent or other equivalent adsorbents.
2. Gas chromatograph: It has a capillary column split/splitless injection port, which can electronically control the carrier gas pressure. Equipped with FID or NPD detector.
3. Chromatographic column: Quartz capillary column, 30m×0.32mm×1.0μm, the stationary phase is polyethylene glycol (PEG-20M), or a chromatographic column with similar performance.
4. Air-tight syringe: 5ml.
5. Micro syringe: 5ul, 10ul, 50ul, 100ul, 500ul, 1000ul.
6. Sampling bottle: 40ml brown wide mouth sampling bottle with Teflon liner screw cap.
7. Brown glass bottles: 1ml and 2ml, with teflon-lined screw caps.
8. Balance: 1/10,000 balance.
9. Volumetric flask: Grade A, 10ml and 100ml.

water sample collection

  In accordance with the regulations, parallel double samples are collected for all water samples, and each batch of samples should include a full program blank and a shipping blank. When collecting water samples, the water samples should overflow in the water sample bottle without leaving space.
water sample preservation
  The water sample should be placed in a refrigerator at about 4°C immediately after collection, and sent to the laboratory for analysis as soon as possible. If it cannot be analyzed in time, it can be stored in a refrigerator at about 4°C. There is no organic interference in the water sample storage area, and the water sample should be completed within 7 days. analyze.

Detection steps

Analysis conditions of direct injection method
Reference chromatographic conditions:
Injection volume: 1.0 μl;
Injection port: splitless, temperature 220℃;
Column temperature: constant temperature 50℃;
Analysis time: 15min;
Column flow: constant flow 7.0ml/min (nitrogen);
NPD detector: temperature 330°C, hydrogen 3.5ml/min, air 80ml/min.

Reference Purge and Trap Conditions

  Take 5ml of water sample for purging, purging time 11min, purging temperature 35℃, desorption temperature 190℃, desorption time 2min, baking temperature 220℃, baking time 7min, purge gas is high-purity nitrogen, purge flow rate 40ml/min.

Reference chromatographic conditions:

Injection port: split ratio 7:1, temperature 200°C; column temperature: FID constant temperature 60°C, NPD constant temperature 100°C;
Analysis time: use FID to detect 10min, use NPD to detect 8min; column flow: constant flow 2.0ml/min (nitrogen);
FID detector: temperature 230°C, hydrogen 40ml/min, air 400ml/min; NPD detector: temperature 330°C, hydrogen 3.5ml/min, air 60ml/min.

Drawing of the standard curve

  NPD direct injection method: Take 6 100ml volumetric flasks and prepare 6 standard series with concentrations of 0.10, 0.50, 1.00, 2.00, 5.00, 10.0 mg/L with acetonitrile standard solution (5.3) in turn. Accurately take 1.0 μl for gas chromatography analysis, take the peak area (peak height) as the ordinate and the concentration of acetonitrile as the abscissa to draw a standard curve.
  FID purge and trap method: Take five 40ml brown VOC tubes, and sequentially prepare five standard series with concentrations of 0.5, 1.0, 2.0, 3.0, and 5.0 mg/L with acetonitrile standard solution (5.3). Accurately take 5.0ml to the purging tube for purging, and then analyze by gas chromatography. Take the peak area (peak height) as the ordinate and the concentration of acetonitrile as the abscissa to draw a standard curve.
  NPD purge and trap method: Take five 40ml brown VOC tubes, and sequentially prepare five standard series with concentrations of 0.025, 0.050, 0.100, 0.200, and 0.500 mg/L with acetonitrile standard solution (5.3). Accurately take 5.0ml to the purging tube for purging, and then analyze by gas chromatography. Take the peak area (peak height) as the ordinate and the concentration of acetonitrile as the abscissa to draw a standard curve. The standard chromatogram of acetonitrile is shown below.

Steps

direct injection

   The water samples were filtered by a 0.45um filter membrane and then directly injected for chromatographic determination according to the conditions of the direct injection method.

Purge and trap

   The detection is carried out according to the analysis conditions of the purge and trap method.

Blank test

   According to the same conditions as the standard curve drawing, take the corresponding volume of experimental water for blank test, and the target compound cannot be detected.

Qualitative Analysis of Acetonitrile

   According to the retention time of the target in the sample and acetonitrile in the standard series, the target is characterized. Before sample analysis, a retention time window t±3S was established. t is the average retention time of acetonitrile at each concentration level during the initial calibration, and S is the standard deviation of the retention time of acetonitrile at each concentration level during the initial calibration. When the sample is analyzed, the target should be peaked within the retention time window.

Quantitative Analysis of Acetonitrile

   After qualitative identification of acetonitrile in the water sample, the concentration of acetonitrile can be directly calculated from the calibration curve according to the measured peak area (peak height). The concentration of acetonitrile in the water sample was calculated according to the relevant formula.