GB 5009.179-2016National food safety standard - Determination of trimethylamine in food (English PDF)

3.1 Reagents

3.1.1 Sodium hydroxide (NaOH).

3.1.2 Trichloroacetic acid (C2HCl3O2).

3.2 Preparation of reagent

3.2.1 50% sodium hydroxide solution. WEIGH 100 g of sodium hydroxide;

DISSOLVE it in 100 mL of water at 20 °C ~ 30 °C.

3.2.2 5% trichloroacetic acid solution. WEIGH 25 g of trichloroacetic acid;

DISSOLVE it in water; MAKE its volume reach to 500 mL.

3.3 Standard substance

Trimethylamine hydrochloride (CAS. 593-81-7), molecular formula. (CH3)3NHCl, purity >= 98%, which is placed in a desiccator and preserved at 4 °C.

3.4 Preparation of standard solution

3.4.1 Trimethylamine standard stock solution. WEIGH 0.0162 g of

trimethylamine hydrochloride standard substance; USE the 5% trichloroacetic acid solution to dissolve and dilute it to 100 mL, which is equivalent to the trimethylamine standard stock solution of concentration 100 µg/mL;

PRESERVE it at 4 °C conditions.

3.4.2 Trimethylamine standard use solution. TAKE a certain volume of

trimethylamine standard stock solution; USE the 5% trichloroacetic acid solution to dilute it stepwise to the trimethylamine standard use solution which has a concentration of 1.0 µg/mL, 2.0 µg/mL, 5.0 µg/mL, 10.0 µg/mL, 20.0

µg/mL, 40.0 µg/mL, respectively.

5.2.2 Mass spectrometry conditions

The mass spectrometry conditions are as follows.

a) Ion source. electron impact ionization source (EI source), temperature.

220 °C;

b) Ionization energy. 70 eV;

c) Transmission line’s temperature. 230 °C;

d) Solvent delay. 1.5 min;

e) Scanning mode. Selective ion monitoring (SIM).

5.3 Determination

5.3.1 Headspace sample injection. EQUILIBRATE the prepared specimen at

40 °C for 40 min. Under the chromatographic mass spectrometry condition of

5.2, USE a syringe to extract 100 µL of the gas above the solution in the headspace bottle; INJECT it into a GC-MS for determination.

5.3.2 Qualitative determination. USE the selective ion monitoring method to

collect data; USE the retention time (RT) of trimethylamine in sample solution, auxiliary qualitative ions (m/z 59 and m/z 42), quantitative ions (m/z 58), the kurtosis ratio (Q) of the auxiliary qualitative ions to quantitative ions, to perform the comparative qualitativeness with the standard solution. The relative deviation between the kurtosis ratio (Q sample) of the auxiliary qualitative ion to the quantitative ion of trimethylamine in the sample solution and the kurtosis ratio (Q standard) of the auxiliary qualitative ion to the quantitative ion of trimethylamine in the standard solution is controlled within ±15%.

5.3.3 Quantitative determination. USE the external standard method for

quantification. USE the peak area of trimethylamine in the standard solution as the ordinate and the concentration of trimethylamine in the standard solution as the abscissa, DRAW the standard curve; USE the calibration curve to calculate the concentration of trimethylamine in the sample solution.

10.4.1 Trimethylamine standard stock solution. WEIGH 0.0162 g of

trimethylamine hydrochloride standard substance; USE the 5% trichloroacetic acid solution to dissolve and dilute it to 100 mL, which is equivalent to the trimethylamine standard stock solution of concentration 100 µg/mL;

PRESERVE it at 4 °C conditions.

10.4.2 Trimethylamine standard use solution. TAKE a certain volume of

trimethylamine standard stock solution; USE the 5% trichloroacetic acid solution to dilute it stepwise to the trimethylamine standard use solution which has a concentration of 1.0 µg/mL, 2.0 µg/mL, 5.0 µg/mL, 10.0 µg/mL, 20.0

µg/mL, 40.0 µg/mL, respectively.

12.1 Preparation of specimen

12.1.1 Pretreatment and preservation of specimen

For livestock, poultry meat and meat products, REMOVE fat and skin. For animal aquatic products such as fish, shrimp and their products, it is necessary to descale or peel it. For all samples, TAKE about 100 g of muscle part; USE the meat grinder to crush it or the knife to cut it; MIX it uniformly. If the prepared specimen is not immediately determined, it shall be sealed in a polyethylene plastic bag and preserved frozen at -18 °C. Before the determination, PLACE it i) Hydrogen’s flow rate. 40 mL/min;

j) Air’s flow rate. 400 mL/min.

12.3 Determination

12.3.1 Headspace sample injection. EQUILIBRATE the prepared specimen at

40 °C for 40 min. Under the above chromatographic mass spectrometry conditions, USE a syringe to extract 250 µL of the gas above the solution in the headspace bottle; INJECT it into a GC-FID for determination.

12.3.2 Qualitative quantification. PERFORM the qualitative analysis based on

the retention time of trimethylamine in the standard chromatogram. USE the external standard method for quantitative analysis. USE the standard peak area as the ordinate and the concentration of the standard solution as the abscissa, to draw the calibration curve. USE the calibration curve to calculate the concentration of trimethylamine in the sample solution.