继续:
QC ELEMENTS:
A 1:1 (v/v) mixture of Sylon BFT and pyridine should be run at the onset of each analysis and then occasionally throughout the analysis to show that there is no carryover.
A method blank which consists of 20 mL of extraction solvent taken through the entire procedure including the addition of the internal standard should be evaluated to make sure that there is no contamination from the reagents, the nylon filter or the containers.
Condition the system at the beginning of each sample set by making two injections of the high standard (see below). These injections will not be used for semi-quantitation.
A low standard, which consists of each analyte at 0.10 µg/mL, should be analyzed at the beginning of the sample set to show that the necessary sensitivity is being attained by the instrument.
A high standard at 1.0 µg/mL of each analyte is analyzed at the beginning of the sequence and after all samples in the batch have been injected to provide a basis for semi-quantitative evaluation and to demonstrate whether the amount of drift during the analysis of the set of samples is tolerable.
A control sample which is representative of the type of samples which are being analyzed is fortified with each analyte (viz. melamine, ammelide, ammeline and cyanuric acid) at a level of 10 µg/g. Analysis of this spiked control sample must indicate that the compounds are present which serves to demonstrate effective system performance at the desired reporting level. See Sample Fortification, below.
PROCEDURE:
This procedure should be used with the
gc operating in the splitless mode for greater sensitivity.
A. Extraction Procedure
Weigh out approximately 0.5 g of a representative portion of the sample into a 50mL polypropylene centrifuge tube.
Add 20 mL of extraction solvent (10:40:50 DEA : H2O : Acetonitrile).
Mix well to thoroughly wet the entire sample.
Sonicate for 30 minutes.
Centrifuge for 10 minutes at 5000 g (or better).
Filter a portion of the supernatant using 0.45µm nylon filter discs (a two-stage or molecular weight cutoff filter may be used for difficult extracts). For example, a 2 mL filtered portion will allow 200 µL for the derivatization step and additional filtrate in the event that the derivatization needs to be repeated or if further work is necessary.
B. TMS-Derivatives
Sample Extracts:
Transfer 200 µL of filtrate from Step A to a
gc vial.
Note: A smaller aliquot may be used provided that the necessary sensitivity level (10 µg/g of sample) is achieved. Reducing the amount of matrix present improves the general performance of the evaporation/derivatization step and saves wear and tear on the instrument.
Evaporate to dryness at 70°C (a low flow stream of dry air or nitrogen may be used).
Note: Taking the filtrate completely to dryness is a critical step in the derivatization process. The presence of water prevents formation of TMS derivatives of the analytes. If the internal standard response is much lower than usual (less than 30%), there may have been problems associated with the derivatization step. In addition, if the vial warms significantly to the touch after addition of the derivatization reagents, residual water was present and a new aliquot of filtrate must be prepared.
Add 200 µL pyridine.
Add 200 µL Sylon BFT.
Add 100 µL of the internal standard solution at 5.0 µg/mL in pyridine. This produces a concentration in the extract of 1.0 µg/mL. If you are not using the internal standard, then add 100 µL of pyridine.
Shake well or vortex to mix.
Incubate at 70oC for 45 minutes.
Ready for Injection.
Note: If insoluble material is observed at the bottom of the vial after 45 minute incubation, transfer liquid portion to another
gc vial or filter before analysis.
C. Instrument Parameters
gc Conditions:
Column 30m DB-5MS 5% phenyl 95% dimethyl-polysiloxane
ID: 0.25mm Film Thickness: 0.25 microns
Inlet Temperature 280oC
Detector Temperature 290oC
Injection Mode Splitless
Injection Volume 1 µL
Carrier Gas Flow He at 35 cm/sec (constant flow)
Oven Program 75oC (hold 1 minute) to 320oC at 15oC/minute (hold 2.67 min) for a total run time of 20 min.
Note: Alternate
gc conditions may be used provided that adequate resolution is obtained between the target analytes. Any such deviations from the method must be noted in corresponding documentation. With small volume liners, some peak splitting has been observed under the above conditions. Using a higher starting temperature (100oC) alleviated the problem. To help overcome interferences, significant additional resolution may be obtained by decreasing the ramp to 4oC/min over the interval 150oC to 200oC, which will shift the retention times.
MS Conditions (Full Scan Mode):
Tune Autotune (to maximize sensitivity across mass range)
A +306V multiplier bump may be added after Autotuning
Acquisition parameters EI; scan mode, 50-450 amu
Sampling Rate 2 (scan rate at ~3.6 scans/sec)
Threshold 100
Filament Delay 6 minutes
MS Temp 230°C (Source); 150°C (Quad)
If the sensitivity which is required to detect analyte spikes at 10 µg/g in the matrix cannot be achieved in full scan mode, use selected-ion monitoring (SIM) parameters below.
MS Conditions (SIM Mode): Select three or four ions to track: M, M+2, M-15 and another
Group Start Time a
(min)
Ions b
M c M + 1 M + 2 M - 15 Other Other
Urea / Biuret from
di- and tri-TMS
derivatives of urea d 6 276
(tri-tms)
204
(di-tms) 189
(di-tms)
261
(tri-tms) 171
Cyanuric Acid 9 345
(100)e 346
(30) 347
(14) 330
(33) 188
(11)
Ammelide 9.7 344
(100) 345
(30) 346
(14) 329
(50) 286
(7) 198
(28)
DACP (internal std.) 10.18 288 289 290 273 275 237
Ammeline 10.5 343
(100) 344
(30) 345
(14) 328
(115) 285
(30) 198
(27)
Melamine 11 342
(54) 343
(16) 344
(8) 327
(100) 285
(12) 197
(13)