Page:NIOSH Manual of Analytical Methods - 5606.pdf/3

 : NOTE: If this method is being combined with NMAM Method 5601 for benomyl, then at the end of the mixing period allow the samples to sit at room temperature overnight. This will allow any possible residual benomyl to convert to carbendazim.

8. Filter an aliquot into a 2-mL autosampler vial through a 4-mm, 0.45-µm PTFE filter.

CALIBRATION AND QUALITY CONTROL:

9. Determine retention times for thiophanate-methyl using the column and chromatographic conditions as shown on page 5606-1. The approximate retention time of thiophanate-methyl using the current chromatographic conditions is 14.1 minutes. (See Figures 1–3.)

10. Calibrate daily with at least six working standards covering the working range for thiophanate-methyl.

11. Determine desorption efficiency (DE) for each lot of OVS-2 tubes used for sampling in the calibration range. Prepare three tubes at each of five levels plus three media blanks.


 * a. Remove cap and PTFE retainer ring from large end of sampler tube (to prevent wicking behind the ring). Apply known volume of calibration solution to face of quartz filter.
 * NOTE:	Spike no more than 30 µL at a time. If more must be applied, connect the sampler to a vacuum pump with a flow ≤1 L/min, then apply spiking solution in aliquots of 15–30 µL. Allow 15 minutes for the solvent to evaporate between each aliquot, to prevent wicking along the sides of the tube in the back-up section (5% or more may deposit on the walls of the tube).
 * b. Cap and allow to stand a minimum of one hour.
 * c. Include an unspiked sampler as a media blank.
 * d. Desorb (steps 5 through 8) and analyze together with working standards (steps 11 and 12).
 * e. Prepare a graph of DE vs. μg thiophanate-methyl recovered.

12. Prepare analyst spike samples when field samples are received and store them together. Analyze with the field samples, blanks, and the liquid standards.

MEASUREMENT:

13. Set the liquid chromatograph according to manufacturer’s recommendations. Set the wavelength for detection at 200 nm and flow rate at 0.200 mL/min.

14. Inject a 5-µL sample aliquot with autosampler or sample injection valve.
 * NOTE: If peak area of a sample is greater than the area of the highest standard, dilute with extraction solvent and reanalyze. Apply the appropriate dilution factor in the calculations.

15. Measure peak area of the analyte.

CALCULATIONS:

16. Determine the mass, µg, of thiophanate-methyl found in the sampler filter and front sorbent section (Wf), back-up sorbent section (Wb), and the media blank front (Bf) and back-up (Bb ) sorbent sections from a standard curve.

17. Calculate concentration, C, of thiophanate-methyl in the air volume sampled, V (L).

EVALUATION OF METHOD:

This method was evaluated with a recovery study over the range of 79.5–567 µg/sample, using spiked laboratory samples, with average recoveries in the range of 89.9–100%. The storage study was completed at 159 µg/sample with recovery averages of 91.6% to 99.8% over the 28 days of the study