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8. Run a spiked urine control specimen with every 10 specimens to maintain quality assurance.

NOTE: Urine used for spiked controls must be analyzed before use to determine background fluoride concentration.

MEASUREMENT:

9. Add 10 mL well-mixed urine and 10 mL TISAB to a 50-mL plastic beaker.

10. Place a small stirring bar into beaker and mix continuously on a magnetic stirrer at room temperature.

11. Immerse electrodes. Allow sample to mix for 2 to 3 min and then record millivolt reading.

12. Rinse electrodes and stirring bar thoroughly with distilled water and wipe dry with tissue before next sample analysis.

CALCULATIONS:

13. Convert the millivolt readings to fluoride concentration using the calibration graph.

14. Express fluoride concentration as mg F /g urinary creatinine

GUIDES TO INTERPRETATION: Urine concentrations of fluorides in normal non-occupationally exposed workers have been reported to range from 0.2 to 3.2 mg/L depending on dietary intake [6]. Preshift levels of less than 4 mg/g creatinine and post-shift levels of less than 7 mg/g creatinine appears to protect workers against bony fluorosis [7]. NIOSH has recommended that post-shift urine specimens should not exceed 7 mg/L (corrected to a specific gravity of 1.024) and pre-shift specimens should not exceed 4 mg/L (1.024) [1,2].

The Biological Exposure Indices for fluoride are 3 mg/g creatinine prior to shift and 10 mg/g creatinine at end of shift [8].

EVALUATION OF METHOD: No formal method evaluation has been reported; however, Tusl [3] reported recoveries of added fluoride from 94 to 100%. Precision based on analysis of 25 specimens in triplicate is estimated to be better than Sr = 0.04.

[1] Criteria for a Recommended Standard...Occupational Exposure to Inorganic Fluorides, U.S. Department of Health, Education, and Welfare, Publ. (NIOSH) 76-103 (1976).

[2] Criteria for a Recommended Standard...Occupational Exposure to Hydrogen Fluoride, U.S. Department of Health, Education, and Welfare, Publ. (NIOSH) 76-143 (1976).

[3] Tusl, J. Direct determination of fluoride in human urine using fluoride electrode, Clin. Chim. Acta, 27, 216-218 (1970).

[4] NIOSH Manual of Analytical Methods, 2nd. ed., V. 1, P&CAM 114, U.S. Department of Health, Education, and Welfare, Publ. (NIOSH) 77-157-A (1977).

[5] Tietz, N. W. Fundamentals of Clinical Chemistry, 2nd ed., W. B. Saunders Co., Philadelphia, PA, 994-999 (1976).

[6] Baselt, R. C. Biological Monitoring Methods for Industrial Chemicals, Biomedical Publications, Davis, CA, 140-143 (1980).

[7] Lauwreys, R. R. Industrial Chemical Exposure: Guidelines for Biological Monitoring, Biomedical