Laboratory: Chlorine Sampling Techniques Essay Example

accurately determine dilution and the final concentration of the gas.

Samples were taken from a glass sampling manifold which was attached to the generation system. The majority of samples were collected in sets of six at one of three concentrations, about 0.5, 1, or 2 ppm chlorine.

Procedure: Samples containing 0.1% sulfamic acid were spiked with standardized chlorine solutions. This test determined the precision and accuracy of the analysis portion of the method.

Results: The results are shown in Table 1. Average analytical recovery was 96.7% and the coefficient of variation (CV1) was 0.03 for the three sets of spiked samples.

Procedure: Samples were collected in 0.1% sulfamic acid solutions at three different chlorine concentrations using the generation system described in Section 1. Midget fritted glass bubblers (MFGB) were used to disperse the air samples in the sulfamic acid solutions. This test determined the precision and accuracy of the sampling and analysis portion of the method.

Results: Sampling and analysis data are presented in Table 2. The NIOSH Statistical Protocol (9.7.) developed for evaluation of methods under the Standards Completion Program (SCP) was used as a guide for the determination of precision and accuracy from this data. The precision for this method using these statistical procedures is:

The average recovery for sampling and analysis over all levels was 99.4%. Any variation from 100% recovery was probably related to difficulties in generating the atmosphere containing the analyte at a given concentration rather than a true bias in the method. The Bartlett's test for homogeneity of variances and an outlier test (9.7.) were applied to this data. The sampling and analysis data passed the Bartlett's test, indicating the CVs could be pooled for

all sets of generated samples. One data point in the third set (2 PEL, n=5) was rejected as an outlier.

Procedure: The collection efficiency (CE) of MFGBs containing 0.1% sulfamic acid was assessed. A chlorine concentration of approximately 2 ppm was generated and samples were collected in series using the MFGBs. A flow rate of 1 L/min and a 15-min sampling period were used. The amount of chlorine collected in each of the two bubblers connected in series was measured.

Results: The results are shown in Table 3. The CE of the single bubbler was determined to be 1.00; therefore, a recovery correction or sampling train is not necessary.

Procedure: Breakthrough is defined as the time the effluent concentration in a second bubbler (containing 0.1% sulfamic acid) connected in series reaches 5% of the concentration of the test gas mixture. A test for breakthrough was conducted at about 2 ppm and at a 1 L/min sample collection flow rate. A sample set consisting of two bubblers in series was used to collect samples for each time interval listed: 15, 30, 60, 120, 180, and 240 min.

Results: No breakthrough occurred at this flow rate, during these sampling times, or at the concentration tested.

Procedure: A study was conducted to assess whether chlorine samples collected at the 1 ppm PEL could be successfully stored in the 0.1% sulfamic acid collection solution. After sample collection using MFGBs, samples were transferred into 20-mL vials, capped with Teflon-lined caps, and stored at ambient laboratory temperatures in a dark environment for 1, 5, 15, or 30 days before analysis.

Results: Samples analyzed after 30 days were found to be within 3% of those analyzed immediately. Thus,

storage does not represent a source of bias that would need to be corrected in the method.

An independent volumetric method  involving a thiosulfate titration was used to determine the concentration of chlorine used for spiking. Chlorine gas was generated at a theoretical concentration which would give a chlorine concentration of 21.70 g/mL in a specified volume of sulfamic acid. This value was calculated using the equation given in Appendix A. The gas was collected in 0.1% sulfamic acid solution which was then used as a chlorine stock solution for spiked samples . All other samples were taken using the generation system. Analyses of the stock solution were performed using the RCE and the thiosulfate titration methods. The following results were obtained: The average value of 20.69 g/ml from both methods was used as the stock solution concentration for the spiked samples mentioned in Section 2.

This sampling and analytical method has been shown to be precise and accurate at exposures near the OSHA PEL of 1 ppm when using 15-L air volumes. Breakthrough or storage stability do not pose significant problems under the conditions tested.

The development and evaluation of this method took place when the OSHA PEL for chlorine was a Ceiling value (15-min samples were taken). Although long-term samples were only taken during breakthrough studies, the data appears to indicate the method is capable of taking samples for chlorine TWA assessments.

Two 4-h samples are recommended for TWA determinations of chlorine. When considering the amount of chlorine collected (as mass), any samples taken near the TWA PEL of 0.5 ppm will have slightly larger mass concentrations of chlorine than those tested for this evaluation. In addition,

during the evaluation no breakthrough was noted after 240 minutes of sampling and collection efficiency was excellent after 15 minutes of sampling. Both of these experiments were conducted at approximately 4 times the TWA PEL of 0.5 ppm.

Bibliography

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