This Backup Report was revised December, 1989

Introduction

The procedure for collection and analysis of sulfur dioxide (SO₂) in air is described in OSHA method no. ID-104 (10.1.). Briefly, SO₂ is collected in a midget fritted glass bubbler (MFGB) containing 0.3 N hydrogen peroxide (H₂O₂). The amount of SO₂ in the H₂O₂ solution is determined by ion chromatography (IC) as sulfate (SO₄^{2 ¯}).

1. Experimental Protocol

This method has been evaluated using 60-L, 60-min samples having concentrations ranging from 2.5 to 10 ppm SO₂ (640 mmHg and 24 °C). At the time this method was evaluated (1981), the OSHA Permissible Exposure Limit (PEL) was 5 ppm as a Time Weighted Average (TWA). The evaluation consisted of the following experiments:

1. Analysis of a total of 18 samples (6 samples at each of three test levels) prepared by spiking known amounts of sodium sulfate in 0.3 N H₂O₂.
2. Analysis of a total of 18 samples (6 samples at each of the three test levels) collected from dynamically generated test atmospheres.
3. Determination of the collection efficiency and breakthrough of the H₂O₂ collection solution in MFGBs.
4. Testing of the storage stability of six collected samples.
5. Determination of the detection limits of the method.
6. Comparison of methods.
7. Conclusions - including a discussion of recent PEL changes for SO₂.

All samples were analyzed using a model 10 ion chromatograph (Dionex, Sunnyvale CA) with a 3 × 500-mm anion separator and a 6 × 250-mm suppressor column.

2. Analysis

Spiked samples were prepared and analyzed to determine analytical precision and accuracy.

Procedure:   Samples were prepared by adding known amounts of a stock sodium sulfate solution to 25 ml of 0.3 N H₂O₂. The concentrations evaluated were 362, 723, and 1,425 µg as SO₂. The step-by-step preparation is listed below.

2.1. Preparation of SO₄^{2 ¯} Stock Solution

A 12,003 µg/mL SO₄^{2 ¯} stock solution was prepared by dissolving and diluting 1.7748 g of anhydrous sodium sulfate (Na₂SO₄) to 100 mL with a 0.3 N H₂O₂ solution. On the day preceding the preparation of this stock solution, the Na₂SO₄ had been heated for approximately 2 h at 110 °C in a drying oven and then allowed to cool overnight in a drying desiccator.

2.2. Preparation of Known Spiked Samples

Three sets of spiked samples were prepared by injecting 45.2, 90.3, and 178 µl, respectively, of the stock solution into 25-mL volumetric flasks. These solutions were then diluted to volume with 0.3 N H₂O₂ solution. The spikes were injected using a calibrated 200-µL adjustable micropipette. Each set consisted of six samples and a sample blank.

2.3. Each sample was analyzed twice by IC.

Results:   The results of the analysis are presented in Table 1. Analytical precision and accuracy was excellent at the concentrations tested.

3. Sampling and Analysis

Samples were collected in 0.3 N H₂O₂ from dynamically generated test atmospheres and then analyzed to determine the overall precision and accuracy of the method.

Procedure:   Test atmospheres of SO₂ gas were dynamically generated at approximately 2.5, 4.8, and 10.0 ppm SO₂ (640 mmHg, 24 °C) by diluting gas from a certified gas cylinder (Matheson, East Rutherford, NJ) which contained 505 ppm SO₂ in nitrogen. The SO₂ was diluted with purified, humid compressed air. A diagram of the generation system is presented in Figure 1.

3.1. The certified SO₂ gas was calibrated against a primary standard of SO₂ (National Bureau of Standards, Gaithersburg, MD) to verify the concentration. The certified gas was determined to be ±2% of stated concentration and the stated value was used in all calculations.

3.2. The humid air and certified SO₂ gas were mixed together in a Teflon mixing tee (7/16 inch i.d.) prior to entering the sampling chamber. The sampling chamber was a twelve port (7/16 inch i.d.) Teflon sampling manifold. There were six sampling ports located on opposite sides of the sampling manifold. The sampling ports were spaced approximately 3 1/4 inch apart.

3.3. Additional connecting tubing and fittings were made of stainless steel or Teflon to avoid any contamination problems.

3.4. The flow rate of the test atmosphere into the sampling manifold was approximately 2.2 times the total sampling flow rate.

3.5. The flow rate of the certified SO₂ gas was monitored and controlled by a model FC 260 mass flow controller (Tylan Corp., Carson, CA). The flow rate, temperature, and relative humidity of the diluent air was controlled by a model HCS-201 Mass Flow, Temperature, and Humidity Control System (Miller-Nelson Research Inc., Monterey, CA).

3.6. The flow rates of the certified SO₂ gas and the air were measured at 2.5 and 4.8 ppm test levels just prior to sampling. At 10.0 ppm, the flow rates were measured just prior to and just after sampling. The certified SO₂ gas flow rate was measured by a model 823-1 Electronic Bubble flowmeter (Mast Development Co., Davenport, IA). The flow rate of the humid air was measured by a dry test meter which had been calibrated against a primary standard.

3.7. A calibrated Sulfur Dioxide Analyzer manufactured by Interscan Co. (Chatsworth, CA) was used to continuously monitor the SO₂ concentration of the test atmosphere in the sampling manifold during the 10.0 ppm test.

3.8. Test atmospheres at 2.5 and 4.8 ppm were generated at 24 °C and 75% RH. The test atmosphere at 10.0 ppm was generated at 24 °C and 50% RH.

3.9. Six samples were collected simultaneously for 60 min (with one exception; one was collected for 90 min) at each of the three test levels. A flow rate of approximately 1 L/min was used. These samples were collected in MFGBs containing 15 mL of 0.3 N H₂O₂ using calibrated Du Pont model P4000 constant flow sampling pumps (E.I. Du Pont De Nemours and Co., Wilmington, DE). The MFGBs were connected to the sampling ports of the manifold with Teflon tubing and fittings, and to the sampling pumps with short pieces of Tygon tubing. Prefilters were not used during sampling.

3.10. After sampling, the samples were transferred to 25-mL volumetric flasks. The MFGB base and stem were rinsed with 4 to 8 mL of unused 0.3 N H₂O₂ and the rinsings were added to the flasks. Samples were then diluted to volume with 0.3 N H₂O₂ and analyzed twice by IC.

Results:   The results of sampling and analysis are shown in Table 2. The precision and accuracy data based on the NIOSH statistical protocol (10.2.) are presented in Tables 1 and 2. The pooled coefficients of variation for spiked (CV₁) and generated (CV₂) samples and the overall CV_T are:

CV1   =   0.018,

CV2   =   0.010,

CVT   =   0.012

The bias of the generated samples over all levels was -0.046. Any variation from zero bias was most likely due to difficulties in generating the atmosphere containing the SO₂ at a given concentration rather than true bias in the method. Overall error was ±7%.

4. Collection Efficiency (CE) and Breakthrough

Procedure:   Samples were taken in series from the generation system to determine the sampling efficiency and if breakthrough could occur. The generation system and conditions described in Section 3 were used. Additional details are listed below.

4.1. Two MFGBs, each containing 15 mL of collection solution, were connected in series with a short piece of Tygon tubing. A sampling pump was then attached to the second MFGB.

4.2. Five samples in series were prepared and collected from the generation system at a concentration of approximately 9 ppm. Sampling times ranged from 240 to 270 min and a flow rate of approximately 1 L/min was used.

4.3. After sampling, the samples were transferred to glass 20-mL scintillation vials. The bubbler base and stem were rinsed and rinsings were added to the vials.

4.4. The amount of SO₂ collected in each MFGB was then measured by IC.

Results:   The results are reported in Table 3. The CE was calculated by dividing the amount of SO₂ collected in the first MFGB by the total amount of SO₂ collected in the first and second MFGB. Breakthrough was calculated by dividing the amount of SO₂ collected in the second MFGB by the total amount of SO₂ collected in the first and second MPGBs. The CE was 100%. No breakthrough into the second MFGB occurred at this concentration, flow rate, or sampling times.

An additional experiment was conducted to examine CE and breakthrough results from SO₂ collection with midget impingers versus MFGBs. Both devices contained 10 to 15 mL of 0.3 N H₂O₂. Results indicated a significant difference did not exist between the two sampling devices for CE or breakthrough.

5. Storage Stability

Procedure:   A study was conducted to assess the storage stability of air samples taken for SO₂ in 0.3 N H₂O₂. These samples were stored in 25-mL volumetric flasks.

5.1. Six samples were collected and analyzed for the storage test. All samples were exposed to about 4.8 ppm SO₂ using the generation system [Note: The same six samples collected in the sampling and analysis experiment (Section 3) were used. The results for these samples represented day 1 of the stability data].

5.2. After collection the samples were transferred to volumetric flasks. These flasks were then closed tightly and stored on a lab bench at normal laboratory temperatures. An aliquot from each of the six samples was analyzed after 1, 24, and 31 days of storage.

Results:   Results are shown in Table 4 and indicate that samples may be stored under normal laboratory conditions for at least 31 days.

6. Detection Limits

Procedure:   Six standards at a concentration of 0.013 µg and six reagent blanks were prepared. The Rank Sum Test (10.3.) was used to determine the qualitative detection limit from the analysis of these samples. The test is a non-parametric or a distribution-free test.

For the quantitative detection limit determination, low concentration standards ranging from 0.033 to 0.33 µg (as SO₂) were prepared by serial dilution of the SO₄^{2 ¯} stock solution. Six standards were prepared at each concentration. Six reagent blanks were also prepared. The standards and blanks were analyzed and peak heights were measured. The quantitative detection limit was determined by using the coefficient of variation (CV) from the results of each set of six standards as a guideline. If the CV for a set of standards was greater than 0.10, then the CV for the next larger concentration was considered. The quantitative detection limit was found when the next higher concentration set of standards had a CV less than 0.10.

Results:   The qualitative detection limit results are shown in Table 5. As shown, the limit was 0.013 µg of SO₂ per 200-µL injection (99.9% confidence level) or 0.65 µg of SO₂ in a 10-mL sample volume.

The analysis of standards (0.033 to 0.33 µg as SO₂¯) and reagent blanks gave a quantitative detection limit of 0.033 µg of SO₂ per 200-µL injection or 1.7 µg of SO₂ in a 10-mL sample volume. The coefficient of variation of replicate determinations of standards at this level was less than 0.10.

7. Method Comparison

The NIOSH barium perchlorate titration (BPT) procedure (10.4.) for analyzing SO₂ was chosen as the reference analytical method to compare to the IC method. The same three sets of spiked samples which were prepared and analyzed by IC in the analysis study (Section 2) were used for the comparison. The NIOSH analytical procedure (10.4.) was followed with slight modifications.

Procedure:   The reagents and procedure used for the BPT method are listed below.

7.1. Reagents

7.1.1. 2-propanol.

7.1.2. Barium perchlorate solution, 0.00521 M (obtained from Hach Chemical Co., Ames, IA).

7.1.3. Thorin indicator solution (0.15% in deionized water).

7.1.4. Perchloric Acid, 1.8%: A 25 mL aliquot of reagent grade perchloric acid (70-72%) was diluted to 1-L with deionized water.

7.2. Analytical Procedure

7.2.1. An aliquot (4.6 to 9.5 ml) of each spiked sample prepared in Section 2 was transferred to an individual 250-mL Erlenmeyer flask and then 100 mL of 2-propanol was added.

7.2.2. The pH of each sample was adjusted to be within the range of 3.2 to 3.5 using 1.8% perchloric acid.

7.2.3. Eight drops of the Thorin indicator were added to each flask and then titrated with 0.00521 M barium perchlorate solution to a pink colored end point.

7.2.4. The amount of SO₂ found in each sample was calculated from the volume (mL) of barium perchlorate solution needed to titrate the sample. Blank corrections were made.

7.3. Results:   The comparison data of the BPT and IC methods are shown in Table 6. This data was statistically analyzed using concepts mentioned in references 10.5. and 10.6. to determine if any significant difference existed in the results of the two methods. A linear regression, plotting the results of the IC versus the BPT method, was calculated and the results are shown:

1. The calculated value of the slope (b) of the linear regression is 1.018 with a standard deviation (S_b) of 0.0102. The range of the slope (95% confidence) is:

   b   ±   (tc   ×   Sb)

   =   1.018   ± 2.11   ×   0.0102   =   0.997  to  1.040

   Where:
       t_c   =   t value at the 95% confidence level

   The calculated value for the slope is not significantly different from an ideal value of 1. On the average, results from either method were similar.

2. The intercept (a) is calculated as -0.01919 mg with a standard deviation (S_a) of 0.00956 mg. The range of the intercept (95% confidence) is:

   a   ±   (tc   ×   Sa)

   =   -0.0192   ±   2.11   ×   0.00956 mg   =   -0.0394  to  0.0010 mg  =  -39.4  to  1.0 µg

   Where:
       t_c   =   t value at the 95% confidence level

   The calculated value for the intercept is not significantly different from an ideal value of 0.

3. The correlation coefficient between the results of the two methods is 0.9992.
4. For an SO₂ value of 831 µg by the barium perchlorate titration method, the IC method will, on the average, give a result of 826 µg.
5. These results indicate excellent agreement between the two analytical methods.

8. Interferences

Particulate sulfate and sulfuric acid will provide a positive interference with the analytical determination of SO₂. This suggests the need for a prefilter when sampling in the presence of these substances. A suitable candidate for the prefilter was considered to be a polystyrene cassette containing a mixed-cellulose ester filter and a cellulose backup pad. Sulfur dioxide was not adsorbed to any significant extent onto this prefilter at low and high RH when collected at a flow rate of 1.5 L/min (10.7.). Another study (10.8.) indicated the cellulose backup pad, the prefilter, and particulate matter collected on the prefilter will not hinder the collection of SO₂.

A later experiment (10.9.) conducted with a lower sampling rate did indicate a slight decrease in SO₂ recovery when using a prefilter in a high humidity atmosphere. Some of the loss appeared to be due to the backup pad because replacement of the pad with a stainless steel (SS) support ring gave higher results. However, the recoveries were still 8 to 10% low when using the SS ring, perhaps indicating a reaction was still occurring between the cellulose filter/cassette and the SO₂. A flow rate of 0.2 L/min was used which may account for the slight loss found in this experiment and not in references 10.7. and 10.8. There is an increase in sample residence time in the cassette and a decrease in total mass at this lower flow rate. The reactive surfaces of the prefilter assembly would not be passivated as rapidly at a lower flow rate if a reaction is occurring.

Although the extent of reactivity of SO₂ with cellulose-type filters appears minimal at a 1.5 L/min flow rate, it is recommended to sample, when necessary, with a modified prefilter when using this method. In areas suspected to contain particulate sulfate or sulfuric acid, sampling should be performed with a prefilter consisting of a cassette containing a glass fiber filter and a circular support ring of cellulose or stainless steel. The support ring should hold the filter in place in the cassette and not interface with the flow of air into the MFGB.

9. Conclusions

This method has demonstrated sufficient precision and accuracy for determining SO₂ at concentrations near the Transitional PEL of 5 ppm. No breakthrough was noted at a sampling rate of 1 L/min and storage stability was adequate to meet normal laboratory needs.

Results in Tables 1 and 2 indicate the sampling and analytical method did not display an increase in imprecision when determining low concentrations (about 2.5 ppm). Therefore, this method should be capable of determining compliance with the Final Rule PEL of 2 ppm SO₂ (TWA) using similar sampling conditions. Sampling times for TWA determinations can be longer than previously recommended in reference 10.10., especially when sampling at lower concentrations. A sampling time of 60 min was recommended in the past.

For the Final Rule limit of 5 ppm (STEL), a 15 min sample should be taken at a flow rate of 1 L/min.

10. References

10.1. Occupational Safety and Health Administration Technical Center: Sulfur Dioxide in Workplace Atmospheres (Bubbler) by T. Wilczek and E. Zimowski (USDOL/OSHA-SLTC Method No. ID-104). Salt Lake City, UT. Revised 1989.

10.2. National Institute for Occupational Safety and Health: Documentation of the NIOSH Validation Tests by D. Taylor, R. Kupel and J. Bryant (DHEW/NIOSH Pub. No. 77-185). Cincinnati, OH: National Institute for Occupational Safety and Health, 1977.

10.3. Dixon, W.J. and F.J. Massey, Jr.: Introduction to Statistical Analysis. 2nd ed. New York: McGraw-Hill Book Co., Inc., 1957. pp. 289-292, 445-449.

10.4. National Institute for Occupational Safety and Health: NIOSH Manual of Analytical Methods. 2nd. ed., Vol. 4 (Method. No. S308) (DHEW/NIOSH Pub. No. 78-175). Cincinnati, OH: National Institute for Occupational Safety and Health, 1978.

10.5. Westgard, J.O., D.J. de Vos, M.R. Hunt, E.F. Quam, C.C. Garber and R.N. Carey: Concepts and Practices in the Evaluation of Clinical Chemistry Methods: III. Statistics. American Journal of Medical Technology 44: 552-571 (1978).

10.6. Westgard, J.O., D.J. de Vos, M.R. Hunt, E.F. Quam, C.C. Garber and R.N. Carey: Concepts and Practices in the Evaluation of Clinical Chemistry Methods: IV. Decisions of Acceptability. American Journal of Medical Technology 44: 727-742 (1978).

10.7. National Institute for Occupational Safety and Health: Backup Data Report No. S308 for Sulfur Dioxide. Cincinnati, OH: National Institute for Occupational Safety and Health, 1977.

10.8. Occupational Safety and Health Administration Health Response Team: Sulfur Dioxide by Edward Zimowski. Salt Lake City, UT. 1981 (unpublished).

10.9. Occupational Safety and Health Administration Analytical Laboratory: OSHA Analytical Methods Manual (USDOL/OSHA-SLCAL Method No. ID-107, Backup Data Report) Cincinnati, OH: American Conference of Governmental Industrial Hygienists (Pub. No. ISBN: 0-936712-66-X), 1985.

10.10. Occupational Safety and Health Administration Analytical Laboratory: OSHA Analytical Methods Manual (USDOL/OSHA-SLCAL Method No. ID-104). Cincinnati, OH: American Conference of Governmental Industrial Hygienists (Pub. No. ISBN: 0-936712-66-X), 1985.

Table 1

Analysis - Sulfur Dioxide
- - - - 0.5 × PEL* - - - -			- - - - - 1 × PEL* - - - -			- - - - - 2 × PEL* - - - -
mg Taken	mg Found	AMR	mg Taken	mg Found	AMR	mg Taken	mg Found	AMR
0.362 0.362 0.362 0.362 0.362 0.362	0.351 0.355 0.353 0.355 0.364 0.366	0.970 0.981 0.975 0.981 1.006 1.011	0.723 0.723 0.723 0.723 0.723 0.723	0.698 0.717 0.679 0.685 0.721 0.710	0.965 0.992 0.939 0.947 0.997 0.982	1.425 1.425 1.425 1.425 1.425 1.425	1.430 1.428 1.396 1.425 1.410 1.432	1.004 1.002 0.980 1.000 0.989 1.005
N Mean Std Dev CV1		6        0.987 0.017 0.017			6        0.970 0.024 0.025			6        0.997 0.010 0.010
*   Transitional PEL of 5 ppm SO2 was used. A 60-L air volume was assumed. AMR   =   Analytical Method Recovery CV1(Pooled)   =   0.018

Table 2

Sampling and Analysis - Sulfur Dioxide
	- - - - - - - - - - Found - - - - - - - - - -				Taken	Recovery
Sample No.	µg	L   Air	mg/m3	ppm	ppm	%
0.5 × PEL*
1 2 3 4 5 6	300.3 308.0 481.5 301.7 281.7 312.3	59.3 59.2 91.9 57.7 55.2 60.5	5.064 5.203 5.239 5.229 5.103 5.162	2.293 2.356 2.372 2.368 2.311 2.337	2.468 2.468 2.468 2.468 2.468 2.468	92.9 95.5 96.1 95.9 93.6 94.7
N Mean Std Dev CV2			6        5.167   0.0707   0.0137			94.8     1.30
1 × PEL*
1 2 3 4 5 6	608.4 607.9 601.3 585.7 564.5 619.7	59.7 59.5 59.0 58.6 56.3 61.1	10.191 10.217 10.192   9.995 10.027 10.142	4.635 4.647 4.635 4.546 4.560 4.613	4.824 4.824 4.824 4.824 4.824 4.824	96.1 96.3 96.1 94.2 94.5 95.6
N Mean Std Dev CV2			6       10.127    0.0939    0.0093			95.5       0.900
2 × PEL*
1 2 3 4 5 6	1268.7 1256.9 1238.0 1217.6 1173.4 1300.6	59.3 59.1 58.9 57.7 55.7 61.2	21.395 21.267 21.019 21.102 21.066 21.252	9.660 9.602 9.490 9.528 9.511 9.595	9.994 9.994 9.994 9.994 9.994 9.994	96.7 96.1 95.0 95.3 95.2 96.0
N Mean Std Dev CV2			6       21.183    0.1440    0.0068			95.7       0.655
*   Transitional PEL of 5 ppm SO2 was used.
CV1(Pooled)   =   0.018		CV2(Pooled)   =   0.01			CVT(Pooled)   =   0.012
Average Recovery   =   95.4%

Table 3

Collection Efficiency (CE) and Breakthrough
Sample No.	Time (min)	1st bubbler ppm found	2nd bubbler ppm found	CE (%)	Breakthrough (%)
1 2 3 4 5	270 270 270 240 240	8.87 8.78 8.69 8.73 8.64	ND ND ND ND ND ND	100.0 100.0 100.0 100.0 100.0	0.0 0.0 0.0 0.0 0.0
			Average	100.0%	0.0%
Taken concentration of 9.38 ppm SO2 (641 mmHg, 25 °C)
ND   =	None Detected, detection limit   =   56.8 µg SO2. A 10 L injection volume and an average sample solution volume of 16.7 mL were used.

Table 4

Stability test - Sulfur Dioxide
	- - - - - - - - - - - Found - - - - - - - - - - -				Taken	Recovery
Sample No.	µg	L   Air	mg/m3	ppm	ppm	%
1 Day
1 2 3 4 5 6	608.4 607.9 601.3 585.7 564.5 619.7	59.7 59.5 59.0 58.6 56.3 61.1	10.191 10.217 10.192   9.995 10.027 10.142	4.635 4.647 4.635 4.546 4.560 4.613	4.824 4.824 4.824 4.824 4.824 4.824	96.1 96.3 96.1 94.2 94.5 95.6
N Mean Std Dev CV			6      10.127     0.0939     0.0093			6   95.5       0.900
24 Days
1 2 3 4 5 6	586.9 601.2 595.9 593.1 567.2 631.8	59.7 59.5 59.0 58.6 56.3 61.1	9.831 10.104 10.100 10.121 10.075 10.340	4.471 4.595 4.593 4.603 4.582 4.703	4.824 4.824 4.824 4.824 4.824 4.824	92.7 95.2 95.2 95.4 95.0 97.5
N Mean Std Dev CV			6      10.095     0.1620     0.0160			6   95.2    1.52
31 Days
1 2 3 4 5 6	596.7 597.7 593.0 583.5 556.3 621.4	59.7 59.5 59.0 58.6 56.3 61.1	9.995 10.045 10.051   9.957   9.881 10.170	4.546 4.568 4.571 4.528 4.494 4.625	4.824 4.824 4.824 4.824 4.824 4.824	94.2 94.7 94.7 93.9 93.1 95.9
N Mean Std Dev CV			6      10.017     0.0979     0.0098			6   94.4       0.939

Table 5

Qualitative Detection Limit - Sulfur Dioxide Rank Sum Test (Nsample = Nblank = 6)
Rank	Sample	Peak Height (mm)
1 2 3 4 5 6 7 8 9   10   11   12	RB1 RB1 RB1 RB1 RB1 RB1 Std Std Std Std Std Std	1.2 1.2 1.2 1.7 1.8 1.9 2.6 2.8 3.4 3.7 4.0 4.2
RB1   =   Reagent Blank Std     =   Standard, 0.013 µg SO2
Rank sum of blank values   =     21 Confidence Level               =   99.9% A significant difference exists between the blank and standard signals. Therefore, Detection Limit   =   0.013 µg SO2 per injection Injection volume of both reagent blank and standard was 200 µL