Analytical Techniques in Occupational Health ... - ACS Publications

13

Downloaded by NORTH CAROLINA STATE UNIV on January 13, 2013 | http://pubs.acs.org Publication Date: April 22, 1980 | doi: 10.1021/bk-1980-0120.ch013

Sampling for Mercaptans by Absorber Tubes

M. W. NATHANS and A. JEONG L F E Corporation, Environmental Analysis Laboratories, 2030 Wright Avenue, Richmond, CA 94804 The common method applied to the measurement of mercaptan concentrations in workroom a i r is to collect the mercaptans in a bub bler or impinger with m e r c u r i c acetate in acetic acid and to analyze the solutions colorimetric ally (1). The disadvantages of sampling by means of impingers are well known, so it was desired to develop a method by which sampling could be accomplished by means of a d s o r ber tubes. Some attempts were made by SRI International who d e veloped and validated a method of collecting butyl mercaptan on s i l i c a gel(2), but who was unsuccessful for the lower-molecular weight m e r captans (3). In this paper, we report on an absorber tube method which we have validated for methyl mercaptan, but which is probably applicable to other mercaptans also. Since colorimetry was used for the analysis, the combined sampling and analysis method is not s p e cific for methyl mercaptan. However, work is currently underway to validate the method with a G C f i n i s h , which is expected to make the method specific for individual mercaptans. Experimental Following Akito s method for sampling f o r mercaptans i n ambient air by means of mercuric-acetate impregnated filter paper (4), we constructed absorber tubes with m e r c u r i c acetate-impregnated f i r e b r i c k . Having been unsuccessful to achieve reproducibility, we s u b sequently were successful with glasswool plugs wetted with the a b s o r ber solution. T

Construction of the T u b e s . Two plugs of glasswool, weighing a p proximately 0.25 g. each, are inserted into a pyrex glass tube, 12.5 c m long χ 4.8 m m diameter, as follows. The first plug is inserted such that one end is about 1 inch from the end of the tube. This plug is wetted with a 0.5 m l . solution prepared by dissolving 50 g. of m e r curic acetate, free of mercurous salts, in about 400 m l . of water, mixed with 25 m l . glacial acetic a c i d , and diluted to 1 l i t e r . The solution is added through the long end of the tube. The second plug is inserted 1 inch into the long end of the tube and also wetted with 0.5 m l . 0-8412-0539-6/80/47-120-231$05.00/0 © 1980 American Chemical Society In Analytical Techniques in Occupational Health Chemistry; Dollberg, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

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of the absorbing solution. The tubes are capped with parafilm or by any other suitable means until use. When in use, the direction of flow is such that the wetted ends of the plugs face the a i r flow. T h i s d i r e c tion is marked on the outer surface of the tubes. Measurement of Pressure D r o p . A i r was pumped through the tubes by means of a personal sampling pump at a flow rate of 300 m l . / minute, as measured by a rotameter between the pump and the tube. A " T " between the rotameter and the tube was connected to an openend mercury manometer. Alternatively, the pump could be adjusted so as to yield the same pressure drop for different tubes and to allow measurement of the flow rate at constant pressure drop. Generation and Sampling of Test Atmospheres. Methyl mercaptan was obtained f r o m Matheson Gas Products in a lecture bottle under 2 atm. pressure at 2 1 ° C and at a stated purity of 99.5%. Atmospheres with known concentrations of methyl mercaptan were generated and sampled in the apparatus shown in Figure 1. A 56-liter T e d l a r bag was filled with a i r metered by a calibrated dry-test meter. During f i l l i n g , a desired quantity of methyl mercaptan was injected by means of a gas syringe into the mixing chamber through a rubber septum. Adequate mixing in the bag was assured by kneading. The test atmosphere was sampled by drawing the gas at 0.3 1 / m i n ute through a t r a i n consisting of rotameter, the absorption tube, a midget impinger filled with 10 m l . of the wetting solution (see above), and an empty impinger by means of a personal sampling pump. L a t e r , a manifold was placed after the three-way stopcock so that three s a m ples could be withdrawn simultaneously. However, the capacity of the bag was such that given the detection limit and the desired concentra tions, no more than four samples could be withdrawn from a single bag filling. A n a l y s i s . The glasswool plugs are carefully pulled out of the glass tube and placed in individual 2 5 - m l . beakers. Fifteen m l . of the H g A c 2 / H A c solution (absorber solution, see above) are added and the contents of the beakers are swirled carefully. The liquid is t r a n s ferred to a 2 5 - m l . volumetric flask. The solution is carefully pressed out of the glasswool with a glass stirring r o d . The glasswool is washed twice with 2 m l . of the H g A c 2 / H A c solution which is added to the v o l umetric flask. F r o m this point, the analysis proceeds essentially by the method of Moore et a l . (1). One-and-one-half m l . of a solution prepared by dissolution of 0.25 g. N , Ν-dimethyl-p-phenyiene diamine dihydrochloride i n 50 m l . concentrated H C 1 , and one-half m l . of Reissner solution is added. The latter is prepared by dissolution of 67.6 g. F e C l 3 « 6 H 2 0 in distilled water, dilution to 500 m l . , addition of 72 m l . boiled concentrated HNO3, and final dilution to 1 l i t e r . The solutions in the flasks to be analyzed are diluted to the mark with d i s tilled or de ionized water, and mixed. The contents of the flasks are transferred to 4 0 - m l . centrifuge

In Analytical Techniques in Occupational Health Chemistry; Dollberg, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


NATHANS

AND

Figure 1.

jEONG

Sampling for

Mercaptans

Apparatus for preparation of gas mixtures and for sampling


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cones and centrif uged for 5 - 1 0 minutes to allow any glass fibers to settle. The absorbance of the supernate is read in a colorimeter at 500 nm between 30 and 45 minutes after the addition of the amine and Reissner solutions. Standards are prepared f r o m lead methyl mercaptide. A stock standard solution, equivalent to 500 Mg C ^ S H / m l . , consists of 156.6 mg P b (SCH3) i n 100 m l . of the H g A c 2 / H A c solution. The working standard solution, equivalent to 10Mg C I ^ S H / m l . , is prepared by a 50-fold dilution of the stock standard solution with H g A c / H A c . The calibration curve is obtained by applying the analytical procedure to aliquots of 0.5, 1.0, 2 . 0 , 3 . 0 , and 5.0 m l . The lower quantification limit of the method is about 0 . 4 m l . , o r 4Mg CH3SH. 2


2

Validation. The validation followed the Standards Completion P r o g r a m Statistical Protocol developed by Busch(5). The pooled coef ficient of variation of the total procedure, C V T which consists of the composite variations in sampling and analysis, 'desorption efficiency, and the pump e r r o r , is given by: CV

Γ —

= JJCV )

T

2

2

—

+ 0.1667 ( C V ^

2

2*71/2

+(0.05) J

where C V is the pooled coefficient of variation based on the date for the generated samples at all concentration levels, C V i is the pooled coefficient of variation of the analysis of spiked samples, and the n u m ber 0.05 represents the pump e r r o r . The coefficient of variation of a set of results generally, is defined as: 2

C V = (standard deviation/mean). In order to test the feasibility of pooling the coefficients of v a r i a tion, Bartlett s test for homogeneity of C V s was applied. The C V £ may be pooled at the 1% significance level for " n " sets of data, if χ < 2.91, where T

T

—

!

η 2

_

f ln(CV ) 2

2

-.fjf.NCV^.)

2

1

X 1 +

M^ÏÏ

f

i = 1

\

where C V is the pooled C V o f all generated samples, CV^ j is the C V of the samples of the i-th set, is the degrees of freedom associated with C V i (= number of data - 1), and 2

2 >

η i=l V In order to determine C V ^ , spikes at each of two levels were p r e pared and analyzed: 9. fyg and 29.4μ% C H S H . F o r the determination 3


13.

NATHANS

AND

JEONG

Sampling

for Mercaptans

235

of C V 2 i sets of six samples each were collected from a i r containing concentrations of CHgSH of 0.52 mg/m , 1.04 mg/m and 1.56 mg/m . 3

3


Results P r e s s u r e drop . The pressure drop across 24 freshly prepared tubes was determined at a flow rate of 300 ml/minute. The range was 25.0 - 44.0 mm Hg. The mean was 36. 7 ± 5.4 (1 σ ) mm Hg. One outlier was excluded. Conversely, the variation of the flow rate was determined at a constant pump setting, such that the flow rate of one tube used as a reference was 300 ml/minute. The pressure drop was 20.5 mm Hg, the range was 275 - 315 ml/minute. The mean was 292 ± 11 ( l g ) ml/minute. Thus, the coefficient of variation was 0.037. Three tubes prepared about 45 days e a r l i e r , capped and stored, were also tested. The pressure drops were 48.5, 47.0 and 60.0 mm Hg respectively. The flow rates at the pump setting which yielded 300 ml/minute through the reference tube were 260, 275 and 275 ml/min ute. These results show that unless special precautions are taken, the tubes should probably not be kept f o r more than 3 or 4 weeks. Breakthrough: The theoretical capacity of the front section of a tube is about 7.5 mg of CH3SH. Experimentally, no breakthrough was observed when 58 μg of CH3SH was absorbed in the front section from air containing CH3SH at about the 2S level (2 mg/m ) at a flow rate of 0.2 1/minute. In no test was CH3SH at about the 2S level (2mg/m ) at a flow rate of 0.2 l/minute. In no test was CH3SH found in the backup section. If it is assumed that each tube can be used to up to one-tenth of its theoretical capacity, one could sample for more than 30 hours at 0.3 1/minute at the IS level. Control over the sample size i s , therefore, not necessary oyer a very wide range. P r e c i s i o n and Accuracy: In order to determine the precision of the analysis, two sets of 15-ml. solutions of H g A c in HAc were spiked with CH3SH and analyzed: one set with 5 μ ΐ . (9. 8 μg) and one set with 15 μ 1. ( 2 9 . 5 μ g ) . The results are shown in Table 1. The r e sults of replicate determinations of CH3SH collected in absorption tubes are shown in Table 2. Recoveries were calculated f r o m the bag compositions, since it was found that sampling directly by means of impingers yielded results that were consistently about 10% lower than those obtained by sampling by means of the absorber tubes. Bartlett s test for homogeneity of variances at 0. 5, 1 and 1.5 times the OSHA standard showed that the variances may be pooled. Therefore, the coefficient of variation was calculated from the pooled variances: 3

2

T

CV-^

0.092

C V = 0.080 2

CV" = T

0.010

where C V i and CVo are the coefficients of variation of the analysis and of sampling ana analysis, respectively, and CV»p is the total coef ficient of variation of the method including the pump e r r o r . The average recovery was 94.7 ± 8.1%, the same as for the spiked samples.


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CHEMISTRY

1

Determination of P r e c i s i o n and A c c u r a c y of the Analysis


Spike:

5 μ 1 . CH SH(9.8Mg)

Spike:

3

15 μ ΐ . C H S H (29.4 Mg) 3

Solution

Recovered

Solution

Recovered

1 2

8.6 8.8 10.1

1 2

28.0 25.5 28.8 25.0 26.5 28.2

3

4

3

4 5 6

7.3

mean std. dev.

8.7

27.0

2Σΐ θν

°· 0.0092

1.15 1

χ

1.56

0.058

3

TABLE

2

Determination of P r e c i s i o n and A c c u r a c y of Sampling and Analysis Cone.

0.52

mg/m

Q 3

13.5 1.

Volume

Collected

3

1.04 m g / m

1.56 m g / m

13.5 1.

13.5 1.

Collected

Collected ^g)

^g)

1 2 3 4 5

7.8 6.3 6.8 7.8 7.6

mean std. dev.

cv

2

recovery CV =0. 080

7.3 0.7 0.091 104%

1 2 3 4 5 6

11.6 11.6 12.9 12.9 12.7 13.8 12.6 0.8 0.067 90%

1 2 3 4 5

2


16.6 18.0 20.7 18.8 20.3 18.9 1.5 0.082 90%

13.

NATHANS AND JEONG

Sampling

for

Mercaptans

TABLE 3


STORAGE PROPERTIES

CHgSH concentration: Flow rate:

1. 04 μ g / l 0.30

l/minute

Volume:

13.5 1

Total sampled :

14.0 μg

μ g C H S H Found 3

Day

Tube 1

Tube 2

Average

1

13.0

12.8

12.9

2

13.6

12.9

13.2

7

13.1

13.6

13.4

15

12.2

12.0

12.1

Mean: Std. CV

Dev.

12.9 0.6 0.044


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OCCUPATIONAL H E A L T H CHEMISTRY

Storage : Samples were collected and analyzed in duplicate after 1, 2, 7, and 14 days storage at room temperature. The concentra tions were about 1 m g / m . The results are presented in Table 3. They show that samples may be stored for at least 7 days. Tubes can probably be stored for significantly longer periods if they are tightly capped. T

3

Discussion The method was validated over the range of 0.54 - 1.62 m g / m at an atmospheric pressure of 760 m m Hg and a temperature of 2 2 ° C . The probable useful range depends p r i m a r i l y on the sample s i z e . F o r a 10-minute sample taken at 300 m l . / m i n u t e , the lower limit of the useful range is probably 0.15 m g / m , but this limit can be extended downward by longer sampling t i m e s . At the same flow rate and a sampling time of 60 minutes, the upper limit of the probable useful range is greater than 2 m g / m . Since the absorbent is an aqueous solution, high humidity is not expected to interfere with the trapping of the compound. However, when condensation occurs such as to increase the volume of the a b s o r bent, the air flow may c a r r y some of the absorbent out of the plug, possibly affecting the recovery of the absorbent adversely. Interferences are those stated by Moore et a l . (4) H2S can produce both turbidity and color with the analytical method. The turbidity may be removed by filtration before addition of the color developing r e a gents. The color interference is insignificant unless appreciably more than 100 μg of H2S are collected. SO2 up to 10 ppm does not interfere. NOtj above6 ppm produces high results. At 14ppm, the NO2 i n t e r ference is approximately 20%. Dimethyl disulfide interferes on a mole-for-mole basis. (6) The method is not specific for individual mercaptans. Specificity as well as a lower detection limit can be achieved by analysis by gas chromatography after reconstitution of the mercaptans and collection in an organic solvent. (4)


3

3

3

Acknowledgment The major part of this work was supported by a contract with Shell Development C o . , of Houston, T e x a s , with M r . D . Morman as Project Monitor, whose suggestions are greatfully acknowledged. We also acknowledge the help and suggestion of M r . H . Y . Gee, Gas L a b o r atory Supervisor, and M r . J . C o r s o , Senior Laboratory Technician, both of L F E Corporation.

Abstract In current industrial hygiene practice, sampling for mercaptans is done by means of an impinger containing 5% HgAc2 in HAc. In or der to circumvent the problems associated with impingers, absorber tubes containing two sections of glasswool wetted with this solution


13.

Sampling

N A T H A N S AND JEONG

for

Mercaptans

239


were constructed and tested. At a flow rate of 300 ml/minute, the pressure drop was 36.7 mm + 14.7%. At contstant pressure drop of 20.5 mm Hg the flow rate was 292 ml./minute ± 3.7%. The coefficient of variation for combined sampling and analysis was determined with standard atmospheres of CH3SH containing 0.5, 1.0 and 1.5 mg/m3 and with a general colorimetric procedure for mercaptans for the analysis. Its value was 0.0010. The average recovery was 95% or better. Literature Cited 1.

Moore, H . , Helwig, H. L., and Graul, R. J., Amer. Ind. Hyg. Assoc. J . (1960) 21, 466.

2.

Taylor, D. G. (Manual Coordinator), NIOSH Manual of Analy tical Methods", 2nd ed. DHEW (NIOSH) Publication No, 77-157C (Method S-350).

3.

Private communication.

4.

Okita, T., Atm. Env. (1970) 4, 93.

5.

Busch, Κ. A . , in Taylor, D. G . , Kupel, R. Ε., and Bryant, J. Μ., "Documentation of the NIOSH Validation Tests", DHEW (NIOSH) Publication No. 77-185 (1977).

6.

California State Department of Public Health, Method No. CAL/ OSHA L-128.

RECEIVED October 23,

1979.


Analytical Techniques in Occupational Health ... - ACS Publications

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