1. Introduction
The purpose of this method is to make a correction for zinc stearate in the
coal tar pitch volatiles analysis as it relates to the carbon and graphite
products industry (SIC code 3624). There are concerns that OSHA is reporting
coal tar pitch volatiles (CTPV) values that are too high due to zinc stearate
and sulfur interferences and if the values for the sulfur and zinc stearate
are subtracted, the CTPV values would be below the standard of 0.2 mg/m³.
This method would be used based on the following rationale: (1) the coal
tar pitch volatiles analysis results must be greater than 0.2 mg/m³
and (2) the benzene soluble fraction must be positive for PAHs and be
confirmed by mass spectrometry. Then zinc stearate and/or sulfur, as
requested, would be analyzed to identify potential false high CTPV analytical
results.
2. Sampling
2.1 Equipment
2.1.1 Calibrated personal sampling pumps capable of sampling within
±5% of the recommended flow rate of 2.0 L/min are used.
2.1.2 A two piece cassette containing a glass fiber filter is the
sampling device.
2.1.3 Other equipment relevant to the initial sample collection is
shown in the method for coal tar pitch volatiles, OSHA Method 58, Coal
Tar Pitch Volatiles (Ref. 1).
2.2 Reagents
No sampling reagents are required. CTPV uses a glass fiber filter.
2.3 Sampling technique
Sampling information is provided as per OSHA Method 58, Coal Tar Pitch
Volatiles (Ref.1) for the air samples.
2.4 Extraction efficiencies
The extraction efficiency of zinc stearate in benzene appears to be
based on the solubility. The solubility is approximately 0.3% for the zinc
stearate. While this is a low solubility for zinc stearate, there may
still be a significant zinc stearate contribution to the coal tar pitch
volatiles mass.
3. Analysis
3.1 Safety Precautions
3.1.1 Observe laboratory safety regulations and practices.
3.1.2 Review any MSDSs provided with reagents and samples.
3.1.3 Review both organic and inorganic methods (OSHA Method 58
and ID-121), particularly that information relating to handling
carcinogenic materials.
3.2 Equipment
3.2.1 Atomic absorption spectrophotometer (Perkin-Elmer Model 5000 or Model
603).
3.2.2 Hotplates for inorganic ashing.
3.2.3 Glassware assortment including conical beakers, volumetric
flasks, 10-mL glass syringes with 5-µm filters.
3.2.4 Vacuum oven
3.2.5 Laboratory hood with sufficient face velocity to
accommodate working with carcinogens (approximately 125 lfpm).
3.3 Reagents
3.3.1 Benzene
3.3.2 Nitric Acid
3.4 Working Standards
See Ref. 5.2.
3.5 Sample Preparation (note the two part prep - organic and inorganic)
3.5.1 Air samples for the zinc analysis are prepared by obtaining
a known volume (approximately 0.25 to 1.0 mL) of the benzene soluble
sample material left over from the CTPV analysis (OSHA Method 58)
and placing that material into a 125 mL conical beaker for
evaporation.
3.5.2 The benzene is evaporated from the above air sample
aliquot(s) by direct nitrogen purge. This is accomplished by passing
a stream of nitrogen through an aspirator and into the Philips
beaker. Note that extreme care must be taken to assure that all
the benzene is evaporated before digesting the samples with nitric
acid !
3.5.3 The residue from the above procedure is taken to a hotplate
where the samples are digested using the inorganic ashing method for
zinc, OSHA ID-121 (Ref. 2).
3.5.4 The digested samples are diluted to volume with deionized
water. It is recommended that 10 ml volume be used for air samples.
Dilutions are prepared with 4% HNO3.
3.6 Analysis
3.6.1 Analyze samples, standards, and blanks according to the
general metals procedure for zinc, OSHA ID-121 (Ref. 2).
3.6.2 Set up one of the atomic absorption spectrophotometers for
flame analysis of zinc.
3.6.3 Typical operating parameters are listed below:
Model PE 5000 |
Element analyzed: |
Zinc |
Flame: |
Air-acetylene |
Lamp current: |
15 mA |
Oxidant Flow: |
40 |
Burner Height: |
7 |
Fuel Flow: |
20 |
Burner depth: |
7 |
Response: |
ABS |
Slit: |
H, 0.7 |
Integration Time: |
3 s |
3.7 Calculations
3.7.1 After the analysis is completed, retrieve the absorbances
or concentrations. Obtain hard copies of raw data from a printer.
3.7.2 Prepare a concentration-response curve by plotting the
absorbance or expanded scale response versus the concentration of
the zinc standards in micrograms per milliliter.
3.7.3 Determine the total zinc concentration in the samples and
blanks. Make a blank correction if necessary by subtracting the
total micrograms for the whole blank sample from the total
micrograms for the entire volume for each field sample.
3.7.4 Calculate the Zn stearate concentration from the zinc
concentration using the gravimetric factor (GF) of 9.671. Then apply
that concentration of zinc stearate to the air samples, in order to
correct the CTPV analyses using:
As = (µg/mL zinc)s(V1)s(DF)(GF)
Ab = (µg/mL zinc)b(V1)b(DF)(GF)
A = As - Ab
Then calculate the air concentration of zinc stearate for each
air sample using the following equation:
where:
As is the total micrograms of zinc stearate in the sample.
Ab is the total micrograms of zinc stearate in the blank.
A is the µg zinc stearate after blank correction.
GF is the gravimetric factor for zinc stearate.
AV is the air volume of the sample in cubic meters.
V1 is the inorganic sample dilution volume for either sample
or blank, in mL.
DF is the dilution factor that converts the sample aliquot that
was used to the original sample volume such that:
DF = |
original benzene volume, typically 3-5 mL
aliquot volume, typically 0.25-1.0 mg/m³
|
At this point, the industrial hygienist will subtract any zinc
stearate result from the CTPV result, assuming the latter was
greater than 0.2 mg/m³.
4. Backup Data
4.1 Solubility of Zinc Stearate
A solubility study of zinc stearate in benzene focused on conditions
recommended in the General Methods Procedure for metals. It was soon
apparent that zinc stearate was not very soluble in benzene.
4.1.1 Procedure: To test the solubility of zinc stearate in
benzene, zinc stearate bulk reagent (ICN Pharmaceuticals, INC., Life
Sciences Group, Plainview, New York, no lot number was listed,
approximately 100% zinc stearate) was spiked directly into a 10 mL
volumetric flask. Six samples were prepared by adding known amounts
of zinc stearate and diluting to volume with benzene. At first,
approximately 20 mg aliquots were added to 4 mL and 10 mL, but the
resulting solutions appeared very cloudy, as if very little material
actually went into solution or as if an emulsion had formed. For
this reason the smaller aliquot weights were used. At least under
these lower weight conditions, the resulting solution/suspension
would filter through the 5 µm syringe filters so a benzene soluble
fraction could be obtained. At the higher weight aliquots, the
suspensions filtered with difficulty.
Another solubility study was done by weighing approximately 20 mg
of zinc stearate into 200 mL benzene. This solution was cloudy, so
the whole volume was filtered into another 200 mL volumetric flask.
Then, 20 mL aliquots were transferred to conical beakers for drying.
It is these results that are contained herein.
4.1.2 Results: Results of the zinc and zinc stearate solubility
study are shown below. As shown, zinc stearate appears to be only
slightly soluble in benzene (approximately 0.3%).
Aliquot |
% Zinc
Recovered |
% Zinc Stearate
Recovered |
|
|
|
ZS1
|
0.0376
|
0.3638
|
ZS2
|
0.0337
|
0.3257
|
ZS3
|
0.0360
|
0.3485
|
ZS4
|
0.0423
|
0.4096
|
ZS5*
|
0.0117
|
0.1132
|
ZS6
|
0.0353
|
0.3409
|
N
|
5
|
5
|
Mean Recovery
|
0.03698
|
0.3577
|
SD
|
0.00329
|
0.0321
|
CV
|
0.0889
|
0.0897
|
|
|
|
*ZS5 is considered to be an outlier and is not used in
these calculations.
|
4.2 Detection Limit
The analytical detection limit for zinc is 0.01 µg/mL. This is the
same as found in OSHA method ID-121 when the samples are ashed in nitric
acid and diluted to volume so that the final matrix concentration is 4%
HNO3.
4.3 Zinc Oxide/Zinc Chloride Solubility Study
Procedure: Several aliquots of zinc oxide and zinc chloride were
prepared in benzene to establish the solubility of these compounds.
Results: The results of the solubility studies are shown below.
Date |
Aliquot Number |
Spiked wt. of
Zn material, mg |
% Recovery as
ZnO or ZnCl2 |
|
|
|
|
7/25/95
|
ZNO
|
5.2
|
0.05
|
8/18/95
|
ZNO1
|
5.59
|
ND
|
8/18/95
|
ZNO2
|
5.59
|
ND
|
8/18/95
|
ZNO3
|
5.59
|
ND
|
8/24/95
|
ZNO1
|
6.36
|
ND
|
8/24/95
|
ZNO2
|
12.72
|
ND
|
8/24/95
|
ZNO3
|
19.08
|
0.0077
|
7/25/95
|
ZNCl2
|
10.6
|
ND
|
8/18/95
|
ZNCL21
|
11.41
|
ND
|
8/18/95
|
ZNCL22
|
11.41
|
ND
|
8/18/95
|
ZNCL23
|
11.41
|
ND
|
8/24/95
|
ZNCL21
|
15.76
|
ND
|
8/24/95
|
ZNCL22
|
31.52
|
ND
|
8/24/95
|
ZNCL23
|
47.28
|
0.0043
|
All of these results have been blank corrected. The preponderance of
these results shows that neither the zinc oxide nor the zinc chloride are
soluble in benzene. If there is a percentage indicated it may be
attributed to a potential low level zinc contamination.
4.4 Other comments
Part of one old air sample was obtained from an organic chemistry
division analyst. An aliquot of this sample was analyzed by both the
benzene extraction and zinc ashing procedures. Approximately 6 µg zinc or
58 µg zinc stearate were recovered from this air sample.
5. References
5.1 Occupational Safety and Health Administration Salt Lake Technical
Center, Coal Tar Pitch Volatiles Method No. 58. July, 1986.
5.2 Occupational Safety and Health Administration Salt Lake Technical
Center, Metal and Metalloid Particulates in Workplace Atmospheres (Atomic
Absorption), Method No. ID-121. 1985(1991).
|