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). |
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