| • Part Number: | 1910 |
| • Part Title: | Occupational Safety and Health Standards |
| • Subpart: | Z |
| • Subpart Title: | Toxic and Hazardous Substances |
| • Standard Number: | 1910.1028 App D |
| • Title: | Sampling and analytical methods for Benzene monitoring and measurement procedures |
| • GPO Source: | e-CFR |
Measurements taken for the purpose of
determining employee exposure to benzene are best taken so that the
representative average 8-hour
exposure may be determined from a single 8-hour sample or two (2) 4-hour
samples. Short-time interval samples (or grab samples) may also be used
to
determine average exposure level if a minimum of five measurements are
taken in a random manner over the 8-hour work shift. Random sampling
means that
any portion of the work shift has the same change of being sampled as
any other. The arithmetic average of all such random samples taken on
one work
shift is an estimate of an employee's average level of exposure for that
work shift. Air samples should be taken in the employee's breathing
zone (air
that would most nearly represent that inhaled by the employee). Sampling
and analysis must be performed with procedures meeting the requirements
of
the standard.
There are a number of methods available for
monitoring employee exposures to benzene. The sampling and analysis may
be performed by
collection of the benzene vapor or charcoal absorption tubes, with
subsequent chemical analysis by gas chromatography. Sampling and
analysis may also
be performed by portable direct reading instruments, real-time
continuous monitoring systems, passive dosimeters or other suitable
methods. The
employer has the obligation of selecting a monitoring method which meets
the accuracy and precision requirements of the standard under his
unique
field conditions. The standard requires that the method of monitoring
must have an accuracy, to a 95 percent confidence level, of not less
than plus
or minus 25 percent for concentrations of benzene greater than or equal
to 0.5 ppm.
The OSHA Laboratory modified NIOSH Method S311
and evaluated it at a benzene air concentration of 1 ppm. A procedure
for determining the
benzene concentration in bulk material samples was also evaluated. This
work, reported in OSHA Laboratory Method No. 12, includes the following
two
analytical procedures:
I. OSHA Method 12 for Air Samples
Analyte: Benzene Matrix: Air Procedure:
Adsorption on charcoal, desorption with carbon disulfide, analysis by
GC. Detection limit: 0.04
ppm Recommended air volume and sampling rate: 10L to 0.2 L/min.
1. Principle of the Method.
1.1 A known volume of air is drawn through a charcoal tube to trap the organic vapors present.
1.2. The charcoal in the tube is transferred to a small, stoppered vial, and the analyte is desorbed with carbon disulfide.
1.3. An aliquot of the desorbed sample is injected into a gas chromatograph.
1.4 The area of the resulting peak is determined and compared with areas obtained from standards.
2. Advantages and disadvantages of the method.
2.1 The sampling device is small, portable, and
involved no liquids. Interferences are minimal, and most of those which
do occur can be
eliminated by altering chromatographic conditions. The samples are
analyzed by means of a quick, instrumental method.
2.2 The amount of sample which can be taken is
limited by the number of milligrams that the tube will hold before
overloading. When the
sample value obtained for the backup section of the charcoal tube
exceeds 25 percent of that found on the front section, the possibility
of sample
loss exists.
3. Apparatus.
3.1 A calibrated personal sampling pump whose flow can be determined within (+ or -) 5 percent at the recommended flow rate.
3.2. Charcoal tubes: Glass with both ends flame
sealed, 7 cm long with a 6-mm O.D. and a 4-mm I.D., containing 2
sections of 20/40 mesh
activated charcoal separated by a 2-mm portion of urethane foam. The
activated charcoal is prepared from coconut shells and is fired at 600
deg. C
prior to packing. The adsorbing section contains 100 mg of charcoal, the
back-up section 50 mg. A 3-mm portion of urethane foam is placed
between the
outlet end of the tube and the back-up section. A plug of silanized
glass wool is placed in front of the adsorbing section. The pressure
drop across
the tube must be less than one inch of mercury at a flow rate of 1 liter
per minute.
3.3. Gas chromatograph equipped with a flame ionization detector.
3.4. Column (10-ft X 1/8 -in stainless steel)
packed with 80/100 Supelcoport coated with 20 percent SP 2100, 0.1
percent CW 1500.
3.5. An electronic integrator or some other suitable method for measuring peak area.
3.6. Two-milliliter sample vials with Teflon-lined caps.
3.7. Microliter syringes: 10-microliter (10-uL syringe, and other convenient sizes for making standards, 1-uL syringe for sample
injections.
3.8. Pipets: 1.0 mL delivery pipets
3.9. Volumetric flasks: convenient sizes for making standard solutions.
4. Reagents.
4.1. Chromatographic quality carbon disulfide
(CS(2)). Most commercially available carbon disulfide contains a trace
of benzene which must
be removed. It can be removed with the following procedure:
Heat under reflux for 2 to 3 hours, 500 mL of
carbon disulfide, 10 mL concentrated sulfuric acid, and 5 drops of
concentrated nitric acid.
The benzene is converted to nitrobenzene. The carbon disulfide layer is
removed, dried with anhydrous sodium sulfate, and distilled. The
recovered
carbon disulfide should be benzene free. (It has recently been
determined that benzene can also be removed by passing the carbon
disulfide through 13x
molecular sieve).
4.2. Benzene, reagent grade.
4.3. p-Cymene, reagent grade, (internal standard).
4.4. Desorbing reagent. The desorbing reagent
is prepared by adding 0.05 mL of p-cymene per milliliter of carbon
disulfide. (The internal
standard offers a convenient means correcting analytical response for
slight inconsistencies in the size of sample injections. If the external
standard technique is preferred, the internal standard can be
eliminated).
4.5. Purified GC grade helium, hydrogen and air.
5. Procedure.
5.1. Cleaning of equipment. All glassware used
for the laboratory analysis should be properly cleaned and free of
organics which could
interfere in the analysis.
5.2. Calibration of personal pumps. Each pump must be calibrated with a representative charcoal tube in the line.
5.3. Collection and shipping of samples.
5.3.1. Immediately before sampling, break the
ends of the tube to provide an opening at least one-half the internal
diameter of the tube
(2 mm).
5.3.2. The smaller section of the charcoal is used as the backup and should be placed nearest the sampling pump.
5.3.3. The charcoal tube should be placed in a vertical position during sampling to minimize channeling through the charcoal.
5.3.4 Air being sampled should not be passed through any hose or tubing before entering the charcoal tube.
5.3.5. A sample size of 10 liters is
recommended. Sample at a flow rate of approximately 0.2 liters per
minute. The flow rate should be
known with an accuracy of at least (+ or -) 5 percent.
5.3.6. The charcoal tubes should be capped with the supplied plastic caps immediately after sampling.
5.3.7. Submit at least one blank tube (a
charcoal tube subjected to the same handling procedures, without having
any air drawn through it)
with each set of samples.
5.3.8. Take necessary shipping and packing precautions to minimize breakage of samples.
5.4. Analysis of samples.
5.4.1. Preparation of samples. In preparation
for analysis, each charcoal tube is scored with a file in front of the
first section of
charcoal and broken open. The glass wool is removed and discarded. The
charcoal in the first (larger) section is transferred to a 2-ml vial.
The
separating section of foam is removed and discarded; the second section
is transferred to another capped vial. These two sections are analyzed
separately.
5.4.2. Desorption of samples. Prior to
analysis, 1.0 mL of desorbing solution is pipetted into each sample
container. The desorbing
solution consists of 0.05 uL internal standard per mL of carbon
disulfide. The sample vials are capped as soon as the solvent is added.
Desorption
should be done for 30 minutes with occasional shaking.
5.4.3. GC conditions. Typical operating conditions for the gas chromatograph are:
1.30 mL/min (60 psig) helium carrier gas flow.
2.30 mL/min (40 psig) hydrogen gas flow to detector.
3.240 mL/min (40 psig) air flow to detector.
4.150 deg. C injector temperature.
5.250 deg. C detector temperature.
6.100 deg. C column temperature.
5.4.4. Injection size. 1 uL.
5.4.5. Measurement of area. The peak areas are measured by an electronic integrator or some other suitable form of area
measurement.
5.4.6. An internal standard procedure is used.
The integrator is calibrated to report results in ppm for a 10 liter air
sample after
correction for desorption efficiency.
5.5. Determination of desorption efficiency.
5.5.1. Importance of determination. The
desorption efficiency of a particular compound can vary from one
laboratory to another and from
one lot of chemical to another. Thus, it is necessary to determine, at
least once, the percentage of the specific compound that is removed in
the
desorption process, provided the same batch of charcoal is used.
5.5.2. Procedure for determining desorption
efficiency. The reference portion of the charcoal tube is removed. To
the remaining portion,
amounts representing 0.5X, 1X, and 2X and (X represents target
concentration) based on a 10 L air sample are injected into several
tubes at each
level. Dilutions of benzene with carbon disulfide are made to allow
injection of measurable quantities. These tubes are then allowed to
equilibrate at
least overnight. Following equilibration they are analyzed following the
same procedure as the samples. Desorption efficiency is determined by
dividing the amount of benzene found by amount spiked on the tube.
6. Calibration and standards. A series of
standards varying in concentration over the range of interest is
prepared and analyzed under the
same GC conditions that will be used on the samples. A calibration curve
is prepared by plotting concentration (ug/mL) versus peak area.
7. Calculations. Benzene air concentration can be calculated from the following equation:
mg/m(3) = (A)(B)/(C)(D) Where: A = ug/mL benzene, obtained from the calibration curve B = desorption volume (1 mL) C = Liters of air sampled D = desorption efficiency The concentration in mg/m(3) can be converted to ppm (at 25 deg. and 760 mm) with following equation: ppm = (mg/m(3))(24.46)/(78.11) Where: 24.46 = molar volume of an ideal gas 25 deg. C and 760 mm 78.11 = molecular weight of benzene
8. Backup Data.
8.1 Detection limit-Air Samples.
The detection limit for the analytical
procedure is 1.28 ng with a coefficient of variation of 0.023 at this
level. This would be
equivalent to an air concentration of 0.04 ppm for a 10 L air sample.
This amount provided a chromatographic peak that could be identifiable
in the
presence of possible interferences. The detection limit data were
obtained by making 1 uL injections of a 1.283 ug/mL standard.
______________________________
| |
Injection | Area |
| Count |
__________|________|__________
| |
1 ....... | 655.4 |
2 ....... | 617.5 |
3 ....... | 662.0 | X = 640.2
4 ....... | 641.1 | SD = 14.9
5 ....... | 636.4 | CV = 0.023
6 ....... | 629.2 |
__________|________|__________
8.2. Pooled coefficient of variation - Air
Samples. The pooled coefficient of variation for the analytical
procedure was determined by 1
uL replicate injections of analytical standards. The standards were
16.04, 32.08, and 64.16 ug/mL, which are equivalent to 0.5, 1.0, and 2.0
ppm for a
10 L air sample respectively.
____________________________________________
|
Injection | Area Counts
|________________________________
| | |
| 0.5 ppm | 1.0 ppm | 2.0 ppm
___________|_________|_________|____________
| | |
1 ........ | 3996.5 | 8130.2 | 16481
2 ........ | 4059.4 | 8235.6 | 16493
3 ........ | 4052.0 | 8307.9 | 16535
4 ........ | 4027.2 | 8263.2 | 16609
5 ........ | 4046.8 | 8291.1 | 16552
6 ........ | 4137.9 | 8288.8 | 16618
X = | 4053.3 | 8254.0 | 16548.3
SD = | 47.2 | 62.5 | 57.1
CV = | 0.0116 | 0.0076 | 0.0034
CV = 0.008.|.........|.........|............
___________|_________|_________|____________
8.3. Storage data - Air Samples
Samples were generated at 1.03 ppm benzene at
80% relative humidity, 22 deg. C, and 643 mm. All samples were taken for
50 minutes at 0.2
L/min. Six samples were analyzed immediately and the rest of the samples
were divided into two groups by fifteen samples each. One group was
stored at
refrigerated temperature of 25 deg. C, and the other group was stored at
ambient temperature (approximately 23 deg. C). These samples were
analyzed
over a period of fifteen days. The results are tabulated below.
PERCENT RECOVERY
________________________________________________________
| |
Day analyzed | Refrigerated | Ambient
_____________|____________________|_____________________
| |
0 ...........| 97.4 98.7 98.9 | 97.4 98.7 98.9
0 ...........| 97.1 100.6 100.9 | 97.1 100.6 100.9
2 ...........| 95.8 96.4 95.4 | 95.4 96.6 96.9
5 ...........| 93.9 93.7 92.4 | 92.4 94.3 94.1
9 ...........| 93.6 95.5 94.6 | 95.2 95.6 96.6
13 ..........| 94.3 95.3 93.7 | 91.0 95.0 94.6
15 ..........| 96.8 95.8 94.2 | 92.9 96.3 95.9
_____________|____________________|_____________________
8.4. Desorption data.
Samples were prepared by injecting liquid
benzene onto the A section of charcoal tubes. Samples were prepared that
would be equivalent to
0.5, 1.0, and 2.0 ppm for a 10 L air sample.
PERCENT RECOVERY
_________________________________________
| | |
Sample | 0.5 | 1.0 | 2.0
| ppm | ppm | ppm
_________|__________|_________|__________
| | |
1 .......| 99.4 | 98.8 | 99.5
2 .......| 99.5 | 98.7 | 99.7
3 .......| 99.2 | 98.6 | 99.8
4 .......| 99.4 | 99.1 | 100.0
5 .......| 99.2 | 99.0 | 99.7
6 .......| 99.8 | 99.1 | 99.9
X = .....| 99.4 | 98.9 | 99.8
SD = ....| 0.22 | 0.21 | 0.18
CV = ....| 0.0022 | 0.0021 | 0.0018
X = 99.4 | | |
_________|__________|_________|__________
8.5. Carbon disulfide.
Carbon disulfide from a number of sources was
analyzed for benzene contamination. The results are given in the
following table. The
benzene contaminant can be removed with the procedures given in section
4.1.
________________________________________________
| |
| ug | ppm
| Benzene/mL | equivalent
Sample | | (for 10 L
| | air sample)
______________________|____________|____________
| |
Aldrich Lot 83017 ....| 4.20.| 0.13
Baker Lot 720364 .....| 1.01.| 0.03
Baker Lot 822351 .....| 1.01.| 0.03
Malinkrodt Lot WEMP ..| 1.74.| 0.05
Malinkrodt Lot WDSJ ..| 5.65.| 0.18
Malinkrodt Lot WHGA ..| 2.90.| 0.09
Treated CS2 |............|............
______________________|____________|____________
II. OSHA LABORATORY METHOD NO. 12 FOR BULK SAMPLES
Analyte: Benzene.
Matrix: Bulk Samples.
Procedure: Bulk Samples are analyzed directly by high performance liquid chromatography (HPLC).
Detection limits: 0.01% by volume.
1. Principle of the method.
1.1. An aliquot of the bulk sample to be analyzed is injected into a liquid chromatograph.
1.2. The peak area for benzene is determined and compared to areas obtained from standards.
2. Advantages and disadvantages of the method.
2.1. The analytical procedure is quick, sensitive, and reproducible.
2.2. Reanalysis of samples is possible.
2.3. Interferences can be circumvented by proper selection of HPLC parameters.
2.4. Samples must be free of any particulates
that may clog the capillary tubing in the liquid chromatograph. This may
require distilling
the sample or clarifying with a clarification kit.
3. Apparatus.
3.1. Liquid chromatograph equipped with a UV detector.
3.2. HPLC Column that will separate benzene
from other components in the bulk sample being analyzed. The column used
for validation
studies was a Waters uBondapack C18, 30 cm x 3.9 mm.
3.3. A clarification kit to remove any particulates in the bulk if necessary.
3.4. A micro-distillation apparatus to distill any samples if necessary.
3.5. An electronic integrator or some other suitable method of measuring peak areas.
3.6. Microliter syringes - 10 uL syringe and other convenient sizes for making standards. 10 uL syringe for sample injections.
3.7. Volumetric flasks, 5 mL and other convenient sizes for preparing standards and making dilutions.
4. Reagents.
4.1. Benzene, reagent grade.
4.2. HPLC grade water, methyl alcohol, and isopropyl alcohol.
5. Collection and shipment of samples.
5.1. Samples should be transported in glass containers with Teflon-lined caps.
5.2. Samples should not be put in the same container used for air samples.
6. Analysis of samples.
6.1. Sample preparation.
If necessary, the samples are distilled or
clarified. Samples are analyzed undiluted. If the benzene concentration
is out of the working
range, suitable dilutions are made with isopropyl alcohol.
6.2. HPLC conditions.
The typical operating conditions for the high performance liquid chromatograph are:
1. Mobile phase - Methyl alcohol/water, 50/50
1. Analytical wavelength - 254 nm
3. Injection size - 10 uL
6.3. Measurement of peak area and calibration.
Peak areas are measured by an integrator or other suitable means. The integrator is calibrated to report results % in benzene by
volume.
7. Calculations.
Since the integrator is programmed to report results in % benzene by volume in an undiluted sample, the following equation is
used:
% Benzene by Volume = A x B Where: A = % by volume on report B = Dilution Factor (B = 1 for undiluted sample)
8. Backup Data.
8.1. Detection limit - Bulk Samples.
The detection limit for the analytical
procedure for bulk samples is 0.88 ug, with a coefficient of variation
of 0.019 at this level. This
amount provided a chromatographic peak that could be identifiable in the
presence of possible interferences. The detection limit date were
obtained by
making 10 uL injections of a 0.10% by volume standard.
_____________________________________
| |
Injection | Area Count |
__________|____________|_____________
| |
1 ........| 45386 |
2 ........| 44214 |
3 ........| 43822 | X = 44040.1
4 ........| 44062 | SD = 852.5
6 ........| 42724 | CV = 0.019
__________|____________|_____________
8.2. Pooled coefficient of variation - Bulk Samples.
The pooled coefficient of variation for
analytical procedure was determined by 50 uL replicate injections of
analytical standards. The
standards were 0.01, 0.02, 0.04, 0.10, 1.0, and 2.0% benzene by volume.
AREA COUNT (PERCENT)
___________________________________________________________________
| | | | | |
Injection | | | | | |
No. | 0.01 | 0.02 | 0.04 | 0.10 | 1.0 | 2.0
__________|_________|_________|________|________|_________|________
| | | | | |
1 ........| 45386 | 84737 | 166097 | 448497 | 4395380 | 9339150
2 ........| 44241 | 84300 | 170832 | 441299 | 4590800 | 9484900
3 ........| 43822 | 83835 | 164160 | 443719 | 4593200 | 9557580
4 ........| 44062 | 84381 | 164445 | 444842 | 4642350 | 9677060
5 ........| 44006 | 83012 | 168398 | 442564 | 4646430 | 9766240
6 ........| 42724 | 81957 | 173002 | 443975 | 4646260 | .......
X = ......| 44040.1 | 83703.6 | 167872 | 444149 | 4585767 | 9564986
SD = .....| 852.5 | 1042.2 | 3589.8 | 2459.1 | 96839.3 | 166233
CV = .....| 0.0194 | 0.0125 | 0.0213 | 0.0055 | 0.0211 | 0.0174
CV = .....| 0.017 | | | | |
__________|_________|_________|________|________|_________|________
|
Next Standard (1910.1028 App E) |
| Regulations (Standards - 29 CFR) - Table of Contents |
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