1. General Discussion
1.1 Background
1.1.1 History of procedure over the past several years the OSHA Laboratory has received
requests to analyze samples for "trimethylbenzene". These have primarily been air samples collected on
charcoal. Since several related aromatic compounds are routinely collected on charcoal, desorbed with carbon
disulfide, and analyzed by gas chromatography, it was decided to try the same with all three trimethylbenzenes. The
three isomers are 1,2,3-, 1,2,4-, and 1,3,5-trimethylbenzene.
1.1.2 Toxic Effects (This section is for information purposes and should not be taken as the basis for OSHA
policy.) Both the 1,2,4- and the 1,3,5- isomers are (Ref 5.1) highly toxic by inhalation and by ingestion. Both are
skin and eye irritants. Both isomers are central nervous system depressants and may cause respiratory disorders.
The 1,2,4-isomer may also be narcotic. Other effects of exposure to these compounds include headache, tension,
nervousness, inflammation and hemorrhaging of mucous membranes, convulsions and ultimately death. No specific
information about the toxic effects of the 1,2,3-isomer was available.
1.1.3. Potential workplace exposure: (Ref. 5.2)
1,2,4-Trimethylbenzene is used as a chemical intermediate in the manufacture of trimellitic anhydride,
pseudocumidine and various dyes and pharmaceuticals. 1,3,5-Trimethylbenzene is used as an intermediate in the
production of anthraquinone vat dyes and ultraviolet oxidation stabilizers for plastics. 1,2,3-Trimethylbenzene is
a precursor for a musk. 1,2,4-Trimethylbenzene is produced at about 22,500 tons/year in the United States. 'The
other Trimethylbenzenes are produced at much lower rates.
1.1.4 Physical properties: (Ref. 5.3)
Compound: |
1,2,3 Trimethylbenzene |
1,2,4 Trimethylbenzene |
1,3,5 Trimethylbenzene |
Molecular Weight: |
120.186 (all isomers) |
|
Density: |
0.8944 |
0.8758 |
0.8652 |
Freezing Point: |
-25.37°C |
-43.8°C |
-44.7°C |
Boiling Point: |
176.1°C |
169.35°C |
164.7°C |
Odor: |
Characteristic of aromatic hydrocarbons. |
Color: |
Each is a clear liquid. |
Molecular Formula: |
C8H12 |
|
|
Flash Point
(Tag closed cup): |
51°C |
46°C |
44°C |
CAS Number: |
526-73-8 |
96-63-6 |
108-67-8 |
IMIS Number: |
T205 |
T306 |
T407 |
|
(2505 for mixture of all three) |
Structure: |
|
|
|
1.2 Limit defining parameters
1.2.1 The detection limit of the analytical procedure is 2 ng/injection for each
isomer. This is the smallest amount of analyte which will produce a peak height 5 times the baseline noise.
1.2.2 The detection limit of the overall procedure is 1 µg/sample for each isomer, assuming a 1 mL sample volume,
a 2 µL injection volume and nearly 100% retention and desorption efficiencies.
1.3 Advantages
1.3.1 The sampling procedure is convenient for both the industrial hygienist and the employee
being sampled.
1.3.2 No reagents and only a small amount of apparatus are required for sampling.
1.3.3 Samples are convenient to ship.
1.3.4 The analytical method is reproducible, sensitive and does not require extensive handling of samples or
standards.
1.3.5 It is possible to analyze at the same time other analytes which are collected and desorbed by the same
procedure and which are separable from the trimethylbenzenes by gas chromatography, through the proper choice of
column and GC parameters.
1.3.6 Samples can be reanalyzed if necessary.
1.4 Disadvantages
None were encountered during these studies.
2. Sampling procedure
2.1 Apparatus
2.1.1 A calibrated personal sampling pump, the flow of which can be determined within ±5% at
the recommended flow.
2.1.2 Charcoal tubes: glass tube with both ends flame sealed, 7 cm long with a 6mm O.D., and a 4 mm I.D.,
containing 2 sections of 20/40 mesh charcoal separated by a 2-mm portion of urethane foam. The front section
contains 100 mg of charcoal, the backup section contains 50 mg. A plug of silane treated glass wool is placed ahead
of the front section and a 3-mm portion of urethane foam is placed behind the backup section
2.2 Sampling technique
2.2.1 The ends of the tube are opened immediately before sampling.
2.2.2 Connect the backup end of the tube to the sampling pump with flexible tubing.
2.2.3 Tubes should be placed in a vertical position to minimize channeling.
2.2.4 Air being sampled should not pass through any hose or tubing before entering the charcoal tube.
2.2.5 Place plastic caps on each end of the tube immediately after sampling. Wrap each sample lengthwise with OSHA
Form-21.
2.2.6 With each batch of samples, submit at least one blank tube from the same lot used for samples. This tube
should be subjected to exactly the same handling as the samples (break ends, seal, & transport) except that no
air is drawn through it.
2.2.7 Send the samples (and corresponding paperwork) to the laboratory for analysis.
2.2.9. If any bulk samples are submitted for analysis they must be shipped in a separate container from the air
samples and blanks.
2.3 Desorption and/or Extraction efficiency
Eighteen charcoal tubes and a blank were studied. Six tubes were injected with 2.8 µL each of a 1/1/1 mixture
(v/v/v) of the isomers. The mixture was then diluted 9:1 (v/v) with carbon disulfide. Each of six tubes were
injected with 7.0 µL of this dilution and six more tubes were injected with 1.4 µL. The tubes were refrigerated
about 18 hours and then desorbed with 1 mL of 0.1% (v/v) n-hexylbenzene in carbon disulfide. The following results
were obtained.
TABLE 1
|
|
1,2,3-isomer
0.835 mg Spike |
1,2,4-isomer
0.817 mg Spike |
1,3,5-isomer
0.808 mg Spike |
|
1. |
92.340% |
94.693% |
96.797% |
2. |
95.651 |
98.666 |
101.239 |
3. |
92.228 |
94.662 |
96.857 |
4. |
95.944 |
98.477 |
100.566 |
5. |
90.102 |
92.432 |
94.375 |
6. |
90.986 |
93.591 |
95.868 |
Ave
S.D. |
92.875%
2.412 |
95.420%
2.579 |
97.617%
2.707 |
|
|
1,2,3-ismoer
0.209 mg |
1,2,4-isomer
0.204 mg |
1,3,5-isomer
0.202 mg |
|
1. |
112.688% |
117.076% |
119.648% |
2. |
108.328 |
112.740 |
115.000 |
3. |
108.436 |
110.928 |
113.664 |
4. |
109.288 |
111.916 |
114.168 |
5. |
114.040 |
116.872 |
119.832 |
6. |
108.992 |
112.048 |
114.780 |
Ave
S.D. |
110.295%
2.441 |
113.597%
2.680 |
116.182%
2.796 |
|
|
0.0417 mg |
0.0409 mg |
0.0404 mg |
|
1. |
111.440% |
117.560% |
116.160% |
2. |
102.680 |
110.400 |
106.900 |
3. |
106.320 |
114.700 |
111.580 |
4. |
107.060 |
111.300 |
113.420 |
5. |
110.680 |
116.160 |
115.920 |
6. |
112.360 |
119.760 |
119.860 |
Ave
S.D. |
108.423%
3.714 |
114.980%
3.619 |
113.973%
4.456 |
2.4 Retention efficiency
A retention efficiency study of six tubes, each injected with 2.8 µL of the 1/1/1 (v/v/v) isomer mixture, and a
blank was then performed. Each tube was exposed to a 0.1 L/min flow of air of approximately 83% relative humidity at
23°C for 100 minutes, corresponding to an air volume of about 10 L. These data were obtained:
TABLE 2
|
|
1,2,3-isomer
0.835 mg Spike |
1,2,4-isomer
0.817 mg Spike |
1,3,5-isomer
0.808 mg Spike |
|
1. |
91.460% |
94.057% |
96.082% |
2. |
92.463 |
94.843 |
96.980 |
3. |
95.020 |
97.714 |
100.132 |
4. |
92.840 |
95.257 |
97.404 |
5. |
91.945 |
94.316 |
97.021 |
6 |
92.648 |
94.505 |
>96.544 |
Ave. |
92.648% |
95.115% |
97.3605% |
These averages were nearly identical to desorption efficiencies for the same loading of the
corresponding isomers. Water vapor appears to have very little effect in dislodging trimethylbenzene from activated
charcoal.
2.5. Storage
Two sets of six samples, each injected with 1.4 µL of the isomer mixture, and one blank, were prepared and stored
under refrigeration for three days. One set was exposed to humid air as was done with the retention study samples
while the other set was not. There was no loss on storage.
TABLE 3 |
No Air Drawn |
|
|
|
1,2,3-isomer
0.417 mg Spike |
1,2,4-isomer
0.409 mg Spike |
1,3,5-isomer
0.404 mg Spike |
|
1. |
90.762% |
93.722% |
95.996% |
2. |
91.360 |
94.332 |
96.504 |
3. |
93.198 |
96.270 |
98.384 |
4. |
91.878 |
95.216 |
97.476 |
5. |
95.754 |
99.244 |
101.642 |
6. |
90.672 |
92.150 |
95.668 |
Ave. |
92.271% |
95.156% |
97.612 |
|
Humid Air Drawn |
|
|
|
1,2,3-isomer
0.417 mg |
1,2,4-isomer
0.409 mg |
1,3,5-isomer
0.404 mg |
|
1. |
91.726% |
93.598 |
96.276 |
2. |
92.750 |
96.595 |
99.209 |
3. |
96.309 |
99.930 |
102.286 |
4. |
99.050 |
102.286 |
105.060 |
5. |
94.915 |
97.717 |
100.458 |
6. |
93.965 |
96.911 |
99.544 |
Ave. |
94.786% |
97.896% |
100.472% |
2.6 Air volume and sampling rate studied
2.6.1 The air volume studied is 10 L.
2.6.2 The sampling rate studied is 0.1 liters per minute.
2.7 Suspected interferences should be listed on sample data sheets.
2.8 Safety precautions
2.8.1 Sampling equipment should be placed on an employee in a manner that does not interfere
with work performance or safety.
2.8.2 Safety glasses should be worn at all times.
2.8.3 All safety procedures that apply to the workplace being sampled should be followed.
3. Analytical Method
3.1 Apparatus
3.1.1 Gas chromatograph equipped with a flame ionization detector.
3.1.2 GC column capable of separating the solvent, an internal standard and three trimethylbenzenes from each other
and from any interferences.
3.1.3 An electronic integrator or some other suitable means of measuring detector response.
3.1.4 Two-milliliter vials with Teflon-lines caps.
3.1.5.A syringe of 2 µL or other convenient size for sample injection.
3.1.6 A repipet for accurately dispensing a known volume of desorbing solution to all the samples. In this work a 1
mL Glenco dispenser was used.
3.1.7 A syringe of 10 µL or other convenient size to inject pure standards into volumetric flasks.
3.1.8 Volumetric flasks of 5 mL or other convenient size in which to prepare standards.
3.2 Reagents
3.2.1 Nitrogen, hydrogen and oxygen of GC grade purity.
3.2.2 Carbon disulfide, reagent grade.
3.2.3 An internal standard, which elutes after the analytes and their most likely contaminants. Reagent grade
n-hexylbenzene seems to be the best choice.
3.2.4 Analytes, reagent grade. 1,2,4-and 1,3,4-trimethylbenzene are readily available at 99% purity each.
However, there seems to be a problem in obtaining 1,2,3-trimethylbenzene above 90X purity. It appears to contain
significant amounts of the 1,2,4-isomer as well as other aromatic compounds.
3.3 Standard preparation
3.3.1 Standards of the trimethylbenzenes are prepared by injecting a known volume of each
isomer, usually 5 µL, into a volumetric flask, usually 5 mL, partly filled with the desorbing solution (carbon
disulfide plus internal standard), then filling the flask to the mark and mixing the analytes with the desorbing
solution by a few successive inversions of the flask.
3.3.3 By preparing standards of various concentrations and running them, a calibration curve may be prepared.
3.3.4 At least two initial standards which are theoretically equal and are within ±5% agreement must be prepared
and run.
3.4 Sample preparation
3.4.1 The front and back sections of charcoal from each tube are transferred to separate
vials.
3.4.2 To each vial is added 1mL of desorbant.
3.4.3 Each vial is sealed immediately and allowed to desorb for 30 minutes with occasional shaking.
3.5 Analysis
3.5.1 Gas chromatographic conditions
Injection size : 2.0 µL
Injector temperature: 200°C
Nitrogen flow rate: 25 mL min.
Detector temperature: 250°C
Hydrogen flow rate: 40 mL min.
Air : 300 mL/ min.
Detector: Flame ionization
Column: 101 × 1/811 FFAP/Chrom. WAW
Oven Program: 120°C/6 min, 5°C increase /min, 150°C/5 min
Attenuation: 7²
Instrument: Hewlett-Packard 5840
Elution times:
1,3,5-trimethylbenzene: 5.37 min.
1,2,4-trimethylbenzene: 6.30 min.
1,2,3-trimethylbenzene: 7.82 min.
n-hexylbenzene (ISTD): 12.70 min.
3.5.2 Chromatogram
3.5.3 Detector response is measured by electronic integration of peak areas or other suitable means.
3.5.4 An internal standard is used to compensate for small differences in injection sizes. The integrator is
calibrated in ppm for a 10L air volume after correction for desorption efficiency.
3.5.5 Precision
A precision study, involving six injections each of standards at four, twice, once, one-half, and one-tenth of the
target concentration was performed.
TABLE 4 |
4X Target |
|
|
|
1,2,3-isomer
0.835 mg/mL |
1,2,4-isomer
0.817 mg/mL |
1,3,5-isomer
0.808 mg/mL |
|
1. |
101.743 |
100.917 |
100.548 |
2. |
99.833 |
99.549 |
99.303 |
3. |
99.833 |
99.334 |
99.092 |
4. |
99.621 |
98.800 |
94.411 |
5. |
98.127 |
97.906 |
97.605 |
6. |
98.131 |
97.875 |
97.640 |
ave.
S.D.
C.V |
99.513
1.336
1.343 |
99.064
1.146
1.157 |
98.766
1.124
1.138 |
|
2X Target |
|
|
|
1,2,3-isomer
0.417 mg/mL |
1,2,4-isomer
0.409 mg/mL |
1,3,5-isomer
0.404 mg/mL |
|
1. |
48.554 |
48.122 |
84.195 |
2. |
48.772 |
48.892 |
48.728 |
3. |
49.189 |
49.386 |
49.198 |
4. |
48.952 |
48.638 |
48.766 |
5. |
48.769 |
48.808 |
48.703 |
6. |
48.905 |
49.046 |
48.962 |
ave.
S.D.
C.V. |
48.845
0.221
0.454 |
48.815
0.424
0.868 |
48.759
0.334
0.684 |
|
1X Target |
|
|
|
1,2,3-isomer |
1,2,4-isomer |
1,3,5-isomer |
|
1. |
24.309 |
24.168 |
24.095 |
2. |
24.356 |
24.269 |
24.179 |
3. |
24.186 |
24.089 |
23.988 |
4. |
24.169 |
24.113 |
24.016 |
5. |
24.323 |
24.289 |
24.153 |
6. |
24.280 |
24.217 |
24.052 |
ave.
S.D.
C.V. |
24.270
0.076
0.314 |
24.191
0.082
0.338 |
24.080
0.076
0.315 |
|
0.1X Target |
|
|
|
1,2,3-isomer
0.0417 mg/mL |
1,2,4-isomer
0.0409 mg/mL |
1,3,5-isomer
0.0404mmg/mL |
|
1. |
3.298 |
3.407 |
3.257 |
2. |
3.312 |
3.446 |
3.253 |
3. |
3.293 |
3.422 |
3.247 |
4. |
3.684 |
3.711 |
3.535 |
5. |
3.679 |
3.867 |
3.604 |
6. |
3.645 |
3.742 |
3.559 |
ave.
S.D.
C.V. |
3.485
0.202
5.804 |
3.599
0.198
5.508 |
3.409
0.173
5.082 |
3.6 Interferences
3.6.1 Any compound which produces a peak which partially or completely overlaps a peak
produced by an analyte or the internal standard is an analytical interference. Possible interferences should
be listed on the sample data sheet. GC parameters should be adusted in whatever way necessary to eliminate any such
interference.
3.6.2 Since the title compounds are most frequently derived from coal tar naphthas, other alkylated benzenes,
styrenes and cycloalkylated benzenes which have similar boiling points and/or molecular formulae may be
interferences. Many such compounds are theoretically possible, but relatively few are available in a high degree of
purity.
3.6.3 Retention time data on a single column does not alone prove chemical identity. Samples over the target
concentration must be confirmed by GC/Mass Spec or other conclusive means.
3.7 Calculations
3.7.1 To calculate the ppm of analyte, based on a 10 L air volume. and a 1 mL desorption
volume, in standards.
ppm = |
1 µL/mL × 1 mL × p × 24.46 × 1000 L
10 L × 1 m³ × DE MW |
24.46 = # liters/mole of any gas at 25°C and 760 mmHg.
MW = molecular weight
p = Density of analyte
1 mL = Recommended desorption volume
10 L = Assumed air volume
DE = Desorption efficiency
3.7.2 Printed values for samples are calculated by:
actual ppm = |
printout - printout blank
(acutal air volume) / 10 L
|
3.8 Safety precautions
3.8.1 All solvents should be handled in a hood.
3.8.2 Skin contact with any solvent is to be avoided.
3.8.3 Safety glasses are to be worn at all times.
4. Recommendations for further study
4.1 Further work should be done to eliminate interferences, since there are many possible
interferences. Capillary columns could be investigated.
4.2 Additional work should be done with each isomer separately to determine whether any given isomer contains trace
amounts of any other isomer. This can't be seen when all three are mixed.
5. References
5.1 Material Safety Data Sheets for 1,2,4- and 1,3,5-Trimethylbenzene from OCIS file.
5.2 Kirk-Othmer Encyclopedia of Chemical Technology (1982), exec.ed. Martin Grayson, vol. 18, pp. 881.5.
5.3 CRC Handbook of Chemistry and Physics, 53rd Edition, ed. by Robert C. Weast, pp. C-165-6.
|