1. General Discussion
1.1 Background
1.1.1 History of procedure
The OSHA Technical Center has received many requests for a sampling and analytical procedure for
di-tert-butyl-p-cresol (BHT). OSHA promulgated an exposure standard on January, 1989, for BHT at a level of 10 mg/m³
TWA. There is a NIOSH method 226 for collection of BHT on silica gel tubes and desorption by 5/95 methanol/carbon
disulfide, and analysis by GC-FID (Ref. 5.1). This is a partially validated method with no retention or collection
studies performed. The lab performed a retention study with 10 liters of 89% RH air, on silica gel tubes spiked
with 100 µg BHT. The average recovery was 30.9%, so another means of collection was explored. OSHA Method 32
collects phenol and cresol on XAD-7 tubes and desorbs them with methanol (Ref. 5.2). BHT is related to these
compounds, and is a solid at room temperature, so a modification of method 32 was tried, using an OVS-7 tube
instead of the XAD-7, and found to be successful. An OVS-7 tube is a glass fiber filter in front of a 270 mg
section of XAD-7 resin followed by a 140 mg section of XAD-7 resin. Desorption, retention, and storage recoveries
were good using OVS-7 tubes.
1.1.2 Potential workplace exposure (Ref. 5.3)
BHT is used as an antioxidant for food, animal feed, petroleum products, synthetic rubbers, plastics, animal and
vegetable oils, and soaps. BHT is used as an antiskinning agent for paints and inks.
1.1.3 Toxic Effects (This section is for information purposes and should not be taken as the basis for OSHA
policy.) (Ref. 5.4)
BHT is a skin and eye irritant. Levels of 5 to 10 times the level in processed foods caused brain and behavioral
changes in mice; the treated mice fought more and slept less than the control group. Large doses of BHT in the food
of rats, mice, cats, and dogs caused liver weight increase, which was reversible in the rat study where 500
mg/kg/day was given for two weeks. The LD50 for male rats was 1.7 g/kg. The FDA limits the BHT level in
food to 2 ppm.
1.1.4 Physical properties (Ref. 5.3):
Synonyms: |
Butylated hydroxytoluene; 2,6-Bis (1,1-dimethyl- ethyl)-4-methyl phenol; BHT; DBPC; Antracine 8;
2, 6-di- tert-butyl-4-methyl phenol; Tenox BHT; Annual CP; Sustane; Dalpac; Impruvol; Vianol |
Molecular weight: |
220.34 |
Melting point: |
70°C |
Boiling point: |
265°C |
Flash point: |
127°C (261°F) |
Odor: |
light cresylic |
Color: |
white to pale yellow crystals |
Molecular formula: |
C15H240 |
CAS: |
128-37-0 |
IMIS: |
2683 |
RTECS: |
27354; G07875000 |
Structure:
![structure of di-tert-butyl-p-cresol structure of di-tert-butyl-p-cresol](https://webarchive.library.unt.edu/eot2008/20081106073344im_/http://www.osha.gov/dts/sltc/methods/partial/pv2108/pv2108str1.gif) |
|
1.2 Limit defining parameters
1.2.1 The detection limit of the analytical procedure is 0.333 µg/mL BHT in the desorbing solvent. This is the
smallest amount that could be detected under normal operating conditions.
1.2.2 The overall detection limit is 0.01 mg/m³ BHT. (All mg/m³ amounts in this study are
based on a 100-liter air volume and a 3-mL desorption.)
1.3 Advantages
1.3.1 The sampling procedure is convenient.
1.3.2 The analytical method is reproducible and sensitive.
1.3.3 Reanalysis of samples is possible.
1.3.4 It may be possible to analyze other compounds at the same time.
1.3.5 Interferences may be avoided by proper selection of column and GC parameters.
1.4 Disadvantages
None known.
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 The sampling media consists of OVS-7 tubes. The OVS-7 tubes are specially made 13-mm O.D.. glass tubes that
are tapered to 6-mm O.D.. These tubes are packed with a 13-mm diameter glass fiber filter then a 270-mg sampling
section followed by a 140-mg backup section of purified XAD-7 resin, available from Alltech. There is a foam plug
between sampling section and backup section and after the backup section. The glass fiber filter is held next to
the sampling section by a polytetrafluoroethylene (PTFE) retainer. These tubes are commercially available
through many sources.
![ovs-7 Figure 1. A diagram of an OVS-7 tube.](https://webarchive.library.unt.edu/eot2008/20081106073344im_/http://www.osha.gov/dts/sltc/methods/images/ovs_7.gif)
Figure 1. A diagram of an OVS-7 tube.
2.2 Sampling technique
2.2.1 Remove the end caps of the OVS-7, immediately before sampling.
2.2.2 Connect the OVS-7 to the sampling pump with flexible tubing.
2.2.3 Place the tubes in a vertical position to minimize channeling, with the smaller section, towards the pump.
2.2.4 Air being sampled should not pass through any hose or tubing before entering the OVS-7.
2.2.5 Seal the OVS-7 with plastic caps immediately after sampling. Seal each sample lengthwise with OSHA Form-21
sealing tape.
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 Transport the samples (and corresponding paperwork) to the lab for analysis.
2.2.8 Bulks submitted for analysis must be shipped in a separate mailing container from other samples.
2.3 Desorption efficiency
2.3.1 Two hundred seventy milligram portions of XAD-7 resin were placed into separate 4-mL vials and six
portions were spiked at each loading of 0.1 mg (1 mg/m³), 0.5 mg (5 mg/m³), 1 mg (10 mg/m³),
and 2 mg (2 mg/m³) BHT. They were allowed to equilibrate overnight at room temperature. They were
desorbed with 3 mL of the desorbing solution for 30 minutes with shaking, and were analyzed by GC-FID. The overall
average was 96.6%.(Table 2.3.1)
Table 2.3.1
Desorption Efficiency
|
Portion # |
|
% Recovered |
|
0.1 mg |
0.5 mg |
1.0 mg |
2.0 mg |
|
1
2
3
4
5
6 |
94.2
94.3
95.4
97.2
95.7
96.2 |
97.6
94.8
96.7
96.3
97.0
96.2 |
96.3
96.4
96.4
95.6
98.8
97.7 |
97.4
98.3
97.0
97.1
98.8
96.8 |
|
|
|
|
|
average |
95.5 |
96.4 |
96.9 |
97.6 |
overall average
standard deviation |
96.6
±1.23 |
|
|
|
2.3.2 Six filters were placed into separate mL vials and spiked at each loading of. 0.1 mg (1 mg/m³),
0.5 mg (5 mg/m³), 1 mg (10 mg/m³), and 2 mg (20 mg/m³) BHT. They were allowed to
equilibrate overnight at room temperature. They were extracted with 3 mL of the desorbing solution for 30 minutes
with shaking, and were analyzed by GC-FID. The overall average was 100%.(Table 2.3.2)
Table 2.3.2
Desorption Efficiency
|
Filter # |
|
% Recovered |
|
0.1 mg |
0.5 mg |
1.0 mg |
2.0 mg |
|
1
2
3
4
5
6 |
99.3
99.4
101
101
100
100 |
99.5
99.5
99.9
101
99.3
100 |
101
99.2
101
99.0
98.5
98.7 |
100
100
101
101
101
98.7 |
|
|
|
|
|
average |
100 |
99.9 |
99.6 |
100 |
overall average
standard deviation |
100
±0.863 |
|
|
|
2.4 Retention efficiency
The filters of six OVS-7 tubes were spiked with 1.0 mg (10 mg/m³) BHT, allowed to equilibrate for 6
hours, and had 100 liters humid air (89% RH) pulled through them. The glass fiber filter was placed before the
Teflon spacer to insure that no BHT spiked onto the filter was in contact with the XAD-7 sections before the humid
air was drawn. They were opened, desorbed, and analyzed by GC-FID. The retention efficiency averaged 99.3%. There
was no BHT found on the backup portions of the tubes. The amount found on the front adsorbent portion of tile OVS-7
tubes indicates that BHT is too volatile to be collected on glass fiber filters. (Table 2.4)
Table 2.4
Retention Efficiency
|
Tube # |
% Recovered
Filter |
% Recovered
'A' |
% Recovered
'B' |
Total |
|
1
2
3
4
5
6 |
65.7
65.9
53.1
65.9
71.7
73.2 |
34.4
34.3
44.7
34.5
28.5
25.0 |
0.0
0.0
0.0
0.0
0.0
0.0 |
100
100
97.8
100
100
98.2 |
|
|
|
|
|
|
|
|
average |
99.3 |
|
2.5 Storage
Glass fiber filters (GFF) from the OVS-7 tubes were removed and spiked with 1.0 mg (10 mg/m³) BHT and
were placed in a 4 mL vial containing the front section of the XAD-7 resin from the OVS-7 tube, and stored at room
temperature, in room light, until opened and analyzed. The recoveries averaged 98.5% for the 14 days stored. The BHT
vaporized off of the glass fiber filters and was absorbed by the XAD-7 resin. The longer the samples were stored,
the more of the BHT was absorbed by the XAD-7 resin. (Table 2.5)
Table 2.5
Storage Study
|
Day |
GFF |
% Recovered
XAD-7 |
Total |
|
7
7
7
14
14
14 |
0.1
0.1
0.0
0.0
0.0
0.0 |
98.5
97.6
98.7
96.4
99.7
100
average |
98.6
97.7
98.7
96.4
99.7
100
98.5 |
|
2.6 Precision
The precision was calculated using the area counts from six injections of each standard at concentrations of 33.3
µg/mL (1 mg/m³), 167 µg/mL (5 mg/m³), 333 µg/mL (10 mg/m³), and 667 µg/mL (2
mg/m³) BHT in the desorbing solvent. The pooled coefficient of variation was 0.00904.(Table 2.6)
Table 2.6
Precision Study
|
Injection
Number |
33.3 µg/mL |
167 µg/mL |
333 µg/mL |
667 µg/mL |
|
1
2
3
4
5
6
|
13243
13093
13313
13170
13229
13118
|
64377
63760
63894
65779
64064
64332
|
129549
129074
130334
132099
130739
132667
|
261860
259408
258086
261404
262641
260757
|
Average
Standard
Deviation
CV
Pooled CV |
13194
±82.9
0.00628
0.00904 |
64368
±732
0.0114 |
130744
±1408
0.0108 |
260693
±1680
0.00644 |
|
where:
![equation 1 equation 1](https://webarchive.library.unt.edu/eot2008/20081106073344im_/http://www.osha.gov/dts/sltc/methods/images/eq_cv.gif)
A(1), A(2), A(3), A(4) = # of injections at each level
CV1, CV2, CV3, CV4 = coefficients at each level
2.7 Air volume and sampling rate studied
2.7.1 The air volume studied is 100 liters.
2.7.2 The sampling rate studied is 1 liter per minute.
2.8 Interferences
Suspected interferences should be listed on sample data sheets.
2.9 Safety precautions
2.9.1 Sampling equipment should be placed on an employee in a manner that does not interfere with work performance
or safety.
2.9.2 Safety glasses should be worn at all times in designated areas.
2.9.3 Follow all safety practices that apply to the workplace being sampled.
3. Analytical method
3.1 Apparatus
3.1.1 Gas chromatograph equipped with a flame ionization detector. A HP 5890 gas chromatograph was used in this
study.
3.1.2 GC column capable of separating the analyte and an internal standard from any interferences. The column used
in this study was a 15-meter DB-WAX capillary column 0.25-µm d.f., 0.32-mm I.D.
3.1.3 An electronic integrator or some other suitable method of measuring peak areas.
3.1.4 Two and four milliliter vials with Teflon-lined caps.
3.1.5 A 10-µL syringe or other convenient size for sample injection.
3.1.6 Pipettes for dispensing the desorbing solution. The Glenco 1-mL dispenser was used in this method.
3.1.7 Volumetric flasks - 5 mL and other convenient sizes for preparing standards.
3.1.8 An analytical balance capable of weighing to the nearest 0.01 mg.
3.2 Reagents
3.2.1 Purified GC grade nitrogen, hydrogen, and air.
3.2.2 Di-tert-butyl-p-cresol, Reagent grade.
3.2.3 Methanol, HPLC grade.
3.2.4 Dimethyl formamide, Reagent grade.
3.2.5 The desorbing solution is 0.25 µL/mL dimethyl formamide in methanol.
3.3 Sample preparation
3.3.1 Sample tubes are opened and the glass fiber filter and the front and back section of each tube are placed
in separate 4-mL vials.
3.3.2 Each section is desorbed with 3 mL of the desorbing solution.
3.3.3 The vials are sealed immediately and allowed to desorb for 30 minutes on a shaker, a roto-rack, or a sample
rocker.
3.3.4 Samples were transferred to two milliliter vials for analysis, as this was the size needed to fit in the
autosampler.
3.4 Standard preparation
3.4.1 Standards are prepared by diluting a known quantity of BHT with the desorbing solution.
3.4.2 At least two separate stock standards should be made. Dilutions of the stock standards are prepared covering
the concentrations in the samples. The analytical standards used in this study ranged from 0.000333 to 1.0 mg/mL of
BHT in the desorbing solution.
3.5 Analysis
![chromatogram chromatogram](https://webarchive.library.unt.edu/eot2008/20081106073344im_/http://www.osha.gov/dts/sltc/methods/partial/pv2108/figure2.gif)
Figure 2. An analytical standard of 333 µg/mL BHT in methanol with 0.25 µL/mL dimethyl formamide
internal standard.
3.5.1 Gas chromatograph conditions.
Flow rates (mL/min) |
Temperature (°C) |
|
Nitrogen (makeup) :
Hydrogen (carrier) :
Air :
Hydrogen (detector):
Injection size :
Elution time :
Chromatogram : |
30
1.5
450
60
1 µL
4.26 min
(See Figure 2) |
Injector:
Detector:
Column: |
220
250
90°C - 1 min
10°C/min - 150°C |
3.5.2 Peak areas are measured by an integrator or other suitable means.
3.6 Interferences (analytical)
3.6.1 Any compound having the general retention time of the analyte or the internal standard used is an
interference. Possible interferences should be listed on the sample data sheet. GC parameters should be adjusted if
necessary so these interferences will pose no problems.
3.6.2 Retention time data on a single column is not considered proof of chemical identity. Samples over the target
concentration should be confirmed by GC/Mass Spec or other suitable means.
3.7 Calculations
3.7.1 The instrument was calibrated with a standard of 0.667 mg/mL BHT in the desorbing solution. The linearity
of the calibration is checked with a standards ranging from 0.000333 to 1 mg/mL BHT in the desorbing solution.
3.7.2 If the calibration is non-linear, a calibration curve is plotted. The area counts for the samples are plotted
with the calibration curve to obtain the concentration of BHT in solution.
3.7.3 To calculate the concentration of analyte in the air sample the following formulas are used:
![equation 3 equation 3](https://webarchive.library.unt.edu/eot2008/20081106073344im_/http://www.osha.gov/dts/sltc/methods/images/eq_analyte_concn2.gif)
3.7.4 The above equations can be consolidated to form the following formula. To calculate the ppm of analyte in the
sample based on a 100 liter air sample:
ppm = |
(µg/mL)(DV)(24.46)(106)(g)(mg)
(100 L)(DE)(MW)(1000 mg)(1000 mg) |
µg/mL
24.46
MW
DV
100 L
DE
|
=
=
=
=
=
= |
concentration of analyte in sample or standard
Molar volume (liters/mole at 25°C and 760 mmHg.
Molecular weight (g/mole)
Desorption volume of 3 mL
100 liter air sample
Desorption efficiency |
3.7.5 This calculation is done for each section of the sampling tube and the results added together.
3.8 Safety precautions
3.8.1 All handling of solvents should be done in a hood.
3.8.2 Avoid skin contact with all chemicals.
3.8.3 Wear safety glasses, gloves and a lab coat at all times.
4. Recommendations for further study
Collection studies need to be performed.
5. References
5.1 "NIOSH Manual of Analytical Methods", U.S. Department of Health, Education, and Welfare, Public
Health Service, Center for Disease Control, National Institute for Occupational Safety and Health, 1976, Method 226.
5.2 Cummins, K., Method 32, "Phenol and Cresol", Organic Methods Evaluation Branch, OSHA Salt Lake
Technical Center, 1986.
5.3 Windholz, M., "The Merck Index", Eleventh Edition, Merck & Co., Rahway N.J., 1989, p.238.
5.4 "Documentation of the Threshold Limit Values and Biological Exposure Indices", Fifth Edition, American
Conference of Governmental Industrial Hygienists Inc., Cincinnati, OH, 1986, p. 227.
|