PROPANE

SOLVENTS BRANCH
 OSHA ANALYTICAL LABORATORY 
SALT LAKE CITY, UTAH 

1. General Discussion 

1.1 Background 

1.1.1 History of procedure
 
Propane has been monitored by combustible gas 
meter (Ref. 5.1). An attempt to collect propane in a gas bag showed a loss of propane from the gas bag atmosphere with time. The gas bag was stored at room temperature for 6 days, and showed an average 30.8% recovery. A solid sorbent tube would be convenient for the industrial hygienist, and allow for quantitation with confirmation. Two Carbosieve S-III tubes in series showed good desorption, retention and storage.

1.1.2 Potential workplace exposure (Ref. 5.2)
 
Propane is used as a household and industrial 
fuel. Propane is used in organic synthesis, the manufacture of ethylene, as an extractant solvent, refrigerant, gas enricher, an aerosol propellant, and in mixtures for bubble chambers.

1.1.3 Toxic Effects (This section is for information purposes and should not be taken as the basis for OSHA policy.) (Ref. 5.2) 

Large quantities of propane in air can be an asphixiant. 

1.1.4 Physical properties (Ref. 5.2):

CAS:	74-98-6
IMIS:	2150
RTECS:	65285 (TX2275000)
DOT:	UN 1075; UN 1978
Molecular formula:	C3H8
Molecular weight:	44.09
Freezing point:	-188°C
Boiling point:	-42°C
Explosive limits:	2.37-9.5% by volume
Flash point:	-105°C (-156°F)
Autoignition point:	467°C (842°F)
Odor:	odorless when pure
Color:	colorless gas
Synonyms:	dimethylmethane; propyl hydride
Structure:

1.2 Limit defining parameters

1.2.1 The detection limit of the analytical procedure is 
4.7 µg/mL propane. This is the smallest amount that could be detected under the operating conditions used in this study. 

1.2.2 The overall detection limit is 0.52 ppm based on a 5 liter air volume. All amounts in this study will be based on a 5 liter air volume.

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 

It is important to use two tubes in series. Both sections of the front tube are used to collect the propane efficiently near the PEL, and the second tube is used to monitor for breakthrough. 

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 Carbosieve S-III tubes, lot 728-39, containing 130-mg adsorbing section with a 65-mg backup section separated by silane treated glass wool, with a silane treated glass wool plug before the adsorbing section and after the backup section. The ends are flame sealed and the glass tube containing the adsorbent is 7 cm long, with a 6 mm O.D. and 4 mm I.D., Supelco tubes (ORBO-91) or equivalent. 

2.2 Sampling technique

2.2.1 The ends of the Carbosieve S-III tube are opened immediately before sampling. 

2.2.2 Connect two Carbosieve S-III tubes in series and connect the second tube to the sampling pump with flexible tubing. 

2.2.3 Tubes should be placed in a vertical position to minimize channeling, with the smaller sections towards the pump. 

2.2.4 Air being sampled should not pass through any hose 
or tubing before entering the first adsorbent tube. 

2.2.5 Seal the Carbosieve S-III tube 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 if submitted for analysis must be shipped in a separate container from the samples.

2.3 Desorption efficiency 

Six tubes were vapor spiked at each loading of 0.793 mg (88.0 ppm), 1.59 mg (176 ppm), 3.97 mg (440 ppm), 7.96 mg (883 ppm), and 15.92 mg (1770 ppm) propane. They were allowed to equilibrate overnight at room temperature. They were opened, each section placed into a separate 2 mL vial, desorbed with 1 mL of the desorbing solution, for 30 minutes with occasional shaking, and were analyzed by GC-FID. The overall average was 105% (Table 2.3). There seemed to be some adsorption of the carbon disulfide onto the Carbosieve S-III, concentrating the solutions, resulting in desorptions greater than 100%. A phase equilibrium study showed the same adsorption of carbon disulfide. A solution of 15.9 mg/mL propane in carbon disulfide was prepared. Three phase equilibrium samples were prepared by adding 1 mL of this solution to 
130 mg of Carbosieve S-III, allowed to equilibrate for 30 minutes with occasional shaking, and analyzed by comparing it to the original solution. The average recovery was 111%. 

2.4 Retention efficiency
 
Six tubes were vapor spiked with 15.9 mg (1770 ppm) propane, and allowed to equilibrate overnight. Breakthrough was measured by placing two Carbosieve S-III tubes in series, then three tubes had either 5 or 10 liters of humid air (91% RH) pulled through them. They were opened, desorbed and analyzed by GC-FID- There was little or no propane found on the backup portions of the second tubes when 5 liters were drawn (Table 2.4). The retention efficiency averaged 102%.

2.5 Storage 

Carbosieve S-III tubes were spiked with 7.93 mg (883 ppm) propane and stored at room temperature until opened and analyzed. The recoveries averaged 98.1% for the 15 days stored (Table 2.5). The recoveries in Table 3 were desorption corrected.

2.6 Precision 

The precision was calculated using the area counts from six injections of each standard at concentrations of 0.793, 3.97, 7.93, and 15.9 mg/mL propane in carbon disulfide with 1 µL/mL p-cymene internal standard. The pooled coefficient of variation was 0.0154 (Table 2.6).

where:

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 5 liters.

2.7.2 The sampling rate studied is 0.1 liters 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. 

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 Hewlett-Packard 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 60-meter DB-5 1.0 µm df capillary column. 

3.1.3 An electronic integrator or some other suitable method of measuring peak areas.

3.1.4 Two milliliter vials with Teflon-lined caps. 

3.1.5 A 10 µL syringe or other convenient size for sample injection. 

3.1.6 A 1000 µl gas-tight syringe for standard preparation. 

3.1.7 Pipets for dispensing the desorbing solution. The Glenco 1 mL dispenser was used in this method. 

3.1.8 Volumetric flasks - 5 mL and other convenient sizes for preparing standards. 

3.2 Reagents 

3.2.1 Purified GC grade nitrogen, hydrogen, and air. 

3.2.2 Propane gas, Reagent grade .

3.2.3 Carbon disulfide, Reagent grade. 

3.2.4 p-Cymene.

3.2.5 The desorbing solution is prepared by adding 250 µL 
p-cymene to I liter of carbon disulfide to obtain a concentration of 0.25 µL/mL p-cymene in carbon disulfide. The p-cymene is used as the internal standard. 

3.3 Sample preparation 

3.3.1 Sample tubes are opened and the front and back section of each tube are placed in separate 2-mL vials. 

3.3.2 Each section is desorbed with 1 mL of the desorbing solution. 

3.3.3 The vials are sealed immediately and allowed to desorb for 30 minutes with occasional shaking. 

3.4 Standard preparation 

3.4.1 Standards are prepared by diluting a known quantity of propane with the desorbing solution. A standard of 1000 µL/mL propane in the desorbing solution at 664 mmHg and 23°C would be 1.59 mg/mL. This was calculated as follows: 

P = Pressure at time of standard preparation. 
T = Temperature at time of standard preparation. 

3.4.2 At least two separate standards should be made. A separate larger standard should be prepared to check the linearity of the response for propane. 

3.5 Analysis

3.5.1 Gas chromatograph conditions.

Flow rates (mL/min)	Temperature (°C)
Nitrogen (makeup): 24 Hydrogen (carrier): 1 Air: 240 Hydrogen (detector): 30	Injector: 180  Detector: 220  Column: 150

Injection size: 1 µL 

Elution time: 4.486 min. 

Chromatogram:

Figure 1. A standard of 1.59 mg/mL propane in carbon disulfide with 0.25 µL/mL p-cymene internal standard.

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 A curve with area counts versus concentration is calculated from the calibration standards. 

3.7.2 The area counts for the samples are plotted with the calibration curve to obtain the concentration of propane in solution. 

3.7.3 To calculate the concentration of analyte in the air sample the following formulas are used: 

*All units must cancel.  

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 5 liter air sample: 

 

µg/mL 24.46 MW DV 5 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 10 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 solvents. 

3.8.3 Wear safety glasses at all times. 

4. Recommendations for further study 

Collection studies from a dynamic atmosphere 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, Second Edition, Vol. 2, Method S-87. 

5.2 Sax, N., Lewis, R., "Hawley's Condensed Chemical Dictionary", Eleventh Edition, Van Nostrand Reinhold Co., New York, 1987, p. 969.