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Related Information: Chemical Sampling - Hydroquinone


Method no.:  PV2094
 
Control no.: T-PV2094-01-9403-CH
 
Matrix: Air
 
Target concentration: 2 mg/m OSHA TWA PEL
 
Procedure: Samples are collected by drawing a known volume of air through an XAD-7 tube coated with 10% phosphoric acid. Samples are desorbed with methanol and analyzed by gas chromatography with a flame ionization detector (GC-FID) using capillary column. Better sensitivity can be obtained using liquid chromatography with an ultraviolet detector (LC-UV).
 
Air volume and
sampling rate studied:
20 liters at 0.2 L/min
 
Status of method: Stopgap method. This method has been only partially evaluated and is presented for information and trial use.
 
Date: September 1992
Revised: March 1994
Chemist: Mary E. Eide
   
Organic Service Branch I
OSHA Analytical Laboratory
Sandy, Utah 84070

1. General Discussion

1.1 Background

1.1.1 History of procedure

OSHA has an exposure standard for hydroquinone at a level of 2 mg/m³ TWA. NIOSH method 5004 collects hydroquinone on a mixed cellulose ester filter and field extraction within one hour of collection with a 1% acetic acid solution (Ref. 5.1). The acetic acid is to prevent hydroquinone from isomerizing to benzoquinone. Retention studies performed at the OSHA lab with humid air (91% RH) showed vaporization of the hydroquinone off of the filters, with only 81% recovery on filters analyzed immediately after the air was drawn. OSHA Method 39 recommends collection of pentachlorophenol on OVS-7 tubes and desorption with methanol (Ref. 5.2), since hydroquinone is similar to pentachlorophenol, this means of collection and analysis were attempted. The hydroquinone sublimed off the glass fiber filter, and collected on the XAD-7 resin. There it isomerized to benzoquinone in the presence of water vapor, and the benzoquinone reacted with the XAD-7 resin. This isomerization continued with storage, with more benzoquinone being formed each day stored. To prevent the isomerization of hydroquinone to benzoquinone, a phosphoric acid coated XAD-7 resin was used for a retention study. No loss of the hydroquinone was observed. This media was further evaluated and found to have good retention, desorption, and storage.

1.1.2 Potential workplace exposure (Ref. 5.3)

Hydroquinone is used as a depigmentor, a photographic reducer and developer, a reagent in determination of small quantities of phosphates, and an antioxidant in oils and greases. It is used as an intermediate in the manufacture of dyes. In human medicine, it is an ingredient in topical creams for blemishes, and in bleaching creams.

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)

Hydroquinone is a skin, eye, mucous membrane, and gastrointestinal irritant. In humans, ingestion of 1 gram of hydroquinone has caused tinnitus, nausea, vomiting, shortness of breath, cyanosis, convulsions, delirium, and collapse. Death has occurred in some individuals after ingestion of 5 grams of hydroquinone. Workplace exposure for greater than 5 years, to levels that had no systemic effects, caused staining and opacification of the cornea.

1.1.4 Physical properties (Ref. 5.3):

Synonyms: 1,4-Benzenediol; Quenelle; p-dihydroxybenzene; Hydroquinol; Aids; Black and White Bleaching cream; Eldoquin; Eldopaque; Quinine; Tecquinol
Molecular weight: 110.11
Melting point: 170-171ºC
Boiling point: 285-287ºC
Flash point: 165ºC (329ºF) (closed cup)
Odor: phenolic
Color: white to light yellow crystals which turn brown with exposure to light and air
Molecular formula: C6H6O2
CAS: 123-31-9
IMIS: 1490
RTECS: MX3500000; 41010
DOT: UN 2662 (Poison)
Structure: structure

1.2 Limit defining parameters

1.2.1 The detection limit of the analytical procedure is 1 µg hydroquinone. This is the smallest amount that could be detected under normal operating conditions.

1.2.2 The overall detection limit is 0.05 mg/m³. (All mg/m³ amounts in this study are based on a 20 liter air volume and 1 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, or LC parameters if liquid chromatography is used.

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 XAD-7 tubes containing 20/60 mesh XAD-7 coated with 10% phosphoric acid, with an 80 mg adsorbing section and a 40 mg backup section, separated by a silane treated glass wool plug before, after, and between the adsorbing sections. 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., SKC tubes or equivalent.

2.1.3 Tubes are available through SKC, catalog number 226-98, or can be prepared from phosphoric acid coated XAD-7 resin, which is prepared in the following manner.

Approximately 100 grams of Amberlite XAD-7 20/60 mesh, a porous polyacrylate adsorbent manufactured by Rolm and Haas, was washed several, times with 100 mL deionized water to remove the fine particles. The resin was washed three times with 100 mL methanol, then three times with acetonitrile, and the excess acetonitrile was removed by vacuum filtration. The resin was placed in a round bottom flask and treated with a solution of 14 mL reagent grade phosphoric acid in 200 mL acetonitrile. It was allowed to stand for 10 minutes, then the resin was dried using a rotary evaporator. The acid-coated XAD-7 resin, with the odor of acetonitrile present, was the stored in a tightly sealed container or packed into tubes.

2.2 Sampling technique

2.2.1 Open the ends of the coated XAD-7 tubes immediately before sampling.

2.2.2 Connect the coated XAD-7 tubes 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 coated XAD-7 tubes.

2.2.5 Seal the coated XAD-7 tubes 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

Six tubes were spiked with loadings of 4.0 µg (0.2 mg/m³), 20 µg (1 mg/m³), 40 µg (2 mg/m³), and 80 µg (4 mg/m³) hydroquinone. 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 methanol with 0.25 µL/mL dimethyl formamide internal standard, desorbed for 30 minutes with shaking, and analyzed by GC-FID. The overall average was 100%.(Table 2.3)

Table 2.3
Desorption Efficiency

Tube# 4.0 µg % Recovered
20 µg
40 µg 80 µg

1 100 101 103 103
2 103 101 103 101
3 102 99.4 98.0 98.6
4 102 97.3 103 98.1
5 100 98.7 98.2 103
6 100 100 95.8 97.6
average 101 99.6 100 100
 
overall average 100
standard deviation ±2.19

2.4 Retention efficiency

The six coated XAD-7 tubes were spiked with 40 µg (2.0 mg/m³) hydroquinone, allowed to equilibrate overnight, and had 20 liters humid air (91% RH) pulled through them. They were opened, desorbed, and analyzed by GC-FID. The retention efficiency averaged 99.7%. There was no hydroquinone found on the backup portions of the tubes. (Table 2.4)

Table 2.4
Retention Efficiency

Tube # %Recovered 'A' % Recovered 'B' Total
1 101 0.0 101
2 100 0.0 100
3 98.7 0.0 98.7
4 101 0.0 101
5 97.5 0.0 97.5
6 99.8 0.0 99.8
       
    average 99.7

2.5 Storage

Coated XAD-7 tubes were spiked with 40 µg (2.0 mg/m³) hydroquinone and stored at room temperature, in room light, until opened and analyzed. The recoveries averaged 97.4% for the 14 days stored. (Table 2.5)

Table 2.5
Storage Study

Day % Recovered

7 97.6
7 96.6
7 95.7
14 100
14 98.3
14 96.0
   
overall average 97.4

2.6 Precision

The precision was calculated using the area counts from six injections of each standard at concentrations of 4.0, 20, 40, and 80 µg/mL hydroquinone in the desorbing solution. The pooled coefficient of variation was 0.0388. (Table 2.6)

Table 2.6
Precision Study

Injection Number 4.0 µg/mL 20 µg/mL 40 µg/mL 80 µg/mL
1 1186 5715 11525 22647
2 1187 5636 11428 22860
3 1149 5650 11525 22782
4 1169 5777 11391 22745
5 1162 5750 11587 22853
6 1136 5814 11612 23155
 
Average 1165 5724 11511 22840
 
Standard Deviation ±88.4 70.5 86.8 173
CV 0.0759 0.0123 0.00754 0.00757
Pooled CV 0.0388

where:
equation

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 20 liters.

2.7.2 The sampling rate studied is 0.2 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 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 df, and 0.32 mm I.D.

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 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.

3.1.9 If liquid chromatography is used for analysis the instrumentation is a liquid chromatograph equipped with an autosampler and an ultraviolet detector, and a C18 column. A Waters 510 pump, 710 B WISP, 440 detector with extended wavelength module were used. The column was a Supelco LC-18-DB, 5µ, 25 cm × 6 mm.

3.2 Reagents

3.2.1 Purified GC grade nitrogen, hydrogen, and air. (for GC analysis only)

3.2.2 Hydroquinone, Reagent grade

3.2.3 Methanol, HPLC grade

3.2.4 Dimethyl formamide, Reagent grade

3.2.5 Desorbing solution is 0.25 µL/mL dimethyl formamide internal standard in methanol. If analysis is performed by liquid chromatography, it may be better to not use an internal standard, as on many C18 columns it is difficult to separate the dimethyl formamide from the hydroquinone.

3.2.6 Deionized water (for mobile phase for LC analysis only)

3.2.7 Phosphoric acid (for mobile phase for LC analysis only)

3.3. Sample preparation

3.3.1 Sample tubes are opened and each section of each tube are placed in separate 2 mL vials, along with the separating glass wool.

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 on a shaker, a roto-rack, or a sample rocker.

3.4 Standard preparation

3.4.1 Standards are prepared by diluting a known quantity of hydroquinone with the desorbing solution.  Stock solutions also had 0.1 mL/L of H3PO4 added to them.

3.4.2 At least two separate stock standards should be made. Dilutions of the stock standards are prepared to bracket the samples. For this study, standards ranged from 1 to 80 µg/mL.

3.5 Analysis

3.5.1 Gas chromatograph conditions.

Flow rates (mL/min) Temperature (ºC)
Nitrogen (makeup): 30 Injector: 240
Hydrogen (carrier): 1.5 Detector: 240
Air: 450 Column: 80º for 2 min
Hydrogen (detector): 30 then 10ºC/min to 200ºC
Injection size: 1 µL
Elution time: 15.69 min
Chromatogram: 

Figure 1

Figure 1. An analytical standard of 40 µg/mL hydroquinone in methanol with 0.25 µL/mL dimethyl formamide internal standard, and analyzed by gas chromatography.

3.5.2. Liquid chromatograph conditions.

Column: 5 µ Supelco LC-18-DB, 6 × 250 mm
Mobile Phase: 1 mL of 0.1:5:95 phosphoric acid:methanol:water (if using methanol with the dimethyl formamide internal standard)
Mobile Phase: 1 mL/min of 0.1:25:75 phosphoric acid:methanol:water (if using methanol only as desorbing solvent)
Injection size: 10 µL
Detector: UV at 219 nm (UV max is 223 nm, secondary max is 200 nm, tertiary max is 291 nm) (note: if the analysis is performed at 219 the DMF can be used as an internal standard, but if 291 nm is used the DMF does not have a response at that wavelength)

Chromatogram:

Figure 2

Figure 2. An analytical standard of 40 µg/mL hydroquinone in methanol with 0.25 µL/mL dimethyl formamide internal standard, analyzed by liquid chromatography with an UV detector at 219 nm, and using a mobile phase of 0.1:5:95 H3PO4:methanol:water.

Figure 3

Figure 3. An analytical standard of 40 µg/mL hydroquinone in methanol with 0.25 µL/mL dimethyl formamide internal standard, analyzed by liquid chromatography with an UV detector at 219 nm, and using a mobile phase of 0.1:25:75 H3PO4:methanol:water.

The following chromatogram was analyzed on an HP5890 Series II gas chromatogram with a flame ionization detector. The column was a 60-m 0.32-mm i.d. capillary column with a 0.25 µm DB-1 film thickness. The temperature program was 80°C for 4 min the 10°C/min to 160°C hold 5 min, with the injector at 200°C and the detector at 250°C.

Figure 4

Figure 4. A chromatogram of 75 µg/mL hydroquinone in methanol with DMF internal standard.

3.5.3 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 dingle column is not considered proof of chemical identity. Samples over the target concentration should be confirmed by GC/Mass Spec or other suitable means.

3.6.3 There is a reaction between the excess phosphoric acid and the methanol to form trimethyl phosphate. On the column used in this study trimethyl phosphate eluted at 10 minutes. The amount formedis approximately 50 µg. If another column is used for this analysis, the trimethyl phosphate should be separated from the other peaks.

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 hydroquinone in solution.

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

(µg/mL)(desorption volume)
desorption efficiency
 =  mass of analyte in sample

mass of analyte in sample
air volume in liters
 =  µg/L of analyte

mg³ of analyte  =  µg x mg x 1000 L
L x 1000 µg x m³

3.7.4 The above equations can be consolidated to form the following formula. To calculate the mg/m³ of analyte in the sample based on a 20 liter air sample:

mg/m³  =  (µg/mL)(DV)(mg)(1000 L)
(20 L)(DE)(1000 µg)(m³)

where:

µg/mL = concentration of analyte in sample or standard
DV = Desorption volume
20 L = 20 liter air sample
DE = 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 study should be performed.

5. References

5.1 "NIOSH Manual of Analytical Methods", U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, Third Edition, Method 5004.

5.2 Cummins, K., Method 39, "Phenol and Cresol", Organic Method's Evaluation Branch, OSHA Salt Lake Technical Center, 1982.

5.3 Windholz, M., "The Merck Index", Eleventh Edition, Merck Co., Rahway N.J., 1989, p. 762.

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. 319.