Abstract
Objectives: The purpose of this research was to develop a simultaneous determination method for monoethanol-amine (MEA) and diethanolamine (DEA) in workplace air for risk assessment. Methods: The characteristics of the proposed method, such as recovery, quantitation limit, reproducibility and storage stability of the samples, were examined. Results: An air sampling cassette containing two sulfuric acid-treated glass fiber filters was chosen as the sampler. The MEA and DEA were extracted from the sampler filters, derivatized with 9-fluorenylmethyloxycarbonyl chloride and then analyzed by a high-performance liquid Chromatograph equipped with a fluorescence detector or photo-diode array detector. The overall recoveries from spiked samplers were 86–99 and 88–99% for MEA and DEA, respectively. The recovery after 5 days of storage in a refrigerator exceeded 95%. The overall limits of quantitation were 0.750 and 0.100 jug/sample for MEA and DEA, respectively. The relative standard deviations, which represent the overall reproducibility defined as precision, were 0.3–1.6 and 0.4–5.7% for MEA and DEA, respectively. Conclusions: The proposed method enables 4-h personal exposure monitoring of MEA and DEA at concentrations equaling 1/3,000–2 times the threshold limit value-time-weighted average (TLV-TWA: 3 ppmfor MEA, 1 mg/m3 for DEA) adopted by the American Conference of Governmental Industrial Hygienists and also by the Japan Society for Occupational Health. The method is useful for estimating worker exposure to MEA and DEA.
Introduction
Monoethanolamine (MEA) was listed as one of the target chemicals in a project on workplace risk assessment carried out by the Ministry of Health, Labour and Welfare (MHLW) of Japan in 20101). The occupational exposure limit (threshold limit value-time-weighted average, TLV-TWA) for MEA adopted by the American Conference of Governmental Industrial Hygienists (ACGIH)2) and also by the Japan Society for Occupational Health (JOH)3) is 3 ppm. Diethanolamine (DEA) is similar to MEA in molecular structure, and its TLV-TWA, adopted by the ACGIH2), is 1 mg/m3.
Filter cassettes containing sulfuric acid-treated glass fiber filters were used as air samplers for alkanol-amines (such as MEA and DEA) in a recent report4). Most analytical methods for alkanolamines require a derivatization procedure because direct analysis is generally difficult5–7). Serbin et al. reported a simple derivatization procedure using 9-fluorenylmethyloxy-carbonyl chloride (FMOC-Cl) for high-performance liquid chromatography (HPLC) with fluorescence detection7).
The purpose of this study was the development of a robust method for simultaneous determination of MEA and DEA in workplace air with the sampling capacity and sensitivity required for the MHLW exposure survey. We modified the quantitation method of Serbin et al., and combined it with a sampling method using sulfuric acid-treated glass fiber filters.
Materials and Methods
Materials
MEA and DEA were purchased from Sigma-Aldrich Japan (Tokyo, Japan), and FMOC-Cl was purchased from Tokyo Kasei Kogyo (Tokyo, Japan). All other reagents were analytical grade or better. A 1 M borate buffer was prepared by dissolving boric acid in water and adjusting the pH to 7.7–8.0 with 6 M NaOH. An air sampling cassette (Catalog No. 225-3LF, SKC Inc., Eighty Four, PA, USA) containing two sulfuric acid-treated glass fiber filters (Catalog No. 303, GASTEC Corporation, Kanagawa, Japan) was used as the sampler. Sampling was performed by drawing air through the sampler using an Air Check 2000 (SKC Inc., Eighty Four, PA, USA).
Instruments
The chromatographic analysis was performed on a Shimadzu (Kyoto, Japan) Prominence UFLC equipped with a RF-20Axs fluorescence detector (FL) or an SPD-M20A photo-diode array detector (PDA). Separation was achieved using an Ascentis RP-Amide (150 mm × 4.6 mm I.D., 3 μm; Sigma-Aldrich Co., St Louis, MO, USA) at a flow rate of 1.0 ml/min and at 50°C. The mobile phase was composed of (A) water and (B) acetonitrile using a step gradient elution of 45% B for 0–8 min, 90% B for 8.01–10 min and then 45% B for 10.01–20 min. Detection wavelengths were set at 272 nm (excitation wavelength) and 311 nm (emission wavelength) for FL and 265 nm for PDA.
Sample preparation
After sampling, the front and back sample filters were placed in separate glass test tubes. After adding 0.15 M NaOH (5 ml) to each tube, the tubes were shaken for 5 min and then centrifuged at 3,000 rpm for 10 min. A 200-μl aliquot of each sample was transferred to another glass test tube. After adding 1 M borate buffer (300 μl) and 15 mM FMOC in acetonitrile (500 μl) to each tube, the tubes were vortex-mixed for 1 min and then allowed to stand for 5 min at room temperature. The tubes were then vortex-mixed for 1 min again after adding 10% (v/v) N-methylmorpholine in acetonitrile (10 ul) and acetic acid (50 μl) to each tube. An aliquot of 5 μl of the samples was injected into the HPLC.
Preparation of spiked samplers for retention efficiency and storage stability tests
A 20 μl of MEA and DEA mixed standard solution diluted with 2.75 M H2SO4 solution was spiked onto the front filter of five samplers. Simultaneously, room air (temperature, 21–25°C; relative humidity, 21–25%) was drawn through the samplers at a flow rate of 1 l/min for 240 min.
For the retention efficiency test, the spiked amounts were varied from 0.747 to 3,580 μg and from 0.104 to 497 μg for MEA and DEA, respectively, for a 240 l of sampling volume; these correspond to air concentrations of approximately 1/3,000–2 times the TLV-TWAs. Small amounts of MEA and DEA were detected in blank samplers, and these amounts were taken into account in the calculations. For storage stability tests, three different amounts (1.79, 17.9 and 1,790 μg for MEA and 0.248, 2.48 and 248 μg for DEA) were spiked onto the filter for a 240 l of sampling volume; these correspond to air concentrations of approximately 0.001, 0.01 and 1 times the TLV-TWAs. Air was drawn through the spiked samplers, and they were then sealed and stored in a refrigerator (4°C) for 5 days.
Results and Discussion
Modification of the published sample preparation procedure and optimization of the analytical conditions for HPLC analysis
The method of Serbin et al. was modified for derivatization of MEA and DEA extracted from the filters used in this method. In their method, the excess FMOC was removed by extraction with pentane7). However, that procedure was not sufficient to remove excess FMOC and may cause loss of the analytes. To overcome this problem, 10% N-methylmorpholine was added as a catalyst for hydrolysis of excess FMOC, and 9-fluorenylmethanol (FMOC-OH) was produced as its hydrolysis product. Serbin et al. also reported that diisopropanolamine and MEA did not completely separate under their HPLC analytical conditions. Therefore, the HPLC analytical conditions (the type of column, column oven temperature and composition of the mobile phase) were optimized to ensure satisfactory separation. The detection wavelengths were also optimized for the modified sample preparation procedure, and a step gradient elution program was adopted to clean the column (Fig. 1).
Retention efficiency of the acid-treated filter
The sampling capacity required for the MHLW exposure survey was 240 l (1 l/min, 240 min), and this was evaluated based on the results of the recovery test. No MEA or DEA was detected on the back filter of any of the spiked samplers. Therefore, the acid-treated filter is suitable as a sampler for the MHLW exposure survey of MEA and DEA. The overall recoveries from spiked samplers were 86–99 and 88–99% for MEA and DEA, respectively (Table 1). We consider these results to be satisfactory.
Table 1.
Recovery from spiked sampler
| Spiked amount (μg) |
Recoverya (%) |
RSDb (%) |
| MEA |
|
|
| 0.747 |
86 ± 1.4 |
1.6 |
| 1.79 |
89 ± 0.7 |
0.8 |
| 17.9 |
97 ± 0.6 |
0.6 |
| 179 |
98 ± 0.3 |
0.3 |
| 1,790 |
98 ± 0.8 |
0.8 |
| 3,580 |
99 ± 0.9 |
0.9 |
| DEA |
|
|
| 0.104 |
89 ± 5.1 |
5.7 |
| 0.248 |
88 ± 2.5 |
2.8 |
| 2.48 |
95 ± 0.9 |
1.0 |
| 24.8 |
96 ± 0.4 |
0.4 |
| 248 |
98 ± 0.8 |
0.8 |
| 497 |
99 ± 0.9 |
0.9 |
A 20 μl of monoethanolamine (MEA) and diethanolamine (DEA) mixed standard solution diluted with 2.75 M H2SO4 solution was spiked onto the front filter of five samplers. Simultaneously, room air was drawn through the samplers at a flow rate of 1 l/min for 240 min. The spiked amounts correspond to air concentrations of approximately 1/3,000–2 times the TLV-TWAs.
a Values are expressed as means ± SD (n=5).
b Relative standard deviation.
Storage stabilities were evaluated by comparing the amounts of MEA and DEA remaining in storage samples with the amounts of MEA and DEA in the samples analyzed immediately after preparation. After 5 days of storage, the recoveries from all the spiked samplers exceeded 95%, indicating that MEA and DEA on an acid-treated filter can be stored for at least 5 days in a refrigerator.
Limit of quantitation and reproducibility
The calibration curves for MEA and DEA by FL exhibited linearity in the ranges of 0.0359–35.9 and 0.00483–48.3 μg/ml, respectively, and those by PDA were 0.0359–718 and 0.0201–96.6 μg/ml, respectively, with correlation coefficients of above 0.999. The FL response was approximately 6–9 times greater than the PDA response; however, the linear range of the FL calibration curve was narrower than for the PDA curve. Therefore, samples that exceeded the linear portion of the FL calibration curve were quantified using the PDA calibration curve. We defined the smallest amount required to obtain a recovery of more than 80% from the results of the recovery test as the overall limit of quantitation (LOQ). The overall LOQs were 0.750 and 0.100 μg/sample for MEA and DEA, respectively, and were more than 6.5-times larger than the sampler blank. Therefore, the measurable air concentration ranges for the proposed method are from 1 to 6,000 ppb for MEA and from 0.3 to 2,000 μg/m3 for DEA, respectively, with a 4-h sample. The relative standard deviations (RSD) of the overall reproducibility of the proposed method, including sampling and analysis, were 0.3–1.6 and 0.4–5.7% for MEA and DEA, respectively (Table 1). This range of RSD values indicates that the proposed method has good reproducibility.
Conclusions
The proposed method enables 4-h personal exposure monitoring of MEA and DEA at concentrations equaling 1/3,000–2 times the ACGIH or JOH TLV-TWA, and will be useful for estimating worker exposures to MEA and DEA.
Acknowledgments
This study was commissioned by the Ministry of Health, Labour and Welfare of Japan.
References
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