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1.1 Background

1.1.1 History

The OSHA SLTC received samples collected on charcoal tubes requesting analysis for dimethyl succinate (DMSU). A desorption study using carbon disulfide showed poor recovery, 72%, when a concentration of 448 µg DMSU was spiked on the tubes. Desorption studies using 1:99 DMF:CS₂ averaged 93.8% recovery over the concentration range of 22.4 to 448 µg DMSU. The retention study showed no loss of DMSU. The storage studies had a loss of DMSU with samples collected with 20 liters humid air (80% RH at 22°C), especially those stored at room temperature, but samples stored under refrigeration had better recoveries. Storage recoveries, corrected for desorption, on day 7 were: dry refrigerated 101%, dry ambient 100%, humid refrigerated 92.8%, and humid ambient 82.2%. Storage recoveries, corrected for desorption, on day 14 were: dry refrigerated 100%, dry ambient 98.8%, humid refrigerated 86.3%, and humid ambient 76.3%. Samples should be refrigerated as soon as possible after sampling, and should be analyzed within one week of receiving them.

1.1.2 Toxic effects (This section is for information only and should not be taken as the basis of OSHA policy.) (Ref. 5.2)

DMSU is a skin, eye, and mucous membrane irritant. The Canadian recommended exposure limit for DMSU is 10 mg/m³. At the time this study was written, there was no PEL or TLV for DMSU.

1.1.3 Workplace exposure (Ref. 5.2 and 5.3)

DMSU is used as a solvent in paints, lacquers, varnishes, nitrocellulose, paint strippers, dyes, fats, photography, and waxes. DMSU is used in perfumes and flavorings for candy, ice cream, and gum. DMSU is used in the manufacture of other succinates.

1.1.4 Physical properties and other descriptive information (Ref. 5.2, 5.3, and 5.4)

Synonyms:	Butanedioic acid, dimethyl ester; Dimethyl butanedioate; Succinic acid, dimethyl ester
CAS number:	106-65-0
DOT:	NA 1993 (flammable liquid)
IMIS:	D917
RTECS:	WM7675000
Molecular weight:	146.1
Flash point:	85°C (185 °F)(cc)
Boiling point:	200°C
Melting point:	18°C
Odor:	sweet winey or fruity odor
Color:	clear liquid
Density:	1.1198
Molecular formula:	C6H10O4
Structural formula:

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The analyte air concentrations throughout this method are based on the recommended sampling and analytical parameters. Air concentrations listed in ppm are referenced to 25°C and 101.3 kPa (760 mmHg).

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1.2 Limit defining parameters

1.2.1 Detection limit of the overall procedure (DLOP)

The detection limit of the overall procedure is 0.484 µg per sample (0.00405 ppm or 0.0242 mg/m³). This is the amount of analyte spiked on the sampler that will give a response that is significantly different from the background response of a sampler blank.

The DLOP is defined as the concentration of analyte that gives a response (Y_DLOP) that is significantly different (three standard deviations (SD_BR)) from the background response
(Y_BR).

Y_DLOP - Y_BR = 3(SD_BR)

The direct measurement of Y_BR and SD_BR in chromatographic methods is typically inconvenient, and difficult because Y_BR is usually extremely low. inconvenient, and difficult because Y_BR is usually extremely low. Estimates of these parameters can be made with data obtained from the analysis of a series of samples whose responses are in the vicinity of the background response. The regression curve obtained for a plot of instrument response versus concentration of analyte will usually be linear. Assuming SD_BR and the precision of data about the curve are similar, the standard error of estimate (SEE) for the regression curve can be substituted for SD_BR in the above equation. The following calculations derive a formula for the DLOP:

Yobs	=	observed response
Yest	=	estimated response from regression curve
n	=	total no. of data points
k	=	2 for a linear regression curve

At point Y_DLOP on the regression curve

YDLOP = A(DLOP) +YBR

A = analytical sensitivity (slope)

therefore

DLOP =

(YDLOP - YBR) A

Substituting 3(SEE) + Y_BR for Y_DLOP gives

DLOP =

3(SEE) A

The DLOP is measured as mass per sample and expressed as equivalent air concentrations, based on the recommended sampling parameters. Ten samplers were spiked with equal descending increments of analyte, such that the lowest sampler loading was 1.12 µg/sample. This is the amount, when spiked on a sampler, that would produce a peak approximately 10 times the background response for the sample blank. These spiked samplers, and the sample blank were analyzed with the recommended analytical parameters, and the data obtained used to calculate the required parameters (A and SEE) for the calculation of the DLOP. Values of 93.7 and 15.11 were obtained for A and SEE respectively. DLOP was calculated to be 0.484 µg/sample (0.00405 ppm or 0.0242 mg/m³).

Table 1.2.1
Detection Limit of the Overall Procedure

mass per sample	area counts
(µg)	(µV-s)
0	0
1.12	125
2.24	241
3.36	354
4.48	419
5.60	544
6.72	670
7.84	770
8.96	855
10.1	955
11.2	1085

Figure 1.2.1. Plot of data to determine the DLOP/RQL.

1.2.2 Reliable quantitation limit (RQL)

The reliable quantitation limit is 1.61 µg per sample (0.013 ppm). This is the amount of analyte spiked on a sampler that will give a signal that is considered the lower limit for precise quantitative measurements.

The RQL is considered the lower limit for precise quantitative measurements. It is determined from the regression line data obtained for the calculation of the DLOP (Section 1.2.1), providing at least 75% of the analyte is recovered. The RQL is defined as the concentration of analyte that gives a response (Y_RQL) such that
 

Y_RQL - Y_BR 10(SD_BR)

therefore

RQL =

10(SEE) A

RQL = 1.61µg per sample (0.013 ppm)
 

Figure 1.2.2. Plot of data to determine the RQL.

Table 1.2.2
Reliable Quantitation Limit

mass per sample	mass recovered	recovery
(µg)	(µg)	(%)
1.12	0.981	87.6
2.24	2.09	93.3
3.36	3.20	95.2
4.48	4.31	96.2
5.60	5.23	93.4
6.72	6.40	95.2
7.84	7.59	96.8
8.96	8.43	94.1
10.1	9.47	93.8
11.2	10.5	93.8

Figure 1.2.3. Chromatogram of the RQL.

2. Sampling Procedure

2.1 Apparatus

2.1.1 Samples are collected using a personal sampling pump calibrated, with the sampling device attached, to within ±5% of the recommended flow rate.

2.1.2 Samples are collected with tubes 7 cm × 4 mm i.d. × 6 mm o.d. glass sampling tubes packed with two sections of charcoal, lot 120. The front section contains 100 mg and the back section contains 50 mg of charcoal, lot 120. The sections are held in place with glass wool plugs and are separated by a urethane foam plug. For this evaluation, commercially prepared sampling tubes were purchased from SKC Inc., (Eighty Four PA) catalog No. 226-01, Lot 120.

2.2 Technique

2.2.1 Immediately before sampling, break off the ends of the sampling tube. All tubes should be from the same lot.

2.2.2 Attach the sampling tube to the pump with flexible tubing. It is desirable to utilize sampling tube holders which have a protective cover to shield the employee from the sharp, jagged end of the sampling tube. Position the tube so that sampled air passes through the front section of the tube first.

2.2.3 Air being sampled should not pass through any hose or tubing before entering the sampling tube.

2.2.4 Attach the sampling tube vertically with the front section pointing downward, in the worker's breathing zone, and positioned so it does not impede work performance or safety.

2.2.5 After sampling for the appropriate time, remove the sample and seal the tube with plastic end caps. Wrap each sample end-to-end with a Form OSHA-21 seal.

2.2.6 Submit at least one blank sample with each set of samples. Handle the blank sample in the same manner as the other samples except draw no air through it.

2.2.7 Record sample volumes (in liters of air) for each sample, along with any potential interferences.

2.2.8 Ship any bulk samples separate from the air samples.

2.2.9 Submit the samples to the laboratory for analysis as soon as possible after sampling. If delay is unavoidable, store the samples in a refrigerator.

2.3 Desorption efficiency

The desorption efficiencies of DMSU were determined by liquid-spiking the charcoal tubes with the analytes at 0.1 to 2 times the target concentration. The loadings on the tubes were 22.4, 112, 224, and 448 µg of DMSU. These samples were stored overnight at ambient temperature and then desorbed with 1 mL of 1:99 DMF:CS₂ with 0.25 µL/mL p-cymene internal standard, and analyzed by GC-FID. The average desorption efficiency over the studied range was 93.8%.

Table 2.3.
Desorption Efficiency of DMSU

		% Recovery
	0.1 X	0.5 X	1.0 X	2.0 X
Tube #	22.4µg	112µg	224µg	448µg
1	93.6	95.4	94.4	92.4
2	92.7	92.1	92.0	93.1
3	92.4	92.8	95.4	96.1
4	91.5	93.6	96.0	94.3
5	91.6	93.8	94.6	95.2
6	92.1	95.0	95.4	95.7
average	92.3	93.8	94.6	94.5
overall average	93.8
standard	±1.50
deviation

2.4 Retention efficiency

The glass wool in front of the front section of the charcoal tube was pulled towards the end, so that none of it was in contact with the charcoal. The glass wool was spiked with 448 µg DMSU, and the charcoal tube had 24 L humid air (80% RH at 21°C) pulled through it at 0.2 L/min. The glass wool was spiked to determine if DMSU would volatize off the glass wool and collect onto the charcoal. They were opened, desorbed, and analyzed by GC-FID. The retention efficiency averaged 98.7%. The values in Table 2.4 were corrected for desorption efficiency. There was no DMSU found on the glass wool indicating that all of it vaporized off. There was no DMSU on the back sections of the tubes, indicating that no breakthrough occurred.

Table 2.4
Retention Efficiency of DMSU

Tube #		% Recovered
	Glass wool	Front section	Back section	Total
1	0.0	99.3	0.0	99.3
2	0.0	99.3	0.0	99.3
3	0.0	100	0.0	100
4	0.0	100	0.0	100
5	0.0	97.8	0.0	97.8
6	0.0	95.9	0.0	95.9
			average	98.7

2.5 Sample storage

The front sections of twelve sampling tubes were each spiked with 448 µg (3.75 ppm) of DMSU, then six tubes were stored in the refrigerator (-10°C), and six were stored at room temperature 23°C. Twelve more tubes were spiked with 448 µg DMSU, and had 20 liters of humid air (80% RH at 21°C) drawn through them, before six tubes were stored in the refrigerator (-10°C), and six were stored at room temperature 23°C. Three of each type of samples were analyzed after 7 days and the remaining three samples of each type after 14 days. The amounts recovered indicate that humidity and temperature affect the ability of charcoal to retain intact the DMSU. The recoveries decreased with time and/or added humidity, with the worst recovery on day 14 day storage with humidity. Results are corrected for desorption efficiency.

Table 2.5
Storage Test for DMSU

Time (days)	%Recovery Humid Ambient	%Recovery Humid Refrigerated	%Recovery Dry Ambient	%Recovery Dry Refrigerated
7	79.6	93.5	101	101
7	79.7	92.3	101	101
7	87.3	92.6	98.8	100
average	82.2	92.8	100	101
14	74.5	87.0	96.7	101
14	76.1	86.1	98.6	100
14	78.5	85.7	101	100
average	76.3	86.3	98.8	100

2.6 Recommended air volume and sampling rate.

Based on the data collected in this evaluation, 20 L air samples should be collected at a sampling rate of 0.2 L/min.

2.7 Interferences (sampling)

2.7.1 It is not known if any compounds will severely interfere with the collection of DMSU on the sampling tubes. In general, the presence of other contaminant vapors in the air will reduce the capacity of the charcoal tube to collect DMSU.

2.7.2 Suspected interferences should be reported to the laboratory with submitted samples.

2.8 Safety precautions (sampling)

2.8.1 Attach the sampling equipment to the worker in such a manner that it will not interfere with work performance or safety.

2.8.2 Follow all safety practices that apply to the work area being sampled.

2.8.3 Wear eye protection when breaking the ends of the glass sampling tubes.

3.1 Apparatus

3.1.1 The instrument used in this study was a gas chromatograph equipped with a flame ionization detector, specifically a Hewlett Packard model 5890.

3.1.2 A GC column capable of separating the analyte from any interferences. The column used in this study was a 60 meter capillary column with a 0.5 µm coating of DB-WAX, with an I.D. of 0.32 mm.

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

3.1.4 Two milliliter vials with Teflon^TM-lined caps.

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

3.1.6 Pipets for dispensing the desorbing solution. A Repipet® dispenser was used in this study.

3.1.7 Volumetric flasks - 5 or 10 mL and other convenient sizes for preparing standards.

3.2 Reagents

3.2.1 GC grade nitrogen, hydrogen, and air.

3.2.2 Dimethyl succinate (DMSU), Reagent grade

3.2.3 Carbon disulfide (CS₂), Reagent grade

3.2.4 Dimethyl formamide (DMF), Reagent grade

3.2.5 p-Cymene (internal standard), Reagent grade

3.2.6 Desorbing solution was 1:99 DMF:carbon disulfide with 0.25 µL/mL p-cymene internal standard.

3.3 Standard preparation

3.3.1 At least two separate stock standards are prepared by diluting a known quantity of DMSU with the desorbing solution of 1:99 DMF:carbon disulfide with 0.25 µL/mL p-cymene internal standard. The concentration of these stock standards was 0.2 µL/mL or 224 µg/mL.

3.3.2 A third standard at a higher concentration was prepared to check the linearity of the calibration. For this study, two analytical standards were prepared at a concentration of 0.2 µL/mL (224 µg/mL), and one at 1.0 µL/mL (1120 µg/mL) DMSU in the desorbing solution.

3.4 Sample preparation

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

3.4.2 Each section is desorbed with 1 mL of the desorbing solution of 1:99 DMF:carbon disulfide with 0.25 µL/mL p-cymene internal standard.

3.4.3 The vials are sealed immediately and allowed to desorb for 30 minutes with constant shaking.

3.5 Analysis

3.5.1 Gas chromatograph conditions.

Injection size:	1 µL
Flow rates (mL/min)
Nitrogen (make-up):	30
Hydrogen(carrier):	2
Hydrogen(detector):	40
Air:	420
Temperatures (°C)
Injector:	200
Detector:	220
Column:	50° for 2 min then 10°/min to 170° for 15 min

Figure 3.5.1 Chromatogram of an analytical standard at the target concentration. Peak identification: (1) carbon disulfide, (2) p-cymene, (3) DMF, and (4) DMSU.

3.5.2 Peak areas are measured by an integrator or other suitable means.

3.6 Interferences (analytical)

3.6.1 Any compound that produces a response and has a similar retention time as the analyte is a potential interference. If any potential interferences were reported, they should be considered before samples are desorbed. Generally, chromatographic conditions can be altered to separate an interference from the analyte.
 

Figure 3.6.1 A mass spectra of dimethyl succinate (DMSU).

3.6.2 When necessary, the identity or purity of an analyte peak may be confirmed by GC-mass spectrometer or by another analytical procedure.

3.7 Calculations

3.7.1 The instrument was calibrated with a standard of 224 µg/mL DMSU in the desorbing solution. The linearity of the calibration was checked with a standard of 1120 µg/mL.

3.7.2 If the calibration is non-linear, two or more standard at different concentrations must be analyzed, bracketing the samples, so a calibration curve can be plotted and sample values obtained.

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

(µg/m) (desorption volume) (desorption efficiency)

= mass of analyte in sample

(mass of analyte in sample) molecular weight

= number of moles of analyte

(number of moles of analyte)

(molar volume at 25°C & 760mm)

=

volume the analyte will occupy at 25°C & 760mm

(volume analyte occupies) (106)* (air volume)

= ppm

* All units must cancel.

3.7.4 The above equations can be consolidated to the following formula.

(µg/mL)(DV)(24.46)(106)(g)(mg) (20 L)(DE)(MW)(1000mg)(1000µg)

= ppm

µg/mL	=	concentration of analyte in sample or standard
24.46	=	Molar volume (liters/mole) at 25° and 760 mm Hg.
MW	=	Molecular weight (g/mole)
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 (analytical)

3.8.1 Avoid skin contact and inhalation of all chemicals.

3.8.2 Wear safety glasses, gloves and a lab coat at all times while in the laboratory areas.

5.1 Trade names Database on CCINFO CD-ROM Disc 95-2, Canadian Centre for Occupational Health and Safety, Hamilton, Ontario.

5.2 Lide, D.R., "Handbook of Chemistry and Physics", 73rd Edition, CRC Press Inc., Boca Raton FL, 1992, p. 3-470.

5.3 Windholz, M., "The Merck Index", Eleventh Edition, Merck & Co., Rahway N.J., 1989, p. 1399.