1. General Discussion
1.1 Background
1.1.1 History
This evaluation was undertaken to establish a suitable sampling
procedure for butyl lactate. A study for butyl lactate collected with
charcoal tubes showed a average recovery of 100% from the desorption
study. This report describes a similar analytical method for sampling
and analysis of butyl lactate.
1.1.2 Toxic effects (This section is for information only and should
not be taken as the basis of OSHA policy.) (Ref. 5.1)
n-Butyl lactate is a poison by intraperitoneal route. Local effects
include irritation to the skin and eyes. Toxic concentration in air for
humans is about 4 ppm.
1.1.3 Workplace exposure (Ref. 5.2)
n-Butyl lactate is used as a solvent for nitrocellulose, ethyl
cellulose, oils, dyes, natural gums, many synthetic polymers, lacquers,
varnishes, inks, stencil pastes, anti-skinning agent, perfumes,
dry-cleaning fluids and adhesives. It is also used as a chemical
intermediate. No data is available on the extent of workplace exposure.
1.1.4 Physical properties and other descriptive information (Ref. 5.2)
Synonyms: |
butyl α-hydroxypropionate;
lactic acid, butyl ester;
2-hydroxypropanoic acid,
butyl ester
|
CAS number: |
138-22-7 |
IMIS: |
0478 |
RTECS: |
OD4025000; 8604 (Ref. 5.3) |
Molecular weight: |
146.21 |
Flash point: |
75.5°C
(168°F)(TOC)
|
Boiling point: |
188°C
|
Melting point: |
-43°C
|
Odor: |
Mild odor |
Color: |
water-white, stable liquid |
Autoignition temp: |
382°C
(720°F)
|
Density: |
0.968 |
Molecular formula: |
CH3CH2OCOOC4H9
|
Structural formula:
|
|
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).
1.2 Limit defining parameters
1.2.1 Detection limit of the overall procedure (DLOP)
The detection limit of the overall procedure is 0.95 µg
per sample (0.016 ppm or 0.095 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 (YDLOP) that is significantly different (three
standard deviations (SDBR) from the background response (YBR).
The direct measurement of YBR and SDBR
in chromatographic methods is typically inconvenient, and difficult because YBR
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 SDBR and the precision of data about the curve are similar,
the standard error of estimate (SEE) for the regression curve can be substituted
for SDBR 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 YDLOP on the regression curve
YDLOP = A(DLOP) + YBR |
A = analytical
sensitivity (slope) |
therefore
Substituting 3(SEE) + YBR for YDLOP
gives
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 highest sampler loading was 14.96 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 112.87 and
35.66 were obtained for A and SEE respectively. DLOP was calculated to be 0.948 µg/sample
(0.016 ppm or 0.095 mg/m³).
Table 1.2.1
Detection Limit of the Overall Procedure |
mass per sample
(µg) |
area counts
(µV-s) |
|
0
1.5
2.99
4.49
5.98
7.48
8.97
10.47
11.96
13.46
14.96 |
0
176
415
537
709
814
1044
1231
1397
1589
1680 |
|
|
Figure 1.2.1 Plot of data to determine the DLOP/RQL |
1.2.2 Reliable quantitation limit (RQL)
The reliable quantitation limit is 3.2 µg
per sample (0.05 ppm or 0.32 mg/m³). 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 (YRQL) such that
YRQL - YBR = 10(SDBR)
therefore
|
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 solid sorbent sampling tubes
containing coconut shell charcoal. Each tube consists of two
sections of charcoal separated by a urethane foam plug. The
front section contains 100 mg of charcoal and the back section,
50- mg. The sections are held in place with glass wool plugs in
a glass tube 4-mm i.d. × 70-mm length. For this evaluation, SKC
Inc. charcoal tubes (catalog number 226-01, Lot 120) were used.
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 sampler 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 sampler 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 n-butyl lactate were determined by
liquid-spiking the charcoal tubes with the analyte at 0.1 to 2 times the
target concentration. The loadings on the tubes were 29.9, 149.6, 299.1,
and 598.2 µg of n-butyl lactate.
These samples were stored overnight at ambient temperature and then
desorbed and analyzed. The average desorption efficiency over the
studied range was 98.25%.
Table 2.3
Desorption Efficiency of n-Butyl Lactate
|
|
% Recovered |
|
0.1 × |
0.5 × |
1.0 × |
2.0 × |
Tube # |
29.9 µg |
149.6 µg |
299.1 µg |
598.2 µg |
|
1
2
3
4
5
6
|
98.49
97.70
97.41
98.26
96.95
97.36
|
97.54
96.99
97.62
96.68
95.38
96.00
|
98.07
98.01
99.03
99.51
99.62
99.93
|
100.03
99.97
98.16
99.67
100.02
99.49
|
average |
97.70 |
96.70 |
99.03 |
99.56 |
overall average |
98.25 |
|
|
|
standard deviation |
±1.29 |
|
|
|
|
2.4 Retention efficiency
Six sampling tubes were each spiked with 598.22 µg
(10.0 ppm or 59.82 mg/m³) of n-butyl lactate, allowed to equilibrate
for 24 hours at room temperature, and then 10 L humid air (80% RH at 21°C)
was drawn through each tube at 0.2 Lpm. They were opened, desorbed, and
analyzed by GC-FID. The retention efficiency averaged 100.16%. There was
no n-butyl lactate found on the back section of the tubes.
Table 2.4
Retention Efficiency of n-Butyl Lactate
|
Tube # |
% Recovered |
|
Front section |
Back section |
Total |
|
1
2
3
4
5
6
|
100.54
98.85
100.56
100.02
100.79
100.22
|
0
0
0
0
0
0
|
100.54
98.85
100.56
100.02
100.79
100.22
|
|
|
average |
100.16 |
|
2.5 Sample storage
The adsorbing sections of twelve sampling tubes were each spiked with
299.1 µg
(10.0 ppm or 29.9 mg/m³) of n-butyl lactate. They were sealed and
stored at room temperature. The next day 10 L of humid air (80% RH at 21°C)
was drawn through each tube at 0.2 L/min. Half of the tubes were stored
in a drawer at ambient temperature and the other half were stored in a
refrigerator at 0°C.
After 7 days of storage three samples from the tubes stored under
refrigeration and three samples from ambient storage were analyzed. The
remaining samples were analyzed after 15 days of storage. The amounts
recovered, which are not corrected for desorption efficiency, indicate
that the samples should be refrigerated. The samples stored in a
refrigerator had an average recovery of 94.5%.
Table 2.5
Storage Test for n-Butyl Lactate
|
Ambient Storage
|
Refrigerator Storage
|
Time (days) |
% Recovery |
Time (days) |
% Recovery |
|
|
7
7
7
15
15
15
|
80.9
79.4
81.1
73.4
76.5
73.6
|
7
7
7
15
15
15
|
94.5
96.2
95.5
92.6
94.9
93.2
|
average |
77.5 |
average |
94.5 |
|
|
2.6 Recommended air volume and sampling rate.
Based on the data collected in this evaluation, 10 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 n-butyl lactate on coconut shell charcoal tubes. In
general, the presence of other contaminant vapors in the air will reduce
the capacity of the charcoal tube to collect n-butyl lactate.
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. Analytical Procedure
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 (HP), 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-m x 0.32-mm i.d.
Rtx-volatiles, 1.5-µm
film thickness.
3.1.3 An electronic integrator or some 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 Pipets for dispensing the desorbing solution. A 1-mL 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 n-Butyl lactate, Reagent grade
3.2.3 Methylene chloride, Reagent grade
3.2.4 Methanol, Reagent grade
3.2.5 n-Heptanol, Reagent grade (internal standard)
3.2.6 Desorbing solution was 95/5 (v/v) methylene chloride/methanol
with 0.25 µL/mL
n-heptanol internal standard.
3.3 Standard preparation
3.3.1 At least two separate stock standards are prepared by diluting
a known quantity of n-butyl lactate with the desorbing solution. The
concentration of these stock standards was 299.1 µg/mL.
3.3.2 A third standard at a higher concentration, 1196.4 µg/mL,
was prepared to check the linearity of the calibration. Dilutions of the
stock standards were made with the desorbing solution to obtain lower
working range standards.
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.
3.4.3 The vials are sealed immediately and allowed to desorb for 60
minutes with intermittent shaking.
3.5 Analysis
3.5.1 Gas chromatograph conditions.
Injection size: |
1 µL |
|
|
|
|
|
|
|
|
Flow rates (mL/min)
|
|
Temperatures(°C)
|
Nitrogen (make-up): |
30 |
|
Injector: |
200 |
Hydrogen(carrier): |
3.0 |
|
Detector: |
225 |
Hydrogen(detector): |
30 |
|
Column: |
50-170 at 10°C/min
|
Air: |
400 |
|
|
|
Figure 3.5.1 A chromatogram of the target concentration,
where the peaks are identified as follows: 1=methoanol, 2=methylene
chloride, 3=ethyl lactate, 4=n-heptanol, and 5=butyl lactate.
|
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.
3.6.2 When necessary, the identity or purity of an analyte peak may be
confirmed by a GC-mass spectrometer or by another analytical procedure.
3.7 Calculations
3.7.1 The instrument was calibrated with a standard of 299.1 µg/mL
n-butyl lactate in the desorbing solution. The linearity of the
calibration was checked with a standard of 1196.4 µg/mL
n-butyl lactate in the desorbing solution.
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:
|
(mg/mL)(desorption volume) |
mass of analyte in sample = |
|
|
desorption efficiency |
|
mass of analyte in sample |
number of moles of analyte = |
|
|
molecular weight |
Volume the analyte will occupy at 25°C
and 760 mmHg is number of moles of analyte times the molar volume at 25°C
and 760 mmHg.
|
(mg/mL)(DV)(24.46)(106)(g)(mg) |
ppm = |
|
|
(10 L)(DE)(MW)(1000 mg)(1000 mg) |
µg/mL = concentration of analyte in
sample or standard
24.46 = molar volume (liters/mole) at 25°C
and 760 mmHg
MW = molecular weight (g/mole)
DV = desorption volume
10 L = 10 liter air sample
DE = desorption efficiency
* All units must cancel.
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.
4. Recommendations for Further Study
Collection studies need to be performed.
5. References
5.1 Sax, N., "Dangerous Properties of Industrial Materials",
Eighth Edition, Van Nostrand Reinhold Co., New York, 1992, p. 622.
5.2 Lewis, R., "Hawley's Condensed Chemical Dictionary",
Twelfth Edition, Van Nostrand Reinhold Co., New York, 1993, p. 187.
5.3 Sweet, D., "Registry of Toxic Effects of Chemical Substances",
1985-86 Edition, U.S. Department of Health and Human Services, Public Health
Service, Center for Disease Control, NIOSH, 1987, Vol. 3A, p. 3023.
|