1. General Discussion
1.1 Background
1.1.1 History
This evaluation was undertaken to establish a suitable sampling and
analysis procedure for ethyl lactate. A study for n-butyl lactate
collected with charcoal tubes showed an average recovery of 100% from
the desorption study. This report describes a similar analytical method
for sampling and analysis of ethyl 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 and 5.2)
Ethyl lactate is a central nervous system depressant and lethal to
animals in high concentration (actual levels not stated) causing
respiratory paralysis. (Ref. 5.1) Ethyl lactate is irritating in a
rabbit eye and guinea pig skin. Ethyl lactate is moderately toxic. The
probable oral lethal dose (human) 0.5-5 g/kg, is between 1 ounce and 1
pint or 1 pound for a 70 kg person (150 lb). (Ref. 5.2)
1.1.3 Workplace exposure (Ref. 5.3)
Ethyl lactate is used as solvent for nitrocellulose, cellulose
acetate, many cellulose ethers, resins; lacquers, paints, enamels,
varnishes, stencil sheets, safety glass and flavoring. No data is
available on the extent of workplace exposure.
1.1.4 Physical properties and other descriptive information (Ref. 5.3
unless otherwise indicated)
Synonyms:
|
Actylol, acytol, ethyl a-hydroxypropionate,
ethyl 2-hydroxypropionate (Ref.5.4) |
CAS number: |
97-64-3 |
IMIS: |
E227 |
RTECS: |
OD5075000; 8612 (Ref. 5.4) |
DOT: |
UN1192 (Ref. 5.4) |
Vapor pressure:
|
0.67 kPa (5 mg Hg @ 30ºC)
(Ref. 5.5) |
Molecular weight: |
118.13 |
Flash point: |
46.1ºC
(115ºF)(CC) |
Boiling point: |
154ºC |
Melting point: |
-25ºC
(Ref. 5.5) |
Odor: |
Mild odor |
Color: |
Colorless, liquid |
Density: |
1.0324 @ 20.4ºC/4ºC |
Molecular formula: |
C5H10O3
|
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 25ºC
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 1.0 µg
per sample (0.021 ppm or 0.10 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 12.07 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 100.89 and 33.56 were obtained for A and SEE
respectively. The DLOP was calculated to be 1.0 µg/sample
(0.021 ppm or 0.10 mg/m³).
Table 1.2.1
Detection Limit of the Overall Procedure
|
mass per sample
(µg) |
area counts
(µV-s) |
|
0
1.21
2.41
3.62
4.83
6.03
7.24
8.45
9.66
10.86
12.07 |
0
145
310
397
526
618
803
903
987
1081
1304 |
|
|
Figure 1.2.1 Plot of data to determine the DLOP/RQL |
1.2.2 Reliable quantitation limit (RQL)
The reliable quantitation limit is 3.33 µg
per sample (0.07 ppm or 0.33 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
Y RQL - YBR = 10 (SDBR)
|
Figure 1.2.3 Chromatogram of the RQL |
therefore
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. x 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 ethyl lactate 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 24.1, 120.7, 214.4, and
482.8 µg of
ethyl lactate. These samples were stored overnight at ambient temperature
and then desorbed and analyzed. The average desorption efficiency over the
studied range was 97.82%.
Table 2.3
Desorption Efficiency of Ethyl Lactate
|
|
% Recovered |
|
0.1 × |
0.5 × |
1.0 × |
2.0 × |
Tube # |
24.1 µg |
120.7 µg |
241.4 µg |
482.8 µg |
|
1
2
3
4
5
6 |
97.25
97.89
98.85
99.43
98.10
98.12 |
96.19
95.88
96.64
95.97
95.19
96.25 |
97.10
98.39
98.52
97.99
99.44
99.67 |
99.37
98.17
98.15
98.19
98.87
97.96 |
average |
98.27 |
96.02 |
98.52 |
98.45 |
overall average |
97.82 |
|
|
|
standard
deviation |
±1.20 |
|
|
|
|
2.4 Retention efficiency
Six sampling tubes were spiked with 482.8 µg
(10.0 ppm or 48.3 mg/m³) of ethyl lactate, allowed to equilibrate
for 24 hours at room temperature, and then 10 L humid air (80% RH at 21ºC)
was drawn through each tube at 0.2 Lpm. They were opened, desorbed, and
analyzed by GC-FID. The retention efficiency averaged 95.7%. There was no
ethyl lactate found on the back sections of the tubes.
Table 2.4
Retention Efficiency of Ethyl Lactate
|
Tube # |
% Recovered |
|
Front section |
Back section |
Total |
|
1
2
3
4
5
6 |
95.64
94.93
95.96
96.79
95.40
95.47 |
0
0
0
0
0
0 |
95.64
94.93
95.96
96.79
95.40
95.47 |
|
|
average |
95.70 |
|
2.5 Sample storage
The front sections of twelve sampling tubes were each spiked with 241.4 µg
(5.0 ppm or 24.1 mg/m³) of ethyl lactate. They were sealed and
stored at room temperature. The next day 10 L of humid air (80% RH at 21ºC)
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 0ºC.
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 90.0%.
Table 2.5
Storage Test for Ethyl Lactate
|
Ambient Storage
|
|
Refrigerator Storage
|
Time (days)
|
% Recovered
|
|
Time (days)
|
% Recovered
|
|
|
|
7
7
7
15
15
15
average
|
64.1
60.1
60.3
50.0
50.7
55.6
56.8
|
|
7
7
7
15
15
15
average
|
91.8
93.0
92.3
86.2
88.9
88.0
90.0
|
|
|
|
|
|
|
|
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 ethyl lactate 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 ethyl 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 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 Ethyl lactate, Reagent grade
3.2.3 Methylene chloride, Reagent grade
3.2.4 Methanol, Reagent grade
3.2.5 n-Heptanol, Reagent grade. This was used as an 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 ethyl lactate with the desorbing solution. The
concentration of these stock standards was 241.4 µg/mL.
3.3.2 A third standard at a higher concentration, 965.6 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)
|
|
Nitrogen (make-up): |
30 |
Hydrogen(carrier): |
3 |
Hydrogen(detector): |
30 |
Air: |
400 |
Temperatures (ºC)
|
|
Injector: |
200 |
|
Detector: |
225 |
|
Column: |
50-170 at 10 ºC/min |
|
Figure 3.5.1 A chromatogram of the target concentration, where the peaks are identified
as follows: 1=methanol, 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 241.4 µg/mL
ethyl lactate in the desorbing solution. The linearity of the
calibration was checked with a standard of 965.6 µg/mL
(20 ppm).
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 Values (µg)
obtained from the blanks are subtracted from air samples.
3.7.4 To calculate the concentration of analyte in the air sample the
following formulas are used:
mass of analyte in sample = |
(mg/mL)(desorption volume)
desorption efficiency
|
number of moles of analyte = |
mass of analyte in sample
molecular weight |
Volume the analyte will occupy at 25ºC and
760 mmHg is number of moles of analyte times the molar volume at 25ºC
and 760 mmHg.
ppm = |
(volume analyte occupies)(106)
air volume |
3.7.5 The above equations can be consolidated to the following formula.
ppm = |
(mg/mL)(DV(24.46)(106)(g)(mg)
(10 L)(DE)(MW)(1000 mg)(1000 mg) |
µg/mL = concentration of analyte in sample or
standard
24.46 = molar volume (liters/mole) at 25ºC and
760 mmHg
MW = molecular weight (g/mole)
DV = desorption volume
10 L = 10 liter air sample
DE = desorption efficiency
* All units must cancel.
3.7.6 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 Browning, E., "Toxicity and Metabolism of Industrial
Solvents", American Elsevier, New York, 1965.
5.2 Gosselin, R.E., H.C. Hodge, R.P. Smith, and M.N. Gleason,
"Clinical Toxicology of Commercial Products", Fourth Edition,
Williams and Wilkins, Baltimore, 1976.
5.3 Lewis, R., "Hawley's Condensed Chemical Dictionary",
Twelfth Edition, Van Nostrand Reinhold Co., New York, 1993, p. 495.
5.4 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. 3024.
5.5 Clayton, G.D. and F.E. Clayton, "Patty's Industrial Hygiene and
Toxicology", Third Edition, John Wiley Sons, New York, 1981, Vol. 2A, pp.
2304-5.
|