Transcript WORKPLACE EXPOSURE ASSESSMENT AND FIELD …

UNIVERSITY OF HOUSTON - CLEAR LAKE
2015
Address the range of analytical techniques for
gases and vapors.
Quantification of individual contaminants
accomplished either by selectivity of analytical
method, or by combining non-selective analytical
method with separation technique.
Need working knowledge and understanding of
analytical methods and procedures. Consider
method selection and requirements/limitations
as well as interferences for investigation.
 Laboratories maintain high degree of analytical proficiency
for substances along with rigorous quality control
programs that are not economically feasible in a small lab.
 Maintain laboratory communication.
 Understand detection limit (DL) criteria.
 Usually combined sampling and analytical method; use
validated methods (e.g. NIOSH, OSHA, EPA).
Given knowledge of analytical limit of detection and a
defined sampling goal (e.g. detect a concentration 10% of
the TLV or PEL for substance):
sample volume calculated to assure that, although the
substance was not detected, the concentration is low and
not of concern.
 Developed to insure analytical reproducibility so results
will be comparable by labs. Also, standard methods
evaluated and tested extensively in terms of measurement
range, precision, accuracy, and interferences.
 Interpret in a statistically meaningful manner.
 Use of non-standard method addressed if documentation
is “at least equivalent”.
American Industrial Hygiene Association
(AIHA) – IH Laboratory Accreditation
Program (IHLAP).
Requires evaluation of:
 Lab personnel qualifications,
 Lab facilities,
 Quality control and equipment,
 Lab recordkeeping, and
 PAT participation.
 Provides blind reference samples for substances (i.e.
asbestos, solvents, metals, silica, etc.) to participating
laboratories quarterly/semiannually.
 Labs are considered proficient if analysis falls within +/3 SD of the reference value.
 Also provide blind external QA samples – e.g. blanks,
duplicates, and also spiked samples or samples of known
concentration.
 NIOSH Manual of Analytical Methods!
 Powerful
tools for separation of gaseous
contaminants and individual analyses.
 Chromatography (GC) involves the process of
separating components of a mixture by using
mobile phase and stationary phase.
 Mobile phase: GAS or LIQUID based on naming
convention.
 Column has a stationary phase.
 Samples
introduced onto column containing the
stationary phase in solution with the mobile phase.
 Repeated interactions differentially retard passage of
individual solutes in a mixture, providing separation.
 Analytes detected to quantify amount present. Output
signal plotted against time.
 Baseline, peak and retention times.
 The size (area) of the chromatographic peak
corresponding to a given contaminant is
directly proportional to the mass of the
contaminant injected.
 Proper calibration can determine the exact
mass of contaminant in unknowns.
 Detectors not respond identically to all
substances.
-Used for low concentration of air contaminants.
-Applicable to compounds with sufficient vapor
pressure and thermal stability to dissolve in carrier
gas and pass through column in sufficient quantity
to be detectable.
-Basic components: carrier gas system; sample
injector system; column; detector; and, a recording
system.
 The contaminant driven from the sorbent at a high
temperature into carrier gas.
 Solvent dilution not involved, so entire mass of
contaminant collected is introduced directly into GC.
 Able to quantify lower concentrations.
 Limitation is only one chance for successful analysis
because entire sample used.
Each sample component repeatedly sorbs/desorbs
from mobile/stationary phases.

Individual compounds elute from column at different
times.

Accurate quantification depends on:

combined abilities of chromatographic column;

carrier gas flow rate; and,

temperature conditions to separate or “resolve” analytes
from other sample components prior to reaching detector.
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Packed - solid support provides a large
uniform
and inert surface areas for
distributing
the
liquid coating with which
contaminants interact; can
use a wide
variety of solvents. Stationary phase
depends on analyte; liquid phase should
be
similar to sample analyzed.
Capillary - better peak resolution due to
low
resistance to flow; smaller injection volumes must
be used.
Columns contained in ovens to use temperature
control for separation to occur isothermally as well as
with programmed changes.

Set temp based on time and separation. Rule:
retention time doubles for every decrease in temp of 30
degrees C.

Temperature programming for separation of
analytes with wide range of boiling points.
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Separation techniques need detector to quantify the
amount of each analyte in column effluent.

Unspecific response and proportional to amount of
analyte present.

Need calibration curve for detector response
comparisons to standard runs.

Selection of the appropriate detector for contaminant of
interest is essential to realize full potential of GC
analysis:
Flame Ionization (FID)
Nitrogen-Phosphorus (NPD)
Flame Photometric (FPD)
Electron Capture (ECD)
Thermal Conductivity (TCD)
Photoionization (PID)
Discharge Ionization Detector (DID)
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Nitrogen
&
Sulfur
Chemiluminescence
Detectors (NCD) & (SCD)
GC with Mass Spectrometry (GC/MS)
High Performance Liquid
Chromatography
(HPLC)
UV-VIS Absorbance (UV-VIS)
Fluorescence Detector
Conductivity Detector (CD)
Electrochemical (ED)
Ion Chromatography (IC)
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Very sensitive to most organic compounds
One of most widely used GC detectors
High sensitivity and exhibits linear response
over wide range (6-7 orders of magnitude).
FID response only to compounds with oxidizable
carbon atoms and NOT to following:
 water vapor;
 elemental gases;
 CO; CO2; HCN; HCOH; formic acid;
 H20, or most other inorganic compounds.
 Little response to carbon disulfide.
-Thermionic or alkali flame detector
-Highly sensitive and selective to nitrogen and
phosphorous compounds, including amines
and
organophosphates.
-Similar to FID principle of detection, except that
ionization occurs on surface of alkali metal salt.
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Used to measure phosphorus- and sulfurcontaining compounds
Examples:
organophosphate pesticides and
mercaptans.
Photomultiplier detects light.
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Selective and highly sensitive for halogenated
hydrocarbons, nitriles, nitrates, ozone, organometallics, sulfur and electron-capturing cpds.
Selectivity based on absorption of electrons
by
compounds
with
affinity
for
free
electrons because of electronegative group.
Use radioactive beta-emitting isotopes (e.g.
tritium).
Non-chlorinated
hydrocarbons
have
little
electron affinity and are not detected.
Limitation is narrow linear range which
necessitates careful calibration range.
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Most universal GC detector since measures most
gases and vapors. Low sensitivity compared with the
other detectors; used primarily for analysis of low
MW gases as CO, CO2, N2, and O2.
Measures differences in thermal conductivity between
column effluent and reference gas (i.e.
uncontaminated
carrier gas). Most common carrier gas used is helium
due to inert and also low MW.
Column effluent and reference gas pass through separate
detector chambers that contain identical electrically
heated filaments.
Differences in thermal conductivity between gases is
proportional to rate of diffusion to/from filament.
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Sensitive to compounds with low ionization
potentials that can be ionized by ultraviolet light.
Used to selectively detect wide range of
compounds
including
aromatics,
alkenes,
ketones, or amines in the presence of aliphatic
chromatographic interferences.
Similar to FID, except that, instead of using a
flame, uses UV for ionization.
Different PID lamps are available to provide
different photon energy levels. Lamp photon
energy is chosen for selectivity of the analyte
over interferences present in sample.
High sensitivity to permanent gases and low MW
compounds (CO, CO2, N2, O2, argon, hydrogen, methane).
Current is proportional to amount of analyte in
effluent amplified/recorded.
Useful in IH labs to analyze gas bag samples for
determining quality of breathing air.
Highly sensitive and selective towards nitrogen
or
sulfur-containing compounds.
NCD – ozone with nitrogen oxide reaction; used for
trace-level analysis of nitrosamines
and
pesticidescontaining nitrogen.
SCD – ozone with sulfur oxide, H2S and/or other
reactions in electrical furnace.
If identify of contaminant is not known, then GC
analysis alone will be insufficient. Therefore, GC
column effluent should pass through a detector that
will provide a qualitative identification of the
numerous peaks exiting the column – use of MS.
- GC column effluent is introduced and ionized
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producing ions that are accelerated and separated by
mass-to-charge ratio.
Mass spectrum is the record of numbers of each
kind of ion; and, relative numbers of each ion are
characteristic for compounds, including isomers.
MS components:
inlet system; ion source;
accelerating system; detector system.
HPLC preferred technique for compounds with high
boiling points (low vapor pressures) and chemicals
that may be unstable at elevated temperatures.
- HPLC uses high pressures (500 - 3000 psi)
required to move the mobile phase through
narrow column with small sorbent particles.
- Separation tool that must be combined with
detector (e.g. UV, fluorescence) to provide
quantitative results.
- PAHs; derivatized airborne organic isocyanates;
bulk samples (e.g. oils, tars, resins, etc.)
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Measures UV or visible light absorbance
of the column effluent.
Especially
sensitive
to
aromatic
hydrocarbons.
Fixed wavelength and variable wavelength
detectors.
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Measures emission of light produced by
fluorescing eluents and is extremely sensitive
to
highly conjugated aromatic compounds (e.g. PAHs).
Some methods use derivatization reagents to
fluoresce analyte.
Detectors vary in sensitivity and selectivity.
Measures conductivity of the total mobile
phase.
Senses all ions present, from solute or mobile phases.
Detector is used for large array of
analyses
which include many already described.
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Responds to compounds that can be readily oxidized
or reduced.
Such as phenols, aromatic amines,
ketones, aldehydes, and mercaptans.
Electrode systems use working and reference
electrodes to quantify analytes over range of six orders
of magnitude.
Form of ion exchange chromatography as a method of
choice for anion analysis (e.g. sulfate; nitrate;
phosphate;
chromate;
chloride;
cyanamide;
isocyanate; sulfite; and thiocyanate).
- Also suitable for analyses of cationic species as
well as used to analyze carcinogens that can be
determined as cations (i.e. beta-naphthylamine,
benzidene, hydrazines, etc.)
- Components: separation column, background
ion suppressor column, various eluents, and a
detector.
- Also use of conductivity detector.
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Wet-chemical methods
Measuring the volume of solution of known
concentration required to react completely with
substance being determined.
Titrimetric methods – detection of endpoint
based on observation of property of the
solution that undergoes a characteristic
change near the equivalence point (e.g. HCl,
H2S, SO2, O3, etc.).
Other examples:
color, turbidity, electrical
conductivity, electrical potential, refractive
index, or temperature of the solution.
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Visible light for “colorimetry”; or “absorption
spectrophotometry”
for
measurement
of
absorption of light at particular wavelength
by solution containing the contaminant or a
material that has been quantitatively derived.
Other methods developed involve use of UV or IR
radiation.
Extent to which light is absorbed by solution is
related to concentration of contaminant in
solution and length of the light beam passing
through the absorbing solution.
Described by Beer-Lambert law.
e.g. Saltzman’s reagent for nitrogen dioxide