UNIVERSITY OF HOUSTON - CLEAR LAKE

2015

INDUSTRIAL HYGIENE -

ANALYSIS OF GASES AND VAPORS

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.

PURPOSE

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).

LAB ANALYSIS

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.

LIMIT OF DETECTION

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”.

STANDARD METHODS

American Industrial Hygiene Association (AIHA) – IH Laboratory Accreditation Program (IHLAP). Requires evaluation of:

Lab personnel qualifications,Lab facilities, Quality control and equipment,Lab recordkeeping, andPAT participation.

QUALIFIED LABORATORY

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!

PAT PROGRAM

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.

CHROMATOGRAPHY METHODS

Figure 12.3

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.

CHROMATOGRAPHY PROCESS

Figure 12.4

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.

CHROMATOGRAPHY RESULTS

-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.

GAS CHROMATOGRAPHY

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.

THERMAL DESORPTION

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.

GAS CHROMATOGRAPHY

- 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.

GC COLUMN TYPES

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.

GAS CHROMATOGRAPHY

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.

CHROMATOGRAPHY DETECTORS

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)

DETECTOR TYPES

- 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)

OTHER DETECTORS/TYPES

- 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.

FLAME IONIZATION DETECTOR (FID)

-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.

NITROGEN-PHOSPHORUS DETECTOR (NPD)

- Used to measure phosphorus- and sulfur-containing compounds- Examples: organophosphate pesticides and

mercaptans. - Photomultiplier detects light.

FLAME PHOTOMETRIC DETECTOR (FPD)

- Selective and highly sensitive for halogenated hydrocarbons, nitriles, nitrates, ozone, organo- metallics, 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.

ELECTRON CAPTUREDETECTOR (ECD)

- 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.

THERMAL CONDUCTIVITYDETECTOR (TCD)

- 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.

PHOTOIONIZATION DETECTOR (PID)

- 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.

DISCHARGE IONIZATION DETECTOR (DID)

- Highly sensitive and selective towards nitrogen or sulfur-containing compounds. - NCD – ozone with nitrogen oxide reaction; used for trace-level analysis of nitrosamines and pesticides-containing nitrogen.- SCD – ozone with sulfur oxide, H2S and/or other reactions in electrical furnace.

NITROGEN AND SULFUR CHEMILUMINESCENCE DETECTORS (NCD & SCD)

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 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.

GC- MASS SPECTROMETRY (GC/MS)

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.)

HIGH PERFORMANCE LIQUID CHROMATOGRAPHY (HPLC)

- Measures UV or visible light absorbance of the column effluent.- Especially sensitive to aromatic hydrocarbons. - Fixed wavelength and variable wavelength detectors.

UV-VIS ABSORBANCE DETECTOR

- 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.

FLUORESCENCE DETECTOR

- 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.

CONDUCTIVITY DETECTOR

- 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.

ELECTROCHEMICAL DETECTOR

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.

ION CHROMATOGRAPHY (IC)

- 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.

VOLUMETRIC METHODS

- 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

SPECTROPHOTOMETRIC METHODS