sampling of atmospheric volatile organic compounds (vocs) by muhammad qasim, aroj bashir university...

SAMPLING OF ATMOSPHERIC VOLATILE ORGANIC COMPOUNDS

(VOCS)

Muhammad Qasim & Aroj Bashir

University of Gujrat

Objectives Introduction Classification of VOCs According to Their Possible Effects on Human

Health and the Environment Reasons for which monitoring of VOCs in air is required VOC monitoring, areas, typical grid sizes and frequencies of data

acquisition Methods for Monitoring VOCs in the Atmosphere Selection of the Capillary Column for

the GC-MS Analysis Of VOCS in Air Sampling of Atmospheric Volatile Organic Compounds The Polarity of the Stationary Phase The Efficiency, Capacity, and Phase Ratio of the Column Identification of VOCS in Air by GC-MS

Introduction Experimental evidence collected over the last three decades has

shown clearly that the accumulation in air of volatile organic compounds might represent an important source of risk for human health.

National directives have been promulgated in the United States and the European Union (EU) to force local and national authorities to control VOC emissions through the best available techniques.

Different abatement strategies need to be followed as a function of the spatial and temporal scales in which potential adverse effects of VOCs and their degradation products can be observed.

Introduction Its need of priority lists is also dictated by the fact that the

environmental effects of VOCs might differ by several orders of magnitude, and some components present at trace levels in the atmosphere can be more effective than the most abundant ones.

The recent development of accurate indices for quantifying the potential impact of individual components on human health, tropospheric ozone production, earth warming, and stratospheric ozone depletion explain

Although unsuitable for alerting the population in the case of accidental release of organic pollutants and for checking the compliance of VOC emissions from stationary sources with national legislation.

Gas chromatograph (GC) monitors have never been replaced by other systems in monitoring networks for VOCs.

GC systems is that any apparatus that has been produced in the last 20 years can be adapted easily to the monitoring of VOCs without losing any of its original capabilities.

The identification of VOCs by this type of monitor is ensured by the selectivity of the GC column and by the specificity of the detectors to which it is connected.

Some detectors, such as the electron capture detector (ECD), the photo ionization detector (PID), and the flame photometric detector (FPD), are used widely because they combine an excellent sensitivity with very high selectivity toward specific components

In complex airsheds where GC not work well. mass spectrometry (MS) has become the preferred method for the monitoring of VOCs with GC systems.

Introduction

Introduction As commonly happens with methods dealing with trace level

determinations, the accuracy of the final results depends not only on the selection of the instrumentation and the quality of the material used but also on the way each individual step is executed.

Photoionization detector Electron capture detector Photometric detector

Classification of VOCs According to Their Possible Effects on Human Health and the EnvironmentVOC-TOX

VOC for which evidence of toxicity to human, animals, and plants at trace levels has been collected through epidemiological studies VOC for which acute and chronic episodes have been documented through in vivo or in vitro laboratory tests carried out with well-recognized and standardized procedures.

VOC-OX VOC characterized by high photochemical ozone and PAN creating

potentials (or equivalent indices) VOC involved in photochemical and acidification processes occurring in the atmosphere.

VOC-STRAT VOC characterized by high depletion potentials of stratospheric ozone.

VOC-CLIM VOC responsible for the thermal trapping of infrared radiation (greenhouse

gases) or for the changing of the optical properties of clouds.

Reasons for which monitoring of VOCs in air is required

To alert the population in the case of accidental release of air toxics in the atmosphere (VOC-TOX).

To assess short- and long-term exposure of humans, animals, and plants to criteria pollutants (VOC-TOX, VOC-OX ).

To validate prediction models, (VOC-OX, VOC-STRAT, VOC-CLIM).

To assess the efficacy of control strategies ( VOC-TOX, VOC-OX, VOC-STRAT, VOC-CLIM).

To investigate the role played by VOCs in affecting short- or long-term equilibrium of the earth (VOC-OX, VOC-STRAT, VOC-CL IM).

VOC monitoring, areas, typical grid sizes and frequencies of

data acquisition

Methods for Monitoring VOCs in the Atmosphere


Optical methods

Differential optical absorption (DOAS).

Fourier transform infrared (FTIR). Advantages

Short response time (seconds).

Unattended operation.

Remote sensing capabilities.

Monitoring over large areas. Disadvantages

Limited number of compounds that can be monitored (DOAS).

Insufficient sensitivity for the monitoring of background concentrations (FTIR).

Difficult calibration.


Mass spectrometric methods

Proton transfer mass.

spectrometry (PT-MS).

Chemical ionization mass.

spectrometry (CI-MS). Advantages

Short response time (from seconds to minutes)

High sensitivity (pptv).

Simultanous monitoring of VOCs in a wide range of molecular weights.

Disadvantages

Impossibility to distinguish isomeric and isobaric components.

High cost for the instrumentation.

Single-point monitoring.

Skilled personnel required.

Spectroscopic methods

Tunable diode laser (TDLAS).

Induced fluorescence (IF).

Chemiluminescence . Advantages

Short response time (seconds).

High sensitivity (pptv). Disadvantages

Limited number of compounds that can be monitored.

High cost for the instrumentation.

Single-point monitoring.



Chromatographic methods

Gas chromatography (GC).

High-performance liquid.

chromatography (HPLC). Advantages

Simultaneous monitoring of a wide

number of VOCs.

Possibility to identify and quantify isomeric

and isobaric components.

General use instrumentation. Disadvantages

Long response time.

Long time for data processing.



Selection of the Capillary Column forthe GC-MS Analysis Of VOCS in Air

General Requirements for the GC-MS Analysis of VOCs

Within the flow constraints posed by the mass spectrometer, the column that better meets identification purposes is the one providing the best resolution (R) and capacity (C) in the widest range of carbon atoms as possible.

In gas chromatography, R defines how well two peaks are separated one from another.

Under isothermal conditions, it is measured as follows:

Sampling of Atmospheric Volatile Organic Compounds

The Polarity of the Stationary Phase

The wide variety of organic components present in air. liquid phase that provides the best resolution. In a specific class of components in air, we must

optimize the column polarity as a function of it. To accurately quantify carboxylic acids in air, we

need, for instance, a very polar column because it provides the best resolution and a linear isotherm of adsorption in a wide range of concentrations for carboxylic acids.

A problem encountered with very polar phases is that not all of them can be bonded chemically to fused silica tubing, and column bleeding can limit their use in GC-MS.

In this case, it is important to see if stabilized phases are available.

The Polarity of the Stationary Phase

The other approach is to optimize the column polarity as a function of the most abundant and frequently observed classes of components in air.

In this case, low or moderately polar columns should be preferred because they allow sufficiently good resolutions for non polar compounds.

These columns also provide acceptable performance for some polar compounds (alcohols).

The polarity index of general-purpose columns range from approximately 20 to 90 (low polar) or 70 to 170 (moderately polar).

low bleeding , these types of columns provide high resistance to water, organic solvents, acids, and bases

The Efficiency, Capacity, and Phase Ratio of the Column

The efficiency effectively can be improved by increasing the column length.

The capacity and retention of these columns can be further optimized by selecting the proper amount of liquid coating inside the column.

Larger capacities indeed can be obtained by increasing the thickness of the liquid film deposited on the internal walls of the capillary tube because more solute molecules can be dissolved in the stationary phase.

Identification of VOCS in Air by GC-MS

The whole effluent of the column is sent to the ion source of the mass spectrometer.

Where molecules are ionized. The products formed transferred by an electrical. Field to a mass analyzer for separation. The potential applied to the ion repelled, negative or

positive ions can be expelled by the source. They can be separated with filters using magnetic

and/or electrical fields. The mass and composition of ions are determined by

recording them with an ion detector.

The Full Scan and Selected Ion Detection in Electron Impact MS

Other MS Ionization Techniques for VOC Identification

The collision of ions with argon or helium is

another way to gain decisive information on the chemical structure of an unknown component.

This technique called MS-MS. The ions formed by electron impact. The oscillation of the ions and keep them in the

center of the electrical field. Helium, is introduced into the chamber. By collision with helium can be extracted and

analyzed. By this can identification of the unknown

compound can be obtained.

chemical ionization is another approach. It is particularly useful for those compounds in which the energy

transferred by electron impact. Electron capture of thermal electrons also can be exploited for the

selective detection. Thermal electrons (electron volts energy) are produced inside the

source by collision of a gas (methane) at high concentration. The experimental setup is quite to the one used in chemical ionization

except for the fact that negative ions are recorded.

Other MS Ionization Techniques for VOC Identification

The Combined Use of Selected Ions and Retention Indices for the Identification of VOCs by Electron Impact MS

Due to their high selectivity and complexity. Techniques cannot be used on a routine basis for the GC-MS analysis

of VOCs in air. Electron impact fragments are selected and used for routine

identification for unknown components. The highest molecular weight of VOCs in air never exceeds a value

of 350 mass units. Reliable results can be obtained with a MS and ion trap analyzers. Ion-trap-based MS provides better sensitivities for to the

identification of precursors and products of photochemical smog pollution in air.

The Combined Use of Selected Ions and Retention Indices for the Identification of VOCs by Electron Impact MS

For positive identification, GC-MS are sufficiently clear to closely match the ones of pure compounds listed in known libraries.

This is possible in practice when the column is able to sufficiently separate all compounds present in the mixture.

The composition of VOCs in air is often so complex. Show substantial overlapping of their chromatographic peaks. If subtraction techniques are unable to clearly separate the

contributions of the ions coming from overlapping compound. Mass spectra obtained in GC-MS will never match those of pure

substances, and positive identification becomes very difficult.

Quantification of VOCS by Mass Spectrometry General Problem

Molar response of individual compounds in electron impact essentially depends on the first ionization potential of the molecule is main difficulty in GC-MS.

Branching and cycling of the molecule also affect the first ionization potential.

Another difficulty is represented by the critical dependence of the conditions existing in the source, the analyzer, and the detector.

Another condition to meet is constancy in concentration and energy of electrons generated inside the source.

All these considerations indicate that the quantitative analysis of VOCs by GC-MS is not easy and may be affected by large errors if specific procedures are not followed strictly.

Standard Solutions for the Calibration of GC- MS for VOCs

Liquid Standard Solutions for the Calibration of VOCs. Permeation and Diffusion Devices for the Calibration of VOCs. Gas Cyli.nders for the Calibration of VOC

Conclusions The use of GC-MS has contributed greatly to our knowledge of the

emission , transformation, and deposition of VOCs in air. There are still very few laboratories around the world using this

method. This technique is too expensive and complex to be used on a routine

basis. Any person with a good chemistry background can learn and

properly use capillary GC-MS Its requires good training and considerable knowledge. For this technique new software packages for automated data

processing are developed. Which drastically reduce the time spent for calculations.