Monitoring of Inorganic Ions in Ambient Air

Mr. S. D. Joshi Scientist

Air Pollution Control Division, NEERI, Nagpur - 440020

Introduction

The air besides containing gaseous pollutants like S02, NOx, CO, HC also

contains particulate matter which is made up of complex organic compounds,

inorganic elements and their oxides and secondary particulates viz, sulphate,

nitrate etc. These secondary particulates are formed from their precursors in

presence of sunlight humidity and metal ion species by oxidation or reduction

processes. The acid precursors viz. sulfate and nitrate, and chloride require

more concern as these have deleterious effects on aquatic and forest eco-

systems and are potentially harmful for a variety of building materials, accelerate

corrosion of metals, damage sculptures and cause chronic respiratory problems

in human beings. In view of possible large social, economic, ecological and

aesthetic value, it has become important to analyse these pollutants in the

ambient air.

These particulates are emitted in the atmosphere by different

anthropogenic and natural sources. Sulphate (SO4-) is formed in the

atmosphere by reduction or oxidation in presence of hydroxy! ion (OH"). The

main sources of sulfur oxides are power houses, sulphuric acid, petroleum

industries, oil refineries, coal burning and domestic use of fuels. Acid

manufacturing, automobile exhaust, explosive industry, fuel burning etc. are the

main sources of nitrogen oxides. Chloride occurs predominantly in coastal cities.

Other sources of chloride are coal burning, waste incineration, automobile

exhaust, burning of chlorinated hydrocarbons. Fluoride is emitted in the

atmosphere by fertiliser industry, aluminium industry, steel making and acid

making industries.

Analytical Techniques

Different methods are available for the analysis of anions. The

conventional methods, viz., titrimetric, colorimetric, spectrophotometric,

gravimetric and ion selective electrode methods though commonly used, are

ion specific, time consuming and less sensitive. Ion Chromatography (IC) is the

only instrumental technique which can be used for rapid, sequential analysis of

different anions simultaneously.

Ion Chromatography

Chromatography encompasses wide range of techniques of separation of

the components of a mixture as a result of differential migration of the component

molecules between stationary phase and mobile phase. It was Hamish, Small of

DOW chemical corporation whose pioneering work in mid seventies in the

analysis of multiple -ion mixture developed into, 'Ion Chromatography" Ion

chromatograph provides a single instrumental technique for rapid sequential

analysis of different anions present in a sample. Ion chromatograph eliminates

the need to use hazardous reagents and it effectively distinguishes among the

halides (Cr,F~) and the oxides (NO3", SO4"). For ambient air monitoring, IC is

the most suitable technique though it is expensive. The anions SO4", NO3", CI",

F" are being monitored in ambient air using IC 2000i/SP and DX-100 models

under NAQM project, NEERI.

Several approaches have been initiated to solve the problem of separation

and detection of ions either independently or in integrated fashion. Three major

approaches are:

i) suppressed ion chromatography

ii) non-suppressed ion chromatography and

iii) indirect detection ion chromatography

Suppressed Ion chromatography

In the group of separation techniques employing ionic interactions ion

exchange is the oldest one. The separation is carried out with the packing that

possess charge bearing functional groups. The most common retention

mechanism is simple ion exchange of sample ions X and mobile phase ion Y

with the charged groups R of the stationary phase.

X" + R+Y_ Y" + R+X" anion exchange

X+ + R"Y+ Y+ + R"X+ cation exchange

For anion exchange separation the sample ions X" are in competition with

the mobile phase ions Y" for the ionic sites R+ of the ion exchanger. Sample ions

that interact weakly with the ion exchanger, in the presence of competing mobile

phase ion will be retained on the column for a shorter duration whereas sample

ion interacting strongly to with the ion exchanger will be retained for larger.

Conductivity being universal property of ionic species in solution, showing

simple dependence on the concentration of ions, has been considered to solved

detection problem since long. However, in ion exchange separation conductivity

response is masked by the mobile phase electrolyte. This problem was tackled

by employing a novel combination of ion exchange columns to remove the

background electrolyte leaving only the ionic solute of interest as conducting

species in the column effluent.

The second column called the stripper or the suppresser complicates the

use of ion chromatography. The stripper column needs periodic regeneration

and furthermore ion exclusion effects and some band broadening in the

suppresser column deteriorates the analysis.

To optimise the regeneration and chromatographic efficiency of

conventional suppressor column, the relative volumes and specific ion exchange

capacities of separator and stripper columns are adjusted. For good

chromatographic efficiency volume ratio of stripper and separator column is kept

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low unity being desirable but values upto 10 acceptable. Regeneration

requirements are minimised by using low capacity pellicular particles or surface

modified resins in separating columns, low ionic strength mobile phase, small

sample size and conventional high capacity porous ion exchange resin in the

stripper. Typical low capacity separator column can be used alongwith

conventional high capacity suppresser column for 8 to 10 hours without requiring

regeneration. Automatic - regeneration feature in commercial instruments

further simplify the analysis. However suppresor should not react with the

sample ions in a way that would remove them from the eluant or reduce their

conductivities or retain them permanently undergoing irreversible changes.

Non Suppressed Ion Chromatography

The ion chromatographic systems with suppresser columns are generally

dedicated and relatively costly. Analysis by non-suppressed systems depends

on existence of a significant measurable difference between sample ions and the

prevailing eiuent ions. For improving sensitivity low capacity exchanges and

proper displacing ions are required. Low capacity exchanger match the low ionic

strength eiuent employed, which enable detection of small amounts of samples

and proper displacing ions can display a useful difference in equivalent

conductance in comparison with common inorganic ions. Low capacity micro

porous anion exchange resin also have been developed for the separation of

inorganic anions. The resins used macro reticular cross-linked polystyrene

beads as substrate. The conductivity detector used for this method requires a

large electronic offset range for nulling the background conductivity of the eiuent

and small cell currents to minimise heat dissipation in the cell and resulting

baseline drift and noise. This detector given best results when used with low

background conductivity of the eiuent and fine temperature stability.

Indirect Detection Ion Chromatography

Many inorganic ions display strong absorbance in UV region but at

wavelengths that were previously inaccessible to liquid chromatography

photometers. With the development of UV detectors that reach down to 190 nm.

These ions are easily amenable to sensitive monitoring and determination. Direct

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UV absorbance has been coupled with several separation modes for variety of

samples. However, the major drawback of these methods is the low sensitivity

even after employing high purity solutes to minimise background absorbance.

Recently the method of indirect detection is gaining applications.

Generally in ion exchange or ion pair separation ultraviolet detector is used in

conjunction with mobile phases with very low absorbance at the monitoring

wavelength.

The sample contains the chromospheres and when they elute and pass

through the detector, absorption of light takes place and positive peak is

recorded.

Generally low capacity ion exchange columns are employed as separators

and an aqueous solutions of potassium hydrogen phthalate (10~4 to 10"3M) as

mobile phase. This salt gives suitable absorption at the wavelength 265 mm.

The choice of the mobile phase and its strength is decided by the anions of

interest and analysis time.

A large number of organic and and inorganic ions have been studied by

this technique. Analogous schemes for cation analysis have also been reported.

Simultaneous analysis of anions and cations in a single chromatogram also can

be done by this system.

Refractive index detector also has been used in this indirect mode. Both

UV & Rl methods are more sensitive than conductivity detection and they give

less baseline noise.

Interference : Any substance that has retention time coinciding with that

of any anion to be determined will interference. For example relatively high

concentrations of low molecular weight organic acids interfere with the

determination of chloride and fluoride. Sample dilution over comes many

interferences. Spurious peaks may result from contaminants in reagent water,

glassware or sample processing apparatus.

Minimum Detectable Limit

The minimum detectable concentration of an anion is a function of sample

size and conductivity scale used. Generally, minimum detectable concentrations

are near 0.1 mg/l for Br, CI", NO3-, N02", P0 43 " and S0 4

2 " with a 25 pi loop

and a 10 pS/cm full scale setting on the conductivity detector.

Air Monitoring/Sampling Collection

For ambient air anion sampling and analytical technique being used for

these pollutants require a large volume of air to be sampled in order to reach

needed detections limits. This has been accomplished utilising the high volume

(1400 Ipm) air sampler for the collection of total suspended particulate matter.

While for respirable suspended particulate matter, respirable dust sampler

is used. This sampler can be adapted with an optional PM-10 aerodynamic

aerosol inlet cut-point design which is insensitive to small variations in sampling

flow rate.

Preparation of Sample Filter

The filter papers are dried in desiccators for hours before, use. To ensure

acceptable filters, they are extracted with water and extracts are analysed by ion

chromatography. A filter blank of less than 0.1 pg per filter is considered

acceptable for field use.

Calibration of Sampling System

Each sampler is to be calibrated i) when new ii) after major repairs or

maintenance iii) whenever any audit point deviates from the calibration curve by

more than 7% iv) when a different sample collection media, other than that which

the sampler was originally calibrate to will be used for sampling, v) before and

after each test series.

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Construct a best fit curve for the points generated and use this relationship

for future work employing the flow sensor device.

Sample Retrieval

At the end of the desired sampling period the power is turned off.

Carefully remove the sampling head containing the filter. Remove filter from the

upper chamber using clean, teflon tipped forceps. Fold the filter in half twice

(Sampled side inward) and place it in an labeled envelope. These filters should

stored in dessicator containing dry silica gel. If the time span between sample

collection and laboratory analysis is to exceed 24 hour samples must be kept

refrigerated at 4°C. At least on field filter should be returned to the laboratory

with each group of samples. A field blank is treated exactly as a sample except

that no air is drawn throughout the filter.

Analysis

Reagents : Deionised or distilled water free from interferences at the

minimum detection limit of each constituent.

Eluant Solution : Eluant solution is a mixture of 1.7 mm sodium

bicarbonate and 1.8 mm Sodium Carbonate.

Regenerant Solution

Regenerant solution is all glass double distilled water for continous

regeneration.

Standard Anion Solution

Prepare a series of standard anion solutions by weighing the inducted

amount of salt, dried to a constant weight at 105°C to 1000 ml. Store in plastic

bottles in a refrigerator. These solutions are stable for at least one month. Verify

stability.

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Combine Working Standard Solution

Prepare a combined working standard solution by mixing appropriate

quantity of stock solution and store in plastic bottle protected from light. Prepare

fresh daily.

Ultrasonic Extraction Method

The extraction of anions is carried out by ultrasonic compact cleaner

model SW 45 (Toshniwal). The tank volume of extractor is 4.5 lits. It has a

sonifer cell disrupter .40 KHz power ultrasonic generator capable of dialing

150/130 w accurately with 127 cm horn distrupter sonabox.

From the exposed filter paper 18 circles of 1.5 cm diameter each were

taken by punching with steel punch (area 31.8 cm2) in a clean 100 ml beaker. To

these circles 20 ml deionised water was added and extracted for 5 min in

ultrasonicator. The extract was vacuum filtered with the help of G4 sintered glass

crucible in a clean plastic bottle. Again 20 ml water was added and extraction

was done for 10 min. The filtrate was collected in same bottle. The extraction

procedure was repeated third time for 15 minutes. The extracted samples were

stored in fridge till analysis.

Calibration Curve

Prepare standards of different concentration by mixing known volume of

different ions. Inject standards containing single anion or a mixture and

determine approximate retention time. Inject atleast three different concentrations

for each anion to be measured and construct a calibration curve by plotting peak

height or area against concentration on linear graph paper. Recalibrate

whenever detector setting is changed. Record the peak height or area and

retention time for calculation of the calibration factor CF.

Calibration Factor CF = Total area of peak

Mass injected (in microgram)

if the percent relative standard deviation (% RSD) of the calibration factor is less

than 20% over the working range, linearity through the origin can be assumed,

and average calibration factor can be used in place of a calibration curve.

The working calibration curve or calibration factor must be verified on each

working day by the injection of one or more calibration standards. If the response

factor for any analyte varies from the predicted response by more than + 20% a

new calibration curve must be prepared for that analyte. Calculate the percent

variance by the following equation-

Percent variance = [(R2 -Ri)/R<|] X 100

Before analysis can be performed the retention time windows must be

established for each analyte. Make three injections of the standard containing all

compounds for retention time window determination. The retention time window

is defined as the plus or minus three times the standard deviation of the absolute

retention times for each standard.

System Equillibration

Turn on ion chromatograph and adjust eluant flow rate. Adjust detector to

desired setting and let system come to equilibrium (20 to 30 min). A stable

baseline indicates equilibrium conditions. Adjust detector offset to zero out eluant

conductivity; with fibre or membrane suppressor adjust the regeneration flow rate

to maintain stability usually 2 to 3 mL/min.

Ion Chromatography with Conductivity Detector

Under the calibration procedures (external)the % RSD of the calibration

factor should be <20% over the linear working range of a five point calibration

curve.

Under the calibration procedures (external) daily working calibration curve

for each analyte should not vary from the predicted response by more than +

20%.

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For each analyte the retention time window must be established, verified

on a daily basis and established for each analyte throughout the course of a 72

hour period.

For each analyte the mid level standard must fall within the retention time

window on a daily basis as a qualitative performance evaluation of the IC system.

The surrogate standard recovery must not deviate by more than 20%.

Calibration

The stock standard solutions were prepared by adding exactly weighed

known amount of compound to a one litre volumetric flask and voiume was made

up by deionised water. A series of different concentrations of standards were

prepared by diluting stock standard solution.

Retention Time

The retention time is the time required for complete elution of a component

from the point of injection of sample. The retention time windows for each analyst

was determined. The retention time window is defined as plus or minus three

times the standard deviation of the absolute retention times for each standard.

Range and Sensitivity

The minimum detectable limit for chloride ion at sensitivity 100 is 2.44 pg.

The minimum detectable concentration of chloride ion for particulates collected

on one glass fiber filter of approximately 400 cm2 is 0.064 pg if 480 m3 of air are

sampled in the ambient atmosphere. The minimum detectable limit for SO4 is

3 pg for fluoride 0.4 pg and for nitrate 0.5 pg at sensitivity 100 on 2000 i/SP

Dionex ion chromatograph.

Precision and Accuracy

The blank fiber filter paper was spiked by known concentration of anions

and extracted ultrasonically. The relative standard deviation (RSD) for 3

ultrasonic extract for SO4 was ±1.35. For nitrate 5.54% for chloride 4.23% and for

fluoride 4.5%. The percentage recovery for SO4 is 98.02 for chloride 97.4, for

nitrate 98.0 and for fluoride 95.0.

Similarly the exposed filter papers were also spiked by known

concentration of anions. The RSD for 2 ultrasonic extract for sulphate was 1.44

for nitrate 5.8 for chloride 5.2 & for fluoride 5.5. The percentage recovery for SO4

is 97.01 for chloride 95.2 for nitrate 95 and for fluoride 90.58%.

Calculations The concentration of different ions is calculated in ug/m3

concentration = peak height concentration Volumeof Total area of 1 (ug/m3) of sample (cm) X of standard (ug) X sample X filter paper(cm2) X

peak height of Area taken for volume of standard (cm) extraction(cm2) air (m3)

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F

SAMPLE

Anions Separation in Rain Water Sample by Ion-chromatograph

SAMPLE

Cat ions Separa t ion in Rain Water Sample

by lon- 'chromasograph