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Analytica Chimica Acta 505 (2004) 231–237

Determination of nitrate in environmental water samples by conversioninto nitrophenols and solid phase extraction−spectrophotometry, liquid

chromatography or gas chromatography–mass spectrometry

Nidhi Lohumi a, Shefali Gosain a, Archana Jain a, Vinay K. Gupta b, Krishna K. Verma a,∗

a Department of Chemistry, Rani Durgavati University, Jabalpur 482001, Madhya Pradesh, Indiab Department of Chemistry, Ravishankar Shukla University, Raipur 492010, Chhattisgarh, India

Received 2 June 2003; accepted 21 October 2003

Abstract

Conversion of nitrate into a nitro-phenol derivative by reaction with 2-methylphenol or 2,6-dimethylphenol allowed at least 100-fold

enrichment of the derivative on Lichrolut EN polymeric cartridge, and it is stable for up to 1 month on the cartridge. The derivative could

be eluted with ammonia–methanol mixture. This reaction for nitrate determination has permitted a choice of final measurement by UV-Vis

spectrophotometry, liquid chromatography or gas chromatography–mass spectrometry when the limits of detection were 10, 6 and 3g l−1,

respectively, and the calibration range 20g to 10mgl−1 nitrate. The method has been validated by spiking natural water samples, when the

recovery of nitrate was 98.5–108.4% (relative standard deviation 2.5–6.1%).

© 2003 Elsevier B.V. All rights reserved.

Keywords: Nitrate; Solid phase extraction; UV-Vis spectrophotometry; Liquid chromatography; Gas chromatography–mass spectrometry; Water samples

1. Introduction

Nitrogen pollution can have potentially harmful effects

in surface and ground waters, and these are causing current

concern [1]. The nitrate content of drinking water is rising

at an alarming rate in both some developed and developing

countries owing largely to an insufficiency of sewage treat-

ment and excessive fertilizer application. Sources of nitrate

pollution are waste from factories dealing with nitrogenous

fertilizers, processed food, dairy and meat products, dis-

charges from secondary treatment systems of domestic and

organic waste, seepage and run off from septic tanks andcesspools, and decomposition of decaying organic matter

buried underground. The current drinking water guideline

is 50 mgl−1 NO3− (11mgl−1 NO3

−−N) [2]; however, the

EU guideline is half of this value [1]. The effect of nitrate

itself is described as primary toxicity [3], its high intake

causes abdominal pains, diarrhea and vomiting. Nitrate is

reduced to nitrite leading to secondary toxicity. Concentra-

tions greater than 50 mg l−1 NO3− in drinking water have

∗ Corresponding author. Tel.:+91-7612323103; fax: +91-7612313252.

E-mail address: [email protected] (K.K. Verma).

been known to cause methemoglobinemia in infants; the ac-

tual toxin is nitrite that is formed by the reduction of nitrate

by intestinal bacteria. In tertiary toxicity, nitrite reacts with

amines in acidic medium to form nitrosamines which are

carcinogens and mutagens [4].

Many spectrophotometric methods are available for the

determination of nitrate. Extensive use has been made of

diazotization and coupling reactions after reduction of ni-

trate to nitrite [5–10]; copper-coated cadmium metal [5–7]

and titanium(III) chloride [8] have been used. However,

reduction should not proceed beyond nitrite. Reduction

of nitrate to nitrite and the catalytic effect of nitrite on

the oxidation of Naphthol Green B by bromate [11], or

reduction to nitric oxide and chemiluminescence detec-

tion by reaction with ozone [12] have been used. A novel

approach was reduction of nitrate to nitrite and reaction

with 2,4-dinitrophenylhydrazine to form an azide, which

is measured by Fourier Transform infrared spectrometry

[13]. This method was used for spiked natural waters.

Reaction of nitrate with 2-sec-butylphenol [14] or with

salicylic acid [15] and spectrophotometry has also been de-

scribed. Various fluorimetric methods have been reviewed

[16]. An integrated enzyme-functionalized field-effect

0003-2670/$ – see front matter © 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.aca.2003.10.060

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232 N. Lohumi et al. / Analytica Chimica Acta 505 (2004) 231–237

transistor device for the sensing of nitrate is an interesting

development [17].

The shortcomings of the reported methods have been

discussed [15]. Nitrate can be utilized in the nitration of

3 phenols, the benzene ring being activated for electrophilic

nitration by the presence of a hydroxyl group, and the re-

sulting nitrophenols are intensely coloured in an alkalinemedium. This principle forms the basis of many spectropho-

tometric methods [2,14,15,18–21]. Strict adherence to re-

action conditions is necessary to avoid the uncertainty of

whether mono- or polynitrophenols are formed; an uncon-

trolled reaction can affect the absorption spectrum of the

product. Removal of organic matter that can undergo a sec-

ondary reaction with nitrate and of any colouring matter is

necessary. High concentrations of chloride vitiate the nitra-

tion reaction owing to the formation of nitrosyl chloride. The

aim of the present study was to evaluate phenols in order

that regioselective mono-nitro compound formation occurs;

to utilize an appropriate pre-treatment method to eliminate

the interference of chloride; to utilize solid phase extrac-tion (SPE) to remove interferences of organic matter and to

pre-concentrate the product in order to enhance the sensitiv-

ity of the determination; and to utilize the reaction chemistry

in, besides spectrophotometry, liquid chromatography (LC)

and gas chromatography–mass spectrometry (GC–MS).

2. Experimental

2.1. Equipment

Solid phase extraction (SPE) cartridges (1 ml) containing200 mg of Lichrolut EN polymeric sorbent were obtained

from Merck (Mumbai, India); other cartridges (CN, C18,

and Amino, 500 mg sorbent each) were obtained from All-

tech, Deerfield, IL. An all-glass 0.45m membrane filter

unit (Millipore-India, Mumbai) was used for filtration of LC

solvents and water samples.

All spectrophotometric measurements were made with

ATI-Unicam UV-2-100 UV-Vis spectrophotometer (Cam-

bridge, UK) using 1 cm matched quartz cells. The LC

instrumentation consisted of a Beckman System Gold 127

solvent module, a Rheodyne 7010 injector (20l loop), a

Shimadzu SPD-2A variable wavelength UV detector (8l

flow-through cell), and a Hewlett-Packard 3395 integrator.

The analytical column was 25 cm × 4.6mm i.d. ODS-2

(5m particle size, Anachem). The mobile phase was ace-

tonitrile:water:glacial acetic acid (50:50:0.5 (v/v)) with a

flow rate of 1 ml min−1; detection was at 295 nm. The peak

area was used for quantitation. The GC–MS instrumentation

consisted of a Hewlett-Packard (Avondale, PA) G1800B

GCD system (HP 5890 Series II gas chromatograph with a

quadrupole mass detector) having a HP-5 (5% phenyl sub-

stituted methylpolysiloxane) 30 m × 0.25 mm i.d. (0.25m

film thickness) capillary column. Helium (99.999%), at

a flow rate of 1mlmin−1, was used as carrier gas. The

injector temperature was 250 ◦C. A sample volume of 1l

was injected. The GC oven temperature was held at 65 ◦C

for 2 min, programmed to rise to 225 ◦C at 25 ◦Cmin−1 and

held for 3 min. The GC-MS transfer line was maintained

at 300 ◦C, and the mass spectrum was scanned from m/z

45 to 450. Electron energy of 70 eV and splitless injec-

tion mode were used. Chromatographic data were acquiredusing HP ChemStation software G1074B version A.01.00

(Hewlett-Packard).

2.2. Reagents, standards and samples

A stock solution of 1000 g l−1 nitrate was prepared by

dissolving 0.1620 g of potassium nitrate (Merck) in 100 ml

of de-ionized water. A stock solution of nitrite was prepared

by dissolving 0.1500 g of sodium nitrite (Merck) in 100 ml of

de-ionized water. Less concentrated working standards were

prepared by sequential dilution of the stock solution with

de-ionized water. HPLC grade acetonitrile and methanolwere used for solution preparation. Separate stock solutions

of phenol, 2-methylphenol and 2,6-dimethylphenol, 5 g l−1

each, were prepared in acetonitrile and stored in the refrig-

erator when not in use. A mixed solution of concentrated

ammonia and methanol (1:9 (v/v)) was used for elution of

the nitro-derivative from the SPE cartridge.

2.3. Determination of nitrate

A sample volume (ca. 3 ml) containing 0.050–10mg l−1

nitrate was mixed with 0.2 ml of a phenol reagent and 1 ml

of 15 M sulphuric acid, and allowed to stand for 10 min in a

water bath at 60 ◦C. The reaction mixture was cooled to room

temperature and diluted to 20 ml with de-ionized water. This

solution was passed through a Lichrolut EN SPE cartridge

that had previously been activated and equilibrated by pass-

ing in sequence 2 ml each of methanol and de-ionized water.

The sorbent was washed with 2 ml of de-ionized water and

the nitrophenol eluted with 2 ml of ammonia–methanol mix-

ture. The absorbance of the yellow eluate was measured at

396 nm in a 1 cm cell against a methanol blank. Or, the eluate

was mixed with 100l of glacial acetic acid and evaporated

under a stream of nitrogen (ca. 40 ml min−1), the residue

was reconstituted into 500l of the mobile phase for LC,

and a 10l aliquot was injected into the LC system. Or,the eluate was mixed with 100l of glacial acetic acid and

evaporated under a stream of nitrogen (ca. 40 ml min−1), the

residue was reconstituted into 500 l of ethyl acetate, mixed

with 10l of the internal standard (500g l−1 thymol) and

1l was injected into the GC system.

2.4. Removal of organic matter

A sample volume (about 3 ml) was passed through a

Lichrolut EN SPE cartridge that had previously been acti-

vated and equilibrated by passing in sequence 2 ml each of

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N. Lohumi et al. / Analytica Chimica Acta 505 (2004) 231–237 233

methanol and de-ionized water. The eluate was subjected to

the determination of nitrate as above.

2.5. Removal of nitrite

To a measured volume (3 ml) of sample solution, 0.1 ml of

0.5% (w/v) urea solution and 1 ml of 5 M sulphuric acid wereadded and the solution was shaken for 5 min. This solution

was subjected to the determination of nitrate as above.

2.6. Removal of chloride

To a measured volume (3 ml) of sample solution, ca.

100 mg of silver sulphate was added and the solution stirred

for 5 min; thereafter, the insoluble mass was removed by

centrifugation. The supernatant was subjected to the deter-

mination of nitrate as above.

3. Results and discussion

3.1. The chemistry

Electrophilic nitration of phenol or its methyl deriva-

tives is a well known reaction for the formation of a

nitrophenol [22]. As the phenolic reagent is present in

excess and, since the benzene nucleus becomes deacti-

vated with the introduction of the first nitro group, the

formation of polynitrated derivatives was considered to

be a remote possibility. Nonetheless, the identity of the

reaction product was confirmed by GC–MS. After deriva-

tization and SPE, the eluate containing the derivative wastreated with 0.1ml of glacial acetic acid and diluted to

4 ml with de-ionized water. This solution was subjected to

solid phase microextraction (SPME) on polyacrylate fibre.

The fibre was retracted into the syringe and the retained

compound was desorbed in the GC injector. The mass

spectrum of the peak corresponding to the nitro-derivative

Fig. 1. Total ion chromatogram for the SPME–GC–MS of nitro-derivative formed by reaction of 1 mg l−1 nitrate with phenol; Peak 1 was identified as

4-nitrophenol, inserts are mass spectra corresponding to peak 1 (upper trace) and of standard 4-nitrophenol in the library (lower trace). Column: HP-5,

30 m× 0.25 mm i.d., 0.25m film thickness, carrier gas helium, flow rate 0.9 ml min−1. Temperature gradient, 65 ◦C for 3 min, increased at 15 ◦Cmin−1

to 170 ◦C and held for 3 min. SPME fibre PA, 85m; sorption time 10 min; desorption period 5 min at 250◦C.

was matched with that in the Wiley275 library of GC–MS.

Side products are formed when phenol is used as a reagent

(Fig. 1), but 2-methylphenol and 2,6-dimethylphenol

gave the expected 2-methyl-4-nitrophenol (Fig. 2) and

4-nitro-2,6-dimethylphenol (Fig. 3), respectively, as sin-

gle nitro-derivatives. Thus, 2-methylphenol and 2,6-

dimethylphenol were used in subsequent studies.

3.2. Optimization of conditions

Owing to the simplicity of the experiments, UV-Vis spec-

trophotometry was used for optimization of reaction con-

ditions. Both 2-methylphenol and 2,6-dimethylphenol were

found to behave identically. Model results are presented here

for a 2mg l−1 nitrate standard when subjected to conversion

into 2-methyl-4-nitrophenol, each parameter being varied in

turn to study its effect. The absorbance was measured at

396 nm.

3.2.1. Effect of sulphuric acid

The reaction was carried out using 0.05–2 ml of 15 M

sulphuric acid, when the absorbance increased from 0.010

to 0.801 for up to 1 ml of acid, and thereafter remained

practically constant when up to 2 ml of acid was used. In

later experiments 1 ml of 15 M sulphuric acid was used.

3.2.2. Effect of 2-methylphenol

The absorbance increased from 0.245 to 0.808 when the

phenol reagent (5 g l−1) was used in the range 0.05–0.2 ml,

and remained constant up to 2 ml of the reagent; 0.2 ml of

reagent was found to be optimum for derivatization.

3.2.3. Effect of heating time

Reaction for about 1 min at ambient temperature produced

an absorbance of 0.138. However, heating at 60 ◦C in a

water-bath had a pronounced effect on the absorbance, a re-

action period of 5 min gave an absorbance of 0.760 which

increased to 0.812 after 10 min and then remained constant

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Fig. 2. Total ion chromatogram for the SPME–GC–MS of the derivative formed by reaction of 1 mg l−1 nitrate with 2-methylphenol. Peak at 7.23min

was identified as 2-methyl-4-nitrophenol. Chromatographic conditions as in Fig. 1.

Fig. 3. Total ion chromatogram for the GC–MS of nitro-derivative formed on reaction of 4 mg l−1 nitrate with 2,6-dimethylphenol; Peak at 6.80 min was

thymol (internal standard) and at 8.72min was identified as 4-nitro-2,6-dimethylphenol; insert are mass spectra corresponding to peak 8.72 min (upper

trace) and that for the standard compound in the library (lower trace). Chromatographic conditions as in the text.

on heating for up to 25 min. The optimal heating time was

10 min at 60 ◦C.

3.2.4. Stability of colour (reaction product)

The reaction mixture after SPE gave an absorbance of

0.810 and, after keeping for 48 h in the dark the value found

was 0.811. Thus, the colour was stable for at least 48 h in

solution.

3.2.5. Off-line SPE

Various SPE sorbents were compared for preconcentra-

tion of the yellow 2-methyl-4-nitrophenol on the basis of

its retention, breakthrough volume and elution behaviour

(Table 1). Lichrolut EN is an excellent sorbent for reten-

tion of polar compounds, such as phenols. The nitro-phenol

retained on the SPE cartridge can be stored for at least 1

month at 18 ◦C after drying the cartridge with nitrogen and

packing in a bottle flushed with nitrogen. In typical experi-

ments, the absorbance or peak area for nitro-phenol was in

range 98.5–101.7%, of the initial value, after storage for 1

month on the cartridge.

In acidic medium, the nitrophenol derivative was mostly

undissociated and therefore both – interaction and

H-bonding was a possible mechanism for strong retention.

The subsequent elution of nitrophenol was found to be very

difficult. Different solvents, viz., methanol, acetonitrile,

methanol:acetonitrile (1:1 (v/v)), ethyl acetate and ethyl

acetate:methanol (1:1 (v/v)), could not fully elute the nitro-

phenol derivative. Finally, 2 ml of methanol:ammonia (9:1

(v/v)), was found to give complete elution of the retained

nitro derivative. There was a deepening in colour after

Table 1

Comparison of various SPE sorbents for retention of 2-methyl-4-

nitrophenol

Sorbent Amount

(mg)

Retention and elution behavioura

Bonded silica

CN 500 No retention

C18 500 Retained as diffused band, V b25ml; elution with 1 ml of

methanol

NH2 500 No retention

LiChrolut EN 200 Strong retention, V b 200ml;

elution with >5 ml of methanolb

SAX 100 No retention

a V b: breakthrough volume.b The retained compound was eluted with 2 ml of 1:9 (v/v) concentrated

ammonia:methanol.

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elution due to the formation of 2-methyl-4-nitrophenoxide

ion (wavelength of maximum absorbance = 396 nm) which

is more resonance stabilized than the undissociated phenol

(wavelength of maximum absorbance = 295 nm).

3.2.6. Preconcentration

The aim of the study on breakthrough volume was torecord the volume of the sample and thus the amount of

analyte that can be pre-concentrated. Insufficient retention of

the analyte by the sorbent and overloading of the sorbent are

two factors responsible for breakthrough of analyte. In this

work 10 ml of 50g l−1 nitrate was subjected to conversion

into the nitrophenol derivative and was mixed with 0, 25,

50, 100, 150 and 200 ml aliquots of distilled water. The

diluted samples were pre-concentrated by SPE on 200 mg

of Lichrolut EN that had been previously been activated

and conditioned with 1 ml each of methanol and distilled

water, respectively. On Lichrolut EN cartridge as much as

200 ml of nitrophenol solution could be preconcentrated with

almost no breakthrough and when eluted (2 ml of elutingagent), as above the average recovery was 99.4% with a

relative standard deviation of 3.5%. Thus, there was at least

a 100-fold enrichment.

An alternative method was also used for preconcentration

of nitrate. An aliquot of 50–100 ml of sample was adjusted

to pH 8 with sodium hydroxide and slowly evaporated to

3–5 ml. After cooling, any insoluble mass formed was re-

moved by centrifugation. This solution was subjected to de-

termination of nitrate as above. Evaporation of the sample

solution after adjustment to pH 8 avoided any side-reaction

of nitrate, such as nitration of humic substances or forma-

tion of volatile nitrosyl chloride; in both cases low recover-ies for nitrate are obtained. Nitrate standards, after dilution

to 100 ml, when analyzed by this method gave a mean re-

covery of 97.9% with a relative standard deviation of 5.1%.

3.2.7. Analysis

The eluate from SPE was either subjected to spectropho-

tometry, with 2-methylphenol as reagent, or evaporated and

the residue reconstituted in a solvent for analysis by GC

or LC. Typical chromatograms of the reaction product ob-

tained by GC–MS (2,6-dimethylphenol as reagent), and LC

(2-methylphenol as reagent), are shown in Figs. 3 and 4. The

phenol reagents can be interchanged for the final technique

of measurement without significantly affecting the results.

3.3. Calibration, limit of detection and validation

Rectilinear calibration graphs were obtained over the

range 0.050 –10 mg l−1 nitrate (conversion into 2-methyl-

4-nitrophenol and spectrophotometry), 0.030–10 mg l−1

(conversion into 2-methyl-4-nitrophenol and LC), and

0.020–10 mg l−1 (conversion into 4-nitro-2,6-dimethyl-

phenol and GC–MS). Over the entire range tested, the

correlation coefficient, r (n = 6), was 0.9967 for spec-

trophotometry and 0.9982 for LC and GC. The maximum

Fig. 4. Chromatogram obtained for 1 mg l−1 nitrate spiked in a Narmada

river water sample (Jabalpur) after derivatization/off line SPE/LC (up-

per trace) and for an unspiked sample (lower trace). Peak designation:

1 = 2-methyl-4-nitrophenol. Column: C18, 25cm × 4.6 mm i.d. (parti-

cle size 5m); Lichrolut EN (200mg) off-line SPE cartridge; detection

wavelength 295nm; AFS 0.08; mobile phase, acetronitrile:water:glacial

acetic acid, 50:50:0.5 (v/v); flow rate 1 ml min−1

; injection volume 20l.

molar absorptivity was found to be 1.27×104 lmol−1 cm−1

at 396 nm. The 3 limit of detection was 10g l−1 in spec-

trophotometry, 6g l−1 in LC, and 3g l−1 in GC–MS.

Interferences have been studied by spiking 2 mg l−1 ni-

trate standards with known amounts of interferents and ana-

lyzing by the present method. Excepting the amount of ana-

lyte handled by the individual method, the basic chemistry of

nitro-derivative formation is the same, and results obtained

by spectrophotometry, LC and GC–MS agreed to within 3%.

Up to 400g l−1 nitrite did not cause more than 1% inter-

ference. This was perhaps due to decomposition of nitrousacid in 15 M sulphuric acid, which was faster than the nitra-

tion of methylphenols. At 1 and 10 mg l−1 nitrite, the results

for the determination of 2 mg l−1 nitrate (without decompo-

sition of nitrite with urea) were respectively about 10 and

100% higher than in the absence of nitrite. Up to 50 mg l−1

nitrite was masked by adding 0.5 ml of 0.5% urea solution.

Up to 1 g l−1 chloride did not cause more than 1% interfer-

ence; at 3 and 5 g l−1 chloride level, the results for nitrate

were 10 and 25% lower. Pretreatment with 100 mg of silver

sulphate could remove the interference from up to 3 g l−1

chloride. Methods were validated by analyzing de-ionized

water and river water samples spiked with known amounts

of nitrate (Section 3.4). Separate experiments were carried

out on river water samples spiked with known amounts of

nitrate and either 10–40 mg l−1 nitrite or 1–3 g l−1 chloride.

The recovery of nitrate (n = 5) was found to be in range

99.0–101.5% (relative standard deviation, R.S.D. 2.5%), and

97.2–99.5% (R.S.D. 5.3%), respectively.

3.4. Application to real samples

The method was applied to determine nitrate in certain

samples of drinking water, river water and ground water. All

water samples were filtered through an 0.45 m membrane

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Table 2

Determination of nitrate in natural water samples

Sample (India) Nitrate founda (mgl−1) R.S.D. (%) Recovery of spiked nitratea (%) R.S.D. (%)

Yamuna river water (Mathura) 2.08 1.9 98.5 2.5

Narmada river water (Jabalpur) 0.28 4.1 104.6 3.9

Ganga river water (Kolkata) 1.43 4.6 107.0 5.1

Ground water (Jabalpur) 0.27 5.2 108.4 4.6Drinking water (Jabalpur) 0.21 5.0 98.7 4.8

Sea water (Mumbai) 0.85 5.8 98.9 6.1

Vehicle factory untreated effluent (Jabalpur) 5730 4.8

a The result are the average of three determinations, spiked at 2 mg l−1.

Table 3

Comparison of features for the determination of nitrate by various methods

Method Detection limit Linear range Reference

Flow injection spectrophotometry

Reduction/diazotization-coupling 12.4g l−1 0.012–8.9 mg l−1 [23]

Reduction/diazotization-coupling ? 4.4−17.7mg l−1 [24]

Reduction/diazotization-coupling 88.6g l−1 0.089–22.1 mg l−1 [8]

Reduction/catalytic oxidation of Naphthol Green B 2.5g l−1 0.01–1mg l−1 [11]

Flow injection chemiluminescence

Reduction/NO–O 3 reaction 0.7g l−1 0.022–4.4 mg l−1 [12]

Sequential injection spectrophotometry

Reduction/diazotization-coupling 0.2 mg l−1 1.2–24.8mg l−1 [10]

Indirect atomic absorption spectrometry

Effect on cadmium 50g l−1 0.5–14mg l−1 [25]

Spectrophotometry

Reduction/diazotization-coupling-extraction ? 30–200g l−1 [26]

Reduction/diazotization-coupling-preconcentration 5.3g l−1 6.8–132.9 g l−1 [6]

Nitration−2-sec-butylphenol 180g l−1 0.5–5mgl−1 [14]

Nitration-sodium salicylate 44.3g l−1 0.44–4.43 mg l−1 [15]

Nitration−2-methylphenol/SPE 10g l−1 0.05–10mg l−1 This work

LC-UV

Nitration of 2-methylphenol/SPE 6g l−1 0.03–10mg l−1 This work

GC–MS

Nitration of 2,6-dimethylphenol/SPE 3g l−1 0.02–10mg l−1 This work

filter prior to analysis. In order to assess the selectivity of

the pre-concentration procedure and to analyze nitrate at the

concentrations found in the water samples, a 5 ml volume of

test sample was analyzed after spiking with 2 mg l−1 nitrate,

and without spiking. The results are given in Table 2. In all

samples analyzed, the recovery was within acceptable limits.

4. Conclusions

Conversion of nitrate into a nitro-phenol derivative results

in its strong retention on Lichrolut EN polymeric cartridge

in standard SPE procedure resulting into at least 100-fold

enrichment. The nitro-phenol derivatives could be stored up

to 1 month on the SPE cartridge at 18 ◦C. This reaction for

nitrate is a useful and convenient alternative to that used in

azo dye formation, and it allows a choice of final measure-

ment by spectrophotometry, LC or GC. A comparison of

features of merit for the determination of nitrate by various

methods is given in Table 3.

Acknowledgements

University Grants Commission, New Delhi, is thanked forfinancial support to this work by a Research Project (No.

F.12-5/2001/SR-I) to KKV, a Junior Research Fellowship to

NL, and an Emeritus Fellowship to VKG.

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