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Available online at www.sciencedirect.com

Journal of Chromatography A, 1174 (2007) 1319

Determination of nitrate esters in water samplesComparison of efficiency of solid-phase extraction

and solid-phase microextraction

Vera Jezova a, Jan Skladal b, Ales Eisner a, Petra Bajerova a,, Karel Ventura a

a University of Pardubice, Faculty of Chemical Technology, Department of Analytical Chemistry, 532 10 Pardubice, Czech RepublicbResearch Institute of Industrial Chemistry, Explosia a.s., 530 50 Pardubice Semtn, Czech Republic

Available online 30 August 2007

Abstract

This paper deals with comparison of efficiency of extraction techniques (solid-phase extraction, SPE and solid-phase microextraction, SPME)

used for extraction of nitrate esters (ethyleneglycoldinitrate, EGDN and nitroglycerin, NG), representing the first step of the method of quantitative

determination of trace concentrations of nitrate esters in water samples.EGDN and NG are subsequently determined by meansof high-performance

liquid chromatography with ultraviolet detection (HPLC-UV). Optimization of SPE and SPME conditions was carried out using model water

samples. Seven SPE cartridges were tested and the conditions were optimized (type of sorbent, type and volume of solvent to be used as eluent).

For both nitrate esters the limit of detection (LOD) and the limit of quantification (LOQ) obtained using SPE/HPLC-UV were 0.23 g mL1 and

0.70g mL1, respectively. Optimization of SPME conditions: type of SPME fibre (four fibres were tested), type and time of sorption/desorption,

temperature of sorption. PDMS/DVB (polydimethylsiloxane/divinylbenzene) fibre coating proved to be suitable for extraction of EGDN and NG.

For this fibre the LOD and the LOQ for both nitrate esters were 0.16g mL1 and 0.50g mL1, respectively. Optimized methods SPE/HPLC-UV

and SPME/HPLC-UV were then used for quantitative determination of nitrate esters content in real water samples from the production of EGDN

and NG.

2007 Elsevier B.V. All rights reserved.

Keywords: Nitrate ester; EGDN; NG; SPE; SPME; HPLC

1. Introduction

Contamination of water and soil is a global environmental

problem and it is important to find analytical techniques to cope

with this challenge. When contaminants are present in natural

samples, their concentration is usually limited to trace levels.

Therefore, as the first step of their quantitative determination the

extraction technique is used, capable of isolating the analyte and

removing matrix interferences. Widely used is the combination

of modern extraction techniques with chromatographic systems.

Nitrate esters have beenmanufactured, stored, tested and used

worldwide over the past centuries. This means that nitrate esters

entered the environment and became soil and water contami-

nants. The Czech Republic, as nitrate esters producer, has to

monitor thequality of thewaters leaving current or formernitrate

esters production plants or the places of military training areas

Corresponding author. Tel.: +420 466037088; fax: +420 466037068.

E-mail address: [email protected](P. Bajerova).

andammunition stores. Also, there exists a risk of contamination

of surface or underground waters.

Ethyleneglycoldinitrate (EGDN) and nitroglycerin (NG) are

typical components of various explosive materials and represent

one group of environmental contaminants. EGDN is transpar-

ent, colourless, liquid explosive, less sensitive than NG, used

mainly in mixtures with NG for low-freezing dynamites. NG is

used in the production of commercial explosives as e.g. dyna-

mites, and as the component of multi-base propellants. In low

concentrations, NG functions as a drug to cure cardiovascu-

lar diseases. Well skin absorbable NG is however toxic. It is

colourless, odourless viscous liquid explosive, very sensitive to

manipulation. EGDN and NG are relatively soluble in water

(EGDN more than NG). Toxic and environmental effects of

EGDN are similar to those of NG. However, because of higher

EGDN vapour pressure, the exposure to EGDN results in more

acute symptoms and clinical manifestations [1].

Extraction methods SPE and SPME are powerful techniques,

widely used in analysing various food, biological and environ-

mental matrices. A few references and publications were found

0021-9673/$ see front matter 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.chroma.2007.08.053
mailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.chroma.2007.08.053http://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.chroma.2007.08.053mailto:[email protected]

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14 V. Jezova et al. / J. Chromatogr. A 1174 (2007) 1319

concerning the use of these methods for extraction of explosive

compounds, namely nitramines [28] and aromatic nitrocom-

pounds [26,8,9]. Only, a minimal number of publications exists,

dealing with utilization of these extraction methods for determi-

nation of nitrate esters [1012].

SPE is a very simple way how to extract the molecules of

interest from complex aqueous matrices. The results are com-

parable with those of classical extraction as e.g. liquidliquid

extraction [2]. SPE has been successfully applied to the extrac-

tion of some types of nitramines and aromatic nitrocompounds

in seawater, river water or well water samples [4,6].

SPME is extraction technique for analysing liquid, solid and

gas samples.Basic component of SPME system is the silica fibre

coated with polymer sorbent (stationary phase), placed in SPME

holder. Several studies describe the coupling of SPME with gas

chromatography (GC) for determination of some nitramines and

aromatic nitrocompounds [3,9,10,13]. Utilization of SPME/GC

coupling is more common than SPME/HPLC, as the latter one

requires the use of SPME/HPLC adapter. SPME/GC determina-

tion of nitrate esters is, however, problematic because of thermalinstability and sorption in the analytical system [14,15].

SPME/HPLC determination of nitramines and aromatic

nitrocompounds in environmental water samples (four seawa-

ter and two groundwater) was described in [4]. Three SPME

fibres were tested for isolation of some nitramines and aromatic

nitrocompounds from water. The results were compared with

SPE/HPLC as SPE is used in routine determinations of explo-

sive compounds in water. It was found that if compared with

SPE, the SPME detection limits were ten times higher.

This paper describes utilization of SPE/HPLC-UV and

SPME/HPLC-UV for quantitative determination of trace con-

centrations of nitrate esters content in water samples. Theefficiency of SPE and SPME was compared.

The aim of these experiments was to compare the efficiency

of extraction techniques at theanalysisof water samples contam-

inated with EGDN and NG. Optimization of various extraction

parameters is necessaryto achieve the best extraction recoveries.

2. Experimental

2.1. Chemicals and reagents

Nitrate esters standards (Supelco, Bellefonte, PA, USA) were

dissolved in methanol. All solvents (methanol, acetonitrile,

acetone) were from Riedel-de Haen, SigmaAldrich (Seelze,Germany). De-ionized water used for model samples was pre-

pared with Demiwa 5-roi purification system (Watek, Ledec nad

Sazavou, Czech Republic) and by Ultra Clear UV system (SG

Water Nashua, NH, USA).

2.2. Water Samples

2.2.1. Model water samples

Model water samples were prepared from de-ionized and

drinking water. EGDN and NG were spiked into 200 mL of de-

ionized or drinking water in the concentration 3 g mL1 of

each compound.

2.2.2. Real water samples

Real water samples used were from the production of nitrate

esters (Explosia, Pardubice, Czech Republic). The wastewaters

leaving production process enter the reactor to be pre-treated.

Substantial decrease in nitrate esters content is achieved by

means of continuous reduction with iron (steel microballs) in

neutral medium [16]. This way pre-treated waters are discharged

to biological wastewater treatment plant. Real water samples

were taken on the outlet of pre-treatment reactor in three various

time intervals (T1, T2, T3).

2.3. SPE procedure

Various polymeric sorbents and solvents were tested to com-

pare their extraction properties in the isolation process of EGDN

and NG from water samples. SPE procedure was tested using

various types of cartridges and several sorbents: Supelco Dis-

covery DSC-18LT-1000mg/6 mL, monomer octadecyl, Supelco

DSC-C8-500mg/3 mL, octyl monomer, and Supelco DSC-PH-

500 mg/3 mL, phenyl monomer (Supelco Bellefonte, PA, USA);three cartridges Phenomenex (Torrance, CA, USA) Strata X

(500 mg/6 mL, surface-modified styrene-divinylbenzene poly-

mer), Strata XC (500 mg/6 mL, 33m caption mixed polymer)

and Strata C18 E (500 mg/3 mL, octadecyl). The last cartridge

tested was Waters Oasis HLB (60 mg/3 mL, divinylbenzene

and N-vinylpyrolidone) (Waters, Milford, MA, USA). SPE pro-

cess was conducted on Visiprep SPE Vacuum manifold system

Supelco (Bellefonte, PA, USA).

The general sample preparation scheme for SPE is: condi-

tioning of the SPE columns, followed by sample loading and

finally elution of the analytes off the solid-phase columns. SPE

cartridges were conditioned immediately prior to each extrac-tion experiment by means of successive washing with methanol

and de-ionized water. 200 mL aliquot of model water sample,

spiked with 3g mL1 of each nitrate ester, was transferred to

SPE cartridgefor extraction. Nitrate esterswere eluated from the

SPE cartridge with solvent (methanol, acetonitrile or acetone).

The eluate was concentrated under a stream of nitrogen to the

final volume of 1 mL with 20L of which being injected into

HPLC system.

2.4. SPME procedure

Solid phase microextraction was used as a reference method

for evaluation of the results obtained by SPE in terms ofquantification limits and accuracy. The manual SPME holder

used and fibres, coated with various polymers: polydimethyl-

siloxane/divinylbenzene (PDMS/DVB, 65m film thickness),

Carbowax/divinylbenzene (CW/DVB, 65m film thickness),

Carboxene/polydimethylsiloxane (CAR/PDMS, 75m film

thickness), Carbowax/templated resin (CW/TPR, 50m film

thickness), as well as SPME/HPLC interface desorption reser-

voir were from Supelco, Bellefonte, PA, USA. Before its first

use, each fibre was conditioned in mobile phase for 30 min.

SPME process was carried out in 8 mL glass vials with

magnetic stirring (500 rpm), using aliquots of water samples

(4 mL) containing 20g mL1

of both nitrate esters and addi-

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V. Jezova et al. / J. Chromatogr. A 1174 (2007) 1319 15

tions of water solution (10%) of sodium chloride. Nitrate esters

were desorbed in desorption chamber of SPME/HPLC inter-

face. To minimize the possibility of carry-over effect the fibre

was washed with methanol for 5 min before the next extraction.

2.5. HPLC-UV system

HPLC analyses were carried out using chromatographic

system with UV-detector (Ecom, Prague, Czech Republic). Sep-

aration of nitrate esters was performed in a column LiChrospher

100 RP-18e, Merck (Darmstadt, Germany). Security Guard C18

Phenomenex was used as well. Isocratic mobile phase acetoni-

trile:water (60:40, v/v) was used for separation.

The analyses were conducted at flow rate of 0.5 mL min1

and column temperature was kept at 25 C. Twenty microlitres

sample was injected through six-port valve into chromato-

graphic column and nitrate esters eluted within 10 min.

UV-detector wavelength was set at 210 nm. Quantification was

carried out using calibration curve and peak area measurements.

2.6. Method validation

Calibration curves were created ranging from 0.7 to

7g mL1 (0.7, 1, 3, 5 and 7) for EGDN and NG in case of

SPE extraction and 0.5 to 7g mL1 (0.5, 1, 3, 5 and 7) in

case of SPME extraction, respectively. The calibration curves

consisted of five different concentrations of analyte per 1 mL of

water. Each concentration point was determined in five repli-

cates. Curves were prepared by adding varying concentrations

of analytes to water.

The precision and accuracy of the method were evaluated

intra- and inter-day by analysis of six replicates of three con-centrations, including the limit of quantification (LOQ) as the

lowest point of calibration curve, middle and highest point. LOQ

was set asthe lowest amount of an analyte in a sample that can be

determined quantitatively with suitable precision and accuracy.

The accuracy of the method was determined as percent error,

while precision was evaluated by intra- and inter-day relative

standard deviation. All parameters were in acceptable range of

15%.

3. Results and discussion

3.1. SPE optimization

3.1.1. Extraction recoveries with various cartridges

Selection of proper SPE cartridge with the most suitable sor-

bent material plays an important role in achieving high and

reproducible recovery of contaminants. Several types of SPE

sorbents from various producers were tested for extraction of

nitrate esters from water samples. Methanol was used as elution

solvent. The best extraction for both compounds was obtained

with Waters Oasis HLB and Strata X cartridges (Fig. 1). The

other cartridges produced high NG extraction recoveries and

only low EGDN yields. Out of all SPE cartridges tested, Waters

Oasis HLB copolymer cartridges were chosen to be used for

further testing.

Fig. 1. Extraction recoveries obtained with tested SPE cartridges.

3.1.2. Elution with different solvents

The recovery of organic compounds by SPE is highly

dependent on the polarity of eluents. Acetone, acetonitrile andmethanol as eluents were tested for the elution of EGDN and

NG from Waters Oasis HLB and Strata X cartridges. Compari-

son of these two SPE cartridges and elution with all solvents is

shown in Fig. 2. The results show the best extraction recoveries

for Waters Oasis HLB cartridges for all solvents and for both

nitrate esters. Acetone and methanol produced almost identi-

cal recoveries (between 70% and 86% for EGDN and 75% and

89% for NG). The best elution recoveries were obtained with

acetonitrile (90% for EGDN and 92% for NG).

Acetonitrile as elution solvent in combination with Waters

Oasis HLB cartridge was chosen for the analyses of real water

samples and measurements of limit of detection (LOD).

3.2. SPME optimization

In order to develop a direct-SPME procedure for the analysis

of EGDN and NG in water samples, several parameters related

to the extraction and desorptionprocesses were evaluated (selec-

tion of SPME coating, effect of sorption and desorption mode

and effect of temperature).

Fig. 2. Comparison of various elution solvents.

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Fig. 3. Comparison of recoveries with various SPME fibres.

3.2.1. Selection of SPME coating

Four SPME fibre coatings were tested for isolation of

nitrate esters from water samples. The fibre coating domi-

nates the recoveries of analytes and influences sorption time.Optimization of SPME conditions for determination of nitrate

esters was accomplished using aliquots of de-ionized water

(4 mL) spiked with both nitrate esters (20g mL1). Each

fibre (CW/TPR, CAR/PDMS, PDMS/DVB and CW/DVB) was

immersed into spiked de-ionized water for 15 min at 50 C.

The other sorption and desorption conditions were as follows:

direct sampling, magnetic stirring static, and desorption in the

desorption chamber for 10 min. Comparison of recoveries with

these fibres is shown in Fig. 3. The best results were obtained

with PDMS/DVB fibre.

3.2.2. Effect of sorption modeInfluence of sorption mode on theefficiency of extraction was

evaluated using stirred de-ionized water spiked with both nitrate

esters. Conditions for direct and headspace sorption were kept

constant in all experiments. Both types of sorption were con-

ducted at 50 C with PDMS/DVB fibre in vessels capped with

a Teflon-lined septum. Sorption time was limited to 15 min and

time for desorption to 10 min in all cases. Extraction efficiencies

at directand headspacesorptionare compared in Fig.4(a). Better

sorption of analytes was achieved when SPME fibre was exposed

directly to water sample. EGDN and NG are not volatilized and

this is why the extraction efficiencies at headspace sorption were

lower than in case of direct sorption. The direct sorption also

showed to be the most suitable extraction mode for nitrate esters.SPME is an equilibrium process, in which analytes are dis-

tributed between the sample matrix and SPME stationary phase.

This is the reason why the sorption time is necessary to be

optimized. The influence of extraction time was measured by

immersing the PDMS/DVB fibre into spiked de-ionized water

for 170 min. In all cases, 4 mLof spiked de-ionized water, mag-

netic stirring, sorption at 50 C and desorption time 10 min were

used. Adsorption profile was defined as the function of time

(Fig.4(b)). Rapid rise in extraction efficiency for NG occurred in

the interval 130 min and after that time the efficiency remained

constant. In case of EGDN extraction efficiency increased with

time (110 min) and then remained constant. Extraction time

Fig. 4. (a) Comparison of direct and headspace sorption. (b) Optimization of

sorption time.

30 min proved to be sufficient to obtain quantitative extraction.

This extraction time is a reasonable compromise between a good

peak area and acceptable extraction time.

3.2.3. Effect of desorption mode

Desorption period (the period during which the nitrate esters

are eluated from fibre coating by means of mobile phase in the

SPME/HPLC-UV interface) was optimized after the sorption

mode optimization. PDMS/DVB fibre was immersed in stirred

spiked de-ionized water at 50 C for 15 min. SPME fibre was

then placed in SPME/HPLC-UV interface. Desorption times

varied within 1 and 20 min. The results (Fig. 5(a)) indicated

fast desorption and 5 min desorption time was then used for all

other SPME/HPLC-UV experiments.

Efficiency of static desorptionwith optimizeddesorptiontimewas then compared with the efficiency of dynamic desorption

(the analytes are immediately injected into the column) that

was, however, lower than that achieved with static desorption

(Fig. 5(b)).

3.2.4. Effect of temperature

Temperature plays an important role in the process of

extraction, because it influences the mass transfer rates and par-

tition coefficients of the analytes. Extraction temperature profile

3070 C for PDMS/DVB fibre was tested under these condi-

tions: 4 mL stirred spiked de-ionized water, magnetic stirring,

sorption 15 min and static desorption 10 min. NG peak area

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Fig. 5. (a) Optimization of desorption time. (b) Comparison of dynamic and

static desorption.

increased with increasing temperature and the amount of EGDN

sorbed decreased somewhat with increase in extraction tem-perature from 50 to 70 C (Fig. 6). Decreased EGDN sorption

with increasing temperature is caused by the decrease of the

distribution constant with increasing temperature. Temperature

50 C was chosen as a compromise to achieve good extraction

efficiencies for both methods and for both nitrate esters.

In summary, the optimized extraction conditions were estab-

lished to be: PDMS/DVB fibre, 30 min sorption and 5 min static

desorption time, 50 C temperature, addition of 10% water solu-

Fig. 6. Effect of temperature.

Fig. 7. (a) Calibration curves for SPE extraction. (b) Calibration curves for

SPME extraction.

tion of sodium chloride to increase the ionic strength, and

magnetic stirring.

3.3. Validation

Optimized extraction conditions (SPE and SPME) were used

for method validation. A five-point calibration curve was cre-

ated as described in Section 2. Good linearity was observed for

both compoundsin mentionedranges (r2 = 0.99).The calibration

curves resulting from analyses and the corresponding equations

are shown in Fig. 7(a) for SPE extraction and Fig. 7(b) for SPME

extraction. Typical HPLC separation of EDGN and NG with UV

detectionis shown in Fig.8. Thelimit of detectionwas arbitrarily

Fig. 8. HPLC separation of EDGN and NG with UV detection.

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Table 1

Concentrations of nitrate esters in model water samples spiked with 3 g/mL of each nitrate ester

Analyte Measured concentrations (g/mL)

SPE/HPLC-UV SPME/HPLC-UV

Spiked de-ionized water Spiked drinking water Spiked de-ionized water Spiked drinking water

EGDN 2.85 0.13 2.91 0.08 2.68 0.12 2.63 0.11

NG 2.80 0.11 2.83 0.09 2.74 0.10 2.69 0.13

Table 2

Concentrations of nitrate esters in realwater samples taken in threetime intervals

Sample Measured concentrations (g/mL)

SPE/HPLC-UV SPME/HPLC-UV

EGDN NG EGDN NG

T1 5.17 0.11 4.13 0.25 4.72 0.17 3.91 0.22

T2 6.97 0.21 1.94 0.23 6.72 0.17 1.68 0.11

T3 6.27 0.19 4.60 0.14 5.83 0.32 4.12 0.30

setat 1/3 of the limit of quantification. Thelimit of quantification

is the lowest concentration that can be determined in a sample

with acceptable precision under the stated operational condi-

tions of the method. The LODs for EGDN and NG obtained on

the basis of the calibration curve were 0.23g mL1, and LOQs

were 0.7g mL1 for both nitrate esters in case of SPE extrac-

tion, LODs for EGDN and NG were 0.16 g mL1 and LOQs

were 0.50g mL1 in case of SPME extraction, respectively.

The obtained LOD values were at the adequate levels for the

identification of target nitrate esters in analysed water samples.

Nitrate esters from 10 different water samples were extracted

and analysed for the determination of the specificity of themethod. No interference was observed in the region of the reten-

tion times of the individual analytes.

3.4. Comparison of SPE and SPME efficiencyapplication

to model water samples

Two hundred millilitres of de-ionized or drinking water were

spiked with EGDN and NG in concentration level 3g mL1

of each nitrate ester. The solutions were analysed by means of

SPE/HPLC-UV as well as SPME/HPLC-UV under optimized

conditions, and the efficiency was compared (Table 1).

3.5. Comparison of SPE and SPME efficiencyapplication

to real water samples

Real water samples were analysed by SPE/HPLC-UV and

SPME/HPLC-UV under optimized conditions. EGDN and NG

were detected in all analysed samples and their concentrations

are shown in Table 2.

4. Conclusion

Efficiency was compared of SPE and SPME at extraction of

EGDN and NG from water samples. By means of model water

samples important parameters SPE and SPME were optimized

and optimal measurement conditions for both extraction tech-

niques were defined. Subsequent quantitative determinations

were performed using SPE/HPLC-UV and SPME/HPLC-UV.

When comparing with the other tested SPE cartridges, the

highest extraction efficiency was achieved with Water Oasis

HLB. The best eluent proved to be acetonitrile.

SPME fibre with polydimethyl/siloxane/divinylbenzene

coating was found to be the most suitable for extraction of

nitrate esters from water samples. The combination of directsorptiondynamic desorption proved to be the best to obtain

high extraction efficiency.

For both extraction methods, the detection limits found were

in g mL1 range with SPME detection limit being higher. If

comparing SPE and SPME extraction efficiency of SPE was

higher and SPME was faster. The SPME method has the advan-

tage of being rapid and organic solvent-free.

The results of the study proved that SPE and SPME

techniques, in combination with HPLC-UV, are suitable for

quantitative determination of lowconcentrations of nitrate esters

in water samples.

The SPE and SPME procedures, both precise and accurate,

satisfy nitrate esters producers requirements. Finally, the SPE

approach is a further successful application in the quantification

of two important nitrate esters in the context of analysis of esters

in water samples; the SPME method, giving results comparable

to those obtained with the SPE method.

Acknowledgements

The experiments were performed thanks to financial support

from the Ministry of Education, Youth and Sports of the Czech

Republic (project MSM0021627502) and the Grant Agency of

the Czech Republic (GACR 203/05/2106).

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