Journal of Chromatography B, 830 (2006) 212–217

Optimisation of an extraction method for the determination ofprostaglandin E2 in plasma using experimental design and

liquid chromatography tandem mass spectrometry

Pedro Araujo∗, Livar FrøylandNational Institute of Nutrition and Seafood Research (NIFES), P.O. Box 2029 Nordnes, N-5817 Bergen, Norway

Received 9 August 2005; accepted 26 October 2005Available online 9 November 2005

Abstract

A new extraction method has been developed for the extraction of prostaglandin E2 (PGE2) from human plasma of patients suffering chronicinflammatory disorders. The extraction solvents were optimised systematically and simultaneously by using a central composite design. Theoptimised method involves precipitation of the protein fraction, centrifugation, evaporation and dissolution of the supernatant in the mobilep em ion-trapm3 ed .5 ng/ml ofi edh 6 to 98%.©

K

1

dna[pnis[ta[ps

ndinsnal-aco-icalre thedins

licity.peci-rinee totialt theh astog-tives

ngled outh as

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hase, screening to confirm the presence of the analyte, and quantification of the positive samples by liquid chromatography tandass spectrometry. Tandem mass spectrometry in negative mode was performed by isolating and fragmenting the ion [PGE2-H]− signal m/z51. Identification and quantification was carried out by extracting the ion fragment chromatograms at 333, 315 and 271m/z. The quantitativetermination was linear for the low nanogram (1–50 ng/ml) and upper picogram (400–1000 pg/ml) range studied, using 15 and 0

nternal standard, respectively. The lower limit of detection was 2.5 pg for an injection volume of 25�l. The optimised extraction method showigh reproducibility (coefficients of variation < 4%) and recovery values, estimated from standard addition experiments, ranging from 92005 Elsevier B.V. All rights reserved.

eywords: Prostaglandin E2; LCMS; Experimental design; Central composite design; Ion-trap mass spectrometry; Plasma extraction method

. Introduction

Prostaglandins are members of the lipid class of biochemicalserived from arachidonic acid by means of the cyclooxyge-ase enzyme. These substances are known for their potencynd varying physiological properties and pathological effects

1,2]. Prostaglandin E2 (PGE2), one of the most widely studiedrostaglandins, has bronchodilator action[3], promotes renalatriuresis and diuresis[4,5], increases motility and secretions

n the gastrointestinal tract[6] and protects the brain cells fromtroke damage[7], but it is also associated with inflammation8,9], pain[10,11], blood vessels constriction[12], blood clot-ing promotion[13], progression and metastases of a variety ofnimal and human tumours including breast, lung, and colon

14,15]. Due to their positive–negative health effect dichotomyrostaglandins have been of significant interest over the lasteveral decades and attracted the attention of pharmaceutical

∗ Corresponding author. Tel.: +47 55905115; fax: +47 55905299.E-mail address: pedro.araujo@nifes.no (P. Araujo).

manufacturers and nowadays there are many prostaglaavailable commercially. Thus, qualitative and quantitative aysis of prostaglandins may be a useful index of pharmlogical, physiological and pathological effects. Immunologassays (radioimmunoassay or enzyme immunoassay) amost widely used methods for the estimation of prostaglandue to their inherent sensitivity, inexpensiveness and simpThe main drawbacks of these assays are their lack of sficity for complex biological fluids, such as plasma and u[16], trend to overestimate the levels of metabolites ducross-reactivity, variability in the quantification of sequensamples and limitation to the detection of a single product atime[17,18]. Single or coupled instrumental techniques, sucmass spectrometry (MS), liquid (LC) and gas (GC) chromaraphy, GCMS and LCMS are suitable and accepted alternato immunoassays in the analysis of prostaglandins[18]. Deter-mination of prostaglandins in biological samples using sior coupled chromatography techniques has been carrieby means of a wide variety of analytical procedures, sucderivatisation, degradation, solid-phase or liquid extractionlayer chromatography and combination of all these proced

570-0232/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.jchromb.2005.10.038

P. Araujo, L. Frøyland / J. Chromatogr. B 830 (2006) 212–217 213

Thus, following the rapid development of analytical techniques,there has been a growing trend in prostaglandin analysis in bio-logical fluids towards faster extraction time, development ofnovel or simpler methods and improvement of the already exist-ing methods in terms of simplicity, routine analysis, improvedquantification and automation. Articles regarding the extractionand determination of prostaglandins in plasma using LCMS arenot widespread; the current literature is focused on samples,such as cultured cell lines[17,19–21], renal tissue[22], livermicrosomes[23], seminal fluids[24], gastric mucosa[25], etc.Recently, a simple and rapid extraction procedure based on theprecipitation of the protein fraction of human plasma samplesspiked with PGE2 and subsequent LC ion-trap MS analysis ofthe supernatant has been proposed[26] but a glaring error, that isspiking the samples with PGE2, standard and internal standardsafter the precipitation step has taken place, precludes any fur-ther conclusion on the accuracy and precision of this particularmethod. An extraction method which requires 1 h to derivatisethe prostaglandins completely prior to LCMS quantification in ahigh concentration calibration range (30–3000 ng/ml) has beenreported[27]. The aim of this study was to develop a rapid, sim-ple and efficient method for the extraction of PGE2 from humanplasma samples and subsequent screening and quantificationin the low nanogram range by using LC ion-trap MS/MS andexperimental design. Screening methods based on LCMS/MSinstrumentation are useful because they provide greater ana-l vels( all tiono

2

2

Ed MI,U adt,G em(

2

eringf withp ga-pPt

2

entsu naryet wasn ting

Fig. 1. Central composite design used in the optimisation of the extractionsolvents.

the white cloud of protein micelles formed in the upper phase(methanol–water) with acetonitrile, the supernatant revealed thepresence of PGE2. A central composite experimental design[29]was used to optimise the proportions of these extraction sol-vents. The experiments were arranged according toFig. 1 andthe studied concentrations of PGE2 standard dissolved in blankplasma were 30 and 0.5 ng/ml. The total number of experimentssuggested by a central composite design is calculated accordingto the expression 2x + 2x + 1 wherex represents the number ofvariables (methanol, water and acetonitrile). A total of 15 exper-iments are required (23 + 2× 3 + 1) in the present study, howeverit is important to note that the methanol and water proportionshave been chosen purposely in order to bring about a regionwhere the methanol:water (v/v) ratio remains constant (whitecircles inFig. 1). In this way it is possible to include in this con-stant region a third factor, namely the influence of the amountof acetonitrile and optimise the 3 variables with 9 instead of thecompulsory 15 experiments.

The optimisation strategy of the extraction procedure is asfollows: Nine test tubes containing 200�l of PGE2 30 ng/ml inacetonitrile standard solution were taken and evaporated to dry-ness under a stream of nitrogen at room temperature. Aliquotsof 200�l of human plasma, drawn from a healthy patient withnot detectable levels of PGE2, were added into the test tubes andvortex-mixed for 2 min. Nine mixtures of methanol and water (I)were prepared according to the central composite design showedi r-ef )a g/mlP otso fol-l dv enceI isr PGEsi ble

ytical efficiency and allow to discriminate endogenous le<0.012 ng/ml PGE2, no detectable by LCMS) from pathologicevels. This allows the analytical effort to focus on quantificaf the positive samples.

. Experimental

.1. Reagents

Prostaglandin E2 and deuterated prostaglandin E2 (PG2-4) were purchased from Cayman Chemical (Ann Arbor,SA). Acetonitrile and methanol were from Merck (Darmstermany). De-ionized water was purified in a Milli-Q syst

Milli-Q system Millipore, Milford, MA).

.2. Plasma samples

Plasma samples were drawn from fasting patients suffrom chronic inflammatory disorders and under treatmentharmacological medication supplemented either with omeolyunsaturated fatty acids (�-3 PUFAs) from seal oil or�-6UFAs from soy oil. Plasma samples were kept at−80◦C prior

o analysis.

.3. Extraction procedure optimisation design

Methanol, water and acetonitrile were the extraction solvsed in this work. Their selection was based on prelimixtraction experiments, using the Bligh and Dyer procedure[28]o extract PGE2 from spiked plasma samples. The analyteot detected in the lower phase (chloroform). After precipita

3n Fig. 1. The data inside the circles inFig. 1represent the diffent methanol:water (v/v) ratios. Portions of 400�l were taken

rom 6 of the I mixtures (indicated inFig. 1 as black circlesnd delivered into six of the nine test tubes containing 30 nGE2 dissolved in plasma and vortex-mixed for 30 s. Aliquf 400�l of acetonitrile (II) were added into these tubes

owed by further 400�l of the aforesaid six mixtures of I anortex-mixed for 30 s after each addition. The addition sequ→ II → I and vortex-mixing for 30 s after each addition,epeated on the remaining three test tubes containing the2tandard dissolved in plasma using 400�l of I (white circlesn Fig. 1 at constant methanol:water (v/v) ratios) and varia

214 P. Araujo, L. Frøyland / J. Chromatogr. B 830 (2006) 212–217

amounts of II (400, 1200, 2400�l). After addition of the sol-vents, the test tubes were centrifuged at 3000 rpm for 10 minat room temperature and the supernatants collected and evapo-rated to dryness under a stream of nitrogen at room temperature.The dried samples are reconstituted in 30�l of acetonitrile, son-icated for 30 s, transferred to autosampler vials and submittedto LCMS/MS analysis.

The procedure described above was also applied to nine sam-ples of blank plasma spiked with 0.5 ng/ml of PGE2.

2.4. Screening of PGE2 in plasma samples

Under optimal extraction conditions the procedure describedabove, without the spiking step, was applied on plasma samples(n = 6) from patients suffering chronic inflammatory disorders.The samples were submitted to LCMS/MS and the screeningcarried out by fragmenting the ion [PGE2-H]− signalm/z 351and identifying the characteristic product ions [PGE2-H2O-H]−,[PGE2-2H2O-H]− and [PGE2-2H2O-44-H]− at 333, 315 and271m/z, respectively.

2.5. Calibration curves

Two calibration ranges 0–1 ng/ml (low range) and 1–50 ng/ml(high range) were studied. A plasma sample from a healthypatient was spiked with different PGEand PGE-d concentra-t atioc el 40,5 .

2

withP oced

2

riesL SI),a d cu 5i1 ,U ts rmica seda r-a r gett lensa Viewo .C werd gi-lf

2.8. Statistics

Data were expressed as mean values and relative standarddeviations. The statistical analysis was done by Statgraphics Plus5.1 software package.

3. Results and discussion

3.1. Solvents optimisation

The analyte [PGE2-H]− m/z 351 was isolated and fragmentedinto the ions [PGE2-H2O-H]−, [PGE2-2H2O-H]− and [PGE2-2H2O-44-H]−. The summation of the intensities of the char-acteristic fragments (333, 315 and 271m/z) in ion counts persecond (icps) was recorded in order to select the optimal extrac-tion solvent condition. The extraction procedure and instrumen-tal precision were monitored by replicating the former threetimes at every experimental point inFig. 1 and the latter bymeasuring repeatedly (n = 3) some of the individual extractions.Table 1shows the experimental results obtained after performingthe extractions of 30 and 0.5 ng/ml PGE2 dissolved in plasmaat different methanol:water (v/v) ratios and different volumesof acetonitrile. It can be seen from this table that the highestmagnitudes of the analytical signal, at low and high PGE2 con-centration, were obtained at a methanol:water (v/v) ratios of3:1 and 2:1 at high and low concentration of PGE. A multipler rdeds gnalsa ant tiost levelo nifi-c ationl d thev s at3 ofmt lts inT ro-c ationi n thep andt ely4 tiono n, atb tionw PGEf

3

ovea ici aded hro-ms to

2 2 4ions as is described above in order to construct the calibrurves. Six equally spaced PGE2 levels were prepared in th

ow (0, 0.2, 0.4, 0.6, 0.8, 1 ng/ml) and high (1, 10, 20, 30,0 ng/ml) range using 0.5 and 15 ng/ml of PGE2-d4, respectively

.6. Plasma samples quantification

Positive samples from the screening step were spikedGE2-d4, treated according to the described extraction prure and submitted to LCMS/MS quantification analysis.

.7. Liquid chromatography ion-trap mass spectrometry

The LCITMS used in this study was an Agilent 1100 seC/MSD trap, SL model with an electrospray interface (Equaternary pump, degasser, autosampler, thermostatte

mn compartment, variable-wavelength UV detector and 2�lnjection volume. The column used a Zorbax Eclipse-C8 RP50 mm× 4.6 mm, 5�m (Agilent Technologies. Palo Alto, CASA) was kept in the column compartment at 40◦C. The solvenystem operated in isocratic mode was acetonitrile with focid 0.1% (v/v) and UV detection at 254 nm. Nitrogen was us nebulizing and drying gas at 350◦C. The ESI source was opeted in negative ion mode and the ion optics responsible fo

ing the ions in the ion-trap, such as capillary exit, skimmer,nd octapoles voltages were controlled by using the Smartption with a resolution of 13,000m/z/s (FWHM/m/z = 0.6–0.7)omplete system control, data acquisition and processingone using the ChemStation for LC/MSD version 4.2 from A

ent. The transitions monitored werem/z 351→ 333, 315, 271or PGE2, m/z 355→ 337, 319, 275 for PGE2-d4.

n

-

ol-

-

e

2ange test that allows the statistical comparison of the recoignals did not reveal significant differences between the sit 30 ng/ml PGE2 when the proportion of methanol is lower th

he proportion of water (ratios 1:3, 1:2 and 3:5). At these rahere was no detection of the analytical signal at the lowf prostaglandin studied in this work. There were not sigant changes in the signal, at the two analytical concentrevels, when the ratio methanol:water was kept constant anolume of acetonitrile was varied. In addition, the signal0 ng/ml of PGE2 recorded at the extraction point 5:3 ratioethanol:water and 400�l of acetonitrile did not differ from

hose found at constant ratio of methanol:water. The resuable 1indicate clearly, that the efficiency of the extraction pedure in terms of signal magnitude and analytical concentrs increased when the proportion of methanol is higher tharoportion of water, hence a methanol:water (v/v) ratio of 3:1

he minimum volume of acetonitrile used in this work, nam00�l can be considered as optimal condition for the extracf PGE2 from plasma samples. The instrumental precisiooth levels of concentration, reported as coefficient of variaas <3.17% and considered acceptable for the analysis of2

rom plasma samples.

.2. Plasma samples screening

Using the optimised extraction condition described abnd plasma samples (n = 6) from patients suffering chron

nflammatory disorders, four positive identifications were muring the LCMS/MS screening step. A representative ion catogram of one of the positive-identified samples at 351m/z is

hown inFig. 2A. Species with identical molecular weights

P. Araujo, L. Frøyland / J. Chromatogr. B 830 (2006) 212–217 215

Fig. 2. Representative ion chromatogram (A), superimposed characteristic ion fragments chromatograms at 333, 315 and 271m/z (B) and MS/MS product ionspectrum of the PGE2 anion (C) of pathological levels of PGE2 extracted from plasma using the optimised protocol outlined in this figure.

[PGE2-H]− are observed in this figure hence a positive iden-tification is accomplished by comparing the elution time ofPGE2 standard dissolved in blank plasma (8.4 min) and thoseshowed inFig. 2A, by performing coelution experiments to con-firm the analyte identity and by extracting the characteristic ionfragment chromatograms fromFig. 2A. The superimposed ionchromatograms at 333, 315 and 271m/z extracted fromFig. 2Aand showed inFig. 2B revealed that the species eluting at 8.4 mincorrespond to [PGE2-H]− effectively and that the presence ofstructural isomers of PGE2, such as PGD2, 8-iso PGE2, 8-iso-15-keto PGE2, which could yield identical product ion spectrawith PGE2, can be discarded. Further experiments aiming ateliminating suspicion of simultaneous elution of PGE2 and itsisomers under the conditions used in this work were conductedby using the extraction and chromatographic protocol reportedby Yang et al.[19] who separated successively PGE2 from its iso-mers. The chromatographic results (not shown) resemble thosein Fig. 2A but with different retention times (9.7 min for PGE2).The MS/MS spectrum at the PGE2 ion chromatogram peak max-imum (Fig. 2C) revealed that under the experimental conditionsused in this work the abundance of the ions 333 and 315m/z,resulting from the loss of one and two molecules of water, did

not show any differences between them. Thus, the summationof these ions was used for quantitative measurements of PGE2.

3.3. Extracted standard calibration curves

The lower range studied containing PGE2 (0–1 ng/ml) andPGE2-d4 (0.5 ng/ml) dissolved in plasma, revealed that the lowerlimit of quantification, defined as the lowest concentration anal-ysed with acceptable accuracy and precision, corresponding toten times the standard deviation of the blank plasma and foran injection volume of 25�l was 0.4 ng/ml. The calibrationgraphs for plasma spiked with PGE2 and internal standard werelinear over the concentration ranges 0.4–1 ng/ml (r2 = 0.988)and 1–50 ng/ml (r2 = 0.988). The analytical characteristics ofthese curves determined by unweighted least-squares regres-sion were ˆy = −0.147+ 2.879× [PGE2] and y = 0.0012+0.081× [PGE2] for the low and high range, respectively. Thetermy in both equations represents the estimated signal as PGE2to PGE2-d4 ratio calculated by using the summation of thecharacteristic fragment peaks of PGE2 (333 and 315m/z) andPGE2-d4 (337 and 319m/z which were also similar in abun-dance). The detection limit corresponding to three times the

216 P. Araujo, L. Frøyland / J. Chromatogr. B 830 (2006) 212–217

Tabl

e1

Sum

mar

yof

the

cent

ralc

ompo

site

desi

gnan

dpr

ecis

ion

resu

ltsat

diffe

rent

PG

E2

conc

entr

atio

ns

Ext

ract

ion

solv

ents

adde

dse

quen

ce(I

→II

→I)

Ext

ract

ion

proc

edur

e(n=

3)In

stru

men

talp

reci

sion

(n=

3)

III

I+II

+I

30ng

/ml

0.5

ng/m

l30

ng/m

l0.

5ng

/ml

CH

3O

H:H

2O

(�l/r

atio

)A

ceto

nitr

ile(�

l)To

talv

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e(�

l)A

vera

gere

spon

se(ic

ps×

105)

Coe

ffici

ento

fvar

iatio

n(%

)A

vera

gere

spon

se(ic

ps×

103)

Coe

ffici

ento

fva

riatio

n(%

)A

vera

gere

spon

se(ic

ps×

105)

Coe

ffici

ento

fva

riatio

n(%

)A

vera

gere

spon

se(ic

ps×

103)

Coe

ffici

ento

fva

riatio

n(%

)

400/

1:3

400

1200

0.63�

6.76

nd–

0.61

1.16

––

400/

1:1

400

1200

2.22�

3.62

2.64

�4.

3540

0/1:

240

012

000.

77�14

.41

nd–

400/

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400

1200

2.67�

3.42

4.12

�3.

942.

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933.

943.

1740

0/1:

112

0020

002.

23�3.

382.

50�

4.22

2.15

0.92

2.61

2.12

400/

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400

1200

0.74�

12.8

3nd

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711.

86–

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0/2:

140

012

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57�3.

013.

85�

5.61

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2400

3200

2.17�

3.27

2.64

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524.

722.

201.

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grou

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d:no

tdet

ecte

d.

Table 2Determination of PGE2 in plasma from patients suffering chronic inflammatorydisorders using extracted standard and standard addition calibration curves

Sample Extracted standard curve (n = 3) Standard addition curve (n = 3)

Mean(ng/ml)

Coefficient ofvariation (%)

Mean(ng/ml)

Coefficient ofvariation (%)

S1 8.060 7.695 8.037 2.212S2 30.556 2.045 29.580 2.004S3 dl

(∼0.1)– – –

S4 27.824 0.807 28.078 0.161

dl: detection limit.

standard deviation of the blank plasma and for an injection vol-ume of 25�l was 2.5 pg of PGE2 on the column.

3.4. Plasma samples quantification

The quantification of the four positive screened samples wascarried out by means of extracted standard calibration curvesand by standard addition curves. The results (Table 2) showedgood agreement between both calibration methods. The recov-ery, estimated according to the 2002 IUPAC recommendations[30] and by using standard addition curves ranged from 96 to98%.

4. Conclusions

The extraction procedure for the analysis of PGE2 in plasmadescribed in this work and based on the precipitation of theproteic fraction and further LC ion-trap high order MS detectionof the supernatant has been shown to be a useful approach fora rapid, simple and efficient screening and quantification of theanalytical prostaglandin in a wide range of concentrations.

The small amount of plasma required coupled with the lowsolvents consumption, fast extraction time and rapidity withwhich the sample is processed make the present approach highlysuitable for routine analysis and clinical investigations of PGE2i

sys-t trac-t thes ntralc threee totaln

A

ingt atoryd pro-v andF thee

n patients suffering from chronic inflammatory disorders.The experimental design has been a valuable tool for the

ematic variation and simultaneous optimisation of the exion solvents used in this work. It is important to highlightubstantial reduction of time and resources when the ceomposite design is manipulated. The optimisation of thextraction solvents has resulted in a 40% reduction of theumber of experiments.

cknowledgements

The authors are grateful to Tormod Bjørkkjær for providhe plasma samples of patients suffering chronic inflammisorders, Beate Klementsen and Benedikte Bjørge for theision of blank plasma and Linn Anne Bjelland Brunborgelicia Dawn Couillard for valuable comments regardingxtraction procedure optimisation design.

P. Araujo, L. Frøyland / J. Chromatogr. B 830 (2006) 212–217 217

References

[1] K. Nithipatikom, N.D. Laabs, M.A. Isbell, W.B. Campbell, J. Chro-matogr. B 785 (2003) 135.

[2] P. Needleman, J. Turk, B.A. Jakschik, A.R. Morrison, J.B. Lefkowith,Annu. Rev. Biochem. 55 (1986) 69.

[3] T. Kerttula, S. Kaukinen, E. Seppala, A. Riutta, I. Mucha, P. Ylitalo, J.Alanko, Prostaglandins Leukot. Essent. Fatty Acids 57 (1997) 555.

[4] M. Rathaus, Z. Greenfeld, E. Podjarny, M. Brezis, J. Bernheim,Prostaglandins Leukot. Essent. Fatty Acids 49 (1993) 815.

[5] J. Ladefoged, Prostaglandins 34 (1987) 611.[6] G. Preclik, E.F. Strange, H. Ditschuneit, Prostaglandins 43 (1992) 545.[7] L. McCullough, L. Wu, N. Haughey, X. Liang, T. Hand, Q. Wang, R.M.

Breyer, K. Andreasson, J. Neurosci. 24 (2004) 257.[8] C.B. Archer, C.P. Page, L. Juhlin, J. Morley, D.M. MacDonald,

Prostaglandins 33 (1987) 799.[9] A. Yermakova, M.K. O’Banion, Curr. Pharm. Des. 6 (2000) 1755.

[10] C. Myers, A. Koki, R. Pamukcu, W. Wechter, R.J. Padley, Urology 57(2001) 73.

[11] M.M. Heinricher, M.E. Martenson, M.J. Neubert, Pain 110 (2004) 419.[12] R.T. Jackson, J.E. Birnbaum, Prostaglandins 21 (1981) 1015.[13] C.A. Herman, X. Wang, K. Gronert, Biochim. Biophys. Acta 1334

(1997) 23.[14] L.M. Kelly, A.D.K. Hill, S. Kennedy, E.M. Connolly, R. Ramanath, S.

Teh, B. Dijkstra, R. Purcell, E.W. McDermott, N. O’Higgins, Eur. J.Surg. Oncol. 29 (2003) 707.

[15] H. Sheng, J. Shao, M.K. Washington, R.N. Dubois, J. Biol. Chem. 276(2001) 18075.

[16] D. Tsikas, J. Chromatogr. B 717 (1998) 201.[17] M. Takabatake, T. Hishinuma, N. Suzuki, S. Chiba, H. Tsukamoto, H.

Nakamura, T. Saga, Y. Tomioka, A. Kurose, T. Sawai, M. Mizugaki,Prostaglandins Leukot. Essent. Fatty Acids 67 (2002) 51.

[18] A. Margalit, K.L. Duffin, P.C. Isakson, Anal. Biochem. 235 (1996) 73.[19] P. Yang, E. Felix, T. Madden, S.M. Fischer, R.A. Newman, Anal.

Biochem. 308 (2002) 168.[20] E.C. Kempen, P. Yang, E. Felix, T. Madden, R.A. Newman, Anal.

Biochem. 297 (2001) 183.[21] H. Tsukamoto, T. Hishinuma, T. Mikkaichi, H. Nakamura, T. Yamazaki,

Y. Tomioka, M. Mizugaki, J. Chromatogr. B 774 (2002) 205.[22] A. Ferretti, V.P. Flanagan, J.M. Roman, Lipids 17 (1982) 825.[23] J. Bylund, J. Ericsson, E.H. Oliw, Anal. Biochem. 265 (1998) 55.[24] J. Abian, E. Gelpı, J. Chromatogr. 394 (1987) 147.[25] J. Abian, O. Bulbena, E. Gelpi, Biomed. Environ. Mass Spectrom. 16

(1988) 215.[26] http://www.thermo.com/eThermo/CMA/PDFs/Articles/articlesFile21281.

pdf.[27] R.D. Voyksner, E.D. Bush, D. Brent, Biomed. Environ. Mass Spectrom.

14 (1987) 523.[28] E.G. Bligh, W.J. Dyer, Can. J. Biochem. Physiol. 37 (1959) 911.[29] P.W. Araujo, R.G. Brereton, Trends Anal. Chem. 15 (1996) 63.[30] D.T. Burns, K. Danzer, A. Townshend, Pure Appl. Chem. 74 (2002)

2201.