Sample purification for spectroscopic high-pressure investigations by dynamic supercritical fluid...

Journal of Supercritical Fluids 16 (1999) 157–165www.elsevier.com/locate/supflu

Sample purification for spectroscopic high-pressureinvestigations by dynamic supercritical fluid extraction

Bjorn Wagner a, Mamoru Nishioka b, Dirk Tuma a, Michael Maiwald c,Gerhard M. Schneider a,*

a Physikalische Chemie, Fakultat fur Chemie, Ruhr-Universitat Bochum, D-44780 Bochum, Germanyb Anan National College of Technology, Department of Mechanical Engineering, Minobayashi-cho, Anan, Tokushima 774-0017, Japan

c Institut fur Technische Thermodynamik und Thermische Verfahrenstechnik, Universitat Stuttgart, D-70550 Stuttgart, Germany

Received 17 March 1999; received in revised form 28 June 1999; accepted 7 July 1999

Abstract

A method for the high resolution purification of 1,4-bis-(n-alkylamino)-9,10-anthraquinone dyestuff samples basedon a modified supercritical fluid chromatography (SFC) equipment was developed to minimize errors in high-pressuresolubility measurements. Impurities and by-products were eluted in a continuous CO2 flow, the purification processcould be observed by a diode array spectrophotometer. The fact that both static and dynamic solubility experimentsof a selected dyestuff gave consistent results turned out to be a credit point for this new method, which can beuniversally used for other purifications, too. Besides, the necessity of calibrating directly in the solvent gas isdemonstrated, since some organic solvents lead to characteristic spectral effects which can strongly disagree with thosein CO2. Our experiments showed a weakening of these effects towards longer alkyl chains of this substance class. Ina separate section, some remarks are given concerning the use of wavenumbers instead of wavelengths in suchexperiments. © 1999 Elsevier Science B.V. All rights reserved.

Keywords: Dyestuffs (disperse); Purification; SFC; Solvent effects; VIS spectroscopy

1. Introduction For some applications, however, the percentageof impurities would have consequences. A typicalexample is the solubility determination of dyestuffsCommercially available dyestuffs often containin compressed gases by using high-pressurea considerable amount of admixtures orspectroscopy, which has been a focus of the workby-products. In the field of technical applicationof our group for several years.this is sometimes intentional, for example to add

The spectroscopic determination of equilibriuma dispersing agent to dispersion dyestuffs or simplydyestuff concentrations (mole/volume) in the fluidfor products made by blending several compo-phase requires a highly purified substance.nents. Another problem is the content ofReliability and reproducibility can be influencedby-products originating from the synthesis.by impurities to a much higher degree than by anyinstrumental parameters and experimental errors.

* Corresponding author. Tel.: +49-234-700-4250;The reasons for this special behavior are as follows.fax: +49-234-7094-293.On the assumption that an impurity exists which isE-mail address: [email protected]

(G.M. Schneider) more soluble in CO2 or any other solvent used and

0896-8446/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved.PII: S0896-8446 ( 99 ) 00028-5

158 B. Wagner et al. / Journal of Supercritical Fluids 16 (1999) 157–165

additionally absorbs in the same wavelength range absorption bands with respect to pressure andas the proper dyestuff itself, the spectra will be temperature applied.superposed and therefore the determination of the In the present paper it is more appropriate toconcentrations becomes difficult or impossible. In use the integrated absorbance A (cm−1) instead,the case that the impurity spectra do not overlap, which is defined as:correct solubility measurements are also hinderedby mutual interaction among more than two com- A=P

n1

n2 logAI

0I Bdn=P

n1

n2 e(n)dncd (2a)

ponents (dyestuff+solvent gas+impurities).In this paper, we want to propose a method

and the handling of dyestuff purification prior to A=Bncd=

Bn

Mcd (2b)

high-pressure phase equilibrium experiments basedon the technique of supercritical fluid chromatog-

withraphy (SFC). The particular method is quite gene-ral and can be used for other purifications, too.

Bn=P

n1

n2 e(n)dn (3)All substances mentioned in the following were

subject to extensive solubility investigations by ourgroup. Publications relevant to this subject are where B

nis the integrated molar absorption

Refs. [1–12]. coefficient (cm mol−1).Evaluation of the band area A between two

wavenumbers n1 and n2 is particularly advanta-2. Spectroscopic determination of concentrations geous rather than peak height measurement, if

solvatochromism occurs due to changes in concen-In general, spectroscopic methods for in situ tration, temperature, and/or pressure. In practice,

investigations of thermodynamic equilibria belong the integrated intensities can be derived directlyto the class of analytical methods. Because phase from absorbance versus wavenumber spectra. Toequilibria remain undisturbed during the experi- determine true band intensities (‘oscillatorment, these methods are particularly useful in strengths’), strictly speaking, wavenumbers mustdealing with equilibrium solubilities or solvation be used instead of wavelengths. For the latter theequilibria [13,14]. integral behaves non-linearly if peak shifts occur.

Normally, the maximum absorbance, Emax, at For this reason absorption coefficients Bl

baseda given wavenumber is used for quantitative on wavelengths cannot be transformed into B

nspectroscopy using the Bouguer–Lambert–Beer based on wavenumbers by any simple multiplica-law: tion. Sometimes mass concentrations c (g cm−3)

are more appropriate, which can be used in combi-Emax=logAI

0I Bmax=emaxc d (1) nation with the molar mass M (g mol−1) in

Eq. (2b).Before calibrating the integrated molar absorp-with molar decadic absorption coefficient

tion coefficient Bn

versus concentration, all instru-emax (cm2 mol−1) at the absorbance maximum,ment adjustments, such as optical distance,intensities I and I0 of probe and reference beam,damping, slit width, scan speed, as well as therespectively, which are dependent on the wave-integration method, should be optimized and heldnumber n (cm−1), the optical path length d (cm),constant.and the concentration of the analyte under investi-

After recording a spectrum, the baseline has togation c (mol cm−1).be subtracted, which is easily performed by aIn many cases Eq. (1) holds and thus is appro-computer. This baseline contains characteristicpriate to a large number of applications. It shouldabsorbances of the optical fibers and sapphirebe accentuated that this equation requires fixed orwindows (see high-pressure cell, Section 3.3) andreproducible baseline absorptions as well as inde-

pendence of energy, bandwidth, and shape of can be recorded before the experiment using the

159B. Wagner et al. / Journal of Supercritical Fluids 16 (1999) 157–165

empty cell. Strictly speaking, the solvent baseline CO2 (99.995 vol%) was purchased from MesserGriesheim GmbH. All organic solvents used, suchmust be considered additionally, as far as this part

of the absorbance exceeds a constant offset. In the as acetone, ethanol, THF, methanol, and heptane,were of analytical grade and delivered from Riedelspectroscopic range investigated, supercritical

fluids showed only baseline shifts due to density de Haen AG.effects. Slightly decreasing baselines have beenfound for increasing pressures. This particular 3.2. Sample purification by dynamic fluid

extractioneffect emerges from an increasing refractive indexof the fluid, resulting in stronger light refractionin the direction of the light beam axis. The purification of the dyestuff samples was

carried out by a dynamic extraction process. TheThe baseline of the empty cell was used in bothcalibration and solubility experiments. The pres- apparatus itself is a slightly modified supercritical

fluid chromatograph (SFC) where the analyticalsure dependent linear offset was negligible, sincefor integration of each spectrum an individual column has been replaced by an extraction column

( length: 10 cm; inner diameter: 2 mm) containingsecond baseline between n1 and n2 was taken.the finely pulverized dyestuff only. This extractioncolumn is also placed in an air thermostat andconnected to a flow-through cell made for in situ3. Experimentalspectroscopic observation. A Polytec X-dap diodearray detector connected with fiber optics to the3.1. Chemicalsflow-through cell was used for spectroscopic meas-urements. CO2 is pumped through the system byThe disperse dyestuffs 1,4-bis-(n-

propylamino)-9,10-anthraquinone (AQ03) and means of a syringe pump (Isco SFC-500).The finely pulverized raw material was packed1,4-bis-(octylamino)-9,10-anthraquinone (AQ08)

were synthesized in our laboratories [2,9,11] into the extraction column. AQ01, AQ02, andAQ03 were filled in as pure substances [4,11],according to a procedure that has been descri-

bed by Mangan et al. for 1,4-bis-(octa- AQ08 was precipitated on a silanized kieselguhr(Merck KGaA) [4,10]. Pressure and temperaturedecylamino)-9,10-anthraquinone [15]. The raw

material of AQ03 had a purity of 99% [11] and conditions had to be selected in such a way thatthe sample in the column was only moderatelythat of AQ08 was 95% [5,10]. 1,4-Bis-

(methylamino)-9,10-anthraquinone (AQ01) and dissolved by CO2, but on condition that the solu-bility of the by-product was significantly higherthe ethyl derivative (AQ02) were purchased from

Aldrich GmbH; these materials were of purity 97% than that of the dyestuff itself in order to preventan excessive loss of the dyestuff during the(AQ01) and 98% (AQ02) before, and 99% after

recrystallization from methanol. Fig. 1 shows the procedure.When the CO2 was pumped through the system,molecular structure of the substances mentioned

here. impurities having a better solubility than theproper dyestuff under test went preferentially intothe mobile phase. The diode array detector (DAD)allowed a continuous spectroscopic observationduring the extraction. With progressive elution theimpurity content decreased and the spectra becamemore and more similar to those recorded in organicsolvents and reported in the literature [9–12,16,17].

The purities of all materials, both as raw pro-ducts and after purification, were controlled usingFig. 1. Structure of 1,4-bis-(n-alkylamino)-9,10-anthraquinone.capillary SFC (Hewlett Packard SFC G1205A).For AQ01 (alkyl=methyl ) n=1, AQ02 (ethyl ) n=2, AQ03

(propyl ) n=3, and AQ08 (octyl ) n=8. The refined substances AQ01, AQ02, and AQ03


Fig. 2. Sectional drawing of the spectroscopic high-pressure cell used for calibrations in CO2.

were all of >99.5% purity [11], while AQ08 was calibration measurements in CO2. The cell isof 99.2% purity [5,10]. designed for pressures up to 100 MPa and has two

For details on apparatus and experiments, see pairs of sapphire windows forming different opticalRefs. [3,4,10,11]. path lengths to enlarge the accessible range for

spectroscopic investigations. Pressure is transmit-ted to the autoclave by compressing heptane by3.3. Calibrationmeans of a spindle press. Temperature is adjustedby water circulation through a mantle around theFig. 2 shows a vertical section through the spec-

troscopic high-pressure cell which we used for cell and measured directly inside the inner volume


Fig. 3. Diagram showing the change in the VIS spectrum of AQ03 during the purification process with dynamic SFE.

by a chromel–alumel thermocouple. The position 4. Results and discussionof the separating piston is detected inductively andgives directly the internal volume of the cell (maxi- 4.1. Sample purificationmum internal volume ~45 cm3) under the actualp, T conditions. The sapphire windows can be When the raw material of the blue anthraqui-

none dyestuff samples was dissolved in CO2, thecoupled to a spectrophotometer (Perkin-Elmermodel Lambda 9) by fiber optics for the investiga- solution always showed a slightly reddish tint.

Consequently, the solubility determinationtions. Supplementary technical details of this equip-ment are published in Refs. [14,18,19]. becomes inaccurate since this particular behavior

did not occur in standard organic solvents. InThe calibration was carried out as follows. Anexactly defined amount of substance is filled into Fig. 4(a) and (b), the SFC chromatogram of AQ03

raw material is given. It is quite striking that thethe cell. In order to minimize weighing errors, itis advantageous to make a stock solution in ace- small amount of impurity causes such a big effect

on the spectrum shown in Fig. 3. In a paper ontone of defined concentration and to pipette aportion of solution into the cell. Acetone was the synthesis of various 1,4-bis-(alkylamino)-

9,10-anthraquinones according to a similartaken since it proved to be a good solvent and nodegradation could be observed, either. After evap- procedure as applied by us, Naiki reported the

formation — again referring to our case — oforating the solvent as well as sealing and evacuat-ing the autoclave, the CO2 is filled in. This time, 1-hydroxy-4-propylamino-9,10-anthraquinone as

the main by-product [16 ]. We could identify thisin contrast to equilibrium measurements, the solidprecipitate must be completely dissolved at all p, particular substance by its characteristic fragments

in MS analysis (Table 1). Fig. 3 shows the purifi-T conditions. By changing pressure the internalcell volume is varied and different concentrations cation of AQ03 (raw material, initial amount of

impurities ~1%) in a CO2 flow. The absorptioncan thus be adjusted. The fluid phase which con-tains the dissolved dyestuff is stirred for a couple spectrum after 100 min is equivalent to that

obtained from tests in standard solvents. A typicalof minutes, then the spectrum is recorded.The entire analysis of the experiments reported indication of the presence of this red by-product

(1-hydroxy-4-propylamino-9,10-anthraquinone) isin this work was done using wavenumbers only.In Figs. 3 and 6(a), however, we switched to the two characteristic peaks at 586 and 630 nm,

with the intensity of the 586 nm peak being largerwavelengths only to illustrate the spectral reso-lution in a more common unit. than that of the 630 nm one. In Fig. 3, this feature


Fig. 5. Solubility of AQ08 in CO2 as a function of density.Density values for CO2 were calculated from an EOS given bySpan and Wagner [20].

This continuous purification had to be done forall members of this substance class before startinghigh-pressure solubility investigations. Fig. 5 is acomparison of selected solubility isotherms of thesystem AQ08+CO2. Our particular purificationmethod was not available to Swidersky [9]. Theresults of Swidersky and coworkers [2,9,10], whoused the sample without fine purification asdescribed here, were about 20–35% higher than

(a)

(b)

those obtained from the refined AQ08 [5,10,12].Fig. 4. (a, b) SFC chromatogram of AQ03 raw material (car-The use of the highly purified material instead gaverier: CO2; pressure: 20 MPa; temperature: 323 K; sample

solvent: acetone; column: capillary column SB cyanopropyl 50, a good correspondence and reproducibility of thelength: 10 m, inner diameter: 50 mm; detection: FID). The chro- data. Swidersky [9] and Tuma and coworker [5,12]matogram in (b) shows the impurity peak (I=1-hydroxy- applied an identical static method, and Kautz [10]4-propylamino-9,10-anthraquinone) at larger scale. Peak integ-

the dynamic method described here, respectively.ration proved a purity of >99%.

4.2. Calibrationcan be seen in the spectrum recorded after 10 min,whereas after 100 min it has disappeared. Through

Calibration directly in the compressed gas is bythis treatment, the purity of AQ03 could beimproved to >99.5%. far more complicated than in the conventional

Table 1Results obtained from the MS analysis of AQ03 samples (Varian MAT CH5, electronic ionization, 70 eV, direct inlet). The particularby-product is 1-hydroxy-4-propylamino-9,10-anthraquinone

Sample m/e Intensity (%) Identification

AQ03 raw material (+by-product) 322 60 [C20H22N2O2]+=M+ (AQ03)293 75 [C18H17N2O2]+281 50 [C17H15NO3]+=M+ (by-product)252 100 [C15H10NO3]+, found in the raw material only

AQ03 after SFC purification 322 90 [C20H22N2O2]+=M+ (AQ03)293 100 [C18H17N2O2]+


Fig. 7. Calibration function of AQ02 in different solvents (refer-ence optical path length d=0.58 cm, flow-through cell ).

a molar extinction coefficient that is similar toCO2 should be used. Thus, in this case ethanoland acetone proved to be applicable. So to speak,the calibration in CO2 is reasonable and necessarybefore starting calibration with organic solvents tocheck their applicability as calibration media.

Fig. 7 is the corresponding calibration diagramfor AQ02. Here, the effects are similar to thosefor AQ01.

For AQ08, however, the results are considerably

(a)

(b)

different. As can be seen from Fig. 8, there is noFig. 6. (a) Spectrum of the purified AQ01 in three different obvious difference between the calibration func-liquid solvents and supercritical CO2. (b) Calibration function

tions in the selected solvents any more. Heptane,of AQ01 in different solvents (reference optical path length d=however, was quite a poor solvent for AQ08 (and0.58 cm, flow-through cell ).other homologous substances with higher molarmass). Neither Swidersky [9], who used fermenta-

way with standard solutions. For that purpose weused the high-pressure autoclave which had beenvolume calibrated up to 100 MPa [18]. The morethe internal volume of the cell (see Fig. 2) can bevaried, the larger the accessible concentrationrange that can be reached.

Fig. 6(a) shows the spectra of AQ01 in threeorganic solvents and CO2. The change from etha-nol to CO2 gives a significant hypsochromic shift.In Fig. 6(b), the integral absorbances versus thecorresponding concentrations of AQ01 are plotted.The slope of the calibration curve in heptanediffers significantly from the other solvents. CO2,however, fits well with acetone and ethanol. Sincethe calibration in CO2 can only be conducted in a Fig. 8. Calibration function of AQ08 in different solvents (refer-

ence optical path length d=1 cm, quartz cuvettes).relatively small concentration range, a solvent with


tion amyl alcohol (3-methyl-1-butanol ), 1,4- Acknowledgementsdioxane, hexane, and also CO2, nor Kautz [10],who used ethyl acetate, 1-pentanol, 1,4-dioxane, One of us (M.N.) acknowledges financial sup-and hexane, respectively, found a different slope port from the Japanese Ministry of Education,in their calibration curves between the non-polar Science, Sports, and Culture (Monbusho). Thehexane and the polar solvents. authors thank Dr. Dietrich Muller for doing the

The reason for this behavior, namely a charac- MS analysis and Joachim J. Masuch for his assis-teristic solvent dependent difference in the spectral tance in the preparation of the figures. The finan-features for AQ01 and AQ02 having short alkyl cial support of the Deutsche Forschungs-chains only, seems to be that the alkyl chains can gemeinschaft (DFG) and the Fonds derrotate freely. The ‘big’ octyl chains of AQ08 shield Chemischen Industrie e.V. is highly appreciated.the molecule against interactions with the polarsolvent molecule, so that an alkane-like sphereexists around the molecule in the solvent. If we

Referencesgradually go from short (e.g. AQ01) to longeralkyl chains (e.g. AQ08 and higher), the difference

[1] U. Haarhaus, P. Swidersky, G.M. Schneider, High-pres-when using alkanes or more polar media to cali-sure investigations on the solubility of dispersion dyestuffsbrate will become smaller and smaller until it in supercritical gases by VIS/NIR-spectroscopy. Part I.

finally disappears. 1,4-Bis-(octadecylamino)-9,10-anthraquinone and Dis-The difficult and ticklish calibration for all- perse Orange 13 in CO2 and N2O up to 180 MPa,

J. Supercrit. Fluids 8 (1995) 100.trans b-carotene, where stereomutation as well as[2] P. Swidersky, D. Tuma, G.M. Schneider, High-pressuresolvatochromism is found and well-known, is sub-

investigations on the solubility of anthraquinone dyestuffsject to ongoing investigations [12].in supercritical gases by VIS-spectroscopy. Part II. 1,4-Bis-(n-alkylamino)-9,10-anthraquinones and Disperse Red 11in CO2, N2O, and CHF3 up to 180 MPa, J. Supercrit.Fluids 9 (1996) 12.5. Conclusions

[3] G.M. Schneider, C.B. Kautz, D. Tuma, High-pressureinvestigations on the solubility of synthetic and naturalA far-reaching exclusion of the influence ofdyestuffs in supercritical gases by VIS-spectroscopy up toimpurities being better soluble in CO2 during high- 180 MPa, in: Ph. Rudolf von Rohr, Ch. Trepp (Eds.), High

pressure solubility measurements of anthraqui- Pressure Chemical Engineering, Process Technology Pro-none-based disperse dyestuffs was achieved ceedings Vol. 12, Elsevier, Amsterdam, 1996, p. 259.

[4] C.B. Kautz, B. Wagner, G.M. Schneider, High-pressurethrough specially purified samples. We could reachsolubility of 1,4-bis-(n-alkylamino)-9,10-anthraquinones inpurities >99% by dynamic supercritical fluidnear- and supercritical carbon dioxide, J. Supercrit. Fluidsextraction using a modified SFC set-up. The qual-13 (1998) 43.

ity of these purifications was proved both by static [5] D. Tuma, G.M. Schneider, High-pressure solubility of dis-and dynamic solubility measurements, which gave perse dyes in near- and supercritical fluids: measurementsequivalent results. up to 100 MPa by a static method, J. Supercrit. Fluids 13

(1998) 37.Concerning the calibration, we noted that it is[6 ] B. Wagner, C.B. Kautz, G.M. Schneider, Investigations onalways recommended to calibrate directly in CO2, the solubility of anthraquinone dyes in supercritical carbonsince in non-polar hexane the spectra of AQ01

dioxide by a flow method, Fluid Phase Equilib. 158–160and AQ02 differ significantly from those in CO2 (1999) 707.and more polar solvents. This deviation becomes [7] D. Tuma, G.M. Schneider, Determination of the solubilit-weaker for AQ08 having relatively long alkyl ies of dyestuffs in near- and supercritical fluids by a static

method up to 180 MPa, Fluid Phase Equilib. 158–160chains.(1999) 743.Generally, it is more advantageous to work with

[8] U. Haarhaus, Spectroscopic high-pressure investigationswavenumbers and integrated absorbances insteadto determine solubilities of dispersion dyestuffs in super-

of wavelengths and absorbances at a single wave- critical fluids at pressures up to 2 kbar and temperatureslength, respectively. A scan over the total spectrum between 300 and 410 K, Doctoral dissertation, Ruhr-Uni-

versitat Bochum, Shaker, Aachen, 1992 (in German).instantaneously indicates solvatochromism.


[9] P. Swidersky, Spectroscopic high-pressure investigations [14] G.M. Schneider, Physico-chemical properties and phaseequilibria of pure fluids and fluid mixtures at high pres-on the solubility of anthraquinone dyestuffs in supercritical

fluids at pressures up to 180 MPa and temperatures sures, in: E. Kiran, J.M.H. Levelt Sengers (Eds.), Super-critical Fluids — Fundamentals for Application, NATObetween 300 and 360 K, Doctoral dissertation, Ruhr-Uni-

versitat Bochum, Shaker, Aachen, 1994 (in German). ASI Series E: Applied Sciences Vol. 273, Kluwer AcademicPublishers, Dordrecht, 1994, p. 91.[10] C.B. Kautz, Fluid chromatographic (SFC) investigations

of organic substances with supercritical carbon dioxide: [15] S.J. Mangan, J. Crouse, F. Calogero, Dyeing polypropyl-ene with anthraquinone derivative disperse dyes, Text.determination of capacity ratios of polyphenylenes and

substituted benzene derivatives as well as spectroscopic sol- Chem. Color. 21 (1989) 38.[16 ] K. Naiki, Studies on the dyes for cellulose acetate VI.ubility investigations of anthraquinone dyestuffs up to

20 MPa, Doctoral dissertation, Ruhr-Universitat Bochum, On the 1-alkylamino-4-hydroxy-anthraquinones and1,4-bisalkylaminoanthraquinones, Yuki Gosei KagakuCuvillier, Gottingen, 1996 (in German).

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bility and stability of anthraquinone dyestuffs and Gottingen, 1997 (in German).[19] M. Maiwald, G.M. Schneider, Solvatochromism inb-carotene in CO2, N2O, CClF3, and SF6, Doctoral disser-

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Series E: Applied Sciences Vol. 273, Kluwer Academic perature to 1100 K up to 800 MPa, J. Phys. Chem.Ref. Data 25 (1996) 1509.Publishers, Dordrecht, 1994, p. 499.

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