Focusing Review

Chromatography, Vol.25 No.3 (2004)

Focusing Review

Pure silica gel as cation−exchange stationary phase inion chromatography for mono− and divalent cations
Kazutoku Ohta
Inhomogeneity Analysis Research Group
Research Institute of Instrumentation Frontier (RIIF)
National Institute of Advanced Industrial Science and Technology (AIST)
2266−98 Anagahora, Simoshidami, Moriyama−ku, Nagoya 463−8560, Japan
Received for review September 21, 2004. Revised manuscript received November 23, 2004. Accepted November 26, 2004.

Abstract
The review article describes the application of a pure silica gel (Pia Seed 5S−60−SIL), synthesized by the hydrolysis of pure teratraetoxysilane [Si(OCH2CHG3)4], as a cation−exchange stationary phase in ion chromatography for common mono− and divalent cations (Li+, Na+,
NH4+, K+, Mg2+ and Ca2+). Using aromatic monoamines at pH 5.0 as eluents, the Pia Seed 5S−60−SIL silica gel acted as an advanced cation−exchange stationary phase for these mono− and divalent cations. Excellent simultaneous separation and highly sensitive indirect−photometricdetection at 275 nm for these cations were achieved on a Pia Seed 5S−60−SIL column (150 × 4.6 mm i. d.) in 20 min with 0.75 mM tyramine[4−(2−aminoethyl)phenol]−0.25 mM oxalic acid at pH 5.0 containing 1.5 mM 18−crown−6 (1,4,7,10,13,16−hexaoxacyclooctadecane) as theeluent. Using dilute oxalic acid (0.2 mM oxalic acid) as the eluent, the Pia Seed 5S−60−SIL silica gel also behaved as an advanced cation−exchange stationary phase for these mono− and divalent cations. Excellent simultaneous separation and highly sensitive indirect−conductimetricdetection for these cations were also achieved on the column in 20 min with 0.2 mM oxalic acid at pH 3.6 containing 1.5 mM 18−crown−6 asthe eluent. The review article also describes the chromatographic behavior of these mono− and divalent cations on the calcinated Pia Seed 5S−60−SIL silica gel columns.
Key words: Pure silica gel, Ion Chromatography, Cations, Indirect−photometric detection, Tyramine, Indirect−conductimetric detection, Oxalic acid, Crown ethers, Calcination.

Kolla et al . [4], and Kondratjonok and Schwedt [5] applied car-

1. Introduction
Ion chromatography (IC) developed by Small et al . [1] isboxylated silica gel (polybutadiene−maleic acid on silica gel), as

widely recognized as a simple, powerful and convenient analyticalthe cation−exchange stationary phase in IC−CD for these mono−technique for the determination of various low−molecular−weightand divalent cations [4, 5]. The greatest advantage of the use of thisionic compounds [2, 3]. Firstly, IC with conductimetric detection

type resin in IC−CD is that the simultaneous separation and highly(IC−CD) was mainly applied for the determination of inorganic

sensitive conductimetric detection for these mono− and divalentanions. This is because a lightly sulfonated styrene−divinylbenzene

cations can be achieved easily with acidic solution as the eluent.

co−polymer (S−DVB) resin was used as the cation−exchange sta-

Now, IC−CD with carboxylated silica column or carboxylated S−

tionary phase in IC−CD for cations and, as a consequence, the si-

DVB resin column is widely utilized for the determination of these

multaneous separation of common mono− and divalent cations

mono− and divalent cations in various real sample waters.

(Li+, Na+, NH4+, K+, Mg2+, and Ca2+) could not be achieved easily

Silica gel is physically stable, unreactive towards organic sol-on this type resin column with acidic solution as the eluent [1−3].

vents and its particle size, pore size, pore volume, and surface area

Phone: +81−52−736−7363, Facsimile: +81−52−736−7224
E−mail: kazu.ohta@aist.go.jp―９３―

Chromatography, Vol.25 No.3 (2004)

can be controlled easily. The silanol group on the surface of silica

silica gel acted as an advanced cation−exchange stationary phasegel is easily bonded chemically to various kinds of functional

for these cations both (a) in IC−IPD with aromatic monoamines asgroups. For these reasons, both unmodified and modified silica gelsthe eluents and (b) in IC−CD with dilute oxalic acid as the eluent

are widely employed as stationary phases in liquid chromatography[28, 29]. The article also describes the effect of calcination (heatfor various organic compounds [6]. Furthermore, since the silanol

treatment) of the Pia Seed−5 S−60−SIL silica gel on chroma-group behaves as a weak acid [7], it is possible to utilize unmodi-

tographic behavior of these mono− and divalent cations [30, 31].

fied silica gel as a cation−exchange stationary phases in IC−CD forvarious cations [8−13]. The most advantage of the use of unmodi-

2. Experiment

fied silica gel in IC−CD is that the simultaneous separation of ma-

2.1. Instruments

jor mono− and divalent cations (Na+, NH4+, K+, Mg2+, and Ca2+)

Ion chromatograph consisted of a Tosoh (Tokyo, Japan) LC−

can be achieved with lithium oxalate as the eluent [11−13]. How-

8020 chromatographic data−processor, a Tosoh CCPM−II solvent

ever, due to very small difference in the equivalent conductivity

delivery pump operated at a flow rate of 1 ml min−1, a Tosoh CO−

between lithium ion (Li+) as the eluent ion and the analyte cations,

8020 column oven operated at 35°C, a Tosoh UV−8020 UV−Vis

the conductimetric detection sensitivities result in very low.

spectrophotometric detector, a Tosoh CM−8020 conductimetric de-

The use of hydronium ion (H+) having the largest equivalenttector, a Tosoh SD−8023 on−line degasser, and a Reodyne (Cotati,

conductivity as the eluent ion was expected to be one of the mostCA, USA) model 9125 injector equipped with a 20 µl sample loop.

effective ways for the enhancement of the detection sensitivities for

A Tosoh RI−8023 refractive index detector was also used for thethe analyte cations in IC−CD. Then, chromatographic behavior ofdetermination of the amounts of crown ethers [18−crown−6

common mono− and divalent cations (Li+, Na+, NH4+, K+, Mg2+,(1,4,7,10,13,16−hexaoxacyclooctadecane) and 15−crown−5 (1,4,and Ca2+) on various commercially available unmodified silica gel

7,10,13−pentaoxacyclopentadecane)] adsorbed on calcinated purecolumns was investigated with 2 mM nitric acid at pH 2.7 as thesilica gel columns.

eluent [14−17]. Some silica gels (Develosil 30−5, Kaseisorb LC−

A Toa Electronics (Tokyo, Japan) IM−40S ion meter with a60−5, and Zorbax BP−SIL) showed cation−exchange behavior andglass electrode was used for the measurements of the pH of eluents.

could be successfully applied in IC−CD for these mono− and diva-

A Toa Electronics CM−20 conductimetric detector was used forlent cations using acidic solution as the eluent. The main cause of

the measurements of conductivities of eluents.

the cation−exchange behavior was attributed to be polyvalent met-

A Perkin−Elmer (Shelton, CT, USA) Lambda 18NS double

als as impurities in silica matrix [18, 19]. However, it should bebeam UV−Vis spectrometer was employed for the measurements

kept in mind that all silica gels could not be utilized as cation−ex-of UV spectra of aromatic monoamine eluents.

change stationary phase in the IC−CD for these cations with acidic
2.2. Silica gelssolution as the eluent.
The use of protonated aromatic monoamines having high mo-

Various commercially available unmodified silica gels [Ma-lar extinction coefficients as the eluent ions is very effective way

cherey−Nargel (Düren, Germany) Nucleosil 50−5, Fuji−Silysia

for the achievement of higher detection sensitivities for these mono−

Chemical (Kasugai, Japan) Super Micro Bead Silica Gel B−5,

and divalent cations in IC with indirect−photometric detection (IC−Nomura Chemical (Seto, Japan) Develosil 60−5, and Pia Tec

IPD) [20, 21]. However, since commercially available silica gels,(Suzuka, Japan) Pia Seed 5S−60−SIL silica gels] were used in thisexcept for pure silica gel, synthesized by the hydrolysis of purework. The Pia Seed 5S−60−SIL silica gel is pure silica gel, synthe-

tetraethoxysilane [Si(OCH2CH3)4] contains various polyvalent met-

sized by the hydrolysis of pure tetraethoxysilane [Si(OCH2CH3)4].

als as impurities in silica matrix, strongly unfavorable interaction

Table 1 shows physical properties of these silica gels. The de-occurs frequently between protonated aromatic monoamines as thetermination of the surface area and pore volume by nitrogen ad-eluent ions and polyvalent metals in the silica matrix [22−25].

sorption isotherms at 77 K was carried out by using a Beckman−

Therefore, the use of aromatic monoamines as the eluent in the IC−

Coulter (Fullerton, CA, USA) Omunisorp 360 gas sorption ana-

IPD with unmodified silica gel column has not been so popularlyzer. The surface area was calculated from the BET equation. The[26, 27].

determination of the particle size was carried out by using a Horiba

The article describes the application of a pure silica gel (Pia

(Kyoto, Japan) LA−920 laser scattering particle size distribution

Seed−5S−60−SIL) as a cation−exchange stationary phase in IC for

analyzer.

common mono− and divalent cations (Li+, Na+, NH4+, K+, Mg2+,

Table 2 shows analytical results of polyvalent metals in theseand Ca2+), in order to demonstrate the effectiveness of the Pia Seedsilica gels. Firstly, the main polyvalent metals were qualitatively

5S−60−SIL silica gel in IC for cations. The Pia Seed−5S−60−SIL

defined as Fe, Al, Ti, Zr, Mg, and Ca by using a Cameca (Cour-―９４―

Chromatography, Vol.25 No.3 (2004)

Kazutoku Ohta

Table 1. Physical properties of various silica gels
Form

Particlesize(µm)

Surfacearea(m2 g−1)

Pore
size
(Å)

Pore
volume(ml g−1)

Packingdensity(g ml−1)

Surface area
per column*(m2 column−1)

Nucleosil 50−5

Spherical

4.8

365

65

0.90

0.51

4.6 × 102

Super Micro Bead Silica Gel B−5

Spherical

5.5

475

60

0.93

0.43

5.1 × 102

Develosil 60−5

Spherical

5.9

417

65

1.04

0.45

4.7 × 102

Pia Seed 5S−60−SIL

Spherical

7.0

619

58

1.04

0.41

6.3 × 102

Silica gel* Column size : 150 mm × 4.6 mm i.d.
Reproduced from Ref. 28 with the permission from Vieweg.
Table 2. Analytical results of polyvalent metal in various silica gels
Content (µmol g−1−silica gel)
Silica gel
Mg

Al

Ca

Ti

Fe

Zr

Nucleosil 50−5

0.99

1.5

3.1

2.1

0.20

1.1

Super Micro Bead Silica Gel B−5

0.08

1.2

0.25

1.3

0.27

0.13

Develosil 60−5

1.7

4.9

12

1.5

0.10

0.16

Pia Seed 5S−60−SIL

0.10< 0.004

0.10

0.01

0.01

0.01

Table 3. Physical properties of calcinated Pia Seed 5S−60−SIL silica gels
Calcinatedtemperature(°
C)

Particlesize(µm)

Surfacearea(m2 g−1)

Pore
size
(Å)

Pore
volume(ml g−1)

Packingdensity(g ml−1)

Surface area
per column*(m2 column−1)

200

7.0

618

58

1.04

0.41

6.3 × 102

400

6.8

581

60

0.94

0.42

6.1 × 102

600

6.7

551

59

0.88

0.45

6.2 × 102

800

6.4

460

56

0.76

0.49

5.6 × 102

1000

5.0

269

42

0.39

0.71

4.8 × 102

* Column size : 150 mm × 4.6 mm i.d.
Adapted from Ref. 30 with the permission from Vieweg.

umn−1) was calculated by use of the equation:

bevoie Cedex, France) IMS−3F secondary ion−mass spectrometer(SIMS) system. Next, the amounts of the above metals were deter-A = (VR –V0 ) × C

mined quantitatively by using a Nippon Jarrel−Ash (Kyoto, Japan)

where VR is the breakthrough volume (ml), V0 is the total dead vol-ICAP−1000 inductively coupled plasma−atomic emission spec-ume (column void volume and connected tube volume, ml) and C

trometer (ICP−AES) system [18].

is the concentration of crown ether in the eluent (mM). Firstly, col-Table 3 shows physical properties of the Pia Seed 5S−60−SIL

umn was equilibrated with 0.75 mM tyramine [4−(2−aminoethyl)silica gels calcinated at 200, 400, 600, 800 and 1000°
C for 5 h.

phenol]−0.25 mM oxalic acid at pH 5.0 as the eluent. A sample of

The separation columns (150 × 4.6 mm i.d., stainless steel)

75 mM tyramine−25 mM oxalic acid at pH 5.0 was injected. Elu-

were packed with these silica gels by the use of the slurry−packing

tion volume of peak corresponding to 75 mM tyramine was re-method.

garded as V0. Next, 0.75 mM tyramine−0.25 mM oxalic acid at pH

The determination of the amounts of crown ethers (18−crown

5.0 containing crown ether as the eluent was passed through col-−6 and 15−crown−5) adsorbed on calcinated Pia Seed 5S−60−SIL

umn and the response of refractive index detector (breakthroughcolumns was carried out. The amount of crown ether (A, µmol col-curve) was monitored. Volume corresponding to breakthrough―９５―

Chromatography, Vol.25 No.3 (2004)

as shown in Figure 1D, symmetrical peaks of the monovalent cat-point in the detector response curve was regarded as VR.

ions were obtained on the Pia Seed 5S−60−SIL column. The peak
destruction was due mainly to strong interaction between proto-

2.3. Chemicals
All chemicals were analytical regent grade. Tyramine, 18−

nated tyramine as eluent ion and polyvalent metals as impurities incrown−6, and 15−crown−5 were purchased from Aldrich Chemi-

silica matrix [22−25]. These chromatograms suggested that the Piacals (Milwaukee, WI. USA), and other chemicals were purchased

Seed 5S−60−SIL silica gel could be applied successfully as the cat-from Wako Pure Chemicals (Osaka, Japan) or Tokyo Kasei Kogyo

ion−exchange stationary phase in the IC−IPD for these cations.
Secondly, chromatographic behavior of these mono− and di-(Tokyo, Japan). Distilled, de−ionized water was used for the prepa-

valent cations on the Pia Seed 5S−60−SIL column was investigatedration of eluents and standard solutions.

with various aromatic monoamines as the eluents. Figures 2A−D
3. Results and discussion

show chromatograms of these cations with (A) 0.5 mM ben-

3.1. Separation and indirect−photometric detection of mono−

zylamine−0.25 mM oxalic acid at pH 5.0, (B) 0.5 mM pheylethyl-and divalent cations on Pia Seed 5S−60−SIL column withamine−0.25 mM oxalic acid at pH 5.0, (C) 0.25 mM 2−methylpyri-aromatic monoamines as eluentsdine−0.1 mM oxalic acid at pH 5.0, and (D) 0.25 mM 2,6−dimeth-

The application of a pure silica gel (Pia Seed 5S−60−SIL),

ylpyridine−0.1 mM oxalic acid at pH 5.0, as the eluents. The pH ofthese eluents was adjusted with 1 M HNO3.

synthesized by the hydrolysis of pure tetraethoxysilane [Si(OCH2
CH3)4], as a cation−exchange stationary phase in ion chromatogra-

As shown in Figure 1D and Figures 2A and B, using proto-

phy with indirect−photometric detection (IC−IPD) for common

nated tyramine, benzylamine or phenylethylamine as the eluentmono− and divalent cations (Li+, Na+, NH4+, K+, Mg2+, and Ca2+)

ions, although symmetrical peaks of these mono− and divalent

was attempted with aromatic monoamines as eluents. Firstly, chro-were obtained, peak resolution between the monovalent cations

matographic behavior of these cations on various commercially

was very poor. The detection sensitivities of these cations obtained

available unmodified silica gel columns (150 × 4.6 mm i.d.) waswith protonated tyramine as the eluent ion were much higher thaninvestigated with tyramine [4−(2−aminoethyl)phenol] as the eluent.

those obtained with protonated benzylamine and phenylethylamine

Table 1 shows physical properties of silica gels used in this work.

as the eluent ions. This is because molar extinction coefficient ofTable 2 shows analytical results of polyvalent metals as impurities

protonated tyramine [1.5 ×103 (mol cm)−1 at 275 nm] was muchin these silica gels. Figures 1A−D show chromatograms of thesehigher than those of protonated benzylamine [2.6 × 102 (mol cm)−1

cations on (A) Nucleosil 50−5 column with 1.0 mM tyramine−0.25

at 256 nm] and phenylethylamine [2.1 × 102 (mol cm)−1 at 257

mM oxalic acid at pH 5.0 as the eluent, (B) Super Micro Bead Sil-nm]. Molar extinction coefficients of protonated 2−methylpyridine

ica Gel B−5 column with 0.75 mM tyramine−0.25 mM oxalic acid

[4.2 × 103 (mol cm)−1 at 253 nm] and 2,6−dimethylpyridine [4.3 ×

at pH 5.0 as the eluent, (C) Develosil 60−5 column with 1.5 mM

103 (mol cm)−1 at 253 nm] were considerably higher than that oftyramine−0.25 mM oxalic acid at pH 5.0 as the eluent, and (D) Pia

protonated tyramine. Unfortunately, as shown in Figures 2C and D,

Seed 5S−60−SIL column with 0.75 mM tyramine−0.25 mM oxalic

largely fronted peaks of the divalent cations were obtained withacid at pH 5.0 as the eluent. The pH of these eluents was adjusted

0.25 mM 2−methylpyridine−0.1 mM oxalic acid at pH 5.0 and 0.25

with 1 M HNO3. This is because nitrate ion (NO3−) has almost nomM 2,6−dimethylpyridine−0.1 mM oxalic acid at pH 5.0, as theeffects on the retention behavior of these cations.

eluents. This might be due to low concentrations of protonated 2−

As shown in Figures 1A−D, it was very easy to elute thesemethylpyridine (0.25 mM) and 2,6−dimethylpyridine (0.25 mM) inmono− and divalent cations in a reasonable period of time (< 20the eluent in comparison to those of protonated tyramine (0.75min). This is because the affinity of these mono− and divalent cat-mM), benzylamine (0.5 mM), and phenylethylamine (0.5 mM) in

ions to the dissociated silanol group as cation−exchanger was very

the eluent. These chromatograms meant that protonated tyramine

similar. The elution order of the monovalent cations was Li+ < Na+

was the most favorable eluent ion in the IC−IPD for these mono−< NH4+ < K+. The order indicated that these silica gels acted as cat-and divalent cations. However, a further study was required forion−exchange stationary phases under the individual eluent condi-

complete separation of these cations on the Pia Seed 5S−60−SIL

tions. Incomplete separation of the monovalent cations and com-column.
18 − Crown − 6plete separation of the divalent cations were achieved. The com-

( 1,4,7,10,13,16 − hexaoxacyclooctadecane )plete separation was due mainly to oxalate in the eluent, whichforms stable complexes with many cations [32, 33]. The addition

acted as a complexing agent for the divalent cations [12, 13]. As

of 18−crown−6 to acidic solution as the eluent is often carried out,

shown in Figures 1A−C, largely destroyed peaks of the monovalent

for improving peak resolution between these mono− and divalentcations were obtained on conventional silica gel columns, whereas,

cations on weakly acidic cation−exchange resin columns [34, 35].

―９６―

Chromatography, Vol.25 No.3 (2004)

Kazutoku Ohta

Figure 1 A

Figure 1 B

Figure 1 C

Figure 1 D

Figure 1. Chromatograms of mono− and divalent cations on various silica gel columns with tyramine as eluent
Column : (A) Nucleosil 50−5, (B) Super Micro Bead Silica Gel B−5, (C) Develosil 60−5, (D) Pia Seed 5 S−60−SIL,
Column size: 150 mm × 4.6 mm i.d., Column temperature : 35 °C,
Eluent : (A) 1.0 mM tyramine−0.25 mM oxalic acid at pH 5.0, (B) 0.75 mM tyramine−0.25 mM oxalic acid at pH 5.0, (C) 1.5 mM tyramine−0.25
mM oxalic acid at pH 5.0, (D) 0.75 mM tyramine−0.25 mM oxalic acid at pH 5.0, The pH of eluents was adjusted using 1 M HNO3.
Flow rate : 1 ml min−1, Detection : Indirect−UV at (A) 270, (B) 275, (C) 265, and (D) 275 nm, Injection volume : 20 µl,
Sample concentration : (A), (B) and (D) 0.2 mM for monovalent cations and 0.1 mM for divalent cations, (C) 0.4 mM for monovalent cations and
0.2 mM for divalent cations
Peaks : 1 = Li+, 2 = Na+, 3 = NH4+, 4 = K+, 5 = Mg2+, and 6 = Ca2+
Reproduced from Ref. 28 with the permission from Vieweg.

―９７―

Chromatography, Vol.25 No.3 (2004)

Figure 2 A

Figure 2 B

Figure 2 C

Figure 2 D

Figure 2. Chromatograms of mono− and divalent cations on Pia Seed 5S−60−SIL column with various aromatic monoamines as eluents
Column: Pia Seed 5S−60−SIL
Eluent : (A) 0.5 mM benzylamine−0.25 mM oxalic acid at pH 5.0, (B) 0.5 mM phenylethylamine−0.25 mM oxalic acid at pH 5.0, (C) 0.25 mM
2−methylpyridine−0.1 mM oxalic acid at pH 5.0, (D) 0.25 mM 2,6−dimethylpyridine−0.1 mM oxalic acid at pH 5.0,
The pH of eluents was adjusted using 1 M HNO3.
Detection : Indirect−UV at (A) 256, (B) 257, (C) 253, (D) 257 nm,
Sample concentration : (A) and (B) 0.2 mM for monovalent cations and 0.1 mM for divalent cations, (C) and (D) 0.1 mM for monovalent cations
and 0.05 mM for divalent cations,
Peaks : 1 = Li+, 2 = Na+, 3 = NH4+, 4 = K+, 5 = Mg2+, and 6 = Ca2+.
Other chromatographic conditions are as for in Figure 1.
Adapted from Ref. 28 with the permission from Vieweg.

―９８―Chromatography, Vol.25 No.3 (2004)

Kazutoku Ohta

Therefore, the addition of 18−crown−6 to 0.75 mM tyramine−0.25

result meant that 18−crown−6 seemed to act as a selective cation−mM oxalic acid at pH 5.0 as the eluent was carried out for com-exchanger (selective retention modifier) for the monovalent cat-plete separation of these mono− and divalent cations on the Piaions. Complete separation of the monovalent cations was achieved

Seed 5S−60−SIL column. Figure 3 shows the relationship between

at the concentration of 18−crown−6 > 1.5 mM. In contrast, as forthe concentration of 18−crown−6 in the eluent and the retention

the divalent cations, with increasing the concentration of 18−crownvolumes of these cations. Figure 4 shows chromatogram of these−6, the retention volumes slightly decreased. The decrease might

cations with 0.75 mM tyramine−0.25 mM oxalic acid at pH 5.0

be due mainly to both (a) low stabilities of complexes formed be-containing 1.5 mM 18−crown−6 as the eluent.

tween the divalent cations and 18−crown−6 (Log KCa2+ < 0.8) [33],As shown in Figure 3, with changing the concentration of 18−

and (b) slight decrease in the cation−exchange capacity caused bycrown−6 in the eluent, the retention volumes of these cations also18−crown−6 adsorbed on the surface of the silica gel. As shown inchanged. As for the monovalent cations, with increasing the con-Figure 4, complete separation of these mono− and divalent cations

centration of 18−crown−6, (a) the retention volumes of Li+ slightly

was achieved in 20 min with 0.75 mM tyramine−0.25 mM oxalic

decreased, (b) the retention volumes of NH4+ and Na+ increased in-acid at pH 5.0 containing 1.5 mM 18−crown−6 as the eluent.

itially and then remained almost the same, and (C) the retention

Various analytical performance parameters were investigatedvolumes of K+ increased drastically. The order of the increase inunder the optimum IC−IPD conditions (eluent : 0.75 mM tyramine

the retention volumes of the monovalent cations (Li+ < Na+ < NH4+

−0.25 mM oxalic acid−1.5 mM 18−crown−6 at pH 5.0). Table 4<< K+) was good agreement with that of the stabilities of com-lists the detection limits (signal−to−noise ratio of 3, injection vol-plexes formed between the monovalent cations and 18−crown−6

ume of 20 µl) of these mono− and divalent cations. Highly sensi-(log KNa+= 0.8, Log KNH4+ = 1.23, and Log KK+ = 2.03) [33]. This

tive indirect−photometric detection at 275 nm was achieved. Thisis because these cations were detected by indirect−photometricallymechanism, in which they displace the protonated tyramine, having
high molar extinction coefficient [1.5 × 103 (mol cm)−1 at 275 nm],

Figure 3. Effect of concentration of 18−crown−6 in 0.75 mM
tyramine−0.25 mM oxalic acid at pH 5.0 as eluent on
retention volumes of mono− and divalent cations on Pia
Seed 5S−60−SIL column

Figure 4. Chromatograms of mono− and divalent cations on Pia
Seed 5S−60−SIL column with 0.75 mM tyramine−0.25
mM oxalic acid−1.5 mM 18−crown−6 at pH 5.0 as eluent

Column : Pia Seed 5S−60−SIL, Eluent: 0−3 mM 18−crown−6 in 0.75 mM
tyramine−0.25 mM oxalic acid at pH 5.0, Detection : Indirect−UV at 275
nm, Sample : 0.2 mM for monovalent cations and 0.1 mM for divalent cations, Symbols : ● = Li+, ▲ = Na+, ■ = NH4+, ◆ = K+, ○ = Mg2+, and △ =Ca2+.
Other chromatographic conditions are as for in Figure 2.
Reproduced from Ref. 28 with the permission from Vieweg.

Eluent : 0.75 mM tyramine−0.25 mM oxalic acid−1.5 mM 18−crown−6 at
pH 5.0, Peaks : 1 = Li+, 2 = Na+, 3 = NH4+, 4 = K+, 5 = Mg2+, and 6 =Ca2+.
Other chromatographic conditions are as for in Figure 3.
Reproduced from Ref. 28 with the permission from Vieweg.

―９９―Chromatography, Vol.25 No.3 (2004)

Table 4. Detection limits (injection volume of 20 µl, signal−tonoise ratio of 3) of mono− and divalent cations with 0.75
mM tyramine−0.25 mM oxalic acid−1.5 mM 18−crown−6 at pH 5.0 as eluent
Detection limits
Cation(µM)(ng ml−1)

Li+

0.31

2.2

Na+

0.32

7.3

NH4+

0.40

7.1

K+

1.4

55

Mg2+

0.22

5.4

Ca2+

0.31

12

Reproduced from Ref. 28 with the permission from Vieweg.
from the eluent. Calibration graphs obtained by plotting peak area
against the concentration of these cations were linear (r2 > 0.99) in
Figure 5 A

the concentration range between 0.005 and 1.0 mM. The relative
standard deviations of the chromatographic peak areas, whose concentrations were 0.2 mM for the monovalent cations and 0.1 mM
for the divalent cations, were < 0.7 % (n=8). Reproducible chromatograms were obtained during repeated chromatographic runs.
From these above results, it was demonstrated that the puresilica gel (Pia Seed 5S−60−SIL) acted as the advanced cation−exchange stationary phase in the IC−IPD for common mono− and divalent cations using tyramine−oxalic acid containing 18−crown−6
as the eluent.
3.2. Separation and indirect−conductimetric detection mono−and divalent cations on Pia Seed 5S−60−SIL column withdilute oxalic acid as eluent
A silica gel (Develosil 30−3) was successfully applied as a
cation−exchange stationary phase in IC with conductimetric detection (IC−CD) for major mono− and divalent cations (Na+, NH4+,
K+, Mg2+ and Ca2+) with lithium oxalate as the eluent [13]. Then,
chromatographic behavior of the cations on the Pia Seed 5S−60−

Figure 5 B

SIL column was investigated with lithium oxalate as the eluent.
Figures 5A and B show chromatograms of the cations with (A) 0.5

Figure 5. Chromatograms of mono− and divalent cations on Pia
Seed 5S−60−SIL column with (A) 0.5 mM lithium oxalate at pH 5.0, and (B) 0.5 mM lithium oxalate−0.75
mM 18−crown−6 at pH 5.0, as eluentsmM lithium oxalate at pH 5.0, and (B) 0.5 mM lithium oxalate at
pH 5.0 containing 0.75 mM 18−crown−6, as the eluents. The pH of
the eluents was adjusted with 1 M HNO3.
As shown in Figure 5A, both incomplete separation of the

Eluent: (A) 0.5 mM lithium oxalate at pH 5.0, (B) 0.5 mM lithium oxalate−0.75 mM 18−crown−6 at pH 5.0, The pH of eluents was adjusted using 1
M HNO3.
Detection: Conductivity, Sample concentration: 0.2 mM for monovalentcations and 0.1 mM for divalent cations, Peaks: 1 = Na+, 2 = NH4+, 3 = K+,
4 = Mg2+, and 5 = Ca2+.
Other chromatographic conditions are as for in Figure 4.
Reproduced from Ref. 29 with the permission from Vieweg.

monovalent cations and complete separation of the divalent cations
were achieved. The complete separation was due mainly to oxalate
in the eluent [12, 13]. The incomplete separation was due mainly tophysical properties (particle size, pore size and surface area, etc.) of
the Pia Seed 5S−60−SIL silica gel. Unfortunately, complete separation of the mono− and divalent cations could not be achieved in a

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Kazutoku Ohta

Figure 6. Effect of concentration of oxalic acid in eluent on retention volumes of mono− and divalent cations on Pia Seed
5S−60−SIL column

Figure 7. Chromatograms of mono− and divalent cations on Pia
Seed 5S−60−SIL column with 0.2 mM oxalic acid at
pH 3.6 as eluent

Eluent : 0.05−1 mM oxalic acid, Detection : Indirect−conductivity, Sample
concentration : 0.1 mM for monovalent cations and 0.05 mM for divalentcations, Symbols : = water dip, ●= Li+, ▲= Na+, ■= NH4+, ◆= K+,○ = Mg2+, and △= Ca2+.
Other chromatographic conditions are as for in Figure 5.
Reproduced from Ref. 29 with the permission from Vieweg.

Eluent : 0.2 mM oxalic acid at pH 3.6, Peaks : 1 = Li+, 2 = Na+, 3 = NH4+,
4 = K+, 5 = Mg2+, and 6 = Ca2+.
Other chromatographic conditions are as for in Figure 6.
Reproduced from Ref. 29 with the permission from Vieweg.

showed cation−exchange behavior at the concentration of oxalic
acid in the eluent < 1 mM. With increasing the concentration of ox-

reasonable period of time (< 20 min) with lithium oxalate at pH 5.0

alic acid, the retention volumes of these cations decreased drasti-

as the eluent. The addition of 18−crown−6 to 0.5 mM lithium ox-

cally. This is due to both (a) a decrease in the cation−exchange ca-alate at pH 5.0 as the eluent was very effective way for improving

pacity caused by suppressing the dissociation of the silanol grouppeak resolution between the monovalent cations. As shown in Fig-

on the surface of the silica gel as cation−exchanger, and (b) an in-ure 5B, complete separation of the cations was achieved in 20 min

crease in the concentration of hydronium ion (H+) as eluent ion inwith 0.5 mM lithium oxalate at pH 5.0 containing 0.75 mM 18−

the eluent. As shown in Figure 7, using 0.2 mM oxalic acid at pH

crown−6 as the eluent. However, since Na+, and NH4+ were de-

3.6 as the eluent, although complete group separation between

tected direct−conductimetrically, and K+, Mg2+, and Ca2+ were de-these mono− and divalent cations was achieved in 10 min, peaktected indirect−conductimetrically, chromatogram was very com-resolutions between the monovalent cations and between the diva-

plicated. Furthermore, due to very small difference in equivalent

lent cations were very poor. These results suggested that the Pia

conductivity between lithium ion (Li+) as the eluent ion and theSeed 5S−60−SIL column could be utilized in the IC−CD for theseanalyte cations, the conductimetric detection sensitivities resulted

cations with dilute oxalic acid as the eluent. However, a further

in very low.

study was required for the complete separation of these cations on
the Pia Seed 5S−60−SIL column.

Dilute oxalic acid was tested as the eluent in the IC−CD with
the Pia Seed 5S−60−SIL column for common mono− and divalent

The addition of 18−crown−6 to 0.2 mM oxalic acid as the elu-

cations (Li+, Na+, NH4+, K+, Mg2+, and Ca2+), in order to eliminate

ent was carried out for improving peak resolution between these

these above drawbacks. Figure 6 shows the relationship between

mono− and divalent cations. Figure 8 shows the relationship be-

the concentration of oxalic acid in the eluent and the retention vol-tween the concentration of 18−crown−6 in the eluent and the reten-umes of these cations. Figure 7 shows chromatogram of these cat-tion volumes of these cations. Figure 9 shows chromatogram ofions with 0.2 mM oxalic acid at pH 3.6 as the eluent.

these cations with 0.2 mM oxalic acid at pH 3.6 containing 4 mM
18−crown−6 as the eluent.

As shown in Figure 6, the Pia Seed 5S−60−SIL silica gel

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Figure 8. Effect of concentration of 18−crown−6 in 0.2 mM oxalic acid at pH 3.6 as eluent on retention volumes ofmono− and divalent cations on Pia Seed 5S−60−SIL
columnFigure 9. Chromatograms of mono− and divalent cations on Pia
Seed 5S−60−SIL column with 0.2 mM oxalic acid−4
mM 18−crown−6 at pH 3.6 as eluent
Eluent : 0.2 mM oxalic acid−4 mM 18−crown−6 at pH 3.6, Peaks : 1 =Li+, 2 = Na+, 3 = NH4+, 4 = K+, 5 = Mg2+, and 6 = Ca2+.
Other chromatographic conditions are as for in Figure 8.
Reproduced from Ref. 29 with the permission from Vieweg.

Eluent : 0−10 mM 18−crown−6 in 0.2 mM oxalic acid, Symbols : ●= Li+,▲ = Na+, ■ = NH4+, ◆ = K+, ○ = Mg2+, and △ = Ca2+.
Other chromatographic conditions are as for in Figure 7.
Reproduced from Ref. 29 with the permission from Vieweg.

Table 5. Detection limits (injection volume of 20 µl, signal−tonoise ratio of 3) of mono− and divalent cations with 0.2
mM oxalic acid−4 mM 18−crown−6 at pH 3.6 as eluentmM oxalic acid at pH 3.6 containing 4 mM 18−crown−6 as theeluent.
Various analytical performance parameters were investigated

Detection limits
Cationunder the optimum IC−CD conditions (eluent : 0.2 mM oxalic acid
−4 mM 18−crown−6 at pH 3.6). Table 5 lists the detection limits(µM)(ng ml−1)

Li+

0.15

0.99

Na+

0.16

3.7

NH4+

0.21

3.9

detected by indirect−conductivity mechanism, in which they dis-

K+

1.0

39

place the hydronium ion, having the highest limiting equivalent

Mg2+

0.17

4.2

ionic conductance, from the eluent. Calibration graphs obtained by

Ca2+

0.25

10

(signal−to−noise ratio of 3, injection volume of 20 µl) of thesemono− and divalent cations. Highly sensitive indirect−conductimetric detection was achieved. This is because these cations wereplotting peak area against the concentration of these cations werelinear (r2 > 0.99) in the concentration range between 0.005 and 0.5

Reproduced from Ref. 29 with the permission from Vieweg.

mM for the monovalent cations and between 0.005 and 0.3 mM forthe divalent cations. Somewhat narrow linear range was attributed

As shown in Figure 8, with changing the concentration of 18−

to be very low cation−exchange capacity of the Pia Seed 5S−60−

crown−6 in the eluent, the retention volumes of these cations

SIL column under the optimum IC−CD conditions. The relativechanged individually. This result meant that 18−crown−6 seemed

standard deviations of the chromatographic peak areas, whose con-to act as a selective cation−exchanger (selective retention modifier)centration were 0.1 mM for monovalent cations and 0.05 mM forfor these cations under the IC−CD conditions. With increasing thethe divalent cations, were < 1.2 % (n=10). Reproducible chroma-concentration of 18−crown−6, the peak resolutions were improved

tograms were obtained during repeated chromatographic runs.

largely. As a result, as shown in Figure 9, complete separation of

From these above results, it was demonstrated that the purethese mono− and divalent cations was achieved in 20 min with 0.2

silica gel (Pia Seed 5S−60−SIL) also acted as the advanced cation−

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Kazutoku Ohta

exchange stationary phase in the IC−CD for common mono− anddivalent cations with dilute oxalic acid containing 18−crown−6 asthe eluent.
3.3. Separation of mono− and divalent cations on calcinated Pia
Seed 5S−60−SIL columns
Calcination (heat treatment) is easy and effective way for themodification of adsorption properties of silica gel for organic compounds [36]. Then, chromatographic behavior of common mono−and divalent cations (Li+, Na+, NH4+, K+, Mg2+, and Ca2+) on columns (150 × 4.6 mm i.d.) packed with the Pia Seed 5S−60−SIL silica gels calcinated at 200, 400, 600, 800 and 1000°
C for 5 h (denoted Pia−Seed−200, −400, −600, −800, and −1000) was investigated, for the expansion the utility of the pure silica gel in IC for
cations. Table 3 shows physical properties of the calcinated Pia
Seed 5S−60−SIL silica gels. Firstly, chromatographic behavior ofthese mono− and divalent cations on the calcinated Pia Seed 5S−60−SIL columns was investigated with 0.75 mM tyramine−0.25 mM
oxalic acid at pH 5.0 as the eluent. Figures 10A−C show chromaFigure 10 B

Figure 10 A

Figure 10 C

Figure 10. Chromatograms of mono− and divalent cations on calcinated Pia Seed 5S−60−SIL silica gel columns with 0.75 mM tyramine−
0.25 mM oxalic acid at pH 5.0 as eluent.
Column : (A) Pia Seed 5S−60−SIL calcinated at 200°
C for 5 h (Pia Seed−200), (B) Pia Seed 5S−60−SIL calcinated at 600°
C for 5 h (Pia Seed−600), (C) Pia Seed 5S−60−SIL calcinated at 1000°
C for 5 h (Pia Seed−1000),
Eluent : 0.75 mM tyramine−0.25 mM oxalic acid at pH 5.0, The pH of eluents was adjusted with 1 M HNO3.
Detection : Indirect−UV at 275 nm, Sample concentration : 0.2 mM for monovalent cations and 0.1 mM for divalent cations,
Peaks : 1 = Li+, 2 = Na+, 3 = NH4+, 4 = K+, 5 = Mg2+, and 6 = Ca2+.
Other chromatographic conditions are as for in Figure 9.
Reproduced from Ref. 30 with the permission from Vieweg.

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Chromatography, Vol.25 No.3 (2004)

Figure 11. Effect of concentration of 18−crown−6 in 0.75 mM
tyramine−0.25 mM oxalic acid at pH 5.0 as eluent on
retention volumes of mono− and divalent cations on
Pia Seed−1000 column

Figure 12. Chromatograms of mono− and divalent cations on Pia
Seed−1000 column with 0.75 mM tyramine−0.25 mM
oxalic acid−0.5 mM 18−crown−6 at pH 5.0 as eluent
Eluent : 0.75 mM tyramine−0.25 mM oxalic acid−0.5 mM 18−crown−6 at
pH 5.0, Peaks : 1 = Li+, 2 = Na+, 3 = NH4+, 4 = K+, 5 = Mg2+, and 6 =Ca2+.
Other chromatographic conditions are as for in Figure 11.
Reproduced from Ref. 30 with the permission from Vieweg.

Column : Pia Seed−1000, Eluent 0−1.5 mM 18−crown−6 in 0.75 mM
tyramine−0.25 mM oxalic acid at pH 5.0, Symbols : ● = Li+, ▲ = Na+, ■ =NH4+, ◆ = K+, ○ = Mg2+, and △ = Ca2+.
Other chromatographic conditions are as for in Figure 10.
Reproduced from Ref. 30 with the permission from Vieweg.

shown in Figure 12, complete separation of these cations was
tograms of these cations on (A) Pia Seed−200, (B) Pia Seed−600,

achieved with 0.75 mM tyramine−0.25 mM oxalic acid at pH 5.0

and (C) Pia Seed−1000 columns.

containing 0.5 mM 18−crown−6 as the eluent. The suitable concen-

As shown in Figures 10A−C, both incomplete separation of

tration of 18−crown−6 on the Pia Seed−1000 column (0.5 mM)

the monovalent cations and complete separation of the divalent cat-

was considerably lower than that on the Pia Seed 5S−60−SIL col-ions were achieved on these columns in 13 min. Chromatographicumn (1.5 mM), as shown in Figure 4. These results suggested thatbehavior of these cations on these columns was very similar. Thesethe effect of 18−crown−6 as the retention modifier for these cations

chromatograms indicated that the calcination was almost no effectenhanced on the Pia Seed−1000 column. Then, the amounts of 18−

for improving peak resolution between these cations with 0.75 mM

crown−6 adsorbed on the calcinated Pia Seed 5S−60−SIL columns

tyramine−0.25 mM oxalic acid at pH 5.0 as the eluent.

were determined with 0.75 mM tyramine−0.25 mM oxalic acid at

Next, chromatographic behavior of these cations on the Pia

pH 5.0 containing 1.5 mM 18−crown−6 as the eluent. Figure 13

Seed−1000 column was investigated in detail with 0.75 mM

show breakthrough curves of the calcinated Pia Seed 5S−60−SIL

tyramine−0.25 mM oxalic acid at pH 5.0 containing 18−crown−6

columns.

as the eluent. Figure 11 shows the relationship between the concen-

As shown in Figure 13, with arising calcination temperature,

tration of 18−crown−6 in the eluent and the retention volumes ofbreakthrough volume of corresponding calcinated Pia Seed 5 S−60

these cations. Figure 12 shows chromatogram of these cations with

−SIL column increased largely. The amounts of 18−crown−6 ad-

0.75 mM tyramine−0.25 mM oxalic acid at pH 5.0 containing 0.5

sorbed on the Pia Seed−200, −400, −600, −800, and −1000 col-mM 18−crown−6 as the eluent.

umns were 9.9, 19, 24, 28, and 30 µmol column−1, respectively.

As shown in Figure 11, with changing the concentration of 18

The amount of 18−crown−6 adsorbed on the Pia Seed−1000 col-−crown−6 in the eluent, the retention volumes of these cations also

umn reached three times larger than that on the Pia Seed−200 col-changed. With increasing the concentration of 18−crown−6 (in theumn. This result revealed that (a) the calcination enhanced the af-

concentration range of 18−crown−6 between 0 and 0.5 mM), peakfinity of the Pia Seed 5S−60−SIL silica gel for 18−crown−6, andresolution between these cations was improved. As a result, as(b) the predominant factor in the effect of 18−crown−6 as the re-

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Kazutoku Ohta

Figure 13. Breakthrough curves of calcinated Pia Seed 5 S−60−
SIL columns with 0.75 mM tyramine−0.25 mM oxalic
acid at pH 5.0 containing 1.5 mM 18−crown−6 as eluent

Figure 14. Effect of concentration of 15−crown−5 in 0.75 mM
tyramine−0.25 mM oxalic acid at pH 5.0 as eluent on
retention volumes of mono− and divalent cations on
Pia Seed−1000 column

Column : (A) Pia Seed−200, (B) Pia Seed−400, (C) Pia Seed−600, (D) Pia
Seed−800, (E) Pia Seed−1000
Eluent : 0.75 mM tyramine−0.25 mM oxalic acid−1.5 mM 18−crown−6 at
pH 5.0, Detection : Refractive index, Sample : 7.5 mM tyramine−2.5 mM
oxalic acid at pH 5.0,
Other chromatographic conditions are as for in Figure 12.
Vo : total dead volume (column void volume + connected tube volume)
VR : breakthrough volume

Column : Pia Seed−1000, Eluent : 0−10 mM 15−crown−5 in 0.75 mM
tyramine−0.25 mM oxalic acid at pH 5.0, Detection : Indirect−UV at 275
nm, Sample concentration: 0.2 mM for monovalent cations and 0.1 mM fordivalent cations, Symbols : ● = Li+, ▲ = Na+, ■ = NH4+, ◆ = K+, ○ =Mg2+, and △ = Ca2+.
Other chromatographic conditions are as for in Figure 13.
Reproduced from Ref. 30 with the permission from Vieweg.

15−crown−5 in the eluent, the retention volumes of these cations
tention modifier for these cations was not the concentration of 18−

increased. The order of the increase in the retention volumes of thecrown−6 in the eluent but the amount of 18−crown−6 adsorbed onmonovalent cations was Li+ < NH4+ < Na+ < K+ and that of the di-silica stationary phase under the IC−IPD conditions.

valent cations was Mg2+ < Ca2+. These orders meant that 15−crown

15−Crown−5 also forms complex with many cations [32, 33].

−5 also behaved as a selective cation−exchanger (selective reten-

Due to low stabilities of complexes formed with 15−crown−5 andtion modifier) for these cations in the IC−IPD conditions. With in-these mono− and divalent cations, the addition of 15−crown−6 tocreasing the concentration of 15−crown−5, peak resolution be-acidic solution as the eluent is not effective for improving peak

tween these cations was improved effectively. As a result, as

resolution between these cations on weakly acidic cation−exchange

shown in Figure 15 A, excellent separation of these cations was

resin columns [35]. Therefore, chromatographic behavior of theseachieved on the Pia Seed−1000 column in 20 min with 0.75 mM

cations on the Pia Seed−200 and −1000 columns was investigatedtyramine−0.25 mM oxalic acid at pH 5.0 containing 5 mM 15−

with 0.75 mM tyramine−0.25 mM oxalic acid at pH 5.0 containing

crown−5 as the eluent. In contrast, as shown in Figure 15 B, the15−crown−5 as the eluent. Figure 14 shows the relationship be-complete separation of the monovalent cations was not achieved on

tween the concentration of 15−crown−5 in the eluent on the reten-

the Pia Seed−200 column with 0.75 mM tyramine−0.25 mM oxalic

tion volumes of these cations on the Pia Seed−1000 column. Fig-

acid at pH 5.0 containing 5 mM 15−crown−5 as the eluent. Whenures 15 A and B show chromatograms of these cations on (A) theusing 0.75 mM tyramine−0.25 mM oxalic acid at pH 5.0 contain-

Pia Seed−1000, and (B) the Pia Seed−200 columns with 0.75 mM

ing 20 mM 15−crown−5 as the eluent, peak resolution between thetyramine− 0.25 mM oxalic acid at pH 5.0 containing 5 mM 15−

monovalent cations remained unsatisfactory on the Pia Seed−200

crown−5 as the eluent.

column. Then, the amounts of 15−crown−5 adsorbed on the calci-

As shown in Figure 14, with increasing the concentration of

nated Pia Seed 5S−60−SIL columns were determined with 0.75

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Chromatography, Vol.25 No.3 (2004)

Figure 16. Breakthrough curves of calcinated Pia Seed 5 S−60−
SIL columns with 0.75 mM tyramine−0.25 mM oxalic
acid−5 mM 15−crown−5 at pH 5.0 as eluent

Figure 15 A

Column: (A) Pia Seed−200, (B) Pia Seed−400, (C) Pia Seed−600, (D) Pia
Seed−800, (E) Pia Seed−1000, Eluent: 0.75 mM tyramine−0.25 mM oxalic
acid−5 mM 15−crown−5 at pH 5.0, Detection: Refractive index, Sample:
7.5 mM tyramine−2.5 mM oxalic acid at pH 5.0,
Other chromatographic conditions are as for in Figure 15.
Vo: total dead volume (column void volume + connected tube volume)
VR: breakthrough volume.

mM tyramine−0.25 mM oxalic acid at pH 5.0 containing 5 mM 15−crown−5 as the eluent. Figure 16 show breakthrough curves of thecalcinated Pia Seed 5S−60−SIL columns.
As shown in Figure 16, with arising calcination temperature,
breakthrough volume of corresponding calcinated Pia Seed 5S−60−
SIL column increased largely. The amounts of 15−crown−5 adsorbed on the Pia Seed−200, −400, −600, −800, and −1000 columns were 19, 32, 40, 44, and 50 µmol column−1, respectively. The
amount of 15−crown−5 adsorbed on the Pia Seed−1000 column
was much larger than that on the Pia Seed−200 column. This result

Figure 15 B

also bore out strongly that the amount of crown ethers adsorbed on
the silica stationary phase was the predominant factor in the effectFigure 15. Chromatograms of mono− and divalent cations on (A)Pia Seed−1000, and (B) Pia Seed−200 columns with
0.75 mM tyramine−0.25 mM oxalic acid−5 mM 15−
crown−5 at pH 5.0 as eluentof crown ethers as the retention modifiers for these mono− and divalent cations under the IC−IPD conditions.
Finally, chromatographic behavior of these mono− and diva-

Column : (A) Pia Seed−1000, (B) Pia Seed−200, Eluent : 0.75 mM
tyramine−0.25 mM oxalic acid−0.5 mM 18−crown−6 at pH 5.0, Peaks : 1 =Li+, 2 = Na+, 3 = NH4+, 4 = K+, 5 = Mg2+, and 6 = Ca2+.
Other chromatographic conditions are as for in Figure 14.
Reproduced from Ref. 30 with the permission from Vieweg.

lent on the calcinated Pia Seed 5S−60−SIL columns was investigated with dilute oxalic acid (0.2 mM, pH 3.6) containing crownethers (18−crown−6 and 15−crown−5) as the eluent. Figures 17 A−
C show chromatograms of these cations on the Pia Seed−1000 column with (A) 0.2 mM oxalic acid, (B) 0.2 mM oxalic acid containing 0.4 mM 18−crown−6, and (C) 0.2 mM oxalic acid containing 4

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Chromatography, Vol.25 No.3 (2004)

Kazutoku Ohta

Figure 17 A

Figure 17 B

Figure 17. Chromatograms of mono− and divalent cations on
Pia Seed−1000 column with (A) 0.2 mM oxalic
acid, (B) 0.2 mM oxalic acid−0.4 mM 18−crown−6,
and (C) 0.2 mM oxalic acid−4 mM 15−crown−5, as
eluents.
Column : Pia Seed−1000, Eluent: (A) 0.2 mM oxalic acid at pH 3.6, (B)
0.2 mM oxalic acid−0.4 mM 18−crown−6 at pH 3.6, (C) 0.2 mM oxalic
acid−4 mM 15−crown−5 at pH 3.6, Detection : Indirect−conductivity,
Sample concentration : 0.1 mM for monovalent cations and 0.05 mM fordivalent cations, Peaks : 1 = Li+, 2 = Na+, 3 = NH4+, 4 = K+, 5 = Mg2+,
and 6 = Ca2+.
Other chromatographic conditions are as for in Figure 16.

Figure 17 C

was achieved with 0.2 mM oxalic acid containing 0.4 mM 18−

mM 15−crown−5, as the eluents.
As shown in Figure 17 A, poor peak resolutions between thecrown−6 as the eluent. The concentration of 18−crown−6 on themonovalent cations and between the divalent cations were obtained

Pia Seed−1000 column (0.4 mM) was much lower than that on thewith 0.2 mM oxalic acid as the eluent. The addition of crown

Pia Seed 5S−60−SIL column (4 mM), as shown in Figure 9. As

ethers to 0.2 mM oxalic acid as the eluent was very effective forshown in Figure 17 C, excellent separation of these cations was

improving the peak resolutions. This is because crown ethers was

also achieved with 0.2 mM oxalic acid containing 4 mM 15−crownadsorbed on the silica stationary phase at first and then acted as se-−5 as the eluent. In contrast, the addition of 40 mM 15−crown−5 tolective cation−exchangers (selective retention modifiers) for these

the eluent was requited for good separation of these cations on the

cations. As shown in Figure 17 B, good separation of these cations

Pia Seed−200 column.

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Chromatography, Vol.25 No.3 (2004)

References

From these above results, it was revealed that the calcinated
Pia Seed 5S−60−SIL silica gels, especially Pia Seed−1000, acted as[1]

Small, H. ; Stevens, T. S. ; Bauman, W. C. Anal. Chem.

[2]

Fritz, J. S. ; Gjerde, D. T. Ion Chromatography−Principals,

1975, 47, 1801−1809.

advanced cation−exchange stationary phases for common mono−and divalent cations in both (a) IC−IPD with aromatic monoamine

2 nd Edition ; Hüthing : Heidelberg, 1987.

(tyramine)−oxalic acid at pH 5.0 containing crown ethers as theeluent, and (b) IC−CD with dilute oxalic acid containing crown[3]

Haddad, P. R. ; Jackson, P. E. Ion Chromatography−Princi-[4]

Kolla, P. ; Köhler, J. ; Schomberg, G. Chromatographia

ethers as the eluent.

pals and Applications ; Elsevier : Amsterdam, 1990.

4. Conclusions

1987, 23, 465−472.

Effectiveness of a pure silica gel (Pia Seed 5 S−60−SIL), syn-[5]

thesized by the hydrolysis of pure tetraethoxysiliane [Si(OCH2

Kondratjonok, B. ; Schwedt, G. Fresenius’ Z. Anal. Chem.
A 1988, 323, 333−337[6]

Scott, R. P. Silica Gel and Bonded Phases ; John Wiley and

K+, Mg2+, and Ca2+) was described. Due to the inert nature of the[7]

Unger, K. K. Porous Silica ; Elsevier : Amsterdam, 1979.

pure silica gel for aromatic monoamines, the Pia Seed 5S−60−SIL[8]Smith, R. L. ; Pietrzyk, D. J. Anal. Chem. 1984, 56, 610−

[9]Brown, D. M. ; Pietrzyk, D. J. J. Chromatogr. 1989, 466,

CH3)4] as a cation−exchange stationary phase in ion chromatography (IC) for common mono− and divalent cations (Li+, Na+, NH4+,

Sons: Chichester, 1993.

614.

silica gel acted as an advanced cation−exchange stationary phase in
IC with indirect−photometric detection for these cations using aro-291−300.

matic monoamines as the eluents. The Pia Seed 5S−60−SIL silica[10] Iwachido, T. ; Ishimaru, K. ; Motomizu, S. Anal. Sci. 1988,

gel also behaved as an advanced cation−exchange stationary phase4, 81−85.

in IC with conductimetric detection for these cations using dilute[11] Iwachido, T. ; Shinomiya, M. ; Zenki, M. Anal. Sci. 1990, 6,

oxalic acid as the eluent. Excellent simultaneous separation and277−282.

highly sensitive detection for these cations were achieved in 20 min
on a Pia Seed 5S−60−SIL column (150 mm × 4.6 mm i.d.) with (a)[12] Iwachido, T. ; Hayama, N. Anal. Sci. 1990, 6, 307−308.

0.75 mM tyramine [4−(2−aminoethyl)phenol]−0.25 mM oxalic[13] Iwachido, T. ; Ikeda, K. ; Zenki, M. Anal. Sci. 1990, 6, 593−
597.

acid at pH 5.0 containing 1.5 mM 18−crown−6 (1,4,7,10,13,16−[14] Ohta, K. ; Sando, M. ; Tanaka, K. ; Haddad, P. R. J. Chro-hexaoxacyclooctadecane) as the eluent, or (b) 0.2 mM oxalic acidmatogr. A 1996, 752, 167−172.

at pH 3.6 containing 4 mM 18−crown−6 as the eluent.

[15] Ohta, K. ; Tanaka, K. ; Paull, B. ; Haddad, P. R. J. Chroma-

Effectiveness of calcinated Pia Seed 5S−60−SIL silica gels as

togr. A 1999, 770, 219−227.

cation−exchange stationary phases in IC for common mono− and[16] Ohta, K. ; Tanaka, K. Water Treatment 1999, 40, 413−421.

divalent cations was also described. Calcination enhanced the affinity of the Pia Seed 5S−60−SIL silica gel for crown ethers [18−[17] Ohta, K. ; Tanaka, K. Analyst 1999, 124, 505−510.

crown−6 and 15−crown−5 (1,4,7,10,13,16−hexaoxacyclooctade-[18] Ohta, K. ; Morikawa, H. ; Tanaka, K. ; Uryu, Y. ; Paull, B. ;cane)] and, as a consequence, the effect of crown ethers as the se-

Haddad, P. R. Anal. Chim. Acta 1998, 359, 255−261.

lective cation−exchangers (selective retention modifiers) for these[19] Ohta, K. ; Morikawa, H. ; Sando, M. Anal. Chim. Acta
2001, 439, 255−263.

cations enhanced effectively. Calcination expanded largely the util-[20] Foley, R. C. ; Haddad, P. R. J. Chromatogr. 1986, 366, 13−

ity of the Pia Seed 5S−60−SIL silica gel in IC for these mono− and26.

divalent cations.

[21] Haddad, P. R. ; Foley, R. C. Anal. Chem. 1989, 61, 1435−

The pure silica gel (Pia Seed 5S−60−SIL) is one of the most1441.

favorable cation−exchange stationary phases in IC for common[22] Sadek, P. ; Koester, C. J. ; Bowers, L. D. J. Chromatogr.

mono− and divalent cations.

Sci. 1987, 25, 489−493.
[23] Ohtsu, Y. ; Shiojima, Y. ; Okumura, T. ; Koyama, J. ; Naka-

Acknowledgements

mura, K. ; Nakata, O. ; Kimata, K. ; Tanaka, N. J. Chroma-

The author thanks Dr. H. Morikawa of Inorganic Powder

togr. 1989, 481, 147−157.

Characterization Research Group, Research Institute of Instrumen-[24] Kimata, K. ; Tanaka, N. ; Arai, T. J. Chromatogr. 1992,

tation Frontier (RIIF), National Institute of Advanced Industrial

594, 87−96.

Science and Technology (AIST), for his cooperation. The author[25] Ohta, K. ; Kusumoto, K. ; Takao, Y. ; Towata, A. ;also thanks Mr. Y. Uryu of Pia Tec Co. Ltd., for his technical co-

Morikawa, H. ; Ohashi, M. J. Chromatogr. A 2002, 956,

operation.

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Chromatography, Vol.25 No.3 (2004)

Kazutoku Ohta

[31] Ohta, K. Unpublished results

173−180.

[32] Pederson, C. J. J. Am. Chem. Soc. 1967, 89, 7017−7036.

[26] Munaf, E. ; Zein, R. ; Takeuchi, T. ; Miwa, T. Anal. Chim.

[33] Izatt, R. M. ; Bradshaw, J. S. ; Nielsen, S. A. ; Lamb, J. D. ;Acta 1996, 334, 39−43.

Christensen, J. J. Chem. Rev. 1985, 85, 271−339.

[27] Ohta, K. Analyst 2000, 125, 269−272.

[34] Laubli, M. W. ; Kampus, B. J. Chromatogr. A 1995, 706, 99[28] Ohta, K. ; Kusumoto, K. ; Takao, Y. ; Ohashi, M. ;−102.

Takeuchi, T. Chromatographia 2002, 55, 95−100.

[35] Ono, M. ; Kumagai, H. ; Sakai, T. ; Date, Y. ; Inoue, Y. Ind.

[29] Ohta, K. ; Kusumoto, K. ; Takao, Y. ; Towata, A. ;Water 1997, 469, 25−31.

Kawakami, S. ; Ohashi, M. ; Takeuchi, T. Chromatographia[36] Mauss, M. ; Engelhardt, H. J. Chromatogr. 1986, 371, 235−

2003, 58, 171−175.

242.

[30] Ohta, K. ; Ohashi, M. ; Takeuchi, T. Chromatographia
2003, 57, 155−160.

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