RAPID COMMUNICATIONS IN MASS SPECTROMETRY

Rapid Commun. Mass Spectrom. 2005; 19: 2809–2818

Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/rcm.2121

Complementary use of Fourier transform laser

microprobe mass spectrometry and time-of-flight static

secondary ion mass spectrometry for the study of the

surface adsorption of organic dyes on silicate materials

Alina-Maria Busuioc1*, Roel De Mondt1, Harri Moisio1, Pegie Cool1, Aurelia Vasile2,

Nicolae Bilba2, Etienne Vansant1 and Luc Van Vaeck1

1Micro- and Trace Analysis Centre (MiTAC), Department of Chemistry, University of Antwerpen (CDE), Universiteitsplein 1, 2610 Wilrijk, Belgium2‘Al.I. Cuza’ University of Iasi, Department of Chemical Technology and Materials Chemistry, B.-dul Carol I 11, RO-700506 Iasi, Romania

Received 6 June 2005; Revised 27 July 2005; Accepted 27 July 2005

The adsorption of organic ionic dyes on different pore size engineered silica materials with poten-

tial application for industrial wastewater treatment has been investigated using Fourier transform

laser microprobe mass spectrometry (FT-LMMS) and time-of-flight secondary ion mass spectrome-

try (TOF-S-SIMS). The complementary use of the two methods with different information depth

allowed data on the subsurface distribution and pore penetration of the adsorbed organic com-

pounds. Macroscopic methods were employed to determine the amount adsorbed on the particles

and the specific external surface area. Local MS analysis allows identification of the organic dyes

exclusively at the outer particle surface when the pore size is inferior to the size of the adsorbing

molecule, or at the surface of the channels inside the material. Specifically, the monolayer informa-

tion depth of TOF-S-SIMS causes a signal to refer essentially to the adsorbate at the outer particle

surface, which is only a fraction of the total adsorption in mesoporous materials, while FT-LMMS

allowed detection of the presence of adsorbates at the outer surface as well as inside the subsurface

of 10 to 50nm depending on the material under study. The observed data open perspectives for the

molecular monitoring of the adsorption behaviour of different materials at the (sub) mm scale.

Copyright # 2005 John Wiley & Sons, Ltd.

Fifteen percent of the total world production of dyes is lost

during dyeing processes and is released in textile effluents.1

The release of these colour wastewaters in the ecosystem is a

dramatic source of aesthetic pollution, eutrophication and

perturbation of aquatic life. As international environmental

standards are becoming more stringent, technological

systems for the removal of organic pollutants such as dyes,

herbicides, pesticides, etc., have been developed. Decolouri-

sation of dye effluents is, today, the principal problem

concerning the textile industry. Physical (adsorption), bio-

logical (biodegradation2) and chemical (chlorination, ozoni-

sation, photodegradation3) methods are the most frequently

used. Among these, traditional physical techniques like

adsorption on activated carbons, ultrafiltration, reverse

osmosis, coagulation by chemical agents, or ion exchange

by synthetic adsorbent resins, can be used efficiently.4–7

Adsorption of dyes on the surface of various materials

represents an important and intensively studied phenom-

enon. The most studied systems are membrane filters,8

polymers,9 biological cells,10 and inorganic sorbents.

Pelekani and Snoeyink11 investigated the competitive adsor-

ption between atrazine and methylene blue on activated

carbon. Khattri and Singh12 used a bio-adsorbent for removal

of synthetic dyes from wastewaters. Demirbas13 studied the

removal of Victoria blue from aqueous solutions using perlite

(a glassy volcanic rock).

Among the use of inorganic sorbents, silicas deserve

attention because of the chemical reactivity of their surfaces,

resulting from the presence of silanol groups. Of particular

interest are the micro- and mesoporous silicate materials

because of their high surface area (300–1400 m2/g) and tune-

able pore sizes that can be tailored to suit specific require-

ments, making them versatile and attractive for these

applications.

One of the prime aspects to be considered in the

development of absorbents for practical applications is the

fundamental relationship between the pore size and the size

of the molecule as a determining factor for the adsorption

behaviour on the specific adsorbent. This work selected a

microporous absorbent (silicalite-1) and three different types

of mesoporous silicate materials (MCM-41, SBA-15 and silica

gel), while quartz was used as a non-porous reference

material for comparative purposes.

Copyright # 2005 John Wiley & Sons, Ltd.

*Correspondence to: A.-M. Busuioc, Department of Chemistry,University of Antwerpen (UA), Universiteitsplein 1, B 2610Wilrijk, Belgium.E-mail: alina-maria.busuioc@student.ua.ac.be

Silicalite-1, one of the most studied pure siliceous micro-

porous molecular sieves in which the pores are created by

interconnected ring-like structures, exhibits non-polar char-

acteristics such as low organophilicity, hydrophobicity, high

thermal stability (at least 7008C), good chemical stability in

acidic conditions, and high size selectivity. The structural

porosity of silicalite-1 consists of sinusoidal channels

(a-direction) with a circular cross-section (5.5� 5.1 A), which

are interconnected with straight channels (b-direction) of

elliptic cross-section (5.6� 5.4 A).

However, microporous materials have several limitations

for the adsorption of large molecules because of steric effects.

For this reason, an important line of research has focused on

the enlargement of the pore sizes into the mesoporous range

(2<d< 50 nm).14 Therefore, the discovery of the mesoporous

silica-based materials such as MCM-41 (i.e., Mobile Corp.

Composite Matter, No 41) in 199215 has expanded signifi-

cantly the possibilities for the adsorption of bulky molecules.

MCM-41 has a honeycomb structure that results from a

hexagonal packing of aligned cylindrical pores. It is an

amorphous hydroxylated silicate consisting of Si(OSi)4 and

Si(OSi)3OH species. The pore sizes can be tailored from 2 to

10 nm using different templating agents, swelling agents,

longer ageing period, or higher synthesis temperature. The

high surface area (>1000 m2 g�1), narrow pore size distribu-

tions and highly ordered structures make these materials

attractive for adsorption of various bulky molecules.

A major drawback of MCM-41 for the removal of dyes from

wastewaters is the lack of hydrothermal stability. An

alternative is the SBA-15 material (named after the Santa

Barbara laboratory that synthesised this material for the first

time) which is also a pure siliceous material with large pores

in the mesoporous range (typically 7–8 nm) and a high

surface area (usually 700–900 m2/g). Its structure consists

also of a honeycomb arrangement of one-dimensional pores.

The thickness and microporosity of the walls improve the

thermal and hydrothermal stability.

In this work methylene blue, crystal violet and rhodamine

6G were used as probing molecules to verify the potential use

of these materials for wastewater treatment. All these dyes

have cationic functions that inherently favour the interaction

with and binding to the silica materials through hydrogen

bridges. The selected dyes, whose structures are represented

in Fig. 1, have different molecular sizes, allowing the possible

steric effects in the adsorption behaviour to be verified.

It is common practice to use the analysis of the adsorbate

concentrations in the solution before and after adsorption to

evaluate the sorption performance of a given material. Few

methods deal directly with the determination of adsorbed

organics on porous silica, and those that do are not molecule-

specific. The recent development of mass spectrometric

methods using focused laser or primary ion beams for local

ionisation of analytes offers unique perspectives for the

sensitive and molecule-specific detection of organic and

inorganic analytes at the surface of solids.

The purpose of this paper is to evaluate the micro-

analytical characterisation of adsorbates on mesoporous

and microporous silica materials. Specifically, distinction

between adsorption at the outer particle surface and inside

the pores of the silica is of major interest. Therefore, the

combination of Fourier transform laser microprobe mass

spectrometry (FT-LMMS)16 and time-of-flight static second-

ary ion mass spectrometry (TOF-S-SIMS)17,18 was used.

While TOF-S-SIMS interrogates an information layer limited

to essentially one monolayer, ions detected by FT-LMMS

originate from the surface and the subsurface up to a depth of

10 nm in compact materials.19 Hence, the combination of

these two methods is potentially important to give informa-

tion about the relation between the pore size of the adsorbent

and the molecular size of the dye.

EXPERIMENTAL

Preparation of silica materialsSilicalite-1 was prepared by hydrothermal synthesis (1508C)

as reported previously20 using fumed silica (Aerosil 380,

Aldrich) as Si source, tetrapropylammonium bromide

(TPABr, Fluka) as templating agent, and HF as mineralising

agent.

The synthesis of mesoporous materials involves the

formation of organic-inorganic composites by a self-assem-

bly process, where the organic phase is organised on a

mesoscopic scale and serves as a template for the inorganic

skeleton. Samples of MCM-41 were prepared according to the

synthetic procedure reported in the literature21 using fumed

silica (Aerosil 380, Degussa) as silica precursor, cetyltri-

methylammonium bromide (CTMAB, Acros Organics) as

templating agent, and tetraethylammonium hydroxide

(TEAOH, sol. 20%, Acros Organics) as mineralising agent.

The composition of the initial gel was SiO2/0.2 TEAOH/0.25

CTMABr/35 H2O. Additionally, SBA-15 was synthesised by

a procedure described earlier22 using tetraethylorthosilicate

(TEOS, 98%, Acros Organics) as silica source, Pluronic P123

((ethylene-oxide)20(propylene-oxide)70(ethylene-oxide)20,

O N+

N

CO

O

H H

Cl-

CN+

N

N

Cl-

S+

N

NNCl-

Rhodamine 6G Methylene blue

Crystal violet

Figure 1. Structures of the dyes studied.

Copyright # 2005 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2005; 19: 2809–2818

2810 A.-M. Busuioc et al.

Aldrich) as templating agent, and HCl (37%, Aldrich) as

mineralising agent.

Characterisation of the materialPorosity and surface area were determined using a Quanta-

chrome Autosorb-1-MP automated gas adsorption system

using nitrogen as the adsorbate at liquid nitrogen tempera-

ture (77 K). All samples were outgassed under vacuum for

16 h at 2008C before measurements. The pore diameter was

obtained from the nitrogen adsorption branch using the

Barrett-Joyner-Halenda (BJH) method. The surface area was

calculated using the Brunauer-Emmett-Teller (BET) method.

Additionally, X-ray diffractograms were recorded using a

Phillips PW 1840 powder diffractometer, with Ni-filtered

CuKa radiation.

Adsorption experimentSiliceous material (100 mg) was added to 100 mL of aqueous

dye solutions with an initial concentration of 1� 10�4 M. The

suspensions were magnetically stirred for 180 min. At given

intervals, aliquots of 4 mL were taken, centrifuged for 15 min

at 3700 rpm, and then analysed by UV-VIS spectroscopy

using a Nicolet e-500 spectrophotometer. The wavelengths

used for the detection of rhodamine 6G, crystal violet and

methylene blue were 526, 590 and 665 nm, respectively.

The remaining suspension was filtrated through a Milli-

pore filter (HAWP 0.45 mm) and the filtered material was used

for surface analysis after drying in a desiccator.

Surface analysisThe S-SIMS experiments were performed using an Ion TOF

IV instrument (Cameca-Ion ToF). The liquid metal ion gun

(LMIG) was used to generate 25 keV Gaþ primary ions. The

gun was operated in the bunched mode with a final pulse

width of 800 ps, allowing nominal mass resolution (10% val-

ley) to be achieved up to m/z 1000. Mass spectra were

acquired from 100� 100mm2 areas during 150 s, resulting in

a total ion dose density below 1013 ions cm�2 on the sample,

which is generally accepted to be under the static limit. As the

impact of a primary ion destroys the molecular structures in

the sample over a given area (typically 10 nm2) around the

point of impact, detection of molecular information in S-

SIMS implies that no spot should be hit twice. The static limit

refers to the maximum allowable ion dose density, and is

determined as the ion dose that makes less than 1% of the sur-

face area inadequate for further analysis. A full discussion is

given elsewhere.17 Mass spectra were calibrated using the

signals for Hþ, Cþ, CHþ, CH2þ, CH3

þ and C3H3þ at m/z 1, 12,

13, 14, 15 and 39, respectively. Electron flooding was used

for charge compensation during the analysis of samples of

silica gel and quartz.

The quartz samples were prepared by pressing particles

into indium foil. For all other materials, pellets were made

using a common die with diameter of 10 mm and a pressure

of 9 tons. Limiting the amount of powder allows pellets with a

thickness of less than 1 mm to be made. Only a small fragment

of the pellet with a flat surface was introduced for analysis to

keep the pump-down time limited.

To account for the possible local heterogeneity, each

sample was measured on at least three different spots.

Absolute intensities at the same primary ion dose and under

identical instrumental conditions were used for quantitative

comparison of the dye concentrations at the surface.

The FT-LMMS instrument with external ion source (here

‘external’ means outside the magnet as opposed to double-

cell instruments in which the sample is ionised inside the

magnetic field) was adapted from the commercial Spectro-

spin CMS 47X Fourier transform mass spectrometer with the

ASPECT 3000 data system.23 An Infinity cell24 in a 4.7 Tesla

magnet is employed, and static electrical fields transport ions

from source to cell. Transfer optics developed in-house25

have been installed, improving the transmission by a factor of

10. Differential pumping permits maintaining about

10�8 Torr in the source and 2� 10�10 Torr in the cell. Samples

are ionised in the reflection geometry, i.e. the laser beam

impinges on the surface at an angle of 458 with the normal on

the sample, and the ions are extracted along the normal from

the same side of the sample. Ionisation is achieved with a

high-power frequency-quadrupled Nd:YAG laser (Quanta-

Ray DCR 2-10) focused to a 5 mm spot. The local power

density on the sample can be adjusted to between 107 and

1010 W cm�2. High-resolution data are collected with a

bandwidth of 2 kHz.

The use of static electrical fields for the transfer of ions

between the external source and the cell, in combination with

ion generation within a time domain of less than a ms, limits

panoramic registration in the broadband mode to given m/z

intervals. During their travel through the flight tubes of the

transport optics, ions of different m/z become separated as in

a TOF mass spectrometer. As a result, ions with low m/z

arrive sooner at the entrance of the cell than highm/z ions. The

first trap plate is grounded to allow ions to enter the cell. Once

inside the cell, ions reflect against the second trap and escape

the cell unless the trapping potential is restored on the front

plate. At that moment, ions can no longer enter the cell. As a

result, the m/z range for simultaneous trapping is limited to

the highest m/z for which the time dispersion during ion

transport is less than the time needed for one back-and-forth

movement of the ions with lowest m/z within the cell. The

Tgate parameter refers to the time between the laser pulse and

the end of the ion injection into the FT cell, and determines in

practice which m/z range can be recorded. A full description

of the influence of the TOF effect on the mass spectra and the

use of the Tgate parameter is given elsewhere.23

The m/z scale was externally calibrated using CsI or Bi2O3.

High-resolution data were used for the quantitative compar-

ison of the local dye concentrations. At least three different

spots were analysed on each sample. The data were

accumulated from 50 individual laser shots.

Small quantities of sample were ground between micro-

scope slides or using a grinding mortar. The fine particle

fraction was mounted on the sample holder by double-sided

tape.

RESULTS AND DISCUSSION

Before going into the details of quantitative assessment for

the local concentrations of dyes at the surface and in the sub-

surface, it will be profitable to address the qualitative infor-

mation obtained by the mass spectra in TOF-S-SIMS and

Surface adsorption of organic dyes on silicate materials 2811

Copyright # 2005 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2005; 19: 2809–2818

FT-LMMS with respect to the detection of structure-specific

ions and to the sensitivity. Although both methods are cap-

able of delivering signals that are characteristic of the molecu-

lar structure of the analyte under study, the use of primary

ions with an impact energy of 25 keV in S-SIMS, and the irra-

diation of the solid with photons each of which impart only a

4.6 eV to the sample, is generally assumed to give rise to fun-

damentally different mechanisms of energy deposition, exci-

tation of the solid, and desorption-ionisation (DI) of the

analytes. Furthermore, there is a fundamental difference

between the two methods in that the highly destructive pri-

mary ion beam destroys the molecular structures in the sub-

surface and hence analysis must be restricted to the ultimate

monolayer at the surface. The interaction between a single

primary ion and the sample remains confined to an area of

typically 10 nm2 at the surface and a depth of 25–50 nm

(depending on the projectile energy). At the low primary

ion currents used in S-SIMS, primary ions deposit their

energy in physically separated nanospots within the bom-

barded spot with a diameter of 100–200 nm. In contrast,

LMMS uses laser pulse irradiation, releasing low energy

photons (4.6 eV for l¼ 266 nm) during 8 ns on a spot with dia-

meter of 1–5mm. The ultra-fast relaxation of the absorbed

energy within the irradiated volume means that the processes

leading to the final desorption are more collective in nature

compared to those in S-SIMS. The collective and softer pro-

cesses in LMMS avoid the in-depth destruction of molecular

structures that is typical of S-SIMS, and imply that the

immediate subsurface can contribute to the detected ions in

LMMS. While the information depth in S-SIMS is 1 mono-

layer, experiments have shown that of FT-LMMS can be as

large as 10–50 nm.16,19 On the other hand, the inherent trans-

mission of FT-LMMS with an external ion source is lower

than that of a TOF analyser because of the unavoidable losses

during ion deceleration and injection through a small orifice

in the front plate, while the inhomogeneous stray field of the

4.7 Tesla magnet complicates ion beam focusing.25 Further-

more, the detection efficiency of ions by means of the image

currents in FTMS is also inferior to that used in ion transport

MS such as TOF-S-SIMS.16

Figures 2(a) and 2(b) show the positive ion mass spectra

recorded with FT-LMMS from a rhodamine 6G reference

sample (pure crystal) and from the dye adsorbed from a

10�4 M solution on quartz particles during 3 h. The Tgate

parameter was adjusted to 500 ms for optimal trapping of ions

between m/z 250 and 500. Prominent signals are due to the

intact cation (ICþ) at m/z 443 and the protonated thermal

decomposition product (IC–C2H5þH)þ at m/z 415. Accord-

ing to the empirical DI model, the ultra-fast desorption of

neutrals leads to the initial population of the selvedge where

Figure 2. Positive ion mass spectra recorded by FT-LMMS from: (a) rhodamine 6G pure dye;

(b) rhodamine 6G on quartz; (c) crystal violet on quartz; and (d) methylene blue on quartz.

2812 A.-M. Busuioc et al.

Copyright # 2005 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2005; 19: 2809–2818

detected ions are formed by either electron ionisation or

adduct ionisation.16 The extremely rapid heating of the solid

by the laser allows a significant fraction of the thermolabiles

to be released intact but still some thermal degradation is to

be expected. The most obvious decomposition of the intact

cation is the heterolytic cleavage of the ethyl group from the

quaternised nitrogen. The resulting neutral is protonated to

form the ions at m/z 415. Scheme 1 illustrates the tentative

pathways leading to the detected ions for rhodamine 6G. The

accurate m/z determinations supporting the structural

assignments in Scheme 1 are listed in Table 1. The spectrum

in Fig. 2(b) recorded by FT-LMMS from the adsorbed dye on

quartz shows that the ions of main diagnostic interest are

detected with good signal-to-noise (S/N) ratio. Based on

measurements of the amount absorbed per g and the specific

surface area, 21.33� 10�5 g rhodamine 6G were adsorbed

per m2 (cf. Table 3). Assuming that the density of rhodamine

6G is 1 g cm�3, this corresponds to a total volume of 22.33�10�5 cm3 per m2, resulting in an average thickness of the

coating (assuming perfectly homogeneous coverage) of

about 0.22 nm, i.e. less than a monolayer. Hence, FT-LMMS

provides the unique combination of an information depth

covering the subsurface over a depth of typically 10 nm

together with a detection sensitivity sufficient to trace back

the presence of analytes in the form of a (few) monolayer(s).

There is a slight difference in the relative intensities of the

signals at m/z 415 and 443 generated from the dye on quartz,

while the fragments atm/z 341 are also substantially higher in

comparison to the results for the pure dye. Both of the relative

increases of the signal intensities at m/z 413 and 341 directly

refer to the relative abundances of the thermal decomposition

products. The presence of the dye as an ultra-thin layer on a

quartz substrate means that the energy deposition primarily

occurs in the inorganic substrate as opposed to the organic

material in the laser irradiation of a pure dye crystal. Apart

from such readily conceived effects on the relative intensities,

there is no matrix effect observed in FT-LMMS, and the mass

spectra of mixtures obtained by this method simply consist of

superpositions of the mass spectra for each individual

component.

The results for crystal violet confirm the previously

observed trends. The signal at m/z 372 refers to the intact

cation. As illustrated in Scheme 2, the main fragmentation is

due to the loss of CH4 yielding the ions at m/z 356. The

formation of the ions at m/z 251 formally involves the loss of

dimethylaniline from the intact cation (cf. Table 1). For the

Table 1. Accuratem/zmeasurements for the major positive

ions in the FT-LMMS spectra from the dyes on silica

Nominalm/z

Elementalcomposition

Experimentalm/z

Theoreticalm/z

Experimentalerror

Rhodamine 6G443 C28H31N2O3 443.2329 443.2326 0.81415 C26H27N2O3 415.2016 415.2016 0.14341 C23H21N2O 341.1648 341.1644 1.36

Crystal violet372 C25H30N3 372.2434 372.2428 1.65356 C24H26N3 356.2121 356.212 0.24341 C23H22N3 340.1808 340.1807 0.34251 C17H19N2 251.1543 251.1543 0.12

Methylene blue284 C16H18N3S 284.1216 284.12159 0.12269 C15H15N3S 269.0993 269.0988 1.42

O N+

H

CH3

N

CH3

C O

H

O

H

O

CH3

N N+

CH2

CH3

CO

H

H

O

O

CH3

N N+

H

H

CH2

CO

H

HH

O N+

H

CH3

N

CH3

H

O N+

H

H

CH3

N

CH3

HN O

CH3

N+

H

H

CO

O

H

CH2CH3

CH4

HCOOCH2CH3

CH4 HCOOCH2CH3

O

427369

m/z = 443

m/z = 415 m/z = m/z =

m/z = 399 m/z = 341

+ H+

Scheme 1. Tentative structural assignments and fragmentation routes for diagnostic positive ions

detected by TOF-S-SIMS from rhodamine 6G. The formation of the ions at m/z 427 from the intact

cations at m/z 443 is discussed in the text.

Surface adsorption of organic dyes on silicate materials 2813

Copyright # 2005 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2005; 19: 2809–2818

sake of simplicity, the complete route is omitted from

Scheme 2. The intensity ratio of the ions at m/z 372 to those

at m/z 356 drops from about 6 when a reference product is

analysed to 0.9 when crystal violet is present on quartz as a

very thin layer. This again reflects the increased occurrence of

thermal degradation. Note, however, that the intensity ratio

of the signals at m/z 372 and 356 approaches much closer to

that for the pure dye on the other substrates used in this

study.

Analysis of methylene blue adsorbed on quartz gives a

positive ion mass spectrum with a base peak atm/z 284 for the

intact cation and a minor fragment ion signal atm/z 268 due to

the selvedge protonation of the thermal decomposition

product (cf. Table 1). Again in this case the relative

abundance of the fragment ions relative to the intact cations

is only that high for quartz substrates, but falls in the range

10–20% for all the other materials and for the pure reference

compound.

In the case of all three dyes and the different substrates, the

m/z range discussed above contains the most significant

information. At lowerm/z values recorded using shorter Tgate

values, no diagnostic signals from the silica matrix were seen

in the positive ion mode as expected from the difference in

absorption between the organic dye and the inorganic

substrate. Increasing the laser power density to detect the

inorganic contribution reduces the efficiency with which the

organic ions are generated and detected.

Figure 3 shows the spectra recorded with TOF-S-SIMS

from the same dye adsorbed samples as those used to obtain

Fig. 1. The high m/z regions of the positive ion mass spectra

are largely reminiscent of those already discussed for FT-

LMMS. As expected the relative intensities of the intact cation

and the different dye-related fragments are different when

using primary ion bombardment or laser microbeam

irradiation. Moreover, TOF-S-SIMS detects small additional

signals from rhodamine 6G, for instance at m/z 427 and 369,

corresponding to the protonated demethylation product of

the dye and the fragment ion arising from loss of ethyl

formate from the intact cation. Although the TOF-S-SIMS

instrument does not provide the same level of mass accuracy

as FT-LMMS, Table 2 shows that reasonable support to the

structural assignment and proposed elemental composition

is provided by the m/z determination within 100 ppm.

Interestingly, TOF-S-SIMS detects a whole series of

fragments in the m/z range up to 200. Most of the important

signals are readily associated with fluorinated hydrocarbon

fragments. Their structural assignment is consistent with the

accurate m/z determinations in Table 2. The presence of these

fluorinated ions is a consequence of the use of a Teflon beaker

for the adsorption experiment. Apparently, microscopic

residues from the Teflon were separated together with the

dye-coated silica. Moreover, the intensity of these fluorinated

fragment ions was substantially higher for quartz and silica

gel substrates that are known to be harder than the silicalite-1,

MCM-41 and SBA-15 fine powder materials, and thereby

scratch more Teflon from the beaker.

The high m/z regions in the mass spectra recorded by TOF-

S-SIMS from crystal violet and methylene blue on quartz

contain no important additional information in comparison

to those already discussed for FT-LMMS.

The adsorption kinetics of the dyes on the different

substrates has been followed using the UV-spectrophotome-

try technique because of the reliable quantification in

solutions. Figure 4 shows representative adsorption curves

for the three dyes on SBA-15. All the dyes and substrates

studied showed fast adsorption behaviour. The time to reach

the equilibrium concentration at the silica surface could well

be shorter than 20 min, i.e. the period required to recover the

solid material by centrifugation.

While the adsorption of the different dyes on the other

silicate materials follows kinetics similar to that for SBA-15,

the adsorbed amounts of the different dyes on the same

CN+

CH3

CH3

N

N

CH3CH3

CH3CH2

H

CN+

CH3

CH3

N

N

CH2CH3

CH2

H

CN+

CH3

CH3

N

NCH2

CH2

CH4CH4

N

NCH3

CH3

N

CH3

CH3

m/z = 372 m/z = 356 m/z = 340

m/z = 251

+

Scheme 2. Tentative structural assignments and fragmentation routes for diagnostic

positive ions detected by TOF-S-SIMS from crystal violet.

2814 A.-M. Busuioc et al.

Copyright # 2005 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2005; 19: 2809–2818

material can be quite different. Specifically, the amount

adsorbed on SBA-15 is highest for crystal violet and lowest for

rhodamine 6G, while the order for silicalite-1 is rhodamine

6G> crystal violet>methylene blue. It is important to realise

that the adsorbed amount not only reflects the efficiency of

chemical interactions (kind and density of active sites) and

the specific area per g of support, but also the size of the pores

relative to that of the adsorbate. The active surface deter-

mined with the BET method uses small molecules that can

penetrate into the small pores of the silica. Whenever the

adsorbate is larger than the pore diameter, it cannot use the

active surface available inside the material. Due to the dif-

ferent information depths probed by FT-LMMS and S-SIMS,

comparison of the results obtained by the two methods might

shed light on the latter aspect of the adsorption process.

Therefore, Table 3 summarises these data together with the

amount adsorbed per unit of surface. The latter information is

of particular importance as both FT-LMMS and TOF-S-SIMS

essentially detect a number of molecules within a laterally

confined spot at the surface.

For all dyes and substrate systems studied, samples were

taken after 3 h, ensuring that complete equilibrium condi-

tions were attained, and analysed in parallel by both FT-

LMMS and TOF-S-SIMS. For convenience the main informa-

tion is plotted in Figs. 5–7 in the form of the peak intensity for

the most intense diagnostic ion in TOF-S-SIMS, and S/N ratio

31

.02

69

.04

93

.02

13

1.0

4

16

9.0

4

23

1.0

1

37

2.3

9x 10

m/z100 200 300

5x10

0.2

0.4

0.6

0.8

1.0

b)

28

.00

69

.03

10

0.0

6

13

1.0

1 16

9.0

6

22

6.1

3

35

5.1

9

41

5.7

3

44

3.2

7

x 10

x 7

m/z100 200 300 400

5x10

0.2

0.4

0.6

0.8

a)

31

.03

58

.11

10

0.0

4

13

1.0

1 16

9.0

4

23

1.0

8

28

4.4

0

x 3

m/z100 200

4x10

1.0

2.0

3.0

4.0

5.0

6.0

7.0

x 3 x 10

c)

Figure 3. Positive ion mass spectra recorded with TOF-S-

SIMS from dyes adsorbed on quartz: (a) rhodamine 6G;

(b) crystal violet; and (c) methylene blue.

Table 2. Accurate m/z measurements for the ions of major

diagnostic interest in the TOF-S-SIMS positive ion mass

spectra for the adsorbed dyes

Nominalm/z

Elementalcomposition

Experimentalm/z

Theoreticalm/z

Experimentalerror (ppm)

Common low m/z signals31 CF 30.9977 30.9984 2069 CF3 69.0034 68.9952 11893 C3F3 93.0027 92.9952 75100 C2F4 100.000 99.9936 64119 C2F5 119.004 118.992 106131 C3F5 131.001 130.992 73143 C4F5 143.007 142.992 106155 C5F5 155.004 154.992 79162 C4F6 162.003 161.990 78169 C3F7 169.006 168.998 44181 C4F7 181.008 180.998 52193 C5F7 193.006 192.998 41219 C4F9 219.005 218.985 91231 C5F9 231.000 230.985 66

Rhodamine 6G characteristic peaks341 C23ON2H21 341.162 341.165 9369 C25ON2H25 369.187 369.167 24399 C25O3N2H23 399.189 399.171 51415 C27O3NH29 415.199 415.214 36427 C27O3N2H27 427.250 427.202 113443 C28O3N2H31 443.234 443.233 2

Crystal violet characteristic peaks251 C17N2H19 251.136 251.154 72328 C22N3H22 328.188 328.181 22340 C23N3H22 340.167 340.181 40356 C24N3H26 356.242 356.212 84372 C25N3H30 372.227 372.244 4

Methylene blue characteristic peaks241 C14H13N2S 241.090 241.079 44268 C15H14N3S 268.095 268.090 19284 C16H18N3S 284.131 284.122 33

0

5

10

15

20

25

30

35

40

0 20 40 60 80 100 120 140 160 180

R6G

CV

MB

Am

. a

ds.

(g d

ye

/ g

SiO

2)

x 1

0-3

Time

Figure 4. Kinetics of adsorption of dyes on SBA-15

measured by UV spectrophotometry.

Surface adsorption of organic dyes on silicate materials 2815

Copyright # 2005 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2005; 19: 2809–2818

for the same ions in FT-LMMS, while additionally the amount

adsorbed per m2 and pore size data from Table 3 are repre-

sented graphically. The ordering of the results according to

the pore size facilitates the assessment of the differences seen

in FT-LMMS and S-SIMS.

Starting with rhodamine 6G, the peak intensities in FT-

LMMS follow a trend similar to that for the amount adsorbed

per g of support. The predominance of the dye ion intensities

on quartz substrates is readily associated with the absence of

pores. Assuming a homogeneous coverage of the dye over

the quartz surface, and using the data in Table 3 together with

the density of the dye, the calculated average thickness of the

adsorbate layer should be less than 1 nm, i.e. far less than the

information depth of FT-LMMS. Examination of the corre-

sponding results for S-SIMS shows that there is a huge

difference in dynamic range (ratio of the most and least

intense signals for the different substrates) compared to FT-

LMMS. The monolayer information depth, and the reduced

ion yield whenever superimposed layers would be locally

formed, means that differences in S-SIMS signal intensities

recorded from a dye on a non-porous material exceed those

for a small dye on a porous material by a larger factor than

that in FT-LMMS, capable of detecting (in part) the subsur-

face adsorbate. Furthermore, there is a striking decrease in

the signal intensity of dye-specific ions in S-SIMS for SBA-15

compared to MCM-41, although the amount of adsorbate

per unit of surface area is similar. The pore size of SBA-15

exceeds that of MCM-41 by a factor of 2, and allows the dye

molecule to adsorb to the inner pore walls inaccessible for

analysis in S-SIMS. In contrast FT-LMMS readily evaporates

Table 3. Summary of the adsorption data for dyes on different silica materials

MaterialSurface area

(m2 g�1)Pore size

(nm)

Amount adsorbed (g dye m�2)

R6G CV MB

Quartz 10 21.33� 10�5 172.2� 10�5 83.61� 10�5

Silicalite-1 370 5 0.75� 10�5 0.55� 10�5 0.18� 10�5

Silica gel 30 22 0.70� 10�5 0.29� 10�5 0.76� 10�5

MCM-41 1040 34.6 2.92� 10�5 3.61� 10�5 1.37� 10�5

SBA-15 810 73.8 2.62� 10�5 4.17� 10�5 3.19� 10�5

R6G¼Rhodamine 6G; CV¼ crystal violet; MB¼methylene blue.

c)

0

5

10

15

20

25

Q S1 SGG MCM SBASamplem Samplem

Am

.Ad

s. (

gD

ye/m

2)

x 10

-5

a)

0

4000

8000

12000

16000

20000

Q S1 SG MCM SBA

Sample

Pea

k In

ten

sity

(co

un

ts)

b)

0

40

80

1600

Q S SG1 MCM SBASample

S/N

d)

0

20

40

60

80

Q S1 SG MCM SBA

Po

reS

ize

(Å)

120

Figure 5. Survey of the results for the adsorption of rhodamine 6G on various silica materials: quartz

(Q), silicalite-1 (S1), silica gel (SG), MCM-41 (MCM) and SBA-15 (SBA); (a) intensity of the positive

ions at m/z 415 measured with TOF-S-SIMS; (b) S/N ratios measured for the positive ions at m/z 415

with FT-LMMS; (c) the amount adsorbed (g dye m�2) calculated from spectrophotometric

measurements on the solution before and after adsorption; and (d) pore sizes of sorbent materials.

2816 A.-M. Busuioc et al.

Copyright # 2005 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2005; 19: 2809–2818

several layers, and samples an additional contribution from

the buried analyte.

Essentially similar trends are observed for crystal violet.

The molecular size of this dye is smaller than that of rho-

damine, and therefore crystal violet is more likely adsorbed

also inside the pores of MCM-41. This would explain the

relative intensity differences seen in FT-LMMS between

SBA-15 and MCM-41 in comparison to those observed in

Fig. 5.

Finally, the peak intensities measured in FT-LMMS and in

S-SIMS for methylene blue, which has the smallest molecular

dimensions of the three dyes studied, clearly reflect the

adsorption inside the pores of even silica gel. This assump-

tion is compatible with the determined pore size. As a result,

TOF-S-SIMS only detects a small and relatively decreasing

fraction (relative to the amount adsorbed per surface area),

finally yielding an almost constant low signal for SG and for

MCM-41. In contrast, FT-LMMS shows a steady increase in

signal intensities from S1 to SBA-15, only interrupted by the

high S/N ratio for MCM-41.

CONCLUSIONS

This study of the surface adsorption of organic cationic dyes

on silica materials with different pore sizes has demonstrated

that the combination of FT-LMMS and S-SIMS provides valu-

able tools to obtain molecular information about the system

with a lateral resolution in the mm range. The availability of

high mass resolution capabilities, together with monolayer

detection sensitivity, gives the measurement of the adsorbed

dyes with FT-LMMS a degree of specificity that is unusually

high in local analysis. On the other hand the method is limited

in the study of aliphatic polymers with low UV absorption. In

this respect, TOF-S-SIMS complements FT-LMMS in that it

allows, for instance, the Teflon contamination of the samples

to be detected.

Molecules with increasing size (methylene blue, crystal

violet and rhodamine G) have been adsorbed from aqueous

solutions on to silica materials with a pore size between 0.5

and 7.2 nm (silicalite-1, silica gel, MCM-41, SBA-15), while

quartz was used as a non-porous reference substrate.

Quantitative information on the adsorbed amount was most

easily obtained from UV spectrophotometry. In all but one

case (methylene blue on MCM 41), the intensity of the

molecule-specific ion signals in FT-LMMS correlate qualita-

tively with the adsorbed amount, regardless of the size of the

pores vs. that of the dye. In contrast, the intensity of dye-

specific ion signals in S-SIMS only scales with the adsorbed

amount when the size of the dye exceeds that of the pores.

Hence, the total amount of adsorbate stays at the surface and

remains fully available for ionisation in S-SIMS. Whenever

the dyes can enter interior pores, the 10–50 nm information

depth of FT-LMMS is needed to recover the qualitative

correlation between adsorbed amount and ion signals.

b)

0

40

80

120

160

Q S1 SG MCM SBA

SampleSample

S/N

a)

0

9000

18000

27000

36000

Q S1 SG MCM SBA

Pea

k In

ten

sity

(co

un

ts)

c)

0

40

80

120

160

200

Q S1 SG MCM SBA

Sample

Am

.Ad

s (g

Dye

/m2)

d)

0

20

40

60

80

Q S1 SG MCM SBASample

Po

re s

ize

(Å)

Figure 6. Survey of the results for the adsorption of crystal violet on various silica materials: quartz (Q),

silicalite 1 (S1), silica gel (SG), MCM-41 (MCM) and SBA-15 (SBA); (a) intensity of the positive ions at m/z

372 measured with TOF-S-SIMS; (b) S/N ratios measured for the positive ions at m/z 372 with FT-LMMS;

(c) the amount adsorbed (g dye m�2) calculated from spectrophotometric measurements on the solution

before and after adsorption; and (d) pore sizes of sorbent materials.

Surface adsorption of organic dyes on silicate materials 2817

Copyright # 2005 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2005; 19: 2809–2818

To the best of our knowledge, this is the first study in which

the local MS method has been exploited to confirm

experimentally the adsorption of dyes at the outside or

inside silicate materials as a function of their size and that of

the pores.

AcknowledgementsAlina Busuioc is indebted to the EC for financial support in

the Marie Curie Training Site Program (HPMT CT 2001

00310). Roel De Mondt is indebted to the Institute for the Pro-

motion of Innovation through Science and Technology in

Flanders (IWT-Vlaanderen) for a PhD grant. The research

is partly conducted within the framework of the IUAP-5

program.

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a)

0

5000

10000

15000

20000

25000

Q S1 SG MCM SBA

Sample

Pe

ak

In

ten

sit

y (

co

un

ts)

b)

0

40

80

120

160

200

Q S1 SG MCM SBASample

S/N

c)

0

30

60

90

Q S1 SG MCM SBA

Sample

Am

.Ad

s.(

gD

ye

/m2)

d)

0

20

40

60

80

Q S1 SG MCM SBASample

Po

re s

ize

(Å

)

Figure 7. Survey of the results for the adsorption of methylene blue on various silica materials: quartz

(Q), silicalite-1 (S1), silica gel (SG), MCM-41 (MCM) and SBA-15 (SBA); (a) intensity of the positive ions at

m/z 284 measured with TOF-S-SIMS; (b) S/N ratios measured for the positive ions at m/z 284 with FT-

LMMS; (c) the amount adsorbed (g dye m�2) calculated from spectrophotometric measurements on the

solution before and after adsorption; and (d) pore sizes of sorbent materials.

2818 A.-M. Busuioc et al.

Copyright # 2005 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2005; 19: 2809–2818