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Miniaturized thermal-assisted purge-and-trap technique coupling with surface-enhanced Raman scattering for trace analysis of complex samples Zhengyi Chen, Gongke Li, and Zhuomin Zhang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b02912 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017

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Analytical Chemistry

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Miniaturized

thermal-assisted

purge-and-trap

technique

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coupling with surface-enhanced Raman scattering for trace

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analysis of complex samples

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Zhengyi Chen, Gongke Li*, Zhuomin Zhang*

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School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China

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* Corresponding authors: Gongke Li, Zhuomin Zhang

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Tel.: +86-20-84110922

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Fax.: +86-20-84115107

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E. mail : [email protected] (Gongke Li)

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[email protected] (Zhuomin Zhang)

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ACS Paragon Plus Environment


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ABSTRACT

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It still remains a great challenge for quantification of trace analytes in complex

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samples by surface-enhanced Raman scattering (SERS) technique due to potential matrix

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influence or weak SERS responses of analytes. In this work, a miniaturized thermal-

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assisted purge-and-trap (MTAPT) device was designed and developed to eliminate matrix

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influence coupled with derivatization method before SERS analysis. The design of

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MTAPT chamber was optimized based on quantitative calculation of its dead volume by

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computational fluid dynamics simulation. The small straight chamber was selected as an

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optimized design with a recovery of 96.1% for formaldehyde. The practical feasibility of

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MTAPT was validated based on four real analytical applications including phenthiol in

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industrial water, formaldehyde in flour, sulfion in waste water and methanol in industrial

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alcohol. The results showed that SERS responses of all analytes dramatically increased

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by eliminating sample matrices after MTAPT process. Phenthiol, formaldehyde, sulfion

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and methanol in real samples could be accurately quantified with recoveries of 80.9%-

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110.0%, and the analytical results were validated by corresponding standard methods.

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The time consumption of MTAPT-SERS for real sample analysis including sample

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preparation and determination was within 16 min. It is highly expected that the

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combination of MTAPT technique with portable SERS instrument can greatly expand the

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range of SERS analysis. The proposed MTAPT-SERS method has high potential for on-

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site analysis of complex samples.

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Keywords: Miniaturized thermal-assisted, Purge-and-trap, SERS, Trace analysis,

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Complex sample.

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Introduction

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In recent years, SERS technique for rapid and on-site analysis of real samples has

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aroused great attention1-3, because it can provide more delicate information of target

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molecular fingerprints with potential single-molecule level of sensitivity4,

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analysis techniques. However, SERS analysis becomes impractical when being applied in

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complex samples, since SERS amplification of Raman signals highly depends on noble

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metal nanoparticles such as Au, Ag or their composites, which are susceptible to the

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matrix of real samples and finally influence the accuracy of SERS analysis6-8. To achieve

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accurate analysis of real samples by SERS, it is crucial to eliminate sample matrices via

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efficient sample preparation

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attaching on the surface of SERS substrates to generate severe interference signals12.

9-11

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than other

in order to avoid large amounts of matrix molecules

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Recently, many efforts have gone to reduce matrix interference and enrich target

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analytes during SERS analysis of real samples by coupling of traditional sample

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preparation or separation methods such as liquid-liquid extraction13, solid-phase (micro-)

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extraction SP(M)E10, magnetic extraction3, headspace microextraction14 or HPLC15, thin

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layer chromatography9, capillary electrophoresis16 and ion-pair chromatography (IC)17 .

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Most of these sample preparation methods are based on liquid-liquid or solid-liquid

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extraction-separation process and usually require large amounts of organic solvent and

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sample solutions with long proceeding time, which usually results in potential

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interference to targets and relatively low extraction efficiency. The ideal sample

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preparation technique coupling to rapid SERS analysis should efficiently eliminate the

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complex sample matrices using few organic solvents with short proceeding time and

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portable instrument scale18. Sample preparation technique, based on liquid-gas or solid-

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gas extraction-separation process, such as purge-and-trap (P&T) and SPME, would be

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potentially suitable for rapid SERS analysis because they avoid using large volumes of

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organic solvent19 and any further clean-up post-treatment20. Thus, it is expected that the

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development of liquid-gas or solid-gas sample preparation methods would benefit the

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enhancement of analytical selectivity and accuracy by SERS. Chen et al.17 developed a

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thin-film headspace microextraction for SERS analysis of trace sulfur dioxide in wine

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samples. This method utilized headspace microextraction process to achieve the efficient

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separation of target volatile SO2 and matrix remaining in the sample solution, which

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resulted in the selective enrichment of SO2 and great enhancement of SRES response

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simultaneously. However, up to now there are few works focusing on the development of

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efficient and portable sample preparation technique coupling with rapid SERS analysis

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for complex samples 11. Moreover, most sample preparation methods coupled with SERS

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were designed for specific volatile compounds within a limited range. Few works are

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devoted to the combination of portable but efficient sample preparation techniques with

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SERS ananlysis for real sample analysis.

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P&T technique is a traditional liquid-gas separation technique with the great

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potential to a suitable sample preparation technique for rapid SERS analysis because

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P&T can efficiently separate the trace analytes from complex sample matrices and has

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been widely applied for the enrichment and analysis of voltile targets from various

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samples20. However, most of commercial P&T devices are adapted to the large-scale

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analytical instruments such as gas chromatography (GC) and GC-mass spectrometry

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(GC-MS). Generally, P&T device includes a sample vessel, a thermal-assisted desorption

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unit and a trapping unit19. Conventional sample vessels are frit purge tubes or common

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frit bottle21. Common thermal-assisted desorption unit and trapping unit is based on the

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manifold heater22 and Tenax-GC trap23, respectively. Thus, commercial P&T devices are

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so bulky that they are not suitable for rapid on-site SERS analysis. To overcome these

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problems, a miniaturized thermal-assisted purge-and-trap (MTAPT) device is developed

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in this work. The main challenge is how to realize the miniaturization of separation

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system in MTAPT but remain its high transferring efficiency at the same time. On one

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hand, it is crucial to design and develop the efficient micro-heater with battery-voltage for

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MTAPT. Recently, as an important unit, micro-heater using battery-voltage can heat to

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300℃ within a short time and has been applied for volatiles monitoring24 or sample

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preparation25 due to its high heating efficiency. An MTAPT device with high heating

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efficiency was designed by using low voltage of micro heater in our work. On the other

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hand, in order to remain high transferring efficiency, it is essential to quantitatively

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investigate the dead volume in the chamber for the optimization of MTAPT design.

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However, until now there are no studies focusing on the investigation of the quantitative

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method for dead volume. Simulation strategies such as computational fluid dynamics

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(CFD) analysis are ideal methods for quantitative calculation of many physical

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parameters of the gas in a specific region26, such as pressure and velocity. Benefit from

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the velocity distribution function of CFD, the dead volume, which is minimal magnitude

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of velocity, could be vividly present during the design of the chamber’s geometry of

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MTAPT. Compared with the traditional P&T device, MTAPT presents not only

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portability but also various functions due to its superior liquid-gas transformation

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performance and efficient heating rate.

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The targets after treatment with MTAPT are volatile and most of them possess weak

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SERS response due to their small Raman cross sections. However, SERS is usually

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suitable for analysis of targets containing highly polarizable aromatic groups6, which are

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large Raman cross sections and yield the strong SERS response

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SERS analysis, derivatization is an efficient strategy to enhance Raman cross section by

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modification of polarizable aromatic groups28,

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derivatization strategy is limited to those chemical active volatile compounds, such as

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hydrazine6, formaldehyde7, 30, 31 and acetone8. Therefore, for some chemically inert or

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non-volatile compounds, derivatization strategy for SERS activated becomes unavailable.

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MTAPT is a good way to solve this problem, since it can transfer some non-volatile

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compounds to volatile or chemically active targets.

29

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. To realize MTAPT-

. However, the applicability of

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In this work, an MTAPT device with high heating efficiency was designed

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containing a micro-heater and specific heating chamber. The miniaturization process was

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optimized in detail based on the CFD simulation. And then four application strategies of

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MTAPT were developed based on various targets with sharply different properties

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including strong SERS response, easily SERS activated and volatile, non-volatile and

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difficultly SERS activated compounds. The application of MTAPT-SERS was illustrated

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by four targets including phenthiol, formaldehyde, sulfion, and methanol. MTAPT-SERS

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technique was actually applied for the rapid analysis of these four trace targets in real

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complex samples.

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Experimental

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Chemicals. 3-Methyl-2-benzothiazolinone hydrazone hydrochloride monohydrate (98%),

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N,N-dimethyl-p-phenylenediamine

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dinitrophenylhydrazine (98%), trifluoroacetic acid (99.5%, TFA) and acetaldehyde

sodium

sulfide

nonahydrate

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(99.99%),

4-

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(99.5%) were purchased from Aladdin Chemistry Co. Ltd (Shanghai, China).

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NH4Fe(SO4)2·12H2O was obtained from Tianjin Chemical Reagent Plant (Tianjin, China).

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Au/SiO2 colloids (2.94×10-4 mol/L) having pinholes consisting of Au NP cores with a

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diameter of 55 nm around and silica shell thickness of 1-2 nm and Au colloids were

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supplied by the groups of Prof. Tian in Xiamen University (Xiamen, China)32.

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Instruments. A portable DeltaNu (Laramie, WY) battery-powered Raman spectrometer

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(model Inspector Raman, diode laser excitation wavelength λex=785 nm) with a liquid-

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N2-cooled CCD detector (Model Spec-10:400B, Roper Scientic, Trenton, NJ) was used

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for SERS analysis. The LTQ-Orbitrap (Thermo-Fisher, San Jose, CA, USA) was used for

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identifying the target analyte and by-product during derivazation. All chromatographic

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analyses for comparison study were performed by a Shimadzu LC-20AB (Shimadzu,

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Japan) which consisted of binary pumps (LC-20AT), an injection valve (Rheodyne model

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7725i) equipped with a 20 µL injection loop, a UV-Vis wavelength detector, a system

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controller (SPD-20A) and an acquisition data software of LC-Solution. Chromatographic

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separation was carried out on a Diamonsil C18 column (250 mm × 4.6 mm i.d., 5 µm

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particle size) from Dikma (Beijing, China), and a 10 mm ×4.6 mm i.d. C18 security guard

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column from Elite (Dalian, China). Current source (ZHAOXIN KXN-3002D DC, China)

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was purchased from local market which could precisely mediate the voltage from 0.0-

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36.0 V.

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Development of MTAPT technique. The MTAPT system mainly included four parts: a

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carrier gas pump, a vapour chamber, a heating vessel and an adoptor (Scheme 1). The

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carrier gas was pumped by a commercial mini-aquarium pump or mini-N2 cylinder. The

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vapour chamber was consisted of a sample injection pore for injecting sample and a

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heater entrance pore for loading vessel. Silicone gasket was plugged in the sample

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injection pore and the heater entrance pore for preventing the leakage of volatile targets.

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A pore on the adoptor was used for gas venting. The heating vessel was well designed

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because it played an important part during the miniaturization. Considering its portability,

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a commercial metal ceramics heater with the size of 1.0 cm × 1.0 cm was employed to

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miniaturize the vapour chamber in MTAPT as much as possible. Considering heating

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efficiency and safety, the resistance and the max voltage of micro-heater was set at 3.0 Ω

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and 17 V, respectively. The micro-heater could heat to 120℃ at 6.0 V simply by use of

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several batteries. The vessel was made of anodic aluminum oxide owing to its much

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higher resistance to corrosion and chemical inertness than the other common metals such

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as copper and iron.

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Computational fluid dynamics analysis of chamber geometry. The effect of the

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chamber geometry on fluid flow conditions and further sampling efficiency were

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investigated by computational fluid dynamics (CFD) analysis. Four different chambers

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consisting of one sample inlet, gas inlet and outlet were designed for obtaining optimal

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chamber geometry. The dimensions of these four chambers were illustrated in Figure S1.

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The Navier–Stokes equation was established for the calculation using a commercial

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software package of ANSYS 14.0/fluent CFD. The modelling was made based on three-

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dimensional steady and incompressible flow. The modelled fluid was air with the density

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of 1.29 kg/m3 and the realizable k-є model (RKE) was used for turbulence model.

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Application strategies of MTAPT-SERS for real sample analysis. MTAPT-SERS

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strategies were developed and applied for analysis of four selected targets including

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phenthiol, formaldehyde, sulfion and methanol in corresponding complex samples in

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order to validate the feasibility of the method. The corresponding strategy was present in

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Scheme 1S. The specific sample preparation and analysis protocols were presented in the

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experimental section of “Supplementary Material”. After being treated by MTAPT, 200

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µL of sample solutions were mixed with 100 µL of corresponding SERS substrate

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followed by SERS detection in the specific acquisition integration time and power. SERS

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acquisition integration time for detection of phenthiol, formaldehyde, sulfion and menthol

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was 5s, 1 s, 1 s and 3 s, respectively. SERS powers for detection of phenthiol,

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formaldehyde, sulfion and menthol were set at high power, medium high power, high

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power and high power.

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Results and discussion

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The Design of MTAPT Device. The presence of dead volumes would make the

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components remain in the sampling chamber, especially in gas recirculation

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illustrate the effect of the miniaturization of chamber on dead volume, CFD simulation

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was carried out for four different kinds of vapour chambers which were circuitous (Type

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a, Figure S1a), straight but big (Type b, Figure S1 b), straight with long heater vessel

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entrance (Type c, Figure S1c) and small with short heater vessel entrance (Type d,

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Figure S1d) respectively. The resulting velocity vectors (Figure 1) showed the gas

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recirculation regions that occurred in the chambers but varied with different dead

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volumes. Especially, it was clearly seen there was gas recirculation region in the inlet of

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circuitous chamber (Figure 1a) in comparison with other straight chambers (Figure 1b-d).

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For further study, the velocity magnitude distribution of these four chambers was

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quantitatively counted based on their CFD simulations data, and their volumes were

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precisely calculated based on their geometry. Dead volume was quantified by the peak

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value (vp) and the distribution percent of velocity magnitude below 0.005 m/s (v

Miniaturized Thermal-Assisted Purge-and-Trap ... - ACS Publications

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