Solid phase analytical derivatization as a sample preparation method

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Solid phase analytical derivatization as a sample preparation method Sanka N. Atapattu ∗ , Jack M. Rosenfeld Department of Pathology and Molecular Medicine, McMaster University, Hamilton, ON L8S 4K1, Canada

a r t i c l e

i n f o

Article history: Available online xxx Keywords: Solid phase analytical derivatization Analytical derivatization Sample preparation Extractive alkylation Phase transfer catalyst

a b s t r a c t Analytical derivatization (AD) is an important procedure in analysis as it improves the sensitivity, selectivity and chromatographic separation. Solid phase analytical derivatization (SPAD) combines extraction and derivatization into a single step fulfilling many aspects of a good sample preparation technique, which includes low organic solvent consumption, economical, ease of automation with any chromatographic system and applicability in a wide range of complicated matrices. In this review we have focused on wide applications of SPAD when used in combination with different sample preparation methods, such as solid phase extraction, ion exchange resins, solid phase microextraction, in-tube, microfluidic devices, and hollow fiber extraction methods. © 2013 Elsevier B.V. All rights reserved.

1. Introduction 1.1. Overview of sample preparation Sample preparation (SPrep) is a critical step in analytical chemistry. It is certainly required in most standard analytical equipment and even the methods based on the advanced instruments benefit by the separation of analyte from matrix components [1,2]. The techniques for this separation generally involve fractionation between an aqueous phase and a water immiscible extracting phase. The extracting phase could be a volatile organic solvent or a pseudophase sorbed on a solid phase. Once partition is complete, the extracting phase is removed and analytes are isolated/concentrated by a number of techniques. A considerable body of research was and remains focused on simplifying and automating sample preparation.

1.2. Liquid–liquid extraction (LLE) LLE is the classical SPrep for biological and environmental samples [2,3]. Developed in the 1950s and used into the mid-70s this technique led to the first analytical methods for measuring drugs and their metabolites in biofluids. While leading to the classical bioanalytical and environmental chemistry LLE, is nevertheless time consuming, labor intensive and requires large amounts of organic solvents. It is also relatively difficult to automate [4,5].

Investigators addressed these problems of this classical with a variety of innovative techniques that are discussed below. Throughout these studies the focus was on high throughput techniques, simplification, automation and miniaturization. 1.3. Solid phase extraction (SPE) Development of SPE overcame some of these disadvantages and reduced solvent consumption, labor requirements and advanced automation [3,5]. In SPE, the analyte is partitioned between the biological or environmental aqueous liquid phase and a reverse phase column typically C18 linked to a silica support or an organic polymer [5]. The solid phase retains the analyte during removal of the aqueous phase and subsequent washing to remove various components. Analyte can then be desorbed by solvent or thermal desorption. Although an improvement over LLE, use of SPE presented other problems [3]. It required careful control of the flow rate through the column both for sorbing analyte from the aqueous phase and eluting the analyte with organic solvent. Although SPE requires less eluting solvent, than the corresponding LLE technique the solvent burden is still large. This would be particularly true for laboratories with high throughput. Matrix solid-phase dispersion extraction (MSDE), “quick, easy, cheap, effective, rugged and safe” (QuEChERS), purge-and-trap extraction and static headspace extraction are some of the recently reported alternatives for SPE. 1.4. Solid phase micro-extraction (SPME)

∗ Corresponding author at: 3N26-1280 Main Street West, McMaster University, Hamilton, ON L8S 4K1, Canada. Tel.: +1 905 818 4008; fax: +1 313 577 2099. E-mail address: [email protected] (S.N. Atapattu).

Elimination of the eluting solvent became possible with the creation of SPME [6–8]. This is a completely solvent-less technique for separating analytes from their matrix and is fully automated

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[8]. SPME is thoroughly investigated for determination of analytes from numerous matrices and is one of the most cited techniques in the analytical chemistry literature. SPME involves the use of a fiber coated with the extracting phase which extracts analytes. After extraction analytes are thermally desorbed from the SPME fiber by transferring into an injection port of a chromatographic instrument. Although widely used, SPME cost of equipment and the fibers themselves can be an issue. A variant of SPME is stir bar sorptive micro extraction (SBSME) in which typically a 0.5–1 mm thick polydimethylsiloxane coating of stir bars is the extraction phase. Stir bars are contacted in aqueous medium to extract analytes [9,10]. After extraction, either thermal desorption or liquid desorption can be used before analysis. 1.5. Single drop microextraction (SDME) This LLE technique was developed as an inexpensive alternative to SPME. Jeannot and Cantwell reported the first SDME method for chromatographic analysis [11]. In their first report few microliters of octane was immersed in aqueous sample aid of a Teflon rod. The sample solution was stirred to accelerate extraction. After extraction the rod was removed and a portion of the octane solvent was injected to a GC/FID for analysis. It found a variety of applications but exhibited some deficits such as a small interfacial area, droplet instability when the aqueous phase was stirred at high speed to provide efficient extraction.

properties of ILs can be varied by varying the structure according to analyte extraction selectivity, efficiency, and sensitivity needs. These features make ILs an excellent extraction media for many liquid phase microextraction techniques such as DLLME, HFLLLME, and SDME. 2. Analytical derivatization Current requirements of analytical methods are high sensitivity, high throughput, ease of use, precision, accuracy and automation. Although instrumentation meets some of these requirements, sample preparation that takes place prior to instrumental analysis is still under active research and development. Despite extra steps reagents, analytical derivatization (AD) of the analyte during SPrep can substantially improve sensitivity of detection, chromatographic separation, and selectivity [2]. Even in the case of mass spectrometry (MS), which is the most sophisticated detector, AD increases sensitivity by one to three orders of magnitude [17]. The use of this technique is limited because it includes an extra step in sample preparation which can be time consuming and cumbersome to execute. Despite these drawbacks advantages of AD generated efforts to simplify, reduce the solvent burden and automate the process [1,2,17,18]. 2.1. Applications with extractive alkylation/phase transfer catalysis

1.6. Hollow fiber liquid–liquid–liquid extraction (HFLLLE) The HFLLLE addressed the issue of droplet instability by using a porous hollow fiber with the impregnated organic solvent acts as an interface between the acceptor and donor phases. PedersenBjergaard and Rasmussen reported the first LLLME method based on the use of porous hollow fibers made out of polypropylene [12]. In HFLLLE methods the analytes of interest are extracted from the aqueous donor phase to the thin layer organic solvent impregnated in the pores of the hollow fiber and then to the acceptor phase inside the lumen of the hollow fiber [12,13]. In the two phase mode the acceptor solution is an organic solvent and therefore more compatible with GC analysis. The acceptor solution is aqueous in the three phase mode which is more compatible with LC analysis. The major advantage of the HFLLLE method is very clean extracts resulting from the small pore size of the hollow fiber which prevents interfering substance particles present in the donor phase entering the acceptor phase and due to the low solubility of organic phase present in the pores. HFLLLE methods have been successfully applied to analyze biological and environmental sample with complex matrices. 1.7. Dispersive liquid–iquid microextraction (DLLME) This is a three solvent component system consisting an aqueous phase, extracting solvent (nonpolar water immiscible solvent), and disperser solvent (polar water miscible solvent) [14,15]. A mixture of disperser and extracting solvents are injected to the aqueous phase to form a cloudy solution. The cloudiness consists of microdroplets of water immiscible solvent and the high surface area speeds the extraction process. Extracting solvent containing the target analytes are then separated from the aqueous phase by centrifugation. 1.8. Ionic liquids (ILs) These ionic solvents possess low melting points, low vapor pressures, high thermal stability, nonflammability, and good solubility for inorganic and organic compounds [16]. Many physical

Classical AD methods involve extraction of the analyte into an organic phase, isolation of the analyte by evaporation of the extracting phase followed by derivatization in a homogenous phase [17]. It is time consuming and labor intensive. The first effort to circumvent these problems was the development of extractive alkylation (EA) or phase transfer catalysis (PTC) which combines extraction and derivatization in a single step [19,20]. In the presence of counter-ions the ionized analytes form a lipophilic ion-pair which is extracted from an aqueous phase to an organic phase containing reagent. Analyte is usually derivatized in the organic phase. Although EA combined the extraction and derivatization step, it is a LLE method and required large amounts of organic solvents. Clearly there was a need to improve the efficiency of this and other early derivatization technique. Table 1 is a compilation of some of the EA/PTC examples. 2.2. Applications with SDME SDME is a very simple, low cost and easy to operate liquid–liquid extraction sample preparation method. Typically 1–5 ␮L of organic extractive solvent suspended with the aid of a need syringe tip. After the analyte of interest is extracted the solvent is retracted into the needle and injected for analysis [30]. Analytes of interest can be derivatized in four ways, derivatizing in the donor aqueous phase, in the extractive solvent droplet, GC injector port in the syringe needle barrel [30,31]. Few examples of AD using SDME as the sample preparation method are listed in Table 2. 2.3. Applications with solid phase extraction Many AD applications have been reported combining SPE and derivatization in literature. There are four modes of AD with SPE: (i) impregnating the solid phase with the derivatizing reagent and then passing the sample solution through the solid phase, (ii) passing the sample solution in order to adsorb on the solid phase and then percolate the derivatizing reagent, (iii) derivatize the analyte before the SPE step, and (iv) derivatize the analyte after the SPE step

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Table 1 PTC/EA methods and reaction conditions. Analyte

PTC ion

Reagent

Instrument

Reference

Carboxylic acids Phenols Phenoxy herbicides Sulphide Phenoxyacetic acids Iodide, cyanide, nitrite and thiocyanate Phenolic acids and flavonoids Organic acid and phenols Phenols Amino acids Phenols

THA TBMPC TBA Benzalkonium TBA TBA TBMPC TBA TBA CTAB CTAB

Dibromoacetone MeI MeI PFBBr MeI PFPES MeI PFBBBr PFBBr 9-fluoreneacetyl chloride Acetic anhydride

GC/MS, GC/FID GC/MS GC/MS GC/MS GC/MS, GC/ECD GC/ECD GC/MS GC/MS GC/MS HPLC GC/MS

[21] [19] [22] [23] [24] [25] [20] [26] [27] [28] [29]

THA: tetrahexylammonium; TBMPC: tributylmethylphosphonium chloride; TBA: tetrabutylammonium; PFPES: 2-(pentafluorophenoxy)ethyl 2-(piperidino)ethanesulfonate; MeI: methyl iodide; PFBBr: pentafluobenzylbromide; PFPE: perfluoropolyether; CTAB: cetyltrimethylammonium bromide.

Table 2 Derivatization methods using SDME. Analyte

Reagent

Analytical instrument

Reference

Chlorophenols Organic acids Parabens Formaldehyde Amino acids Aliphatic amines

Dimethylsulphate BSTFA BSA Acetylacetone NBD-F PFBAY

HPLC/UV GC/MS GC/MS Fluorescence spectrophotometer Laser-induced fluorescence GC/MS

[32] [31] [33] [34] [35] [36]

BSTFA: bis(trimethylsilyl)-trifluoroacetamide; pentafluorobenzaldehyde.

BSA:

N,O-bis(trimethylsilyl)acetamide;

NBD-F:

4-fluoro-7-nitro-2,1,3

benzoxadiazole;

PFBAY:

2,3,4,5,6-

Table 3 Derivatization methods using SPE. Analyte

Reagent

Analytical instrument

Reference

Alkylphenols, halogenated phenols Alkylphenols and BPA Alkylphenols and halogenated phenols Alkylphosphonic acid Amphetamines Dialkyl phosphates

Acetic anhydride PFBBr PFPdy BSTFA Fluorescamine PFBBr

GC/MS GC/MS GC/MS GC/MS CE/Fluorescence GC/MS

[29] [27] [38] [39] [40] [37]

PFBBr: Pentafluorobenzylbromide; PFPdy: Pentafluoropyridine; BSTFA: bis(trimethylsilyl)-trifluoroacetamide.

[27,29,37–40]. Few examples of SPE based AD methods are listed in Table 3. 2.4. Applications with HFLLLME Coupling of HFLLLM with AD improved chromatographic separation, sensitivity and selectivity of analytes. Three different approaches provide the coupling. That is, in situ derivatization where the analyte is derivatized in the donor phase, derivatization in the hollow fiber lumen and injector port derivatization [41]. Varanusupakul et al. [42] used HFLLLME in head space (HS) mode to simultaneously extract and derivatize haloacetic acids (HAAs) into methylesters using methanol as the reagent. These combinations are listed in Table 4. The HAAs were derivatized in acidic

aqueous donor phase with methanol. The derivatized methylesters were extracted in HS mode by a hollow fiber which had 1-octanol as the acceptor phase. Chiang and Huang [43] used a similar HS mode to simultaneously extract and derivatize amphetamine and methylenedioxyamphetamine from urine samples. In their work the analytes were extracted in a HS mode by a hollow fiber which had n-nonanol as the acceptor phase containing the derivatizing reagent pentafluorobenzaldehyde. 2.5. Applications with SPME Many SPME and AD combinations have been utilized in analyzing a wide range of biological and environmental samples. Few examples are listed in Table 5. On-fiber AD can be carried out by

Table 4 Derivatization methods using HFLLLME. Analyte

Reagent

Chromatographic instrument

Reference

Haloacetic acids Amphetamine and methylenedioxyamphetamine Cyanide ions Polyamines Ibuprofen, Naproxen, Ketoprofen Chlorophenols Pharmaceutically active acids Pharmaceutically active acids Gabapentin Biogenic amines

Methanol Pentafluorobenzaldehyde Ni(II)–NH3 Tosyl chloride Tetra butyl ammonium sulphate Acetic acid anhydride MTBSTFA MTBSTFA FDNB Dansyl chloride

GC-ECD GC/MS CE/UV HPLC/UV GC/FID GC/MS GC/MS GC/MS HPLC/UV HPLC/UV

[42] [43] [44] [45] [46] [47] [41] [48] [49] [50]

MTBSTFA: N-(tert-butyldimethylsilyl)-N-methyl-trifluoroacetamide; FDNB: 1-fluoro-2,4-dinitrobenzene.

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4 Table 5 Derivatization methods using SPME. Analyte

Reagent

Analytical instrument

Reference

Phenol Volatile fatty acids Anti-inflammatory drugs Fatty acids Fatty acids Acidic herbicides Fatty acids, phenols and indoles Fatty acids Aldehydes

Acetic anhydride 1-Pirenyldiazomethane MTBSTFA Diazomethane 1-Pirenyldiazomethane Diazomethane MTBSTFA Methanol PFBHA

GC/MS GC/MS GC/MS GC/ECD/MS GC/FID GC/MS GC/MS GC/MS GC/MS

[51] [52] [53] [6] [7] [54] [55] [56] [57]

MTBSTFA: N-methyl-N-(tert-butyldimethylsilyl)trifluoroacetamide; PFBHA: O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine.

Table 6 Derivatization methods using in-tube.

2.7. Solid phase analytical derivatization

Analyte

Reagent

Analytical instrument

Reference

Histamine Histamine Amino acids dimethylamine Aliphatic amines

OPA NDA FMOC FMOC FMOC

HPCE/Fluorescence HPCE/Fluorescence HPCE/UV HPLC/UV HPLC/UV

[58] [59] [60] [61] [62]

OPA: o-phthalaldehyde; NDA: Naphthalene-2,3-dicarboxaldehyde; FMOC: 9fluorenylmethyl chloroformate.

impregnating the fiber with the derivatizing reagent before or after the extraction of the analyte. Two other modes of SPME associated AD are: (i) Carrying out derivatization in solution, and (ii) GC injector port derivatization.

An alternative to miniaturization and automation is to use solid phases rather than liquids as the extraction/derivatization phase. A combination of SPE and AD, termed solid phase analytical derivatization (SPAD) offers the advantages of SPE and simultaneous derivatization [1,26]. In this technique termed SPAD, the extraction/derivatization takes place on the solid phase or in the interface between the liquid and solid phase. SPAD fulfills many aspects of a good sample preparation technique, which includes low organic solvent consumption, economical, ease of automation with any chromatographic system and applicability in a wide range of complicated matrices [1]. Since SPAD is an extension of SPE the isolation procedures are similar and this facilitates automation. The focus of this review is to report various advances in SPAD methods and applications in analyzing phenol, organic acid, carbonyl and amine compounds.

3. Phenols by SPAD 2.6. Applications with in-tube methods 3.1. Polar phenols In-tube solid-phase extraction is an efficient sample preparation method using an open tubular fused-silica capillary GC column to extract analytes of interest. Analytes are extracted, pre-concentrated and derivatized if needed on the stationary phase of capillary column. Continuous sample extraction, pre-concentration, desorption, and injection are performed in combination with a GC or LC. Usually the capillary column is impregnated with the derivatizing reagent before sample extraction. Recent biological and environmental sample analysis methods are listed in Table 6.

Analytical methods for phenols include GC-MS, highperformance liquid chromatography (HPLC) equipped with ultraviolet and mass spectrometry detectors. However the most common method reported is GC–MS using wall-coated opentubular (WCOT) columns due to its high sensitivity and separation power [27,29,38,52,63–66]. Many of these phenols and alcohols can interact with accessible silanol groups on the fused-silica surface and at low concentrations require analytical derivatization (AD) to reduce tailing of peaks which reduce resolution and sensitivity

Table 7 Phenol analysis by SPAD. Analyte

Reagent

Instrument

Solid phase

Reference

Alkylphenols, halogenated phenols Alkylphenols and BPA Alkylphenols and halogenated phenols Alkylphenols and BPA Alkylphenols and halogenated phenols Phenol Alkylphenols and fatty acids Phenol, hydroquinone, catechol Phenols, acids and indoles Estrogens 4-Hydroxynonenal testosterone and epitestosterone Alkyl phenols Recreational drugs THC, THCCOOH THC, cannabidiol and cannabinol

AA PFBBr PFBBr PFPdy PFPdy AA PDAM HMDS MTBSTFA DSC AA AA BSFTA MTBSTFA MTBSTFA MSTFA

GC/MS GC/S GC/MS GC/MS GC/MS GC/MS GC/MS GC/MS GC/MS LC/MS/MS GC/MS GC/MS GC/MS GC/MS GC/MS GC/MS

C18 bonded silica Bond Elute PPL XAD-4 XAD-4 Oasis MAX PDMS SPME fiber CAR/PDMS SPME fiber SPME fiber SPME fiber Oasis HLB column PDME coated Stir Bar PDME coated Stir Bar SPE column On-fiber On-fiber On-fiber

[29] [27] [26] [63] [38] [51] [52] [70] [55] [79] [9] [10] [4] [80] [81] [77,82]

AA: acetic anhydride; PFBBr: pentafluobenzylbromide; PFPdy: pentafluoropyridine; PDAM: 1-pirenyldiazomethane; THC: 9-tetrahydrocannabinol; THCCOOH: 11-nor-9-carboxy-9-tetrahydrocannabinol; DSC: dansyl chloride; HMDS: hexamethyldisilazane; MSTFA: N-methyl-N-trimethylsilyltrifluoroacetamide; MTBSTFA: N-(tertbutyldimethylsilyl)-N-methyltrifluoro acetamide.

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of detection [67–69]. The simplest and the most common AD methods for phenol are silylation and acetylation which addresses the issues interaction with free silanol groups and volatility [1,17]. Many variants of SPAD have been utilized to determine phenols in complicated sample matrices. The most common approaches are, use of a strong anion exchange resin [29,38], derivatizing on a poly styrene–divinylbenzene cross linked macroreticular resin [26,63], SPME on-fiber [51,52,55,70] and stir bar polymer coatings [9]. Some of the significant SPAD applications are listed in Table 7. 3.1.1. Anion exchange resin and solid phase extraction cartridges In case of anion exchange resin approach, first the phenolates were immobilized in the resin followed by a drying and derivatizing steps. Li and Lee coated the ionic surfactant cetyltrimethylammonium bromide (CTAB) on a C18 boned SPE cartridge [29]. The surfactant CTAB is adsorbed on the C18 bonded silica solid phase acting as ion-exchange sites. Phenolates were immobilized by the surfactant molecules from aqueous samples. After phenolates were adsorbed the SPE cartridge was dried using a stream of nitrogen gas then derivatized using acetic anhydride on-column before GC/MS analysis. This method was validated in humic acid and surrogate sea water conditions. This method can be modified using a strong anion exchange SPE cartridge instead of coating a surfactant on the solid phase. Kojima et al. passed an alkaline solution of low molecular weight alkyl and halogenated phenols through a strong anion exchange Oasis MAX SPE cartridge for the analytes to get adsorbed [38]. Next the column was dried using a stream of nitrogen and derivatized on column passing a solution of the derivatizing reagent pentafluoropyridine (PFPdy). Jonsson et al. utilized a Discovery DSC-18 SPE cartridge to adsorb lipophilic alkylphenols from fish bile [4]. An ionic surfactant or a strong anion exchange cartridge was not required in their work as higher molecular weight more lipophilic alkylphenols were readily adsorbed onto the solid phase due to strong hydrophobic interactions. After adsorption of alkylphenols the cartridge was dried using the stream of nitrogen gas and derivatized on-column with the derivatizing reagent BSTFA for analysis. Another approach is to impregnate the derivatizing reagent on a solid phase followed by sorption of the phenolate on to the surface for derivatization. Kuklenyik et al. used tetrabutlylammonium (TBA) cation as a PTC to transfer ionized alkylphenols of from alkalinized urine onto a solid phase impregnated with pentafluorobenzyl bromide (PFBBr) [27]. The phenolates reacted with the reagent to form the pentafluorobenzyl (PFB) ethers of the analytes. 3.1.2. SPME on-fiber derivatization A common variant of SPAD is on fiber derivatization in SPME. Es-haghi et al. used an on fiber derivatization method to detect phenol in air samples [51]. A SPME fiber was exposed to air for phenol adsorption. The adsorbed phenol was derivatized exposing the fiber to the derivatizing reagent acetic anhydride. Finally the fiber was thermally desorbed in a GC injection port for analysis. Similar on-fiber derivatization approaches have been reported for analysis of polar phenol compounds in urine [70] and cow slurries [55]. Larreta et al. reported a HS-SPME on fiber derivatization method for fatty acid and phenol analysis in cow slurries [52]. In this method the SPME fiber was first immersed in a solution of 1pirenyldiazomethane, the derivatizing reagent. Phenols, indoles and fatty acids were simultaneously extracted and derivatized onfiber and injected to a GC inlet for thermal desorption and analysis. 3.1.3. Derivatizing on a poly styrene–divinylbenzene crosslinked macroreticular resins XAD phases have been extensively used in SPAD method development in a wide range of applications [1,71–76]. Usually before

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the sample is added the solid phase is impregnated with a suitable derivatizing reagent. Analyte is diffused from the bulk aqueous phase to the surface of the solid phase. The derivatization reaction occurs on the surface of the solid phase. High molecular weight lipophilic analytes are readily adsorbed on to the solid surface due to strong hydrophobic interactions [75,76]. However polar low molecular weight analytes form weak interactions with the solid phase and poorly adsorbed on to XAD solid phases. One solution is the use of a PTC; the ionized analyte forms a lipophilic ion pair with a counter-ion of the opposite charge [26,72,73]. As expected the addition of a counter-ion increased the yields of hydrophilic phenols, carbonyls, and acids on XAD resins. However, counter ions inhibited the pentafluorobenzylation of higher molecular weight alkylphenols, chlorophenols and carboxylic acids on the XAD-4 resin [26]. This could be due to slow diffusion of bulky ion-pairs. To overcome these issues a two-step derivatization method was reported [26]. In the first step of the method the lipophilic analytes were derivatized without any counter ions. After a reaction time of 20 min, TBA counter-ion was added to the reaction mixture and reacted for further 15 min. This two-step derivatization method showed good yields for both hydrophilic and lipophilic alkylphenols, chlorophenols, and carboxylic acids. The method was validated in humic acid, surrogate Sea water and surrogate lake water conditions. 3.2. Lipophilic phenols Lipophilic phenols have the ability to form strong hydrophobic interactions with solid supports. Thus, these compounds are readily adsorbed on solid phases. Some of these compounds posses more than one functional group therefore require more than derivatizing reagent. Analysis of hormones, cannabinoids and biomarkers with the hydroxyl functional group is important is forensic, bio-medical and environmental sample analysis. In early SPAD work on cannabinoids was carried out on XAD-2 phases [75,76]. First the conditioned XAD-2 phase was impregnated with the derivatizing reagent. After adding the sample solution to the XAD-2 phase the container was shaken for a specified period of time for the derivatization reaction to occur. Next the supernatant was removed and the XAD-2 phase was dried and extracted with an organic solvent of GC analysis. The derivatizing reagent PFBBr has been widely used in SPAD applications on XAD resins. The reagent PFBBr forms pentafluorobenzyl ethers with phenols and pentafluorobenzyl esters with carboxylic acids [26]. This reagent has a high boiling point of 174 ◦ C making difficult to remove excess reagent. It is well known that PFBBr excess reagent degrade GC columns. An alternative is use of PFPdy reagent instead of PFBBr [38]. The reagent PFPdy forms pentafluoropyridyl ethers with phenols. Atapattu et al. reported a SPAD method for the determination of four endocrine disrupting chemicals 4-tert-butylphenol, 4-tertoctylphenol, bisphenol A and 17␤-estradiol using PFPdy as the derivatizing reagent [63]. These lipophilic phenols did not require any PTC as they form strong hydrophobic interactions with the XAD-4 resin. The method was validated with human urine samples. Most of the recent SPAD work on cannabinoids was carried out with SPME on-fiber derivatization. Cannabinoids can be easily extracted from biological and environmental samples in HS-SPME mode. After extraction the fiber is exposed to a suitable derivatizing reagent for on-fiber derivatization. Finally the fiber is thermally desorbed in a GC inlet for analysis. Cannabinoids in human hair and waste water samples have been analyzed successfully in this mode. Although SPME on-fiber derivatization has been widely used suffers from two weaknesses: (i) the fragile fused silica and (ii) the outer surface of the stationary phase coating is exposed when extending through the syringe needle [77]. To overcome these issues solid-phase dynamic extraction (SPDE) was developed [78].

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6 Table 8 Acid analysis by SPAD. Analytes

Reagent

Instrument

Solid phase

Reference

Volatile fatty acids Acidic herbicides Volatile organic acids Fatty acids, amino acids Alkylphosphonic acids Anti-inflammatory drugs Organic acids Fatty acids Fatty acids Organic acids Chlorinated acid herbicides Volatile fatty acids Organic acids Fatty acids Fatty acids Fatty acids Acidic herbicides Pharmaceutically active compounds

PDAM BF3–MeOH/TMSD BSTFA MTBSTFA BSTFA MTBSTFA PFBBr MTBSTFA Methanol Methyl iodide Methyl iodide PFBBr Methyl iodide PFBBr Diazomethane PDAM Diazomethane MTBSTFA

GC/MS GC/ECD GC/MS GC/MS GC/MS GC/MS GC/MS GC/MS GC/MS GC/MS GC/ECD GC/MS GC/FID GC/ECD/MS GC/ECD/MS GC/FID GC/MS GC/MS

SPME fiber C18/PS-DVB In syringe derivatization Metal surface SPE anion exchange disk SPME fiber XAD-4 SPME fiber SPME fiber Anion exchange resin Anion exchange disk Anion exchange resin Anion exchange resin XAD-2 SPME fiber SPME fiber SPME fiber Hollow fiber

[52] [89] [31] [88] [39] [53] [26] [55] [56] [84] [85] [86] [87] [71] [6] [7] [54] [48]

PDAM: 1-pirenyldiazomethane; TMSD: trimethylsilyldiazomethane; PFBBr: pentafluobenzylbromide; BSTFA: bis(trimethylsilyl)-trifluoroacetamide; MTBSTFA: N-methylN-(tert-butyldimethylsilyl)trifluoroacetamide.

In this method the stationary phase is coated inside a stainless steel needle. Musshoff et al. extracted cannabinoids in HS-SPDE mode and derivatized the adsorbed analytes exposing the fiber to MSTFA [77]. When SPDE and SPME were compared in cannabinoid analysis showed similar results. Another alternative to SPME is SBSE. In SPAD methods similar to SMPE on-fiber derivatization, first the analytes are extracted from aqueous sample to the stir bar coated stationary phase [9,10]. Next the stir bar coating is exposed to the derivatizing reagent for analyte derivatization. Finally the derivatives are desorbed thermally or using a solvent for analysis. Sandra and co-workers extracted testosterone and epitestosterone from human urine to a PDMS coated stir bar [10]. After extraction the stir bar was dried and exposed to acetic anhydride for derivatization. In the final step the stir bar was placed in a glass desorption tube of an autosampler for GC analysis. 4. Organic acids by SPAD Analysis of analytes with the carboxyl functional group is very important in environmental chemistry and biomedical applications due its wide presence. It is a common practice in both GC and LC analysis to derivatize acids [83]. In GC analysis carboxyl group is derivatized in order to improve volatility and to avoid interaction of accessible silanol groups in WCOT columns [67,68]. In LC analysis carboxylic acids are derivatized to add a chromophore to improve sensitivity and selectivity. Many SPAD methods have been utilized to determine analytes with the carboxyl functional group in environmental and biological samples (Table 8). The most common approaches are, use of a strong anion exchange resin [84–87], derivatizing on a poly styrene–divinylbenzene cross linked macroreticular resin [71], SPME on-fiber, hollow fiber membranes [6,7,54], metal surfaces [88], and GC injection syringes [31]. The use of strong anion exchange resin to isolate carboxylates is a common practice in analytical chemistry [84–87]. The derivatization after isolating the carboxylates can have two approaches: (i) derivatization on the anion exchange resin (SPAD) after drying the resin or (ii) derivatizing after eluting the carboxylate from resin. However analyte elution followed by derivatization is a tedious process and time consuming thus the SPAD approach is more widely applied. Chatfield et al. [84] isolated chlorinated acid

herbicides on an anion exchange resin. The isolated carboxylates were derivatized using supercritical carbon dioxide containing alkylation reagent methyl iodide. Later derivatives were eluted in a supercritical fluid extraction (SFE) system as methyl esters. Cummins and Wells reported a similar method to determine low molecular weight fatty acids using the derivatizing reagent pentafluorobenzyl bromide [86]. Field and Monohan [85] demonstrated an “in-vial” elution technique for the determination of chlorophenoxy acid herbicides in aqueous samples. A strong anionexchange solid-phase extraction disks were used to concentrate the chlorinated acid carboxylate. The chlorinated acids were eluted from the disk and derivatized with methyl iodide a single step to their methyl esters with an in-vial elution technique. In a more recent report Subramaniam et al. [39] used an anion exchange disk and in-vial elution technique to determine alkylphosphonic acid nerve agent markers. In their work a wide range of alkylphosphonic acids were derivatized using BSTFA as the chemical reagent. Many SPAD based methods have been developed derivatizing and extracting fatty acids in a single step on poly styrene–divinylbenzene crosslinked macroreticular resins and solid phases of SPE cartridges [26,71,89]. In methods where XAD phases were utilized, the derivatizing reagent is impregnated on the solid phase and the aqueous sample was made to contact with the solid phase [26,71]. Higher molecular weight acids react at a faster rate than lower molecular weight acids in this mode [26]. In order to increase the reaction rates for lower molecular weight analytes counter ions are used. The ion-pair formed brings the carboxylate on to the solid surface to react due to stronger inter molecular interactions with the solid phase. However, when analyzing a mixture of low and high molecular weight acids this approach could cause difficulties. Higher molecular weight carboxylates form a bulky ion pair with counter ions, as a result hinders the diffusion process. To overcome this issue a two-step derivatization method was reported [26]. First higher molecular weight acids were reacted without any counter ions and in the second step low molecular weight acids were reacted with the aid of a phase transfer catalyst. Two variants of SPAD based on-fiber SPME methods have been reported in literature. In the first mode the SPME fiber impregnated with the reagent is immersed in the aqueous solution for simultaneous derivatization and extraction [7]. In the second approach analytes are first extracted from aqueous solution and then the fiber is exposed to the chemical reagent for on-fiber derivatization [6,8,52,53,55,56].

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7

Table 9 Carbonyl analysis by SPAD. Analyte

Reagent

Instrument

Solid phase

Reference

Hexanal, heptanal Airborne carbonyls Airborne carbonyls Hexanal, heptanal Glutaraldehyde Airborne carbonyls Carbonyls in cigarette smoke Ozone and airborne carbonyls Hexanal, heptanal Airborne carbonyls Formaldehyde Aldehydes Aldehydes Malondialdehyde Butanone, 2-pentanone malondialdehyde

DNPH PFBHA PFPH DNPH DNPH DNPH DNPH BPE& DNPH DNPH DNPH PFPH/PFBHA PFBHA PFPH/PFBHA Dansyl hydrazine DNPH

HPLC/DAD GC/MS GC/MS HPLC/DAD HPLC/UV HPLC/DAD HPLC/DAD HPLC/UV HPLC/DAD HPLC/DAD GC/MS GC/MS GC/FID HPLC/Fluorescence HPLC/UV

PDMS frit Tenax sorbent packed tube Tenax sorbent packed tube Monolith frit LpDNPH Rezorian cartridge DNPH-coated silica particles DNPH-coated silica particles Impregnated silica particles Monolith capillary column GC capillary column Microfluidic chip SPME fiber SPME fiber XAD-2 XAD-2

[90] [91] [92,93] [94] [95] [96] [97] [98–100] [101] [102] [103] [57] [8] [72,73] [74]

DNPH: 2,4-dinitrophenylhydrazine; PFBHA: O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine; BPE: trans-1,2-bis-(4-pyridyl)ethylene; PFPH: pentafluorophenyl hydrazine.

Sha et al. derivatized volatile organic acids in a syringe with BSFTA [31]. The volatile organic acids were extracted from aqueous samples in a head space SDME mode. After extraction the acids were in-syringe derivatized. This method was validated by analyzing tobacco samples. A NASA based research group recently reported an interesting SPAD variant where organic and amino acids were derivatized on a derivatization cup made out of stainless steel [88]. The acids were derivatized with the reagent MTBSTFA at 300 ◦ C for several minutes. The reported method has potential applications in analyzing mineral samples in space missions in the future. Guo and Lee compared three variants of SPAD, solvent bar microextraction (SBME), HF-LPME and SPME on-fiber derivatization in a recent report [48]. In SBME and HF-LPME the analytes were derivatized on the hollow fiber surface. In the SBME method 2.8 cm long hollow fiber segment was taken and one end was heatsealed. The acceptor phase and derivatizing reagent was introduced with the aid of a needle tip. The remaining end of the hollow fiber was then heat sealed to form the solvent bar. The solvent bar was then placed in an aqueous solution for extraction stirring at 700 rpm. After extraction the derivatized analyte enriched acceptor phase was extracted using a needle tip and 1 ␮L of this extract was injected to a GC/MS for analysis. When the three derivatization procedures were compared SBME method superior in extraction efficiency and less time consuming. The method was validated analysis pharmaceutically active compounds in drain water samples.

5. Carbonyl compounds by SPAD A wide range of carbonyl compounds are found in environmental and biological samples. Analysis of carbonyl compounds at trace level concentrations has become an important topic in research due to demanding applications in environmental monitoring and medical diagnosis. The analysis of trace levels of some of these compounds challenges existing analytical methods because their concentrations are lower than most instrument limits of detection. Research groups in recent times have focused on new sample preparation and derivatization methods to improve selectivity, sensitivity and speed of analysis. The two most common methods utilized in carbonyl compound analysis are GC/MS and HPLC/UV. A wide variety of SPAD methods have been employed combining both GC/MS and HPLC/UV (Table 9). Many carbonyl compounds are present in the atmosphere as pollutants resulting from anthropogenic activities and natural processes. Most SPAD methods employed in analyzing airborne carbonyls focus on derivatizing the analytes on a 2,4dinitrophenylhydrazine (DNPH) impregnated solid material in a

cartridge. The formed DNPHydrazone derivatives are then analyzed by HPLC/UV. However these approaches suffer from two weaknesses. One is Ozone in the atmosphere react with DNPHydrazone derivatives interfering with airborne carbonyl analysis and the other is E- and Z-geometrical isomer presence of DNPHydrazone derivatives giving multiple peaks in HPLC analysis [95,96,98–100]. To address the first issue Uchiyama and co-workers used a two bed cartridge system to trap Ozone [98]. The first cartridge bed had an Ozone scrubber impregnated silica particles and the second cartridge bed consisted DNPH impregnated silica particles. Three Ozone scrubbers 1,2-bis-(4-pyridyl)ethylene (4-BPE), 1,2-bis-(2-pyridyl)ethylene (2-BPE) and hydroquinone (HQ) were compared. HQ and 2-BPE showed more promising results than 4-BPE. The same research group addressed the second issue of E- and Z-geometrical isomer presence by transforming the C N double bond of the DNPHydrazone derivatives using 2-picoline borane to give a C N single bond [95,96]. Although derivatizing airborne carbonyls on a DNPH impregnated solid material and analyzing by HPLC/UV is a well establish approach, these methods suffer from several disadvantages; limited HPLC peak capacity, overlap of peaks due to different isomers, and difficulties in analyzing higher molecular weight carbonyl compounds. In GC based method development many researchers focused on impregnating reagents pentafluorophenyl hydrazine (PFPH) or O-(2,3,4,5,6pentafluorobenzyl)hydroxylamine (PFBHA) on a solid material before analysis [91–93]. Ho and Yu used a PFBHA impregnated Tenax sorbent packed tube size of a GC inlet to trap airborne carbonyls [91]. The derivatives were then released to a GC column by thermal desorption. Methods based on impregnating PFPH on solid material and analyzing using GC/MS have been compared with classical DNPH coated solid sorbent HPLC/UV analysis methods [92,93]. Results showed GC/MS methods are superior to classical methods based on HPLC/UV especially for higher molecular weight carbonyl compounds. Xu et al. explored two types of frits for the simultaneous derivatization and extraction of hexanal and heptanal in biological samples [90,94]. This research group first utilized a monolith frit and evaluated the method analyzing hexanal and heptanal in human urine and serum samples [94]. In their second method a porous polypropylene frit with a polydimethylsiloxane (PDMS) coating was utilized to analyze hexanal and heptanal in human serum [90]. In both these methods DNPH was used as the reagent and analysis was performed using HPLC/DAD. Other variants of SPAD in analysis of carbonyl compounds include SPME on fiber

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8 Table 10 Amine analysis by SPAD. Analyte

Reagent

Instrument

Solid phase

Reference

Aromatic amines Carbamates Aliphatic amines Methylamine Amphetamines Aliphatic amines Primary amines Trimethylamine Diamines Biogenic amines Dimethylamine Aliphatic amines Amphetamines Biogenic amines Aromatic amines

Allyl isothiocyanate TMPAH/TMSH PFBOC FMOC PCF FMOC PFBAY FMOC TFAA Dansyl chloride FMOC FMOC Fluorescamine DTAF TFANA

GC/MS GC/MS GC/MS HPLC/UV GC/MS HPLC/UV GC/MS/MS HPLC/UV GC/MS HPLC/UV HPLC/UV HPLC/UV CE/fluorescence Fluorescence GC/MS

SPME fiber GC column In syringe SPME fiber Monolithic silica spin column SPME fiber SPME fiber SPME fiber SPME fiber Polypropylene hollow fiber GC column GC column Oasis HLB On-microfluidic device Oasis HLB

[107] [113] [110] [106] [111] [104] [108] [105] [109] [50] [61] [62] [40] [112] [114]

TMPAH: trimethylphenylammonium hydroxide; TMSH: trimethylsulfonium hydroxide; PFBOC: pentafluorobenzoyl chloride; FMOC: 9-fluorenylmethyl chloroformate; PCF: propyl chloroformate; PFBAY: 2,3,4,5-pentafluorobenzaldehyde; TFAA: 1,1,1-trifluoroacetylacetone; DTAF: dichlorotriazine fluorescein; TFANA: trifluoroacetic anhydride.

derivatizations [8,57], microfluidic on chip derivatizations [103] and simultaneous derivatization and extraction on XAD phases [72–74]. 6. Amine compounds by SPAD The most common separation technique in amine analysis is HPLC. The molar absorptivity coefficients for amines are low especially due to lack of chromophores except aromatic amines. These compounds show little or no electrochemical or fluorescence activity. The hydrogen atom attached to the nitrogen in amine group has a partial positive charge due to the electro-negativity difference between the two atoms. Analysis of amine compounds by GC using WCOT columns is a difficult task at low concentrations. Polar analytes can interact with accessible silanol groups on the fusedsilica surface and result in tailing of peaks, especially at low analyte concentrations and temperatures thus lowering the sensitivity of detection [67–69]. Therefore derivatization is very important in before HPLC or GC analysis. Many variants of SPAD applications have been utilized in amine analysis in biological and environmental samples. Recently reported methods include on-fiber, in-tube derivatization, hollow fiber derivatization, simultaneous extraction and derivatization on SPE cartridges, on-chip derivatization and in-syringe methods (Table 10). 6.1. SPME on-fiber derivatization There are two SPME on-fiber derivatization SPAD variants: (i) after the analyte is adsorbed impregnate the fiber with the derivatizing reagent and (ii) impregnating the fiber with the reagent and then extracting the analyte of interest [104]. SPME on-fiber derivatization methods have been utilized in combination with both HPLC [104–106] and GC [107]. Campins-Falco and co-workers explored the combination SPME on-fiber derivatization and LC analysis [104–106]. The SPME fiber was first impregnated with the derivatizing reagent FMOC and then exposed to the sample for adsorption and derivatization. After derivatization the fiber was inserted into the SPME–HPLC interface filled with acetonitrile for solvent desorption and injection to the HPLC system. The method was validated analyzing water and air samples [105,106]. Llop et al. demonstrated extraction of amines from sewage sludge by pressurized hot water as the extraction solvent [108]. The extracted amines were then derivatized head space on-fiber with 2,3,4,5-pentafluorobenzaldehyde followed by GC/MS/MS analysis.

Awan et al. used a head space on-fiber derivatization approach in biogenic diamine analysis with trifluoroacetylacetone as the derivatizing reagent [109]. The method was validated analyzing diamines in food samples. Verma and co-workers reported a similar approach for aromatic amine analysis in food samples [107]. The SPME fiber was impregnation with allyl isothiocyanate. The extracted aromatic amines form allyl thiourea derivatives. These allyl thiourea derivatives were pyrolyzed in the inlet of the GC to form aryl isothiocyanates. 6.2. In tube and in-syringe derivation In tube derivation has been coupled with LPME and SPME techniques in aliphatic amine and carbamate pesticide analysis. Zhang et al. combined hollow fiber LPME with in-tube derivatization for carbamate pesticide analysis [113]. The carbamate pesticides were extracted from aqueous sample in a hollow fiber LPME technique. Both the organic extract and the derivatizing reagent were introduced to a GC/MS for on-column derivatization and analysis. Campins-Falco and co-workers combined chemical derivatization with automated in-tube SPME for the analysis of dimethylamine [61]. A GC capillary coated with 95% polydimethylsiloxane and 5% polydiphenylsiloxane was impregnated with the derivatizing reagent FMOC. Derivatized dimethylamine was analyzed using LC. The developed method was validated analyzing aliphatic amines in aqueous samples [62]. In-syringe derivatization of short chain aliphatic amines from aqueous samples after SDME with PFBOC containing nitrobenzene as the extraction solvent has been carried out. After in-syringe derivatization the organic extract was injected to a GC/MS for analysis. The method was validated using lake, river and industrials waste water samples [110]. 6.3. SPE, HF-LPME and on-microfluidic methods Many SPAD variants combining SPE and derivatization can be found in literature. When analyzing samples with complicated matrices presence of interfering compounds cause difficulties. In order to improve selectivity in amphetamine analysis in biological samples Rudaz and co-workers derivatized adsorbed analytes with fluorescamine on a SPE cartridge [40]. The derivatives after elution were analyzed using a laser induced fluorescence which is very selective. Nakamoto et al. utilized a monolithic silica spin column for simultaneous extraction and derivatization of amphetamines and 3,4-methylenedioxyamphetamines in human urine. Sample extracts were analyzed using GC/MS [111].

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Simultaneous extraction and derivatization of biogenic amines have explored with HF-LPME. Biogenic amines were derivatized with dansyl chloride, after derivatization the acceptor phase was with-drawn into a syringe and analysis was carried out using HPLC/UV. The proposed method was validated with food samples. Derivatization on microfluidic device for biogenic amine analysis was reported by deMello and co-workers [112]. Biogenic amines were derivatized with dichlorotriazine fluorescein, which showed two orders of lower LOD values compared to commonly used derivatizing reagents o-phthaldehyde and fluorescein isothiocyanate. 7. Conclusions Solid phase analytical derivatization (SPAD) is an attractive tool for determination of wide range of analytes in complicated biological and environmental sample matrices. Low organic solvent consumption, ease of automation, low cost and efficiency in SPAD has helped immensely gain its popularity over the years. More focus on automation can be expected in SPAD method development as time factor is becoming critical in sample preparation methods. Analytical derivatization (AD) increases sensitivity, selectivity and separation. As a result of immerging derivatizing reagents more SPAD variants are expected in analytical chemistry literature. These attractive features will ensure SPAD as an important sample preparation method in research and industrial applications. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

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