determination of aromatic amines from azo dyes reduction by liquid-phase sorbent trapping and...

Qing ZhangChao WangHua BaiXing WangTing WuQiang Ma

Institute of Industrial ProductInspection, Chinese Academy ofInspection and Quarantine,Beijing, China

Original Paper

Determination of aromatic amines from azo dyesreduction by liquid-phase sorbent trapping andthermal desorption-gas chromatography-massspectrometry

A novel method based on thermal desorption was developed for the trapping of aro-matic amines from azo dyes reduction in liquid phase. Combining GC-MS, the detec-tion limits for 21 kinds of aromatic amines ranged from 0.1 to 3 mg/kg. The recov-eries of most aromatic amines were F70% with RSDs f10%. This method also offerslow sample and organic solvent requirements, effective preconcentration and con-venient procedure. It has been applied to determine aromatic amines from the realsamples of textile and yielded good results.

Keywords: Aromatic amine / Azo dyes / Liquid-phase trapping / Sorbent / Thermal desorption /

Received: February 10, 2009; revised: May 6, 2009; accepted: May 6, 2009

DOI 10.1002/jssc.200900089

1 Introduction

Aromatic amines are used in the chemical industry inhigh amounts [1], and many of them are toxic to humans[2–5]. One of the major origins of aromatic amines is azodyes, which are widely used as colorants in a variety ofconsumer goods such as textiles, toys, leather, paper,food and paints (The Environment Agency, PollutionInventory, England and Wales, 2003). Some researcheshave shown that these dyes can break down to muta-genic and carcinogenic aromatic amines during usethrough reduction of the azo group (-N=N-) [6, 7]. There-fore, the health issues of azo dyes and their determina-tion have received much attention in recent years. Forexample, to prevent the potential risk from consumers,the European Parliament has issued the European Direc-tive 2002/61/EC [8] to regulate the use of azo dyes. Accord-ing to this directive, azo dyes that will form any of 22listed harmful aromatic amines in textile and leather areto be banned.

Several analytical methods have been developed fordetermination of aromatic amines from azo dyes reduc-tion [9–16]. In brief, the analytical procedures involvedin these methods can be divided into three steps: reduc-

tive reaction of azo dyes in an aqueous solution, followedby extraction of the amines from the solution and thefurther determination by LC, GC, or CE. Due to the highsolubility of aromatic amines in water, the extractionstep is of great difficulty. Also, it is of great importance toachieve high performance in the following separationand detection. Liquid-liquid extraction and SPE are twofrequently used methods for the extraction of theseamines from aqueous samples. To reduce the require-ments of sample and organic solvent, some miniaturizedprocedures such as solid-phase microextraction [17–19]and liquid-phase microextraction [20–22] have also beenapplied. However, for practical applications of thesemethods in routine inspection work, there are still someproblems to resolve such as special interface, limitedadsorption capacity, and flavorsome procedures.

Here we proposed an alternative approach based onsorbent trapping followed by thermal desorption (TD).Sorbent trapping is the most widely used strategy for pre-concentration of analytes prior to analysis [23–26]. How-ever, in most applications, only analytes in gaseousphase were handled. In this work, aromatic amines fromazo dye reduction in liquid-phase are directly entrappedon Tenax-TA, which then was subjected to TD for analyterecovery and the subsequent determination by GC-MS.Using this approach, determination of 21 aromaticamines has been obtained with high sensitivity and goodreproducibility. The method was applied to determinearomatic amines in textile samples and the resultsshowed good agreement with conventional solid extrac-tion method.

Correspondence: Dr. Qing Zhang, Institute of Industrial Prod-uct Inspection, Chinese Academy of Inspection and Quarantine,Beijing 100123, ChinaE-mail: [email protected]: +86-10-85772625

Abbreviations: TD, thermal desorption

i 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com

2434 Q. Zhang et al. J. Sep. Sci. 2009, 32, 2434 – 2441

J. Sep. Sci. 2009, 32, 2434 –2441 Sample Preparations 2435

2 Experimental

2.1 Standards and reagents

Standards for each of the 21 aromatic amines (F98%) con-sidered in this work were purchased individually fromChemService (West Chester, PA, USA). The detailed infor-mation of these compounds is listed in Table 1. Stock sol-utions of each compound (100 mg/L) were prepared inHPLC-grade methanol obtained from Baker (Phillipsburg,NJ, USA). The calibration solution of each compound wasprepared by diluting the stock solution to 10 and 1 mg/Lwith sodium citrate buffer solution (0.06 mol/L, pH 6.0).

Tenax-TA (60–80 mesh) and Carbopack B (60–80mesh) was obtained from RUIZHX (Beijing, China). GDX-102 (60–80 mesh, polystyrene-divinylbenzene) was fromTianjin Chemical Reagent (Tianjin, China). The waterwas purified through a Mill-Q water system (Millipore,Bedford, MA, USA) prior to use. All other chemicals wereof analytical grade and used without further purifica-tion. Textile samples made of cotton was used in thisstudy.

2.2 Instrumentation

The analysis of GC-MS was carried out with an Agilent6890 Series GC enquipped with an Agilent 5975 massselective detector (Agilent Technologies, CA, USA)coupled to an Ultra-UNITY automated thermal desorber(Markes, UK). The desorber contains a cold trap to focusthe analytes before entering the analytical column. Glasssorbent tubes (8966 mm id, Kinglass, China) are filledwith 150 mg Tenax-TA (60–80 mesh) to form packed sam-

ple tubes. A TC-20 sample tube conditioning/dry-purgerig (Markes) was used to condition packed sample tubesand remove excess water from packed sample tubes.Before sampling, packed sample tube was conditioned at3258C for 30 min. A Visiprep SPE vacuum manifold(Supelco, PA, USA) was used to perform analytes extrac-tion. Sorbent tube was connected to the manifold via amanually prepared connector. The flow chart diagram ofthe procedure of sample treatment and the device usedin extraction procedure are shown in Fig. 1. For GC sep-aration, a HP-5ms column (30 m60.25 mm id 0.25 lmfilm thickness) was used.

2.3 Experimental conditions of GC-MS and TD

The following is a typical set of experimental conditions.

2.3.1 GC

The column temperature program was as follows: 1008Cfor 1 min, 88C/min to 2008C, 108C/min to 2308C, 2 to2508C, and held for 8 min. Helium (99.999%) was used ascarrier gas at a column flow of 1.0 mL/min.

2.3.2 MS parameters

Transfer line temperature: 2808C; electron impact ion-ization at 70 eV; Qualitative analysis was performed inscan mode and the scan range was 50–350 (m/z). Quanti-tative analysis was performed in SIM and the characteris-tic ion of each aromatic amine was listed in Table 1.


Table 1. Aromatic amines studied in this work.

No. Aromatic amine CAS number Characteristic ion Boiling point (8C)

1 o-Toluidine 95-53-4 107 2002 o-Anisidine 90-04-0 123 2253 4-Chloroaniline 106-47-8 127 2324 p-Cresidine 120-71-8 137 2355 2,4,5-Trimethylaniline 137-17-70 135 2336 4-Chloro-o-toluidine 95-69-2 141 2417 2,4-Diaminotoluene 95-80-7 122 2848 2,4-Diaminoanisole 615-05-4 138 a2509 2-Naphtylamine 91-59-8 143 306

10 2-Amino-4-nitrotoluene 99-55-8 152 a30011 4-Aminobiphenyl 92-67-1 169 30212 4-Amioazobenzene 60-09-3 197 36013 4,49-Oxydianiline 101-80-4 200 19014 4,49-Methylenedianiline 101-77-9 198 39915 Benzidine 92-87-5 184 40216 o-Aminoazotoluene 97-56-3 225 a30017 3,39-Dimethyl-4,49-diaminobiphenylmethane 838-88-0 226 a25018 3,39-Dimethylbenzidine 119-93-7 212 30119 3,39-Dichlorobenzidine 91-94-1 252 36820 4,49-Methylene bis(2-chloroaniline) 101-14-4 266 20221 3,39-Dimethoxybenzidine 119-90-4 244 415


2.3.3 TD parameters

Sample tube desorption was performed at 3008C for30 min while the cold trap was held at 58C; desorb flow:50 mL/min; cold trap desorption: 3608C for 5 min; thetotal split ratio was about 40:1.

A typical GC-MS chromatogram of 21 aromatic aminesunder the above conditions is shown in Fig. 2.

2.4 Sample preparation

2.4.1 Reductive cleavage

Textile material was cut into small pieces (about565 mm2). Then, a quantity of 17 mL sodium citrate buf-fer solution (0.06 mol/L, pH 6.0) preheated to 708C wasadded to a portion of 1.0 g textile sample and kept for30 min at 708C. Subsequently, 3.0 mL aqueous sodium


Figure 1. Schematic illustration of the system for aromaticamines extraction.

Figure 2. Typical GC-MSchromatogram of the 21 aro-matic amines. The upperchromatogram is the totalion chromatogram (TIC) andthe lower one is the SIMchromatogram to each of the21 aromatic amines.


dithionite solution (200 mg/mL) was added to above solu-tion for reductive cleavage of the azo groups, shaken vig-orously and kept at 708C for another 30 min. Then, it wasquickly cooled to room temperature and filteredthrough a 0.22 lm filter for subsequent use.

2.4.2 Extraction

Packed sample tube (Tenax-TA) was coupled to the vac-uum manifold. Then, it was firstly rinsed twice with1 mL of methanol and twice with 1 mL of distilled water,respectively. Then, 200 lL of the above reaction solutionwas added into the sample tube and the sample loadingflow rate should be carefully controlled not to exceed200 lL/min. The sample tube was then washed with 1 mLdistilled water and dried under vacuum for 1 min. Then,the sample tube was placed on TC-20 and dried at 508Cwith N2 flow rate of 50 mL/min for 30 min. After that, thesample tube was immediately analyzed.

3 Results and discussion

3.1 Selection of sorbents

Selection of a correct sorbent is one of the most impor-tant factors, when developing a valid and robust TDmethod. Considering the nature of aromatic amines andthe requirements of TD, a suitable sorbent in our workshould at least meet two requirements: i) It should havean appropriate strength of adsorption. Since most of aro-matic amines have high boiling point (over 2008C) andhigh solubility in water, the sorbent should have notonly the ability to capture them from water but also theability to release them. ii) It should show poor adsorptionof water, which is a footstone for successful extraction ofaromatic amines from water. A large water backgroundwould result in some problems in GC detection, such asinterference with detector performance, retention timeshifts and faster deterioration of columns.

Based on the above consideration, we tested threehydrophobic sorbents including GDX-102, Carbopack Band Tenax-TA. The parameters of these sorbents werelisted in Table 2. All of the sorbents were appropriatelyconditioned prior to test and the calibration solutions(1 mg/L) of each aromatic amine were used. In our experi-ments, GDX-102 was proven unsuitable for TD of aro-matic amines since the maximum use temperature of

GDX-102 was not high enough. About half of aromaticamines (No.12 –21) cannot be sufficiently desorbed onGDX-102 even at its max temperature. Carbopack B wasnot either an appropriate choice since its absorbancestrength was too strong for aromatic amines. It requiredrelatively harsh conditions (desorption temperatureA3508C and desorption time A40 min) to perform a com-plete desorption. Compared with the above two sorbents,Tenax-TA has a compatible adsorption strength and anadequate maximum temperature. Most of the 21 aro-matic amines can be completely desorbed under a de-sorption temperature of 3008C. Hence in our experi-ments, we chose Tenax-TA as the sorbent for aromaticamines.

3.2 Desorption efficiency

The selection of desorption parameters such as desorp-tion temperature and desorption time was based on thedesorption efficiency of aromatic amines, which was cal-culated using Eq. (1):

Desorption efficiency ¼ A1

A1 þ A2 þ � � � þ An6100%

ð1Þ

A1: peak area obtained from the first desorption;

A2: peak area obtained from the second desorption;

An: peak area obtained from the n desorption thatAn/An-1 less than 5%.

We take 3,39-dimethoxybenzidine and benzidine asrepresentatives to build a preliminary desorption condi-tion, since they maintain the highest boiling pointsamong all the target amines in this work with the leastdesorbable properties (which can be seen in Table 1). Fiftymicroliters of 10 mg/L calibration solution of 3,39-dime-thoxybenzidine and benzidine were added into sampletubes and desorbed. Their desorption efficiency at differ-ent desorption temperature is shown in Fig. 3. Benzidinecan be completely desorbed at 3008C with the time of30 min. And the efficiency of 3,39-dimethoxybenzidine is80% at the same condition. Nevertheless, we test theother aromatic amines at this preliminary condition andthey all can be completely desorbed. Therefore, wechoose 3008C and 30 min as the desorption condition inthis work.


Table 2. Characteristics of the sorbents tested in this work.

Sorbent Surface type Mesh size Specific surfacearea (m2/g)

Max temp.(8C)

Tenax-TA Hydrophobic (diphenylene oxide polymer) 60 –80 35 320GDX-102 Hydrophobic (polystyrene-divinylbenzene) 60 –80 680 250Carbopack B Hydrophobic (carbon) 60 –80 100 400


3.3 Measurement of water adsorption

In this work, adsorption of water on sorbents should becarefully estimated since aromatic amines were directlyentrapped in water. Overfull absorbed water may bringsome problems such as clog in cold trap, interferencewith detector performance and faster deterioration ofthe chromatographic columns. Studies about water de-sorption on a variety of sorbents have been carried out bysome researchers [27, 29]. In their experiments, Tenax-TAshowed very poor adsorption to water (a5 mg/g) in highlyhumid air. The adsorption of water on Tenax-TA was alsostudied using water instead of humid air in our experi-ments. The mass of absorbed water on sorbents withdifferent states was measured using a DL31 titrator (Mett-ler-Toledo, Switzerland). The results are shown in Table3. From these results we may draw two conclusions: i)The adsorption of water on Tenax-TA was so weak that itcan be easily removed by N2 flow. ii) After activation ofmethanol, the capacity of Tenax-TA for water adsorptionwas increased, this was helpful to entrap aromaticamines from aqueous solution.

3.4 Breakthrough volume and safety samplingvolume

The determination of breakthrough volume and safetysampling volume was on the basis of European standard[29] and the technical support of Markes (Note 5 and 8 ofThermal Desorption Technical Support). Two sampletubes were linked in tandem. The connecting regionbetween the two tubes was a Teflon fitting and the backtube was connected to the SPE vacuum manifold. A series

volume (1, 2, 3, 4, and 5 mL) of aqueous solution contain-ing aromatic amines (2 mg/L) was sampled through thetandem sample tubes. The flow rate was controlled atabout 50 lL/min. After that, the two tubes were driedunder N2 and analyzed. The breakthrough volume isdefined as the volume for which the proportion of ana-lyte present in the back tube (g) is 5%. And g was calcu-lated using Eq. (2). The safety sampling volume is twothirds of the breakthrough volume. The experimentshave shown the breakthrough volume of all 21 aromaticamines were at least 5 mL (To simplify the experiment,the exact values of each aromatic amines were not meas-ured) and the safety sampling volume was 3.3 mL at leastaccordingly.

g ¼ At2

AT¼ At2

At1 þ At26100% ð2Þ

At1: peak area obtained from front tube;

At2: peak area obtained from back tube;

AT = At1 + At2

3.5 Linear range, detection limit and recovery

For calibration curve of each aromatic amine, 10, 50, and200 lL of 1 mg/L calibration solution and 50, 100, and500 lL of 10 mg/L calibration solution of each aromaticamines (the amount of each aromatic amine was 10, 50,200, 500, 1000 and 5000 ng, respectively) were added tothe sorbent tube in a flow of N2 gas using a calibrationsolution loading rig and then determined. The resultswere shown in Table 4. For 1 g textile sample, 20 mLreductive reaction solution and 200 lL sampling solu-tion, the LOD (S/N = 3) of each aromatic amines was calcu-lated, ranging from 0.1 mg/kg to 3 mg/kg. The recoveriesof each aromatic amine were determined in reductivereaction solutions of textile samples, which were spikedwith standards of each aromatic amine. The results wereshown in Table 5. The recoveries of 21 aromatic amineswere between 56.3 l 106.1%.


Figure 3. Desorption efficiencies of benzidine and 3,39-dichlorobenzidine at different temperatures. The desorptiontime was 30 min.

Table 3. Mass of water adsorbed on Tenax-TA sorbents(averages from three measurements) of different state (state1: sorbents was rinsed with 3 mL water; state 2: state 1 fol-lowed by N2 drying at 508C with a flow rate of 50 mL/min for30 min).

A: sorbent was used without methanolactivation;B: sorbent was used after methanol activation

Mass of adsor-bed water(mg/g)

A State 1 96 l 8.2State 2 7.6 l 0.6

B State 1 1397 l 128State 2 5.5 l 0.4


3.6 Application to textile samples

The developed SPE-TD approach was applied to real sam-ples and the results were compared with the SPE method[10], in which the aromatic amines released in the reduc-tion reaction were transferred to a t-butyl methyl etherphase using diatomaceous earth columns.

Five textile samples were tested and one was found tocontain benzidine of 10 mg/kg as shown in the chroma-togram of Fig. 4. This sample was tested five times usingeach method. The difference between the results wasanalyzed using Origin 6.0 and no significant differencewas found. Therefore, we considered that the resultsobtained by the SPE-TD method and the SPE methodhave shown good agreements.

4 Concluding remarks

A SPE-TD method was proved efficient for the determina-tion of aromatic amines from azo dye reduction in textilesamples. With the liquid-phase trapping protocol, themain advantages of this method were the low organicsolvents consumption and convenient procedure. Forexample, 70–80 mL organic solvents were required inSPE [10] and liquid-liquid extraction method [30], whileonly 2 mL methanol was used in our method. It is helpfulto protect the operator's health by reducing the use ofsolvents. On the other hand, this method was more dura-ble than other miniaturized approaches such as solid-phase microextraction and liquid-phase microextrac-tion, since the sorbent tube is durable enough to bereused many times.

There are also some disadvantages to this present strat-egy. An additional device is needed to perform TD thatwould increase the cost of instrument. In addition, somearomatic amines with high boiling point have severepeak tailing since it is difficult to desorb them quickly.Nevertheless, this method may provide some new ideas


Table 4. Linear ranges and LOD of 21 aromatic amines

Aromatic aminesNo.

LOD(mg/kg)

Linear range (ng) Typical linear equation R2

1 0.2 10 –5000 Y = 2927X –45968 0.99972 0.3 10 –5000 Y = 1239X –20993 0.99963 0.2 10 –5000 Y = 959X + 94226 0.99824 0.3 10 –5000 Y = 862X + 83925 0.99605 0.2 10 –5000 Y = 1312X + 20251 0.99646 0.3 10 –5000 Y = 887X + 65389 0.99827 2 50 –5000 Y = 70.5X + 5947 0.99568 0.5 10 –5000 Y = 457X+ 23434 0.99609 0.2 10 –5000 Y = 1223X + 16897 0.9960

10 0.5 10 –5000 Y = 435X + 43562 0.995611 0.1 10 –5000 Y = 3658X –45586 0.992012 0.5 10 –5000 Y = 526X + 35684 0.998213 0.6 10 –5000 Y = 343X–20415 0.995614 3 50 –5000 Y = 56.8X + 7552 0.992015 0.3 10 –5000 Y = 2203X + 52562 0.998216 0.5 10 –5000 Y = 398X + 35412 0.996017 0.6 10 –5000 Y = 356X–15896 0.992018 0.3 10 –5000 Y = 854X + 56982 0.995619 2 50 –5000 Y = 82.5X + 6235 0.992020 0.6 10 –5000 Y = 326X + 26531 0.995621 2 50 –5000 Y = 68.8X + 5835 0.9964

Table 5. Recoveries of 21 aromatic amines in reductivereaction solutions of textile samples (n = 5).

Aromaticamines

Spiked level: 300 ng Spiked level: 1000 ng

No. Recovery(%)

RSD(%)

Recovery(%)

RSD(%)

1 85.2 5.2 81.2 5.12 90.2 4.6 85.4 4.83 96.5 3.8 106.1 5.04 80.6 4.3 83.7 6.05 87.2 8.2 85.1 6.96 89.3 7.2 100.8 5.57 80.1 6.8 76.2 7.68 62.8 8.3 70.9 8.79 76.9 9.2 70.1 8.6

10 88.2 4.6 100.5 4.211 56.3 7.2 60.2 6.312 90.3 5.3 98.2 4.713 65.6 6.5 72.2 5.514 58.2 7.2 59.2 7.615 78.6 5.9 83.0 5.616 74.8 4.3 79.2 7.217 71.2 5.9 76.2 4.618 72.6 7.2 75.6 6.419 76.3 8.2 73.9 6.920 66.3 8.5 70.3 7.921 72.9 10.0 71.6 8.3



Figure 4. GC-MS chromatogram of a textile sample containing benzidine. The upper chromatogram is a TIC chromatogram andthe lower one is an SIM chromatogram.


on the extraction of aromatic amines from azo dyesreduction, and the studies of its application for otheranalytes are on going.

The authors acknowledge the Ministry of Science and Technologyof People's Republic of China (2006BAK10B03) and Ministry ofFinance of People's Republic of China (200810953) for the financialsupport of this project.

The authors declared no conflict of interest.

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determination of aromatic amines from azo dyes reduction by liquid-phase sorbent trapping and...

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