2,4,6-Trichlorophenyl alkyl ethers as retention index markers in capillary gas chromatography with electron-capture and mass spectrometric detection

451

Analytica Chimica Acta, 286 (1994) 451-456 Elsevier Science B.V., Amsterdam

2,4,6-Trichlorophenyl alkyl ethers as retention index markers in capillary gas chromatography with electron-capture and mass spectrometric detection Marco Morosini and Karlheinx Balls&miter Abteilung Analytische Chemie und Umweltchemie, UniversitZt Ulm, D-89069 Ulm (Germany) (Received 17th February 1993; revised manuscript received 19th July 19931

Abstract A homologous series of eight 2,4,6_trichlorophenyl alkyl ethers (TCPEsl was synthesized to provide a retention index (RI) reference system for routine analysis by high-resolution gas chromatography (HRGC) with electroncapture (ECD) or mass spectrometric (MS) detection. The chemical structure of the RI markers was chosen in order to mimic to some extent the molecular interaction between the GC system and important families of environmental contaminants such as halogenated, low-volatile compounds. Owing to the three chlorine atoms in the TCPEs, a good signal is obtained with ECD. With MS detection the different TCPEs give a common strong signal due to the 2,4,6&chlorophenol fragment. This allows the monitoring of all the TCPBs with a unique single ion monitoring (SIM) trace. A reference scale of retention indices was set (RI T8IiJ with absolute rounded values for the TCPEs (e.g., RI-T2 - 1400, RI-T4 = 1600). The retention indices of the TCPEs referred to n-alkanes were also calculated (stationary phase: 5% diphenyl-95% dimethylpolysiloxane). In addition to their use as RI markers, the TCPEs can be used as single or multiple internal standards and as test substances to ascertain possible discrimination phenomena in the GC part (injection block and column) of a GC-MS system. keywords: Gas chromatography; Gas chromatography-mass alkyl ethers

Thirty-five years after its introduction by Kovits [l], the retention index (RI) system is still “perhaps the most significant contribution to the theory and practice of gas chromatography (GC)” [2l. The RI system can contribute to validating the identity of signals in GC and also in liquid chromatography (LC) [3-51. This is particularly true for GC, where capillary colums and industrial production have led to a dramatic increase in the resolution potential and to better reproducibility. Nevertheless, the RI system is rarely used routinely. One reason is the lack of a univerCorreqwndence to: M. Morosini, Abteilung Analytische Cbemie und Umweltchemie, Universitiit Ulm, D-89969 Ulm (Germany).

spectrometry; Retention index markers; Trichlorophenyl

sally accepted RI marker series, detectable by electron-capture detection (ECD). As low concentrations of the classical RI markers, the n-alkanes, are not detectable by ECD, some ECDdetectable RI markers have been proposed: n-alkyl trichloroacetates (ATA) [6,7], monobromo-nalkanes 181, chiorobenxenes and chlorobiphenyls [9l, dichlorobenzyl alkyl ethers [lo], alkyl bis(trifluoromethyl)phosphine sulphides (M series) [ll], nitroalkanes [12] and alkyl nitrates [13]. The choice of possibly a unique and generally accepted RI reference series for the high-resolution (HR) GC-ECD would be desirable. In view of this choice and in order to broaden the range of candidate RI markers, in this work a homologous series of eight 2,4,6-trichlorophenyl alkyl

0003~2670/94/$07.00 Q 1994 - Elsevier Science B.V. All rights reserved SSDI 0003-2670(93)E0423-5

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M. Morosini and K Ballschmiter /Anal Chim. Acta 286 (1994) 451-456

ethers (TCPEs or T2, T4, T6, T8, T10, T12, T14, T16, with alkyl---C2-C16) was synthesized. The TCPEs can be detected not only by flame ionization detection (FID), atomic emission spectrometry (AES) and mass spectrometry (MS), but also by ECD. A "problem-oriented" approach, aimed at the separation of semi-volatile halocarbons and to their detection by ECD and MS, was preferred to a "universal marker" approach. Owing to some structural parameters in common with important families of halogenated environmental contaminants (biphenyls, terphenyls, biphenyl ethers, dibenzodioxins, dibenzofurans and some classes of pesticides), the TCPEs are likely to interact with apolar or semi-polar stationary phases to some extent in a similar way to the analytes. Better reproducibility is therefore to be expected.

EXPERIMENTAL

Synthesis The WiUiamson synthesis of the alkyl aryl ethers was applied in eight separate reactions: 2,4,6-trichlorophenol (4.2 g) and potassium iodide (10 nag) as catalyst were dissolved in dry ethanol (32 ml), stirred and heated under reflux. After addition of the alkyl bromide (2-6 g), the solution was stirred and held at boiling temperature for 6 h. The ethanol was distilled with a Vigreux column under a water-pump vacuum. The reaction products were dissolved in a 5% NaOH solution (15 ml), followed by extraction in a separating funnel with diethyl ether (3 × 15 ml). The organic phase was washed with distilled water until a neutral reaction of the water was attained. The solvent was evaporated in a rotary vacuum system. The total yield of the ether synthesis was 40-60%, The product was weighed and stored at -180C. The purity of the synthesized compounds, checked by H R G C with FID, ECD and MS detection, was >t 95%. A further purification step involving distillation or sublimation will be applied before using the TCPEs in routine analytical work. The molecular weights of the eight TCPEs ranged between 225.35 (T2) and 421.35 g mol- ~ (T16).

Instrumentation An HP 5840A gas chromatograph (HewlettPackard, Palo Alto, CA) was used with FID and ECD. The column was DB-5 (5% diphenyl-95% dimethyipolysiloxane) (J & W Scientific, Folsom, CA) (25 m x 0.25 mm i.d.; 0.1 ~m; phase ratio /3 -- 625). The carrier gas was hydrogen at 15 psi. Splitless injection was used. The temperatures were injector 250"C, FID 280"C and ECD 300"C and the column was programmed from 90°C (3 min) at 2, 3 and 5°C min-1 to 270°C (held for 5 min). A GC HP 5890 gas chromatograph (HewlettPackard), was used with a VG TS 250 doublefocusing mass spectrometer with EBE configuration (VG Analytical, Manchester), with electron impact (El) ionization at 30 and 70 eV. The column was Ultra 2 (5% diphenyl-95% dimethylpolysiloxane) (Hewlett-Packard) (50 m x 0.20 mm i.d.; 0.33 ~m; phase ratio /3--151). The carrier gas was helium at 33 psi. Splitless injection was used. The temperatures were injector 280°C and detector source 180"C and the column was programmed from 90°C (3 min) at 2.5"C min -1 to 310°C (held for 5 min). Chemicals 2,4,6-Trichlorophenol (purity 98%) and eight n-alkyl bromides (C2, C4, C6, C8, Cl0, C12, C14 and C16) (purity 97%) were obtained from Fluka (Buchs, Switzerland). Polychlorobiphenyls (PCBs) (IUPAC [14] Nos. 28, 52, 101, 118, 138, 153 and 180) (purity 99%) were purchased from Promochem (Wesel, Germany). RESULTS AND DISCUSSION

The individual TCPEs and a mixture of all of them were diluted in isooctane at different concentrations (1 /~g, 10 ng, 1 ng, 100 pg and 10 pg /~1-1). Storage of the solutions for 6 months on the laboratory bench in dear glass bottles did not alter the ECD signal. Injection temperatures of 250 and 280"C were used without significant thermal degradation of the compounds. Signals much over the quantification limit were obtained on injecting the following amounts of a single TCPE: FID 10 ng, MS 1 ng (SIM 50 pg) and ECD 10 pg.

M. Morosini and K BalLvchmiter/AnaL Chins. Acta 286 (1994) 451-456

The presence of three chlorine atoms in the TCPE molecule ensures a good ECD response also in the lower picogram range. Peaks with very

453

little tailing were obtained using a 5% diphenyl95% dimethylpolysiloxane stationary phase (DB-5, Ultra 2).

IL 35.

(b)

3a. 85.

w. 75.

a. 65. 6s. 55. 50. 15. (8. 35. 1.

a. 8. 15. IS. 5. I_

(c)

a3 37

11: 11

I*

f

159

Fig. 1. Fragmentation of (a) 2,4,6-trichlorophenyl ethyl ether 02, M.W. 225.351, (b) 2,$,6-trichlorophenyl decyl ether (TlO, M.W. 337.35) and (c) 2,4,6-trichlorophenyl hexadecyl ether (T16, M.W. 421.35). EI ionization at 70 eV. Trichlorophenol fragment at m/z 196, 198 and 200.

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itf. Morosti and K Ballschmiter /Anal. Chim. Acta 286 (I!%) 451-456

With MS detection (EI, 70 eV> the signal of the molecular ion is weak or at the limit of detection for all the synthesized TCPEs; its intensity decreases with increasing alkyl chain length. The predominant signal is the 2,4,6-trichloropheno1 ion fragment (m/z 196, 198, 200) (Fig. la, b and c). The fragmentation pattern obtained at 30 eV is very similar to that obtained at 70 eV, indicating that the 2,4,6-trichlorophenol fragment is particularly stable. 2,4,6-Trichlorophenol itself with EI ionization gives a strong molecular ion pattern (m/z 196, 198,200). The strong signals of the 2,4,6-trichlorophenol fragment are useful for monitoring the homologous TCPEs. The retention indices of the TCPEs and of some selected PCBs were calculated with Van den Do01 and Kratz’s equation for temperatureprogrammed elution [15]. Using the n-alkanes as a reference system, a linear temperature programme of 25°C min-’ and an Ultra 2 column, the TCPEs give RIalkane values between 1405.6 021 and 2895.4 (T16) (Table 1). The TCPEs T4-T16 show increments of the RIalkane values greater than 200 RI units for each CH,CH, group (AZ between 213 and 2161. The RIalkane increment between T2 and T4 is smaller (202 RI a,kaneunits), indicating a non-linear progression of the RI values of the lowest members of the series. This is also the case with other RI markers series [7] and with the n-alkane series itself [16]. The purpose of this work being to substitute the n-alkanes as a reference system, rounded values of retention indices (RI,,,) were assigned to the single TCPEs (e.g., RI-T2 = 1400, RI-T16 = 2800). When the logarithm of the adjusted retention time (log tJ of the TCPEs is plotted against their retention index (RI,,,,, isothermal elution at 130, 150, 170, 190 and 21O”Q a linear relationship is observed. Plotting the gross retention time of the TCPEs obtained with a temperature programme of 2°C min- ’ value, the eight TCPEs lay against the RI,,, very close to a linear regression through eight points (Fig. 2). A more precise calibration of the RI values would require optimization of the GC conditions oriented for this purpose. However, this is not the case in routine environmental analysis, where the operating parameters are

TABLE

1

Retention indices of 2,4,6-trichlorophenyl alkyl ethers (TCPEs), alkyl trichloroacetates (ATAs) and polychlorobiphenyls (PCBs.1 on a 5% phenyl-95% dimethylpolysiloxane column with temperature programming at 25°C min-’ Substance

RI akaanes

T2 T4 T6 T8 TlO T12 T14 T16

1405.6 1608.0 1822.2 2035.8 2251.9 2466.8 2682.5 2895.4

Mean RIalkane increment for CH,CH, in the TCPE series ATAI ATA ATA ATAlO ATAll ATA ATA ATA ATA1.5 ATA ATA ATA ATA ATA Mean RI,,,, increment for CH, in the ATA series PCB PCB PCB PCB PCB PCB PCB

28 52 101 118 153 138 180

a Al = RI,,

AI= RI alkanes (202.4) 214.2 213.6 216.1 214.9 215.7 212.9

R~TCPE

1400 16oo 1800 2OtKl 2200 2400 2600 2800

214.6

(114.6) 96.8 102.3 97.7 96.2 98.2 95.9 97.8 96.4 98.1 96.7 98.2 97.3

1474.8 1589.4 1686.2 1788.5 1886.2 1982.4 2080.6 2176.5 2274.3 2370.7 2468.8 2565.5 2663.7 2761.0

97.6 1853.4 1920.4 2079.7 2192.9 2241.0 2292.2 2428.9

- RInC-Xij.

rather regulated so as to obtain time and separation efficiency, e.g., using double ramp heating of the column instead of a linear temperature programme. Table 1 reports the retention indices of the TCPEs with reference to the n-alkanes (RI alkanes)and those of thirteen trichloroacetates

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M. Momini and K. Bah!schmiter/Anal. Chim. Acta 286 (1994) 451-456

SW’

1600

Iwo

1900 2200 wcm

2aoo

TCPEs (normalized to 100.0 RI units). This effect is probably due to the different structural features of the two series: a carboxylate group in the ATAs and an aromatic ring in the TCPEs. When the ATAs and the TCPEs are injected together, the members of the TCPE series show an increasing delay with respect to the ATA compounds, which increases with increasing length of the alkyl chain. Owing to their structural analogy to some families of analytes (e.g., PCBs, polyhalogenated dibenzodioxins and dibenzofurans or PHDD/F), the TCPEs are better suited than the ATAs as RI markers for these analytes. In the analysis of the PCBs the use of the two congeners, PCB 52 and PCB 180, as internal reference compounds has been proposed [17] for the identification of other congeners through relative retention data (RR,,,,,,) based on the equation

mo

Fig. 2. Plot of the retention index (RITCPn) of eight TCPEs (T2-T16) against their retention time. The dotted line is the linear regression obtained using the eight points.

(ATAS) and of seven PCB congeners with reference to the TCPE (RIr&. CI-I, increment of the The average RI,, homologous series of ATAs (97.6 RI units) is lower than the RI,,, CH, increment of the

RR 52+180 =

+ ~PcBl80

wc!Es/~PcB,,

(1)

On plotting the retention indices of the PCBS (RI TCPE) against their relative retentions

3000,o

2800,O

180

138

2400,O

118 =;,A/' lo1 DDE A' _A' TCN 52,/A’

3 2000,o t. a

28

1800,O

r/*

Hc:/ 1600.0

I 1400.0$

1000,0 I 0.0000

I I o.!iOoo

1

1 I

1.0000

1,SOOO

I I

2,OOOo

I

2,sooo

3,0000

RR (52+180)

Fig. 3. Plot of RI,, against the relative retention (RR -r+~tst,) for seven PCBs and of some other chlorinated compounds. PCBz = pentachlorobenxene; HCB = hexachlorobenxene; DDE = 4,4’-dichlorodiphenyldichloroethene; TCN = tetrachloronaphthalene. HRGC-ECD with a DE5 column (25 m X 0.25 mm i.d.; 0.1 pm) programmed at 2°C mm-‘.

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M. Morosini and K. Ballschmiter /Anal. Chim. Acta 286 (1994) 451-456

(RR 52+1sJ (Fig. 3), a linear correlation

is-obtained, confirming the advantage of a homologous series structurally similar to the analytes. A single TCPE or few of them can also be used as internal standards to be added to environmental samples before extraction and cleanup. The elution range covered by the T2-T16 series on low-polarity stationary phases allows the bracketing of several families of environmental contaminants. The synthesized series of TCPEs is also suitable for monitoring the performance of the GC part of a GC-MS system. Mass-dependent discrimination of the response of the MS detector can be ruled out by detecting the common trichlorophenol signal; this enables one to ascertain possible discrimination phenomena in the remaining part of the system (injection block and column). Conchsions The TCPEs are a useful RI reference system in the GC analysis of halogenated low-volatile, non-polar compounds. They could probably be used as RI markers also for other nonhalogenated ECD-detectable analytes (e.g., nitroaromatic compounds, alkyl nitrates, quinones). A good ECD signal and a strong trichlorophenol fragment signal (m/z 196, 198, 200 amu) ensure good detectability using ECD and MS-SIM. The authors are grateful to Mr. Thomas Wiedmann for his assistance with the mass spectrometric analyses. M.M. is grateful to the Community

Bureau of Reference (BCR) of the Commission of the European Communities (CEC) for a bursary (B/BCR*-913002).

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