Original

Pap

er

O’Connor, Rowland, Evans 839

Gavin O’Connor1),Steve J. Rowland,E. Hywel Evans

University of Plymouth,Plymouth EnvironmentalResearch Centre, Departmentof Environmental Sciences,Drake Circus, Plymouth,PL4 8AA, UK

Evaluation of gas chromatography coupledwith lowpressure plasma sourcemass spectrometry for thescreening of volatile organic compounds in food

A number of ion sources, namely low pressure helium MIPs, and low pressure heliumand argon ICPs, coupled to GC and mass spectrometry have been evaluated for theanalysis of volatile organic compounds. The most promising of these was the argonLP-ICP-MS. Detection limits of between 7–206 pg monitoring m/z 35 for 35Cl detec-tion, and between 92–7800 pg monitoring m/z 13 for 13C detection, were obtained fora range of chlorinated and unchlorinated organic compounds. The potential for deter-mining empirical formulae was demonstrated by an excellent correlation between pre-dicted and measured Cl/C ratios. The technique was successfully used for the head-space analysis of chlorinated and unchlorinated volatile and semi-volatile compoundsspiked into olive oil.

Key Words: Foods; Gas chromatography; Plasma mass spectrometry; Elementselective detection; Plasmamass spectrometry; Volatile organic compounds; Foods;

Received: March 1, 2001; accepted: April 30, 2002

1 Introduction

Plasma source mass spectrometry has proven itself as apowerful detector for chromatography, enabling tracemetal speciation analysis [1–4]. The success of suchinstrumentation can be partly attributed to the ease of cou-pling the chromatographic instrumentation to an atmo-spheric pressure argon inductively coupled plasma [1, 4].Such instrumentation has been successful for the analy-sis of organometallic species, but it has been less suc-cessful in the analysis of other organic compounds, suchas halogenated compounds. The major reasons for thisare: a) the atmospheric pressure argon plasma does notpossess sufficient ionisation power to ionise the halogens(I.P. 10.5 to 17.42 eV); b) the occurrence of many poly-atomic interferences below 80 m/z. This has fuelledfurther investigation into alternative plasma ion sources,with a higher ionising power than that of the atmosphericargon ICP and with less polyatomic interferences. Themost common method of achieving this has been the useof helium plasmas, because helium has a higher ionisingpotential (I.P. 24.59 eV) than argon (I.P. 15.76 eV); how-ever, atmospheric pressure helium ICPs are difficult tosustain.

One way of overcoming the problem is to generate theplasma under lower than atmospheric pressure. Lowpressure (LP) ICPs have been used, in conjunction withmass and optical spectrometry, as element-selectivedetectors for gas chromatography (GC). Evans et al. [5]have developed an argon LP-ICP, sustained with1 L min– 1 and 200 W power, for the production of elemen-tal ions for a series of organometallic and organohalidecompounds. The same instrument was also used for theproduction of molecular and fragment ions of a series oforganohalides by simply reducing the forward power to25 W. However, large amounts of analyte were requiredfor the production of the molecular signals. Castillano etal. [6] have investigated the use of a low pressure (LP)ICP, sustained with 0.5 L min– 1 helium at 100 W forwardpower, for the analysis of organometallic and organoha-lide compounds. Detection limits of 25 pg and 15 pg werereported for chlorobenzene and hexachlorobenzenerespectively. More recently the problems associated withmolecular ion formation in a low pressure helium ICPhave been addressed by the addition of reagent gases tothe LP-ICP. This has enabled linear calibration of the LP-ICP in the molecular mode as well as the atomic modewith detection limits of 4 pg and 76 pg for iodobenzeneand dibromobenzene, respectively, when operating in theatomic mode, and 100–229 pg for chlorobenzene, iodo-benzene, and dibromobenzene when operating in themolecular mode [7].

By far the most common helium plasma is the microwaveinduced plasma (MIP). The success of theMIP canmostlybe attributed to its ability to sustain stable helium plasmasat atmospheric pressure. However, MIPs are intolerant to

Correspondence: E. Hywel Evans, University of Plymouth,Plymouth Environmental Research Centre, Department ofEnvironmental Sciences, Drake Circus, Plymouth, PL4 8AA,UK.E-mail: hevans@plymouth.ac.ukFax: +44 (0)1752 2330401) Present address: LGC, Queens Road, Teddington, Middle-sex TW11 0LY, UK.

J. Sep. Sci. 2002, 25, 839–846

i 2002WILEY-VCH Verlag GmbH&Co. KGaA,Weinheim 1615-9306/2002/1309–0839$17.50+.50/0

840 O’Connor, Rowland, Evans J. Sep. Sci. 2002, 25, 839–846

solvent loading, therefore the majority of work performedwith MIPs has been with gaseous sample introduction [8].Helium MIPs coupled to atomic emission detectors havebeen applied to the analysis of halogenated compoundswith detection limits of between 10 and 40 pg for chlori-nated and brominated compounds [2]. Many groups haveinvestigated coupling helium MIPs to mass spectrometry.This has normally involved the removal of the ICP sourcefrom a commercially available ICP-MS and replacing itwith the MIP source. Much work has been devoted toplasma torch design and improving the robustness of theplasma, thereby enabling liquid sample introduction [3].Typical detection limits for a tangential flow torch,8 L min– 1 helium MIP-MS with GC sample introductionare 9–20 pg for chlorinated organic compounds [7].Suyani et al. [9] investigated the use of a tantalum injector,in conjunction with a tangential flow torch, which confinedthe sample eluent of the GC to a narrow region in theplasma. This was shown to improve detection limits oforganotin compounds by a factor of ten, to 0.09 pgdepending of the species of tin [9].

In this work, several plasma types coupled with massspectrometry have been evaluated as element selectivedetectors for gas chromatography, with a view to screen-ing for volatile organic compounds in foods.

2 Experimental

2.1 Low pressure inductively coupled plasma

2.1.1 Argon plasma

An inductively coupled plasma mass spectrometer (ICP-MS, VG PQ2, VG Elemental, Winsford, UK) was modifiedto accept a low pressure torch. The low pressure torchwas a 140 mm long quartz tube of 1/2 in. OD, with a 1/4 in.OD side arm. The quartz plasma torch was placed into theICP load coil and connected to the ion sampling interfacevia a low pressure sampling cone (Machine shop, Univer-sity of Plymouth), which was machined from aluminium,and had a 2 mm orifice and an Ultra-torr fitting for a 1/2 in.pipe. This enabled a vacuum seal to be formed betweenthe low pressure torch and the ion sampling interface).Plasma gas was supplied using a 1 L min– 1 mass flowcontroller (Tylan) from a liquid argon source (BOC)(Table 1). This instrument was different from that usedpreviously [5] (i.e. a later model by the same manufac-turer).

2.1.2 Helium plasma

A detailed description of the design and optimisation of theGC-LP-ICP-MS, used in this study, has been given pre-viously [10]. In brief, a Hewlett Packard Mass SelectiveDetector (MSD) was modified to enable it to analyse and

detect ions from the LP-ICP. This was achieved by using acustom made ion sampling interface. The low pressureplasma was sustained using a modified RF generator andmatching network, in a 140 mm long quartz tube of 1/2 in.OD, with a 1/4 in. OD side arm to which a calibration vialcontaining perfluorotributylamine (PFTBA) was attached.The quartz plasma torch was connected to the ion sam-pling interface in the same way as described for the argonplasma. The helium plasma gas was added to the lowpressure torch using a 100 mL min– 1 mass flow controller(Unit Instruments, Dublin, Ireland).

Data were acquired on a Hewlett PackardMSworkstation,with HP5970A (Version 3.1) software, which was inter-faced to the MSD. The instrument was operated in selec-tive ion monitoring (SIM) mode, monitoring the two iso-topes of carbon, at 12 and 13m/z, and chlorine at 35 and37 m/zwith a dwell time of 100 milliseconds.

2.2 Low pressuremicrowave plasma

An inductively coupled plasma mass spectrometer (ICP-MS, VG PQ2, VG Elemental, Winsford, UK) was con-verted to accept a microwave induced plasma (MIP) ionsource. This was achieved by replacing the RF power sup-ply and torch box with a 2115 MHz, 1500 W AstexAXmicrowave generator and a surfatron plasma cavity with arectangular waveguide. Extra pumping was applied to thesampling interface by attaching an extra vacuum pump(Edwards ES2000) to the existing vacuum line via a throt-tle valve (Edwards High Vacuum, Crawly, Sussex, UK).The low pressure MIP torch was identical to that used forthe low pressure ICP, except that the torch was positionedin the microwave discharge cavity rather than the RF loadcoil. Helium plasma gas (99.99% Air Products PLC,Crewe, Cheshire, UK) was supplied to the torch by a20 L min– 1 mass flow controller (Tylan). Typical operatingconditions are shown in Table 1.

2.3 Gas chromatograph

Two different gas chromatographs were interfaced withthe low pressure torches.

The low pressure argon MIP and argon ICP were inter-faced with a Hewlett Packard 5890 GC fitted with an split/splitless injector, cryogenic oven control and a DB10.32 mm630 m capillary column with a 0.1 lm film thick-ness (J & W, Merck, UK). The low pressure helium ICPwas interfaced with a PU 4550 GC (Pye Unicam, Cam-bridge, UK) fitted with an on-column injector and a DB50.32 mm630 m capillary column with a 0.1 lm film thick-ness (J & W, Merck, UK). The interface was via a heatedtransfer line maintained at a constant temperature of2508C. The capillary column was passed through theheated transfer line and into the low pressure torch, the

J. Sep. Sci. 2002, 25, 839–846 Screening VOCs in food 841

vacuum seal being made using a combination of Ultra-torrand Swagelock fittings. This configuration has previouslybeen described in more detail [5]. The injection volumewas 1 lL in all cases. Operating conditions and tempera-ture programs are shown in Table 1.

3 Results and discussion

3.1 Low pressuremicrowave plasma

The plasma gas flow rate and power were optimised toobtain maximum analyte signal for 35Cl. Increasing thegas flow rate from 0.7 to 1.6 L min– 1, at 60 W power,caused a decrease in analyte signal, and below0.7 L min– 1 the plasma was unstable and extinguishedwhen the analyte solvent eluted from the column. Forwardpower (at a gas flow of 0.7 L min– 1) had little effect on the35Cl ion analyte signal though a maximum was observedat 60 W. Hence, the plasma was operated at a gas flowrate of 0.7 L min– 1 and a power of 60 W.

A selected ion chromatogram for atm/z 35, 37, and 13 fora 25 ng on-column injection of 1,2-dichlorobenzene,1,2,3-trichlorobenzene, and trichlorophenol is shown inFigure 1. It was necessary to include a 2 minute delay,prior to ramping the oven temperature, in order to allowthe solvent time to separate from the analyte compounds.When the delay was not included the solvent memoryeffect caused an extremely high carbon background sig-nal at m/z 13 and masked the peaks of the early elutingcompounds. While this was possible for all the semivola-tile compounds the volatile compounds still eluted tooquickly so that detection at m/z 13 was not possible. It

should be noted that this problem did not arise when mon-itoring the chlorine isotopes at m/z 35 and 37, and nodelay is necessary if only Cl-selective detection isrequired. Figures of merit for Cl-selective detection atm/z 35 are shown in Table 2. Detection limits for C-selec-tive detection were extremely poor because, even aftercomplete elution of the solvent, the carbon backgroundwas considerably higher than the background observedfor the chlorine isotopes. This was attributed to back-streaming of organic vapours from the rotary vacuumpumps used to evacuate the expansion interface of theinstrument, and possibly residual solvent contamination inthe torch. In an effort to remove this background a fore-line vacuum trap was placed between the vacuum pumpsand the expansion interface. The fore-line trap consistedof a liquid nitrogen cold finger made from stainless steelwhich could be interfaced to the vacuum line by KF 40 andKF 50 vacuum flanges. The cold finger was kept topped

Table 1. Operating conditions for the various low pressure plasma source mass spectrometers used.

Ar LP-ICP-MS He LP-ICP-MS He LP-MIP-MS

Gas chromatograph Hewlett Packard 5890 Pye Unicam PU 4550 Hewlett Packard 5890Injector Cryogenic split/splitless On-column Split/splitlessColumn DB1 30m60.32 mm DB5 30m60.32 mm DB1 30m60.32 mm

0.1 lm film 0.1 lm film 0.1 lm filmHead pressure/kPa 30 30 30Oven conditions 08C for 3 min, to 308C for 2 min, to 308C for 2 min, to

2208C@ 208 min– 1 1808C @ 208min– 1 1808C @ 208 min– 1

Plasma-MSForward power/W 100 6 60Reflected power/W 8 0 2Plasma gas/L min – 1 0.45 Helium, 0.7Torch pressure/mbar not measured 0.3 2.0Expansion pressure/mbar 0.3 0.04 0.4Intermediate pressure/mbar a10– 4 n/a a10– 4

Analyser pressure/mbar 4.0610– 7 mbar a10– 7 3.5610– 6

Selected ions m/z 13, 35, 37 13, 35, 37 13, 35, 37Dwell time/s 0.25 n/a 0.25Solvent delay/min 0 3 3

n/a not applicable

Figure 1. Selected ion chromatogram for a 25 ng on-columninjection of chlorinated standards by helium GC-LP-MIP-MS.

842 O’Connor, Rowland, Evans J. Sep. Sci. 2002, 25, 839–846

up with liquid nitrogen while the instrument was operating.Along with the fore-line trap ultra-high purity helium waspurchased (GC+ grade, Air Products) to reduce thechance of carbon contamination from poor quality helium.The use of the high purity gas and fore-line trap made nosignificant difference to the Cl-selective detection limits atm/z 35, though the coefficient of variation on repeat injec-tions was reduced from an average of 9.5% to 2.9%. Thebackground carbon signal at m/z 12 and 13 was still toohigh to enable detection of the analyte compounds below10 ng on-column.

3.2 Low pressure inductively coupled plasma

3.2.1 Helium plasma

The plasma gas flow rate and operating power were indivi-dually optimised. The gas flow was ramped from 3 to15 mL min– 1, while the power was kept stable at 6 W, andthe signal for a 1 ng on-column injection of hexachloro-benzene was monitored. Three repeat injections wereperformed at each flow rate. The peak area of each injec-

tion was obtained by integration and the average peakarea was plotted against plasma flow rate. The optimisa-tion indicated that a 3 mL min– 1 plasma gas flow was theoptimum yielding an overall helium plasma flow, includingthe GC column gas flow, of 6 mL min– 1. The maximumsignal was observed at 6 W forward power. Figures ofmerit for the LP-ICP-MS obtained using these optimisedconditions are shown in Table 3. The figures of meritreported are based on the integrated peak areas of the35Cl ion as this gave the most intense signal. The 12C sig-nals only became visible on injecting 500 ng on-column,due to a large background caused by the solvent. The 12Csignal increased with oven temperature, suggesting thatcolumn bleed also contributed to the large carbon back-ground.

3.2.2 Argon plasma

Plasma gas flow rate and operating power were individu-ally optimised. The gas flow was ramped from 0.35 to0.65 L min– 1, while the power was kept stable at 100 W,

Table 2. Analytical figures of merit obtained for Cl-selective detection at m/z 35 for a 0.7 L min – 1, 60 W helium LP-MIP.

Compound Limit of detectiona) Slope R 2 Linear range Coefficient of(ng on-column) (counts/ng) studied (ng) variation %

Hexachlorobenzene 0.059 12394 0.9992 0.1 to 100 10.01,2,4-Trichlorobenzene 0.036 8201 0.9994 0.1 to 100 8.41,2-Dichlorobenzene 0.031 6382 0.9997 0.1 to 100 6.51,4-Dichlorobenzene 0.070 6328 0.9968 0.1 to 100 10.8Pentachlorobenzene 0.072 16982 0.9984 0.1 to 100 10.81,2,3-Trichlorobenzene 0.051 9677 0.9968 0.1 to 100 8.22,4,6-Trichlorophenol 0.301 2331 0.9986 0.5 to 100 12.0

a) LOD = 3r/slope.

Table 3. Analytical figures of merit obtained for Cl-selective detection at m/z 35 for a 6 mL min– 1, 6 W helium, LP-ICP.

Compound Limit of detectiona) Slope R 2 Linear range(ng on-column) (counts/ng) studied (ng)

Carbon tetrachloride 0.408 2629 0.9994 0.5 to 500Hexachlorobenzene 0.156 108969 0.999 0.1 to 100Tetrachloroethylene 0.105 38424 1.00 0.1 to 5001,2,4-Trichlorobenzene 0.481 6286 1.00 0.5 to 5001,2-Dichlorobenzene 0.317 9021 0.9972 1 to 5001,4-Dichlorobenzene 0.072 12681 0.9996 0.5 to 500Pentachlorobenzene 0.091 30133 0.9975 0.5 to 5001,2,3-Trichlorobenzene 0.729 11437 0.9994 0.5 to 5001,3,5-Trichlorobenzene 0.150 12965 0.9992 0.5 to 500Trichloroethylene 0.546 890 0.9822 1 to 100Chloroform 0.451 1561 0.9981 1 to 1001,2-Dichloroethane 4b) 1128 0.9997 5 to 1001,1,1-Trichloroethane 1b) 879 0.9810 1 to 1002,4,6-Trichlorophenol 0.200 12413 0.9927 1 to 100

a) LOD = 3r/slope.b) Compounds eluted close to solvent front.

J. Sep. Sci. 2002, 25, 839–846 Screening VOCs in food 843

and the signal for a 1 ng on-column injection of penta-chlorobenzene was monitored. The plasma power wasoptimised within the range 90 to 140 W. Optimum condi-tions are shown in Table 1. The figures of merit for theargon LP-ICP-MS are shown in Table 4 and Table 5 forCl-selective and C-selective detection at m/z 35 and 13respectively.

Table 5 does not show figures of merit for the three mostvolatile compounds because the 13C signal was maskedby the solvent, despite the fact that a cryogenic on-columninjector was used which allowed the injection temperatureto be lowered to 08C. Nonane and octane were also inves-tigated as possible solvents. These eluted after the ana-lyte compounds. However, they left large carbon depositson the inside surface of the torch which led to increasedcarbon background for repeat injections.

A Cl-selective chromatogram of selected volatile andsemivolatile standard is shown in Figure 2. The relation-ship between the number of chlorine atoms and signalintensity for equimolar concentrations of the differentchlorinated compounds is shown in Figure 3. The linearresponse indicates that the instrumental response was

independent of the nature of the molecule, which high-lights one of the advantages of using atomic spectrometricdetection, namely that calibration is possible by injection

Table 4. Analytical figures of merit obtained for Cl-selective detection at m/z 35 for a 0.45 L min – 1, 100 W argon LP-ICP.

Compound LODa) Slope R 2 LDR(ng on-column) (counts/ng) studied (ng)

Carbon tetrachloride 0.045 60894 0.9999 0.1 to 15Hexachlorobenzene 0.019 560925 0.9987 0.05 to 10Tetrachloroethylene 0.206 70394 0.9993 0.06 to 101,2,4-Trichlorobenzene 0.011 384880 1.00 0.05 to 101,2-Dichlorobenzene 0.012 323007 1.0000 0.05 to 101,4-Dichlorobenzene 0.010 335732 0.9999 0.05 to 10Pentachlorobenzene 0.014 488238 1.0000 0.05 to 101,2,3-Trichlorobenzene 0.007 456372 1.0000 0.05 to 101,3,5-Trichlorobenzene 0.013 356577 1.0000 0.05 to 10Trichloroethylene 0.141 37125 0.9981 0.2 to 16Chloroform 0.046 40420 0.9998 0.2 to 321,2-Dichloroethane 0.0001 33753 1.0000 0.05 to 101,1,1-Trichloroethane 0.127 45644 0.9984 0.05 to 102,4,6-Trichlorophenol 0.017 273650 0.9976 0.05 to 10

a) LOD = 3r/slope

Table 5. Analytical figures of merit obtained for C-selective detection at m/z 13 for a 0.45 L min – 1, 100 W argon LP-ICP.

Compound Limit of detectiona) Slope R 2 Linear range(ng on-column) (counts/ng) studied (ng)

1,2,4-Trichlorobenzene 0.092 80273 0.9998 0.5 to 101,2-Dichlorobenzene 0.527 92500 0.9997 0.5 to 101,4-Dichlorobenzene 0.487 98345 0.9969 0.6 to 13Pentachlorobenzene 0.915 52213 0.9996 0.5 to 101,2,3-Trichlorobenzene 0.692 89125 0.9989 0.6 to 101,3,5-Trichlorobenzene 1.2 74926 0.9994 0.5 to 102,4,6-Trichlorophenol 7.8 44277 0.9978 5 to 50

a) LOD = 3r/slope.

Figure 2. Selected ion chromatogram of a 0.5 ng on-columncryogenic injection of a mixture of chlorinated compoundsmonitored at m/z 35, using 100 W Ar LP-ICP-MS, GC condi-tions 08C for 3 min to 2208C @ 208 min– 1: a, trichloro-methane; b, dichloroethane; c, trichloroethane; d, carbon tet-rachloride; e, trichloroethylene; f, tetrachloroethylene; g, 1,4dichlorobenzene; h, 1,2 dichlorobenzene; i, 1,3,5-trichloro-benzene; j, 1,2,4-trichlorobenzene; k, 1,2,3-trichloroben-zene; l, trichlorophenol; m, pentachloronezene; n, hexa-chlorobenzene.

844 O’Connor, Rowland, Evans J. Sep. Sci. 2002, 25, 839–846

of a single chlorinated standard. A plot of the ratio of peakarea, atm/z 35 and 13, versus the known Cl/C ratio for thechlorinated compounds is shown in Figure 4, indicatingthat the determination of empirical formulae is possibleusing this detection method.

3.3 Comparison of plasma sources

A comparison of sensitivity and limit of detection for Cl-selective detection obtained using the three types ofplasma source is shown in Figure 5. It is evident that thelowest limits of detection were obtained for the 100 W ArLP-ICP (Figure 5.a), primarily because the highest sensi-tivity was obtained using this plasma (Figure 5.b). It isgenerally assumed that helium plasmas yield lower detec-tion limits for the halogens due to enhanced ionisationleading to higher sensitivity. However, in this case highersensitivity was obtained using an argon plasma (Figure 5).There was very little difference in sensitivity between the6 W He ICP and the 60 W He MIP, so it may be that thereis a critical power density necessary for efficient ionisationregardless of the gas used. This is probably related to thephenomenon whereby plasmas, which are formed by an

induced field at low pressure, operate as capacitive dis-charges at low power rather than as inductive discharges.When the power is increased to a particular level, depend-ing on pressure and plasma gas, the induced field takesover and the power density increases making moreenergy available for ionisation [11]. Much higher sensitiv-ities (ca. 100,000 counts ng– 1) for Cl-specific detectionhave previously been achieved using a 100 WHe LP-ICP-MS system [6], which seems to confirm this view. Detec-tion limits were also much lower in this previous study,caused by a combination of higher sensitivity and lowerbackground.

3.4 Analysis of oil samples

Based on the figures of merit, the 100 W Ar LP-ICP wasidentified as the most promising technique for furtherinvestigation; hence, a series of spiked olive oil sampleswere analysed using GC-LP-ICP-MS and the operatingconditions shown in Table 1. The compositions of thespiked sample are shown in Table 6.

Figure 3. Relationship between the number of chlorineatoms and signal intensity for equimolar concentrations ofdifferent chlorinated compounds in a mixed standardobtained using 100 W Ar LP-ICP.

Figure 4. Ratio of peak area, at m/z 35 and 13, versus theknown Cl/C ratio for the chlorinated compounds. Figure 5. Comparison of (a) detection limit and (b) sensitivity

for chlorinated compounds at m/z 35 using the three differentplasma types.

J. Sep. Sci. 2002, 25, 839–846 Screening VOCs in food 845

Selected ion chromatograms atm/z 35 and 13, for a 10 lLheadspace sample of spiked oil “a” (volatile compounds)taken at room temperature and injected on-column, areshown in Figure 6. The equivalent set of chromatogramsfor a 50 lL headspace sample of spiked oil “b” (semi-vola-tile compounds) taken at 1508C are shown in Figure 7.

There was no solvent present for this analysis so it waspossible to monitor the 13C, 35Cl, and 37Cl ions for both thevolatile and semi-volatile analytes. The background atm/z 13 was elevated compared to that of them/z 35. How-ever, all the analyte compounds, and some unknown con-taminants, were observed using C-selective detection inthis case. Identification of the analyte peaks in the chro-matogram was achieved by comparison with the Cl-selec-tive chromatogram atm/z 35.

4 Conclusions

A number of ion sources coupled to GC and mass spec-trometry have been evaluated for the analysis of volatileorganic compounds, namely a low pressure pressurehelium MIP, and low pressure helium and argon ICPs.The most promising of these was the argon LP-ICP-MS,which was effective for the detection of chlorinated com-pounds by Cl-selective and C-selective detection, thoughoverlap of the solvent tail with early eluting compoundsstill remains a problem in the latter case. The use of theargon LP-ICP-MS showed great promise for headspaceanalysis of volatile organic compounds in oil and the abilityof the instrument to provide empirical formulae also lookspromising.

Table 6. Analyte contents of spiked olive oil samples.

Sample “a”Head space 10 lL 228CWeight of olive oil 100.038 gCarbon tetrachloride 10 lLTetrachloroethylene 10 lLTrichloroethylene 10 lLTrichloromethane 10 lLTrichloroethane 10 lLDichloroethane 10 lL

Sample “b”Head space 50 lL 1508C for 5 minWeight of olive oil 100.011 g1,2-Dichlorobenzene 10 lL1,3-Dichlorobenzene 10 lL1,4-Dichlorobenzene 12.6 mg1,2,3-Trichlorobenzene 17.9 mg1,2,4-Trichlorobenzene 10 lL1,3,5-Trichlorobenzene 9.7 mgPentachlorobenzene 12.5 mgHexachlorobenzene 10.7 mg

Figure 6. Selected ion chromatograms for 10 lL headspaceof spiked oil sample “a” monitored at (a) m/z 35; (b) m/z 13.GC conditions 08C for 3 min to 2208C @ 208min– 1.

Figure 7. Selected ion chromatograms for 50 lL headspaceof spiked oil sample “b” monitored at (a) m/z 35; (b) m/z 13.GC conditions 08C for 3 min to 2208C @ 20 8min– 1.

846 O’Connor, Rowland, Evans J. Sep. Sci. 2002, 25, 839–846

Acknowledgements

GOC would like to thank the Ministry of Agriculture Fish-eries and Food for provision of postdoctoral funding.

References

[1] N.P. Vela, L.K. Olson, J.A. Caruso, Anal. Chem. 1993,65, 585.

[2] P.C. Uden, J. Chrom. A 1995, 703, 393.

[3] E.H. Evans, J.J. Giglio, T.M. Castillano, J.A. Caruso,Inductively Coupled and Microwave Induced PlasmaSources for Mass Spectrometry, The Royal Society ofChemistry, Cambridge, 1995.

[4] K. Sutton, R.M.C. Sutton, J.A. Caruso, J. Chromatogra-phy A 1997, 789, 85.

[5] E.H. Evans, W. Pretorius, L. Ebdon, S. Rowland, Anal.Chem. 1994, 66, 3400.

[6] T.M. Castillano, J.J. Giglio, E.H. Evans, J.A. Caruso, J.Anal. At. Spectrom. 1994, 9, 1335.

[7] G. O’Connor, L. Ebdon, E.H. Evans, J. Anal. At. Spec-trom. 1997, 12, 1263.

[8] L.K. Olson, J.A. Caruso, Spectrochim. Acta. 1994, 49B,7.

[9] H. Suyani, J. Creed, J.A. Caruso, J.A., R.D. Satzger, J.Anal. At. Spectrom. 1989, 4, 777.

[10] G. O’Connor, L. Ebdon, E.H. Evans, H. Ding, L.K. Olson,J.A. Caruso, J. Anal. At. Spectrom. 1996, 11, 1151.

[11] P.A. Miller, G.A. Hebner, K.E. Greenberg, P.D. Pochan,B.P. Aragon, J. Res. Natl. Inst. Stand. Technol. 1995,100, 427.

[JSS 1210]