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Abstract

A retention time locked (RTL) method for the determination of trace level volatile organic

compounds (VOCs) in environmental waters was developed on the Agilent 7697A

Headspace and 7890B/5977A GC/MS. Analysis on the GC/MS was performed in

simultaneous Scan/SIM mode. This method can determine 66 VOCs over a calibration

range of 1 – 20 µg/L with some compounds being detectable at lower levels.

This method is therefore suitable for routine analysis, as part of a monitoring strategy

that includes volatile organic compounds, as required by the Water Framework Directive

(2000/60/EC) and detailed in the update to Annex I: Environmental Quality Standards for

Priority Substances and certain other Pollutants [1].

ENVIRONMENTAL ANALYSIS

ANALYSIS OF VOLATILE ORGANIC COMPOUNDS IN

ENVIRONMENTAL WATERS USING THE AGILENT 7697A

HEADSPACE AND 7890B/5977A GC/MS

Solution Note

Author

Bernhard Rothweiler

Agilent Technologies

Waldbronn, Germany

Introduction

The analysis of volatile organic compounds (VOCs) in environmental water samples is

usually performed by either headspace (HS) or purge and trap (P&T), with separation

by gas chromatography (GC) and detection by mass spectrometry (MS).

The P&T technique uses a continuous ﬂ ow of carrier gas to purge any volatiles from the

matrix onto an adsorbant trap. The trap is then heated, releasing the adsorbed compounds

onto the GC/MS for analysis. This technique offers very good sensitivity as the sample

is exhaustively extracted. However, P&T systems are more complicated to run and

maintain compared to HS systems. They can also suffer from water

carryover problems which can lead to reduced sensitivity, loss of peak

shape and cross-contamination. Environmental samples containing

detergents can also suffer from foam formation issues.

The other approach is the HS technique, which uses a closed sample container and a sampling system. The water sample in the sealed vial is

heated and agitated to help drive the volatiles from the sample matrix and into the gaseous headspace. An equilibrium is formed between the

two phases. This equilibrium can be shifted by the addition of salt to the sample. After a speciﬁed time, a portion of the headspace is transferred

onto the GC/MS via a valve with a sample loop. This technique is robust and experiences few carryover problems as less water is transferred to

the GC/MS.

This application note details the use of an Agilent 7697A HS coupled to a 7890B/5977A GC/MS for the analysis of VOCs in environmental

waters. The MS system features a new inert source with an extractor lens, which provides additional focus to the ion beam into the mass

analyser, resulting in a signiﬁcant increase in the number of ions analysed and better sensitivity of the instrument. The GC/MS was run in

Scan/SIM mode and the data analysis was accomplished using MassHunter software.

Analytical Technique

Sample Preparation

Sample Preparation: 10 mL of sample and 2 g of salt were added

to each vial. An aliquot of Stock Internal Standard (prepared in

Methanol) was spiked into the solution and the vial sealed.

Standard Preparation: 10 mL water and 2 g of salt were added to

each vial. An aliquot of Stock Standard (prepared in Methanol) and

Stock Internal Standard (prepared in Methanol) were spiked into the

solution and the vial sealed. Standards were prepared at 1, 5, 10 and

20 µg/L, with 66 analytes and 3 internal standards at each level. Two

sets of standards were prepared and run at the beginning and end of

the sequence.

The Internal Standards used were D4- 1,2-Dichloroethane

(CAS 17060-07-0), D8-Toluene (CAS 2037-26-5) and

4-Bromoﬂuorobenzene (CAS 460-00-4). The method was retention

time locked to D8-Toluene at 11.890 minutes.

Instrumentation

Agilent 7697A Headspace Sampler, Agilent 7890B Gas

Chromatograph with an Agilent VF-624 MS 60m capillary column

and an Agilent 5977A Mass Selective Detector with MassHunter

workstation.

Results and Discussion

The method was developed using the 10 µg/L standard solution.

The method is run in Scan/SIM mode, where selective ion

monitoring (SIM) is used for quantitation and Scan could be used for

identiﬁcation purposes if necessary. To maximise sensitivity the run

has been divided into 16 SIM groups, with 2 ions for each compound,

one for quantitation, the other for qualitation. Figure 1 shows the

Scan (Total Ion Chromatogram, TIC) and SIM traces for the 10 µg/L

standard and Figure 2 the Scan and SIM traces for the 1 µg/L

standard.

Calibration standards at 20, 10, 5 and 1 µg/L were analysed at the

beginning and end of each sequence. Figures 3 and 4 show typical

8-point calibrations for bromochloromethane and 1,2-Dichloroethane.

All VOCs could be detected in the 1 µg/L standard. Some compounds

were even visible in the blanks, mostly at very low levels, indicating

that the detection limits for some of the VOCs may even be below the

0.1 µg/L level.

A spiked surface water sample at the 1 µg/L level was prepared and

analysed to assess reproducibility. Table 1 shows the relative standard

deviation data for this solution (total number of samples = 5). With

the exception of Vinyl chloride, all of the compounds tested for had

RSDs of less than 3% at this level.