GC/MS gets out in the field

In situations such as an oil spill there are many emergency protocols that spring into action – and now a new one is set to monitor the immediate after effects of sudden toxin release. Amy Li discusses new technology allowing rapid monitoring of nitrobenzene levels

Nitrobenzene [C₆H₅NO₂] is an important synthesised chemical that is primarily used in industrial processes as a chemical intermediary for the production of aniline¹. It may also be produced in the environment by the nitration of benzene, an air pollutant¹. It is an extremely toxic chemical that is harmful to human health – in the UK it is classified by the Environment Agency as a hazardous volatile organic compound (VOC), and by the Health and Safety Executive as toxic and dangerous to the environment². Nitrobenzene is also one of the target compounds in the US Environment Protection Agency (EPA) Method 625³. It may be discharged into the water at low levels by industrial production processes, resulting in the pollution of surface water and groundwater. The annual release of nitrobenzene to the environment by US industries has been estimated at over 900,000 pounds¹. With industrial development and increasing discharge of industrial wastewater, determination of nitrobenzene has become one of the most important monitoring targets for major rivers in the world.

Perhaps of more concern with regards to environmental impact is the possibility of urgent water pollution cases caused by petrochemical plant explosion or oil spill accidents. The 2005 Jilin chemical plant explosion in China resulted in an estimated 100 tonnes of pollutants benzene and nitrobenzene entering the nearby Songhua River, with nitrobenzene concentrations in river water measured at 33 times higher than permissible levels, and the water supply in nearby cities was shut down as a safety measure^4,5. Therefore, emergency nitrobenzene monitoring with transportable instruments remains an urgent requirement in environmental and public health sectors. Tests for benzene and nitrobenzene are typically conducted using gas chromatography-mass spectrometry (GC/MS); however the biggest challenge for field-transportable GC/MS systems in the past has been method performance with sensitivity and accuracy comparable to that obtained from stationary laboratory instruments.

The 5975T LTM GC/MSD (Agilent Technologies) has been specifically created for portability and deployment in the field by addressing some of the critical factors limiting use of GC/MS in the field:

•    Use of proprietary Low Thermal Mass GC Technology (LTM) resistively-heated column module in place of the typical air bath oven, to reduce size, lower power consumption and enhance transportability. LTM technology also allows for faster GC analysis and higher sample throughput. By providing dramatically faster temperature ramp rates, LTM technology shortens GC cycle times, and makes it easier to deal with the varied and sometimes difficult analytical challenges faced in the field.

•    Incorporation of an anti-vibration base for safer transport and design for easy installation or exchange of columns while on site.

•    Power consumption is only 50% of a comparable benchtop GC/MS system and the footprint is one-third smaller, meaning it requires less energy to run and takes up less space in the mobile laboratory.

•    Vacuum-keeping technology keeps the system in a vacuum state even after the system is turned off. This allows the instrument to configure in less time, saving around an hour when compared to conventional systems.

•    Specialist software (Agilent MSD Productivity ChemStation and Deconvolution Reporting Software) suitable for non-expert use, as trained operatives may not be available in the field. For field operation the unit is controlled with the data system software installed on a laptop connected by a LAN cable.

•    The use of a standard high performance quadrupole mass spectrometer provides the same mass spectral performance in a transportable instrument with data that is fully compatible with standard reference library data over a wide range of concentrations. This is especially critical for the analysis of unknown chemicals that will be introduced to the GC/MS in unknown concentrations.

A recent application report for the 5975T LTM GC/MSD has outlined a fast method for determining nitrobenzene in water following a simple sample extraction procedure6. The experiment proved that this method is accurate, rapid and sensitive enough for determining nitrobenzene in water at low ppm levels, and can be applied in an emergency monitoring situation.

The experimental method used a narrow bore (0.18mm id) column. LTM column technology provides fast heating and cooling capability, reducing the run time to four minutes. In this method, the cool down time (from 250°C to 60°C) was no more than one minute. The machine was calibrated at five different concentration levels (0.1, 0.5, 1, 5 and 10µg/mL) of the standard solution. The resultant calibration curve showed a wide range of linearity, with r2=0.9998. The calibration curve is shown in Figure 1.

The method detection limit (MDL) was used to assess the sensitivity of the method. The MDL is defined as the minimum concentration of a substance that can be measured and reported with 99% confidence that the value is above zero3. The MDL of nitrobenzene was 0.017 µg/mL using this approach.

Repeatability was tested by measuring two different concentration levels of nitrobenzene standard solutions, each with 10 runs. The relative standard deviations (RSD) of MSD responses were calculated and used in Table 1.

Recovery and concentration of nitrobenzene in water were calculated using the following formulae:

Recovery (%) = (conc of spiked sample–conc of unspiked sample) × 100
                               conc added (2.5µg/mL)

Conc of nitrobenzene in water (µg/mL) = calculated conc by calibration curve × volume of solvent (2mL)
                    volume of water (50mL)

Sample preparation and recovery used a running water sample – 50mL of running water sample was placed in the 5975T LTM GC/MSD separating funnel and 2mL of methylene chloride were added. The mixture was manually agitated for one minute to extract nitrobenzene into the methylene chloride layer and the methylene chloride layer was collected. A 1-µL volume of collected sample solution was injected into the 5975T LTM GC/MSD. The resulting chromatogram is shown in Figure 2. A 2mL of 2.5-µg/mL amount of standard solution was spiked into 50 mL water to test the recovery. The calculated recovery was 108%, and the resultant chromatogram is shown in Figure 3.

The described experiment demonstrates a simple and fast method for the determination of nitrobenzene in water using the 5975T LTM GC/MSD with a narrow-bore LTM column and easily operable sample preparation. This solution was developed for field emergency use with five minutes of injection-to-injection cycle time. The method performance shows sensitivity and accuracy comparable to that obtained from stationary laboratory instruments.

Author: Amy Li, Agilent Technologies
Amy is a product specialist from Agilent Technologies Shanghai responsible for application support on GC and GC/MS product.