Comparison of chromatographic measuring methods for PAH analysis

Status:

completed 10/2021

Aims:

Polycyclic aromatic hydrocarbons (PAHs) are formed during various thermal processes and can appear in a wide range of workplaces (e.g. coking plants, tar refineries, during the production of refractory materials). The majority of PAHs are classified as (potentially) carcinogenic. For this reason, the MGU risk assessment measuring system of the German Social Accident Insurance Institutions includes the measurement of PAHs in air samples as a standard measurement method. In addition to the marker substance benzo[a]pyrene (BaP), a wide range of other PAHs are routinely studied, including the 16 PAHs that were classified as priority pollutants by the Environmental Protection Agency (EPA). The carcinogenicity of the various PAHs can differ significantly. Recent studies indicate that higher-molecular-weight PAHs in particular, those with 6-ring or 7-ring structures, have a higher carcinogenic potential than the marker substance BaP. These types of PAHs are currently not analysed in the Institute for Occupational Safety and Health of the German Social Accident Insurance (IFA).

The possibility of expanding the standard measurement method in the MGU should be reviewed. To do so, a gas chromatography (GC) method and a high-performance liquid chromatography (HPLC) method will be compared in order to determine the optimal analytical technique for measuring both low-molecular-weight PAHs and high-molecular-weight PAHs. The expanded method should be adopted as a standard measurement method in the MGU.

Activities/Methods:

As part of a Master’s thesis, the functionality of the measurement system will first be assessed on the basis of preliminary tests utilising PAH standards.

During the next stage, the measurement methods of the two analytical systems for a mixed sample from the EPA-PAH list will be optimised to enable optimal separation of the individual substances along with the highest possible level of sensitivity. Identical samples will be used for both analytical techniques (GC and HPLC). As part of the method validation process, measurement parameters such as the limit of detection (LOD), limit of quantification (LOQ) and recovery rate are determined. During the next stage, the selection of analytes should be expanded to include alkylated PAHs and higher-molecular-weight PAHs. A method validation process is also carried out separately for both analytical techniques at the stage.

After this, a statistical evaluation of the measurement results is carried out for both analytical techniques in order to compare them. The method that is suitable for both the previously analysed PAHs and the higher-molecular-weight PAHs will be selected.

Results:

The preliminary tests confirmed that both measurement techniques were suitable for the task at hand. Both methods were optimised to enable isolation of all PAHs – both those from the EPA-PAH list and the additionally selected substances. As a result, it was possible to separate all 34 substances and four internal standards using the GC method, while the HPLC method was able to separate 32 substances and three internal standards. The measurements required for validation were carried out using a standard sample containing all 34 PAHs.

The results demonstrated that the GC method has a better level of sensitivity and a better concentration range. With the HPLC method, the concentration range is significantly smaller due to the fluorescence detector used and reaches its maximum at 100 ng/ml. As a result, the sample has to be diluted in the event of higher concentrations. No significant differences were observed with regards to the quality of the calibration. As suspected, there were differences between the two measurement systems with regard to the analysis of higher-molecular-weight PAHs. These were more difficult to analyse than lower-molecular-weight PAHs using the GC method. The advantages of the methods are reversed here, as the HPLC method is better at analysing higher-molecular-weight PAHs. The biggest disadvantage here was the very long time (~2 hours) needed to achieve good separation of these PAHs using the GC method. Using the HPLC method, it was possible to separate and analyse these PAHs in half the time.

Both methods delivered excellent results; the GC method may be preferable for lower-molecular-weight PAHs in particular due to the lower limit of detection, however the measurement times are significantly longer when this method is used with highly condensed samples. This study demonstrated that both methods currently meet the requirements for measurement methods. If the acceptable concentration for benzo[a]pyrene (BaP) is further reduced, a review should be carried out to assess whether a switchover to the more sensitive gas chromatography method is possible. However, the HPLC method is preferable when ring structures with a higher molecular weight need to be considered during the assessment of PAHs in workplaces.

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