Food & Beverage Analysis


The automation of vacuum assisted headspace solid phase microextraction (Vac-HS-SPME) and its application for analysis of volatiles in food


Kathy Ridgway, Element Lab Solutions, kathy.ridgway@element.com


Headspace solid phase microextraction (HS-SPME) is a well-established technique for the determination of volatiles in food. It is a solvent free sample preparation approach that can provide extraction and enrichment of analytes from a variety of matrices.


The rate-limiting step is the analyte transfer from the matrix, which means that for semi-volatile analytes methods can require long extraction times and/or high extraction temperatures to obtain the required sensitivity. For some food applications it is preferred to keep temperatures lower, to avoid formation of additional compounds, for example, due to Maillard reactions. Applying vacuum during HS-SPME has been shown to improve extraction kinetics for some semi-volatile sample components, resulting in higher extraction effi ciencies and analyte sensitivity, with shorter sampling times and at milder sampling temperatures.


Publications to date have used manual application of vacuum, offl ine, followed by analysis on automated HS-SPME-GC-MS instrumentation. Automating the entire procedure will bring signifi cant benefi ts, including speed, throughput, and the ability to run the system unattended.


Theoretical aspects


HS-SPME involves three partitions: sample matrix, sample headspace and fi bre. This results in two concurrent thermodynamic processes and therefore two partition coeffi cients are involved in achieving a fi nal equilibration. However, the majority of HS- SPME methods do not wait for the system to achieve the equilibrium state and typically extraction is performed for a defi ned period of time. In this scenario, the analytical performance depends more on the kinetics associated with the mass transfer of the analyte into the sample headspace.


Bias has been observed in HS-SPME methods towards the more volatile analytes, which have a higher affi nity for the vapour phase therefore transition more readily into the headspace and subsequently onto the fi bre. Common approaches employed to speed up the mass transfer kinetics for the semi-volatile components are the use of agitation or an increase in the temperature of extraction.


An additional or alternative way of accelerating extraction kinetics for semi-volatiles in HS-SPME is applying vacuum conditions, performing vacuum-assisted HS-SPME (Vac- HS-SPME).


The advantages of performing Vacuum Assisted Headspace solid phase microextraction (Vac-HS-SPME), compared to atmospheric pressure headspace solid phase microextraction ( AP-HS-SPME) have been described extensively in the literature [1- 7].


Figure 1 shows how Vac-HS-SPME gives faster extraction times at a given sampling temperature, or increased sensitivity at reduced sampling temperatures when compared to atmospheric pressure HS-SPME (AP-HS-SPME).


For aqueous based samples, the effect of vacuum can be related to Henries Law constant – relating to the partition between the water and gas phase. The reduced pressure will have the most impact in terms of improved extraction effi ciencies for those compounds with a KH value less than 1.6 x 10-4 atm m3 mol-1.


Experimental


Compared to standard HS-SPME, the only additional step required for Vac-HS-SPME is the application of vacuum to the sample container. When performed manually this can be achieved using a gastight syringe or a vacuum pump with tubing and Luer Lock attachment. The vacuum is maintained during sampling by the use of custom-made vial closures which can be used with standard SPME headspace vials (Figure 2).


In order to automate the evacuation process, a GERSTEL robotic multipurpose sampler was used with a selection of modules that were modifi ed and tested to compare to the manual process. A Dual head Robotic/ robotic Pro GERSTEL MPS was used with a Preparation Syringe Module (PSM) fi tted with a 2.5ml syringe (no needle) and a SPME tool for sample extraction/introduction. Incubation was performed in a GERSTEL cooled Agitator. Evacuation was achieved using a modifi ed purge tool connected to a vacuum pump with multiple headspace extraction (MHE) station. Details of this and how it is controlled in the GERSTEL Maestro software are detailed in Application note Element Lab Solutions [8].


Figure 2: Extractech vacuum closures for Vac-HS-SPME. Evaluation of food volatiles test mixture


Following testing of the automated evacuation procedure, experiments were conducted on a custom-made test mix to assess the performance compared to atmospheric pressure headspace SPME (AP-HS-SPME). Compounds chosen are detailed in Table 1.


Table 1: Analytes included in custom test mixture to assess performances on Vac-HS-SPME # NAME


CAS NUMBER


1 2 3 4 5 6 7


8 9


2-pentanone 2-pentanol


2,5-dimethylpyrazine Ethyl hexanoate 2-nonanone


Ethyl octanoate Whiskey Lactone


Ethyl decanoate Delta Deca lactone


10 Ethyl tetra decanoate 11 Octadecane


Figure 1: Extraction profi le of compounds with low affi nity for the headspace for regular atmospheric pressure HS-SPME and vacuum-assisted Vac-HS-SPME - kindly provided by E. Psillakis from publication Anal. Chim. Acta, 2017, 986, 12-24


107-87-9 6032-29-7 123-32-0 123-66-0 821-55-6 106-32-1


39212-23-2


110-38-3 705-86-2 124-06-1 593-45-3


MOLECULAR WEIGHT [amu]


86.1 88.1


108.1 144.2 142.2 172.2 156.2


200.3 198.3 256.4 254.4


BOILING POINT [°C]


102 115 155 167 195 208 94


241 170 295 316


KH


[atm m3 mol-1]


2.1E-04 2.1E-05 3.5E-06 1.1E-03 7.1E-04 1.6E-03 Not


available 3.2E-03 6.2E-06 2.3E-02 1.9E-2


2.41 2.52 6.35 8.41


10.72 13.33 16.18


17.86 19.85 25.64 25.78


A bulk test mixture was prepared in water and analysed using the same methods for Vac-HS-SPME and AP-HS-SPME. Peak areas for the target analytes were recorded and compared (Figure 3).


RETENTION TIME [min]


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