CarbonX for QuEChERS Removes 50% More Matrix as Compared to GCB
Introduction:
Graphitized Carbon Black (GCB) is generally added to QuEChERS dispersive solid phase extraction (dSPE) kits when the food extract is highly pigmented. This is due to the fact that these pigments and sterols cause fouling of the liner and column as well as decreased signal to noise when quantitating samples on GC/MS and LC/MS. United Science is offering a product, CarbonX for QuEChERS, that works similarly to GCB without the large losses of planar pesticides seen with GCB. In addition, the CarbonX for QuEChERS removes more total mass of contaminants than GCB leading to decreased fouling of the column and liner.
1. Samples
Extracts of homogenized spinach with 1% Acetic Acid in Acetonitrile using QuEChERS AOAC method.
2. Standards and Reagents
Acetonitrile and Acetic Acid were purchased from Aldrich Chemical Co.Spinach used was Birds Eye brand spinach purchased from Cub Foods. QuEChERS 50mL centrifuge tubes, salt packets, GCB, PSA, and MgSO4 were purchased from Agilent Technologies.
3. Equipment
Balance: Mettler-Toledo AE 163 Balance
GC/MS: Agilent 5890 GC coupled to 5973 MSD
4. Procedure
4.1 Examination of column and liner fouling after dSPE cleanup
1. Three 2 mL dSPE tubes were each filled with 50 mg PSA and 150 mg MgSO4. One tube had 50 mg CarbonX for QuEChERS added and a second had 50 mg GCB added. The third tube had nothing added.
2. The two tubes with carbon in them had 1 mL of the pigmented 50 ppb QuEChERS AOAC pesticide mix added. The tube without carbon (the standard) had 1 mL of the same 50 ppb QuEChERS AOAC pesticide mix but in a matrix standard with the pigment already removed before the addition of the pesticides.
3. All three tubes were vortexed for 1 min followed by centrifugation at 5000 rpm for 5 min.
4. The supernatant from each tube was put into a GC vial.
5. The recoveries of the pesticides were evaluated independently for the CarbonX CoA tubes and the GCB tubes. A new liner and column was installed for each set of injections, which followed the order:
3 Standard Injections
3 Sample Injections
3 Check Standard Injections
3 Sample Injections
3 Check Stand Injections
3 Sample Injections
4.2 Determination of remaining matrix mass after dSPE cleanup
1. 15 g of Birds Eye spinach was homogenized
2. The homogenized spinach was added to a 50 mL centrifuge tube along with a salt extraction packet containing 6g MgSO4 and 1.5g NaAcetate. Finally, 15 mL of the 1% Acetic Acid in ACN solution is added.The tube is shaken for 1 min, then centrifuged at 5,000 rpm for 5 min.
3. The extract is poured off into a separate container.
4. Two six dram vials were filled with 250 mg PSA and 750 mg MgSO4 each. One vial then had 250 mg of CarbonX for QuEChERS added and the other had 250 mg GCB added.
5. To each vial 5.00 mL of spinach extract was added.
6. The vials were vortexed for 1 min and then centrifuged at 5000 rpm for 6 min.
7. 2.0 mL of supernatant from each vial were placed into two tarred vials and evaporated to dryness, and then the vials were weighed.
5. Instrumental Method
GC conditions
Inlet: Splitless
Inlet liner: Agilent Ultra Inert splitless single taper wool p/n 51903163
Carrier gas: Helium
Inlet pressure: 19.6 psi (constant pressure mode) during run
Inlet temperature: 250 ºC
Injection volume: 1.0 µL
Purge flow to split vent: 30 mL/min at 0.75 min
Oven temperature program: 70 ºC (1 min), 50 ºC/min to 150 ºC (0 min), 6 ºC/min to 200 ºC (0 min), 16 ºC/min to 280 ºC (6 min)
Column: Agilent J&W HP-5MS Ultra Inert 15 m × 0.25 mm, 0.25 µm (p/n 19091S-431UI)
MSD
MS conditions
Tune file: Atune.u
Mode: SIM (refer to QuEChERS AOAC method)
Source, quad, transfer
line temperature: 230 ºC, 150 ºC and 280 ºC respectively
Solvent delay: 2.30 min
Multiplier voltage: Autotune voltage
6. Results
6.1 Examination of column and liner fouling after dSPE cleanup
Figure 1. A comparison of the average recoveries and check standards for spinach matrix spiked with 17 AOAC QuEChERS pesticides follow by dSPE cleanup with GCB (yellow and red, respectively) and CarbonX for QuEChERS (purple and blue, respectively). Both recoveries and check standards are normalized to the initial values obtained. The chart demonstrates that repeated injections with matrix cleaned up with GCB decreases in intensity more rapidly than matrix cleaned up with CarbonX for QuEChERS.
6.2 Determination of remaining matrix mass after dSPE cleanup
Figure 2. Mass remaining after pigment removal with GCB and CarbonX for QuEChERS. Approximately twice as much mass remains after GCB cleanup as CarbonX for QuEChERS cleanup.
7. Conclusion
CarbonX for QuEChERS removes more total mass of pigments and sterols when used in dSPE than GCB, leading to an increased column and liner lifetime.