What dSPE works with spinach in GC-MS/MS analysis? #NationalSpinachDay

Today is a National Spinach Day! What is a better way to celebrate than to talk about spinach analysis?

During part 1 of my blog series, I discussed what dSPE was best for celery. I found that most dSPE, with the exception of dSPE containing high amounts of graphitized carbon black (GBC), showed acceptable analyte recovery and matrix removal. The same cannot be said for spinach. Spinach was more complicated although initially the data looked similar (Fig. 1). The planar pesticides have lower recoveries while the rest of the analytes are unchanged.

Figure 1: Comparison of mean responses of pesticides spiked in spinach using individual dSPE. The planar pesticides, such as Thiabendazole and Cyprodinil, have low recoveries.

Examining spinach-related pesticides is more challenging because the planar pesticides in our mix are not commonly found on spinach (Fig. 2).

Figure 2: Comparison of mean responses of selected pesticides spiked in spinach using individual dSPE

So, can we use any dSPE since the recoveries are comparable? Well, not really. Unlike celery, spinach has high levels of pigmentation. If we choose a clean-up method that doesn’t address the pigmentation, the GC-MS/MS system (e.g. liner, inlet seals, MS source), will suffer. As you can see in Figure 3, the differences in pigment removal are striking.

Figure 3: Visual comparison of spinach extract after clean-up with individual dSPE

The high GCB dSPE (#26123 and #26219) are the best at removing the pigments. Table 1 provides a list of the type and amount of sorbent contained within the dSPE evaluated for this work.

Table 1: Description of dSPE used for optimization

As I mentioned in my celery blog, high GCB can also lead to removal of planar pesticides. Therefore, either #26123 or #26219 are viable options for spinach analysis. There are ways to mitigate this effect, mainly using internal standards, such as anthracene, deuterated standards, or take advantage of the matrix-matched calibration. Although these approaches work well, take care when you add the internal/calibration standards. Matrix-matched calibration won’t correct losses in the dSPE step if the internal standard or surrogate is added after the clean-up step. I’ve also looked into which QuEChERS salts are the best to use with spinach (Fig 4). I got the best results with AOAC 2007.1 salts (6 g MgSO4, 1.5 g NaOAc).

Figure 4: Comparison of mean responses of selected pesticides using individual QuEChERS salts

To summarize, based on the selected pesticides, the best recoveries and matrix clean-up were achieved using AOAC 2007.1 salts in combination with dSPE with high GCB content (#26123 or #26219). Next time I’ll tackle cleanup techniques for the analysis of oranges!