Using LPGC to Speed Up Pesticide Analysis in Cannabis Products

The recent launch of our low-pressure gas chromatography (LPGC) kit has spurred a lot of interest among scientists that work on routine testing of GC amenable compounds, particularly, in the analysis of pesticides in diverse food matrices. Faster instrumental analysis, while still getting good separation and reliable data are highly desirable features for novel chromatography-based analytical methods. Briefly, LPGC allows for faster GC run times by using a 0.53 mm analytical column that is directly connected to the vacuum of the MS analyzer. To maintain positive pressure at the GC inlet, a short and narrow restriction capillary is used to connect the GC inlet to the analytical column [1,2]. By keeping the column under vacuum conditions, there is a decrease in the viscosity of the carrier gas, which allows for faster separations while maintaining acceptable peaks resolution [3–5]. If you are interested in getting additional information about how LPGC works, these links can be very helpful: 

An Introduction to Low-Pressure GC-MS (LPGC-MS)

Speed Up Multiresidue Pesticides Analysis in Food with Low-Pressure GC-MS

LPGC - Fast way to your pesticide analysis!

Now, taking into account all the great features that LPGC offers, we decided to evaluate the performance of this technology for the analysis of those pesticides not easily detected via electrospray ionization but that are regulated by the state of California in cannabis products. Those compounds are quintozene (pentrachlornitrobenzene), methyl parathion, captan, chlordane (trans and cis-chlordanes), chlorfenapyr, cyfluthrin, and cypermethrin. First, it should be noted that we developed full workflows for the analysis of pesticides and mycotoxins in cannabis edibles such as brownies, gummies, and more recently chocolate. As you can see in the links provided, the analysis of GC amenable pesticides in brownies and gummies extracts was successfully carried out by running a conventional GC method of 23 min. On the other hand, in our recent chocolate work we demonstrated the suitability of using LPGC-MS/MS for the quantitation of the above mentioned pesticides at LOQs that comply with California regulations. Based on these results, and to have a better understanding of the performance of LPGC, we ran chocolate and gummy extracts under both GC and LPGC-MS/MS conditions. A summary of the instrumental parameters is presented in Table 1.

	Conventional GC-MS/MS	LPGC-MS/MS
Instrument	Thermo Trace 1310-TSQ 8000
Column	Rxi-5ms, 30 m, 0.25 mm ID, 0.25 µm (cat.# 13423)	Low-pressure GC column kit (factory-coupled restrictor column [5 m x 0.18 mm ID] and Rtx-5ms analytical column [15 m, 0.53 mm ID, 1 µm plus 1 m integrated transfer line on the outlet end]) (cat.# 11800)
Injection Mode	Splitless
Inj. Vol.	1 µL
Liner	Topaz 4.0 mm ID Single Taper Inlet Liner with wool (cat# 23447)
Inj. Temp.	250°C
Split Flow	14.0 mL/min	20 mL/min
Splitless Time	0.50 min
Purge Flow	5 mL/min
Oven	90°C (hold 1 min) to 310°C (hold 10 min) by 25°C/min	80 °C (hold 1 min) to 330 °C (hold 5.50 min) by 45 °C/min
Carrier Gas	He, constant flow
Flow Rate	1.40 mL/min	2.0 mL/min
Detector	MS/MS
Method Type	Acquisition - timed
Ionization Mode	EI
Transfer Line Temp.	290°C
Source Temp.	330 °C	325 °C

Figure 1 shows a comparison of two chromatograms corresponding to extracts obtained from a chocolate sample spiked at 100 ng/g. The chromatogram on top was acquired using the LPGC setup, whereas the one on the bottom was obtained with the conventional GC method.

Figure 1. Chromatograms corresponding to GC amenable pesticides in a chocolate extract run via LPGC-MS/MS (top) and via GC-MS/MS (bottom).

As shown, by using LPGC all the target analytes eluted before 7 min which is significantly faster than the elution under the conventional GC conditions. In addition to the elution profile, we collected data on the LOQs obtained under each set of conditions (Table 2). For our work, LOQs were estimated as the lowest concentration with a signal-to-noise ratio of at least 10, a difference of <25% between the spiked concentration and the estimated concentration, and a <25% precision value.

Table 2. Retention times and LOQs for GC amenable pesticides from the California list analyzed in chocolate and gummy extracts using conventional GC and LPGC.

Compound	Rt. LPGC, min	Rt. GC, min	Action levels, ng/mL*	LOQs
				LPGC, chocolate, ng/mL	GC, chocolate, ng/mL	LPGC, gummies, ng/mL	GC, gummies, ng/mL
Quintozene (PCNB)	4.7	7.0	200	10	5	20	10
Methyl parathion	5.0	7.5	100	5	5	5	5
Captan	5.5	8.4	5000	25	10	50	10
Chlordane	5.6	8.5	100	50	25	50	20
Chlorfenapyr	5.7	8.8	100	10	5	20	10
Cyfluthrin	6.7	10.6	1000	5	5	5	5
Cypermethrin	6.8	10.9	1000	10	5	5	5

* Action levels stated by the California Bureau of Cannabis Control - Testing Laboratories - BCC

As can be seen in Table 2, compounds such as methyl parathion and cyfluthrin showed the same LOQs under the conventional GC method and LPGC conditions. However, for the rest of the pesticides higher LOQ values were observed in the case of LPGC in comparison to the longer GC method. It should be noted that faster GC runs are attainable at the expense of some overall method resolution, and this is could lead to higher LOQ levels in some cases. For these particular matrices, the LOQs attained using both methods are still below the action levels required by the state of California, which demonstrates the convenience of LPGC in providing faster intrumental turnaround times. We are currently evaluating the performance of LPGC-MS/MS for the analysis of these pesticides in hemp, so please stay tuned for new content coming soon.

1. De Zeeuw, J., Peene, J., Jansen, H. G., Lou, X., A simple way to speed up separations by GC-MS using short 0.53 mm columns and vacuum outlet conditions. HRC J. High Resolut. Chromatogr. 2000, 23, 677–680.
2. de Zeeuw, J., Peene, J., de Nijs, R. C., Gas Chromatographic Device, US6301952B1, publ. date 2001.
3. Lehotay, S. J., de Zeeuw, J., Sapozhnikova, Y., Michlig, N., Rousova Hepner, J., Konschnik, J. D., Low-Pressure Gas Chromatography– Mass Spectrometry for Fast, Sensitive, Robust GC–MS Analysis. LCGC North Am. 2020, 38, 457–466.
4. Giddings, J. C., Theory of Minimum Time Operation in Gas Chromatography. Anal. Chem. 1962, 34, 314–319.
5. Sapozhnikova, Y., Lehotay, S. J., Review of recent developments and applications in low-pressure (vacuum outlet) gas chromatography. Anal. Chim. Acta 2015, 899, 13–22.