Pushing the Limits: Achieving ppb-level Detection Limits for As, Cd, Pb, and Hg in Hemp Samples on ICP-OES

Recent developments in the medical and recreational use of cannabis products have created the need for testing these products for toxic elements. Maximum Allowable Limits for these toxic elements are currently set by many states at very low concentrations, particularly for Arsenic, Cadmium, Mercury, and Lead - the “Big Four”. These low concentrations have led to ICP-MS becoming the most widely used and recommended technique for trace level analysis of heavy metals in cannabis samples. However, ICP-MS is quite complex in operation, associated with a high cost of ownership, and needs significant routine maintenance. The simpler and less costly ICP-OES technique is an attractive alternative to ICP-MS. However, reaching the detection limits at trace levels by ICP-OES, while keeping high productivity and high sample throughput, is a challenging analytical task. The demand for increased sensitivity was accomplished by employing a high efficiency and robust sample introduction system combined with an optimized microwave sample digestion methodology. The sample introduction system included a high efficiency nebulizer and cyclonic spray chamber. The increased efficiency was accomplished by combining an improved dual channel technology where the gas and sample channels are separated through the entire body of the nebulizer with an external impact surface positioned close to the nebulizer gas orifice under an optimized angle. Required accuracy was achieved by applying an innovative analytical approach to matrix matching of the calibration standards. Hemp samples were digested in a high efficiency microwave digestion system. Analyses were carried out on two separate ICP-OES instruments: one equipped with an echelle and the other with a Rowland circle (ORCA) optical design. The results show that the developed methodology allows for the accurate and reliable analysis of real-world cannabis samples at and below the levels defined by strictest detection limits requirements.