Sampling Protocol for Cannabis in the Field

Before a sample can be prepared for analysis as will be described below, the sample must be collected and submitted to the laboratory. If the initial sample collection and preparation are flawed then the final answers will be biased. Collecting a representative sample of blood or urine from a patient for chemical analysis is fairly easy to do. Typically, a sample of urine is collected or blood via venous puncture, the sample is stored in the cold to mitigate chemical instability whereupon sample preparation followed by chemical analysis is performed. But how does one get a representative sample of cannabis grown in a 100-acre field or a greenhouse where hundreds of plants are growing? A key point here is the representative sample! Representative samples are selected to accurately reflect the larger group and should represent the characteristics of the group as a whole. Ideally, representative samples are homogeneous or similar in nature, but when that is not possible, the best attempts must be made to achieve samples that represent the majority of the characteristics of the larger grouping.19 It is well known that the chemical composition of cannabis plants can differ significantly within a field as well as within a given plant itself. For example, the cannabinoids of interest are often present at much higher levels in the flower of the plant compared to the stem or seeds. Cannabis is a very complicated plant. So, what portion of the plant does one collect for chemical analysis? Different amounts of compounds can occur in different locations within the plant. In some cases, it has been reported that higher THC concentrations are found in buds located high on the plant as opposed to buds located lower in the plant.20 Adding to the challenge is the fact that THC content of hemp generally peaks as the plant ripens, so the timing of when sampling occurs is important to accurately measure THC concentration and monitor compliance with the US Department of Agriculture (USDA) hemp production program.21 One of the most common complaints of growers of cannabis is the long delays from the time of collecting a sample to the availability of the laboratory results (author’s personal correspondence). Often this can be from 15 to 30days depending upon the state jurisdiction and the availability of laboratory facilities.21 If a hemp grower wishes to harvest when the crop’s CBD content is at a maximum, how does he or she know when that optimal time is for maximum profits? Finally, there is the question of how many plant samples and what portion of the plant should be collected per unit area or agricultural plot of cannabis. The Massachusetts sampling protocol suggests that a sample is more likely to accurately represent the production batch if the material is homogenous (i.e., well mixed).22 Mixing or other homogenization steps help to homogenize the product before sample collection. The latter jurisdiction also suggests that because production batches may vary in scale (i.e., volume or weight), varying numbers or sizes of samples may be required to promote representativeness. In section 6.0 of the Massachusetts guidance, a nine-step detailed sample collection protocol is provided.22 However, there is little guidance to the person collecting the samples as to how many plant samples per unit area, how much and what portion of the plant to collect, or how best to maintain a chain of custody, etc. Suffice it to say those people currently collecting cannabis samples are provided some form of local guidance from their particular jurisdiction and or should follow the recommendations of the recent USDA guidance until further updates are published.21 In the end, sampling protocols are not yet well defined and each state tends to use their own interpretation of best practices.

Sample Preparation of Cannabis Plant Materials and Cannabis-Derived Products

Before we move on to overview some of the modern analytical techniques used for the analysis of cannabis and cannabis-derived samples, it is worth highlighting the importance of sample preparation prior to chemical analyses. As noted above, the breadth of sample types relevant to modern cannabis analyses is very broad. It is well known by those experienced in the chemical analysis of plant materials, biological samples, and commercial products such as edibles that an optimized sample preparation method is very important for accurate and precise chemical measurements. Thus, there is a very big difference in the interfering sample matrix components within a cannabis plant flower compared to the matrix of a chocolate brownie. Thus, optimized sample preparation for each of the different sample types must be employed to obtain accurate quantitative determination of the targeted cannabinoids, pesticides, etc. that are of interest. Although there are currently many different methods used by participating laboratories, the one by Giese et al. represents perhaps one of the preferred approaches for the preparation of cannabis-derived samples.23 Despite this suggested sample preparation procedure in practice, there are currently no standard sample preparation procedures. Each laboratory tends to develop its own method or tweak a referenced procedure to suit its needs. Consequently, there can be considerable variability with interlaboratory results even on the same samples. There remains much work to be done going forward to develop more standardized sample preparation as well as analysis procedures.

The above introductory discussion on the chemical analyses applicable to cannabis brings us to the modern day where we will describe the current methods and techniques being used. Given that this book focusses on the analytical horsepower of inductively coupled plasma (ICP)-MS and other atomic spectroscopic techniques for the ultratrace determination of heavy metals, we will begin with why this important modern analytical technique is very important to the field of cannabis analyses.

 
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