Phytoremediation

Careful and thorough digestion of samples for trace metals is critical to obtaining accurate results. For many applications, ultra-trace determinations of micronutrients and/or highly toxic elements are required. Minimizing contamination during the digestion process is critical. In addition, potential complications from incomplete carbon removal can impact the results.

Plant tissue digestion is generally handled consistently the same. Once final milling of the homogenate is complete, the sample is ready for sub-sampling for digestion. Digestions are typically performed using a nitric acid procedure conducted in closed Teflon vessels at 105^oC. The heating is done in convection ovens over a period of approximately 16 hours. The extended digestion time allows complete oxidation of organic material so dissolved carbon levels are minimized, which reduces or eliminates several carbon induced interferences for both ICP/OES and ICP/MS determinations.

Instrumental procedures usually follow EPA Methods 6020A & EPA 6020B for ICP-MS, 6010C for ICP-OES, 7470/7471 for CVAAS mercury, 1631 for P&T-CVAFS mercury and 1630 for methyl-mercury. ALS also offers As speciation (Inorganic-As, As-III, As-V) in tissue by EPA 1632A.

Determinations of organic compounds are typically performed by following published methods, but refined processes are incorporated into the procedures within the scope of the methods to maximize sensitivity and selectivity. In addition, ALS offers a number of procedures outside the scope of published methods. In these cases, in-house SOPs are developed. Note that ALS holds certifications for a number of laboratory specific SOPs.

Following sample extraction and cleanup, instrumental analysis is performed for organic constituents using a combination of Gas Chromatography/Mass Spectrometry (GC/MS), GC/MS with Selected Ion Monitoring (GC/MS/SIM), GC/MS with Large Volume Injection (GC/MS/LVI), GC with tandem mass spectrometry (GC/MS/MS), GC with High Resolution Mass Spectrometry (GC/HRMS), GC with Electron Capture Detection (GC/ECD), GC/ECD with LVI, GC with Flame Photometric Detection (GC/FPD), GC with Flame Ionization Detection (GC/FID), Liquid Chromatography with Ultraviolet Detection (LC/UV), and LC with tandem mass spectrometry (LC/MS/MS).

ALS is using a number of sample preparation techniques to ensure representative subsampling and homogenization. The methods applied will also prevent cross contamination and loss of analytes:

• Cutting
• Grinding
• Blending
• Cryogenic Grinding

Plant tissue generally undergoes a gross homogenization to obtain a large sub-sample for drying at 60^oC as the preliminary step to subsequent trace metals determinations (other than mercury and methyl mercury). The dried material is further homogenized via various techniques depending on the type of tissue. Mechanical milling is normally required. Note that ALS also has the capability to perform cryogenic grinding of plant material, which is the preferred option for preparing samples for determination of mercury and/or organic constituents. Although mercury in plant tissue is generally not volatile at 60^oC, aliquots for analysis are taken prior to drying to alleviate doubts. Thus, cryogenic milling of the “as received” plant tissue provides a superior homogenate. Homogenates are air-dried for organics.