Samples are aliquoted into 15 mL or 50 mL VWR Sterile Metal-Free Centrifuge Tubes (Catalog # 89049-172 or 89049-176, respectively). Samples are homogenized before sub-sampling and vortexed if necessary. The weight of the empty tube + cap and the weight of the aliquot are recorded using a four place Mettler balance. The balance is interfaced to a Dell laptop equipped with Balance Keys software such that weights are transferred to an Excel spreadsheet digitally. If your sample volume is small or if sample transfer is difficult, we may recommend you provide us with the sample in the centrifuge tube along with a manifest containing the empty tube weight and sample weight.
For every 20 samples, one blank, one standard reference material (SRM), one duplicate, and one spike is prepared. Reference materials are used from NIST and NIES (Japanese reference materials) and participates in the Quebec Multi-element External Quality Assessment Scheme for biological samples and the USGS Inter-laboratory Comparison Study.
Samples are diluted using MilliQ DI water, Triton X-100 (CAS9036-19-5) and Omnitrace acids (nitric acid CAS 7697-37-2, hydrochloric acid CAS 7647-01-0). Acids are dispensed using a Fortuna OPTIFIX Basic bottle-top dispenser (Catalog # 612-2112 or 612-2114) or an Eppendorf Repeater M4 Multi-Dispenser pipette (Catalog # 4982000322) using Eppendorf Combitips advanced dispenser tips. Solutions are dispensed using a 10 mL Brandtech Dispensette S Bottle top dispenser, digital volume adjustment, with recirculation valve (Catalog # 4600341). The exact dilution factor for each sample is calculated in Excel by dividing the final weight by the sample aliquot weight.
Toenail and Hair Analysis
Visible dirt is removed from the nails, and they are transferred to a 7 mL polyethylene vial. Samples undergo several washing steps. Each washing solution is added using an Eppendorf Repeater pipette and subsequently removed using a Foxx Life Science Vactrap vacuum aspirator system (SKU: 30R-2201-FLS). Samples are sonicated in a Branson Ultrasonics Cleaning Bath (Branson Ultrasonics Brookfield, CT).
1X wash with 2 mL of ACS grade acetone (CAS 67-64-1) followed by 20 min ultrasonic bath.
1X wash with 2 mL 1% Triton X-100 followed by 20 min ultrasonic bath
5X wash with 2 mL MilliQ deionized water
After washing, samples are dried in a clean dry box. This washing procedure removes all external contamination (including nail polish and dirt) without extracting metals from inside the nail. When the toenails are completely dry, the samples are transferred to and weighed in 15 mL vials. Samples are digested following trace metals in solid protocol.
Trace Metals in Solution (multi-element or single element)
Water analysis: samples are acidified using HNO3 to 0.5% HNO3.
Serum and plasma analysis: samples are diluted 10X with 0.2% HNO3, 0.01% Triton-X 100 and 1 ppm gold.
Urine analysis: samples are diluted 10X with 1% HNO3
Blood analysis: samples are diluted 50X with 0.2 % HNO3, 0.01% Triton-X 100 and 1 ppm gold
Trace Metals in Solid: Open Vessel
Our standard digestion method is as follows. Volumes and microwave settings may be adjusted based on sample type.
Samples are digested in 15 mL or 50 mL centrifuge tubes. Generally, samples with ≤ 100 mg equivalent dry weight are digested in 15 mL tubes and 1 mL acid; samples with ≥ 100 mg dry weight are digested in 50 mL tubes. For analysis not requiring Hg, 1 or 5 mL of HNO3 is added. For analysis requiring Hg, 1 or 5 mL of HNO3/HCl is added as a 9:1 ratio. Samples undergo a cold digestion for a minimum of 12 hours. Then, 100 μL or 200 μL of trace metal grade H2O2 is added to the samples before microwave digestion. Samples are digested in a CEM MARS 6 Microwave (Matthews, NC) using a fiber optic temperature probe immersed in one sample to control the temperature of the whole batch. Samples are ramped up to 105 ºC over 15 min and held at 105 ºC for 45 min. Samples are then diluted with MilliQ DI water to 10% acid. The final weight is recorded after dilution.
Trace Metals in Solid: Closed Vessel
Our standard digestion method is as follows. Volumes and microwave settings may be adjusted based on sample type.
Samples (up to 200 mg dry weight) are weighed into MarsXPress Teflon digestion vessels and 2 mL of HNO3or 9:1 HNO3/HCl (if Hg is requested as an analyte) is added. A PFA stopper and Teflon screw cap are used to close the vessel and can hold pressure up to 200 psi. The digestion carousel is heated to 220 ºC with a 20 min ramp time and 20 min hold time. After cooling, the digested sample is poured into a pre-weighed 50 mL tube, and milliQ DI water is used to rinse the Teflon vessel three times, with the rinsate added to the 50 mL tube. The volume is brought to 50 mL and the final weight is recorded.