Metal Analysis

Sample Preparation

o Acid digestion/dilution (typically HNO­₃)

§ Can cause proteins to crash out and interfere with nebulizer

o Alkaline digestion + chelating agent (EDTA) + Surfactant (Triton-X100)

§ Proteins more tolerant in alkaline solution

§ EDTA added help metal solubility/stability

§ Triton-X100 added to disperse and solubilize proteins

o Total Dissolved Solids (TDS) content of sample should be < 0.2%

§ Typically dilute samples between 10 and 50 times

o Internal standards

§ Should be similar in mass and ionization potential to target analyte

§ Common standards, not found in biological samples

· Lithium (⁶Li)

· Scandium (⁴⁵Sc)

· Germanium (⁷²Ge)

· Yttrium (⁸⁹Y)

· Rhodium (¹⁰³Rh)

· Indium (¹¹⁵In)

· Tellurium (¹²⁵Te)

· Terbium (¹⁵⁹Tb)

· Rhenium (¹⁸⁵Re)

· Iridium (¹⁹¹Ir)

· Bismuth (²⁰⁹Bi)

o Useful for high mass analytes (i.e. Pb)

o Bismuth still used in some medications, take into consideration when designing experiments

Sample introduction

o Infused via peristaltic pump into a concentric pneumatic nebulizer

Interference

o Spectroscopic: non-analyte ions with the same m/z as target

§ Isobaric elements: isotopes of two elements that have the same m/z within the resolution of the MS

§ Double charged ions: occurs when the second ionization potential of the analyte is less than that of Argon

· Rarely occurs, but important to consider in specific circumstances (i.e. selenium and gadolinium in biological samples exposed to MRI contrast solution)

§ Polyatomic ions: form due to recombination reactions or incomplete atomization

· Highly problematic, frequently occurs due to reactions with sample matrix, reagents used in prep, plasma gasses, and atmospheric gasses (i.e. ⁴⁰Ar³⁵Cl and ⁷⁵As)

· Can monitor degree of oxide formation through tuning with cerium (<3% considered acceptable)

· Careful selection of sample treatment reagents required to avoid this interference (i.e. HCl can interfere with ²³V and ³³As)

§ Tailing interference: due to spectral overlap from an adjacent mass

· i.e. ⁵⁵Mn can experience overlap from ⁵⁴Fe and ⁵⁶Fe when the concentration of Fe is much greater than that of Mn in a given sample

o Non-spectroscopic: due to sample matrix or instrument drift

§ Matrix effects: enhancement or suppression due to contents of sample matrix

§ Sample introduction effects: Overloading the plasma with aerosol can cause cooling and decreased ionization.

· Can often be mitigated through further sample dilution

§ Plasma effects: enhancement or suppression of signal due to other ions present in the sample

· Suppression: High levels of easily ionizable elements (i.e. K and Na) will suppress ionization of the target analyte

· Enhancement: Presence of carbon in a sample can enhance the signal of analytes with higher ionization potentials

§ Space-charge: Most significant matrix effect, caused by electrostatic repulsion in the plasma beam.

· Lower mass analytes are more greatly affected by this phenomenon; Important to choose an internal standard with a similar m/z to the analyte of interest

o Mitigation by dynamic reaction cell (DRC): use of a reactive gas (i.e. O₂) in a reaction cell prior to analysis in quadrupole

§ On mass detection: Interfering ion reacts with gas to form a polyatomic ion with a different m/z from target analyte

§ Off mass detection: Analyte reacts with gas to form a polyatomic ion with a different m/z from interfering ion

§ Neutralization: Interfering ion is neutralized by the reactive gas

Section detailing elements we are currently set up to run

o Will need to be updated as we complete new projects