Instruments and Techniques
PerkinElmer – SCIEX ELAN DRCe ICP-MS
Norwalk, CT USA
In an OES instrument the Inductively Coupled Plasma (ICP) needs careful tuning in order to create excited atom species, not ions, which are undesirable. As the excited atom decays back to ground charge, it emits characteristic wavelengths of light, the intensity of which is used to determine the analyte concentration against a set of suitable standards.
The detection limits of an ICP-MS can be two to three orders of magnitude lower than those of an OES instrument.
In an ICP-MS instrument, the plasma is run at much higher RF-power levels. The intent here is to create ions. In a very simplified form, this is what happens:
After suitable preparation, which can be rather time consuming, samples are introduced via the sample introduction system to a nebulizer and spray chamber, where a high velocity argon flow creates a very fine mist of the sample solution. Only the very smallest droplets are transported to the argon plasma; the rest goes directly to waste.
The plasma, at temperatures in excess of 10,000 K, desolvates, vaporizes, and ionizes the sample. The ion stream goes from atmospheric pressure, via a set of sampler and skimmer cones, into a high vacuum environment. It is then focused via an ion-lens and guided into a quadro-pole mass filter which, in combination with the tunable autolens, is tuned to allow only entities of a specific charge to mass-ratio to pass. All this is computer controlled and happens on the fly, so actual sample runs can be quite short. After passing the mass filter, the remaining ions are guided into a dual-stage detector, which converts the ion impacts into electron-cascades, which are counted and, along with the initial sample information, converted to concentration information.
We practice matrix matching and utilize internal standards, which are added at a constant rate via the third channel of the on-board peristaltic pump. The internal standard must be an element that does not exist in the sample. Thus it is vital to know as much about the sample as possible, including the presence of any possible impurities. Theoretical concentrations, even best guesses, aid us in designing appropriate standard sets.
This instrument employs a Dynamic Reaction Cell, (DRCe), consisting of a small quadrupole into which certain reaction gases can be introduced, thereby chemically eliminating certain polyatomic interferences, that could otherwise mimic masses if they were to reach the quadrupole mass filter. In certain instances, an element such as 32S is thus measured as 48SO, sidestepping the interferences encountered by the mass 32 isotope.
Despite its technical complexities, our ELAN DRCe is a robust instrument, maintainable largely by the operator and relatively easy to tune and operate.
The single most crucial aspect of ICP-MS analysis is sample preparation. Since only small amounts of sample are used, homogeneity is of utmost importance. Total dissolved solids may not be more than a maximum of 0.25%, and in most real life applications samples must be drastically diluted. We typically like to keep most analyte concentrations at or below 80 ppb.
Sample amounts of 1 to 2 mg are usually sufficient. But that depends entirely on the expected analyte concentrations.
Protein samples present a particularly difficult challenge, since we often get only single digit µL liquid samples, in buffers that might themselves carry interfering species, such as P, S, Ca, Cl and organics. In such cases we would like to see sufficient material to perform a digestion, which also eliminates most organic components that might cause variability in sample transport, such as varying viscosity and surface tension.
Our digestions are done in a Discover SPD 80 (CEM Corporation) Microwave Digestion System.
Discover SPD 80 (CEM Corporation)
The high purity acid cocktails used depend on the analytes in the sample. In rare instances we are face with acid insoluble materials, such as metallic ruthenium, which requires a NaOH/Na2O2 fusion.
In most instances we need to do two dilution steps from the digester to the final sample presented to the instrument. We are keenly aware that each dilution represents an error amplification and exercise great care in sample preparation.
DETECTION LIMITS (pdf) — Our instrument, an Elan DRCe, falls somewhere between the Elan 9000 and DRC II.
Potential users are invited to visit the laboratory for consultations, preferably between 8:30 am and 10:00 am.