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Crustal Geophysics and Geochemistry Science Center

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High Resolution ICP-MS Laboratory

Photo of sample preparation laboratory.
Class 100 Sample Preparation Lab
This facility is designed for precise and accurate analysis of geological and biological materials. The laboratory includes a Class 100 clean room for sample preparation and a Class 1000 instrumentation room. Clean sample handling techniques are necessary for the ultratrace determination of particular elements and for precise isotope ratio determinations. The laboratory contains two high resolution (HR) ICP-MS systems - a multicollector ICP-MS used primarily for isotope ratio determinations and a single collector system used for accurate elemental concentration determinations of difficult elements.

Single collector HR-ICP-MS

Typical quadrupole mass spectrometers used in ICP-MS have resolutions between 0.7 - 1.0 amu. This is sufficient for many routine applications. However, there are some instances where this resolution is NOT sufficient to separate overlapping molecular or isobaric interferences from the elemental isotope of interest. Table 1 below shows some commonly occurring interferences that make ultratrace determinations of several important elements difficult, particularly in specific geological matrices. The resolving power (R) of a mass spectrometer is calculated as R = m/(|m1-m2|) = m/Δm, where m1 is the mass of one species or isotope and m2 is the mass of the species or isotope it must be separated from; m is the nominal mass. For example, as Table 1 shows, As and Cr are elements that suffer from interferences due to chloride species that form in the plasma, making accurate determinations of low concentrations of these elements in high chloride matrices difficult by quadrupole ICP-MS (which generally have R values less than 300). Photo of Thermo Finnigan Element HR-ICP-MS.
Thermo Finnigan Element HR-ICP-MS

Table 1. Example interferences and resolving power required.
Analyte Interference |Δ m| m R
75As = 74.92160 40Ar35Cl = 74.93123 0.00963 75 7788
52Cr = 52.94065 37Cl16O = 52.96081 0.02016 53 2629
56Fe = 55.93494 40Ar16O = 55.95729 0.02235 56 2505
40Ca = 39.96259 40Ar = 39.96238 0.00021 40 190476
87Sr = 86.90889 87Rb = 86.90918 0.00029 87 300000

For this reason, the use of high resolution or magnetic sector mass spectrometers has become more common in ICP-MS, allowing the analyst to eliminate or reduce the effect of interferences due to mass overlap. Although HR-ICP-MS can effectively reduce many interferences, it is not without some cost. For every 10-fold increase in resolving power, there is a concomitant decrease in sensitivity. This can serve to limit detection capabilities where the analyte concentrations are very low.

The laboratory currently employs a Thermo Finnigan Element HR-ICP-MS which is capable of resolving powers of 300, 3000, and 8000. The instrument has the capability to perform multielemental analyses in multiple resolutions where interferences may be an issue for accurate concentration determinations or to verify the presence or absence of a suspected interference. It also can be used for performing highly accurate analyses utilizing isotope dilution methodologies and for performing rapid isotope ratio analyses of simple systems. Samples are generally introduced as a solution as for quadrupole ICP-MS (see Solution ICP-MS Laboratory page for sample preparation information). For applications where dissolution is difficult or where spatial variation of elemental composition is of interest, direct solid sampling via laser ablation is also possible. (For more general information on HR-ICP-MS click here)

Multicollector ICP-MS

Photo of Nu Instruments Multicollector ICP-MS.
Nu Instruments MC-ICP-MS
The laboratory currently contains a Nu Instruments HR multi-collector ICP-MS system. The instrument employs a high resolution double-focusing mass spectrometer. An electrostatic zoom lens system then focuses the ion beams into an array of static Faraday and ion counters that simultaneously measure isotope (ion beam) intensities for the isotopic system being studied. Photo of New Wave UP 213 Laser Ablation System.
New Wave UP 213
Laser Ablation System
The MC-ICP-MS is capable of determining isotope ratios with external precisions on the order of 10-20 ppm which is much more comparable to the precisions obtained by TIMS than those previously achieved by quadrupole ICP-MS or single-collector HR-ICP-MS systems. MC-ICP-MS also has speed as an advantage. Isotope ratio measurements can be performed in just a few minutes per sample as compared to the several hours per sample required for TIMS analysis. In addition, due to the efficiency of the high temperature ICP (Inductively Coupled Plasma) ion source, MC-ICP-MS can be used to study both "traditional" (e.g. Sr, Nd, Pb, Hf) and "non-traditional" (e.g. Li, B, Be, Fe, Cu, Zn, Cr, S, Hg, Mo) isotope systems.

Generally, samples are introduced into the MC-ICP-MS in solution or as a solid ablation stream from a laser for in situ analysis. Aqueous samples can be introduced as a wet aerosol using a Peltier cooled spray chamber, as a dry aerosol using a desolvation nebulizer, or as a vapor using a cold vapor generator. The use of Laser Ablation (LA) sampling can give added information on the spatial variations of elemental composition in solid samples. (See Laser Ablation ICP-MS laboratory page for specific examples.)

The use of firm, trade, and brand names is for identification purposes only and does not constitute endorsement by the U.S. government.

For further information contact Michael Pribil (mpribil@usgs.gov).

Mineral Resources Program
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