ICP-MS Features and Analytes

1. Analyte Elements

ICP-MS can analyze the approximately 70 elements highlighted in the periodic table below and can measure many of these elements at ppt levels.
Although sensitivity differs by element, ICP-MS tends to offer good sensitivity when measuring elements with the following three characteristics:
(1) A mass number with a high isotopic ratio
(2) An element with a high mass number (less interference)
(3) An element with low ionization energy
Because ICP-MS can also measure the Halogen elements (except fluorine), it can measure a broader range of elements than inductively coupled plasma atomic emission spectroscopy (ICP-AES) and atomic absorption (AA) spectroscopy.

2. Comparing Elemental Analysis Methods

Table 1 below compares ICP-MS with other elemental analysis methods that measure liquid samples. Because AA measures a single element at a time, it is ideal for the low cost, routine analysis of a small number of elements. However, being a single-element method, AA cannot be used for qualitative analysis and analysis times increase in proportion with the number of elements that need to be measured.
ICP-AES(OES) and ICP-MS incur a greater initial cost compared to AA but are more efficient techniques as they can measure multi-element.
ICP-MS is generally more efficient than ICP-AES because it performs higher sensitivity measurements; however, because ions are introduced directly into the instrument, ICP-MS has lower matrix tolerance and more maintenance than ICP-AES. Accuracy can also be an issue when introducing a wide variety of ions into the instrument. Taking arsenic and selenium as examples, many fields of study require high-sensitivity analysis of these elements due to their toxicity. Both ICP-AES and AA require a pretreatment process based on hydride generation that is specific to each element to achieve these highly sensitive measurements, while ICP-MS can multi-element analysis both arsenic and selenium in addition to many other elements in a sample without complex pretreatment. However, like spectral interference in ICP-AES, mass interferences can occur in ICP-MS due to polyatomic clusters or mass (isobaric) overlap. Modern ICP-MS are specially designed to deal with these issues.

Table 1: Comparison of elemental analysis instrumentation

3. Samples Analyzable by ICP-MS

ICP-MS is typically used to analyze aqueous sample solutions, with the samples normally acidified to stabilize the analyte elements in solution with 1-2 % (v/v) nitric acid and Table 2 reviews some general guidelines. The addition of acid prevents element precipitation and absorption to the container. Hydrochloric acid is added for better solubilization and stabilization of elements like mercury and noble metals, though acids other than nitric acid cause spectral interference and their use should be limited when possible.
Solid samples require dissolution in solution to prepare them for ICP-MS analysis. Recently, microwave digestion systems have been used to prepare samples for ICP-MS analysis by performing acidic decomposition in sealed high-pressure vessels. This form of sample preparation is fast and effective and is becoming more common in analytical laboratories, especially for difficult to digest matrices. Hydrofluoric acid is used to decompose silicate samples and samples with strong oxide bonds. However, hydrofluoric acid is highly caustic and requires caution as it damages glass in the sample introduction system along with causing spectral interference. (If hydrofluoric acid must be used, a dedicated chemically resistant sample introduction system should be installed.)
Samples that contain sediment can clog the nebulizer and require filtering before introduction. Samples with a high salt concentration or large amounts of organic material can also clog the sample introduction system (due to salt deposition and soot generation in plasma), causing fluctuations in sensitivity. When the sample matrix and its effect on analysis is a concern, either the sample is diluted within the limits allowed by measurement sensitivity or the organic materials in the sample are decomposed.
ICP-MS can also analyze samples in organic solvents, but the sensitivity and accuracy of this analysis is lower than for aqueous solutions. Moreover, there is a risk of interference from carbon atoms. Analyzing samples in organic solvents also requires a sample introduction system designed for organic solvents with an additional Ar/O₂ gas stream to reduce soot generation.

Table 2: General guidelines for good ICP-MS sample preparation