Factors Determining Sensitivity in ICP-MS

Nov 01, 2009

Inductively coupled plasma–mass spectrometry (ICP-MS) is renowned for its phenomenal sensitivity as an elemental analyzer. It is the most sensitive elemental spectrometric technique, with detection limits in the low parts-per-trillion or even parts-per-quadrillion range for many elements. This article examines the reasons for this sensitivity and considers how it might eventually be further improved.

There are basically three types of commercially available inductively coupled plasma–mass spectrometry (ICP-MS) instruments that differ mainly in the type of mass analyzer they use. They are quadrupole, magnetic sector field (also called high resolution ICP-MS), and time of flight. Of these, quadrupole ICP-MS is by far the most commonly used, and this article will be limited to this type of instrument. Most of the relevant principles, however, generally apply to other mass analyzer types as well.

The ICP-MS instrument consists of four main components. The inductively argon coupled plasma ICP serves as the high temperature ion source. The ICP essentially is the same as that used in ICP optical emission instruments with one important difference — the plasma in optical devices is primarily an atom source whereas in a mass spectrometer it produces ions. The interface and ion optic components extract the ions from the plasma at atmospheric pressure and focus them into the mass analyzer, which is at high vacuum. The mass analyzer (quadrupole in this discussion) separates the ions based on their mass-to-charge ratio (m/z) and delivers them sequentially to the detector. The detector is an electron multiplier that generates an electrical pulse to the counting circuitry for each ion that strikes it. Most newer quadrupole ICP-MS instruments also include a collision–reaction cell between the interface and analytical quadrupole. The function of this cell is to remove polyatomic (molecular) interferences before mass analysis. Some loss of signal can occur in this cell due to collisions between analyte ions and gas in the cell.

The argon ICP is a highly efficient ion source. Most metals are ionized at 80 to >95% efficiency, while a few high ionization potential metals such as mercury have ionization efficiencies as low as 40%. Even nonmetals such as sulfur, phosphorus, silicon, and all of the halogens except fluorine are sufficiently ionized to allow for highly sensitive measurement. Only five elements cannot be directly measured by ICP-MS: hydrogen, helium, fluorine, neon, and argon. The first four are not ionized because their first ionization potentials are higher than that of argon, and the last argon is not measurable in an argon plasma.


Figure 1: Block diagram depicting the functional components of a modern quadrupole ICP-MS system in order from left to right.
Let's examine the actual efficiency of a typical quadrupole ICP-MS system. We'll use 1 part per trillion (ppt) as an example detection limit, though some elements will be lower, some higher. A 1-ppt (w/v) concentration is equivalent to 1 ng/L or 10-9 g/L. The equivalent number of atoms (ions) will depend on the atomic weight. Applying the Avogadro constant (6.02 X 1023 atoms/mole), 1 ppt is ~6.68 X 1013 atoms of Be (atomic mass 9) or ~2.53 X 1012 atoms of U (atomic mass 238) per liter. Using a convenient nebulizer flow rate of 1 mL/min would result in between 1.1 X 109 and 4.2 X 107 atoms per second per ppt being introduced to the instrument at the nebulizer. ICP-MS sensitivity is commonly measured in terms of millions of counts per second at the detector per ppm introduced to the nebulizer (Mcps/ppm, also called MHz/ppm). This value varies depending on the element as well as instrument design and operating conditions but is typically ~100–500. So if we work back to our 1-ppt example, this translates to 100–500 cps/ppt. Using the calculation above for the number of atoms introduced to the nebulizer (we'll use an average value of 108 ) for a 1-ppt solution, we can calculate the efficiency of generating a pulse at the detector from a given atom introduced to the nebulizer as 100/108 or 10-6 . About one in a million atoms introduced to the nebulizer actually reach the detector. Where do the rest go? If we examine all of the components of a typical ICP-MS system (Figure 1), we will see that there are inefficiencies (some intentional, some unavoidable) at each stage.

Table I shows the efficiencies that can be assigned to the various stages of ICP-MS analysis in very general terms.


Table I: Approximate efficiencies for the various components and processes of a modern quadrupole ICP-MS instrument
Multiplying the efficiencies in Table I results in an overall estimated efficiency of about 0.00002% (1 in half a million), which is in general agreement with the estimate based on measured response. So, the most sensitive elemental analyzer we have is only about 0.00002% efficient. What if anything can we do about that? And, do we really need to do anything? There are two schools of thought on whether we need to further improve ICP-MS sensitivity. The first school, typified by the commercial service laboratories, would generally say that 1-ppt detection limits are good enough for most applications, and that in fact it is very difficult to routinely produce blanks at that level for many elements. Therefore the actual method detection limits are not instrument limited but are limited by the cleanliness of the sample preparation and laboratory environment.


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