MCQs in ICP-MS
What is ICP-MS? Inductively coupled plasma mass spectrometry (ICP-MS) is a flexible chemical analysis method that can be tailored to meet the
In its simplest form, ICP-MS takes a pre-prepared liquid containing the analyte, pumps it through a nebulizer to create an aerosol which is introduced into an argon gas plasma. The high temperature of the plasma (~5500-6500 K) is sufficient to atomize and ionize almost all elements, including those with the highest ionization potentials. The analyte ions thus produced can be steered with electrostatic ion optic elements into a mass spectrometer where the ions can be separated into their mass to charge (m/z) ratio and detected. Since ionization is close to 100% efficient for most elements, the counts detected for an elemental ion is representative of its concentration in the analyte.
The method of sample pre-preparation has concurrently matured from the initial step of simple acid digestion to today where laser ablation (LA) is commonly combined with ICP-MS for spatially resolved analysis and elemental mapping of solid samples. Furthermore, separation techniques have been coupled to ICP-MS to permit the measurement of molecules, and in particular biomolecules. The ionization process would typically destroy all molecular information from the sample, but this problem has been largely conquered by introducing an element of sample selectivity through pairing separation techniques (e.g. liquid chromatography (LC), gas chromatography (GC), capillary electrophoresis (CE), gel electrophoresis (GE)) with ICP-MS.2, 3 The separation technique can isolate out all the species containing the element of interest and sequentially forward them on to the ICP-MS system for detection.
How does ICP-MS work? A schematic diagram showing the various components of an ICP-MS system is illustrated in Figure Liquid samples are introduced into the nebulizer by a peristaltic pump or self-aspiration where they form an aerosol of fine droplets. Not all nebulizers are the same, and the type chosen is based on factors such as the viscosity, volume and cleanliness of the sample to be analyzed.
Figure 1: Schematic showing main components of an ICP-MS system described in more detail in text. The fine droplets that are created by the nebulizer are passed through a spray chamber before entering the plasma. Different types are again available, but the function remains the same: to allow a high number of the small droplets to enter the plasma while discriminating against the larger droplets which can create analytical issues if permitted into the plasma.
The argon plasma generated in the ICP reaches temperatures of between 5500-6500 K and is generated by passing argon gas through concentric quartz tubes (commonly referred to as the ICP torch) that are contained at one end within a radio frequency (RF) coil. Energy supplied to the coil by an RF generator couples with the argon gas to produce the plasma. As the liquid droplets enter the high temperature plasma, they are converted to the gaseous state. As they absorb more energy, they will eventually release an electron to form a single, positively charged ion.
The interface region where the ions produced by the plasma are introduced to the mass spectrometer presents an engineering challenge. In the first instance, the torch region reaches temperatures of ~6000K, the other side of the interface remains at room temperature. Secondly, the torch will have been necessarily backfilled with the Ar gas required to generate the plasma, while the mass spectrometer will be under high vacuum conditions. Each manufacturer will have different solutions, but two or more cone structures (lenses) are used to prevent a wide divergence of the ions as they enter a region of high vacuum, and to focus them into the collision cell or directly into the mass spectrometer.
Watch this video from the Teach Me in 10 series to get an introduction to ICP-MS from Abe Gutierrez and Ed McCurdy.
Ions will not be the only species exiting the plasma – neutral atoms and photons will be present. Photons can give rise to false ion counts so it is important that they are removed from the path of ions. There are variations on a theme as to how the different manufacturers deal with this issue, but a common solution is to place some form of lens element that will selectively bend only the ions into the quadrupole mass spectrometer.
Most modern instruments will have a “universal” or “reaction/collision” cell located between the ion optic elements and the mass spectrometer to help reduce the problem of mass interferences. This occurs when two ions, an elemental ion (e.g. 56Fe+) and a molecular ion that may result from the interaction between the sample matrix and the Ar gas in the plasma (e.g. 40Ar16O+), have ostensibly the same m/z value – 56 amu (atomic mass units). Many of these interferences will be difficult to separate based only on the mass resolving power of the mass spectrometer. Unless this mass interference is eliminated, the resultant measurement will have a high background and lower detection limit directly attributable to the 40Ar16O+ or other interfering ions.
In collision mode, the cell is backfilled with a partial pressure of inert gas, and both the elemental and molecular ions will lose some of their kinetic energy through collisions with the inert gas atoms as they travel through the cell. The likelihood of such collisions will be greater for the much larger 40Ar16O+, so when it reaches the end of the cell, it will have lost far more kinetic energy. By placing a kinetic energy band pass filter at this point, the two types of ions can be effectively separated by their differences in kinetic energy, and only 56Fe+ will continue its journey into the quadrupole mass spectrometer. However, the 56Fe+ intensity will have been slightly cut as well by the energy filter, albeit at a much smaller proportion. Nonetheless, the detection limit will be adversely affected.
In a reaction cell, the inert gas atoms are replaced by a reactive gaseous species. The rationale is that the introduced gas will react with the interfering species to produce a neutral species that can no longer be influenced by the electrostatic fields of the ion optics or the quadrupole. It will be effectively filtered out. The analyte ion is not affected, and thus compared to the collision cell it is a more powerful method for eliminating mass interferences. However, care needs to be taken to ensure that no “new” mass interferences are generated during this process.
Quadrupole mass spectrometers are most commonly found in ICP-MS instruments, although others based on magnetic sectors and time-of-flight are available. Quadrupole mass spectrometers measure one mass at a time, with the RF voltage and direct current (DC) offset voltage set to allow only ions of a single specific m/z value oscillate through the region between the four poles that make up the spectrometer. Ions of other masses will collide with the rods and be eliminated. The RF and DC voltages can be serially scanned to allow for detection through the desired m/z range in an analysis. Depending upon the configuration, magnetic sector mass spectrometers work in a similar fashion, except the magnetic field is scanned to bend the trajectory of ions within the desired m/z range through to the detectors.
As mass spectrometers can have significant length through which the ion must travel to achieve mass separation, it is absolutely imperative that this region is under high vacuum. Otherwise, analyte ions could collide with gas molecules leading to possible charge exchange reactions and decreasing overall sensitivity and increasing unwanted mass interferences.
How do you analyze ICP-MS data and what does it tell you?
Data in ICP-MS is generally analyzed either quantitatively or semi-quantitatively, as isotope ratio measurements or in isotope dilution analyses.
With this knowledge, full quantitative analyses can be planned with appropriate standards, and instrument operation protocols adjusted in the event of identified potential mass interference problems. Quantitative analysis proceeds by deriving calibration curves that will convert measured analyte counts to a concentration. To create this curve, a set of reference standards that have a verified concentration of the analyte are measured under the exact same instrumental conditions as the unknown sample. For these types of analyses the quadrupole RF and DC voltage settings or the magnetic sector magnetic fields need only be set for the m/z values of interest. There is no need to scan through the entire mass range. Finally, the method of isotope dilution is a means of achieving the highest quantitative accuracy. For example, if the analyte is 56Fe+, the sample could be diluted by adding a known quantity of 57Fe, a stable isotope which has a natural abundance of 0.29% and the same chemical and physical properties of 56Fe. It therefore acts as an internal standard. By measuring the isotope ratio 57Fe/56Fe, and since the amount of added 57Fe is known, the concentration of 56Fe can be calculated. The beauty of this method is that the analysis, standardization and quantification is all done in a single experiment, and the end result is arrived at by a ratio, so any instrumental effects will be ruled out.
FAQ
What is J value in ICP-MS?
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