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Inductively Coupled Plasma Ionization Sources

The most common application of ICP ionization system is to determine element speciation. In principle, the different compounds can be separated by liquid chromatography, and the mass spectrometer can select those separated components that contain the specific element or elements of interest. The IC ionization system is applicable to this type of analysis, as it can provide charged atoms of all the heavy elements present, and it is the speciation of such elements that is important. The details of the ICP torch, that is used to produce the charged atoms, has already been described and so some examples of its use will be given.

Shum and Houk [33] employed size exclusion and ion exchange chromatography to separate certain metal proteins , and two species of selenium , SeO32- and SeO43-. 15 pg of selenium could be easily detected. Due to its high sensitivity and selectivity, Atomic Spectroscopy is the preferred method for the determination of trace elements. However, Atomic Spectroscopy cannot differentiate between the chemical forms and oxidation states of the element, and these can be very important, as they determine the toxicity of the substance, and the role the element plays in biosynthesis. If a separation technique is used to differentiate between the different species of the element or its oxidation state, then the mass spectrometer can unambiguously identify the peaks that contain the element of interest. In addition, if isotope ratioing is employed, a quantitative assay can also be accomplished.

A GPC column was used for the separation of the metal proteins and an ion exchange column was used for the separation of the selenium . The mobile phase from the column passed through a fused quartz capillary to a direct-injection nebulizer; the distance between the inner capillary and the nebulizer tip was about 25 μm. The ICP/MS tandem instrument was the Elan Model 250 (Perkin Elmer Sciex).

A sample of blood serum containing metal proteins of lead, cadmium , zinc , barium , copper, iron and sodium was separated on the size exclusion column, and the elements monitored by the mass spectrometer. The results obtained are shown in figure 51. It should be noted that results for all seven metals were obtained from a single injection. The separation of the different oxidation states of selenium on an ion exchange column is shown in figure 52

Figure51. The Separation of Metalloproteins in Human Serum

Figure 52. The Separation of SeO32- and SeO42-

Figures 51 and 52 demonstrate the general efficacy of the system. The detection limits for iron, copper, zinc , cadmium and lead were reported to be 3 pg, 0.7 pg, 1 pg, 0.5 pg and 0.5 pg, respectively.

Although the elements are unambiguously identified, the chemical nature of the eluents can only be assessed from the chromatographic data. It would be possible to employ an electrospray interface to another mass spectrometer, in parallel with the ICP/Ms instrument, and this could provide further information on the structure of the associated protein.

Powell et al. [34], described a sensitive technique, employing an API interface to determine the speciation and the quantitative assay of Cr(III) and Cr(VI). The API interface used a direct injection nebulizer, and a sensitivity of 30, 60 and 180 ng/l was shown for the total Chromium , Cr(III) and Cr(VI) respectively.

The sample volume was 10 μl and, thus, the actual mass sensitivity appears to be 0.3, 0.6, and 1.8 pg of the total Chromium , Cr(III) and Cr(VI) respectively. A diagram of the sampling arrangement is shown in figure 53.

Figure 53. Valve Configuration for the Simultaneous Monitoring of Total and Speciated Chromium

The valves were connected so that the 10 μl sample could either be directed through the LC column, which would separate the different Chromium species, before the eluent passed to the direct inlet nebulizer, or to a by-pass loop, which would allow the total sample to be passed directly to the nebulizer.

Figure 54. The Separation of the Different Valency States of Chromium .

The same procedure was used as described previously, the liquid chromatograph separated the different species of Chromium , and the ICP/MS identified those peaks that contained the Chromium . The two oxidation states of Chromium were well separated and, as shown in figure 54, both the 52 and 53 isotopes could be clearly monitored and differentiated.dissolved in natural waters are important to the metabolic processes that take place in aquatic ecosystems. Such material acts as a substrate in heterotrophic processes (metabolic assimilation of decaying vegetable or animal tissue) or as enzymes , vitamins or toxins in different organisms. In natural waters, the dissolved organic material largely comprise of humic substances. Humic substances contain a range of polar and dispersive functional groups and can, therefore, interact strongly with heavy metal pollutants as well as PCBs and PAHs and pesticides . Thus, heavy metals can be transported by the humic substances over large distances. Alternatively, the humic substances can trap the heavy metals and act as a detoxification agent. It is clear that the speciation of the different heavy metals in water can have very important ramifications. On the one hand, a particular species can contribute to water toxicity, and on the other, it can help in the purification of water for drinking purposes.

Rottman and Heumann [35] developed an apparatus to examine heavy metal interactions with dissolved organic substances in natural waters. They employed a LC/MS combination with an API interface and a special sample system. A diagram of their sampling arrangement is shown in figure 55.The sample volume was 500 μl, which, on injection, passed through a guard column packed with TopOffGel 3PW, and then through a TSK 3000 PW, glass, size exclusion, analytical column.

The mobile phase was monitored by a UV detector and the exit flow passed through a second sample valve (used for calibrating the spike flow) to a Y junction where it was mixed with the spike flow. The mixture then passed directly to the nebulizer of the ICP interface and was then monitored by the mass spectrometer. The results obtained from the analysis of a number of natural waters are shown in figure 56.

Figure 55. The LC/MS Tandem Apparatus for Measuring the Speciation of Heavy Metals in Natural Water

Figure 56. UV Absorption and Elemental Chromatograms of Different Water Samples

All the chromatograms show clearly fractionated organic substances. The components from the bog water are eluted fairly early in the chromatogram, and as the separation was carried out on an exclusion column, this means that the organic materials were of significantly higher molecular weight (having larger molecular volume) than those from the river and lake water. It is also seen that the distribution of the different heavy metal elements, within the range of humic compounds present, differs quite considerably between the different water sources. The technique is obviously ideal for examining the speciation of the heavy elements, even at the low concentrations normally found in natural waters.


Figure 57. The Micro-Flow Injection Apparatus for the Assay of Arsenic Compounds

Pergantis et al. [36] designed a microscale system, using microbore columns and the ICP ionization system for the detection and estimation of arsenic compounds at the fentogram level. The system was designed to minimize any band dispersion that could take place between the sample valve, or column exit, and the nebulizer. A diagram of the microscale flow injection system is shown in figure 57.


Figure 58. The Separation of Some Arsenic Compounds Using Different Chromatographic Conditions.

The micro-flow nebulizer was designed to operate at low flow rates, i.e. in the range 10 to 150 μl/min. The nebulizer is placed inside the ICP torch, and is designed to give a very fine droplet spray, at the low flow rates demanded by microbore columns.

The system was used to separate and identify a range of arsenical animal feedstock additives, from naturally occurring organic and inorganic arsenic compounds. The separations obtained are shown in figure 58. The upper chromatogram (A) shows the separation achieved by employing dispersive interactions only, the lower chromatogram (B) was obtained by exploiting a mixture of dispersive and ionic interactions by using an ion pair reagent contained in the mobile phase. The lower limit of detection was reported as 4 ng/l, which would be equivalent to 4 pg/ml. The volume of sample placed on the column is not clear, but if it were 1 μl, then this would be equivalent to a mass of 4 fentograms. The high resolution of the liquid chromatograph, coupled with the high sensitivity of ICP-MS, makes the system a very powerful tool for use in many contemporary analytical applications.

 

 

About the Author
RAYMOND PETER WILLIAM SCOTT was born on June 20 1924 in Erith, Kent, UK. He studied at the University of London, obtaining his B.Sc. degree in 1946 and his D.Sc. degree in 1960. After spending more than a decade at Benzole Producers, Ltd. Where he became head of the Physical Chemistry Laboratory, he moved to Unilever Research Laboratories as Manager of their Physical Chemistry department. In 1969 he became Director of Physical Chemistry at Hoffmann-La Roche, Nutley, NJ, U.S.A. and subsequently accepted the position of Director of the Applied Research Department at the Perkin-Elmer Corporation, Norwalk, CT, U.S.A.
In 1986 he became an independent consultant and was appointed Visiting Professor at Georgetown
University, Washington, DC, U.S.A. and at Berkbeck College of the University of London; in 1986 he retired but continues to write technical books dealing with various aspects of physical chemistry and physical chemical techniques. Dr. Scott has authored or co-authored over 200 peer reviewed scientific papers and authored, co-authored or edited over thirty books on various aspects of physical and analytical chemistry. Dr. Scott was a founding member of the British chromatography Society and received the American Chemical society Award in chromatography (1977), the M. S. Tswett chromatography Medal (1978), the Tswett chromatography Medal U.S.S.R., (1979), the A. J. P. Martin chromatography Award (1982) and the Royal Society of Chemistry Award in Analysis and Instrumentation (1988).
Dr. Scott’s activities in gas chromatography started at the inception of the technique, inventing the Heat of Combustion Detector (the precursor of the Flame Ionization Detector), pioneered work on high sensitivity detectors, high efficiency columns and presented fundamental treatments of the relationship between the theory and practice of the technique. He established the viability of the moving bed continuous preparative gas chromatography, examined both theoretically and experimentally those factors that controlled dispersion in packed beds and helped establish the gas chromatograph as a process monitoring instrument. Dr. Scott took and active part in the renaissance of liquid chromatography, was involved in the development of high performance liquid chromatography and invented the wire transport detector. He invented the liquid chromatography mass spectrometry transport interface, introduced micro-bore liquid chromatography columns and used them to provide columns of 750,000 theoretical plates and liquid chromatography separations in less than a second. Dr. Scott has always been a “hands-on” scientist with a remarkable record of accomplishments in chromatography ranging from hardware design to the development of fundamental theory. He has never shied away from questioning “conventional wisdom” and his original approach to problems has often produced significant breakthroughs.

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