ICP-MS

This chapter reviews and expands on the material presented by the authors at the 1987 Eastern Analytical Symposium. A short review of the work done in this area by other researchers will be presented for completeness. The primary focus will be on chromatographic sample introduction for plasma source MS. The use of ICP-MS for the detection of high performance liquid chromatography (HPLC) eluates will be discussed. Speciation of several As and Sn compounds found in the environment demonstrates the ability of ICP-MS to provide element specific detection at ultra-trace levels. In addition, gas chromatographic detection using He microwave-induced plasma mass spectrometry (MIP-MS) is shown to be a promising technique, particularly for the determination of halogenated species. 

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If a sample solution is introduced into the center of the plasma, the constituent molecules are bombarded by the energetic atoms, ions, electrons, and even photons from the plasma itself. Under these vigorous conditions, sample molecules are both ionized and fragmented repeatedly until only their constituent elemental atoms or ions survive. The ions are drawn off into a mass analyzer for measurement of abundances and mJz values. Plasma torches provide a powerful method for introducing and ionizing a wide range of sample types into a mass spectrometer (inductively coupled plasma mass spectrometry, ICP/MS). 

Fundamentally, introduction of a gaseous sample is the easiest option for ICP/MS because all of the sample can be passed efficiently along the inlet tube and into the center of the flame. Unfortunately, gases are mainly confined to low-molecular-mass compounds, and many of the samples that need to be examined cannot be vaporized easily. Nevertheless, there are some key analyses that are carried out in this fashion the major one i.s the generation of volatile hydrides. Other methods for volatiles are discussed below. An important method of analysis uses lasers to vaporize nonvolatile samples such as bone or ceramics. With a laser, ablated (vaporized) sample material is swept into the plasma flame before it can condense out again. Similarly, electrically heated filaments or ovens are also used to volatilize solids, the vapor of which is then swept by argon makeup gas into the plasma torch. However, for convenience, the methods of introducing solid samples are discussed fully in Part C (Chapter 17). 

To examine a sample by inductively coupled plasma mass spectrometry (ICP/MS) or inductively coupled plasma atomic-emission spectroscopy (ICP/AES) the sample must be transported into the flame of a plasma torch. Once in the flame, sample molecules are literally ripped apart to form ions of their constituent elements. These fragmentation and ionization processes are described in Chapters 6 and 14. To introduce samples into the center of the (plasma) flame, they must be transported there as gases, as finely dispersed droplets of a solution, or as fine particulate matter. The various methods of sample introduction are described here in three parts — A, B, and C Chapters 15, 16, and 17 — to cover gases, solutions (liquids), and solids. Some types of sample inlets are multipurpose and can be used with gases and liquids or with liquids and solids, but others have been designed specifically for only one kind of analysis. However, the principles governing the operation of inlet systems fall into a small number of categories. This chapter discusses specifically substances that are normally liquids at ambient temperatures. This sort of inlet is the commonest in analytical work. 

Solutions can be examined by ICP/MS by (a) removing the solvent (direct and electrothermal methods) and then vaporizing residual sample solute or (b) nebulizing the sample solution into a spray of droplets that is swept into the plasma flame after passing through a desolvation chamber, where excess solvent is removed. The direct and electrothermal methods are not as convenient as the nebulization inlets for multiple samples, but the former are generally much more efficient in transferring samples into the flame for analysis. 

Suffice it to say at this stage that the surfaces of most solids subjected to such laser heating will be heated rapidly to very high temperatures and will vaporize as a mix of gas, molten droplets, and small particulate matter. For ICP/MS, it is then only necessary to sweep the ablated aerosol into the plasma flame using a flow of argon gas this is the basis of an ablation cell. It is usual to include a TV monitor and small camera to view the sample and to help direct the laser beam to where it is needed on the surface of the sample. 

Samples to be examined by inductively coupled plasma and mass spectrometry (ICP/MS) are commonly in the form of a solution that is transported into the plasma flame. The thermal mass of the flame is small, and ingress of excessive quantities of extraneous matter, such as solvent, would cool the flame and might even extinguish it. Even cooling the flame reduces its ionization efficiency, with concomitant effects on the accuracy and detection limits of the ICP/MS method. Consequently, it is necessary to remove as much solvent as possible which can be done by evaporation off-line or done on-line by spraying the solution as an aerosol into the plasma flame. 

The term nebulizer is used generally as a description for any spraying device, such as the hair spray mentioned above. It is normally applied to any means of forming an aerosol spray in which a volume of liquid is broken into a mist of vapor and small droplets and possibly even solid matter. There is a variety of nebulizer designs for transporting a solution of analyte in droplet form to a plasma torch in ICP/MS and to the inlet/ionization sources used in electrospray and mass spectrometry (ES/MS) and atmospheric-pressure chemical ionization and mass spectrometry (APCI/MS). 

Many designs of nebulizer are commonly used in ICP/MS, but their construction and mode of operation can be collated into a small number of groups pneumatic, ultrasonic, thermospray, APCI, and electrospray. These different types are discussed in the following sections, which are followed by further sections on spray and desolvation chambers. 

The size of the droplets formed in an aerosol has been examined for a range of conditions important in ICP/MS and can be predicted from an experimentally determined empirical formula (Figure 19.6). Of the two terms in the formula, the first is most important, except at very low relative flow rates. At low relative velocity of liquid and gas, simple droplet formation is observed, but as the relative velocity increases, the stream of liquid begins to flutter and to break apart into long thinner streamlets, which then break into droplets. At even higher relative velocity, the liquid surface is stripped off, and the thin films so-formed are broken down into... 

Using Poiseuille s formula, the calculation shows that for concentric-tube nebulizers, with dimension.s similar to those in use for ICP/MS, the reduced pressure arising from the relative linear velocity of gas and liquid causes the sample solution to be pulled from the end of the inner capillary tube. It can be estimated that the rate at which a sample passes through the inner capillary will be about 0.7 ml/min. For cross-flow nebulizers, the flows are similar once the gas and liquid stream intersection has been optimized. 

For a longitudinal disturbance of wavelength 12 pm, the droplets have a mean diameter of about 3-4 pm. These very fine droplets are ideal for ICP/MS and can be swept into the plasma flame by a flow of argon gas. Unlike pneumatic forms of nebulizer in which the relative velocities of the liquid and gas are most important in determining droplet size, the flow of gas in the ultrasonic nebulizer plays no part in the formation of the aerosol and serves merely as the droplet carrier. 

Plasma torches and thermal ionization sources break down the substances into atoms and ionized atoms. Both are used for measurement of accurate isotope ratios. In the breakdown process, all structural information is lost, other than an identification of elements present (e.g., as in inductively coupled mass spectrometry, ICP/MS). 

Samples to be examined by inductively coupled plasma and mass spectrometry (ICP/MS) are frequently in the form of a solution of an analyte in a solvent that may be aqueous or organic. 

ICP/MS. inductively coupled plasma and mass spectrometry used as a combined technique ICR. ion cyclotron resonance (spectroscopy)... 

To measure trace metals to the levels required in the guidelines involves the use of state-of-the-art instmmentation such as inductively coupled plasma/mass spectrometry (icp/ms). 

Miscellaneous. Trace analyses have been performed for a variety of other materials. Table 9 Hsts some uses of electrothermal atomic absorption spectrometry (etaas) for determination of trace amounts of elements in a variety of matrices. The appHcations of icp /ms to geological and biological materials include the following (165) ... 

Obtaining of data concerning the chemical composition of water is critical significance for monitoring water reservoirs and forecasting the quality of drinking water from different water supply sources. A dry residue is commonly used with the methods AAS, ICP-AES, ICP-MS (analysis of liquid) widely applied for determination of water composition. So it is vital to create a standard sample of the composition of dry residue of ultra-fresh Lake Baikal water, its development launched since 1992 at the Institute of Geochemistry SB RAS. 

At present time the use of oxide single erystals sueh as bismuth germanate (Bi Ge O, ) and pai atellurite (TeO,) as deteetors in opto-eleetronies stimulate produetion of high purity Bi, Te, Ge and their oxides Bi O, GeO, TeO,. This requires development of analytieal teehniques for purity eontrol of these materials. For survey traee analysis atomie emission speetrometry (AES) and mass speetrometry (MS) with induetively eoupled plasma (ICP) is widely used. However, the deteetion limits of impurities aehievable by these methods for the analysis of high purity solids are limited by neeessity of sample dissolution in pure aeids and dilution up to 5 10 times for ICP-MS and 50-100 for ICP-AES. One of the most effeetive ways to improve the analytieal performanees of these methods is pre-eoneentration of miero-elements. 

The use of pre-eoneentration in eombination with ICP-AES and ICP-MS let us to diminish a degree of sample dilution up to two orders of magnitude and essentially reduee the limits of deteetion of about 30 elements. In addition, low eontent of matrix element in the solutions prepai ed for ICP-analysis reduees the matrix influenee and minifies the speetral interferenees. Main limitation, espeeially for determination of widespread elements (Si, Ca, Fe, ete.) at the ppb and ppt levels is the purity of ehemieals used for eoneentrate dissolution and dilution. 

In the report analytieal figures of merit of ICP-MS and -AES teehniques in eombination with pre-eoneentration of traee elements for survey analysis of high purity metals and their oxides used as the preeursors for oxide monoerystals produetion will be presented and eompared to that of direet ICP-MS and -AES teehniques. 

Inter-laboratory control for collecting samples of soils from cell N36E46 was performed, with AEA, XRE, ICP-MS employed, at the Institute of Geochemistry, Irkutsk, Russia and Kingston University, England. 

The complex of the following destmctive and nondestmctive analytical methods was used for studying the composition of sponges inductively coupled plasma mass-spectrometry (ICP-MS), X-ray fluorescence (XRF), electron probe microanalysis (EPMA), and atomic absorption spectrometry (AAS). Techniques of sample preparation were developed for each method and their metrological characteristics were defined. Relative standard deviations for all the elements did not exceed 0.25 within detection limit. The accuracy of techniques elaborated was checked with the method of additions and control methods of analysis. 

SELECTION OF CONDITIONS OF SOIL MICROWAVE DECOMPOSITION FOR ICP-MS ANALYSIS... 

Analysis of soils is an important task in the environmental researches. Reliability of ICP-MS results of soil analysis mainly depends on chemical sampling. Recently microwave systems are widely used for preparation of different samples. Influence of microwave radiation on sample ensures a complete decomposition of sample, greatly increases the mineralization, and allows possible losses of volatile elements to be minimized. In the given study to intensify decomposition of soils we applied the microwave sample preparation system MULTIWAVE (Anton Paar, Austria and Perkin-Elmer, USA) equipped with rotor from 6 autoclaves with TEM reaction chambers of 50 ml volume. 

HR-ICP-MS EEEMENT-2 (Pinnigan MAT, Germany) equipped with a standard introduction system (quartz water-cooled spray chamber, concentric nebulizer, torch with 1.5 mm i.d. injector and nickel cones) was used for measurements. The following operating conditions were used RP power 1150 W, coolant gas flow rate 16 1 min k auxiliary gas flow rate 0.85 1 min nebulizer gas flow rate 1.2 1 min k Sample uptake rate was 0.8-1 ml min k Measurements were performed with low and middle resolutions. Rh was used as an internal standard. Por calibration working standard solutions were prepared by diluting the multielemental stock solutions CPMS (SPEX, USA) with water to concentration range from 5 ng to 5 p.g I k... 

The most common detectors in HPLC are ultraviolet, fluorescence, electrochemical detector and diffractometer. However, despite all improvements of these techniques it seems necessary to have a more selectivity and sensitivity detector for the purposes of the medical analysis. It should be therefore improvements to couple analytical techniques like infrared IR, MS, nuclear magnetic resonance (NMR), inductively coupled plasma-MS (ICP-MS) or biospecific detectors to the LC-system and many efforts have been made in this field. 

Laser based mass spectrometric methods, such as laser ionization (LIMS) and laser ablation in combination with inductively coupled plasma mass spectrometry (LA-ICP-MS) are powerful analytical techniques for survey analysis of solid substances. To realize the analytical performances methods for the direct trace analysis of synthetic and natural crystals modification of a traditional analytical technique was necessary and suitable standard reference materials (SRM) were required. Recent developments allowed extending the range of analytical applications of LIMS and LA-ICP-MS will be presented and discussed. For example ... 

LA techniques using UV YAG-Nd laser (wave length - 266 nm) for trace analysis of natural and artificial diamonds and for detenuination of chemical composition of micro-inclusions (5 - 50 p.m ) in natural quartzites were developed by ICP-MS ELEMENT (Germany). 

DETERMINATION OF TRACE ELEMENTS IN GEOLOGICAL SAMPLES BY MAGNETIC SECTOR ICP-MS... 

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