ICP torch interface

Ions produced in an ICP torch interfaced to a quadrupole mass spectrometer. Sample introduction by nebulizer, laser vaporization or electrical heating. 

SFC-ICP-MS requires rather expensive and complicated instrumental design [473,474]. Interfacing the SFC restrictor with the ICP torch follows different approaches for pSFC and cSFC [469]. Polar modifiers, however, do not have a serious deleterious effect on the ICP plasma, which enables the polarity of the mobile phase to be changed with no significant loss of sensitivity or resolution. This enables analysis of compounds which are too polar for adequate separation with pure C02 as the mobile phase. SFC is still in its infancy as far as speciation analysis of metal-containing additives is concerned. 

The layout of an ICP-MS is shown schematically in Figure 8.17 and comprises three essential parts the ICP torch, the interface and the mass spectrometer. The ICP torch differs little from that discussed earlier and the mass spectrometer is very similar to those used for organic mass spectrometry and discussed in Chapter 9. Typically a quadrupole instrument would be used. The construction of the interface is shown in Figure 8.18 and is based on the use of a pair of water-cooled cones which divert a portion of the sample stream into the ion optics of the mass spectrometer whence the mass spectrum is produced by standard mass spectrometer operation. Some modern instruments also incorporate a so-... 

One of the first reported couplings of GC-ICP-MS was by Van Loon et al. [115], who used a coupled system for the speciation of organotin compounds. A Perkin-Elmer Sciex Elan quadrupole mass filter instrument was used as the detector with 1250 or 1500 W forward power. The GC system comprised a Chromasorb column with 8 ml min 1 Ar/2 ml min-1 02 carrier gas flow with an oven temperature of 250°C. The interface comprised a stainless-steel transfer line (0.8 m long) which connected from the GC column to the base of the ICP torch. The transfer line was heated to 250°C. Oxygen gas was injected at the midpoint of the transfer line to prevent carbon deposits in the ICP torch and on the sampler cone. Carbon deposits were found to contain tin and thus proved detrimental to analytical recoveries. Detection limits were in the range 6-16 ng Sn compared to 0.1 ng obtained by ETAAS, but the authors identified areas for future improvements in detection limits and scope of the coupled system. 

Carey and Caruso [126] also summarised the two main approaches to interfacing the SFC restrictor with the ICP torch. The first method, used with packed SFC columns, introduces the restrictor into a heated cross-flow nebuliser and the nebulised sample is subsequently swept into the torch by the nebuliser gas flow. Where capillary SFC systems are used, a second interface design is commonly employed where the restrictor is directly introduced into the central channel of the torch. This interface is more widely used with SFC-ICP-MS coupling [20]. The restrictor is passed through a heated transfer line which connects the SFC oven with the ICP torch. The restrictor is positioned so that it is flush with the inner tube of the ICP torch. This position may, however, be optimised to yield improved resolution. The connection between the transfer line and the torch connection must be heated to prevent freezing of the mobile phase eluent after decompression when exiting the restrictor. A make-up gas flow is introduced to transport the analyte to the plasma. This... 

SFC has received attention as an alternative separation technique to liquid and gas chromatography. The coupling of SFC to plasma detectors has been studied because plasma source spectrometry meets a number of requirements for suitable detection. There have been two main approaches in designing interfaces. The first is the use of a restrictor tube in a heated cross-flow nebuliser. This was designed for packed columns. For a capillary system, a restrictor was introduced into the central channel of the ICP torch. The restrictor was heated to overcome the eluent freezing upon decompression as it left the restrictor. The interface and transfer lines were also heated to maintain supercritical conditions. Several speciation applications have been reported in which SFC-ICP-MS was used. These include alkyl tin compounds (Oudsema and Poole, 1992), chromium (Carey et al., 1994), lead and mercury (Carey et al., 1992), and arsenic (Kumar et al., 1995). Detection limits for trimethylarsine, triphenylarsine and triphenyl arsenic oxide were in the range of 0.4-5 pg. 

The principal methods of interfacing SFC with ICP-MS have been discussed by Carey and Caruso [94]. Where packed SFC columns are used, the SFC restrictor is connected to a heated cross flow nebulizer and the nebulizer gas flow carries the sample to the plasma. For the more commonly used capillary columns, the SFC restrictor is passed through a heated transfer line that is connected directly to the torch of the ICP-MS. For optimal resolution of peaks, the restrictor should be positioned so that it is level with the injector of the ICP torch. This position may be varied slightly (Fig. 10.15). Heat is applied where the transfer line and torch connect to prevent freezing of the mobile phase when it decompresses after exiting the restrictor. To transport the analyte to the plasma efficiently, a gas flow of approximately 0.8-1.0 mL/min is used. This gas flow may also be heated to improve peak resolution. 

Although there has been limited use with CE interfaces, the direct injection nebulizer (DIN) was first described by Shum et al. - and later used by Liu et al. for CE (Fig. 2E). In this design, the nebulizer introduces the sample very near the plasma inside the ICP torch and eliminates the spray chamber assembly. Close to 100% analyte transport efficiency can theoretically be obtained with the DIN, but the nebulizer is restricted to very low liquid flow rate and thus is well matched to CE interfacing. This design does induce local plasma cooling due the lack of desolvation and detection limits are only slightly improved over other nebulizer designs. 

A modified version of the Grimm-type GDL has been described by Shao and Horlich [594], The source has a floating restrictor and is designed so as to replace an ICP torch in an ICP-MS. Therefore, the anode is slightly positive with respect to the earthed skimmer interface of the MS system. The simultaneous analysis of an unknown sample and a reference material was carried out by means of a system based on two pulsed GD sources housed within the same tube [595], Optimization of the relative position of the two cathodes was achieved by evaluating the signals produced in GD-MS when using the same specimen for each of them. 

Figure 11 - 2 shows schematically the components of a commercial ICPMS system. A critical part of the instrument is the interlace that couples the ICP torch, which operates at atmospheric pressure with the mass spectrometer that requires a pressure of less than lO tnrr. This coupling is accomplished hy a differentially pumped interface coupler that consists of a sampling cone, which is a vsater-coolcd nickel cone with a small... 

Describe the interface between the ICP torch and the mass speclrotncicr in an ICPMS instrumetu. 

Figure 7.29 Axial (end-on) ICP torch position showing how the shear gas cuts off the cool plasma tail plume. Light emitted from the plasma passes through the ceramic interface into the spectrometer on the right side of the diagram (not shown). [From Boss and Fredeen, courtesy of PerkinElmer Inc. (www.perkinelmer.com).]...

The combination of ICP torch and mass spectrometric resolution resulted in a much more powerful technique than ICP-AES in terms of sensitivity, selectivity and precision. Further, the coupling of MS to ICP results in the analysis of isotopes of various elements. In ICP-MS, the plasma torch is in a horizontal position and it works under normal pressure. An interface cone is placed between a plasma source and mass spectrometer. Ions produced in ICP are transferred to the mass spectrometer through a small hole (about 1 mm in diameter) in the cone. The mass spectrometer is usually a quadrapole analyser. 

Solid deposition on the interface cone and skimmer is a problem in ICP-MS. Deposition decreases with increased power of the ICP torch. Major interference effects in ICP-MS may be divided into two groups (3.1) signal enhancement and suppression effects, and (3.2) spectral interferences. 

The ICP source operates at atmospheric pressure, whereas a mass spectrometer requires a pressure of <10 ton. Therefore, the coupling of an extremely hot atmospheric pressure ICP torch with a mass spectrometer that operates at high vacuum is a challenge. A suitable interface is required to overcome this challenge. ICP-MS coupling was first attempted in the early 1980s [17,18]. Conceptually,... 

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