By T. De Smaele, J. Vercauteren, L. Moens, R. Dams, Laboratory of Analytical Chemistry, Ghent University, Institute for Nuclear Sciences; P. Sandra, Laboratory of Organic Chemistry, Ghent University, Gent (Belgium)

We obtained detection limits on the order of low-ng/L for metal (or fg absolute) by coupling an HP 6890 gas chromatograph (GC) to an HP 4500 inductively coupled plasma mass spectrometer (ICP-MS) by means of a heated stainless-steel transfer line. Construction of the transfer line allows coupling and decoupling within a few minutes, which guarantees high sample throughput for both total-metal analysis and metal speciation.

Why couple GC to ICP-MS?
Notwithstanding the possibilities of capillary gas chromatography coupled to atomic emission detection (CGC/AED) for metal speciation, there are a number of features that make the hyphenation CGC/ ICP-MS unique. State-of-the-art ICP-MS instrumentation provides sensitivities that no other technique can offer and allows multi-element detection in a single run. Moreover, the argon (Ar) plasma is more stable than the helium (He) microwave-induced plasma in AED, and solvent venting in CGC/ICP-MS becomes redundant. In addition, ICP-MS offers isotopic information of the elements of interest, allowing even isotope dilution techniques as calibration method.

Typical operating conditions for both instruments are summarized in Table I. The coupling^1-3 is schematically outlined in Figure 1.

Table I. Operating parameters of CGC/ICP-MS

All parameters such as make-up gas flow rate, torch box position, and lens settings can be easily optimized through the software by measuring a continuous xenon (Xe) signal. Hydrogen used as CGC carrier gas is therefore doped with xenon in a concentration of 0.1 % (v/v). The ¹²⁶Xe⁺ signal of the less abundant Xe isotope is measured and the different parameter settings adjusted for the highest possible intensity and the most stable signal.

Figure 1. Schematic of the transfer line between GC and ICP-MS.

A very important and critical parameter is the torch position. The easiest way to optimize the torch position is to use the Autotune facility in the HP ChemStation software with conventional pneumatic nebulization and the GC decoupled. Once the Autotune performed the optimization, the torch position is locked and the GC coupled to the ICP-MS. To fine-tune the torch position, manual tuning in the ChemStation software and measurement of the continuous ¹²⁶Xe⁺ signal can be used.

During CGC/ICP-MS analyses, the ¹²⁶Xe⁺ signal is also used both as an internal standard to correct for changes in instrument sensitivity and signal drift and to check performance and condition of the system in general. One small leak will deteriorate the ¹²⁶Xe⁺ signal immediately.

Figure 2. Fast CGC/ICP-MS of organometallic compounds at the 100 ng/L level

Different Masses Measured Simultaneously in 3.5 Minutes
The multi-element capabilities of the HP 4500 ICP-MS in its hyphenation with GC is demonstrated in Figure 2. Using the splitless mode, we manually injected 1 µL of a 100 ng/L (as metal) organometallic standard, ethylated with sodium tertaethylborate and containing methyl mercury (MM), trimethyl lead (TML), dimethyl lead (DML), monobutyl tin (MBT), dibutyl tin (DBT), tributyl tin (TBT), and tripropyl tin (TPT) which in the analysis of real samples is used as internal standard. The HP 6890 GC has some unique features, like electronic pneumatic control (EPC) and high-temperature ramps for temperature programming that can be employed to obtain short analysis times. The chromatogram shown was obtained on a conventional capillary column (30 m x 0.25 mm x 0.25 µm methylsilicone film) at a program rate of 100ºC/min in the constant-flow mode. The separations were completed in 3.5 minutes! The chromatogram was recorded in the time-resolved analysis mode at a scan rate of 30 ms/mass and 1 point/mass. The continuous signal was scanned at 0.05 ms/mass. With these parameters, four different masses can be measured simultaneously. The peaks obtained were sharp, and no distortion was observed.

Table II. Linearity and limits of detection for organometallics

Linearity
The linear dynamic range was checked by subsequent injections of ethylated organometal standard solutions in the concentration range 10 ng/L to 5 µg/L. The regression coefficients for the different species range between 0.9922 and 0.9998 (Table II). The linear dynamic range extends over at least three orders of magnitude.

Lowest LODs Ever Reported
The limits of detection (LOD) for organomercury, -tin and -lead compounds were determined as 3 times the standard deviation of the background measured after 10 successive injections of 1 µL isooctane containing the internal standard tripropyl tin. The results are summarized in Table II. The LODs are in the low-ng/L range for organotin and organomercury compounds and even at the sub-ng/L level for organolead compounds. These values correspond to absolute amounts in the fg- and sub-fg-range. These values are the lowest instrumental LODs for CGC/ICP-MS ever reported.

Figure 3. Detail of the T-joint.

The "Mechanics" of Coupling GC and ICP-MS The transfer line between the two instruments consists of two stainless-steel tubes (3 o.d. mm and 2 mm i.d.) that are resistively heated by applying an a.c. voltage from a variable power supply. The stainless-steel tubes are joined by welding in a miniaturized T-joint (Figure 3), which consists of a stainless-steel plate (30 x 10 x 5 mm) with apertures to connect the stainless-steel tubes. A Swagelok® reducing union (1/8-to-1/16-inch) was welded on one side. The T-joint is placed in the GC oven wall to reduce cooling as much as possible. The shortest stainless-steel tube (1 m) is used to guide a fused-silica capillary (in fact, the proper transfer line) to the ICP-MS torch. The second stainless-steel tube (2 m) is folded around the transfer line and acts as an argon gas heater. Heating for Ar Gas Essential Since the CGC effluent is on the order of a few mL/min while the ICP-MS operates at a gas flow rate of approximately 1000-1500 mL/min, additional Ar make-up gas is added to the GC effluent. Heating this gas is essential to prevent condensation of the analytes. Two electrical contact points are welded on the stainless-steel tubes: one at the beginning of the Ar gas heater, the second one 180 mm from the tip of the transfer line. In the middle of the transfer line, a thermocouple (type K) is attached for temperature control. With this design, an equable temperature is obtained over the entire transfer line. The most critical part of the interface is the part that is inserted into the torch. The last 180 mm of the stainless-steel tube should be adequately heated to prevent condensation of the solutes eluting from the analytical capillary column in the ICP-MS torch by the plasma gas and cooling gas. Since the inner diameter of the classical injector tube of the ICP-MS torch is only 38 mm and the outer diameter of the stainless-steel tube 30 mm, the latter must be heated directly as applied for the other part of the transfer line. To accomplish that, the last 180 mm are cut lengthwise into two halves. Insulating and Rinsing The two halves of the tubing are electrically insulated over the entire length by covering the inner surfaces with polyimide insulating tape. At the end of the transfer line, the two halves are electrically connected. Finally, a thin polyimide layer is wrapped around the two halves of the transfer line to obtain the outer diameter and shape of the original tube. By applying an a.c. voltage over the two halves, this part of the transfer line can be uniformly heated while positioned in the torch. All stainless-steel tubes are rinsed with 10% chloric acid and acetone to remove all greases and metal particles, then flushed with Ar and heated to 300ºC to dry. The entire transfer line is insulated with glass and rock wool and wrapped into industrial pipe lagging. The completed transfer line is compact and approximately 1 m in length. The transfer fused-silica capillary (0.25 mm i.d.) is part of a deactivated retention gap and is coupled to the analytical column with a deactivated glass press-tight connector. To strengthen the transfer capillary, it is slipped into a 0.53-mm-i.d. deactivated fused-silica capillary. To prevent discharges between the transfer line and the RF coil of the ICP-MS, the transfer line is inserted into the torch to approximately 20 mm from the end of the injector tip and attached to the torch with a teflon balljoint piece and clip. The fused-silica capillary, however, is inserted into the tip of the injector tube and immobilized by tightening the vespel ferrule in the reducing union of the T-joint. Swagelok is a U.S.-registered trademark of Swagelok Company

Click for more information on the HP 4500 ICP-MS

References
1. De Smaele, T., Verrept, P., Moens, L., and Dams, R. Spectrochim. Acta, 50B, 1409 (1995).
2. De Smaele, T., Moens, L., Dams, R., and Sandra, P. Fresenius' J. Anal. Chem., 6, 355, 778 (1996).
3. De Smaele, T., Moens, L., Dams, R., and Sandra, P. LC-GC Int., 9 (3), 138 (1996); LC-GC, 14 (10), 876 (1996) and LC-GC Asia Pacific, 1 (2), 28 (1998).
4. Moens, L., De Smaele, T., Dams, R., Van Den Broeck, P., and Sandra, P. Anal. Chem., 69, 1604 (1997).
5. De Smaele,T., Moens, L., Dams, R., Sandra, P., Van der Eycken, P., and Vandyck, J.
J. Chromatogr. A, 793, 99 (1998).

It is not our intent to discuss sampling and sample preparation for real samples (e.g. water, soil, sediment, grass, fish tissue, etc.). We refer the interested reader to References 4 and 5.