Liquid sample introduction

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. 

Approximately 70 different elements are routinely determined using ICP-OES. Detection limits are typically in the sub-part-per-billion (sub-ppb) to 0.1 part-per-million (ppm) range. ICP-OES is most commonly used for bulk analysis of liquid samples or solids dissolved in liquids. Special sample introduction techniques, such as spark discharge or laser ablation, allow the analysis of surfaces or thin films. Each element emits a characteristic spectrum in the ultraviolet and visible region. The light intensity at one of the characteristic wavelengths is proportional to the concentration of that element in the sample. 

Samples must be introduced into the plasma in an easily vaporized and atomized form. Typically this requires liquid aerosols with droplet diameters less than 10 pm, solid particles 1-5 pm in diameter, or vapors. The sample introduction method strongly influences precision, detection limits, and the sample size required. 

A versatile Laser-SNMS instrument consists of a versatile microfocus ion gun, a sputtering ion gun, a liquid metal ion gun, a pulsed flood electron gun, a resonant laser system consisting of a pulsed Nd YAG laser pumping two dye lasers, a non-resonant laser system consisting of a high-power excimer or Nd YAG laser, a computer-controlled high-resolution sample manipulator on which samples can be cooled or heated, a video and electron imaging system, a vacuum lock for sample introduction, and a TOF mass spectrometer. 

Ionisation in an API source can take place in a variety of ways depending on the type of applications, namely by gas-phase ionisation, liquid- and plasma-based ionisation. At present, there are three major application areas of API-MS air or gas analysis (industrial emissions), on-line LC-MS (largest commercial application), and ICP-MS. A wide variety of sample introduction devices are available for gas analysis by API-MS. For use in ICP-MS, ions are sampled directly from the inductively... 

In addition to standard liquid injection there are many GC accessories which can provide different methods of sample introduction to the column, such as HS, SPE, SFE, TD, TG, Py, etc. Examples of such GC-FTIR devices are TD-GC-FT1R (with a cryostat interface) and PyGC-FTIR. 

In ICP-AES and ICP-MS, sample mineralisation is the Achilles heel. Sample introduction systems for ICP-AES are numerous gas-phase introduction, pneumatic nebulisation (PN), direct-injection nebulisation (DIN), thermal spray, ultrasonic nebulisation (USN), electrothermal vaporisation (ETV) (furnace, cup, filament), hydride generation, electroerosion, laser ablation and direct sample insertion. Atomisation is an essential process in many fields where a dispersion of liquid particles in a gas is required. Pneumatic nebulisation is most commonly used in conjunction with a spray chamber that serves as a droplet separator, allowing droplets with average diameters of typically <10 xm to pass and enter the ICP. Spray chambers, which reduce solvent load and deal with coarse aerosols, should be as small as possible (micro-nebulisation [177]). Direct injection in the plasma torch is feasible [178]. Ultrasonic atomisers are designed to specifically operate from a vibrational energy source [179]. 

The //PLC system described in this chapter is equipped with 24 parallel columns for liquid chromatography, each with its own sample introduction port and exit port for connection to detectors of choice (UV absorbance and/ or fluorescence). Flow from a binary solvent delivery system is divided evenly across 24 channels and results in 1/24 of the programmed pump flow rate through each column (i.e., total flow of 300 /zL/ min will produce a flow of 12.5 /zL/ min in each column). Samples are introduced to the columns by a multichannel autosampler configured to sample from either 96-or 384-well SBS standard plates. Figure 6.2 depicts a general view of the system. 

As mentioned previously, introducing the sample to the flowing mobile phase at the head of the column is a special problem in HPLC due to the high pressure of the system and the fact that the liquid mobile phase may chemically attack a rubber septum. For these reasons, the use of the so-called loop injector is the most common method for sample introduction. 

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