ICP-OES Sample Preparation

A variety of sample types can be analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES), including aqueous and organic liquid and solid samples. These have to be brought into a state that the ICP-OES instrument as a whole can process for elemental analysis. The most typical sample form is a liquid. A liquid sample is introduced using a peristaltic pump to ensure constant, stable flow. Commonly, a nebulizer uses a high-speed flow of gas (usually argon) to shatter small droplets of liquid into an aerosol. This aerosol is then introduced into a spray chamber which removes the larger droplets. Only the aerosol is then transported to the plasma. Solid samples are typically ablated into small particles either using a laser or spark ablation system and then transported directly to the plasma by a carrier gas.

Samples analyzed by ICP-OES cover many application areas, including those in the environmental, metallurgical, geological, petrochemical, pharmaceutical, materials, and food safety.

When the amount of dissolved solids in a sample (e.g., in waste or sea water, metallurgical digests) rises above a certain level (typically >3%), either the sample needs to be diluted or specific sample introduction components, which have a high solids tolerance, need to be used for analysis. A typical high-solids sample introduction system is comprised of a parallel path nebulizer and wide bore torch center tube to prevent blockage due to crystallization of salts, either at the tip of the nebulizer or at the center tube. A baffled spray chamber is also used to reduce the matrix load (the amount of sample transported to the plasma) on the plasma. When analyzing samples with even higher percentages of dissolved solids (>15%), a sheath gas must be used to prevent complete blockage of the center tube. It is also good practice to use a ceramic rather than a quartz torch, because quartz has a tendency to undergo devitrification, leading to faster wear of the ICP torch, especially if the sample contains high concentrations of group I or group II elements.

The required analytical performance for hydride-forming elements can normally be achieved using a standard sample introduction setup (nebulizer, spray chamber, etc.). However, as the primary wavelengths for these elements are in the UV end of the spectrum, they undergo a higher degree of transmission loss to the absorption, resulting in reduced sensitivity. 

Analysis of samples containing toxic elements can be enhanced by the use of a hydride generation sample introduction system. This is due to the chemical properties of these elements, which enable the formation of volatile gaseous hydrides when reacted with reducing agents such as sodium borohydride. These gaseous hydrides can be separated from the liquid sample and introduced into the plasma. The removal of the liquid portion of the sample concentrates the analyte, reduces plasma loading, and removes interfering species, which in turn reduces the background signal and increases sensitivity.

Because solid samples cannot be introduced into the plasma directly, they must be either transferred into the plasma using a solid-sample accessory (e.g., electrothermal vaporization, laser ablation), or they must be dissolved or digested into a solution. The main techniques used to dissolve solids are acid digestion and fusion.

When using fusion for sample preparation, complete digestion takes place. It also results in high solid content. As a result, sample dilution is needed to bring the total dissolved solid content into the working range of the instrument, reducing the method detection limit.

Acid digestion usually involves the dissolution of a sample in a hot acid, or a mixture of hot acids. Heating acids is performed on a hot plate, or by using a microwave digestion system that employs pressurized vessels to produce even higher reaction temperatures. 

If a total digestion of the soil is required, then hydrofluoric acid (HF) is added to the digestion protocol. HF is not only highly corrosive and toxic: it also dissolves glassware used in the process of sample preparation and introduction into the ICP-OES. In such cases, a special sample introduction system, which is inert to HF, must be used.

For samples that contain high amounts of organic material, e.g., food, hydrogen peroxide can be added during digestion to accelerate the dissolution process. For the digestion of plastic material, a strong oxidizing agent such as sulfuric acid is applied; however, many resulting sulfates are insoluble.

The analysis of organic samples is typically not a straightforward process. Because of the physical properties of these samples, plasma stability can be adversely affected. Parameters that influence the plasma's overall performance include the volatility and viscosity of the organic sample. To overcome such obstacles, a special sample introduction system is used, which employs a baffled spray chamber and a less efficient nebulizer (e.g., V-groove nebulizer). A small bore torch center tube is used to reduce the load on the plasma. Typical organic ICP-OES applications include the analysis of oils and greases for wear metals and additives, the composition of petrochemicals at refineries, and purity determination of paints and inks.

A plasma that contains an organic solvent will appear green due to the emission of carbon and carbon species (C, C₂, CN). The edges of the plasma are also more clearly defined when compared to those of aqueous plasmas. The emissions from carbon-based molecules may interfere with signals emitted from other analytes, and especially alkali elements that emit light in the visible region of the spectrum.

To reduce the carbon emissions, compressed air can be added to the argon gas of the plasma, helping to convert carbon species into oxides such as carbon monoxide and carbon dioxide. This helps reduce these interferences in the visible region and improve sensitivity for group I and group II elements.

Volatile samples have the tendency to develop a very dense aerosol when nebulized and increase the pressure in the spray chamber. This can cause plasma instability and can even extinguish a plasma. To introduce a very volatile sample (e.g., gasoline) into the plasma, certain countermeasures must be applied. One option is to dilute the sample with a less volatile solvent. However, this also reduces the detection capacity of the method. Another solution is to cool down the sample with the help of a cooled spray chamber, thus reducing volatility. In so doing, the vapor pressure of the sample is reduced, resulting in greater plasma stability. Also, the application of a low flow nebulizer can reduce the load of aerosol reaching the plasma, increasing plasma stability.

Highly viscous samples (e.g., plant oils, lubricating oils) must be diluted prior to analysis; alternately, they may be heated before and during sample introduction to reduce their viscosity. Heating of the sample introduction components can take place in the same unit that is used for cooling of volatile samples, however, this is not always practical as the heating of the sample takes place only in the spray chamber. Dilution of viscous organic samples is typically performed using an organic solvent (e.g., xylene, kerosene) with a dilution factor of 1:10.