The Role of Optical ICPs in Environmental Analyses

Over the last 50 years, there has been increasing legislation relating to environmental protection and quality. The main reason for this is that the general public and environmental awareness groups have been putting pressure on governments (from local to international) and global organizations to improve the quality of the environment. Common themes emerging from this legislation are the need to reduce levels of contaminants in the environment and to improve environmental monitoring. Many of the Environmental Quality Standards (EQS) that are used to assess the state of the environment have been tightened in recent years. This has led to problems for scientists who carry out analyses of environmental samples, since methods and instrumentation must be adapted and improved to be able to detect, with confidence, the specified contaminant at and below the EQS to ensure the necessary EQS are met.

All analytical methods that involve a form of instrumentation are limited by the working range of the instrument’s detector. Optical inductively coupled plasma (ICP) instruments are no exception, but they generally have a much greater working range (sub-ppb to %) than other elemental techniques such as graphite furnace atomic absorption spectrophotometry (GF-AAS) (sub-ppb to low ppm). A solution to the problem posed by lower EQS is to carry out a preconcentration step in the sample preparation before the sample is introduced to the instrument. In many cases, this is not practical because large sample volumes are needed and the number of steps and reagents required will increase. One such example is the Analysis of Seawater for Trace Elements, a method published by the Directorate of Fisheries Research (U.K.) in 1994. This is used for routine analysis and involves complexation of the elements of interest, extracting them with an organic solvent and then back-extracting the elements into an aqueous solution for analysis.

Faced with the prospect of carrying out complex sample preparation methods, many laboratories have changed the instrumentation used so that the analysis they carry out remains compliant with the relevant environmental legislation. On the whole, GF-AAS has traditionally been used for environmental analyses and is still the instrument of choice for many smaller laboratories in which funding is limited. However, in the early to mid-1990s, optical emission ICP, and later ICP-MS, took a firm hold due to the falling cost of the instruments, the multielement capabilities, and simplification of operation. To some extent, optical ICPs are perceived as inadequate for certain environmental analyses due to the detection limits that are possible in comparison to ICP-MS. In reality, this is not true because the limits of quantification that are stipulated in environmental legislation can be achievable by optical ICPs, as recent developments in the technology have shown.

The design stage of modern optical ICPs now makes full use of the latest software modeling packages to improve performance. Some examples of these are:

  • Finite element analysis (FEA), which allows the structural design of the instrument to be optimized to maximize rigidity and stability.
  • Computational fluid dynamics (CFD), which enables modeling of gas and heat flows within the instrument. This permits designers to optimize the design to give maximum thermal stability by reducing heat transfer between the different components of the instrument.
  • Zemax (Zemax Development Corp., Bellevue, WA) can also be used to optimize the optical components of the instrument to allow for maximum light throughput, optimum image quality and resolution, and minimum stray light.

The use of these tools in combination with sound engineering practices allows for the design of instruments that are reliable; require minimal servicing; and have excellent stability, sensitivity, resolution, and ease of use.

The majority of optical ICPs on the market are available in different configurations depending on the application for which the instrument is to be used. Components that are application dependent are the sample introduction system and the orientation of the plasma view. To some extent, the polychromator and the type of detector used will have an influence on the performance of an instrument in relation to the application, but these two components are determined by the instrument supplier and are not user defined.

Technology

An iCAP 6500 Duo ICP spectrometer (Thermo Fisher Scientific, Cambridge, U.K.) was used for the analysis of several different environmental matrices to assess the performance of a modern optical emission ICP. In most cases, these results have been compared to a relevant piece of legislation and/or to the performance of an ICP-MS instrument.

This series of spectrometers employs the use of a charge injection device (CID) detector. A CID is a photosensitive device comprising doped silicone wafers containing a two-dimensional array of pixels that can be addressed individually by electrodes embedded in the device.

CID detectors are unique in that the accumulated charge (generated by photons striking the surface of the device) can be read both destructively and nondestructively. This has many advantages for atomic spectrometry since it allows matrix elements and trace elements to be read in the same exposure of the CID.

By continually monitoring the accumulated charge of a pixel, the pixel can be read before it becomes full and the result digitized and stored. Once this has occurred, the pixel will be injected with a negative charge, setting the pixel back to zero and permitting it to accumulate more charge for the remainder of the exposure time. This is the mode of operation when elements are present at high concentrations, and it allows a wide dynamic range.

When elements are present at lower concentrations, the pixel can be left to accumulate charge for the entire exposure time and only read at the end, thus maximizing the signal-to-noise ratio. CIDs are inherently nonblooming. This means that charge will not “spill” from one pixel to the next when a pixel becomes full, causing peak broadening. This is not the case with some other types of solid-state detectors. Other types of detectors such as charge coupled devices (CCD) either have to be segmented to prevent blooming, which involves removing large portions of the detector area to prevent charge spilling from one pixel to the next, or, alternatively, drains can be integrated into the pixel structure, which dump the excess charge from the saturated pixels.

In the first case, important wavelengths may be missing from active areas of the detector, while in the second case, data for wavelengths falling on saturated pixels cannot be reported. These issues may limit the applicability of CCD detectors for complex analyses.

For environmental samples, a common type of sample introduction system is a glass concentric nebulizer in conjunction with a cyclonic spray chamber. This combination has the advantage of high efficiency and maximum sensitivity, and uses low volumes of sample solution. For environmental applications where sensitivity is of paramount importance, another common type of nebulizer is the ultrasonic nebulizer (USN). This instrument has much improved efficiency and provides a dense sample aerosol. Use of a USN can improve the sensitivity of an instrument by a factor of 10–15.