Simultaneous multi-element detection

Specific multi-element monitoring is possible in various ways. Sequential switching between elemental wavelengths of a monochromator is useful if speed is compatible with peak elution rates [5]. However, the most widely utilized multi-element detection has been by means 

Classes of atomic plasma emission chromatographic detectors 

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Benkhedda et al demonstrated the unique performance of ICP-TOF-MS for the simultaneous multi-element detection of 22 isotopes of rare earth elements (REEs) in a El peak of about 5 s width (at 50% height) without the introduction of any spectral skew. Eigure 2.24 represents the profiles obtained by USN-ICP-TOE-MS for three replicates of 250 ptL injections of a 0.1 mg/L multi-elemental standard of REE s after FI on-line pre-concentration in a fkr (KR) pre-coated with the chelating reagent l-phenyl-3-methyl-4-benzoyl-pyrazol-5-one (PMBP). 

The linear dynamic range of an ICP-MS instrument is typically about 4-6 orders of magnitude, while the sensitivity is often 3 orders of magnitude better than the ICP-AES technique. While the detection limits for many elements with ICP-MS are better or comparable to AES and graphite furnace atomic absorption spectrometry, the simultaneous multi-elemental detection afforded by ICP-MS clearly places this technique in the lead for analytical utility and versatility. 

Plasma sources were developed for emission spectrometric analysis in the late-1960s. Commercial inductively coupled and d.c. plasma spectrometers were introduced in the mid-1970s. By comparison with AAS, atomic plasma emission spectroscopy (APES) can achieve simultaneous multi-element measurement, while maintaining a wide dynamic measurement range and high sensitivities and selectivities over background elements. As a result of the wide variety of radiation sources, optical atomic emission spectrometry is very suitable for multi-element trace determinations. With several techniques, absolute detection limits are below the ng level. 

Sequential and simultaneous multi-element ultratrace detection... 

The current generation of inductively coupled plasma emission spectrometers provide limits of detection in the range of 0.1-500pg L 1 in solution, a substantial degree of freedom from interferences and a capability for simultaneous multi-element determination facilitated by a directly proportional response between the signal and the concentration of the analyte over a range of about five orders of magnitude. 

It is also possible to use an internal standard to correct for sample transport effects, instrumental drift and short-term noise, if a simultaneous multi-element detector is used. Simultaneous detection is necessary because the analyte and internal standard signals must be in-phase for effective correction. If a sequential instrument is used there will be a time lag between acquisition of the analyte signal and the internal standard signal, during which time short-term fluctuations in the signals will render the correction inaccurate, and could even lead to a degradation in precision. The element used as the internal standard should have similar chemical behaviour as the analyte of interest and the emission line should have similar excitation energy and should be the same species, i.e. ion or atom line, as the analyte emission line. 

Vinas et al. (1993a), determined copper in biscuits and bread using a fast-program slurry electrothermal atomic absorption procedure and Miller-Ihli (1988) also used EAAS after slurry preparation for simultaneous multi-element AAS, as did Littlejohn et al. (1985) who introduced slurried food samples into the graphite furnace for analysis. Haswell and Barclay (1992), carried out on-line microwave digestion of slurry samples with direct flame atomic absorption spectrometric elemental detection. 

Today s analysts no longer focus on just trace elemental analysis, but rather they face the challenge of being able to speciate and quantify many high profile elements. ICP-MS coupled with suitable chromatographic separation techniques offers a unique combination of simultaneous multi-element capabilities with ppb and ppt detection limits and the ability to perform elemental speciation. In this paper, we will present how Inductively Coupled Plasma -Mass Spectrometry (ICP-MS) has been successfully used with liquid chromatogri hy to provide trace level speciation information. 

Although the ICP is somewhat more sensitive in terms of reported detection limits than DCP, the former cannot tolerate as high a dissolved solids content as the latter. Therefore, on the original silicate materials, the detection limits are similar. Another advantage of PES compared to AA is that commercial PES spectrometers can be configured for simultaneous multi-elemental analysis, while the current commercial multi-elemental AAs are sequential. The base price of PES equipment is higher than AA but if the sample load is high, the increased productivity of multi-elemental PES may result in a lower cost per analysis. 

Plasma AES has several advantages (possibility for the qualitative and simultaneous multi-element analysis, measurements in the vacuum UV region, high sensitivity, low detection limits, less chemical interferences, low running costs) and it has become more and more important for the determination of traces in a great variety of samples. On the other hand, it does not compensate totally for any other instrumental method of analysis, but it compensates for those faults which might exist in other techniques. The complementary nature of plasma AES and AAS capabilities for trace elemental analysis is an important feature of these techniques. Plasma AES exhibits excellent power of detection for a number of elements which cannot be determined or are difficult to determine at trace levels by flame AAS e.g. B, P, S, W, U, Zr, La, V, Ti) or by electrothermal AAS (B, S, W, U). Thus, optical plasma emission and atomic absorption are not actually alternatives, but in an ideal way complement one another. 

ICP-MS is being used in many branches of science. Many desirable analytical characteristics, such as superior detection limits, spectral simplicity, possibility for simultaneous multi-element analysis, and isotope ratio determinations, are reasons for its widespread popularity. However, not even this technique is free from interferences. Particularly, spectral (polyatomic) and non-spectral (suppression and enhancement) interferences cause analysts to consider carefully the sample preparation procedure and finally the matrix. Most of the fundamental research papers published deal with the suppressive and spectral interference effects. 

Atomic absorption methods possess a number of advantages (i) high specificity (ii) low detection limits (hi) easy to use (iv) low investment and running costs (v) interferences and methods for their elimination are well documented (vi) easy sample preparation (vii) a number of special techniques available for the determination of non-metals and organic compounds. However, AAS is a single element method. One element is determined in a series of samples and the instrumental parameters are optimized for the next element and the series is repeated. Thus, simultaneous multi-element analysis is technically difficult to perform. In addition, qualitative analysis is impractical. A further disadvantage is that often dangerous gas mixtures must be used. 

Advantages of plasma atomic emission spectrometry are (i) wide linear dynamic range (10 -10 orders of magnitude) (ii) easy and rapid qualitative analysis (iii) simultaneous multi-element analysis (iv) low running costs (v) good precision, low detection limits, and high sensitivity (RSD values FAAS 0.3-1%, GF-AAS 1-5%, ICP-AES 0.5-2%) (vi) minimized chemical interferences (vii) analysis of more than 70 elements including refractories... 

Although the presently realised continuum-source AA spectrometers are still operated sequentially for multi-element detection, it may be anticipated that, with use of suitable optics and multi-array detectors, this method wiU become a truly simultaneous multi-element technique [10]. 

Background correction by wavelength-modulation is not widely used, but seems to have considerable potential for simultaneous multi-element AAS with continuum sources. Alternatively, high-resolution echelle gratings may be used that can detect both the elemental line and the background at the same time. 

Since AAS is classically a single-element technique, there is an increasing trend to overcome this Hmitation by the development of simultaneous multi-element spectrometers with multichannel detection. Such instmments are presently available on the market, and allow the simultaneous determination of up to six elements. As these are GF instruments, they must however be used under compromise conditions for the ashing and atomisation steps. 

We have demonstrated an extremely sensitive sensor platform for groundwater monitoring. The sensitivity of the cantilever sensor depends on cantilever dimension, while the selectivity depends on die selectivity of the surface coating for chemical interactions. Research is underway to develop cantilever arrays for simultaneous multi-analyte detection. The primary advantages of the microcantilever method are (1) the sensitivity of microcantilevers based on their ability to detect cantilever motion with subnanometer precision (2) their ability to be fiibricated into multi-element sensor arrays and (3) their ability to work in a liquid environment. 

Note The analytical problems of inorganic MS often require only certain selected isotopes or narrow m/z ranges to be measured. Multicollector systems, for example, are adjusted to simultaneously detect a few isotopes for the purpose of accurate isotope ratio determinations or to quantify a low-abundant isotope together with an isotopic standard for internal reference. Thus, the data is more often presented in tabular form or in plots of concentration versus variables such as depth of invasion, age of samples, or location on a surface. Mass spectra covering a wider range are only acquired for survey multi-element detection. 

Using a combination of analytical techniques, the nuclear microprobe can provide simultaneous multi-elemental analysis over the entire Periodic Table with a spatial resolution of 1 pm, a minimum detection limit of 1-100 ppm depending on the conditions and a quantitative accuracy of 5-20% depending on the type of analysis. Although the penetration depth of MeV protons can be in the region of 100 pm in some materials, the nuclear microprobe is a surface-biased technique since signals are detected preferentially from the near surface region ( 10 pm depth). 

Maia et al. [332] eliminated interferences in the direct determination of Hg in powdered coal samples by means of analyte transfer during the pyrolysis step from the platform to a graphite tube wall. This graphite tube was permanently modified with Pd, and detection limits in the range of 0.025-0.05 pg/g were obtained. In simultaneous multi-element determinations of Cu, Cr, Al, and Mn in urine, Pd was also very successfully used as a matrix modifier [333]. The use of Pd as a modifier, and also in combination with dynamic background correction techniques such as the Smith-Hieftje technique (see Section 4.6.3), enables a considerable enhancement of the analytical accuracy of AAS, as shown in the case of As determinations [334]. 

Tseng, C. M., Amouroux, D., Brindle, I. D., and Donard, O. F. X. (2000) Field cryofo-cussing hydride generation applied to the simultaneous multi-elemental determination of alkyl-metal(loid) species in natural waters using ICP-MS detection. J. Environ. Mon., 2, 603-12. 

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