Multielement detection limits

Because different elements have different spectrochemical properties, optimum analytical conditions may vary from element to element (69,70). Since all elements are determined simultaneously with an image detector spectrometer, compromise analytical conditions must be employed. Brost, et al. (71 ) have described a response parameter which can be used to determine the optimum compromise analytical conditions. Because the optimum compromise analytical conditions for a given determination depend on the expected analytical concentrations of the elements present in the sample, meaningful multielement detection limits cannot be reported without reference to a particular sample type. The many reported multielement detection limits which appear in the literature simply indicate the detection limits obtained under a particular arbitrary set of conditions, and do not necessarily represent the detection limits obtainable under optimum analytical conditions for a particular sample type. Thus the detection limits achieved in an actual multielement determination are more often likely to be compromise-limited rather than instrument-limited. 

Comparisons with other systems. Data presented in Table VI provide a comparison of results obtained with the image dissector with results reported by others with other systems. Results in the second column represent multielement detection limits observed in this work. Results in the third and fourth columns represent detection limits reported for single element determinations with conventional optics and a silicon vidicon (12J and a commercial atomic absorption instrument (33). 

The multielement detection limits with the echelle/image dissector are comparable to, or better than, single element detection limits reported for a silicon vidicon and conventional optics. Detection limits for Cr, Cu, and Mn with the echelle/ image dissector compare favorably with single element data reported for a conventional atomic absorption instrument with a photomultiplier detector, but detection limits obtained here for Ni and Co are higher by factors of 10 or more than for the conventional instrument. The echelle/image dissector system should be adaptable to a so-called flameless atomizer and be subject to the same improvements in sensitivities and detection limits as conventional detector systems. 

An energy dispersive spectrometer is cheaper and faster for multielement analytical purposes but has poorer detection limits and resolution. 

Inductively coupled plasma-mass spectrometry (ICP-MS) is a multielement analytical method with detection limits which are, for many trace elements, including the rare earth elements, better than those of most conventional techniques. With increasing availability of ICP-MS instalments in geological laboratories this method has been established as the most prominent technique for the determination of a large number of minor and trace elements in geological samples. 

Multielement analysis, excellent detection limits for heavy metals quantitative measurement of heavy-metal trace contamination on silicon wafers... 

Atomic absorption spectroscopy of VPD solutions (VPD-AAS) and instrumental neutron activation analysis (INAA) offer similar detection limits for metallic impurities with silicon substrates. The main advantage of TXRF, compared to VPD-AAS, is its multielement capability AAS is a sequential technique that requires a specific lamp to detect each element. Furthermore, the problem of blank values is of little importance with TXRF because no handling of the analytical solution is involved. On the other hand, adequately sensitive detection of sodium is possible only by using VPD-AAS. INAA is basically a bulk analysis technique, while TXRF is sensitive only to the surface. In addition, TXRF is fast, with an typical analysis time of 1000 s turn-around times for INAA are on the order of weeks. Gallium arsenide surfaces can be analyzed neither by AAS nor by INAA. 

ICP-OES is one of the most successful multielement analysis techniques for materials characterization. While precision and interference effects are generally best when solutions are analyzed, a number of techniques allow the direct analysis of solids. The strengths of ICP-OES include speed, relatively small interference effects, low detection limits, and applicability to a wide variety of materials. Improvements are expected in sample-introduction techniques, spectrometers that detect simultaneously the entire ultraviolet—visible spectrum with high resolution, and in the development of intelligent instruments to further improve analysis reliability. ICPMS vigorously competes with ICP-OES, particularly when low detection limits are required. 

A recent extension of the scope of SPE-GC and SPE-GC-MS concerns the use of AED detection with its multielement detection capability and unusually high selectivity. Hankemeier [67] has described on-line SPE-GC-AED with an on-column interface to transfer 100 iL of desorbing solvent to the GC. The fully on-line set-up is characterised by detection limits of 5-20 ngL because of quantitative transfer of the analytes from the SPE to the GC module. On-line coupling of SPE with GC is more delicate than SPE-LC, because of the inherent incompatibility between the aqueous part of the SPE step and the dry part of the GC system. 

Mass spectrometry is the only universal multielement method which allows the determination of all elements and their isotopes in both solids and liquids. Detection limits for virtually all elements are low. Mass spectrometry can be more easily applied than other spectroscopic techniques as an absolute method, because the analyte atoms produce the analytical signal themselves, and their amount is not deduced from emitted or absorbed radiation the spectra are simple compared to the line-rich spectra often found in optical emission spectrometry. The resolving power of conventional mass spectrometers is sufficient to separate all isotope signals, although expensive instruments and skill are required to eliminate interferences from molecules and polyatomic cluster ions. 

Although ICP-AES is a multielement technique, its inferior detection limits relative to GFAAS would necessitate the processing of large volumes of seawater, improvements in the preconcentration procedures in use thus far, or new, alternative preconcentration procedures such as carrier precipitation (see below). 

Prange et al. [809,810] carried out multielement determinations of the stated dissolved heavy metals in Baltic seawater by total reflection X-ray fluorescence (TXRF) spectrometry. The metals were separated by chelation adsorption of the metal complexes on lipophilised silica-gel carrier and subsequent elution of the chelates by a chloroform/methanol mixture. Trace element loss or contamination could be controlled because of the relatively simple sample preparation. Aliquots of the eluate were then dispersed in highly polished quartz sample carriers and evaporated to thin films for spectrometric measurements. Recoveries (see Table 5.10), detection limits, and reproducibilities of the method for several metals were satisfactory. 

Actinide metal samples are characterized by chemical and structure analysis. Multielement analysis by spark source mass spectrometry (SSMS) or inductively coupled argon plasma (ICAP) emission spectroscopy have lowered the detection limit for metallic impurities by 10 within the last two decades. The analysis of O, N, H by vacuum fusion requires large sample, but does not distinguish between bulk and surface of the material. Advanced techniques for surface analysis are being adapted for investigation of radioactive samples (Fig. 11) ... 

The interest in the graphite furnace as an emission source stems from the desire to achieve a multielement detection capability whilst retaining the low limits of detection and the possibility of sample pretreatment. Several... 

Stripping analysis is the best-known analytical method that incorporates an electrolytic preconcentration step [2-5]. The technique couples the advantages of extremely low detection limits ( 10 10-10-11 M), multielement and speciation capabilities, suitability for on-line and in situ measurements, and low cost. 

Multielement determination (sequential or simultaneous) faster analysis time minimal chemical interaction detection limits and sensitivity fall in between that of flame and graphite furnace measurements. 

Multielement determination sensitivity and detection limits exceptionally good (over 100 times greater than furnace techniques for some metals) isotopes also may be measured also has the capability to determine nonmetals (at a much lower sensitivity) broad linear-working range high cost. 

Microwave-induced plasma (MIP), direct-current plasma (DCP), and inductively coupled plasma (ICP) have also been successfully utilized. The abundance of emission lines offer the possibility of multielement detection. The high source temperature results in strong emissions and therefore low levels of detection. Atomic absorption (AA) and atomic fluorescence (AF) offer potentially greater selectivity because specific line sources are utilized. On the other hand, the resonance time in the flame is short, and the limit of detectability in atomic absorption is not as good as emission techniques. The linearity of the detector is narrower with atomic absorption than emission and fluorescence techniques. 

The fluorescence technique combines the advantages of the large dynamic range of emission techniques with the simplicity and high selectivity of absorption techniques. Flame sources have been extensively used, however, for elements with refractory oxides, the ICP source has been found to be more satisfactory for AFS. A system for hollow cathode lamp excited ICP-AFS, as proposed by Demers and Allemand (1981), is commercially available as a modular simultaneous multielement ICP system. Although fluorescence techniques often offer two orders of magnitude sensitivity improvement over absorption, the multielement approach for AFS has not yet been commercially successful. Also promising for the future is the laser-excited furnace AFS where the detection limits for most elements are comparable to those of ICP-AES and for some elements, for eg, As, Cd, Pb, Tl, Lu, even lower (Omenetto and Human, 1984). The future for AFS techniques has been discussed by Stockwell and Corns (1992). 

The FAAS method offers similar detection limits to NAA and is suitable for the determination of low levels of lead. Equipment costs are reasonable and the instrumentation is commonplace in many analytical laboratories. A large number of metallic elements, over a wide concentration range, extending down to ultra-trace level, can be analyzed, thus making the technique versatile and useful for other forensic applications as well as FDR detection. Apart from cost, the main advantages are simplicity, speed of analysis, and in house operation. One disadvantage of FAAS is that it is not capable of simultaneous multielement analysis. 

Since the introduction of the first commercial instrument in 1983, inductively coupled plasma mass spectrometry (ICP-MS) has become widely accepted as a powerful technique for elemental analysis. Two excellent books on ICP-MS have been published [1,2]. ICP-MS provides rapid, multielement analysis with detection limits at single parts part trillion or below for about 40 to 60 elements in solution and a dynamic range of 104 to 108. These are the main reasons most ICP-MS instruments have been purchased. Two additional, unique capabilities of ICP-MS have also contributed to its commercial success elemental isotope ratio measurements and convenient semiquantitative analysis. The relative sensitivities from element to element are predictable enough that semiquantitative analysis (with accuracy within a factor of 2 to 5) for up to 80 elements can be obtained using a single calibration solution containing a few elements and a blank solution. 

The low detection limits and rapid, multielement analysis provided by ICP-MS make it particularly attractive for environmental applications, in which high sample throughput is often essential. For several elements, including Pb, the maximum acceptable levels have decreased as the ability to measure lower and lower concentrations has improved. ICP-MS measurements are used to assess environmental quality, including meeting legislated requirements to investigate the natural sources and transport of elements and to identify sources of pollutants. 

Inductively coupled plasma mass spectrometry is becoming more and more popular because of the low detection limits, high selectivity, and rapid multiele-... 

Instrument and maintenance costs are high. Prices range from about 180,000 (U.S. dollars, 1998) for quadrupole instruments to almost 1,000,000 (U.S. dollars, 1998) for a fully capable multicollector sector-based instalment and laser ablation sampling. About 10 to 20 L/min of Ar is used by the ICP. Sampling and skimmer cones cost 800 to 3000 (U.S. dollars, 1998), depending on material. Detector lifetime may be less than 1 year. Vacuum pumps have limited lifetimes. Of course, the rapid multielement analysis capabilities, low detection limits, and isotope measurements often provide information that makes ICP-MS successful financially as well as scientifically. 

The ICP-MS has several analytical attractions including very low detection limits [parts per billion to parts per trillion (ppb to ppt) levels], a large linear dynamic range, relatively simple spectra, excellent stability, multielement determination capability, and ability to measure isotopic abundances. Disadvantages are mainly due to the formation of polyatomic interferences from the plasma gas, entrained gases, and matrix elements such as Cl [16]. 

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