Preconcentration detection limits

Sensitivity, Accuracy, and Reliability. In order to perform analyses on many real samples with little pretreatment (such as sample preconcentration), detection limits obtained with the ideal system should be as low as possible to permit the direct determination of trace components. Since an analytical technique is of... 

Van Os et al. [41] achieved complete separations in 6min of l-30mg L concentrations of bromide, chloride, nitrite, nitrate and sulphate using a Zipax SAX separation column, with eluent suppression and electrical conductivity detection [42]. The necessary high pressure packing techniques for packing the separation column have been described [43]. With sample preconcentration, detection limits were reduced to about 5pg L but calibration graphs for chloride and nitrate were not Unear. Sodium adipate and 1.4 X 10 disodium succinate are used as eluants, both at pH7. 

The term direct TXRF refers to surface impurity analysis with no surface preparation, as described above, achieving detection Umits of 10 °—10 cm for heavy-metal atoms on the silicon surface. The increasit complexity of integrated circuits fabricated from silicon wafers will demand even greater surfrce purity in the future, with accordingly better detection limits in analytical techniques. Detection limits of less than 10 cm can be achieved, for example, for Fe, using a preconcentration technique known as Vapor Phase Decomposition (VPD). 

Shipping analysis is an extremely sensitive electrochemical technique for measuring trace metals (19,20). Its remarkable sensitivity is attributed to the combination of an effective preconcentration step with advanced measurement procedures that generate an extremely favorable signal-to-background ratio. Since the metals are preconcentrated into the electrode by factors of 100 to 1000, detection limits are lowered by 2 to 3 orders of magnitude compared to solution-phase voltammetric measurements. Hence, four to six metals can be measured simultaneously in various matrices at concentration levels down to 10 10 i. utilizing relatively inexpensive... 

The presence of redox catalysts in the electrode coatings is not essential in the c s cited alx)ve because the entrapped redox species are of sufficient quantity to provide redox conductivity. However, the presence of an additional redox catalyst may be useful to support redox conductivity or when specific chemical redox catalysis is used. An excellent example of the latter is an analytical electrode for the low level detection of alkylating agents using a vitamin 8,2 epoxy polymer on basal plane pyrolytic graphite The preconcentration step involves irreversible oxidative addition of R-X to the Co complex (see Scheme 8, Sect. 4.4). The detection by reductive voltammetry, in a two electron step, releases R that can be protonated in the medium. Simultaneously the original Co complex is restored and the electrode can be re-used. Reproducible relations between preconcentration times as well as R-X concentrations in the test solutions and voltammetric peak currents were established. The detection limit for methyl iodide is in the submicromolar range. 

This procedure is concerned with narrowing the sample zones on the start hue on the chromatographic plate. In analytical separation the preconcentration procedure is applied to increase the efficiency and decrease detection limit. In preparative planar... 

When pushed to the limit by overriding human health concerns, residue chemists have achieved detection limits of Ippt (Ingkg ) or even into the low ppqr (1 pg kg ) range. An example at the 1 ppt level is provided by methods for 2,3,7,8-tetrachlorodibenzodioxin (TCDD) in milk and TCDD in adipose tissue. Eor relatively clean matrices such as water and air, preconcentration on solid-phase adsorbents followed by GC or gas chromatography/mass spectrometry (GC/MS) can provide detection limits of 1 ng m and less for air (examples in Majewski and Capel ) and 1 ngL and less for water (examples in Larson et A summary of units of weight and concentration used to express residue data is given in Table 1. 

Similar comments apply in the case of organics in seawater, because it has now become possible to resolve the complex mixtures of organics in seawater and achieve the required very low detection limits. Only since the advent of sample preconcentration and mass spectrometry coupled with gas chromatography and high-performance liquid chromatography, and possibly derivatisa-tion of the original sample constituents to convert them into a form suitable for chromatography, has this become possible. 

Anion Preconcentration method Analytical finish Detection limit (pg/1) Section Reference... 

Methods of improving the detection limits for anions by preconcentration are reviewed in Table 2.6. 

The background concentrations at which metals occur in seawater are extremely low, and much work has been done on preconcentration procedures in attempts to improve detection limits for these metals. Various preconcentra-... 

Sakamoto [243] determined picomolar levels of cobalt in seawater by flow injection analysis with chemiluminescence detection. In this method flow injection analysis was used to automate the determination of cobalt in seawater by the cobalt-enhanced chemiluminescence oxidation of gallic acid in alkaline hydrogen peroxide. A preconcentration/separation step in the flow injection analysis manifold with an in-line column of immobilised 8-hydroxyquinoline was included to separate the cobalt from alkaline-earth ions. One sample analysis takes 8 min, including the 4-min sample load period. The detection limit is approximately 8 pM. The average standard deviation of replicate analyses at sea of 80 samples was 5%. The method was tested and inter calibrated on samples collected off the California coast. 

Falkner and Edmond [334] determined gold at femtomolar quantities in seawater by flow injection inductively coupled plasma quadrupole mass spectrometry. The technique involves preconcentration by anion exchange of gold as a cyanide complex, [AulCNjj], using 195Au radiotracer (ti/2 = 183 days) to monitor recoveries. Samples are then introduced by flow injection into an inductively coupled plasma quadrupole mass spectrometer for analysis. The method has a detection limit of 10 fM for 4 litres of seawater preconcentrated to 1 ml, and a relative precision of 15% at the 100 fM level. 

In contrast, the coupling of electrochemical and spectroscopic techniques, e.g., electrodeposition of a metal followed by detection by atomic absorption spectrometry, has received limited attention. Wire filaments, graphite rods, pyrolytic graphite tubes, and hanging drop mercury electrodes have been tested [383-394] for electrochemical preconcentration of the analyte to be determined by atomic absorption spectroscopy. However, these ex situ preconcentration methods are often characterised by unavoidable irreproducibility, contaminations arising from handling of the support, and detection limits unsuitable for lead detection at sub-ppb levels. 

In many applications, such as the analysis of mercury in open ocean seawater, where the mercury concentrations can be as small as 10 ng/1 [468,472-476], a preconcentration stage is generally necessary. A preliminary concentration step may separate mercury from interfering substances, and the lowered detection limits attained are most desirable when sample quantity is limited. Concentration of mercury prior to measurement has been commonly achieved either by amalgamation on a noble-metal metal [460,467, 469,472], or by dithizone extraction [462,472,475] or extraction with sodium diethyldithiocarbamate [475]. Preconcentration and separation of mercury has also been accomplished using a cold trap at the temperature of liquid nitrogen. 

Petit [563] has described a method for the determination of tellurium in seawater at picomolar concentrations. Tellurium (VI) was reduced to tellurium (IV) by boiling in 3 M hydrochloric acid. After preconcentration by coprecipitation with magnesium hydroxide, tellurium was reduced to the hydride by sodium borohydrate at 300 °C for 120 seconds, then 257 °C for 12 seconds. The hydride was then measured by atomic absorption spectroscopy. Recovery was 90 - 95% and the detection limit was 0.5 pmol/1. 

Fang et al. [661] have described a flow injection system with online ion exchange preconcentration on dual columns for the determination of trace amounts of heavy metal at pg/1 and sub-pg/1 levels by flame atomic absorption spectrometry (Fig. 5.17). The degree of preconcentration ranges from a factor of 50 to 105 for different elements, at a sampling frequency of 60 samples per hour. The detection limits for copper, zinc, lead, and cadmium are 0.07, 0.03, 0.5, and 0.05 pg/1, respectively. Relative standard deviations are 1.2-3.2% at pg/1 levels. The behaviour of the various chelating exchangers used was studied with respect to their preconcentration characteristics, with special emphasis on interferences encountered in the analysis of seawater. 

Tony et al. [951] have discussed an online preconcentration flame atomic absorption spectrometry method for determining iron, cobalt, nickel, magnesium, and zinc in seawater. A sampling rate of 30 samples per hour was achieved and detection limits were 4.0,1.0,1.0,0.5, and 0.5 xg/l, for iron, cobalt, nickel, magnesium, and zinc, respectively. 

Chang et al. [952] used a miniature column packed with a chelating resin and an automatic online preconcentration system for electrothermal atomic absorption spectrometry to determine cadmium, cobalt, and nickel in seawater. Detection limits of 0.12,7 and 35 ng/1 were achieved for cadmium, cobalt, and nickel, respectively, with very small sample volume required (400-1800 xl). 

Winge et al. [730] have investigated the determination of twenty or more trace elements in saline waters by the inductively coupled plasma technique. They give details of experimental procedures, detection limits, and precision and accuracy data. The technique when applied directly to the sample is not sufficiently sensitive for the determination of many of the elements at the low concentrations at which they occur in seawater, and for these samples preconcentration techniques are required. However, it has the advantages of being amenable to automation and capable of analyzing several elements simultaneously. 

Owing to inadequate detection limits by direct analysis, various workers examined preconcentration procedures, including dithiocarbamate preconcentration [447,732-734], ion exchange preconcentration [735-737], chelation solvent extraction [736], coprecipitation [738], and preconcentration in silica-immobilised 8-hydroxyquinoline [129]. 

Berman et al. [735] have shown that if a seawater sample is subjected to 20-fold preconcentration by one of the above techniques, then reliable analysis can be performed by ICP-AES (i.e., concentration of the element in seawater is more than five times the detection limit of the method) for iron, manganese, zinc, copper, and nickel. Lead, cobalt, cadmium, chromium, and arsenic are below the detection limit and cannot be determined reliably by ICP-AES. These latter elements would need at least a hundredfold preconcentration before they could be reliably determined. 

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). 

Warnken et al. [956] have reported an online preconcentration - ultrasonic nebulisation - ICP-MS method that achieved detection limits of 0.26,0.86,1.5, 10, and 0.44 ng/1 for manganese, nickel, copper, zinc, and lead in seawater. This online preconcentration method compares favourably to the state of-the-art off-line methods. 

Brief mention has been made, particularly in connection with the inductively coupled plasma atomic absorption spectrometric technique, of the need to preconcentrate seawater samples prior to the determination of metals, in order to achieve adequate detection limits. 

An ideal method for the preconcentration of trace metals from natural waters should have the following characteristics it should simultaneously allow isolation of the analyte from the matrix and yield an appropriate enrichment factor it should be a simple process, requiring the introduction of few reagents in order to minimise contamination, hence producing a low sample blank and a correspondingly lower detection limit and it should produce a final solution that is readily matrix-matched with solutions of the analytical calibration method. 

The benefits imparted by preconcentration to improved sensitivity are illustrated in the example of lead preconcentration on Chelex 100 resin [871,872], followed by analysis by ICP-AES. Without preconcentration the best detection bmit achievable is 60 ng/1, via direct nebubsation. When the Chelex 100 preconcentration step is included, the detection limit improves to 0.6 ng/1, i.e., 100 times better, which is a very important improvement achieved in the analysis of seawaters. Examinaton of Table 5.12 reveals that the following metals can be determined with detection limits in the 1 -10 ng/1 range beryllium (0.6 ng/1),... 

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