Detection limits in the ppm

The most common application of dynamic SIMS is depth profiling elemental dopants and contaminants in materials at trace levels in areas as small as 10 pm in diameter. SIMS provides little or no chemical or molecular information because of the violent sputtering process. SIMS provides a measurement of the elemental impurity as a function of depth with detection limits in the ppm—ppt range. Quantification requires the use of standards and is complicated by changes in the chemistry of the sample in surface and interface regions (matrix efiects). Therefore, SIMS is almost never used to quantitadvely analyze materials for which standards have not been carefiilly prepared. The depth resoludon of SIMS is typically between 20 A and 300 A, and depends upon the analytical conditions and the sample type. SIMS is also used to measure bulk impurities (no depth resoludon) in a variety of materials with detection limits in the ppb-ppt range. 

Photolytic derivatisation in LC-EC (HPLC-hi/-EC) has been proposed for the trace assay of organophosphorous agricultural chemicals. The detection limits in the ppm range are said to be sufficiently low that the method can be applied to crop extracts at levels routinely encountered. 

A gas (usually 90%Ar- -10%CH4) filled proportional counter is an alternative ED detector (except at short wavelengths where a Xe sealed proportional counter is preferred) particularly in industrial applications. This detector provides resistance to vibrations, mechanical reliability, and minimal temperature dependency. A proper choice of the filling gas of proportional counter is of great importance in minimizing the background caused by the wall effect. Although the resolution of this detector is relatively very poor, the detection limit in the ppm range can be obtained. 

An immediate distinction needs to be made between dynamic and static SIMS experiments. In the former, a primary ion current density of typically > 10 pA cm is used and this leads to rapid sputtering of material. The surface is eroded at a rate of order nm s and by following the intensity of chosen peaks in the mass spectrum as a function of time, a concentration depth profile can be constructed. In this mode SIMS can be very sensitive, with trace element detection limits in the ppm-ppb range. However, quantification is not straightforward. Secondary ion intensities are strongly matrix-dependent and extensive calibration procedures involving closely related standards of known composition and under identical experimental conditions must be used to extract quantitative concentrations. 

Instrumental methods for the determination of water in polymeric materials often rely on heat release of water from the polymer matrix. However, in some cases (e.g. PET) the polymer is hydrolysed and a simple Karl Fischer method is then preferred. Small quantities of water (10 pg-15mg) of water in polymers (e.g. PBT, PA6, PA4.6, PC) can be determined rapidly and accurately by means of a coulometric titration after heating at 50 to 240 °C with a detection limit in the order of 20 ppm. 

At ID18F beamline simultaneous p-XRF (excitation energy of 28 keV spectrum collection using a Si(Li) solid state detector with detection limits in the range 0.01 ppm for 3025) and p-XRD (monochromatic X-rays... 

One popular configuration is the Grimm source, which accepts samples in the form of discs. Such sources usually operate at 500-1000 V, 25-100 mA and 1-5 Torr, wiA detection limits of approximately 0.1 ppm. Another configuration is the hollow-cathode lamp in which the sample can be either machined as a hollow cathode, evaporated to dryness (if a solution) or pressed (if a powder) into a hollow cathode made of pure graphite. Typical operating conditions are 200-500 V, 10-100 mA, and 0.1-1.0 Torr, with detection limits in the range 0.1-10 ppm. 

Kalinoski et al. [32] has applied this method to the determination of tri-chothecene mycotoxins in wheat. The methods were based on chemical ionisation MS and collision-induced dissociation tandem MS and enabled the rapid identification of ppm levels of several trichothecene mycotoxins. Supercritical carbon dioxide is shown to allow identification of mycotoxins with minimum sample handling in complex natural matrices such as wheat. Tandem MS techniques are employed for unambiguous identification of compounds of varying polarity, and false positives from isobaric compounds are avoided. Capillary column SCFC-MS of a SCF extract of the same sample was also performed, and detection limits in the ppb range appear feasible. 

Detection limits in the range of low parts per billion (ppb) can be achieved by immunoassay testing for certain parameters in aqueous samples. For soil samples, detection limits of <10 ppm can be achieved for many contaminants... 

Flame OES can be used to determine the concentrations of elements in samples. The sample usually must be in solution form. Generally, one element is determined at a time if using an AAS system in emission mode. Multichannel instruments are available for the simultaneous determination of two or more elements. Detection limits can be very low as seen in Appendix 7.1, Table Al. Detection limits for the alkali metals are in the ppt concentration range when ionization suppression is used. One part per trillion in an aqueous solution is 1 pg of analyte per mL of solution or 1 x g/mL. Most elements have detection limits in the high ppb to low ppm range. 

Samples must be in the form of liquids or gases for analysis. Sample preparation may include buffering the sample to an appropriate pH for some ISE and gas measurements or adding ionic strength buffer to make all samples and standards of equivalent ionic strength. The detection limit for most common electrodes is about 10 M, while gas probes have detection limits in the low ppm range. 

The method of choice, which has been established in several companies within the last three years, uses ion chromatography (see Fig. 2.2-2). This comparatively convenient method can achieve detection limits in the low ppm range [6]. For applications where an even lower chloride content is required, ICP-MS (inductively coupled plasma-mass spectrometry), which has a detection limit at the 10 ppb level, can be used [14]. 

In practical terms, it is suggested that, in any application where the presence of halide ions may cause problems, the concentration of these be monitored to ensure the purity of ILs. The VoUhard procedure for halide ions or the use of a chloride-selective electrode are two methods which are well suited for the detection of amounts of hahde in the region of 100 ppm [9]. The advantage of the chemical procedure is the ability to detect any hahde anion. Recently ionic chromatography was developed to monitor the level of hahde in ILs [10,11]. The method enables a better detection of hahde impurities, as the authors claim a detection limit in the range of a few parts per million. 

The computing treatment of a PIXE spectrum can give the elementary concentration of more than 20 elements with a detection limit near the ppm. The reality or the confidence in these absolute values given by the PIXE method depends on many factors such as the counting statistics, the background under the particular peak and the spectrum interpretation (refer to Fig. 1.20 for spectrum background). 

As the complex reaches the column, it is retained at the top of the packing and its absorbance monitored with a fiber-optic spectrophotometer. The output is a curve where the peak height is proportional to the nitrite concentration in the sample. This procedure affords detection limits in the sub-ppm range. 

Table 4.11 compares SIMS and SNMS (cfr. also Table 8.57 of ref. [110a]). Detection limits in the sub-ppm range are accessible under optimised analytical conditions. A lateral resolution of less than 100 nm and an in-depth resolution of a few nm can be achieved. One of the unique features of SNMS is the ease of analysis of insulators. This is at variance to SSMS, GD-MS and SIMS, which are handicapped by electrical charging effects. Laser SNMS is not strictly restricted to elemental analysis, but can also be applied to the characterisation of molecular surfaces. For an optimum yield of intact molecular ions and characteristic fragments it is necessary to optimise laser power density, wavelength, and pulse width [112],... 

---