Precision sample introduction

Easily controlled and precise sample introduction ensures quantitative precision. 

A short guard column containing the same stationary phase as the analytical column is placed before the analytical column to protect it from contamination with particles or irreversibly adsorbed solutes. A high-quality pump provides smooth solvent flow. The injection valve allows rapid, precise sample introduction. The column is best housed in an oven to maintain a reproducible temperature. Column efficiency increases at elevated temperature because the rate of mass transfer between phases is increased. Mass spectro-metric detection provides quantitative and qualitative information for each substance eluted from the column. Ultraviolet detection is most common and it can provide qualitative information if a photodiode array is used to record a full spectrum of each analyte. Refractive index detection has universal response but is not very sensitive. Evaporative light scattering responds to the mass of each... 

As FI and SI share the same principles (precise sample introduction, controlled dispersion, and reproducible timing), the costs are very similar and lower than those of commercial analyzers providing a similar (or even poorer) performance. For the same reason, the... 

The reduction of the yellow-colored Mo(VI) complex to the blue-colored Mo(V) complex is a slow reaction. In the standard spectrophotometric method, it is difficult to reprodudbly control the amount of time that reagents are allowed to react before measuring the absorbance. To achieve good precision, therefore, the reaction is allowed sufficient time to proceed to completion before measuring the absorbance. In the FIA method, the flow rate and the dimensions of the reaction coil determine the elapsed time between sample introduction and the measurement of absorbance (about 30 s in this configuration). Since this time is precisely controlled, the reaction time is the same for all standards and samples. 

Samples must be introduced into the plasma in an easily vaporized and atomized form. Typically this requires liquid aerosols with droplet diameters less than 10 pm, solid particles 1-5 pm in diameter, or vapors. The sample introduction method strongly influences precision, detection limits, and the sample size required. 

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. 

Precision FIA measurements typically show low relative standard deviations (RSD) on replicate measurements, mainly due to the definite and reproducible way of sample introduction. This is a very important feature especially for CL, which is very sensitive to several environmental factors and sensitivity relies greatly on the rate of the reaction. 

An HPTC injector allows the introduction of a precise sample volume onto the column. A typical manual injector consists of a 6-port valve with a rotor, a sample loop and a needle port (Eigure 9). A sample solution is introduced into the sample loop using a 22-gauge blunt tip syringe in the TOAD position. The sample is then injected into the column by switching the valve to INJECT. The typical external sample loop size ranges from 6 pT to 2 mT. For many years, the Rheodyne 7125 injector was the industry-standard. In the early 1990s, it was replaced by the Rheodyne 7725 injector, which injects samples without momentary flow disruptions. ... 

Flow injection analysis is based on the injection of a liquid sample into a continuously flowing liquid carrier stream, where it is usually made to react to give reaction products that may be detected. FIA offers the possibility in an on-line manifold of sample handling including separation, preconcentration, masking and color reaction, and even microwave dissolution, all of which can be readily automated. The most common advantages of FIA include reduced manpower cost of laboratory operations, increased sample throughput, improved precision of results, reduced sample volumes, and the elimination of many interferences. Fully automated flow injection analysers are based on spectrophotometric detection but are readily adapted as sample preparation units for atomic spectrometric techniques. Flow injection as a sample introduction technique has been discussed previously, whereas here its full potential is briefly surveyed. In addition to a few books on FIA [168,169], several critical reviews of FIA methods for FAAS, GF AAS, and ICP-AES methods have been published [170,171]. 

Mass spectrometry is a sensitive analytical technique which is able to quantify known analytes and to identify unknown molecules at the picomoles or femto-moles level. A fundamental requirement is that atoms or molecules are ionized and analyzed as gas phase ions which are characterized by their mass (m) and charge (z). A mass spectrometer is an instrument which measures precisely the abundance of molecules which have been converted to ions. In a mass spectrum m/z is used as the dimensionless quantity that is an independent variable. There is still some ambiguity how the x-axis of the mass spectrum should be defined. Mass to charge ratio should not lo longer be used because the quantity measured is not the quotient of the ion s mass to its electric charge. Also, the use of the Thomson unit (Th) is considered obsolete [15, 16]. Typically, a mass spectrometer is formed by the following components (i) a sample introduction device (direct probe inlet, liquid interface), (ii) a source to produce ions, (iii) one or several mass analyzers, (iv) a detector to measure the abundance of ions, (v) a computerized system for data treatment (Fig. 1.1). 

Flow-injection sample introduction has been successfully applied in the analysis of standard reference materials and in the measurement of accurate and precise isotope ratios, and, hence, isotope dilution analysis. The rapid sample throughput possible with FI should allow a four-fold increase in the sampling rate compared with conventional nebufization techniques. Also, the amount of sample consumed per analytical measurement by FI is considerably less than continuous nebufization. TTiese considerations are of particular importance for the cost-effective operation of ICP-MS. 

Automation is especially advantageous for analysing large numbers of samples on a routine basis. The flow injection method requires low sample volumes, hut even the recommended 600 pi loop size can be reduced to approximately 100 pi without substantial losses in sensitivity, accuracy or precision. In certain applications involving ICPs samples as small as 20 pi have been reported [7]. There is Httle doubt that sample introduction with a flow-injection valve and driven by a peristaltic pump or another... 

HPLC has more or less supplanted GC as a method for quantifying drugs in pharmaceutical preparations. Many of the literature references to quantitative GC assays are thus old and the precision which is reported in these papers is difficult to evaluate based on the measurement of peak heights or manual integration. It is more difficult to achieve good precision in GC analysis than in HPLC analysis and the main sources of imprecision are the mode of sample introduction, which is best controlled by an autosampler, and the small volume of sample injected. However, it is possible to achieve levels of precision similar to those achieved using HPLC methods. For certain compounds that lack chromophores, which are required for detection in commonly used HPLC methods, quantitative GC may be the method of choice, for analysis of many amino acids, fatty acids, and sugars. There are a number... 

The whole atomizer may be water cooled to improve precision and increase the speed of analysis. The tube is positioned in place of the burner in an atomic absorption spectrometer, so that the light passes through it. Liquid samples (5-100 mm ) are placed in the furnace, via the injection hole in the centre, often using an autosampler but occasionally using a micro-pipette with a disposable, dart-like tip. Solid samples may also be introduced in some designs, this may be achieved using special graphite boats. The sample introduction step is usually the main source of imprecision and may also be a source of contamination. The precision is improved if an autosampler is used. These samplers have been of two types automatic injectors and a type in which the sample was nebulized into the furnace prior to atomization. This latter type was far less common. 

Because ICP-MS with different instrumentations and sample introduction systems (besides solution nebulization, also laser ablation or hyphenated methods, such as HPLC, CE, SPME) is today the most frequently used analytical technique for precise and accurate isotope ratio measurements, the following section will mainly focus on this form of mass spectrometry with an inductively coupled plasma source. 

Sr). Over the past 30 years, lead and strontium isotope ratios have been measured with thermal ionization mass spectrometry (TIMS). Elemental salts are deposited on a filament heated to produce ionized particles, which are then sent into a mass spectrometer where they are detected by multiple Faraday cups arrayed such that ions of several masses are collected simultaneously. TIMS is capable of high precision isotope discrimination, but the instruments tend to be large and expensive, and extensive sample preparation is required prior to sample introduction. Newer ICP-MS-based technologies like multi-collector ICP-MS (especially laser ablation) circumvent some of the sample preparation issues while exploiting the precision of simultaneous mass discrimination, but they are still limited by the number and configuration of ion collectors. 

There are several drawbacks to ultrasonic nebulizer/desolvation systems. Precision is typically somewhat poorer (1% to 3% relative standard deviation) than for pneumatic nebulizers (0.5% to 1.0% relative standard deviation) and washout times are often longer (60 to 90 sec compared to 20 to 30 sec for a pneumatic nebulizer/spray chamber without desolvation). Furthermore, chemical matrix effects are dependent on the amount of concomitant species that enter the ICP per second. Therefore, use of any sample introduction device that increases the amount of sample entering the plasma per second also naturally leads to more severe matrix effects when the sample contains high concentrations of concomitant species. 

The major limitation of LA-ICP-MS is the need for standards that closely match the properties of the samples. In some cases it is possible to use NIST glass standard reference materials for calibration in the analysis of geological materials [67,68], Internal standardization employing MS signals from elements at known concentrations has been used to improve precision and accuracy. Other techniques, such as acoustic [69] and light scattering [70] measurements, have been used in an attempt to monitor the relative amount of material ablated. These approaches seem to work well for variations in the amount of material sampled for similar sample matrices but not for very different types of solids. Dual-sample introduction systems with either wet [71] or dry [72] aerosol introduction in addition to laser ablation have also been reported. 

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