Direct injection of aqueous samples

Future efforts in the field of environmental analysis will be focused on several fronts, including analyte enrichment and measurement, on-line and on-site techniques, multi-residue methodology, direct injection of aqueous samples into LC/MS/MS... 

New techniques in application of capillary columns are also of great interest and very helpful in GC analyses. Direct injection of aqueous samples onto the column is an appealing and simple method which does not require sample preparation. Headspace injection onto a column, with or without cryofocusing, is a very promising technique for determination of low concentrations (submicrogram per... 

Wastewater Direct injection of aqueous sample Shinde 1988 GC/FID No data No data Patil and... 

One recent development that offers scope for direct injection of aqueous samples into an MS system is the high-field, asymmetric waveform ion mobility spectrometer (FAIMS). By interfacing the FAIMS technology to an ESI—MS instrument a significant reduction in detection limits has been achieved for molecules of <300 Da. Preconcentration is unnecessary and analytical times are significantly shortened. Recent appHcations of this technique include naphthenic and haloacetic acids in water and amphetamines, morphine and codeine... 

The advantages of distillation techniques over direct injection of aqueous solutions includes concentration and elimination of dissolved solids. Injection of samples containing dissolved solids can rapidly plug the gas chromatographic injection port with salts and have deleterious effects on both the column and subsequent samp-ples. 

Today, lEC is used infrequently in comparison with other chromatographic methods. In most cases, IPC is more convenient because of its higher column efficiency, more stable and reproducible columns, and easier control over selectivity and resolution. There are, however, cases for using lEC instead of RP- or IP-HPLC, especially when organic ions have poor UV absorbance and need other detection (conductivity or MS). Then, completely volatile components of mobile phase are required. In such cases, lEC with volatile buffers fulfil this requirement, whereas ion-pair reagents are not sufficiently volatile in most cases also, when compounds are isolated or purified by HPLC separation, the removal of mobile phase is necessary. When multistep separation is required, the aqueous buffer-salt mobile phase used for ion-exchange allows direct injection of a sample fraction onto an RP column for the next step of separation. This may be difficult with IP systems. 

Volatilised tin species were trapped from the same or replicate water samples following rapid injection of aqueous, excess sodium borohydride solution directly into the P/T sparging vessel immediately prior to beginning the P/T cycle. 

Vinyl alcohol copolymer gel is hydrophilic and has been developed for aqueous-phase size-exclusion liquid chromatography however, it is less polar than the polysaccharides. Its specificity permits the direct injection of a biological sample without deproteinization. For example, blood serum from a patient suffering from chronic nephritis has been injected directly as a measure of the degree of dialysis (Figure 3.17). Adenosine triphosphate, adenosine diphosphate, and adenosine monophosphate in red blood cells have also been separated directly (Figure 3.18). Theophylline in blood serum has been... 

Silica columns can tolerate relatively heavy loads of triglyceride and other nonpolar material. Such material is not strongly adsorbed and can easily be washed from the column with 25% diethyl ether in hexane after a series of analyses (83). Procedures for determining vitamins A and E have been devised in which the total lipid fraction of the food sample is extracted with a non-aqueous solvent, and any polar material that might be present is removed. An aliquot of the nonpolar lipid extract containing these vitamins is then injected into the liquid chromatograph without further purification. Direct injection of the lipid extract is possible because the lipoidal material is dissolved in a nonpolar solvent that is compatible with the predominantly nonpolar mobile phase. Procedures based on this technique are rapid and simple, because there is no need to saponify the sample. 

Alternatively, an aliquot of aqueous sample directly injected onto an appropriate GC column for FID determination. 

The bioanalyst can be required to analyse most biofluids although the most common are urine and the aqueous phase of blood, i.e. plasma or serum. Other samples may be cell and tissue extracts, synovial fluid, cerebrospinal fluid (CSF) and saliva. In the case of urine and CSF with their very low protein content it might be possible to directly inject the sample into an HPLC column. With most silica-based packing materials, direct injection of blood proteins will rapidly lead to column deterioration. HPLC columns are expensive and their efficiency is easily lost so correct preparation of samples will not only improve column life but also improve the results. At its simplest it is only necessary to remove particulate matter from samples to prevent clogging of the column and frits. Modern HPLC packings are very susceptible to contamination by proteins, fats and other macromolecules from biological samples and it is necessary to remove these (except of course for protein analysis). 

Despite the potential for direct aqueous injection of water samples into reverse phase systems, there are very few cases where this is possible due to the low detection levels normally required for environmental analysis. Using direct aqueous injection and coulometric electrochemical detection, the analysis of phenol and chlorophenols and 2-mercaptobenzothiazole have been achieved at trace levels (methods with limits of detection for phenol 0.034 ngp and 0.8 pgl for mercaptobenzothiazole have been achieved). There is a potential for the use of direct aqueous injection for the analysis of phenol in effluents using fluorescence detection which would be expected to detect down to low mg T. Direct aqueous injection has been used in an automated system similar to that shown in Figure 11.1. The trace enrichment cartridge was replaced by a large sample loop (50 pi) and a coulometric electrochemical detector used instead of the UV detector. 

One limitation of atomic absorption spectrophotometry is that the samples generally have to be in solution, preferably aqueous. Thus, either the sample must be directly soluble in a suitable solvent or some type of pretreatment, such as acid digestion, is necessary. One exception is that some instruments using a graphite furnace can be modified for direct injection of solids. Another limitation is that only one metal can be analyzed at a time. There are four primary methods of accomplishing this ... 

It is particular concern in the analysis of aqueous samples, because the solvent effects involved during the direct aqueous injection (DAI) allow the direct analysis of levels ()ig l) of trihalomethanes and related compounds in water, using well deactivated thick film columns and E(D detection [19, 20 ]. 

The overriding principle in many extractions of aqueous samples is for the analyte molecules to be as near neutral as possible by adjusting the pH of the sample matrix. This guiding principle applies to most liquid—liquid and reverse-phase solid-phase extractions. If a compound cannot be made electrically neutral through pH adjustment, then an alternate approach could be considered. A compound such as a zwitterion, which cannot be made electrically neutral could be isolated from the matrix using an approach which is insensitive to the ionization state of the molecule, such as direct injection, protein precipitation, microdialysis, or ultrafiltration. 

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