External standard Extraction

Eatty acids from commercial fats and oils, such as peanut oil, are extracted with methanolic NaOH and made volatile by derivatizing with a solution of methanol/BE3. Separations are carried out using a capillary 5% phenylmethyl silicone column with MS detection. By searching the associated spectral library students are able to identify the fatty acids present in their sample. Quantitative analysis is by external standards. 

A matrix extract was prepared from poison-free scallop and spiked at the level of 200 ngg of scallop hepatopancreas. The toxins were determined by using LC-MS with calibration employing external standards prepared in methanol. The matrix extract was then spiked further with 300 ngg of each of the toxins and redetermined. The results obtained for each analyte are summarized in Table 5.17 and show that, when using the external calibration method, the values obtained range from 138 to 170 ngg a reduction in accuracy of between 15... 

An internal standard method gives more reliable results when elaborate sample preparation is required, as in extraction of a drug substance from biological fluids, or extraction of pesticides and herbicides from soil and plant matter. The addition of internal standard (IS) to the sample and standard acts as a marker to give accurate values of the recovery of the desired compound(s). Since the determination of wt% involves the ratio of the detector responses in the two chromatograms, the injection volume is not critical as in an external standard method. 

Buchberger et al. [104] carried out a selective determination of iodide in brine. The performance of a potentiometric method using an ion-selective electrode and of liquid chromatography coupled with ultraviolet detection at 230 nm were compared as methods for the determination of iodide in the presence of other iodide species. Satisfactory results were obtained from the potentiometric method provided the solution was first diluted tenfold with 5 M sodium nitrate, and external standards were used. Better reproducibility was, however, achieved with HPLC, provided precautions were taken to prevent reduction of iodine to iodide in the mobile phase, for which extraction of iodine with carbon tetrachloride prior to analysis was recommended. This was the pre-... 

In the analysis of seawater, isotope dilution mass spectrometry offers a more accurate and precise determination than is potentially available with other conventional techniques such as flameless AAS or ASV. Instead of using external standards measured in separate experiments, an internal standard, which is an isotopically enriched form of the same element, is added to the sample. Hence, only a ratio of the spike to the common element need be measured. The quantitative recovery necessary for the flameless atomic absorption and ASV techniques is not critical to the isotope dilution approach. This factor can become quite variable in the extraction of trace metals from the salt-laden matrix of seawater. Yield may be isotopically determined by the same experiment or by the addition of a second isotopic spike after the extraction has been completed. 

True profile analysis requires scanning over the whole mass range for the acquisition of all data on excreted compounds. Quantitation has been more challenging on a quadrupole instrument because total ion current peaks are seldom a single component and extracted-ion chromatograms (EICs) when recovered from scanned data are of poor quality due to the lower sensitivity of scanning GC-MS. Thus, we developed profile analysis based on SIM of selected analytes but tried to ensure the components of every steroid class of interest were included. For ion traps the fundamental form of data collection (in non-MS/MS mode must be full -scans). Thus, the quantitative data produced are EICs obtained from scanned data. The EICs are of the same ions used for SIM in quadrupole instruments and the calibration external standards are the same. 

External standard procedure using ChromStar software or manual calculation using an integrator. Measure the creatinine in urine and calculate the results as mmol/mol creatinine. Measure Hb in dried blood extracts and calculate results as nmol/g Hb. 

An accurately prepared standard solution of retinyl acetate (i.e., a solution of known concentration) is taken through the saponification and extraction procedure along with each batch of samples, and the resultant retinol solution is used as an external standard without spectrophotometric standardization. This technique, which is recommended by COST 91 (149), compensates for losses of vitamin A incurred during the saponification and subsequent manipulations (i.e., the calculated vitamin A value is recovery-corrected). 

AA Margarine butter Two procedures reported to be equally effective based on extraction into warm water or hexane Analytical Spherisorb-ODS (150 X 4.6 mm, 3 /zm Tracer Analytical). Isocratic water acidified with sulphuric acid to pH 1.95. 0.7 ml/min. UV absorbance 254 nm. External standardization. Linear range = 5-100/zg/ml, r > 0.999. Recoveries 96-101% (n = 6). 53... 

AA and IAA simultaneously Yeast Metaphosphor ic acid/cold perchloric acid extraction Precolumn ODS-IO (40 X 2.6 mm Bio-Rad). Analytical Spherisorb ODS-2 (250 X 4.6 mm, 5 jum Rainin). 35°C. Isocratic 0.1 mM EDTA and 1.0 mM tetrabutyl-ammonium phosphate in 0.08 M acetate buffer, pH 4.2 + methanol (95 + 5, v/v). No flow rate reported. Electro- chemistry + 0.72 V vs. Ag/AgCl reference electrode, glassy carbon working electrode. External standardization. Linear range = 0-60 /ug/g yeast (dry weight). Reproducibility CV 2.3% for IAA, 1.2% for A A. Recoveries quantitative for both vitamers. 59... 

Total thiamine Milk Enzymatic hydrolysis of protein with trypsin and thiamine phosphates to thiamine with claradiastase oxidation of thiamine to thiochrome using ferricyanide (derivatization stopped with sodium sulphite) thiochrome extracted with 1-butanol Analytical Nucleosil Phenyl (150 mm, 5 fi Macherey-Nagel). Isocratic methanol + acetonitrile + isobutanol + water (80 +10+10+5 v/v/v/v). 0.7 ml/min. Fluorescence 375/430 nm (ex/em). External standardization. 76 Recoveries 95% thiamine as thiochrome from milk. 

Total thiamine Rice Extraction by refluxing Analytical Zorbax Isocratic phosphate- Online postcolumn derivati- External standardization. 77... 

Total thiamine Baby food cereals cookies Acid hydrolysis with 0.1 M hydrochloric acid at 100°C for 30 min enzymatic hydrolysis of thiamine phosphates to thiamine with takadiastase at 47°C for 3 h weak ion-exchange (methyl-carboxylatein, add form) solid-phase extraction/ cleanup Analytical Lichrospher 100RP-18 (125 X 4 mm, 5 /u.m Merck ). Isocratic methanol + 10 mM phosphate buffer, pH 2.8 containing 5 mM sodium hexane-sulphonate + tri-ethylamine (15 + 84.9 + 0.1, v/v/v). 1.0 ml/min. UV absorbance 254 nm. External standardization. 79 Linear range = LoD to 7 /zg/ml thiamine, r = 0.9995. Reproducibility CV < 3% using food samples (n = 10). Recoveries 92.1-96.0% thiamine using baby food (n = 3). Samples spiked before hydrolysis. 

Total Ready-to-eat Extraction by auto- Precolumn Isocratic acetoni- UV absorbance External standardization. 90... 

Total niacin Fortified foods Sulfuric acid extraction Analytical PRP-X100 Isocratic dilute UV absorbance External standardization. 101... 

Total folacin Egg yolk lima Extraction with Analytical A aqueous solution UV absorbance External standardization. 

CNCbl, HOCbl, Milk dairy Extraction and protein Analytical A acetonitrile. Detection of vitamers in External standardization. 177... 

Biotin and biotin analogs Infant formula Protein precipitation using concentrated hydrochloric acid neutralization with 6 M NaOH lipid extraction with n-hexane Precolumn Microsorb C18 (15 X 4.6 mm, 5 jam Rainin). Analytical Microsorb C18 (250 X 4.6 mm, 5 /zm Rainin). Isocratic 100 mM phosphate buffer, pH 7.0 + methanol (80 + 20, v/v). 0.4 ml/min. Postcolumn reaction system UV absorbance at 220 nm followed by streptavidin-fluorescein isothiocyanate (2.0 mg/L) knitted open tubular reaction system (10.0 m x 0.5-mm ID) at a flow rate External standardization. 184 Linear range = 0.08-1.00 fjM biotin. LoD = 0.02 /zM or 97 pg biotin at SNR = 3. Repeatability CV 3.5% for biotin in infant formula. 

Thiamine and Broccoli Extraction by auto- Analytical /zBondapak Isocratic Fluorescence time- External standardization. 190... 

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