Urea determination

Following this procedure urea can be determined with a linear calibration graph from 0.143 p.g-ml To 1.43 p.g-ml and a detection limit of 0.04 p.g-ml based on 3o criterion. Results show precision, as well as a satisfactory analytical recovery. The selectivity of the kinetic method itself is improved due to the great specificity that urease has for urea. There were no significant interferences in urea determination among the various substances tested. Method was applied for the determination of urea in semm. 

The above discussion does not mean that the use of urease and subsequently the use of an ammonia electrode is not practicable for a urea determination. Unfortunately, the commercial company that produced the urea analyzer, chose a conductivity procedure, which happens to be unsuitable for the laboratory of Neonatology. Had they chosen the ammonia electrode, which happens to be a relatively good electrode, and is especially specific, since only ammonia and not potassium can pass an air space, then the instrument could have been made highly specific for urea. In this case an ammonia determination would be done initially and then subtracted by the computer, from the amount which has been generated subsequently. In any case, with present technology, sensitivity is not adequate to use less than approximately 15fil of serum. 

Screen-printed electrodes manufactured with the use of carbon-containing ink were used, (i) Catalytic system was inserted into carbon ink prior to ink immobilization on polymer substrate (ii) catalyst or polymer matrix (nation), containing catalyst, was immobilized on the working surface of the transducer. Table 27.1 displays the results of urea determination with the application of catalytic systems inserted into ink or immobilized on the transducer by different techniques. Optimum results (minimum mean square deviation and maximum correlation between the introduced and determined concentrations) are... 

Urea determination with sensors differently modified with catalysts... 

Table 27.2 illustrates urea determination with the use of sensors, manufactured as described in point 3 of Table 27.1. Initial concentrations of aqueous NiS04 solution and chloroform NaDDC solution were varied. The optimum results (minimum mean square deviation and maximum correlation between the introduced and determined concentrations) are observed for the case when the initial concentration of NiS04 exceeds four times the initial chloroform NaDDC concentration. For further assays, the sensor was used meeting the conditions described in point 3 of Table 27.1 and in point 3 of Table 27.2. Solutions contained 0.04 g NaDDC in 2 ml of chloroform and 0.2 g NiS04 in 2 ml of H20.

Urea determination with Catalyst 3 containing sensor for different initial concentrations of chloroform NaDDC solution and aqueous NiS04 solution... 

Enzyme-free urea sensor Nano carbon-containing transducer, covered with chemical catalytic system, is used as electrochemical enzyme-free sensor for urea determination in biological materials. 

Sulfonylurea herbicides and metabolites C02-methanol Using Ci8 solid phase extraction disk High-performance liquid chromatography with UV detection. Dimethyl sulfonyl ureas determined by GC with detection by electron capture or N-P or MS. [162,163]... 

The value for the molar mass of urea determined in this experiment is thus 59.7 g/mol. Urea has the formula (NH2)2CO and a molar mass of 60.0, so the result obtained in this experiment agrees fairly well with the known value. 

Enzyme electrodes for lactate determination using immobilized lactate dehydrogenase 16), for urea determination using immobilized urease 17), for L-amino acids using immobilized L-amino acid oxidase 18), and for various amines using the appropriate immobilized deaminase system (19) have also been prepared. A urease electrode is commercially available from Beckman,... 

Urea can be readily determined by using urease. The ammonia produced is determined by Nesslers reagent (55), by coupling to the NADH dependent glutamate dehydrogenase reaction (56), or by the use of a cation sensitive electrode (57). The latter method permits urea determination over the range of 0.1-50 fxg/ml in a few seconds (19). 

Koncki, R. Chudzik, A. Walcerz, I. Urea determination using pH-enzyme electrode. J. Pharm. Biomed. Anal. 1999, 21, 51-57. 

Rovida E, Mosca A, Dossi G. Serum and whole blood urea determination by the use of a microprocessor controlled differential pH analyzer. Eur J Chn Chem Chn Biochem 1981 19 820. 

Al. Alexander, P. W., and Joseph, J. P., A coated-metal enzyme electrode for urea determinations. Anal. Chim. Acta 131, 103-109 (1981). 

Kulys et al. (1986b) studied urea determination by difference measurement between two antimony electrodes covered with exchangable membranes (Fig. 68). Urease was attached in the pores of a macroporous membrane (thickness, 10 pm, pore diameter, 0.1 pm) by glutaraldehyde. This layer was covered with a monoacetylcellulose membrane. The membrane for the auxiliary electrode was prepared analogously, but using BSA instead of urease. The assay of urea was carried out with a differential amplifier which simultaneously differentiated the time course of the potential difference between enzyme and auxiliary electrode (kinetic method). Thus, a response time of only 20 s was possible. 

Hamann (1987) employed a potentiometric urea electrode in an enzyme difference analyzer for urea determination in serum. The difference between the potential changes of a urease-covered and a bare pH glass electrode is evaluated 30 s after sample injection. This fixed-time regime provides a measuring frequency of 20-25/h the linear range for 1 120 diluted samples is 1-20 mmol/l. These results are better than those of common potentiometric enzyme sensors. 

Fig. 136. Urea determination with the urea module during dialysis treatment of a patient.

An ultramicro modification of the enzymatic glucose determination has heen described by Kingsley and Getchell (K2). Their method requires 20 pi sample and needs no deproteinization. Fawcet and Scott (FI) developed an ultramicro modification of the urea determination with urease (Sample 10 pi). 

Urea is the most important end product of protein degradation in the body. Its concentration in blood depends on the protein catabolism and nutritive protein intake and is regulated by renal excretion. Thus the estimation of blood urea nitrogen is important in the assessment of kidney failure. The normal level of urea ranges from 3.6 mM to 8.9 mM. All enzymatic methods for urea determination are based on the principle of urea hydrolysis by urease ... 

Most biosensors described in the literature for the determination of urea are potentiometric based on NH4 or HCOj" sensitive electrodes [181, 182]. Osaka and co-workers constructed a highly sensitive and rapid flow injection system for urea analysis with a composite film of electropolymerised inactive PPy and a polyion complex [183]. Pandey and co-workers fabricated a urea biosensor based on immobilised urease on the tip of an ammonia gas electrode (diameter 10 pm) made from a PPy film coated onto a platinum wire [170]. The enzymatic response was achieved in the wide range of 0.001-0.05 M with a stability of more than 32 days. Cho and co-workers [184] developed a procedure for urea determination by crosslinking urease onto PANI-Nafion composite electrodes, which could sense the ammonium ions efficiently. Such a urea biosensor has a detection limit of about 0.5 pM and a response time of 40 seconds. 

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