Overload volume

The width of a band eluting from a column in linear chromatography may be calculated from the sum of the variances of the different processes which influence it. For the present purposes we assume that the load is low and the predominant band spreading mechanism is due to the injection. The band variance may be considered as the sum of the variances due to the column and the injection volume. (It should be remembered that the variance is the square of the standard deviation of the band the standard deviation can be measured as 1/4 of the baseline width, so variance is calculated as = w Vlb.) If the column efficiency is high, and the column variance is therefore small compared with that of the injection, the latter will dominate and the peak will become rectangular, with a width equal to that of the injection. Under such circumstances, two peaks will be just resolved if their centres are separated by an elution volume equal to that of the injection. 

The sample concentration which corresponds to the volume overload example above is approximately 0.3 mg/mL Normally a sample solubility of between 10 and lOOmg/mL is desirable for preparative separations. A very poor solubility such as this usually is unacceptable and changes in temperature, mobile phase or the entire phase system are made in order to improve it to the level where mass effects predominate. There is a further need to reduce the volume overload effects other than those noted here. This is a consequence of the interactions between the solutes, which are considered in the following section. 

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The major cause of peak asymmetry in GC is sample overload and this occurs mostly in preparative and semi-preparative separations. There are two forms of sample overload, volume overload and mass overload. 

Volume overload results from too large a volume of sample being placed on the column, and this effect will be discussed later. It will be seen that volume overload does not, in itself, produce asymmetric peaks unless accompanied by mass overload, but it does broaden the peak. Mass overload, however, frequently results in a nonlinear adsorption isotherm. However, the isotherm is quite different from the Langmuir isotherm and is caused by an entirely different phenomenon. 

If the sample is relatively insoluble in the mobile phase, then it can be dissolved, as a dilute solution, in a relatively large volume of solvent. A large volume of the solution can then be placed on the column, a procedure that results in volume overload. 

The effective use of column volume overload for preparative separations was experimentally demonstrated by Scott and Kucera [1]. These authors used a column 25 cm long, 4.6 mm I.D. packed with Partisil silica gel 10 mm particle diameter and employed n-heptane as the mobile phase. The total mass of sample injected was kept constant at 176 mg, 8 mg and 0.3 mg of benzene, naphthalene and anthracene, respectively, but the sample volumes used which contained the same mixture of solutes were 1 pi, 1 ml, 2 ml and 3 ml. The chromatograms of each separation are... 

The problem is made more difficult because these different dispersion processes are interactive and the extent to which one process affects the peak shape is modified by the presence of another. It follows if the processes that causes dispersion in mass overload are not random, but interactive, the normal procedures for mathematically analyzing peak dispersion can not be applied. These complex interacting effects can, however, be demonstrated experimentally, if not by rigorous theoretical treatment, and examples of mass overload were included in the work of Scott and Kucera [1]. The authors employed the same chromatographic system that they used to examine volume overload, but they employed two mobile phases of different polarity. In the first experiments, the mobile phase n-heptane was used and the sample volume was kept constant at 200 pi. The masses of naphthalene and anthracene were kept... 

To demonstrate the effect in more detail a series of experiments was carried out similar to that of volume overload, but in this case, the sample mass was increased in small increments. The retention distance of the front and the back of each peak was measured at the nominal points of inflection (0.6065 of the peak height) and the curves relating the retention data produced to the mass of sample added are shown in Figure 7. In Figure 7 the change in retention time with sample load is more obvious the maximum effect was to reduce the retention time of anthracene and the minimum effect was to the overloaded solute itself, benzene. Despite the reduction in retention time, the band width of anthracene is still little effected by the overloaded benzene. There is, however, a significant increase in the width of the naphthalene peak which... 

Preparative chromatography involves the collection of individual solutes as they are eluted from the column for further use, but does not necessarily entail the separation of large samples. Special columns can be designed and fabricated for preparative use, but for small samples the analytical column can often be overloaded for preparative purposes. Columns can be either volume overloaded or mass overloaded. Volume overload causes the peak to broaden, but the retention time of the front of the peak... 

The easiest way for an analyst to obtain small quantities of a component of a mixture is to overload an analytical column. In order to exercise this technique, the solute of interest must be well separated from its closest neighbor. The column can then be overloaded with sample until the peak dispersion resulting from the overload, causes the two peaks to touch at their base. There are two types of column overload, volume overload and mass overload. In practice, it is often advantageous to employ a combination of both methods and a simple procedure for doing this will be given overleaf. 

Volume overload can be treated in a simple way by the plate theory (8,9). In contrast, the theory of mass overload is complicated (10-12) and requires a considerable amount of basic physical chemical data, such as the adsorption isotherms of the solutes, before it can be applied to a practical problem. Volume overload is useful where the solutes of interest are relatively insoluble in the mobile phase and thus, to apply a sample of sufficient size onto the column, a large sample volume is necessary. If the sample is very soluble in the mobile phase then mass overload might be appropriate. 

To determine the band dispersion that results from a significant, but moderate, sample volume overload the summation of variances can be used. However, when the sample volume becomes excessive, the band dispersion that results becomes equivalent to the sample volume itself. In figure 10, two solutes are depicted that are eluted from a column under conditions of no overload. If the dispersion from the excessive sample volume just allows the peaks to touch at the base, the peak separation in milliliters of mobile phase passed through the column will be equivalent to the sample volume (Vi) plus half the base width of both peaks. It is assumed in figure 10 that the efficiency of each peak is the same and in most cases this will be true. If there is some significant difference, an average value of the efficiencies of the two peaks can be taken. 

Volume overload employing a solution of the material in the mobile phase at a level of about 5% w/v is a recommended method of sampling for preparative columns if the system is not optimized. However, a combination of volume overload and mass overload has also been suggested as an alternative procedure by Knox (13). 

Knox and Piper (13) assumed that the majority of the adsorption isotherms were, indeed, Langmuir in form and then postulated that all the peaks that were mass overloaded would be approximately triangular in shape. As a consequence, Knox and Piper proposed that mass overload could be treated in a similar manner to volume overload. Whether all solute/stationary phase isotherms are Langmuir in type is a moot point and the assumption should be taken with some caution. Knox and Piper then suggested that the best compromise was to utilize about half the maximum sample volume as defined by equation (15), which would then reduce the distance between the peaks by half. They then recommended that the concentration of the solute should be increased until dispersion due to mass overload just caused the two peaks to touch. 

Volume overload (TACO—transfusion-associated cardiac overload)... 

Improper blood pressure measurement Volume overload... 

Volume overload (e.g., valvular regurgitation, shunts, high-output states)... 

Heart failure patients exist in one of two clinical states. When a patient s volume status and symptoms are stable, their HF condition is said to be compensated. In situations of volume overload or other worsening symptoms, the patient is considered decompensated. Acute decompensation can be precipitated by numerous etiologies that can be grouped into cardiac, metabolic, or patient-related causes (Table 3-3).5... 

Hypoperfusion of skeletal muscles leads to fatigue, weakness, and exercise intolerance. Decreased perfusion of the central nervous system (CNS) is related to confusion, hallucinations, insomnia, and lethargy. Peripheral vasoconstriction due to SNS activity causes pallor, cool extremities, and cyanosis of the digits. Tachycardia is also common in these patients and may reflect increased SNS activity. Patients will often exhibit polyuria and nocturia. Polyuria is a result of increased release of natriuretic peptides caused by volume overload. Nocturia occurs due to increased renal perfusion as a consequence of reduced SNS renal vasoconstrictive effects at night. In chronic severe HF, unintentional weight loss can occur which leads to a syndrome of cardiac cachexia. This results from several factors, including loss of appetite, malabsorption due to gastrointestinal edema, elevated metabolic rate, and elevated levels of proinflammatory cytokines. 

Serum creatinine May be increased owing to hypoperfusion preexisting renal dysfunction can contribute to volume overload. 

In patients with evidence of mild to moderate volume overload, diuretics should be initiated at a low dose and titrated to achieve a weight loss of up to 2 pounds (0.91 kg) per... 

Dopamine is most commonly reserved for patients with low systolic blood pressures and those approaching cardiogenic shock. It may also be used in low doses (less than 3 mcg/kg per minute) to improve renal function in a patient with inadequate urine output despite high filling pressures and volume overload, although this indication is controversial. 

Monitor changes in hemodynamic variables if available. Cardiac index should increase, with a goal to maintain it above 2.2 L/minute per square meter. Pulmonary capillary wedge pressure should decrease in volume overloaded patients to a goal of less than 18 mm Hg. 

Loop diuretics are the diuretics of choice for the management of volume overload in acute renal failure. 

Develop a plan to provide symptomatic care of complications associated with ARF, such as diuretic therapy to treat volume overload. Monitor the patient s weight, urine output, electrolytes (such as potassium), and blood pressure to assess efficacy of the diuretic regimen. 

The kidney is unable to adjust to abrupt changes in sodium intake in patients with severe CKD. Therefore, patients should be advised to refrain from adding salt to their diet, but should not restrict sodium intake. Changes in sodium intake should occur slowly over a period of several days to allow adequate time for the kidney to adjust urinary sodium content. Sodium restriction produces a negative sodium balance, which causes fluid excretion to restore sodium balance. The resulting volume contraction can decrease perfusion of the kidney and hasten the decline in GFR. Saline-containing intravenous (IV) solutions should be used cautiously in patients with CKD because the salt load may precipitate volume overload. 

Fluid restriction is generally unnecessary as long as sodium intake is controlled. The thirst mechanism remains intact in CKD to maintain total body water and plasma osmolality near normal levels. Fluid intake should be maintained at the rate of urine output to replace urine losses, usually fixed at approximately 2 L/day as urine concentrating ability is lost. Significant increases in free water intake orally or intravenously can precipitate volume overload and hyponatremia. Patients with stage 5 CKD require renal replacement therapy to maintain normal volume status. Fluid intake is often limited in patients receiving hemodialysis to prevent fluid overload between dialysis sessions. 

Diuretic therapy is often necessary to prevent volume overload in patients with CKD. Loop diuretics are most frequently... 

Sodium bicarbonate tablets are administered in increments of 325 and 650 mg tablets. A 650 mg tablet of sodium bicarbonate contains 7.7 mEq (7.7 mmol) each of sodium and bicarbonate. Sodium retention associated with sodium bicarbonate can cause volume overload, which can exacerbate hypertension and chronic heart failure. Patient tolerability of sodium bicarbonate is low because of carbon dioxide production in the GI tract that occurs during dissolution. 

When determining the dose of bicarbonate replacement, the goal for therapy is to achieve a normal serum bicarbonate level of 24 mEq/L (24 mmol/L). The dose is usually determined by calculating the base deficit [0.5 L/kg X (body weight)] x [(normal C02) - (measured C02)]. Because of the risk of volume overload resulting from the sodium load administered with bicarbonate replacement, the total base deficit should be administered over several days. Once the goal serum bicarbonate level is attained, a maintenance dose of bicarbonate is necessary and should be titrated to maintain serum bicarbonate levels. 

Initiation of dialysis is dependent on the patient s clinical status. Symptoms that may indicate the need for dialysis include persistent anorexia, nausea, vomiting, fatigue, and pruritus. Other criteria that indicate the need for dialysis include declining nutritional status, declining serum albumin levels, uncontrolled hypertension, and volume overload, which may manifest as chronic heart failure, and electrolyte abnormalities, particularly hyperkalemia. Blood urea nitrogen (BUN) and serum creatinine (SCr) levels may be used as a... 

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