Sample dilution

Before enumeration, samples may have to be serially diluted because the number of microorganisms is too high for a valid measurement. The volume of diluent into which the sample is added is referred to as a dilution blank. Although dilution blanks are most commonly made using sterile 0.1% w/v peptone, other media can be used such as 0.85% w/v saline or Butterfield s phosphate buffer (Swanson et al., 1992). Use of sterile distilled water for purposes of dilution should be avoided due to osmotic stress, which decreases viability. Normally, 18 x 150 mm autoclav-able test tubes equipped with polypropylene or metal closures or caps are used for making dilution blanks rather than cotton stoppers. 

In a dilution series, the minimum dilution is 10-fold (1 mL + 9mL, 1 10, or 10 ). Practically, dilutions are performed by aseptically transferring 1 mL of sample into a tube containing 9mL diluent. Dilutions of higher magnitude are then made by sequential 1 10 dilutions of previous dilution as shown in Fig. 14.1. For example, a 1 100 dilution of a wine can performed by two sequential 1 10 dilutions. However, a 1 100 dilution can also be made by adding 1 mL of the wine to 99 mL diluent (e.g., milk dilu- 

When preparing a dilution series, it is important to maintain aseptic conditions (Section 13.8). For example, dilution blanks must be flamed when opened to transfer sample (Fig. 13.2). Sterile 1.0 mL pipettes (0.1 mL graduations) or variable volume pipettors equipped with sterile plastic tips must be used to aseptically transfer liquid between dilution blanks. Sterile pipettes or pipette tips should be used only once, and then a new pipette or tip is used. Pipettors can be sterilized by wiping down their surfaces with 70% v/v ethanol and should be calibrated yearly. 

One problem in preparing a dilution series is the potential agglomeration of cells. For instance, Pediococcus as well as some yeasts secrete a sticky capsule that allows cells to clump together in aggregations of a few to 20 

Once the compound of interest has been isolated from the sample matrix, it is often necessary to dilute the sample in order to analyse it within the validated assay range (see Chapter 8). A test injection may have to be performed in order to assess the sample concentration in the first instance. Once it has been established that the sample does require dilution, this can easily be performed using calibrated glass (larger volumes) or variable volume pipettes (smaller volumes). 

---

The use of pre-eoneentration in eombination with ICP-AES and ICP-MS let us to diminish a degree of sample dilution up to two orders of magnitude and essentially reduee the limits of deteetion of about 30 elements. In addition, low eontent of matrix element in the solutions prepai ed for ICP-analysis reduees the matrix influenee and minifies the speetral interferenees. Main limitation, espeeially for determination of widespread elements (Si, Ca, Fe, ete.) at the ppb and ppt levels is the purity of ehemieals used for eoneentrate dissolution and dilution. 

As a method of research, has been used high-performance liquid chromatography in reversed - phase regime (RP HPLC). The advantage of the present method is the following the additional information about AIST and FAS composition (homologous distribution) simple preparation of samples (dilution of a CS sample of in a mobile phase). 

Two methods of sample preparation were investigated. The former is dilution of blood semm with 0.1% Triton X-100, the latter is aeid mierowave digestion. As evaluated, the most adequate mineralization proeedure for determining the majority of elements in blood semm by ICP AES is aeid mierowave digestion. However, the ICP AES determination of abundant elements (B, Si, Mn), whieh present in semm at 0.001-0.01 ppm levels should be follow sample dilution with Triton X-100. 

The sensory technique used for assessing human perception of odors is called olfactometry. The basic technique is to present odorants at different concentrations to a panel of subjects and assess their response. The process favored by the U.S. National Academy of Sciences is dynamic olfactometry (16). This technique involves a sample dilution method in which a flow of clean, nonodorous air is mixed with the odorant under dynamic or constant... 

Minimize stocks and segregate from other chemicals and work areas. Where appropriate, keep samples dilute or damp and avoid formation of large crystals when practicable. Add stabilizers if possible, e.g. to vinyl monomers. Store in specially-designed, well-labelled containers in No Smoking areas, preferably in several small containers rather than one large container. Where relevant, store in dark and under chilled conditions, except where this causes pure material to separate from stabilizer (e.g. acrylic acid). 

Fig. 7-11. Separation of amino acids after derivatization with OPA and A -isobutyryl-L-cysteine. Column Superspher 100 RP-18 (4 ptm) LiChroCART 125-4, mobile phase 50 mM sodium acetate buffer pH 7.0/sodium acetate buffer pH 5.3/methanol, flowrate 1.0 ml min temperature 25 °C detection fluorescence, excitation 340 nm/emission 445 nm. Sample diluted orange juice. (Merck KGaA Application note W219189 reproduced with permission from H. P. Fitznar, Alfred-Wegener-Insti-tute for Polar and Marine Research.)...

Example 6-2 The following standard addition plot was obtained for a competitive electrochemical enzyme immunoassay of the pesticide 2,4-D. A ground water sample (diluted 1 20 was subsequently assayed by the same protocol to yield a current signal of 65 nA. Calculate the concentration of 2,4-D in the original sample. 

Solid-state, esr spectra of [Cu(Et2dtc)] and [Cu S2P(OPr )2 2] dissolved in coordinating and noncoordinating solvents have been compared with single-crystal and powdered samples diluted with the corresponding complexes of divalent nickel and zinc. With noncoordinating... 

Co concentration was determined by spectrophotometer (Varian Cary 500) at 692 nm wave length, with the sample diluted with a 9 mol/L concentrated HCl solution. NO content in gas phase was obtained by an on-line Fourier transform infrared spectrometer (Nicolet E.S.P. 460 FT-IR) equipped with a gas cell and a quantitative package, Quant Pad. 

It must be pointed out, that at present, this type of equipment cannot be readily purchased, but must be assembled from materials which are commercially available. The cost of a sampling station would be less than 3,000. This would include 3 or 4 multiple dispensers, a single sample diluter and possibly a micromanipulator. If the micromanipulator is omitted, then the cost would be less than 2,000. 

FIGURE 11.22 Densitograms illustrating the effect of sample dilution with methanol. System silica/T + AcOEt -l- MeOH (83 15 2). Plates triple developed (a) 2 mg of Chelidonium majus extract dissolved in 0.2 ml of methanol (b) 2 mg of the extract dissolved in 1.6 ml of methanol. For abbreviations, see Figure 11.16. (For details, see Waksmundzka-Hajnos, M., Gadzikowska, M., and Hajnos, M.., J. Planar Chromatogr., 15, 289-293, 2002.)... 

LC/MS/MS is used to measure the ratio of analyte to internal standard in the sample and standard (the relative response determination). Once the internal standard has been added to the sample extract or standard solution, the analyte/IS ratio will not change. Subsequent sample losses will not change the analyte/IS ratio, nor will sample dilutions, solvent evaporation, changes in instrumental response or loss of chromatographic resolution. 

If this criterion still cannot be met by changing the operating parameters, run higher level standards until a signal-to-noise ratio of 9 1 is obtained. This will require adjusting the method final sample dilution such that this standard level corresponds to the required LOQ. 

Experiments with aldicarb sulfone in ground beef involved simple extraction with acetonitrile during tissue homogenization and resulted in a definite immunoassay response at the tolerance level of 10 qg kg A moderate, but rather consistent, matrix effect was observed. A more severe matrix effect was observed in bovine milk, blood, and urine. For the liquid matrices, sample dilution was not a satisfactory strategy, because the assay variability increased at lower concentrations, negating any benefit of reducing the matrix effect. This work clearly demonsttated that matrix effects are... 

Viscous Sample Dilute to reduce processing time... 

Concentration assays are often the least demanding, since usually the component to be measured is abundant and minor components scarce. Even if resolution is poor or there is detector noise, accurate measurements of concentration can still be obtained. In concentration assays, the principal requirements are stringency in the precision of sample dilution and measurement of column losses of the major component. Detector calibration, another important issue in concentration assays, has been discussed above. 

The 1/16" x 0.02" i.d. transfer line also functioned as a sample dilution device in other applications, a stainless steel column packed with glass beads has been found to be useful for dilution. This simple dynamic dilution technique has been used extensively in flow injection analysis.3 A refractive index detector is typically used to measure the sample transfer time. As shown in Figure 4, approximately 5 minutes is required to transfer the sample plug to the Rheodyne valve. As the apex of the sample band passes though the Rheodyne valve, the valve is activated and 1 pi injected onto the liquid chromatographic column. The sample transfer time was checked periodically over 1 year of operation and found to be stable. 

Polymers, which are slow to dissolve, represent a significant problem for conventional sampling. Recirculation of sample and solvent is one approach, but excessive sample dilution may occur. In the system described in the present section, a flow-through valve was modified to contain an internal dissolution chamber, as is shown in Figure 15. A known volume of solvent... 

Choice of phase type Phase weight Particle size pH of sample Dilution of sample... 

As GC is not only used as a separation medium but also as an analytical technique detection has an important function. Even if the column tolerates high-solute levels, detector requirements may determine the best injection technique or they may dictate adding a sample dilution step before injection to bring injected quantities within the optimal operating range. GC instruments accommodate an extremely wide range of solute concentrations. Minimum and maximum solute... 

Soil, wastes, and groundwater Acid digestion of sample, dilution with water, and filtration AAS (EPA method 7420) GFAAS (EPA method 7421) 0.1 mg/L 1 pg/L No data No data EPA 1986e... 

Calculating the P/S ratio from the corresponding peak area ratios or by using one point calibration method leads to erroneous interpretations [54] because the P/S ratio depends on sample dilution. An accurate quantitation of FA is thus needed to evaluate this important parameter correctly. 

To ensure that the slow magnetization relaxation is an intrinsic molecular property, the measurement could be carried out for a sample diluted in a diamagnetic matrix. Indeed [TbPc2] and [DyPc2] compounds were investigated along with... 

Campbell and Ottaway [672] have described a simple and rapid method for the determination of cadmium and zinc in seawater, using atomic absorption spectrometry with carbon furnace atomisation. Samples, diluted 1 + 1 with deionised water, are injected into the carbon furnace and atomised in an HGA-72 furnace atomiser under gas-stop conditions. A low atomisation temperature... 

Samples diluted 10-fold with blank plasma prior to analysis. 

---