In chromatography, why does the retention time axis not start from zero?
398743
Sometimes it does, sometimes it doesn't.  It depends how the data are being collected and processed.\n\nThe is an "unretained peak" delay time between injection of the sample and arrival of the first molecules from the injector to reach the detector device, whatever it may be.  Depending on the carrier gas, the column type & dimensions, and to some extent to carrier flow, it will take x amount of time for an unretained molecule to get from injector to detector.  The retention time is only a relative measurement anyway, for comparing standards to sample for identification purposes.  Hope this helps...
656703
In thin layer chromatography or TLC the stationary phase consists of a thin layer of adsorbent like silica gel, alumina, or cellulose on a flat carrier like a glass plate, a thick aluminum foil, or a plastic sheet.\n\nThe process is similar to paper chromatography with the advantage of faster runs, better separations, and the choice between different adsorbents. TLC is a standard laboratory method in organic chemistry. Because of its simplicity and speed TLC is often used for monitoring chemical reactions and for the qualitative analysis of reaction products.\n\nTLC plates are made by mixing the adsorbent with a small amount of inert binder like calcium sulfate (gypsum) and water, spreading the thick slurry on the carrier, drying the plate, and activation of the adsorbent by heating in an oven. The thickness of the adsorbent layer is typically around 0.1–0.25 mm for analytical purposes and around 1–2 mm for preparative TLC.\n\nSeveral methods exists to make colorless spots visible:\n\nOften a small amount of a fluorescent dye is added to the adsorbent that allows the visualization of UV absorbing spots under a blacklight ("UV254"). \nIodine vapors are a general unspecific color reagent. \nSpecific color reagents exist into which the TLC plate is dipped or which are sprayed onto the plate. \nOnce visible, the Rf values of the spots can be determined. These values should be the same regardless of the extent of travel of the solvent, and in theory are independent of a single experimental run. They do depend on the solvent used, and the type of TLC plate.\n\nThin layer chromatography is also used in finding which pigments a plant contains. By taking extract of the plants cellulose and applying the technique, one can adequately find the pigments. It may also be used to detect pesticides or insecticides in food, or in forensics to analyze the dye composition of fibers.
339093
http://www.csc-scc.gc.ca/text/pblct/forum/e133/v13n3a17e.pdf\n\nThis document has a table of meds and their retention times.
679000
time= 2.303/(rate constant) log( initial conc / final conc)\n\nthis is for a first order reaction...
1027922
Typically, all GC and HPLC columns are made of a silicaceous material, which is then coated on the interior surfaces with a variety of materials to modify their absorptive/attractive qualities.  A C8 column has a variety of 8-carbon alkyl groups (could be n-octyl, or perhaps methyl-heptyl, etc) bonded to the silica surface; a C18 column has 18-carbon alkyl groups bonded; a C4 column has butyl-groups.\n\nThe differing nature of the treatments alters the attractive qualities of the column walls, thereby altering the retention times of the various analytes.  If you have an idea what your analytes are, you can then chose the specific column that will do a better job of separating them.
949907
It has to do with the number of turning points a graph has and the end behavior.  For example the graph of x^2-4x+4 "bounces off" the x-axis at (0,2). This graph's end behavior is it approaches + infinity on the right(the leading coefficient is +), and since it is of an even degree it must do the same on the left. If it went through the x-intercept, instead of bouncing off, it would approach - infinity on the left. The graph of x^3-6x^2+12x-8 goes through the x-intercept of (0,2). This graph's end behavior is it approaches + infinity on the right(the leading coefficient is +), and since it is of an odd degree it must do the opposite on the left. If it bounced on the x-intercept like that of x^2-4x+4, it would approach + infinity on the left instead of - infinity.
310782
one of your solvents is more polar than the other (in this case acetone) the substances to be separated by chromatography will move the most in a solvent mixture they are most similar to. Remember like dissolves like. So if your unknown is easily dissolved in a non-polar solvent like hexanes then a non polar solvent will move it the most on the paper. If your substance is very polar, IE. an organic acid. It will stay near the bottom of your chromatography paper, when using a non-polar solvent mixture. 9:1 ether:acetone is actually quite non-polar, you can progressively increase your solvent polarity by making multiple solvent mixtures with changing ratios 8:1, 5:1 etc.   Obviously, you must use a new piece of chromatography paper each time you run a new experiment (unless your spots didn't move at all!). The basic concept of these experiments is polarity and how it relates to separating a mixture. Remember, like dissolves like!
409824
When what is mixed is not fully disolved into the liquid medium.  It usually settles.  A cenrifuge will not separate out the components that are not part of the solution.
152635
You program the data in to a calculator (like a TI-83), graph, and model the data points using an exponential profile.  It will then give you the form of the equation that best fits your data points in the form of a*e^(b*x), which you can then graph over your data points.  In your case, "b" should be negative to indicate decay.
444550
The difference in solubility of the components allows for separation.  I don't remember the specifics, but that is the reason.  Water will not have a big enough difference.
705668
the retention times are longer for everything. The gas flow rate is the only variable decider in the speed of transit through the columns. Slower flow means slower movemnt independent of column interactions\n\nThe separation should theoretically be better for everything besides the components that don't interact at all with the column. They come out at dead/void volume anyway. The other components are better separated because the slower flow rate means they have more time to interact with the stationary phase and so the differences in elution rate become more pronounced.
854810
Since benabean took the absolute difference in the periods, the value of K IS relevant; in fact, crucial to the solution.  My solution:\nTake the ratio of the 2 periods: T2/T1 = √(g1/g2) = 1.00095426\nMultiply this by the seconds in a day (86400)→ 86482.4\n\nSo, a difference of 82.4 sec lost per day.
29264
Yuo can't solve this.Because it is a formula and not an equation..If u assign some value equal to this formula such as 0 u have the solution.
207923
I believe you run a blank sample and assume the resulting spectograph as 0  then using the spectrograph of your unknown using the Peaks  calculate the response factors then concentration.
180398
It varies depending whether you want the final product as an active protein or denatured.\n\nFrom cells\n\nHomogenisation\nCentrifugation (assuming you know what fraction the protein will be in)\n\nYou could then possibley use centrifugation across a glucose gradient that would further separate proteins or more simpley...\n\nFind a pH or temperature where the protein is insoluble. (Extremes will denature the protein)\nor...\nUse a size exclusion coloumn to filter out larger smaller proteins (depending on the coloumns specifications and your needs, won't denature).\n\nFor further purification:\n\nUse isoelectric focusing across a gel.\nUse a affinity chromatography coloumn specific to your protein.\n\nHope that helps. It's from memory and should be ok, I study biochemistry so if I've things wrong then I should be quite worried!
728081
i'm going to assume that the x axis is marked time and the y axis is marked displacement (that's a posh vector way of saying distance)\n\nWhen the particle is above the x axis (y is +ve) the particle is forward from it's starting point\n\nConversely for moving backward it is below the x axis (y is -ve)\n\nIt speeds up when the line has +ve gradient (from bottom left to top right)\n\nconversly it slows down when gradient is -ve (top left to bottom right)\n\nyou get these last 2 by differentiation.\n\nnow double differentiate to get the acceleration\n\nVelocity is at a max when there is a peak (maximum where gradient = 0) \n\nVelocity is at a min where there is a trough (minimum where gradient is =0)\n\nThese max and min are like Vs and inverted Vs, so the +ve and - just to the left and right of these determins which is which.\n\nOn a graph, you can tell by looking.\n\nIt stands still, when the line is horizontal in a line rather than a max or min
855472
gas chromatographt (GC) and High Performance Liquid Chromatography (HPLC) are different , and to understand why you must think about what chromatography is:\n\nChromatography in its simplest form is like putting ink on blotting paper and watching the colours separate.\nLiquid chromatoraphy uses a "column" which is made from bare or bonded silica, it separates a mixture of compounds by how polar they are. You can use a gradient of different  solvents.\n\nGC also uses a column, but it is a capillary column and instead of using a liquid to carry your mixture which needs to be separated it uses a carrier gas, like nitrogen. You can vary the temperatures in both LC and GC to aid better resolution.\n\nGC is used for more volatile compounds and LC is used more less volatile. HPLC usually refers to reversed phase, normal phase is where the column is vare silica which is very polar. Bonded silica is bonded with hydrocarbons which is non polar.\n\nThe thing to remember is that "like attracts like" so if the column in non polar, the compound to elute first will be the most polar.\n\nTo summarise, they are both separation techniques, one uses gas and the other liquid. You would choose which one to uese depending on how volatile the compounds which you want to separate are.\n\n(^_^)