Abstract:
The invention relates to a method of preparing a reaction mixture for Polymerase Chain Reaction (PCR) assay and a solution set for PCR. The method comprises providing a sample solution comprising a biological sample to be amplified in said PCR assay and first colorant providing the solution a first color, providing a reagent solution comprising at least one other substance required for performing said assay and second colorant providing the solution a second color different from the first color, and mixing the sample solution and the first reagent solution for providing a mixed solution to be subjected to the PCR process, the mixed solution having, due to said first and second colorants, a third color different from the first and second colors. The invention significantly aids in pipetting PCR assays.

Description:
RELATED APPLICATIONS 
     This application claims priority to copending U.S. application Ser. No. 13/499,518, which is a national stage application of PCT/FI2010/050772 filed Oct. 4, 2010, which claims priority to Finnish application Serial No. 20096013 filed Oct. 2, 2009, each of which is expressly incorporated by reference herein in its entirety. 
     FIELD OF THE INVENTION 
     The invention relates to pipetting of (bio)chemical reagents. In particular, the invention relates to a method of pipetting of reagents to microwells for Polymerase Chain Reaction (PCR) amplification, in particular quantitative PCR (qPCR) amplification. In addition, the invention relates to new products for aiding pipetting. 
     BACKGROUND OF THE INVENTION 
     When pipetting the PCR reaction, all necessary components, i.e. reagents, can be added to the reaction tube one by one, or preferably by first combining at least some of them as a master mix followed by dividing this mixture to multiple samples. One of the components that usually must be added separately is the sample under study. The number of samples, tubes or sample wells in a microtiter plate can be hundreds or even thousands per setup. 
     Adding reagents correctly, i.e. in the right order and amount, is crucial for obtaining valid results not only in PCR but also in many other (bio)chemical reactions. Failed experiments result in loss of time and money. Economic importance can be huge. This is because of waste of ingredients, plastic ware and personnel working hours. Moreover, delays in obtaining the results of the experiments may be significant. There have been various solutions to this well-known problem. 
     There are mechanical solutions to the problem. The art acknowledges various automated pipetting robots, multichannel pipettes and guidance systems (e.g. Finnzymes PIKO® Light Plate, BIOTX WELL AWARE™) used with sample tubes and plates. 
     For several years there have also been available PCR master mixes or buffers that contain some visible dye to help pipetting and tracking of electrophoresis runs. These mixes typically also have some component to increase density of the solution to help in electrophoresis gel loading. 
     U.S. Pat. No. 6,942,964 discloses a product using a pipetting aid dye which is also used as a gel loading and tracking dye. The colorant has been incorporated with the polymerase, which helps the user to see whether the polymerase has been pipetted to the PCR mix or master mix. A similar product is BioLine Accuzyme Red. 
     USB Corporation&#39;s RubyTaq features a polymerase including a mixture of two dyes which are separated during the agarose gel run: magenta (runs between 500 bp [2% gels] and 1500 bp [0.8% gels]) and yellow (runs less than 10 bp). 
     Fermentas and Promega have also added a colorant to the enzyme reaction buffer. Fermentas&#39; DREAMTAQ™ Green reaction buffer can be seen as green, but the color separates into a blue and yellow bands during gel electrophoresis. Promega GOTAQ® and GOTAQ® Green Mastermix behave similarly. 
     There are also products available where the dye added to the polymerase is not intended to help in the electrophoresis phase. Examples include ABgene RED HOT®. 
     NEB provides a product (CRIMSON TAQ®) featuring a dye Acid red added to the DNA polymerase reaction buffer. The product also uses 6% dextran as a density enhancer. 
     Qiagen&#39;s CORALLOAD® dye is available both as a concentrate in a separate tube for being added to an uncolored master mix, and also as an optional ready-made 10×PCR buffer. It contains two gel tracking dyes (orange and red). 
     KR 2002/0045167 discloses freeze-dried PCR mixes containing a colorant to confirm dissolution of the PCR components. U.S. Pat. No. 6,153,412 discloses also freeze-died reaction mixtures which are used for identifying the existence of a lyophilized PCR reagent and to ensure complete mixing of the PCR reagent and the test sample. U.S. Pat. No. 5,565,339, on the other hand, discloses the use of a dye in a hot start wax, which does not dissolve into the reaction mixture. 
     ABSOLUTE™ Blue QPCR Master Mix contains an inert blue dye to ease pipetting in reaction set-up. 
     Also WO 2007/088506 discloses a dyed master mix. 
     Applied Biosystems has ROX passive reference dye included in their qPCR products. The purpose of the dye is to provide a steady fluorescence level which can be used to normalize against any non PCR related fluorescence variation between the different reactions and in one sample during a reaction. The method is also suggested to normalize at least partly against deviations in pipetting accuracy. 
     All but the three last ones of the products mentioned above are suggested to be used only in traditional end point PCR. In addition to the colorants, they typically contain a density enhancer to get the sample material into the bottom of the gel wells (see e.g. U.S. Pat. No. 6,942,964). Without the density enhancer, samples would disperse into the surrounding liquid. 
     Colorants used in end-point PCR are generally not compatible with quantitative PCR (qPCR). This is usually because they prevent real-time optical measurements of the ongoing reaction. In particular, the dyes typically have a spectrum which overlaps with the detection wavelengths of qPCR fluorescence or their absorbance is too high. General requirements for the dyes used include non-inhibitory effect on the PCR reaction or stability in the reaction pH. 
     An additional disadvantage of the abovementioned solutions in which the dye is provided in the master mix or in the polymerase is that they are not able to provide help for pipetting the samples (i.e. in PCR the material to be amplified). Sample pipetting is, however, the step where keeping track of the process is most important, and most difficult. 
     The various mechanical systems available cannot solve the problem entirely. They are expensive and pipetting errors cannot always be seen visually mainly because of volume differences, which means the error might not be detected until after obtaining failed results. 
     Thus, there is a need for enhanced pipetting aids. In particular, there is a need for such pipetting aids which can also be applied for the sample pipetting phase. 
     SUMMARY OF THE INVENTION 
     It is an aim of the invention to provide a novel solution for aiding pipetting during preparation of PCR assays and in particular for making the detection of errors in various stages of pipetting easier. 
     The aim of the invention is achieved by the invention as defined in the independent claims. 
     In the pipetting stage of PCR assays, at least two reagent solutions are mixed for obtaining the final mixture which is subjected to PCR. The invention is based on the idea of coloring the reagent solutions with different initial colorants, which, upon mixing produce a distinguishable color different from the colors of the initial colorants. 
     Thus, one can tell directly by the color of the solution, whether it is the first reagent solution, the second reagent solution or the mixture of these. 
     More specifically, the method comprises
         providing a first reagent solution comprising at least one substance required for performing the assay and a first colorant providing the solution a first color,   providing a second reagent solution comprising at least one other substance required for performing said assay and a second colorant providing the solution a second color different from the first color,   mixing the first and second reagent solutions for providing a mixed solution to be subjected to the PCR process, the mixed solution having, due to said first and second colorants, a third color different from the first and second colors.       

     In a typical application, one of the reagent solutions is a sample solution, that is, a solution containing or intended to receive a biological sample to be amplified in the PCR assay, and the other of the reagent solutions contains some other at least one other substance required for performing the assay, for example the polymerase solution or master mix. The sample solution may be a buffered solution (hereinafter “a sample buffer solution”). Thus, a microwell having a first color indicates that there is only sample solution without other reagents, e.g. master mix, in the well. A microwell with second color indicates that master mix has been added but there is no sample yet. Finally, a microwell with third color implies that sample has been properly added to the master mix. The inspection of the color can be made visually or by automatic optical means. 
     In one embodiment, one of the reagent solutions is an elution buffer, such an elution buffer used in combination with a nucleic acid purification kit. 
     In one embodiment, one of the reagent solutions is a dilution buffer used to facilitate lysis of a solid-state sample to release nucleic acids. The reagent solution can also be used to dilute, digest or precipitate released components before PCR. Thus, it is possible to use the invention when pipetting direct PCR assays. 
     In further embodiments, one of the reagent solutions is a solution used in cDNA synthesis reaction, reverse transcriptase reaction or bisulphite reaction. 
     In one embodiment, one of the reagent solutions is some other solution used for preparing the sample for the PCR process. 
     The invention also provides a new use of dyes for producing two or more colored PCR reagent solutions, which are capable of forming a mixed solution having a color distinguishable from the initial colors of the reagent solutions. 
     Further embodiments are the subject of the dependent claims. 
     A particular aim of the invention is to achieve a pipetting aid solution which is compatible with quantitative PCR. This is achieved by using such colorants and colorant concentrations which do not significantly disturb the fluorescent processes, i.e. excitation and emission, or optical detection used in qPCR. In particular, the reaction mixture subjected to qPCR is transparent or translucent at least at the qPCR excitation and emission wavelengths. This generally means that the maximum absorbance of the of the reaction mixture is less than 0.5, in particular less than 0.15 (measured using 1 mm light path) and that the absorption window of the colorant does not overlap, at least significantly, with the excitation or emission wavelengths of the fluorescent qPCR dye(s) or modified DNA oligonucleotide probe(s) used. 
     In one embodiment, a reaction mixture for quantitative PCR is prepared, the reaction mixture comprising fluorescent dye, primer or probe, and wherein the absorbance peaks of any of said colorants do not overlap with the emission or excitation wavelength of said fluorescent dye, primer or probe. If overlap exists, it should not significantly weaken the qPCR signal, generally implying that the total absorbance of the reaction mixture at said wavelengths is less than 0.05, preferably less than 0.03, in particular less than 0.1. 
     The invention provides considerable advantages. As the initial solutions and the resultant solution are mutually of different colors, one not only distinguish between the initial solutions, but also between the initial solutions and their mixture. 
     In addition, from colored solutions one can be quickly perceive whether the solutions have been properly mixed and whether there are significant deviations from the desired reaction volume. 
     Moreover, the colors make it easier to see if there is any liquid splashed or spilled in wrong places where they could potentially cause contamination, microwell sealing problems etc. Especially with adhesive sealing films applied on microtiter plates before thermal cycling, any liquid in the sealing contact can compromise the seal and thus the whole PCR assay. 
     The present invention can also be used together with the mechanical solutions to lower their error rate even more, if the pipetting steps performed are visualized using the colorants. When using pipetting robots, it is possible to add a quality check step based on optical detection after desired steps, or the volume and color of the reagents can be quickly checked visually. 
     For the above reasons, dyes and other colorants used in the present manner can help keeping track in reaction setup and especially during loading reagents into reaction plate. Thus, the approach provides considerable help and increased certainty for pipetting samples. 
     In qPCR there is no need to load the amplified products into a gel after a PCR reaction. Thus, a density enhancer is not needed. Consequently, the present solutions may be free of density enhancer or contain only minor amounts of density enhancer (i.e. less than required for gel electrophoresis). 
     According to one embodiment, there is provided, in addition to the abovementioned first and second reagent solutions, one or more additional reagent solutions comprising additional colorants providing the solutions different colors. The solutions are capable of forming, on mixing, additional solutions having, due to said additional colorants, further distinguishable colors. Thus, the invention can be used not only for aiding the pipetting in one particular stage of the process, e.g. pipetting of the of the sample to master mix, but also for aiding the pipetting during other steps, in particular the steps previous or subsequent to the sample pipetting step. 
     In more detail, the method may comprise providing a third reagent solution comprising at least one further substance required for performing said assay, the third reagent solution containing a third colorant providing the solution a fourth color different from the first, second and third colors mentioned above, and mixing the third reagent solution with the first and second reagent solutions for providing a mixed reagent solution having, due to said first, second and third colorants, a fifth color different from the first, second, third and fourth colors. In particular, the first reagent solution may be a sample, the second reaction solution the master mix and the third reagent solution may be a primer solution. The order of application is not essential, unless otherwise defined in assay instructions. 
     Alternatively, to the above the method may comprise providing a third reagent solution containing third colorant providing the solution a fourth color different from the first, second and third colors, and individually mixing the first reagent solution with said second and third reagent solutions for obtaining first and second mixed solutions having third and fifth colors, respectively, different from each other and the first, second and fourth colors. For example, the second reagent solution may contain one set of primers and the third reagent solution may contain a second set of primers. In this embodiment too, the colors of all initial ingredients and all resultant mixtures are unique. 
     The two abovementioned embodiments can also be chained such that the second and third reagent solutions are ultimately individually mixed with a reagent solution which is itself prepared by mixing at least two different colored reagent solutions. Other kinds of combinations are possible too. 
     A “reagent solution” is any solution containing at least one reagent needed or advantageously used for PCR purposes. Most typical ingredients are polymerase, nucleotide, primer, ions, magnesium, other salt, pH buffering agent, dNTPs or fluorescent qPCR dye or probe, oligonucleotide, nucleic acid binding agent, a nucleic acid template. The reagent may also be other polymerase reaction additive, which has an influence on the polymerase reaction or its monitoring. 
     The term “sample solution” covers both buffered and non-buffered sample solutions which are still free of template or into which the template to be amplified using PCR has already been added, unless otherwise specified. The term “sample solution” is covered by the term “reagent solution”. 
     The term “master mix” refers to a mixture of all or most of the ingredients or factors necessary for PCR to occur, typically all except for the template and primers which are sample and amplicon specific. Commercially available master mixes are usually concentrated solutions. A master mix may contain all the reagents common to multiple samples, but it may also be constructed for one sample only. Using master mixes helps to reduce pipetting errors and variations between samples due to differences between pipetted volumes. It also minimizes the time spent for pipetting. 
     A qPCR master mix is a master mix intended for performing a qPCR reaction. Thus, it may contain fluorescent dye or fluorescently tagged oligonucleotide primers or probes. 
     The term “premix” refers to a master mix that contains all the necessary components for a PCR reaction except for the template. 
     The term “color” herein means any detectable spectral response (of a solution) to white light in the visual range. Thus, there is at least one wavelength range in the absorbance spectrum of the solution which provides a colored visual appearance for the solution (in contrast to the nearly 100% transmittance of water). White, black, and shades of gray are herein counted as colors. As will be shown later, an absorbance higher than about 0.01 (1 mm light path) gives a visually perceivable color for a solution whereas an absorbance higher than about 0.001 (1 mm light path) can be relatively easily detected by hardware-based spectral detection means. 
     The term “different colors” means that the colors are distinguishable, preferably by the naked eye, but at least with spectral detection means. In particular, “different colors” may have maximum peaks in their absorption spectrum separated by at least 30 nm. Preferably, the different colors are selected from the groups of: red, yellow, blue or cyan, magenta, yellow and visually distinguishable combinations and shades thereof, such as green, orange, and violet. 
     The term “colorant” means any substance which is capable of being homogeneously mixed or dissolved within a solution and capable of giving the solution a perceivable color. According to one embodiment, the colorant is a dye, in particular an aqueous dye, preferably a non-oxidizing aqueous dye. 
     The terms “transparent” or “translucent” colorant-containing solution refers, in particular, to a solution which has an optical transmission window at at least some fluorescence excitation and/or emission wavelengths that can be used for performing qPCR, the wavelengths depending on the fluorophores, fluorescent dye(s), and/or modified DNA oligonucleotide probe(s) contained in the reaction mixture. Typically, the excitation wavelength is between 350 and 690 nm, in particular between 490 and 650 nm. The emission wavelength is typically between 350 nm-730 nm, in particular 515 nm-680 nm. A transparent solution is optically essentially non-diffusive, whereas a translucent solution passes light diffusely. 
     The term “sample” refers to a solid material or a solution that contains the nucleic acid of interest or is to be analyzed for the presence of nucleic acid of interest. 
     The term “dilution buffer” refers to a solution that can be used for sample pretreatment before PCR setup. Pretreatment can include sample lysis for releasing nucleic acids, dilution, binding, chemical lysis, precipitation and enzymatic digestion of some components. 
     The term “preparative process” refers to any reaction, pipetting step or pretreatment which yields a product which can in total or in part be used as a sample in a subsequent PCR reaction. 
     Typically the third color, achieved by mixing the solutions with the first and second colorants, is a chromatic combination of the first and second colors of the solutions. Thus, the third color may be produced as a sum spectrum of the spectra of the first and second colors. However, it is not excluded that the third color is formed through a more complex process, e.g. reaction of the first and second colorants, or due to a fluorescent process, e.g. fluorescence resonance energy transfer (FRET), provided that the fluorescence wavelengths differ from those of qPCR fluorophores used. 
     Next, embodiments and advantages of the invention are described in more detail with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows in cross-sectional view three microwells containing colored sample buffer, colored reagent solution and their colored mixture, respectively. 
         FIG. 1B  shows a top view of a microtiter plate containing empty wells and wells containing colored sample buffer, colored reagent solution, their colored mixture and wells with optically clear liquid. 
         FIGS. 2 a  and 2 b    illustrate as a flow chart exemplary ways of carrying out the invention. 
         FIG. 3  shows a flow chart of the present process according to one embodiment of the invention. 
         FIGS. 4 a -4 d    show standard series obtained with master mix and sample with ( 4   a  and  4   b ) and without ( 4   c  and  4   d ) pipetting aid dyes. 
         FIGS. 5 a -5 d    show absorbance spectra relating to a absorbance measurement example. 
         FIGS. 6 a -6 c  and 7 a -7 c    illustrate the use of colorant in a cDNA synthesis reaction. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     To make plate setup easier, the invention provides, according to one embodiment, a dye combination that helps keeping track of pipetting master mix, samples and mixing of these, in the pipetting phase of (q)PCR process. Thus, the dyes are preferably optimized so that they will have minimal effect on qPCR reaction (e.g. will not influence the sample or DNA polymerase used) and will not significantly affect optical detection of fluorescence. In other words, the dyes used are compatible with the qPCR assay. 
     Typical fluorophores used for qPCR purposes include Alexa 350, FAM™, TET™, VIC™, JOE™, HEX™, CY® 3, TAMRA™, ROX™, TEXAS RED®, CY® 5, CY® 5.5 and QUASAR® 705, the emission and excitation wavelengths of which are shown in Table 1. 
     
       
         
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Dye 
                 Ex 
                 Em 
               
               
                   
                   
               
             
             
               
                   
                 Alexa 350 
                 350 
                 440 
               
               
                   
                 FAM 
                 494 
                 518 
               
               
                   
                 JOE 
                 520 
                 548 
               
               
                   
                 VIC 
                 538 
                 554 
               
               
                   
                 HEX 
                 535 
                 556 
               
               
                   
                 Cy3 
                 552 
                 570 
               
               
                   
                 TAMRA 
                 565 
                 580 
               
               
                   
                 Cy3.5 
                 581 
                 596 
               
               
                   
                 Texas Red 
                 583 
                 603 
               
               
                   
                 ROX 
                 585 
                 605 
               
               
                   
                 Cy5 
                 643 
                 667 
               
               
                   
                 Cy5.5 
                 675 
                 694 
               
               
                   
                 Quasar705 
                 690 
                 705 
               
               
                   
                   
               
             
          
         
       
     
       FIG. 1A  illustrates the basic principle of the invention. The microwell  12 A contains colored sample buffer  14 A having a first color (horizontal lines). The microwell  12 B contains colored master mix  14 B having a second color (vertical lines). The microwell  12 C contains colored mixture of the sample buffer and colored master mix, having a third color (horizontal and vertical lines) resulting from the first and second colors. 
       FIG. 1B  illustrates a microtiter plate  10  which, in addition to the solutions shown and marked as in  FIG. 1A , contains empty wells  12  and non-colored liquid  14 D having no color (dots). In addition, there is shown a diluted reaction mixture  14 E, which is achieved by diluting the initial reaction mixture  14 C with the non-colored liquid  14 D, the diluted reaction mixture having the same basic color as the initial reaction mixture  14 C, but with increased transparency, i.e. reduced absorbance (sparse horizontal and vertical lines). As will be described later in more detail, not only the color, but also the degree of dilution can be monitored according to one embodiment of the invention. 
     The dyes can preferably be both detected and distinguished from each other visually, i.e. by naked eye. Thus, the different colors are spectrally relatively densely distributed and no special equipment is needed. However, in automatic devices utilizing optical spectral detectors or computer vision, also colors more finely distributed on the spectral scale can be used, without compromising the ability to distinguish between different colors. 
     According to one embodiment, the combination comprises a blue master mix and yellow sample buffer. When mixed together these form clearly green solution. Blue color in the plate indicates that master mix has been added but there is no sample yet. When sample is added color turns green. If solution in the well is yellow it means that there is only sample without mastermix. According to one embodiment, the blue dye comprises Xylene cyanol. According to one embodiment, the yellow dye comprises Quinoline yellow. These dyes have been found to be compatible with the polymerase and sample buffer, respectively. 
     Other potential dyes comprise Brilliant Blue, Patent Blue, Indigocarmine, Acid Red 1, m-Cresol Purple, Cresol Red, Neutral Red, Bromocresol Green, Acid Violet 5, Bromo phenol blue, and Orange G. Other potential dyes are listed in U.S. Pat. No. 6,942,964. 
     According to one embodiment, the dyes are strong enough to give a visually perceivable color for the respective solutions, but weak enough not to disturb fluorescence detection and/or weak enough not to interfere with gel electrophoresis migration tracking with other dyes commonly used for that purpose. For example, the abovementioned Xylene cyanol and Quinolene yellow belong to this group of dyes. Thus, if the colored amplified reaction mixture is subjected to end-point gel electrophoresis analysis, the colorants do not have an influence on the analysis. Instead of that, a conventional loading buffer with electrophoresis dye can be added to the amplified mixture. Moderate dyeing also maintains the general visual appearance of the solutions transparent or translucent. 
     A suitable concentration of the dye depends on the dye itself. According to one embodiment, directed to machine-aided color detection, the concentration of the dye in the initial solution is adjusted to result in an absorbance of 0.001-0.5 at its maximum absorption wavelength (1 mm light path). According to an embodiment directed to visual color detection, the concentration of the dye is adjusted to result in an absorbance of 0.01-0.5, in particular 0.03-0.5, at its maximum absorption wavelength (1 mm light path). According to a most preferred embodiment, the absorbance is 0.03-0.15, which ensures both visual detectability of the color and negligible or small effect on the qPCR measurement even if the absorbance peak would slightly overlap with the qPCR excitation and/or emission wavelengths. It is preferred, if such overlap exists, that the total absorbance at the qPCR excitation and/or emission wavelength is less than 0.05, preferably less than 0.03, in particular less than 0.01 (1 mm light path), regardless of the maximum absorbance. As the two (or more) initial solutions have differently colored dyes, there is no significant cumulative absorbance at any particular wavelength. It should also be noted that the above absorbances are the preferred absorbance of solutions diluted to the desired PCR processing concentration. If the solutions are delivered as concentrates, the preferred absorbances are respectively higher. 
     According to an alternative embodiment, at least one of the solutions is provided with dye, which is both suitable to be used in qPCR (i.e. does not affect the fluorescence detection at the wavelengths used) and strong enough to detected in gel electrophoresis, and runs on an appropriate distance at the gel with respect to the samples. Thus, a separate loading buffer is not necessarily needed. 
     The sample buffer containing a dye may be delivered either as a dilute or concentration, depending on the intended use. 
       FIG. 2  illustrates the general concept of pipetting a colored sample solution (step  20 ) and at least one reagent solution (step  21 , optionally  22 ) into a single container and mixing the solutions (step  23 ). The color of the mixed solution is checked (step  24 ) before subjecting the mixture to PCR (step  25 ). It should be noted that there may be other pipetting and processing steps which are not shown in  FIG. 2  for simplicity. 
     Several embodiments taking advantage of the general idea of the invention are explained below. 
     According to one embodiment, there are provided a plurality of colored sample buffer solutions, in which different colorants are used to give the sample buffers different colors. According to a further embodiment, mixing the plurality of colored sample buffers with the same colored reagent mixture yields reaction mixtures of different colors. Thus, in a multi-sample PCR assay, one can distinguish between different samples based on the color of the solution. For example, a yellow sample buffer and a red sample buffer mixed with a blue master mix could give green and magenta reaction mixtures, from which one can immediately verify not only the proper mixing, but also the type of the sample. 
     According to one embodiment, there are provided a master mix and a plurality of colored primer mixes, in which different colorants are used to give the mixes different colors (master mix: color  1 , primer mixes: color  2  and color  3 ). Combining the primer mixes with the mister mix yields still different colored mixes (colors  4  and  6 ). Further, by adding a colored sample (color  7 ) to the mixes obtained, distinguishable PCR solutions are obtained (colors  8  and  9 ). In each case of the process, the color of the solutions is indicative of the contents of the solution. 
     As illustrated in  FIG. 3 , according to one embodiment, there are provided a plurality of master mixes or other premixes (steps  31 ,  32 ) which are provided with different colorants to render the premixes differently colored (say, premix  1 : color  2 , premix  2 : color  3 , . . . premix n: color n+1) and a sample having a further color (color  1 ) (step  30 ). The premixes are individually mixed with the sample (steps  33   a ,  33   b ) and the colors of the resulting solutions are checked (steps  34   a ,  34   b ). The colors are preferably chosen so that each combination of a premix solution and sample solution solutions yields a unique color rendering the solutions distinguishable from each other and the initial premix and sample solutions. After checking ( 34   a ,  34   b ) the solutions are, in principle, ready for PCR. It should be noted that there may be other pipetting and processing steps which are not shown in  FIG. 3  for simplicity. 
     More generally, there may be provided a plurality of initial solutions (each containing a component needed in the PCR reaction, e.g. polymerase, primer, ions, dNTPs or fluorescent qPCR dye or probe, or other additives) which are provided with different colorants to render the solutions differently colored (say, solution  1 : color  2 , solution  2 : color  3 , . . . solution n: color n+1) and a sample having a further color (color  1 ). The colors are preferably chosen so that each combination of solutions yields a unique color rendering the solutions distinguishable from the other solutions. 
     Selected variations of the invention having high utility value are described below. 
     Use of Dye in Elution Buffer 
     Nucleic acids for molecular biology experiments are usually purified from complex sample material. There are various methods for purification including methods based on extraction, precipitation, hybridization and different modes of chromatography or filtering etc. In most of the techniques nucleic acids are either dissolved or eluted in selected solution. Precipitated nucleic acids can be dissolved in a variety of solutions. When using other method that are based on other DNA interactions there are more requirements for the elution buffer such as suitable ionic strength. Purification methods based on DNA binding to silica in high ionic strength conditions are widely used. Bound DNA is eluted from silica matrix with low ionic strength buffer or with pure water. Many kits based on the silica binding method are available and usually the kit contains the elution buffer. To reduce the number of pipetting steps in the experiment workflow, the colorant can be included in the elution buffer or the buffer provided with the kit can be replaced with the colored buffer. By doing this, the user does not have to add the color in a separate step. 
     For example, the sample buffer containing the dye can be used as a sample elution buffer in combination with many commercial or homebrew DNA purification kits. The elution buffer provided with many of the available kits can be just simply replaced with the sample buffer containing the dye. Alternatively, a small amount of dye concentrate can be added in the elution buffer provided with the kit without diluting the sample too much. 
     The other colored reagent solution may be any other solution needed for the process, as discussed above. 
     Use of Dye in Dilution Buffer (Direct PCR) 
     New enzyme technology has made it possible to significantly simplify sample preparation for PCR and it is even possible to put the unpurified sample directly to the PCR. However in many experiments the sample needs to be separated in to several reactions and it is often preferable to be able to store some of the sample for possible repeats or other purposes. Thus direct PCR protocols where sample is lysed and dissolved in a special sample buffer are very popular. In these so called dilution protocols the dilution buffer may contain different agents that lyse the sample. In addition to these agents a colorant can be added to the dilution buffer to make subsequent pipetting steps easier. 
     The other colored reagent solution may be any other solution needed for the process, as discussed above. 
     To demonstrate this embodiment, two set of extractions from bovine milk samples were done with a kit based on DNA binding to silica. One following the guidelines and the other set where the elution buffer was replaced with 1× sample buffer with yellow dye. Purified samples were used in qPCR and qPCR results of the described two sets were compared. No significant difference was observed. 
     Use of Dye in Reverse Transcription Reaction 
     Majority of real time PCR is done for gene expression studies. In these experiments the nucleic acid of interest is RNA and thus not directly suitable template for normal qPCR. Before qPCR the RNA sample must be reverse transcribed before the qPCR step. Reverse transcription and qPCR reaction can be combined and performed subsequently in same reaction mixture. However usually the condition is a compromise and not optimal for either of the two reactions. In most cases it is more optimal to do separate reverse transcription reaction and use the created cDNA as a template in a separate qPCR reaction. In reverse transcription reaction setup there are the same challenges of keeping track of samples during pipetting as described for qPCR. An embodiment of the invention describes how colorant can be used in cDNA synthesis reaction to overcome this challenge. 
     The use of colorant in cDNA synthesis reaction has also been demonstrated as follows: 
     Two cDNA synthesis reaction series were prepared one with the yellow colorant, in final concentration of 10 fold compared to the concentration instructed for the qPCR, the other without the additional dye. HeLa total RNA dilution series with 1000 ng, 500 ng, 10 ng 1 ng, 100 pg and 10 pg dilutions were used as template. Reactions were otherwise done according to the manual (Product number F-470, Finnzymes). A 1.5 μl aliquot of each reaction was then used as a template in qPCR with DyNAmo SYBR Flash qPCR master mix. 
     With reference to  FIGS. 6 a -6 c  and 7 a -7 c   , two standard curves were created, the first ( FIG. 6 c   ) representing the series with the added dye and the other ( FIG. 7 c   ) without the dye. The results show that cDNA synthesis can be performed in presence of colorant and the quantitative nature of the reaction is maintained. 
     In practice, the dye can be brought into the reaction with the transcriptase, with the sample, with the buffer solution_or separately as a concentrate. 
     Use of Dye in Bisulphite Reactions 
     Similarly to what is discussed above, dye can also be added to any component taking part in a bisulphite treatment prior to mixing the sample thereby obtained with a second reaction solution. 
     As can be seen from the above examples, the dye can be present not only when mixing the final PCR reaction mixture but also in preparative process steps, in particular those relating to sample preparation, such as reverse transcription reaction (e.g. in cDNA synthesis), bisulfite reaction, sample elution or sample dilution. These examples are not limiting and, as understood by a person skilled in the art, the dye can be introduced also into these reactions in various ways, for example, with the enzyme, with reaction buffer, with the sample or separately. 
     As there is color present also in the preparative process steps, pipetting of these steps is facilitated too. However, according to a preferred embodiment, at least one colored solution is brought when mixing the final reaction solution, e.g. with the polymerase or master mix. 
       FIG. 2 b    illustrates, at a general level, the principle of introducing at least one colored reagent solution (step  20 ′) to the process prior to at least one preparative process step  29 ′. The pretreatment may involve the introduction of one or more other substances too (step  28 ′). After pretreatment, the process can be continued similarly to as explained above, by mixing the product (or aliquot thereof) of the preparative process step with second colored reagent solution (steps  21 ′ and  23 ′), checking the color of the mixed solution (step  24 ′) and proceeding to (q)PCR (step  25 ′). 
     Monitoring of the pipetting process is described below. 
     Preferably, the different colors are distinguishable by the naked eye. However, hardware-aided optical measurements capable of distinguishing between colors can be utilized too. 
     According to one embodiment, the condition of one or more microwells to be pipetted is checked at least once during the pipetting process automatically by optical means capable of spectral resolution. 
     According to one embodiment, the condition of the microwells is automatically checked at least two times in different stages of the pipetting process. Preferably such checking is carried out after every separate pipetting step. 
     The concentration of the colorant decreases and the color of a colored solution become weaker due to every addition of non-colored, i.e. optically clear, liquids. On the other hand, addition of colored substances changes the shade of the color. Thus, the strength and/or shade of color of a solution within a well is indicative of the stage of pipetting. By automatic measurement of the spectral response of the microwell(s), the progress of the pipetting process can be monitored. 
     According to one embodiment, the abovementioned monitoring is carried out using a computer program, which is adapted to compare the measured spectral responses after the desired pipetting steps with predefined limits for these steps. Such limits are designed to reflect the correct shade and/or strength of the color of the solution, taking into account the reagents added. A microwell, for which the measured value is not within an accepted range, is flagged incorrectly pipetted. 
     The detection of the color of a solution is preferably based on absorbance measurement. The detection instrumentation, which may be an in integral part of an automatic pipetting apparatus or a (q)PCR thermal cycling apparatus, for example, contains absorbance measurement means, i.e. a light source, a light detector and means for determining the absorbance of the contents of a microwell at least at one wavelength or wavelength range. Preferably, the absorbance measurement means comprise a spectrophotometer. By means of the invention, the reliability of pipetting and PCR assay can be improved, as even small changes in shades and strengths of colors, and thus in the contents of the wells can be detected. 
     According to an alternative embodiment, the detection instrumentation is contained in a separate apparatus to which the reaction solutions can be transferred either automatically or manually after critical pipetting steps. In the separate apparatus, a quick plate read is carried out before the plate is transferred for further processing. 
     Thus, the invention also provides an apparatus for monitoring pipetting, comprising means for receiving a microtiter plate containing a plurality of microwells and means for measuring the optical absorbance of contents of the microwells. The means for detecting the absorbance are adapted to detect the spectral absorbance profile of the sample (for color detection) and/or color intensity of the sample (for dilution detection). Preferably, the apparatus is capable of both the abovementioned functions for being able to monitor the entire pipetting process. The optical detection means are preferably connected to a computing unit which analyses and stores the measured absorbances and performs a calculation or comparison of the measurement data with pre-stored data. 
     The detection instrumentation may contain a microplate-receiving block which can be cooled for keeping the temperature of the reactions solutions low enough. For most hot-start polymerases cooling is not necessary. 
     Automatic detection is of particular assistance when the volume of the reaction vessel is small, that is, less than 5 μl, in particular less than 1 μl, as reliable visual observation of both the color and volume of the solutions is more difficult in these cases. 
     The microwells may be separate or be contained in microtiter strips or plates of any known type. Preferably, the wells are manufactured from transparent material, allowing the visual inspection or spectral measurements to be carried out through the wall of the well. 
     Dyeing Example 
     Xylene cyanol as a colorant was added to DyNAmo Flash SYBR® green qPCR and DyNAmo Flash Probe qPCR master mixes from (Finnzymes, Finland) in the concentration of 0.0026% (w/v). The result was a clearly blue transparent solution. Quinolene yellow was added to a sample buffer in the concentration of 0.00174% (w/v). The sample buffer contained 1 mM Tris-HCl pH 8.5 and 0.1 mM EDTA. As a result, a clearly yellow transparent solution was obtained. 
     The colored sample buffer and the colored master mix were mixed, resulting in a clearly green transparent mixture. 
     Amplification Example 
       FIGS. 4 a -4 d    show standard series obtained with master mix and sample with ( FIGS. 4 a  and 4 b   ) and without ( 4   c  and  4   d ) the pipetting aid dyes of Example 1. Both series were done by amplifying human genomic DNA sequence with DyNAmo Flash Probe master mix according to the protocol in the product manual. Primer sequences were ACCTCCAAACTGGCTGTAAC (SEQ ID NO: 1) and ATCTCCTCCTCATTGCAAAG (SEQ ID NO: 2). Detection was based on hydrolysis probe with a sequence TGGCCCCTTGAAGACAGCAG (SEQ ID NO: 3). Amplicon size was 121 bp. 
     From the mutual similarity of the amplification curves ( FIGS. 4 a  and 4 c   ) and standard curves ( FIGS. 4 b  and 4 d   ) can be seen that presence of the dye does not affect the reaction efficiency or significantly affect fluorescence intensities. 
     Pipetting Example: 
     The present invention was utilized to implement the following pipetting sequence:
         A colored (blue) 2× mix was thoroughly mixed with primers and probes, additives and water for obtaining a premix for several reactions.   The premix was pipetted to several wells of a microtiter plate (15 μl/well).   Colored (yellow) DNA sample solutions were pipetted onto the premixes (5 μl/well).   The color of the resulting solution was manually checked to be correct (green).       

     After the above steps, the resulting solution is ready to be subjected to (q)PCR. For qPCR, the reagents are preferably centrifuged to the bottoms of the wells. 
     Absorbance Measurement Examples 
     Absorbance of different dilutions of the dyes used in the examples above and dilutions of existing colored master mixes were measured and compared to visual observation to assess the visually perceivable range in different wavelengths. The purpose was in particular to determine visually perceivable absorbance range with different dyes and also check if commercially available dyes would be suitable to be used with FAM and SYBR fluorescent dyes which are probably the most popular dyes used in qPCR. 
     Measurements were performed with Nanodrop ND-1000 spectrophotometer, which uses 1 mm and 0.1 mm light paths. 
     The results, including visual detectability of color, absorbance maxima and absorbances of the samples as well as types of the solutions and dyes used in the experiments are shown in Tables 1-7 below. Tables 1-3 show the results obtained with preferred dyes to be used with FAM or SYBR, whereas Tables 4-7 show comparative examples obtained with commercially available colored PCR solutions. 
     In the Tables, the following denotations are used: 
     +++ strong color 
     ++ color easy to see 
     + weak color but visible in normal laboratory environment by naked eye 
     − color not visible in normal laboratory environment by naked eye 
     For cases denoted with asterisk (*), the absorbance peak was not completely well-defined or clear. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Absorbance 
                   
                   
               
               
                   
                   
                 Visual 
                 maximum 
               
               
                 Product 
                 Dilution 
                 color 
                 nm (λ max ) 
                 A (1 mm)   
                 A (1 cm)   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Reagent color 
                 500x  
                 +++ 
                 615 
                   
                   
               
               
                 Finnzymes 
                 50x  
                 +++ 
                 615 
                 1.63 
               
               
                   
                 5x 
                 ++ 
                 615 
                 0.179 
               
               
                 Xylene cyanol 
                 1x 
                 ++ 
                 615 
                 0.037 
                 0.302 
               
               
                 (XC) 
                   0.5x 
                 + 
                 615 
               
               
                   
                   0.2x 
                 − 
                 615 
                   
                 0.013 
               
               
                   
                   0.04x 
                   
                 615 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Visual 
                 nm 
                   
                   
               
               
                 Product 
                 Dilution 
                 color 
                 (λ max ) 
                 A (1 mm)   
                 A (1 cm)   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Sample buffer 
                 500x  
                 +++ 
                 413 
                   
                   
               
               
                 Finnzymes 
                 50x  
                 +++ 
                 413 
               
               
                   
                 5x 
                 ++ 
                 413 
                 0.578 
               
               
                 Quinolene yellow 
                 1x 
                 ++ 
                 413 
                 0.124 
                 1.163 
               
               
                 (QY) 
                   0.2x 
                 + 
                 413 
                 0.026* 
                 0.186 
               
               
                   
                   0.04x 
                 − 
                 413 
                 0.022* 
               
               
                   
                   0.02x 
                   
                 413 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                 Visual 
                   
                   
                   
               
               
                 Product 
                 Dilution 
                 color 
                 nm (λ max ) 
                 A (1 mm)   
                 A (1 cm)   
               
               
                   
               
             
             
               
                 Colored reaction mix 
                 1x 
                 ++ 
                 413 
                 0.128 
                   
               
               
                 Finnzymes 
                 0.2x 
                 + 
                 413 
                 0.032 
                 0.186 
               
               
                   
                 0.1x 
                 + 
                 413 
                 0.015 
                 0.059 
               
               
                 G7 
                 0.04x 
                 − 
                 413 
               
               
                   
               
             
          
         
       
     
     The absorbance maxima of the solutions of Tables 1-3 are relatively far from the fluorescent wavelengths of FAM and SYBR dyes. The absorbance spectrum (1× dilution) of the reaction mixture of Table 3 is shown in  FIG. 5 a   . From the data is can be concluded, that the dyes tested proved to be suitable to be used in the initial solutions and also together in a qPCR reaction mixture as colorants with these fluorescent dyes. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                 Visual 
                   
                   
                   
               
               
                 Product 
                 Dilution 
                 color 
                 nm (λ max ) 
                 A (1 mm)   
                 A (1 cm)   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Crimson Taq buffer 
                 5x 
                 +++ 
                 510 
                 1.804 
                   
               
               
                 NEB 
                 1x 
                 ++ 
                 510 
                 0.406 
               
               
                   
                 0.5x 
                 ++ 
                 510 
                 0.213 
               
               
                 CT 
                 0.1x 
                 + 
                 510 
                 0.046 
               
               
                   
                 0.02x 
                 + 
                 510 
                 0.013* 
               
               
                   
                 0.01x 
                 +/− 
                 510 
               
               
                   
                 0.005x 
                 − 
                 510 
               
               
                   
               
             
          
         
       
     
     The buffer of Table 4 has absorbance maximum at 510 nm which is close to FAM and SYBR fluorescence maxima. Absorbance would decrease qPCR signals with these dyes significantly. Thus, the use of this mix in qPCR would not be feasible. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                 Product 
                 Dilution 
                 Visual color 
                 nm (λ max ) 
                 A (1 mm)   
                 A (1 cm)   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Green GoTaq 
                 5x 
                 +++ 
                 419 
                   
                   
               
               
                 Promega 
                 1x 
                 ++ 
                 419 
                 1.183 
               
               
                   
                 0.5x 
                 ++ 
                 419 
                 0.527 
               
               
                 GT 
                 0.1x 
                 ++ 
                 419 
                 0.130 
               
               
                   
                 0.02x 
                 + 
                 419 
                 0.029* 
               
               
                   
                 0.01x 
                 + 
                 419 
                 0.014* 
               
               
                   
                 0.005x 
                 − 
               
               
                   
               
             
          
         
       
     
     The mix of Table 5 has very strong absorbance at 419 nm. As the absorbance peaks are not very sharp it also has significant absorbance at 495 nm (0.17 with 1 mm light path), which is the range where FAM and SYBR dyes are excited. Absorbance would decrease qPCR signals with these dyes. The absorbance spectrum (1× dilution) is shown in  FIG. 5 c   . 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                   
                   
                 Visual 
                   
                   
                   
               
               
                 Product 
                 Dilution 
                 color 
                 nm (λ max ) 
                 A (1 mm)   
                 A (1 cm)   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Quick-Load mm 
                 2x 
                 +++ 
                 478 
                 1.922* 
                   
               
               
                 NEB 
                 1x 
                 ++ 
                 485 
                 1.000 
               
               
                   
                 0.5x 
                 ++ 
                 485 
                 0.516 
               
               
                 QL 
                 0.1x 
                 + 
                 485 
                 0.103 
               
               
                   
                 0.02x 
                 + 
                 485 
                 0.025 
               
               
                   
                 0.01x 
                 − 
                 485 
                 0.012* 
               
               
                   
                 0.005x 
               
               
                   
                 0.001x 
               
               
                   
                 0.0005x 
               
               
                   
               
             
          
         
       
     
     The mix of Table 6 has very strong absorbance at 485 nm. As the absorbance peaks are not very sharp it also has significant absorbance at 495 nm (0.982 with 1 mm light path), which is the range where FAM and SYBR dyes are excited. Absorbance would decrease qPCR signals with these dyes significantly. The absorbance spectrum (1× dilution) is shown in  FIG. 5 d   . 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 7 
               
               
                   
               
               
                 Product 
                 Dilution 
                 Visual color 
                 nm (λ max ) 
                 A (1 mm)   
                 A (1 cm)   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Coral Load 
                 10x   
                 +++ 
                   
                   
                   
               
               
                 Qiagen 
                 1x   
                 ++ 
                 475 
                 0.407 
               
               
                   
                 0.5x 
                 ++ 
                 475 
                 0.202 
               
               
                 CL 
                 0.1x 
                 + 
                 475 
                 0.043* 
               
               
                   
                  0.04x 
                 + 
                 475 
                 0.018 
               
               
                   
                  0.02x 
                 − 
               
               
                   
               
             
          
         
       
     
     The mix of Table 7 has very strong absorbance at 475 nm. As the absorbance peaks are not very sharp it also has significant absorbance at 495 nm (0.393 with 1 mm light path), which is the range where FAM and SYBR dyes are excited. Absorbance would decrease qPCR signals with these dyes significantly. The absorbance spectrum (1× dilution) is shown in  FIG. 5   b.    
     Perceivable range is dependent on the wavelength but in general color providing a absorbance above 0.01-0.1 with 1 mm light path seems to be visually distinguishable from the clear liquid. When absorbance is raised to 0.1-0.2, the color appears very clear to the eye. However, sophisticated instruments are more sensitive and thus dyes providing absorbance above 0.001 could be used when e.g. a spectrophotometer is used for color measurement. 
     Use of instrument for checking the reaction setup volume by color detection enables more diluted colors to be used for that purpose minimizing possible negative effects that the colors might have. For example in qPCR the range of dyes that could be used without significantly affecting fluorescence detection would be increased. 
     The embodiments and examples above and the attached drawings are for illustrative purposes. The scope of the invention should be evaluated on the basis of the following claims taking equivalents into account. 
     Applicants incorporate by reference the material contained in the accompanying computer readable Sequence Listing identified as Sequence Listing 078623_13_ST25.txt, having a file creation date of Aug. 24, 2015 10:18 A.M. and file size of 1,033 bytes.