Automated Sample Dilutors

One important consideration is the decision to go with popular off-the-shelf systems from well-established vendors, similar systems from smaller vendors, or custom systems. It is important to keep in mind some of the vendor selection criteria mentioned earlier in the chapter vendor support, vendor stability, and experience working with GLP customers. Smaller vendors or custom software are not necessarily bad, but they can have more software bugs since they have fewer customers using the software and reporting problems. Given the need for validation and change control, the system with the fewest problems is often the best investment. 

Sample dilution is one of the most labor-intensive tasks in running an LBA. Add standard curve and quality control dilution (which these systems can easily do) [8], and it becomes over half the effort to run an assay. Depending on the specific practices of a laboratory, samples can be diluted in duplicate, triplicate, at four serial dilutions, at eight serial dilutions, with one or more sample diluents, at different dilutions for each sample, and from different size tubes (e.g., clinical versus nonclinical). Similarly, standard curve and control dilutions can be performed in a variety of ways. For these reasons, it can be challenging to implement an automated dilutor to meet all of these requirements. 

A spreadsheet can be created with the answers to the above questions. Using the 80% criteria, requirements should be categorized as in-scope or out-of-scope. In addition, it is important to look for opportunities for standardization. A finalized list of requirements should be created, reviewed, and approved by end users and laboratory management. The above process can be repeated for standard curve and control dilution if these steps are to be automated. 

A more advanced approach is to create a more complex protocol that can dilute each sample to an independent dilution factor. This eliminates the segregation of samples by dilution factor, but adds considerable complexity. The dilutor needs to know the specific dilution factor for each sample in a run, which requires that this information be imported into the protocol for each run from an electronic file. In addition, it is no longer obvious how each sample was diluted. This means that some sort of audit trail report must be created to report the dilution factor for each sample. Most popular automated dilutors have this capability, but it often requires custom programming by the vendor. This approach has the potential for the greatest efficiency, so it is worth further discussion. 

Pick up 1 mL tips and add 300 990 pL of diluent to the first intermediate locations. 

---

FIGURE 11.2 To be workflow example for an automated sample dilutor. 

Next, add an independent sample dilutor or two. This is likely the major bottleneck in sample analysis since the stand-alone workstations increased assay capacity. Automated sample dilutors are discussed in more detail below. 

Automated sample dilutors and assay robots can independently have a beneficial impact on a laboratory s efficiency and throughput. Further improvements can come from integration of the two types of systems and a LIMS. There are many ways of accomplishing this, based on the needs of an individual laboratory however, two general approaches are described ... 

In order to determine chemical elements in soil, samples of the soil must undergo a solid-liquid extraction. Sometimes the extracts resulting from this procedure have analyte concentrations that are too high to be measured accurately by the chosen method. Therefore, they must be diluted. At the Natural Resources Conservation Service (NRCS) Soil Survey Laboratory in Lincoln, Nebraska, an automated diluting device is used. Using this device, the analyst accurately transfers aliquots of the extract and a certain volume of extraction solution to the same container. This dilutor may also be used to pipet standards and prepare serial dilutions. 

Protocols that accept electronic inputs such as the sample dilution factor or number of diluents present one additional complication knowing what the instrument actually did. Just as a manual dilution process requires documentation of the various parameters used in the dilution, so does an automated dilutor. This information may be captured in an audit trail, but it is generally necessary to store this information with the assay documentation where it can easily be cross-checked against the parameters in a LIMS. This creates a requirement for the automated dilutor to generate a report of sample IDs, actual dilutions performed, number of... 

Most automated dilutors have capacitance liquid level detection. The conductive pipette tips sense a change in capacitance when they come in contact with a liquid. This feature is critical for these instruments since excess sample may be transferred if the tip submerges too far into the sample. On the other hand, if the liquid level detection setting is too sensitive, the pipette tips may touch the wall of the sample tube and... 

The first approach is based on the fact that the LIMS contains information such as sample ID, run number, sample location, and sample dilution factor. This information can be extracted from the LIMS and used to create worklists for automated dilutors (see Fig. 11.12). This eliminates the need for an analyst to re-enter this information into the dilutor, thereby increasing efficiency and reducing transcription errors. Implementing this requires the creation of a custom application that can query the LIMS and translate the information into a worklist that can be read by the automated dilutor. 

One potential drawback with this approach is that the automated dilutor may not dilute the samples exactly as instructed by the LIMS. There could be samples with insufficient volume, clots, errors, and others. For this reason, a manual QC step must ensure that the actual sample dilution information is reflected in the LIMS. If a particular sample was not diluted, then it must be deactivated or removed from the run in the LIMS. 

The second approach overcomes this issue by starting with a report from the automated dilutor showing how samples were actually diluted. This report is then... 

Dedicated Analyzers. A wide choice of such systems, with varying degrees of automation, are available for clinical applications. One of the most prevalent is an atomic-emission spectrometer that generates both Na and K concentrations simultaneously on separate readouts a matter of seconds after a sample is aspirated. Automatic dilutors are commonly built in. Auto samplers and printers are generally available as optional attachments. Some of these systems are readily converted to Li assays when needed. 

---