Fraction collectors are well-known devices intended for collecting liquid samples originating from slow-flowing sources having variable liquid compositions. Fraction collectors are typically used in chromatographic separations such as high performance liquid chromatography (HPLC), gas chromatography (GC), supercritical fluid chromatography (SFC), column chromatography, and liquid-liquid separations, and for the collection of distillates from various distillation processes. The size of each individual sample or fraction dispensed by the fraction collector is determined by conventional measuring equipment such as a timer, a drop counter, a level sensing device, or by a variety of spectrophotometric methods. Fraction collectors typically sequentially align a dispensing head, carry a sample delivery conduit or tube over individual collection vessels part of an array of collection vessels, and deliver sample to collection vessels.
Fraction collectors may be broadly categorized into two groups. The first group includes fraction collectors in which a plurality of collection vessels are mounted on a generally circular turntable. These fraction collectors are commonly called “turntable collectors.” The turntable collectors all generally operate to fill a plurality of collection vessels by the combination of a rotatably mounted turntable and a rotatably mounted arm carrying a dispensing head. The dispensing head is typically aligned over a single collection vessel, the liquid is dispensed into the collection vessel, and the turntable then rotates to align with another collection vessel. In turntable collectors, the plurality of collection vessels are typically arranged in a concentric circular or spiral pattern. In order to manipulate the dispensing head over collection vessels in different circles of a concentric pattern or with an expanding or contracting spiral pattern, the rotatably mounted arm moves to align the dispensing head with each of the individual collection vessels in the arrangement.
The second group of fraction collectors includes collectors with the collection vessels arranged in a grid pattern on a stationary stage, and a dispensing head manipulated in either a 2-dimensional plane or in all three dimensions to selectively dispense liquid into the individual collection vessels. Fraction collectors in the second group are commonly called “X-Y collectors.”
The two groups of fraction collectors each have distinct advantages. For example, when handling a large number of liquid collection vessels, fraction collectors with rectangular grid patterns more effectively use bench space. X-Y collectors are also better suited to handling large scale collection vessels than are the rotatably mounted turntables. Furthermore, X-Y collectors may be adapted to popular standardized receptacle plates, such as microtiter plates, or other standard or custom arrangements. In contrast, turntable-type fraction collectors may be more advantageous when used in high resolution liquid chromatography as they require shorter attachment tubing between the slow-flowing source and the dispensing head, thus limiting diffusional re-mixing of the separated components within the liquid flow and resulting in better separation.
Both X-Y and turntable fraction collectors must be adjusted to fit the collection vessels used for sample collection, which commonly come in various sizes. Generally, vertical adjustment of the fraction collector has typically been done by either manual adjustment of the legs or incorporation of a vertical adjustment (Z-axis) of the dispensing head. The first method, utilized in some X-Y collectors, allows for manual adjustment of the height of the dispensing head by removing the legs of the base of the fraction collector and putting on longer or shorter legs as desired. However, while this method can be effective, if the legs of the base are adjusted manually, the racks containing the collection vessels need to be removed during adjustment, as the legs can only be adjusted individually. Furthermore, it is difficult to balance the collection bed during the adjustment, creating the opportunity for spillage to occur. Moreover, manual adjustment requires realignment of the dispensing head with the collection vessels and collection bed, introducing additional steps that could damage the fraction collection apparatus. As fraction collectors have become more advanced, the tolerances have become tighter, and therefore manual adjustment of the dispensing head assembly threatens to damage some of the fragile electrical parts.
In addition, manual vertical dispensing head adjustment may knock the dispensing head out of horizontal alignment, causing the liquid fractions to miss the sample collection vessels. This can result in lost time in cleaning up the liquid, damage to the instrument, or present a hazardous environment for the operator, depending on the contents of the liquid. Furthermore, when a dispensing assembly is knocked out of alignment, the precision alignment procedures required to realign the assembly can be very time consuming.
Incorporation of a vertical adjustment (Z-axis) axis into the dispensing head is known in the art. However, fraction collectors with a Z-axis require complex mechanics, and sometimes software, in order to get the dispensing head to move reliably and precisely in all three directions. These complex mechanics often add both size and cost. Furthermore, because collection vessels used in a single fraction collection run are commonly of uniform size, a dispensing head that moves in a vertical direction is often unnecessary.
Regardless of whether the fraction collector is an X-Y collector or a turntable collector, it is of primary importance in the design that the mechanism for aligning the dispensing head and collection vessels be as simple and as sturdy as possible, while being reliably capable of precisely positioning the dispensing head over sequential collection vessel. Although many fraction collectors known in the art can reliably and precisely position the dispensing head over a sequential collection vessel, many are quite complex and therefore not only expensive but difficult to adjust and maintain. In particular, known fraction collector arrangements may include dispensing head adjustment mechanisms that are extremely complicated and delicate.
Therefore, a need exists for a fraction collector that reliably and precisely aligns a dispensing head and collection vessel without adding significant cost or fragility.