Pickup error is a geometry problem reported as a fluidics problem

A 96-well plate that sat 0.8 mm low in its tray produced 47 “no sample” aborts on a 60-injection sequence, and the analyst spent two hours replacing a syringe that was never the problem. The instrument was a Thermo Scientific Vanquish with a plate gripper, and the fault was a worn tray locator pin that let the plate settle below the pickup window. That is the shape of most lab chromatography autosampler pickup error and misalignment events I have dealt with: a mechanical fit failure that the firmware reports as a fluidics or sample problem. The chemistry was fine. The plate was not where the needle thought it was.

This article is about the geometry of that failure. Tray height, gripper tolerance, cap seating, and the error codes each one produces on Agilent, Waters, Shimadzu, and Thermo instruments. If your sequence is aborting on pickup, the fix is almost always a measurement, not a part swap.

The autosampler has a narrow window in which the needle can enter a vial or well and still draw a valid aliquot. On a standard 2 mL vial with a 12 x 32 mm body and a septa cap, the usable needle travel is roughly the top 6 to 8 mm of the vial interior. If the vial sits 1 mm proud of its nominal seat, the needle can bottom out on the septum or short-stroke. If it sits 1 mm low, the needle can enter but the sample volume drawn is short by the dead volume above the needle tip.

Most instruments do not measure the vial position. They infer it. The firmware knows the commanded Z-axis travel and the expected motor current profile. When the needle contacts something harder than liquid at the expected depth, or fails to reach the expected depth, it raises a pickup or “sample not found” error. The message names the symptom, not the cause. Your job is to convert the error back into a dimension.

The three dimensions that matter:

• Tray height: the vertical distance from the tray datum to the vial or plate seat. Nominal is set by the instrument, typically with a calibration gauge or a factory-set offset. Drift here is the single most common cause of pickup error I see.
• Gripper tolerance: the positional accuracy with which the plate handler places a microplate into the nest. On instruments with a robotic gripper, this is a repeatability figure, usually specified at ±0.2 to ±0.5 mm. Wear in the gripper jaws opens that up.
• Cap seating: how far the septum cap is pressed onto the vial neck. A cap that is not fully seated sits high, and the needle short-strokes against it. A cap that is cross-threaded sits at an angle, and the needle glances off.

Each of these produces a different error signature. Learning to read the signature is faster than pulling the syringe.

The error codes each misalignment produces, by instrument family

Firmware messages differ, but the mapping from geometry to code is consistent. Here is what I have logged across the common platforms. Exact strings vary by firmware revision, so treat the code families as a guide and check your own service manual.

The pattern to internalize: a height fault gives you a no-sample or short-draw message, because the needle reaches its commanded depth but the liquid is not there. A tilt or crash fault gives you a needle or Z-axis message, because the needle hits something it should not. Height faults are quiet and intermittent. Crash faults are loud and usually bend the needle. If you are chasing intermittent aborts, you are almost certainly looking at a height or seating problem, not a crash.

Why tray height drifts, and how to measure it back to nominal

Tray height does not drift on its own. It drifts because something in the stack moved. The common causes, ranked by how often I have found them:

• Worn or missing tray locator pins. The pins that register a sample tray or plate nest to the deck wear down or get bent during tray swaps. A pin worn by 0.3 mm lets the tray settle that much lower. This was the Vanquish case above.
• Wrong tray or adapter for the vial format. A tray cut for 2 mL vials used with 1 mL vials sits the smaller vial too low. The firmware does not know the vial format changed unless the tray ID says so.
• Deck contamination. Dried buffer or salt under a tray lifts one corner. A 0.2 mm salt crust under one edge tilts the whole tray and produces position-dependent aborts.
• Gripper jaw wear. On plate handlers, worn jaws place the plate a fraction low or off-center. The plate looks seated but the well centers are shifted.

Measure it directly. Put a dial indicator or a height gauge on the deck datum and check the top of a vial or plate at four corners of the tray. The four readings should agree within ±0.2 mm. If they do not, you have a tilt or a height fault, and you can localize it to a corner. On a plate handler, check the plate seat with a feeler gauge: the gap under the plate at each corner should be uniform and under 0.1 mm.

For vial-based instruments, the calibration gauge that ships with the autosampler is the reference. Run the height calibration procedure after any tray swap, any locator pin replacement, and any time you change vial format. Do not skip it because the tray “looks the same.” The 0.8 mm that cost the Vanquish run two hours was invisible to the eye.

Gripper tolerance: the part of the alignment chain nobody checks

On instruments with a robotic plate gripper, the gripper is a second alignment stage stacked on top of the tray. The tray positions the plate nest; the gripper positions the plate within the nest. Both must be in tolerance, and a fault in either produces the same “plate not detected” family of errors.

Gripper repeatability is specified by the manufacturer, typically at ±0.2 to ±0.5 mm at the plate seat. That tolerance is consumed by jaw wear, by a bent gripper finger, and by debris on the jaw faces. A gripper that places a plate 0.4 mm off-center shifts every well center by 0.4 mm. On a 96-well plate with a 9 mm well pitch, 0.4 mm is not enough to miss the well, but it is enough to put the needle close to the well wall, which raises the risk of a needle glancing off and bending on the next injection.

Check gripper alignment with a plate that has been marked at the nest datum. Place it, remove it, and measure the offset between the plate edge and the nest edge at two adjacent sides. The offset should be under 0.3 mm and consistent across repeated placements. If it varies placement to placement, the jaws are worn or the gripper is loose on its mount. Torque the gripper mount per the service manual (commonly in the 0.5 to 1.0 N·m range for the small fasteners) and re-check.

The other gripper failure I see is a plate that is placed correctly but not fully seated. The gripper releases before the plate drops onto the nest datum, so the plate rests on a corner of the nest. The well centers are then high on one side and low on the other. This produces position-dependent aborts across the plate, which is the signature that separates a gripper seating fault from a tray height fault (which is uniform across the plate).

Cap seating: the 0.5 mm that turns into a bent needle

Cap seating is the alignment dimension analysts control most directly and get wrong most often. A crimp cap or screw cap that is not fully seated sits high by the thickness of the unseated skirt, commonly 0.3 to 0.8 mm. The needle then short-strokes against the septum, and the firmware reports a short draw or a no-sample condition on that position only.

The failure modes, with the numbers:

• Under-crimped cap. The crimp jaw did not close fully, so the cap sits high. A properly crimped 11 mm cap has the skirt rolled under the vial neck bead with no visible gap. Check with a go/no-go gauge or by eye at 10x. Under-crimped caps are the leading cause of position-specific short draws I have logged.
• Cross-threaded screw cap. The cap sits at an angle of a few degrees. The needle enters the septum off-axis and can glance off the aluminum or plastic cap edge, bending the needle. This is a crash fault, not a short draw.
• Septum not pre-punctured on a reseal cap. Some caps need the septum pre-slit or the needle can coring. A cored septum drops a plug into the vial, which then blocks the needle on the next draw from that vial. The puncture count matters: a standard silicone/PTFE septum holds up for roughly 3 to 5 punctures before coring risk rises, and a pre-slit septum for more. Track punctures per vial if you re-inject.
• Wrong cap for the vial neck. An 11 mm cap on a 13 mm neck, or a snap cap on a crimp vial, sits high or loose. Match the cap to the neck finish every time.

For the consumable side of this, the septum material and the cap geometry are specified together. If you are reviewing what the septum is made of and how it behaves under repeated puncture, the polymer and extraction behaviour is covered elsewhere on the site. Here, the point is mechanical: seat the cap fully, check it with a gauge, and track puncture count per vial.

Two field cases, with the measurement that found each one

Case 1: the intermittent “no vial” on a 1260

Symptom. An Agilent 1260 autosampler aborted on “no vial detected” for 6 to 9 positions out of every 100, always in the same tray region. The analyst had replaced the needle and the syringe.

Measurement. I put a height gauge on the deck and read the top of a vial at the four corners of the tray. Three corners agreed within 0.1 mm. The fourth corner read 0.7 mm low. The locator pin at that corner was bent, so the tray settled on that side. The firmware’s position check passed (the tray was present) but the needle short-stroked on the low side.

Fix. Replaced the locator pin, re-ran the height calibration, and the aborts stopped. The needle and syringe were never the problem. The lesson: a uniform-across-tray fault points at the tray datum, a corner-localized fault points at a pin or a contamination crust, and a position-dependent-across-plate fault points at the gripper.

Case 2: the bent needle on an e2695

Symptom. A Waters e2695 bent a needle every few hundred injections, always after a batch of samples prepared in a different lab. The “needle obstruction” fault came up mid-sequence.

Measurement. I measured cap seating on 50 vials from the incoming batch with a go/no-go gauge. 14 of 50 were under-crimped by 0.4 to 0.6 mm. The needle was short-stroking on the high caps and, on the worst ones, glancing off the cap edge. The incoming lab had a crimper with a worn jaw.

Fix. Re-crimped the batch, replaced the worn crimper jaw, and added a cap-seating check to the incoming sample acceptance criteria. The needle bend was a downstream symptom of an upstream cap problem. This is the case that convinced me to put a go/no-go gauge next to every crimper on the bench.

A fit-and-alignment checklist you can run in 20 minutes

When a sequence aborts on pickup, work this list in order. It moves from the cheapest check to the most expensive, and it usually stops at step 3.

1. Read the error, classify it. No-sample or short-draw means a height or seating fault. Needle or Z-axis means a crash or tilt fault. Write down which positions failed and whether they cluster by tray region or by plate position.
2. Check the caps on the failed positions. Go/no-go gauge on 10 vials from the failing region. Under-crimped or cross-threaded caps explain position-specific faults.
3. Measure tray height at four corners. Dial indicator or height gauge, top of vial to deck datum. Agreement within ±0.2 mm is the pass criterion. A low corner is a pin or a contamination crust.
4. Check the tray locator pins. Visual and a straightedge. A bent pin is visible; a worn pin is not, so measure the tray seat height against a known-good tray.
5. For plate handlers, check gripper placement. Marked plate, repeated placements, offset under 0.3 mm and consistent. Varying offset means worn jaws.
6. Re-run the height calibration. After any tray swap, pin replacement, or vial format change. This is not optional.
7. Only now look at the fluidics. Syringe, needle, and tubing. If steps 1 through 6 passed and the fault persists, the problem is downstream of the geometry.

The order matters because the geometry checks are fast and non-destructive, while the fluidics checks involve removing parts and introducing new variables. I have watched analysts replace a syringe, a needle, and a loop before measuring tray height, and the fault was a bent pin the whole time.

Specifying consumables so alignment problems do not come back

Most of these faults are preventable at the point of purchase. When you specify vials and caps, ask for the dimensions that govern seating, not just the material.

• Vial neck finish and cap match. 11 mm crimp, 13 mm crimp, 9 mm screw, or snap, specified as a pair. Do not mix.
• Crimp cap skirt dimensions. Ask the supplier for the seated height tolerance. A cap that seats to within ±0.1 mm of nominal is what you want.
• Septum puncture rating. Silicone/PTFE for 3 to 5 punctures, pre-slit for more. Match to your re-injection policy.
• Plate flatness. For plate handlers, ask for the plate flatness spec (commonly under 0.2 mm across the footprint). A warped plate defeats a perfectly aligned gripper.

For the fluid path downstream of the needle, the filter and membrane choices matter to the same alignment story because a partially clogged filter changes the draw profile and can mask a short-draw fault as a pressure fault. The HPLC filter and PTFE membrane filter selections are covered on their own pages. The COD vial format is a separate geometry again, with its own cap and septum dimensions, and it is worth confirming the seat height against your autosampler’s tray before you commit a sequence to it. For instrument-specific service procedures and consumable dimensions, the manufacturer resources are the reference: Agilent chromatography consumables and columns, Thermo Fisher chromatography and sample handling, and Restek technical resources for chromatography. For method-level context on sample handling in regulated work, U.S. EPA SW-846 test methods is the reference for the waste and materials methods that most often drive these sequences.

If you are building out a bench and want the broader consumables context, the Lab Chromatography resource covers the instrument and consumable categories this article sits inside.

The decision rule

When a lab chromatography autosampler pickup error appears, do not touch the fluidics until you have measured the geometry. Classify the error first: a no-sample or short-draw message means a height or cap-seating fault, and a needle or Z-axis message means a crash or tilt fault. Then measure tray height at four corners against a ±0.2 mm criterion, check cap seating with a go/no-go gauge on the failing positions, and for plate handlers check gripper placement against a 0.3 mm offset. Re-run the height calibration after any tray swap or vial format change. If all of that passes and the fault persists, then and only then move to the syringe, needle, and tubing. The alignment chain is short, measurable, and almost always where the fault lives.

References & further reading

Related resources from our lab consumables network:

• Lab Chromatography at labchromatography.com
• Hplc Filter at hplcfilters.com
• Ptfe Membrane Filter at ptfemembranefilter.com
• Cod Vial at codvial.com

External references and standards:

• Agilent chromatography consumables and columns
• Thermo Fisher chromatography and sample handling
• U.S. EPA SW-846 test methods for waste and materials
• Restek technical resources for chromatography