UV-Vis Spectrophotometer

A spectrophotometer answers a different kind of question from most of the instruments it stands beside. It does not report a property of the sample directly; it reports how much light the sample absorbed at a wavelength someone chose, and everything else - concentration, purity, whether a reaction went where it was supposed to - is inferred from that. The choice that fixes everything downstream is therefore which wavelengths the instrument can reach and how finely it can separate them.

This range covers nine models in two series. The V series works in the visible region on a tungsten lamp; the UV series adds a xenon lamp and reaches down to 190 nm. Across them the bandwidth runs from 5 nm to 1.8 nm, the wavelength accuracy from 2 nm to ±0.5 nm, and all but one model are single beam.

What the instrument measures

Four quantities are reported: transmittance, absorbance, concentration and the slope factor that connects the last two. Transmittance is the fraction of the incident light that passes through the sample. Absorbance is its negative logarithm, which is the quantity that is proportional to concentration in the working range and the one most measurements are actually taken in. Concentration follows from absorbance through a factor established against standards of known concentration, and the slope factor is that conversion held by the instrument.

Because the relationship between absorbance and concentration is linear only within limits, the useful measuring range is narrower than the scale suggests. A sample that is too concentrated reads as though it were dilute, because the relationship stops being linear before the detector runs out of signal. That is a property of the method rather than of the instrument, and it is why the choice of path length and of dilution matters as much as the model chosen.

How the measurement works

Light from the lamp is dispersed by a 1200 line per millimetre holographic grating in a Czerny-Turner monochromator, which turns the broad output into a narrow band at a chosen wavelength. That monochromatic beam passes through the sample, and the intensity reaching the detector is compared with the intensity that reached it through a reference. The ratio is transmittance, and the logarithm of its reciprocal is absorbance.

Three things follow from that arrangement. The first is that the source and the detector are as much a part of the measurement as the sample is, which is why source life and drift appear on the specification at all. Drift is quoted as ≤0.003 A on every model, and it is the figure that decides whether a small absorbance difference can be trusted over a long run. The second is that the monochromator sets the resolution: the grating is common to the range, but the slit width behind it, quoted as spectral bandwidth, is not, and it is the bandwidth rather than the grating that decides whether two close peaks stay separate. The third is that the reference matters. Every measurement is a ratio, so anything that changes between the reference and the sample reading lands in the result unless the optics measure both at the same time or the instrument is re-zeroed often enough.

Choosing between the visible and ultraviolet series

The visible series reaches 320-1100 nm, except on LC-V1200E where the span is 350-1050 nm. The ultraviolet series reaches 190-1100 nm on every model, the additional span coming from a xenon lamp alongside the tungsten one. Nothing else in the specification decides this: a method written below about 320 nm cannot be run on a visible instrument, because there is no light there to absorb, and a method written above that wavelength does not need the ultraviolet series unless something else in the same laboratory does.

Where both regions are needed from one instrument, the UV series covers the visible region in the same box, so there is no reason to buy the visible series as well. Where the work is entirely in the visible region and the laboratory has no intention of moving below 320 nm, the visible models reach the same wavelengths with the same grating at a lower specification cost.

Bandwidth, and why it decides whether two peaks resolve

Spectral bandwidth is the width of the band of wavelengths the instrument actually passes at any one setting, and it runs here from 5 nm to 1.8 nm. A wide bandwidth is not simply a coarser reading: it averages together everything inside that 5 nm window, so two absorption peaks sitting within a few nanometres of each other are reported as one broad shoulder, and no amount of care elsewhere recovers them. A bandwidth comfortably narrower than the peak being measured is the requirement, and where a method states a peak width the bandwidth can be chosen against it directly.

That is the reason the range spans a factor of nearly three in bandwidth across nine models. Where a method reads a broad visible peak at a fixed wavelength, a 4 nm or 5 nm bandwidth is not a compromise. Where the method involves a sharp peak, a shoulder, or two peaks that have to be separated, the 1.8 nm or 2 nm models are the ones that can do it.

Single wavelength or scanning

The base models are set to a single wavelength and read the sample there, which is what a routine determination needs and is the fastest way to run one. The N and NC models and the Pro and 1800 models scan: the wavelength moves continuously and the spectrum is recorded across a range, which is what is required to locate a peak, to compare a sample against a reference spectrum, or to establish that nothing unexpected is absorbing elsewhere in the range.

Scanning is not only for research work. Any method that has to be validated in the laboratory, or that may be disturbed by an interfering substance, is easier to defend with a spectrum than with a single point, and a spectrum taken once often removes a later argument. Where a method is fixed at one wavelength and the answer is simply the number at that wavelength, single wavelength operation is not a limitation and the scanning models are not needed.

Single beam and double beam

Eight of the nine models are single beam, which measures the reference and the sample at different moments and relies on re-zeroing to keep the two comparable. LC-UV1800 is double beam, which splits the light so that the sample and the reference are measured at the same time and any change affecting both cancels continuously. In a single beam instrument, source drift, detector drift and a temperature change in the compartment all enter the result between the reference reading and the sample reading; periodic re-zeroing removes most of that, which is why single beam instruments remain perfectly adequate for routine work.

The difference becomes worth paying for in two situations: a measurement that has to be held stable over a long period, and a method in which the absorbance difference between the sample and the reference is small enough that drift is a significant part of it. LC-UV1800 also carries the finest bandwidth in the range at 1.8 nm, so it is the instrument for a laboratory that needs both a narrow bandwidth and stability rather than one that simply wants the top of the list.

Cuvettes, the compartment and the consumables

Every model is supplied with a four position cuvette holder, either a fixed 1 cm holder or on some models a 0.5-5 cm adjustable one, and a box of four 1 cm glass cuvettes. The UV models add two quartz cuvettes. That is the set, and the point to hold on to is that glass does not transmit in the ultraviolet: any measurement below about 320 nm needs quartz cells, they come only with the UV models, and they have to be ordered separately. Extra quartz cells are an order item in their own right rather than an accessory that can be borrowed from another instrument.

The path length of the holder matters in the same way the bandwidth does: absorbance is proportional to the distance the light travels through the sample, so a 0.5 cm and a 5 cm holder measure different amounts of the same solution. Where a method states a path length, the holder has to match it, and where a dilute sample has to be read without concentration, a longer path length raises the absorbance into the useful range.

Measurement limitations and selection considerations

Several parameters that matter in a laboratory are not stated anywhere in the material: the mains supply, the warm-up time, the ambient temperature and humidity limits, the source replacement interval, the ingress protection of the enclosure, and the light source life per model. Nothing here is inferred to fill those gaps, and the material’s own note that its figures are the supplier’s nominal values, to be confirmed against the delivered nameplate and manual, applies to the whole page.

Two operational limits are worth stating plainly. The instrument weighs between 7 kg and 12.5 kg depending on the model and is a bench instrument whose optical path is sensitive to vibration, so it belongs on a stable bench rather than a shared trolley. And the lamps are consumables with a rated life of 1000 h or 2000 h by model, which the material notes is shortened by frequent switching, so an instrument that is used throughout a working day is better left running than switched on and off around each measurement.

Where the instrument is to be used for regulated testing, the qualification documents are a separate requirement. The material records that certification material was removed at source and that no certificate information is included, and it advises requesting the type approval certificate and the 3Q validation documents from the supplier for pharmaceutical and food testing. That is a question for the enquiry rather than an assumption from the delivery.

Ordering

The items that decide the answer are the wavelength or wavelengths the method uses, the peak width and therefore the bandwidth required, whether single wavelength operation is enough or a spectrum has to be recorded, whether the ultraviolet region is needed at all, the path length of the cells, whether the results have to be captured on a computer, and which documents the laboratory’s quality system requires. State the method rather than only the model where possible, because the wavelength and the peak width are what fix the model list rather than the other way round. Quantity, destination port and required delivery date complete the enquiry.