Clinton Hayes

Temporal and environmental sensitivity of a photodiode array spectrophometric system

The effect of the spectral variation in quartz tungsten lamp output with respect to elapsed time from power-up and variation in environmental temperature, and the variation in readout in the front-end electronics (FEE) and spectrometer with temperature, on predictive model performance of total soluble solids (TSS) in intact fruit was assessed for a silicon photodiode spectrometer-based system. Lamp (10 each of OSRAM HLX64623 and Sylvania 521995 12 V 100 W GY6.35 quartz tungsten halogen) output was assessed at 10 s intervals over a 4 h period, and 10 min intervals over approximately 3000 h. The environmental temperature of each component in a near infrared (NIR) spectroscopy system (lamp, FEE, spectrometer) was incrementally adjusted in 10°C intervals between 10°C and 60°C. The lamp output was spectrally stable within the time of the first measurement (10 s), although total illumination was not stable until approximately 40 min from start-up. Thus, the performance of the predictive models based on second derivative of absorbance data was not significantly impacted by lamp warm-up time. Noise on measurement associated with the use of a single white reference resulted in a mean increase in root mean square error of prediction (RMSEP) as high as 0.22% TSS and individual increases as high as 0.82%. Averages of white reference measurements significantly improved performance. When predictive models were developed using second derivative absorbance data and averaged (10) white references, there was no statistically significant impact in RMSEPs on time of lamp warm-up (after 10 s), even during the last hours of lamp life. Spectral variation resulting from changes of NIR system components (lamp and FEE) also affected lamp output quantity rather than quality and thus did not affect the predictive performance owing to the second derivative absorbance pretreatment. Some lamps displayed start-up output characteristics on their first use that were not repeated in subsequent trials. This result indicates the need for a short lamp “burn-in” period.

Light-emitting diodes as light sources for spectroscopy: Sensitivity to temperature:

Understanding of light-emitting diode lamp behaviour is essential to support the use of these devices as illumination sources in near infrared spectroscopy. Spectral variation in light-emitting diode peak output (680, 700, 720, 735, 760, 780, 850, 880 and 940 nm) was assessed over time from power up and with variation in environmental temperature. Initial light-emitting diode power up to full intensity occurred within a measurement cycle (12 ms), then intensity decreased exponentially over approximately 6 min, a result ascribed to an increase in junction temperature as current is passed through the light-emitting diode. Some light-emitting diodes displayed start-up output characteristics on their first use, indicating the need for a short light-emitting diode ‘burn in’ period, which was less than 24 h in all cases. Increasing the ambient temperature produced a logarithmic decrease in overall intensity of the light-emitting diodes and a linear shift to longer wavelength of the peak emission. This behaviour is consistent with the observed decrease in the IAD Index (absorbance difference between 670 nm and 720 nm, A670–A720) with increased ambient temperature, as measured by an instrument utilising light-emitting diode illumination (DA Meter). Instruments using light-emitting diodes should be designed to avoid or accommodate the effect of temperature. If accommodating temperature, as light-emitting diode manufacturer specifications are broad, characterisation is recommended.

Improving calibration transfer between shortwave near infrared silicon photodiode array instruments

The use of a model developed on spectra of one (master) instrument with spectra collected using another (slave) instrument requires differences in spectra of master and slave units to be orthogonal to the calibration model. The more spectral similarity is achieved in hardware, i.e. by matching the optical characteristics of the devices, the less chemometric correction is required. The transfer of partial least squares models for total soluble solids (TSS) of intact apple fruit between instrumentation based on silicon photodiode arrays was improved by use of more accurate wavelength assignments over the wavelength range used in the model. Several transfer methodologies were trialled, including piecewise direct standardisation (PDS), transfer by orthogonal projection, model updating (MU) and difference spectrum adjustment. The difference spectrum method combined with new wavelength assignments and MU gave results comparable to the performance of the master instrument and to models directly developed on the slave instruments (r2 = 0.95, SEP-b = 0.47 and bias = −0.03% TSS, for a population of mean = 14.45 and SD = 1.64% w/v). The use of average difference spectrum adjustment combined with MU was preferred over PDS because of ease of implementation.

Spectrophotometer ageing and prediction of fruit attributes

Deterioration of lamp output quality over time and degradation of detector signal-to-noise ratio are issues associated with ageing of a spectrophotometer. To document the effect of instrument ageing on short wavelength near infrared spectroscopy-based assessment of internal attributes of fruit quality prediction (total soluble solids, TSS, of juice), an assessment was conducted of several handheld photodiode array-based spectrophotometers over several years, with repeated spectra of a reference Teflon (PTFE) tile and spectra of 20 apple fruit acquired at yearly intervals. The repeatability of each instrument was assessed as the standard deviation of absorbance of repeated measures of a reference, typically around 0.2 mAbs. Instrument changes were identified in performance and in principal components analysis plots, but performance (apple TSS model) was not related to instrument repeatability. A piecewise direct standardisation method is recommended to maintain a multivariate calibration model across spectrometers.