However, intra-laboratory studies of analytical variability have been conducted. The results have been analyzed by separating data from these laboratories using automatic methods from those using manual techniques. Reasons for dispersion in both cases are poor standardization and photometric variability. It therefore seems to suggest that the photometric performance of automatic instrumentation should be studied to assess the magnitude of the photometric component of the analytical error. At the very least, linearity specifications and standards seem necessary. If the results of the CAP are representative of the spectrophotometric performance range in this country, then we should be concerned about the effect of that variable yield on accuracy and precision.
Sometimes very little sample is available for research and shorter path lengths of only 1 mm are needed. Where quantification is required, the absorption values shall be kept within the dynamic range of the instrument below 1. This is because an absorption of 1 implies that the sample absorbed 90% of the incident light, or is indicated as 10% of the incident light was let through the sample. With so little light reaching the detector, some UV-Vis spectrophotometers are not sensitive enough to reliably quantify small amounts of light. Two possible simple solutions to this problem are diluting the sample or reducing the length of the pad.
An experiment was conducted to assess a self-analyzer’s ability to distinguish small changes in concentration. Phosphate solutions in the range of 3.23 to 4.73 mg/dl were prepared by dilutions of heavily reactive phosphate salt. These were analyzed in a car analyzer (modified Sumner technique), on three separate days. On each day, the standards used in the routine laboratory to prepare calibration curves were used. These were linear in the concentration range from 1 mg/dl to 12 mg/dl (absorption range 0.065 to 0.710).
Some idea of the extent of the errors involved can be obtained by studying Table 6. This table summarizes an experiment in our laboratory in which a diluted solution of nicotinamide adenine dinucleotide reduced in phosphate buffer at pH 7.4 was made. These solutions were placed in internal test tubes with a diameter of 19 mm and read against a phosphate white space in 10 and 20 nm broadband network instruments. The same test tubes were used on both instruments and the measurements were completed within minutes. As far as this author knows, no studies have been published of the photometers used in automatic instrumentation.
The detector takes advantage of the fact that the extinction coefficients of oxy and deoxy hemoglobin are almost equal in the wavelength range from 930 nm to 800 nm. By emitting a beam of light in this range, the detector is relatively insensitive to oxygenation of the blood. Commercially available solid-state light sources and detectors that respond to wavelengths of 820 nm can be selected for use in the sensor. Therefore, quartz sample holders are required for UV research because quartz is transparent to most UV light. Air can also be seen as a filter because wavelengths of light shorter than about 200 nm are absorbed by molecular oxygen in the air. A special and more expensive configuration is required for measurements with wavelengths of less than 200 nm, usually involving an optical system filled with pure argon gas.
Chemical standards should also be available to verify the linearity and sensitivity of photometers used in automatic analyzers. Our UV fish range quartz cells and spectrophotometer cells are manufactured from UV quality quartz glass. This material offers 83% (HTR & CRF-H) or 80% (CRF & NRC) transmission from 190 to 2500 mm and is the best choice for testing within that range. The transmission of our IR quartz veins is 80% in the usable range of 220 to 3500 nm. Glass buckets are usually intended for use in the wavelength range of visible light, while molten quartz is usually used for ultraviolet applications.
The absorption is equal to the logarithm of a fraction that includes the intensity of the light before the sample is reviewed divided by the intensity of the light after passing through the sample. Fraction I shared by Io is also called transmission, which expresses how much light has passed through a sample. However, Beer-Lambert’s law is often applied to obtain the concentration of the sample after measuring absorption when molar absorption (ε) and trajectory length are known.
The data represent the absorption of acid dichromate calculated from the CAP survey for three concentrations. The high, low and average values are shown, as well as the data obtained in our laboratory. It is clear that any analysis requiring absorption for the conversion of absorption into concentration would be a serious error, if a value from the literature were used.
Traditional ultraviolet-visible spectroscopy or fluorescence spectroscopy uses liquid samples. Often the sample is a solution, with the spectrophotometer cuvettes substance of interest dissolved inside. The sample is placed in a bucket and the bucket is placed in a spectrophotometer for testing.
Spectrophotometric measurements, both manual and automated, are widely used in the clinical chemistry laboratory. In this country, at least 1,000,000 measurements of this type are carried out per day in a wide variety of equipment; however, there are few rules. The results of intra-laboratory studies suggest that performance can be improved.