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K E Y w O R D S . P/B-ratio, X-ray microanalysis, bulk specimens, surface roughness, quantitative analysis, background subtraction, peak intensity, biological microanalysis, scanning electron microscopy, energy dispersive analysis, tilt angle, epoxy resin. S U M M A R YThe use of a net peak intensity and of a peak to background (P/B)-ratio of sulphur and chlorine is examined in the X-ray microanalysis of a 2.40, w/w S bulk standard in Spurr's epoxy resin. In calculating the P/B-ratio, the background intensity is calculated for the same energy region as for the net peak. Analyses were carried out on the flat top of the standard and on the slope running down from the top on the side not facing the X-ray detector. The results obtained for the peak to local background ratios from the top and the slope yielded a relatively small mean deviation (1 1 "/,,) while net peak intensities ultimately were reduced to 7% or less of the initial value for the flat top. This indicated that a peak to local background ratio is to be preferred in the quantitative analysis of bulk specimens which have poorly defined local tilt and takeoff angles. A second advantage is the inherent correction for beam current fluctuations. I N T R O D U C T I O NIn quantitative X-ray microanalysis of thin biological specimens (i.e. transparent to the beam electrons) both net peak intensities as well as peak to background (P/B) ratios are used in order to quantify the spectral information on elemental composition (Hall, 1975). In the quantification of bulk specimens, in general, the net peak intensity is used ever since the beginning years of electron probe microanalysis (early fifties). However, in the last decade the advantages of using a P/B ratio instead of a net peak intensity, especially in biological bulk specimen X-ray microanalysis, are increasingly recognized (Statham, 1981;Echlin et al., 1982). As early as 1971 Cobet & Traub found empirical relations between the P/B ratio and the concentration of a certain element in methacrylate standards. Zs-Nagy et al. (1977) showed the same results for measurements on organic crystals. Pawley (1978) and Small et d. (1979) approached the use of P/B ratios for bulk specimens more theoretically and developed a special method to analyse irregularly-shaped specimens. Wroblewski et al. (1978) and Statham (1979) used the ratio of the P/B ratio of an element in the specimen to the P/B ratio of the same element in the standard, as starting data for a ZAF-correction.In biological bulk specimen X-ray microanalysis the use of a P/B ratio can be advantageous because one of the inaccuracies involved is the poorly defined surface topography (Boekestein et al., 1980a, b). The specimen preparation techniques for X-ray microanalysis will include (Q 1984 The Royal Microscopical Society 327

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Stems of cut rose flowers (Rosa hybrida L. cv. ‘Sonia’) were placed in water to study the development of a population of micro‐organisms at the cut surface and in the xylem vessels. The cut surface became covered with bacteria within 2 d of vase life. The bacteria were accompanied by an amorphous substance which was apparently bacterial slime. After 7 d of vase life many fungal hyphae were also found at the cut surface. Inside the xylem vessels the bacteria were often clustered at the inter‐vessel pits. After 4 d of vase life most of the vessels that had been opened by cutting contained bacteria. Only a few xylem elements, located several centimeters from the cut surface, contained an amorphous substance. A few fungal hyphae were observed inside the xylem vessels. No yeasts were found, either at the cut surface or inside the xylem. Pseudomonas species accounted for more than 70% of the total bacterial population of the cut surface and the xylem vessels, and Enterobacter species (mainly Ent. agglomerans) for less than 10%. Acinetobacter, Aeromonas, Alcaligenes, Bacillus, Citrobacter and Flavobacterium were occasionally observed.

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